2002(A)Using Soluble Polymers To Recover Catalysts and Ligands

Using Soluble Polymers To Recover Catalysts and Ligands

David E.Bergbreiter*

Department of Chemistry,Texas A&M University,P.O.Box30012,College Station,Texas77842-3012

Received March5,2002

Contents

I.Introduction3346

II.Early Work3346

A.Linear Polystyrene-Bound Catalysts3346

B.Asymmetric Catalysis with Linear

Polymer-Supported Catalysts

3347

C.Water-Soluble Polymer Supports for Catalysts3347

1.Proteinaceous Supports3347

2.Polar Polymer Supports3348

D.Polymers Supporting Catalysts with Thermally

Reversible Solubility

3348

1.Sirotherm Resins3348

2.Poly(vinylpyridine)Supports3348

E.Biphasic Systems Employing Soluble

Polymer-Supported Catalysts

3348

F.Inorganic Polymer Supports3349

1.Polyphosphazene Supports3349

2.Silicon-Containing Polymers as Catalyst

Supports

3349

G.General Features Common to Early Work3350

III.Characterization Advantages of Soluble Polymer

Supports

3350

A.1H NMR Spectroscopy3350

B.13C NMR Spectroscopy3351

C.Relaxation Time Effects3352

D.UV?Vis,Fluorescence,and IR Spectroscopic

Studies of Soluble Polymer-Bound Ligands

and Catalysts

3353

IV.General Features of Soluble Polymer-Supported

Catalyst Activity

3354

V.Strategies for Separation and Recovery of

Soluble Polymer-Bound Ligands and Catalysts

3355

A.Solid/Liquid Separations of

Polyethylene-Bound Catalysts

3356

1.Rhodium(I)Hydrogenation Catalysts on

Polyethylene Supports

3357

2.Polyethylene-Bound Nickel(0)Diene

Cyclooligomerization Catalysts

3358

3.Polyethylene-Bound Palladium(0)

Catalysts

3359

4.Polyethylene-Bound Palladium(II)

Catalysts

3359

5.Polyethylene-Bound Hydroformylation

Catalysts

3360

6.Polyethylene-Bound Ruthenium Catalysts3360

7.Polyethylene-Bound Europium(III)

Complexes 3361

8.Polyethylene-Bound Rhodium(II)

Cyclopropanation Catalysts

3362

9.Polyethylene-Bound Chiral Rhodium(II)

Catalysts

3362

10.Polyethylene-Bound Phase-Transfer

Catalysts

3362

11.Polyethylene-Bound Tin Catalysts3363

12.Polyethylene-Bound Polymerization

Catalysts

3363

B.Solid/Liquid Separations of Catalysts on

Polymers with Inverse

Temperature-Dependent Solubility

3364

1.“Smart”Thermally Responsive Catalysts3365

2.Poly(N-isopropylacrylamide)-Bound

Catalysts

3365

C.Solid/Liquid Separations of Catalysts on Acid-

and Base-Soluble Polymers

3366

D.Solid/Liquid Separations of Catalysts after

Solvent Precipitation of Polymers

3367

1.Poly(alkene oxide)-Bound Catalysts3367

2.Polyacrylonitrile-Bound Scandium

Catalysts

3368

3.Chiral Rigid-Rod Polymers Containing

Binaphthyl Groups as Catalyst Supports

3368

4.Binaphthyl-Containing Dendritic

Polymer-Supported Catalysts in

Asymmetric Hydrogenation

3370

5.Linear Polystyrene-Bound Catalysts3370

6.Dendrimer-Supported Catalysts3370

7.Poly(dimethylsiloxane)-Supported

Catalysts

3371

8.Polyacrylate-Supported Catalysts3371

E.Liquid/Liquid Separation of Water-Soluble

Polymer-Bound Catalysts under Aqueous

Biphasic Conditions

3371

1.Water-Soluble Polymer-Bound

Hydrogenation Catalysts Derived from

Poly(acrylic acid)or Polyethylenimine

Used under Biphasic Conditions

3372

2.Water-Soluble Polymer-Bound

Hydroformylation Catalysts Derived from

Poly(acrylic acid)or Polyethylenimine

Used under Biphasic Conditions

3372

3.Water-Soluble Polymer-Bound Asymmetric

Hydrogenation Catalysts Used under

Biphasic Conditions

3373

4.Modified Polymers as Supports for

Hydroformylation Catalysis under Biphasic

Conditions

3373

5.Proteins as Supports for Hydroformylation

Catalysis under Biphasic Conditions

3374

*E-mail:bergbreiter@https://www.doczj.com/doc/0613817246.html,.

3345 Chem.Rev.2002,102,3345?3384

10.1021/cr010343v CCC:$39.75?2002American Chemical Society

Published on Web08/30/2002

6.Poly(acrylic acid)-Supported Ruthenium

Transfer Hydrogenation Catalysts

3374

7.Polymers with Thermoregulated Solubility

as Supports in Catalysis under Biphasic

Conditions

3374

8.Dendrimers as Supports for Catalysts

under Biphasic Conditions

3375

F.Liquid/Liquid Separation of Fluorinated

Polymer-Bound Catalysts under Biphasic

Conditions

3375

1.Fluorinated Acrylate Polymers as

Fluorous-Phase-Soluble Supports

3375

2.Fluorinated Polyacrylates as Supports for

Catalysts in Supercritical Carbon Dioxide

3375

3.Fluorinated Polyacrylate-co-Polystyrene

Polymers as Fluorous-Phase-Soluble

Supports

3375

G.Liquid/Liquid Separation of Polymer-Bound

Catalysts with Thermomorphic Solvent

Mixtures

3376

1.Polar Polymers as“Normal”

Thermomorphic Biphasic Catalyst

Supports

3376

2.Dendrimers as“Normal”Thermomorphic

Biphasic Catalyst Supports

3378

3.Nonpolar Polymers as“Inverse”

Thermomorphic Biphasic Catalyst

Supports

3378

H.Membrane Separations of Soluble

Polymer-Bound Catalysts

3378

I.Separations of Soluble Catalysts on

Dendrimers

3380 VI.Summary3381 VII.References3381 I.Introduction

The use of soluble polymers to recover catalysts and ligands has its origins in the synthetic approaches to peptide and oligonucleotide synthesis that were developed in the labs of Merrifield and Letsinger in the1960s.1,2Those discoveries revolutionalized the synthesis of biomolecules.3,4They also led to a significant amount of work in a variety of industrial and academic laboratories s work that was in the case of catalysis directed toward developing systems to immobilize or heterogenize homogeneous catalysts.5 In most cases,this work focused on the same in-soluble polymers Merrifield used s divinylbenzene (DVB)-cross-linked polystyrene.5-10

Soluble polymers were common35years ago,but they did not receive as much attention for use as catalyst supports as was the case for their cross-linked,insoluble analogues.Indeed,through the 1960s and1970s reports describing the use of soluble polymers as supports for recovery and reuse of homogeneous catalysts were scattered.10,11This re-view first highlights selected examples of this early work because this work illustrates many of the ideas and separation strategies that have continued to receive attention in more recent work.12-14The review then discusses in a general way some of the common features of soluble polymers that differenti-ate them from their insoluble cousins.The review then goes on to outline in a broad way the separation strategies used to recover soluble polymeric ligands or soluble polymer-ligated homogeneous catalysts. Finally,it focuses on more recent work with several classes of soluble polymers,discussing their advan-tages,the separation strategies employed,and the limitations of these supports.This review does not cover biocatalysts and emphasizes examples where soluble polymers are used to recover and reuse a catalyst.Poly(alkene oxide)supports are reviewed elsewhere and are discussed to a limited extent here. II.Early Work

One of the first groups to use soluble polymers and to point out their advantages was Bayer’s group.15-17 Much of this group’s interest was in peptide synthesis and metal complexation using soluble polymers.18,19 However,Bayer’s group was among the first to describe soluble polymer alternatives to cross-linked polymers as catalyst supports.This work involved a variety of polymers,poly(ethylene oxide),polystyrene, poly(vinylpyrrolidine),and poly(vinyl chloride),al-though their initial published work focused mainly on the first two polymers.In general,Bayer’s group used relatively simple polymer modification to pre-pare the necessary ligands.

A.Linear Polystyrene-Bound Catalysts

In Bayer’s work,linear polystyrene(MW)100000) was chloromethylated,and the resulting benzylic chloride was then substituted by diphenylphosphine using KPPh2,chemistry analogous to that used earlier with cross-linked polystyrene resins(eq1).15,20 David E.Bergbreiter is an“immigrant”Texan with two“native Texan”daughters,a hyperactive chocolate Laborador,16acres,and a“very small”share of an oil well.After public schooling in Chicago,undergraduate work at Michigan State University,and graduate work with Professor Whitesides at the Massachusetts Institute of Technology,he and his wife Lynne moved to Texas A&M in1974.There he rose through the ranks and is currently a professor of chemistry with interests in catalysis,organic chemistry,and polymer and surface chemistry.

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The resulting polymer1contained varying amounts of phosphine on the basis of elemental analyses.A typical loading is shown in eq 1.Catalysts were loaded onto these soluble diphenylphosphine-contain-ing polymers by allowing an excess of this linear polymer to exchange with ligands of preformed Rh or Pd complexes.This exchange reaction produced a polymer-bound catalyst.[(Rh(CO)2(acac)],Rh2(CO)4-Cl2,Rh(PPh3)3Cl,RhH(CO)(PPh3)3,and PdCl2(PhCN)2 are examples of molecular complexes used in reaction with this diarylphosphinated chloromethylated poly-styrene to form polymeric catalysts.The resulting polymer-bound complexes(e.g.,2and3)were not extensively characterized.In some cases(e.g.,in the Pd case)the products may have been polymer-modified forms of colloidal metal.Both homogeneous hydrogenation and hydroformylation reactions were carried out using these polymer-bound catalysts.For example,the rhodium hydrogenation catalyst2pre-sumed to be analogous to Wilkinson’s catalyst was used for five cycles of hydrogenation of1-pentene, with catalyst recovery being effected by ultrafiltration using a polyamide membrane and a10000molecular weight cutoff.Solvent precipitation too was used to isolate these polymer-bound catalysts.The rhodium hydroformylation catalyst3had ca.3:1selectivity for the normal product in hydroformylation of terminal alkenes.This catalyst too was recycled twice using ultrafiltration to separate the catalysts and products. The authors report separation of the soluble polymer-bound Rh catalysts was quantitative.

B.Asymmetric Catalysis with Linear

Polymer-Supported Catalysts

Bayer and Schurig16along with Ohkubo et al.21 were the first to describe soluble polymers as sup-ports for asymmetric catalysts.In Bayer’s case,a DIOP(4,5-bis(diphenylphosphinomethyl)-2,2-dimethyl-1,3-dioxolane)ligand was attached to a linear poly-styrene,the resulting polystyrene-bound version of DIOP(e.g.,4)was allowed to react with HRh(CO)-(PPh3)3(eq2),and the resulting polymer-bound Rh

complex5was used to hydroformylate styrene. Hydroformylationproductswereobtained(thebranched product was the predominant product),but the ee was only2%.The catalyst5was separable from low molecular weight materials by membrane filtration. Asymmetric hydrogenation was also studied by Ohkubo,Fujimori,and Yoshinaga using other poly-meric catalysts.21This group used the same soluble polystyrene-bound version of DIOP used earlier by Bayer’s group(e.g.,4)to prepare a rhodium(I) catalyst to hydrogenate prochiral unsaturated acids such as2-methyl-3-phenylpropenoic acid,(Z)-2-meth-yl-2-butenedioic acid,and(E)-2-methyl-2-butenedioic acid(eq3).In this case,these authors used rather small polystyrene oligomers as supports,with the largest polymer having a degree of polymerization of about50.The smallest oligomer had a degree of polymerization of<10.Some differences in conver-sion and ee were noted for the differently sized polymers,but the ee values were generally<40%and the conversions were modest.Recovery and separa-tion of the polymeric catalysts from the products was not described.

C.Water-Soluble Polymer Supports for Catalysts

1.Proteinaceous Supports

The use of environmentally more benign solvents such as water is now a subject of considerable interest in the context of“green chemistry”.22The use of water as a solvent was also featured in some of the early work with soluble polymers.In this case,a natural polymer s a protein s was used as a water-soluble chiral support to prepare an asymmetric cationic Rh(I)hydrogenation catalyst.23This work also featured the use of noncovalent interactions for catalyst immobilization,taking advantage of the affinity of the protein avidin for the N,N-bis(2-diphenylphosphinoethyl)biotinamide6to immobilize a chiral chelating phosphine ligand.In this example, the asymmetric induction in hydrogenation of pro-chiral enamides using the Rh(I)catalyst prepared in eq4was not high enough to be practical(ca.40-44%ee),but this work illustrates how soluble poly-mers confer their own solubility behavior on a ligand that would otherwise be insoluble in a particular solvent.In this case,the size of this proteinaceous

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polymer support in comparison to the products provided a way to effect catalyst separation using membrane filtration(a10000molecular weight cutoff ultrafiltration membrane was used).Finally,this report showed that the enantioselectivity was a result of a specific macromolecule-catalyst interaction.The presence of other proteins(e.g.,lysozyme,bovine serum albumin,or carbonic anhydrase)had no influ-ence on the enantioselectivity of7though these other proteins did alter7’s reactivity.

2.Polar Polymer Supports

A later report by Bayer and Schumann described using linear polymers such as poly(vinylpyrrolidi-none),polyethylenimine,polyacrylonitrile,and poly-(ethyleniminodiacetic acid)as polychelatogens with Rh,Pd,Pt,and Ni as the actual metal catalysts.17 These catalysts had much higher activities than the earlier catalysts described by this group and could be recovered by ultrafiltration(or by biphasic separa-tions if water-soluble polymers were used).However, it is likely that the actual catalysts are colloidal metal rather than molecular complexes in these cases.24 D.Polymers Supporting Catalysts with Thermally Reversible Solubility

Temperature-dependent properties of polymers were used in early work to design a recoverable catalyst system.While the polymer in this example is never soluble,this report is relevant in the context of this review because the polymer serves as a reversible reservoir for the transition-metal catalyst. In these examples,a transition-metal salt was re-leased from the polymer on heating.In the first instance,this involved using a thermally regenerable ion exchange polymer that reabsorbed the transition-metal salt on cooling to room temperature.25Here the properties of the polymer support lead to a thermally soluble/insoluble homogeneous catalyst s a scheme used subsequently by our group and others in de-signing soluble polymer supports for homogeneous catalysts.

1.Sirotherm Resins

The basis of the thermally soluble/insoluble cata-lyst system developed by Pittman was the Sirotherm ion exchange material developed earlier for desali-nation applications.26Sirotherm ion exchange resins consist of a mixture of amphoteric polymers in a water-permeable polymer matrix.The amphoteric polymers include both an acidic polymer(a poly-(acrylic acid))and a basic polymer(a polyamine such as poly(p-diethylaminostyrene)or poly(N-alkyldiall-ylamine)).This thermally regenerable ion resin works because the proton-transfer equilibrium5is temper-

ature dependent.For example,at ambient temper-ature,the equilibrium favors the ionic form of the polymer and500mg/L sodium chloride can be absorbed from an aqueous solution that contains 1000mg/L sodium chloride.At elevated temperature (80-90°C),this equilibrium favors the neutral polymers and this sodium chloride is released to solution.Pittman used this same behavior to bind and release an ionic transition-metal catalyst.

In Pittman’s case,both hydroformylation and Reppe carbonylation reactions were studied.The Reppe reaction and the hydroformylation both used RhCl3/ Me3N or RhCl3solutions as catalyst precursors. Rhodium carbonyl hydrides or rhodium clusters presumably form in the reaction.Pittman speculated that the active catalyst in the Reppe reactions was a rhodium carbonyl hydride anion such as8.This anion would complex to the Sirotherm resin like chloride does.Pittman further noted that this complexation and catalyst recovery was only efficient under neutral conditions.Under acidic(CH3CO2H present)or basic ((CH3)3N present)conditions,catalyst recycling was not efficient as judged by the decreasing1-pentene conversion in a second or third catalyst recycling. However,under neutral conditions,a suspension containing1g of Sirotherm resin and30mL of a solution of the rhodium catalyst8in a1:5(v/v) mixture of water and THF was successfully used for 16cycles for the carbonylation shown in eq 6. Approximately equal amounts of normal and branched products formed.About10-20%of the product was in the form of an alcohol.The total turnover was2500 in these16cycles.However,each reaction cycle required a relatively long time(10days each),the 1-pentene conversion was modest in each cycle(ca. 60%),and a relatively high catalyst loading was present in each cycle.Nonetheless,the polymer in this example afforded a thermally reversible way to generate a homogeneous catalyst at high tempera-ture that could then be recovered by cooling and filtration.

2.Poly(vinylpyridine)Supports

Insoluble forms of poly(vinylpyridine)have also been used to chemically bind or release a homoge-neous hydroformylation catalyst in response to the presence or absence of hydrogen.27The polymer in this work is chemically analogous to the temperature-dependent ion exchange polymers used by Pittman. This work is one of the earliest examples where the properties of a polymer were used to recover a homogeneous catalyst.

E.Biphasic Systems Employing Soluble Polymer-Supported Catalysts

Another early example of a soluble polymer-sup-ported catalyst from Hodge’s group provided

an

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Bergbreiter

illustration of the use of a soluble polymer as an organic catalyst in a biphasic reaction.28In this work, a phosphine-containing polystyrene(MW)100000) was prepared as a copolymer of styrene and p-diphenylphosphinostyrene(ca.7:1mol/mol).Alkyla-tion of this phosphine by benzyl chloride or by 2-bromomethylnaphthalene led to formation of the phosphonium salt9.This relatively acidic polymeric phosphonium salt is insoluble in aqueous base,but as a polyvalent phosphonium salt,it acts as a phase-transfer catalyst in this toluene/water mixture.Depro-tonation of the phosphonium salt under these bipha-sic reaction conditions then produces an ylide that reacts with an aldehyde acceptor to form an alkene (eq7).The soluble phase-transfer polymeric catalysts here s the phosphonium salts on the polystyrene s were also substrates in the subsequent Wittig reac-tions.However,if the polymer were used in excess, it and the byproduct polymeric polystyrenediphe-nylphosphine oxide could both be recovered and separated from the product alkene by precipitation using methanol.

F.Inorganic Polymer Supports

1.Polyphosphazene Supports

Linear inorganic polymers too were used early on as supports for homogeneous catalysts.Examples of such chemistry include Allcock’s use of polyphos-phazene-bound phosphine ligands to prepare olefin isomerization and hydroformylation catalysts,29Awl’s use of linear and ladder polysiloxanes as supports for hydrogenation catalysts,30and Farrell’s use of silox-ane polymers to support hydroformylation catalysts.31 In Allcock’s case,the polyphosphazene-bound tri-arylphosphine10was used to prepare various metal complexes using AuCl,CuI,H2Os3(CO)10,and[RhCl-(CO)2]2.Soluble products formed in most cases though coordination of phosphines from multiple polymers to the same metal was thought to lead to cross-linking and insolubility in some cases,with a polymer precipitate forming.31P NMR spectroscopy or IR spectroscopy(metal carbonyl complexes)served to establish that complete complexation of the polymeric phosphines by metal had been achieved.Polymer isolation by hexane precipitation often led to materi-als that did not redissolve.These materials may also have had coordinative cross-links involving phos-phines of more than one polymer chain to the complexed metals.

Reactions where the polyphosphazene-bound poly-meric metal complexes served as catalysts were studied.In these reactions,catalyst/product separa-tion was effected by simple distillation.However, catalyst reuse was not completely successful because of catalyst decomposition s a general problem that was recognized in part because of early attempts to recycle hydroformylation catalysts.32

2.Silicon-Containing Polymers as Catalyst Supports Decomposition of a polymer-bound catalyst also frustrated the repetitive use of poly(phenylsiloxane)-complexed-Cr(CO)3as a stereoselective hydrogena-tion catalyst for hydrogenation of methyl sorbate to methyl(Z)-3-hexenoate.30In this case,the-Cr(CO)3 groups were attached to the pendant phenyl rings of the ladder phenyl-containing siloxane polymer or to phenyl groups of a phenyl methyl silicone as shown in eq8.

The product polymer was characterized by IR spectroscopy,elemental analysis,viscosity,GPC,and thermal analysis.However,while these soluble si-loxane polymer-bound chromium carbonyl derivatives were more stable on the basis of thermal gravimetric analysis than a similar polystyrene polymer,signifi-cant loss of catalyst activity due to loss of>90%of the-Cr(CO)3groups occurred after this polymer was used in a hydrogenation and recovered by precipita-tion/filtration.30

Siloxane-supported hydroformylation catalysts were also prepared by Farrell et https://www.doczj.com/doc/0613817246.html,ing the chemistry shown in eq9.31The resulting polymers prepared by this chemistry had an M v of21000and were used to prepare soluble ligands to support a rhodium carbo-nyl complex.However,while>90%of the polymer could be recovered after a1-hexene hydroformylation by solvent precipitation(gravimetric analysis),el-emental analysis of the recovered polymer showed

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3349

that significant phosphine ligand and rhodium metal loss occurred.IR spectroscopy confirmed these results on the basis of the loss of the rhodium complex’s carbonyl band at1970cm-1.While earlier work by this group had shown significant polymer degrada-tion in the phosphination step(especially using LiPPh2and the chloromethylated polymer),GPC showed no molecular weight loss for the polymeric catalyst11during a hydroformylation reaction. Soluble rhodium complexes were also prepared by Sanger using methyl phenyl silicone polymers that were first chloromethylated(SnCl4,ClCH2OCH3)and then phosphinated(KPPh2).33This group reported IR studies not only of this polymer but also of other rhodium carbonyl complexes prepared using poly-mers such as-[CH2CH(C6H4PPh2)]n-,-[CH2CH-(C6H4CH2PPh2)]n-,and-[CH2CHOPPh2)]n-.These complexes were used as hydroformylation catalysts. Issues of recycling and catalyst/product separation were not addressed and would likely have been problematic due to catalyst/ligand decomposition.

G.General Features Common to Early Work These early papers where soluble polymers were used discuss in a general way many of the issues that are still important today in the design of new systems where polymers facilitate homogeneous catalyst re-covery and reuse.34,35Many or most of the concepts described in more recent work have precedent in these early reports using soluble polymers as catalyst supports.However,this early work like much other work with“heterogenized”homogeneous catalysts was not widely adopted either in industry or in academia.Most often this reflected modest catalyst activity or selectivity.The separation processes were also not always well described or as mature as similar separations used today.Ultrafiltration membranes, for example,were used early on for separation and recovery of soluble polymer-bound catalysts and were even recognized as potential tools for conventional catalyst recovery.36However,the early examples that used ultrafiltration to recover a soluble polystyrene-bound Rh catalyst were inefficient.For example, hydrogenation of pentene with a polymer-bound Wilkinson’s catalyst used over200mL of solvent in the overall process to hydrogenate a relatively small amount of pentene.15The use of membranes in reactors is now much improved with better materials, newer membranes,and practical continuous mem-brane reactors.37Catalyst loss and catalyst stability were other problems encountered in much of this early work.31Catalyst and ligand stability issues continue to be problems,but advances in spectroscopy make it easier to study these effects and to charac-terize the catalysts and ligands.Finally,it is not always clear in this early work what the catalyst was or how efficient catalyst recycling was since recycling efficiency was most often based on catalyst activity in a second or third cycle and not on trace metal analysis or studies of ligand stability.Moreover,few reports described successful use of a polymer-bound catalyst more than three times.

Most industrial and academic labs still mainly use insoluble polymer supports for catalysis and synthe-sis.However,the original scattered reports of the use of soluble polymer systems have substantially ex-panded in recent years to the point that soluble polymers are no longer uncommon as supports for catalysts and ligands.38-40This review’s focus is these soluble polymers and the schemes that use soluble polymers for polymer-bound ligand and catalyst recovery and reuse.These issues are discussed in detail for several systems below.However,there are certain common features of all the systems,features that significantly impact the utility of soluble poly-mers in catalysis,catalyst recovery,and ligand synthesis.These features are briefly discussed first. III.Characterization Advantages of Soluble Polymer Supports

One of the most significant features of soluble polymer-supported ligands and catalysts is the facil-ity of ligand and catalyst synthesis and characteriza-tion afforded by the soluble polymer support.The use of a soluble polymer support offers advantages of homogeneity during synthesis and during character-ization.In synthesis,this often means that it is possible to use synthetic procedures that directly mimic those used to prepare low molecular weight ligands and catalysts.In characterization,this means that it is relatively possible to use the range of solution-phase techniques routinely used in any synthetic operation.For example,spectroscopic pro-cedures that work best with solutions are1H NMR spectroscopy and UV-vis spectroscopy,two promi-nent examples of techniques less readily used with an insoluble polymer support that work well with soluble polymer supports.Other NMR techniques such as13C and31P NMR spectroscopy are also very useful with soluble polymers though the advent of gel-phase NMR analyses for insoluble polymer-sup-ported catalysts and ligands means that these other NMR procedures are not uniquely useful for soluble polymer supports.41

A.1H NMR Spectroscopy

1H NMR spectroscopic characterization of soluble polymer-bound ligands and catalysts is most facile for ligands and catalysts present as end groups for polymers that have an equivalent weight of less than 10000.Figure1illustrates this with an example of a terminally bound Pd(II)catalyst,12,on poly(ethylene glycol)(PEG).This catalyst was prepared from an

amine-terminated monomethoxypoly(ethylene glycol)via amidation 42and was analyzed by 1H NMR spectroscopy using conditions like those used for a low molecular weight species (a 20mg sample,7min of total analysis time).In this case,the poly(ethylene glycol)-bound Pd(II)catalyst with an SCS (sulfur -carbon -sulfur)ligand has an obscuring signal due to the polymer,the -CH 2O -groups at δ3.5.How-ever,the 1H NMR spectrum of the catalyst (and ligand)has aryl peaks in the δ6.5-8.5region that show up clearly.

1

H NMR spectroscopic analysis of other ligands bound to terminal sites on polymers is equally feasible and makes monitoring the success of syn-thetic reactions easier.This has been illustrated in work with linear polyethylene-bound ligands and ligand precursors.43For example,when a polyethyl-ene oligomer such as 13with a terminal -CH 2OH group was converted into a terminal acetate ester (-CH 2OCOCH 3)or into a phosphite ligand,a change in chemical shift of the characteristic triplet due to this hydroxymethyl group in the 1H NMR spectrum at δ3.3provided a convenient way to follow the success of the reaction.In the example shown where an azo dye probe or ligand was attached to a polyethylene oligomer (eq 10),the -CH 2OH methyl-

ene triplet at δ3.3shifted to δ4.2in 14.The change in the methylene H’s chemical shift,the absence of residual signals at δ3.3,and the absence of other peaks that could have arisen from side reactions as well as the integration of the -CH 3end group peak relative to the new -CH 2O -and aryl C -H peaks provide conclusive evidence for the success of this reaction.In these examples,the polyethylene polymer support has a large background peak at δ 1.3.Fortunately this is a region of little interest in 1H NMR spectroscopy.The ligand peaks show up down-field with a resolution like that of a low molecular weight compound as is the case with the catalyst 12.NMR spectroscopic analyses of soluble polymeric ligands and catalysts can be limited by polymer solubility because even an oligomer support’s solubil-ity is dictated by the solubility characteristics of the polymer.In the case of a polyethylene oligomer,the ligands and subsequent catalysts are only soluble in hot solvents such as toluene-d 8at 100°C.This requirement for a 100°C environment introduces some problems in analysis especially for catalysts that have dynamic behavior.For example,ligand exchange in polyethylene-bound catalysts such as 15

results in an averaged solution-state 31P NMR spec-trum.Dynamic behavior that averages an NMR signal in the low molecular weight analogue of 15can be suppressed by using a lower temperature for the NMR analysis.In a polymer such as 15,this option is precluded by the polymer’s temperature-dependent solubility.As a result,dynamic behavior of this Wilkinson’s catalyst analogue attached to a polyethylene oligomer can only be resolved by resort-ing to solid-state 31P NMR spectroscopy.44

Soluble polymers with ligands and catalysts on pendant side chains are more common than soluble polymers with terminally bound ligands and cata-lysts.Fortunately,NMR analysis of soluble polymers with pendant side chains is also feasible.In this case,the main chain protons and carbons of the polymer encompass a broader chemical shift range and are more likely to complicate the NMR spectrum.None-theless,it is possible to get good 1H and 13C NMR spectra for pendant substrates,ligands,or catalysts on such polymers.If the substituents that are being added have 1H NMR signals that do not overlap the polymer,solution-state quality spectra can be ob-tained when the substituent is attached with a short (e.g.,10-atom)tether.This is illustrated by the 1H NMR spectrum shown in Figure 2a,which is a 1H NMR spectrum of the vinyl group of an undecenyla-mide,16,on a soluble poly(N -isopropylacrylamide)(PNIPAM).The vinyl protons here have a line width of ca.2Hz,and the characteristic coupling pattern for the three protons of a vinyl group is easily discernible.45

B.13C NMR Spectroscopy

Solution-state 13C NMR spectroscopy is also useful in following the course of reactions on soluble

poly-

Figure

1.1H NMR spectrum of a polymer that contains a poly(ethylene glycol)-bound SCS -Pd(II)catalyst as the terminal group of the polymer (12).

mers.This is illustrated with the spectra in Figure 2b,c.These spectra of the poly(N-octadecylacryla-mide)-bound1,3-bis(thioarylmethyl)arene ligand1746 and a similar poly(N-isopropylacrylamide)-bound SCS-Pd(II)Heck catalyst,18,show that quaternary carbons of a relatively lightly loaded(10mol%) soluble polymer still are easily detectable.These polymers have also been characterized as solids by CP-MAS solid-state NMR spectroscopy.However,the solution-state spectra are more readily obtained and provide a direct comparison to low molecular weight compounds.

C.Relaxation Time Effects

Several studies have shown how soluble and in-soluble polymers affect relaxation times of bound groups and how metal complexation affects31P NMR signals.44,47,48In the case of soluble polymers,such effects have been studied both with terminally func-tionalized polymers and with polymers containing catalysts or ligands as pendant groups(e.g.,poly-acrylamides).

In the case of a soluble poly(ethyldiphenylphos-phine),T1values for phosphorus of the oligomeric phosphine19were similar to those of an electroni-cally equivalent octadecyldiphenylphosphine,20,un-der the same conditions(3and9s,respectively).44 T1values for phosphines in soluble,insoluble,and low molecular weight ligands were also studied by Stille.48Stille’s work showed that a soluble polysty-rene-bound DIOP ligand(e.g.,4)and a DVB-cross-linked-polystyrene-bound DIOP ligand had similar T1values for the phosphines in the DIOP groups, values that were an order of magnitude smaller than for the phosphines in DIOP itself.Stille’s work did agree with studies of19in that T1values were generally larger for a soluble polymer-bound phos-phine than for an insoluble polymer-bound phos-phine.

T1relaxation times of the terminal carbon of the pendant N-alkyl isopropyl group of the N-isopropy-lacrylamide(NIPAM)and of methyl groups of the side chains of poly(N-isopropylacrylamide)-co-poly-(N-hexylacrylamide)or poly(N-isopropylacrylamide)-co-poly(N-octadecylacrylamide)measured for solu-tions of these polymers in CDCl3at ambient tempera-ture are listed in Table1.This work and earlier work by Bayer and Fyfe suggest that the apparent differ-ences in studies of19and in relaxation times seen for polystyrene-bound DIOP ligands in Stille’s work are a result of the flexibility of pendant groups on a soluble(or insoluble)polymer.NMR studies of re-laxation in methyl groups on polyacrylamides show that the methyl substituents on a polymer can have shorter T1relaxation times than analogous small molecules(Table1).However,if a suitable spacer group is present,there is no measurable difference in the T1relaxation time as can be seen if one

Figure2.NMR spectra of pendant groups on soluble polymers:(a)a1H NMR spectrum of a poly(N-isopropyl-acrylamide)-bound11-undecenylamine showing solution-like resolution for a vinyl group with a modestly sized spacer;(b)a13C NMR spectrum of a1,3-thiophenylmethyl-substituted arene ligand on a poly(N-octadecylacrylamide) polymer showing eight distinct aryl carbons in theδ110-150range;(c)a13C NMR spectrum of an SCS-Pd(II)Heck catalyst on poly(N-isopropylacrylamide)showing the aryl carbons,the ispo carbon attached to Pd(δ150),and larger peaks upfield due to the polymer and spacer.Table1.T1Relaxation Times for Methyl Groups on Pendant Chains of Soluble Polyacrylamides

molecule T1(s) hexylamine 6.8(0.4 octadecylamine 4.6(0.7 poly(N-isopropylacrylamide)0.4(0.1 poly(N-isopropylacrylamide)-co-

poly(N-hexylacrylamide)

2.7(0.8

poly(N-isopropylacrylamide)-co-

poly(N-octadecylacrylamide)

4.1(

0.7

compares T1values for the-CH3group of the free and polymer-bound octadecylamine.45

The results in Table1and for polymers19and20 are similar to earlier results that directly compared T1values for low molecular weight compounds,a soluble PEG support,a DVB-cross-linked polystyrene with a PEG graft,and a DVB-cross-linked polysty-rene(21;R)-CH3,-(CH2CH2O)n CH3,PEG-polystyrene,and polystyrene,respectively).49The effect of these different polymers or a simple methyl group on T1values for the various carbons of the tripeptide21are illustrated in Figure3.

D.UV?Vis,Fluorescence,and IR Spectroscopic Studies of Soluble Polymer-Bound Ligands and Catalysts

UV-vis or fluorescence analyses of polymers with UV-vis or fluorescence spectroscopic labels is also feasible.For example,solutions of polymer14or of polymers such as22or23can be studied to deter-mine the concentration of the polymer.Such dyes also provide information about solvation when the ab-sorption maximum or emission maximum of a poly-mer-bound dye or fluorophore is sensitive to solvent polarity.

Dye-labeled soluble polymers also have other ad-vantages.This is illustrated by recent work with22, 23,27,and28.50In the case of22,the extent of recovery of the organic base catalyst,a dimethylami-nopyridine analogue,is easy to assay because of the presence of a small amount of dye label.A similar approach reported recently used a colored dendrimer, 24,that was used to bind substrates and which also monitored polymer recovery.51In both cases,a simple colorimetric assay is facilitated by the presence of an azo dye chromophore in the soluble polymer or dendrimer.Recent work by our group has also shown that chromophoric catalyst ligands such as the azo dye ligand in27and28have additional functional-ity.50Not only does the dye label in27or28prepared in eq11provide a probe of where the polymer and

catalyst is during recovery of the polymer and catalyst,it also provides evidence about catalyst stability.The metalated polymers27and28exhibit a pronounced bathochromic shift in their visible spectrum vis-a`-vis the precursor nonmetalated poly-

Figure3.T1values for five selected carbons in a BOC-protected tripeptide,21,attached to various polymer supports(R)-CH3,PEG,PEG-PS,or PS):b,A C in21; 1,B C in21;0,C C in21;2,D C in21;O,E C in21.

meric dyes25and26,and the loss of Pd from this azo ligand leads to changes in the UV spectrum of 27and28that signal catalyst decomposition has occurred.

IR analysis of metal carbonyls on both soluble polymers is another useful analytical tool.20,34IR spectroscopy has also been useful with insoluble polymers.Such spectroscopic studies have the ad-vantage that they can simply discern the coordination environment of a polymer-bound metal complex.IR spectroscopy can also be useful in analyzing catalysts after a reaction to determine catalyst stability and recyclability,especially in the case of metal carbonyl catalysts.

IV.General Features of Soluble

Polymer-Supported Catalyst Activity

Soluble polymer-bound catalysts can be designed to have activity equivalent to that of their low molecular weight analogues.This is especially true for catalysts attached at the terminus of a linear polymer in a good solvent.Through the use of suitable spacer groups,catalysts attached to pendent chains too can have essentially the same reactivity as their low molecular weight counterparts.

A comparison of the selectivity of nickel(0)diene cyclooligomerization catalysts ligated by hindered triaryl phosphite ligands attached to the termini of polyethylene oligomers(29)to that of similar cata-lysts ligated by low molecular weight ligands(30) shows how similar polymer-bound and low molecular weight catalysts can be.52These results,illustrated in Figure4,show the similarity in selectivity that can be achieved with a soluble polymer-bound cata-lyst with a wide variation in the relative amounts of ligand and metal.

Pendant-chain-bound catalysts,especially when they contain a short spacer,can also have reactivity like that of their low molecular weight analogues. However,while soluble polymer-bound catalysts can have activities identical to those of their electronically similar low molecular weight analogues,the proper-ties of a polymer can affect catalyst activity.Such effects were seen in early studies of asymmetric catalysis by polystyrene-DIOP-Rh(I)complexes.21 Sometimes these effects arise because the random coil of a high molecular weight polymer can behave like a solvent or cosolvent.34For example,our work has shown that cationic Rh(I)hydrogenation cata-lysts attached to a polymer polyacrylamide polymer have activities that are lower than those of their low molecular weight analogues.This lower activity for a catalyst such as31was attributed to solvation of and coordination to the Rh(I)centers by amides of the polymer.Evidence for this effect is seen in experiments where32,a low molecular weight ana-logue of31used in the presence of PNIPAM,had a lower turnover frequency(TOF)in allyl alcohol hydrogenation.44Slightly lower activity in a PNIPAM organic catalyst was also noted in our recent study of PNIPAM-bound dimethylaminopyridine ana-logues.50A slight effect of the polymer on the reactiv-ity of a terminal catalyst was also noted in our work on the asymmetric C-H insertion reaction using a polyethylene-bound Rh(II)catalyst,33.53

While the polyamide support in31decreased TOF values for allyl alcohol hydrogenation,a neutral Rh(I)catalyst,34,was unaffected by the polymer.45 At25°C,the TOF values for hydrogenation of allyl alcohol by the low molecular weight catalyst35in the presence or absence of added poly(N-isopropyl-acrylamide)were2.7and2.6mol of H2/(mol of Rh h),respectively.These values are essentially the same as the TOF for the same hydrogenation using

Figure4.Selectivity in butadiene cyclodimerization to form1,5-cyclooctadiene as a function of the ratio of a polyethylene-bound phenyl bis(2-methylphenyl)phosphite ligand to Ni(O)or as a function of the ratio of nona-decylphenyl bis(2-methylphenyl)phosphite to Ni(b).

the poly(N-isopropylacrylamide)-bound neutral Rh(I)catalyst34(2.2mol of H2/(mol of Rh h)).

It is important to note that a soluble polymer does not always have a negative effect on catalyst selectiv-ity even if the polymer may affect catalyst reactivity. The amides of the polymer support in the soluble poly(N-isopropylacrylamide)-bound chiral Rh(I)cata-lyst36,for example,have essentially no effect or a minimal effect on the percent ee in an asymmetric hydrogenation such as eq12where the phenylalanine product was obtained in88%ee with a poly(N-isopropylacrylamide)-bound chiral hydrogenation cata-lyst.54

Soluble polymers whose main chain contains chiral catalyst ligands have also been studied,and their reactivity too can be compared to that of low molec-ular weight catalysts.For example,Chan has de-scribed an asymmetric hydrogenation catalyst pre-pared using the copolymer37derived from an axially chiral5,5′-diamino-BINAP(BINAP)2,2′-bis(diphe-nylphosphino)-1,1′-binaphthyl)ligand,a chiral diol, and terephthalic acid.55The90+%ee values seen with this catalyst in the hydrogenation shown in eq 13are excellent and comparable to what would be obtained with a low molecular weigh Ru-BINAP catalyst.

V.Strategies for Separation and Recovery of Soluble Polymer-Bound Ligands and Catalysts Separation schemes for recovering a soluble poly-mer-bound catalyst or ligand,separating it from the product,and reusing it are primarily based on gas/ liquid,liquid/solid,or liquid/liquid separation tech-niques(Figure5).

Distillation(Figure5a)is a simple and effective procedure.It is generally applicable to many sorts of catalytic reactions.Polymeric ligands are not necessary in this case,and such separations are equally effective with ordinary homogeneous cata-lysts.A potential problem with gas/liquid separations is catalyst or ligand thermal instability. Macromolecules can also be separated from low molecular weight components on the basis of size. Membrane filtration(Figure5b)provides a useful liquid/liquid separation technique that was used in some of the earliest work using soluble polymer-bound catalysts.15Semipermeable membranes have a long history in macromolecular chemistry and are ubiquitously used in separations of biomacromol-ecules from low molecular weight species.37,56Such techniques are particularly well adapted to separa-tion of dendrimer-bound catalysts because of the globular structure of these hyperbranched polymers. Membrane systems also offer an engineering advan-tage in that they can be designed so that catalysts can be used in continuous reactions,a possible process advantage over other separations that de-pend on using catalysts in batch-type processes.37,56,57 Liquid/liquid separations(Figure5c)that are based on the selective solubility of a polymer in one phase of a biphasic mixture can also be used to recover Figure5.General schemes for separation,recovery,and reuse of soluble polymer-bound catalysts:(a)gas/liquid separation of products by distillation;(b)separation of low molecular weight products from a high molecular weight soluble polymer-bound catalyst and ligand using a semi-permeable membrane;(c)use of a physical or chemical stimulus to induce formation of two immiscible liquid phases that are subsequently separated so as to separate products from a solution of a soluble polymer-bound catalyst and ligand;(d)use of a reaction mixture that is always biphasic,with the catalysts residing in a phase different from that in which the substrates/products reside;

(e)use of a physical change,a chemical reaction,or a solvent change to induce formation of a precipitate of a polymer-bound catalyst and ligand followed by a solid/ liquid separation using centrifugation or filtration to isolate a reusable catalyst.

soluble polymer-bound catalysts and ligands.For example,fluorinated polymers that have selective solubility in a fluorous phase of a biphasic organic/ fluorocarbon system can be designed to recover and reuse catalysts.In this case,the system would be biphasic throughout the process.Alternatively,it is possible to design polymers that have thermomorphic phase-selective solubility in either a polar or nonpolar phase of a biphasic system.46,58-60If an appropriate solvent system were then chosen,a soluble polymer-bound catalyst or ligand could be used under monophasic conditions(e.g.,at elevated tempera-ture).If,after reaction,this miscible mixed solvent system were perturbed(e.g.,by cooling),a biphasic mixture could re-form.Then liquid/liquid separation could be used to recover the catalyst and separate it from the products.Examples of such liquid/liquid separations with acrylates,polar acrylamides,and nonpolar acrylamides are all described below. Precipitation of a polymeric ligand or polymeric catalyst(Figure5d)followed by a filtration or some other sort of liquid/solid separation is the most common separation scheme used with soluble polymer-bound catalysts.The way in which a soluble polymer-bound catalyst is precipitated can vary.Precipitation of a soluble polymer can almost always be ac-complished by the addition of an excess of a poor solvent.Alternatively,if the soluble polymer is am-photeric,precipitation can be achieved by protonation or deprotonation.Soluble polymers that precipitate on cooling can also be used and recovered by filtration or centrifugation.Finally,there are examples where soluble polymers have inverse temperature-depend-ent solubility.In these latter cases,increasing tem-peratures can be used to precipitate a soluble polymer-bound catalyst.Examples of each of these sorts of separations are discussed below.

A.Solid/Liquid Separations of

Polyethylene-Bound Catalysts

Polymers can have profound temperature-depend-ent solubility.Polyethylene is a common example of a polymer with strong temperature-dependent solu-bility.Even in what would be considered a“good”solvent,polyethylene has no detectable solubility at room temperature.However,on heating,polyethyl-ene readily dissolves to form solutions that can contain as much as1g of polymer/10mL of solvent. Such concentrations of polymer are much more than sufficient to provide a platform for a catalyst support since0.4mmol of catalyst/g of polyethylene is a loading that can readily be achieved.For example, solutions of the polyethylene-bound diphenylphos-phine-ligated Rh(I)hydrogenation catalyst15in toluene could be prepared such that the catalyst concentration was>10-2M at100°C.44

Our group was the first to take advantage of the temperature-dependent solubility of polyethylene in designing recoverable but soluble polymer supports for catalysts.The examples below show that poly-ethylene oligomers are effective and general supports for ligands and catalysts.The necessary functional oligomers are available by anionic polymerization of ethylene.43While such anionic polymerization does not lead to polymers with good materials properties, the oligomeric ligands that derive from the synthesis shown in eqs14and15have chemistry that is equivalent to that of their low molecular weight octadecyl or nonadecyl analogues.As noted above, multistep syntheses such as those in eqs14or15can be followed by1H NMR spectroscopy since the end group concentration is relatively high for ethylene oligomers such as these(n e200)in hot deuterated toluene.Studies with dye-labeled polyethylene oli-gomers14,spin-labeled polyethylene oligomers39,

and pyrene-labeled polyethylene oligomers23veri-fied that these materials are soluble in hot solvent but have no detectable solubility in any solvent at room temperature.The lack of room-temperature solubility in these spectroscopically labeled polymers, in the phosphine and phosphite ligands19,29,and 38above,and in catalysts attached to these polymers is a direct consequence of the solubility characteris-tics of the polyethylene portion of this polymer.In these cases,the terminally functionalized polyolefin functions as a solubility handle in much the same way as the poly(ethylene oxide)polymers used early on by Bayer served as solubility handles for catalysts and amino acids.16The anionic oligomerizations used to prepare these polyethylene oligomer ligands are suitable for synthesis of small quantities(<10g)of the necessary polymers.More recent chemistry in-cluding living polymerization of ylides and function-alization of vinyl-terminated ethylene or propylene oligomers yields more monodisperse oligomers that would be an alternative more economic route to larger quantities of the necessary end-functionalized polyolefins.61,62

Many sorts of catalysts have been attached to polyethylene polymers by a number of groups.Ex-amples are listed in Table2.In all cases,the catalysts dissolve in a solvent suitable for polyethylene at elevated temperature.However,while such catalysts are homogeneous at elevated temperature,all such catalysts are completely insoluble at room tempera-ture.The general scheme used in catalysis and catalyst recovery with the polyethylene-bound cata-lysts and ligands listed in Table2is illustrated in Figure6.Typically,the catalyst is used homoge-neously in a suitable solvent at high temperature and recovered by filtration or centrifugation as a solid polyethylene powder at room temperature.In these examples,the polyethylene ligands selectively pre-

cipitate.The precipitates that form include all poly-ethylene-containing species.However,these semi-crystalline solids do not entrain extraneous materials, so products or starting materials that are not associ-ated with a polyethylene oligomer do not contaminate the recovered catalyst.

1.Rhodium(I)Hydrogenation Catalysts on Polyethylene Supports

Rh(I)hydrogenation was one of the first reactions studied for a polyethylene-bound catalyst.44In this case,direct comparisons were made between the activity and selectivity of this soluble polymer-bound catalyst15,its low molecular weight analogue(C18H37-PPh2)3RhCl,40,and an insoluble DVB-cross-linked polystyrene-bound catalyst.The polyethylene-bound catalyst15was prepared from a diphenylphosphine-terminated polyethylene oligomer using one of three processes(eqs16a-c)s phosphine ligand exchange

with Wilkinson’s catalyst,phosphination of a chloro-rhodium ethylene dimer,or exchange of polyethyl-enediphenylphosphine for phosphine and ethylene ligands in a monomeric catalyst.Each route gener-ated an equivalent catalyst.Solution-state31P NMR spectroscopy showed that no triphenylphosphine was present after the catalyst had been recycled several times,a result predicted by the expected favorable entropy for polymer-rhodium complexation.69,79The use of[ClRh(H2C d CH2)]2(eq16a)was deemed to be the best synthesis procedure for this catalyst,and this route was used to prepare the low molecular weight analogue40(eq17).

The(CH3CH2(CH2CH2)ca.100CH2CH2PPh2)3RhCl so prepared was an order of magnitude more reactive than a commercially available insoluble polystyrene-bound catalyst.44It was about as active as its octa-decyl analogue,typically having80%of the activity of the octadecyldiphenylphosphine-ligated catalyst. The polyethylene-bound catalyst could be reused multiple times.Recycling experiments showed that the catalyst could be used up to18times with no significant loss in activity.Recycling in these cases used the protocol shown in Figure6,with the polyethylene-bound catalysts typically being recov-ered by filtration.

These polyethylene-bound catalysts were charac-terized by NMR spectroscopy as described above.The reactivity of these catalysts with various alkenes was also compared in kinetic studies.The relative reac-tivity with mono-,di-,and trisubstituted alkenes and toward a larger alkene was comparable to that of a low molecular weight electronically equivalent Rh catalyst,(C18H37PPh2)3RhCl(40).“Used”catalyst was examined spectroscopically(31P NMR at elevated temperature)and was found to be equivalent to “fresh”catalyst.The only change after multiple recycle/recovery experiments was a small(ca.10%) increase in the intensity of the phosphine oxide peak. The extent of recovery of the polymeric ligand(and catalyst)in this reaction was estimated to be>99.9% on the basis of earlier results with analogous poly-mers that contained spectroscopic labels.Analysis of the filtrate for Rh by inductively coupled plasma analysis confirmed this estimate and showed that there was no detectable rhodium in the filtrate.This corresponded to loss of<0.1%of the charged rhodium catalyst.These quantitative measures of rhodium

Table2.Polyethylene-Bound Transition-Metal Catalysts Used Homogeneously and Recovered at Room Temperature by a Solid/Liquid Separation a

reaction catalyst recovery(%) alkene hydrogenation44L3RhCl>99.9 alkene hydroformylation63,64L2Rh(CO)Cl>99.9 allylic acetate substitution65L4Pd>99.98 alkene cyclopropanation66[(LCO2)2Rh]2>99.9 asymmetric cyclopropanation/CH insertion53[(LCO2)2Rh]2

alcohol carbonylation67L2PdX2>99.98 butadiene polymerization68(LCO2)3Nd>99.98 butadiene cyclodimerization/cyclotrimerization52LC6H4OP(OAr)2Ni>99.99 primary alcohol oxidation69(LCO2)Ru6+LCO2->99.9 tin halide reductions70LSn(C4H9)2Cl

ATRP polymerizations71-73LCu

Kharasch reactions74L3RuCl2

alkyne carboxylations75L3RuCl2

butadiene dimerization65L2PdX2>99.98 multistep oxidation/reduction76L3RhCl/cross-linked PVP-Cr6+

phase-transfer catalysis77,78LXR3+(X)N,P(or crown ether))

a L is a polyethylene-oligomer-bound ligand.

Figure6.Scheme used with polyethylene-ligated catalysts

that are homogeneous in a hot solution of substrate and

that are insoluble and separable from a room-temperature solution of

products.

recovery were in agreement with estimates of cata-lyst recovery based on persistent activity in the recycling experiments.

a.Three-Phase Test of Catalyst Stability.An important issue in recycling catalysts is catalyst stability.A concern with hydrogenation catalysts is the possible formation of colloidal metal.24Earlier and continuing reports describe many examples of polymer-ligated metal colloids that are active hydro-genation catalysts.80-83In the case of the polyethylene-bound Rh(I)hydrogenation catalyst a three-phase test was used to test for the formation and presence of rhodium metal colloids and,indirectly,to test catalyst stability.84In this case,a2%DVB-cross-linked polystyrene was modified to contain a styrene group.Then this insoluble alkene substrate was treated with H2and either with the polyethylene-bound Rh(I)catalyst15or with(C18H37PPh2)3RhCl.

Both catalysts reduced the alkene on the polymer. Both fresh and used polyethylene-bound catalysts were measurably less reactive than the low molecular weight catalyst,but this relative difference in reac-tivity is expected since a macroscopic substrate is being reduced by a polymeric catalyst.Similar effects of solvent and polymer size in reactivity have been seen previously.85

b.Simultaneous Multistep Oxidation/Reduc-tion Using a Polyethylene-Bound Rhodium(I) Hydrogenation Catalyst.The use of polymeric systems to achieve site isolation has been recognized in many contexts.86The role of site isolation in carrying out multistep reactions with insoluble poly-mers was recognized early on as a promising aspect of polymer-supported chemistry that was not feasible in solution-phase synthesis.87Soluble polymers also lend themselves to such chemistry.It is well-known that mixing of polymers with one another is often energetically unfavorable.This is especially true if the polymers have very different characters.For example,a nonpolar polymer such as polyethylene is unlikely to be miscible or compatible with a polar polymer.

The incompatibility of polyethylene with polar polymers was used to advantage using polyethylene-bound catalysts and oxidation reagents on polar ion exchange polymers to design a multistep simulta-neous oxidation/reduction procedure.76In this mul-tistep reaction scheme(Figure7),the polyethylene-bound Rh(I)hydrogenation catalyst and phosphine ligand were used in the presence of an insoluble poly-(vinylpyridinium chlorochromate)(PVPCC)reagent. Both the Rh(I)and the phosphine are always associ-ated with a poly(vinylpyridinium)-incompatible poly-ethylene oligomer and are thus kinetically isolated from this insoluble polymer-bound oxidant.Control experiments with ClRh(PPh3)3showed that this low molecular weight catalyst was decomposed by this same PVPCC oxidant.With ClRh(PPh3)3,no hydro-gen uptake occurred and the phosphine ligands were shown to have been oxidized to form triphenylphos-phine oxide.Since the penultimate reductant,H2,is kinetically inactive with the poly(vinylpyridinium chlorochromate)oxidant,this mixed phase system consisting of15and PVPCC could be used to carry out a simultaneous oxidation/reduction(eq18).After the reaction,filtration of the hot solution separated the solution containing product and polyethylene-oligomer-bound ligand and catalyst from the in-soluble oxidant.Cooling and filtration or centrifuga-tion then separated the recyclable polyethylene-bound catalyst from the aldehyde product.

2.Polyethylene-Bound Nickel(0)Diene Cyclooligomerization Catalysts

The separation and reuse of soluble polyethylene-bound diaryl phosphite or polyethylene-bound phenyl diaryl phosphite to ligate nickel(0)diene oligomer-ization catalysts provided an early illustration of the utility of polyethylene ligands to recover active homogeneous catalysts whose activity mirrored that of low molecular weight analogues.52In these recy-cling experiments,a solution of the nickel(0)poly-ethylene phosphite catalyst was prepared by in situ formation of a Ni(0)catalyst from a mixture of the polyethylene ligand,Ni(acac)2,and triethylalumi-num.The amber-colored solution so prepared was visually homogeneous.Catalysts such as29or42 formed in this way(eq19)converted butadiene to

cyclooligomerization products in3-5h at100°C in toluene.Cooling typically produced a light yellow polyethylene solid and a clear toluene solution of product.Centrifugation or filtration avoiding oxygen exposure produced a product solution and a solid polyethylene-bound catalyst that could be reused without triethylaluminum activation.Up to three cycles were carried out.Analysis of the product solution from the second cycle after catalyst precipi-Figure7.Simultaneous use of a recoverable polyethylene-bound Rh(I)hydrogenation catalyst containing a polyeth-ylene-bound phosphine,19,with an insoluble,poly-(vinylpyridinium)-bound Cr(VI)oxidant.

tation using inductively coupled plasma analysis showed that<0.01%of the charged nickel catalyst had been leached.This corresponded to a<5ppb concentration of Ni in the product solution.Experi-ments analyzing for nickel in control experiments where no nickel catalyst was present show that this level of Ni corresponded to the minimum detectable level of Ni under these conditions.

Catalyst and ligand integrity is a general concern and proved to be problematic in the case of these Ni(0)-catalyzed diene oligomerizations.Catalyst sta-bility was initially tested simply by monitoring catalyst reactivity in the recycled catalyst.A slight decrease in activity through three cycles indicated some loss of catalyst or catalyst decomposition.The absence of leached nickel suggested that the observed gradual decrease in catalyst effectiveness was not due to catalyst leaching.Subsequent experiments heating the polyethylene-bound catalysts in the absence of substrate suggested that phosphite ligands such as 41were not as thermally stable under our reaction conditions as desired.Indeed,heating the polyethyl diaryl phosphite41in toluene for extended periods showed that irreversible changes in the ligand oc-curred on the basis of31P NMR spectroscopy.Such changes included the formation of some phosphonate and formation of a trialkyl phosphite.The nickel catalysts too are very reactive.Such catalysts could decompose by adventitious oxidation in the repetitive recovery and recycling oxidations.The lability of these nickel complexes to oxygen and moisture impurities had been established previously.88

As noted above,these polyethylene-bound Ni(0) catalysts have reactivity that is essentially the same as that of their electronically similar octadecyl ana-logues(Figure4).Product selectivity maps show that this similarity in activity is maintained for wide variations in the ratio of ligand to nickel.However, only at ligand:metal ratios that are>1:1is catalyst recovery and reuse practical.Unlike the case where the polyethyl diaryl phosphite-ligated catalyst is used,catalysts in reactions using the octadecyl diaryl phosphite-ligated catalyst cannot be recycled.

3.Polyethylene-Bound Palladium(0)Catalysts Diphenylphosphine-terminated polyethylene oligo-mers have been used with a variety of Pd(0)and Pd(II)catalysts to prepare allylic substitution,65diene dimerization,65and carbonylation catalysts64,67that can be successfully recovered and reused using the protocol in Figure6.Such recycling was most exten-sively studied with the Pd(0)catalyst prepared using the exchange reaction in eq20.This preparative reaction worked very well.Heating a4:1(mol/mol) ratio of the PE Olig PPh2with the yellow-orange solu-tion of the triphenylphosphine-ligated Pd(0)formed a yellow-orange solution that in turn formed a light yellow precipitate and a colorless solution on cooling to room temperature.The resulting polyethylene powder had a faint yellow color.This precipitation process was quantitative.No polyethylene ligand remained in solution.

When the initially formed polyethylene-bound cata-lyst43was allowed to react with an allyl ester such as allyl benzoate and a secondary amine such as morpholine or piperidine,a quantitative conversion of the allylic ester into an allylic amine occurred in 10min at100°C in toluene(eq21).Cooling precipi-tated the polyethylene catalyst,and the precipitate was isolated by centrifugation.Centrifugation was used instead of filtration to separate the catalyst from the product solution to minimize possible oxygen exposure of this oxygen-sensitive catalyst.The cata-lyst isolated in this fashion could be reused through 10cycles in reaction21with no diminution in activity.ICP analysis of the filtrate after the first reaction showed no detectable Pd in comparison to a blank.The estimated loss of palladium to solution was<0.001%of the charged palladium. Polyethylene ligands were also successfully used with Pd/C catalysts to prepare in situ catalysts that mimic the reactivity of(Ph3P)4Pd in allylic substitu-tion chemistry.83,89-91While the identity of the cata-lyst in these experiments is not clear,the PE Olig PPh2 ligand functions at elevated temperature with the Pd/C catalyst and the mixture of Pd/C and polyeth-ylene ligand can be recovered by filtration and reused as long as adventitious oxidation of the PE Olig PPh2 by oxygen is avoided.

The polyethylene-ligated Pd(0)catalyst43was also successfully used and recycled in10cycles of a decarboxylative allylic substitution(eq22)using a variety of substituted acetoacetic acid allyl esters. Typical catalyst loadings were2mol%.However, since the reaction was complete within5min,these loadings could be reduced significantly.

4.Polyethylene-Bound Palladium(II)Catalysts

Pd(II)catalysts ligated by polyethylene can be successfully prepared and recycled too.In this case, Pd(OAc)2was first added to a reactor.Then2equiv of PE Olig PPh2was added.Heating the resulting mixture to100°C formed the catalyst44,which was successfully used through five cycles in dimerization of butadiene to form a mixture of allylic acetates(eq 23).65Recycling and recovery of44was possible

after

cooling using centrifugation to separate44from the allyl ester product mixture.

The Kharkhanov group at Moscow State University also used polyethylene-complexed Pd(II)catalysts in carbonylation chemistry(eq24).64,67The necessary Pd(II)catalyst ligated by an oligomeric phosphine was obtained by heating Cl2Pd(PPh3)2with a sto-ichiometric quantity of the oligomeric phosphine ligand19at100°C in toluene.The oligomeric catalyst was then separated from triphenylphosphine and any uncomplexed catalysts by cooling and filtra-tion.

When a stoichiometric amount of the oligomeric phosphine(relative to Pd)was used in aryl iodide carbonylations(eq24),the catalyst45was about60% as active as Cl2Pd(PPh3)2.This activity was main-tained for11runs with a gradual decrease in catalyst activity in cycles10-12(Figure8).The decrease in catalyst activity was attributed to inadvertent phos-phine ligand oxidation.In accord with this sugges-tion,the authors noted that a catalyst mixture that was prepared using excess phosphine ligand19was slightly less active than the complex prepared with stoichiometric amounts of the oligomeric phosphine ligand but the resulting homogeneous catalyst mix-ture was more stable(Figure8).

5.Polyethylene-Bound Hydroformylation Catalysts Hydroformylation catalysts ligated by polyethyl-enediphenylphosphine ligands were studied both by the Bergbreiter group and by the Moscow State University group.63,64For example,64the Moscow State University group used an oligomerically ligated Rh(I)catalyst in hydroformylation of alkenes includ-ing1-dodecene,cyclododecene,styrene,and1,5-cyclooctadiene(eq25).The ratio of linear to branched product in the1-dodecene hydroformylation was sensitive to syn gas pressure and to the presence of excess oligomeric ligand.In the case of the stoichio-metric complex46,the linear product was about70% of the product mixture.Increasing the amount of oligomeric ligand relative to the rhodium catalyst46 increased the percentage of linear product to87%and also increased the stability of the catalyst solution. Catalyst solutions containing excess phosphine ligand were recycled at least10times successfully using the precipitation/filtration protocol shown in Figure

6. Such recycling maintains the phosphine:Rh(I)ratio because both the excess ligand and catalyst are equally efficiently recovered on cooling.

6.Polyethylene-Bound Ruthenium Catalysts

a.Catalysts for the Kharasch Reaction.Re-coverable polyethylenediphenylphosphine-bound ru-thenium(II)catalysts have been studied by several groups.Joint work between Weinreb’s group and Bergbreiter’s group has shown that Cl2Ru(PE Olig-PPh2)3catalysts can be prepared by exchange of PE Olig PPh2with the triphenylphosphine groups of Cl2-Ru(PPh3)3.74This phosphine exchange reaction pro-duced a clear solution that contained triphenylphos-phine on the basis of31P NMR spectroscopy.The catalyst so formed was completely insoluble at room temperature.These groups showed that the resulting soluble polyethylene-bound Karasch reaction cata-lysts have activity and selectivity similar to those of their low molecular weight analogues.These soluble polyethylene-bound Ru(II)catalysts are also practical catalysts.Such catalysts readily dissolve in hot toluene and were successfully recycled several times without any measurable change in reactivity(eq26). For example,in reaction26the yield of the haloge-natedγ-lactone using Cl2Ru(PPh3)3as catalyst was 81%and was80%in each of three cycles with the Cl2Ru(PE Olig PPh2)3catalyst.The soluble polymer-bound catalyst was also shown to be comparable to the low molecular weight catalyst in intramolecular Kharasch cyclizations.There were slight changes in product stereoselectivity seen in intramolecular cyclizations s changes that were either due to the use of an alkyldiphenylphosphine or due to environmen-tal effects of the nonpolar polyethylene ligand.How-ever,the catalyst47was recyclable both in intra-molecular Kharasch reactions and intermolecular reactions such as reaction26.The effectiveness of the polymer-bound catalyst in these reactions can be judged by the scale of the reactions;a typical Khara-sch reaction was carried out on a50mmol scale using <1mol%catalyst.

b.Catalysts for Alkyne Carboxylation.Dimeric ruthenium catalysts containing both phosphine ligands and bridging carboxylate groups were also prepared by reaction of19with[Ru(μ-O2CH)(CO)2)]n.75The resulting catalyst,[Ru(μ-O2CH)(CO)2(PE Olig PPh2]2

Figure8.Recovery of catalyst45as a stoichiometric polyethylenediphenylphosphine-Pd(II)complex(black bars) or in the presence of excess polyethylenediphenylphosphine ligand(gray bars)in iodobenzene carboxamidation(eq24).

(48),was then used to prepare vinyl esters by carboxylation of terminal alkynes with various car-boxylic acids(eq27).The effectiveness of the catalyst recycling protocol shown in Figure6for these poly-ethylene-ligated alkyne carboxylation Ru catalysts can be seen in results where the same recovered catalyst was used successively in a series of reactions. Specifically,one0.1mmol sample of the catalyst48 was successively recovered and reused in a series of reactions,first forming vinyl diesters of propyne and adipic acid,hexyne and succinic acid,propyne and adipic acid,hexyne and succinic acid,propyne and adipic acid,propyne and succinic acid,propyne and succinic acid,hexyne and adipic acid,hexyne and phthalic acid,hexyne and phthalic acid,and hexyne and adipic acid.This sequence of successive reactions led to55mmol of the various diesters,none of which were contaminated by vinyl esters from earlier reac-tions.

c.Ruthenium Carboxylate Cluster Catalysts for Alcohol Oxidation.The work described above used polyethylene ligands to recover catalysts and relied on phosphorus-containing polyethylene ligands to coordinate to a transition metal.Polyethylene with terminal anionic carboxylate groups too can be used as a counterion to recover metal salts active in catalysis.66,68,69,79Indeed,carboxylic acid-terminated polyethylene oligomers have proven to be especially useful in transition-metal catalyst recovery.This was apparent in early work designed to prepare a recov-erable,reusable polymer-bound oxidation catalyst.69 The formation of the catalyst in this case relied on the exchange equilibrium shown in eq28.Heating a 1:1(equiv/equiv)ratio of PE Olig CO2H to[(CH3CH2-CO2)6Ru3O(H2O)3]+CH3CH2CO2-in toluene at100°C followed by cooling to room temperature converted the initially dark green solution of the propionate ruthenium complex49into a suspension of a green polyethylene powder in a colorless supernatant.ICP analysis showed that this green powder had1.09% Ru,suggesting that the powder contained a mixture of polyethylene carboxylate and propionate ligands on the recovered polymer-ligated Ru3cluster50. When the polyethylene-bound catalyst so formed was redissolved in chlorobenzene with a primary or secondary alcohol in the presence of oxygen,aldehyde or ketone forme

d.While recovery of these polyeth-ylene carboxylate triruthenium clusters was efficient, the oxidation activities of these catalysts were mod-est.Recycling of this catalytic oxidant was ac-complished by cooling(reprecipitation)and redisso-lution(heating).ICP analysis of the filtrate after the second cycle showed<2%of the charged Ru was lost to solution.Control experiments showed that the propionate cluster was not entrapped in virgin poly-ethylene;a polyethylene ligand was required if the clusters were to be recovered and recycled.However, catalyst decomposition limited catalyst recyclability. Such thermal decomposition had been previously noted for the propionate complex92and also affected the polyethylene-bound clusters.The catalyst deac-tivation in this case could be followed by UV-vis spectroscopy.The polyethylene carboxylate-bound Ru3cluster’s UV-vis spectrum had aλmax of658(this λmax was shifted from the686nmλmax of the propi-onate cluster,possibly because of the nonpolar PE Olig CO2-ligands).Thisλmax of658nm changed to 640nm after prolonged heating of the catalyst, indicating catalyst decomposition had occurred. 7.Polyethylene-Bound Europium(III)Complexes

The color change seen in the exchange reaction28 (dark green solution to colorless solution with a green polyethylene precipitate)could have been the result of the polyethylene carboxylate being a better ligand than a propionate ligand.However,separate experi-ments with octadecanoate and polyethylene carboxy-late-bound Eu(III)clusters showed that the effective-ness of complexation of this Ru cluster by polyethylene ligands in the presence of low molecular weight propionate ligands is a result of entropic effects s entropic effects that can be used to advantage in catalyst recovery and use.

Fluorescence spectroscopic analyses of europium-(III)carboxylate solutions containing as carboxylate ligands mixtures of PE Olig CO2-or C17H35CO2-were used to demonstrate the equivalence of a polyethyl-ene and low molecular weight ligand and to quantify the entropic advantages for metal complex recovery that result from the use of these room-temperature insoluble polyethylene ligands.79The amount of eu-ropium(III)carboxylate remaining in solution after heating and precipitation of mixtures of(C18H37CO2)3-Eu and PE Olig CO2H was used to estimate equilibrium constants for eqs29-31.By varying the molar ratios of(C18H37CO2)3Eu to PE Olig CO2H from1:6to6:1, mixtures were prepared that had from66%to<0.09% of the original Eu(III)left in solution as measured by fluorescence.These experimental values for the percentage of remaining soluble(C18H37CO2)3Eu after dissolution,equilibration,and precipitation of the (C18H37CO2)3Eu and PE Olig CO2H mixtures were then compared to calculated percentages for the remaining (C18H37CO2)3Eu assuming various equilibrium con-stants for equilibria29-31.The results indicated that C18H37CO2H and PE Olig CO2H were roughly equivalent as ligands.Further experiments showed that quantitative recovery of the Eu(III)by PE Olig-CO2H was possible after2-5cycles of dissolution, equilibration,and precipitation even when stoichio-

metric amounts of the two carboxylic acids were used in the initial exchange reaction.If the octadecanoate ligand was a minor component,no detectable Eu(III) remained in solution because entropic factors favor formation of one of the boxed complexes in equilibria 29-31.These complexes in boxes in eqs29-31all contain at least one polyethylene ligand that ensures the complex precipitates.

8.Polyethylene-Bound Rhodium(II)Cyclopropanation Catalysts

The use of a polyethylene carboxylic acid to support a transition-metal salt was successfully used in the preparation of practical catalysts using Rh2(OAc)4as a catalyst precursor.Rhodium(II)carboxylates are known as effective catalysts for cyclopropanation reactions of alkenes,and using PE Olig CO2H as the oligomeric ligand and Rh2(OAc)4as the Rh(II)source, we successfully prepared Rh2(PE Olig CO2)4(51)cata-lysts that were effective in cyclopropanation reactions such as that shown in eq32.66In this case,the

stereoselectivity(trans/cis)seen with the oligomeric catalyst at100°C mirrored that of the low molecular weight Rh2(OAc)4as catalyst at25°C(values in parentheses in eq32).Copper(II)salts of this car-boxyl-terminated polyethylene oligomer,Cu(PE Olig-CO2)2,were equally effective in this olefin cyclopro-panation chemistry.These catalysts could be recycled by precipitation and filtration and used through10 cycles without any loss in activity.

9.Polyethylene-Bound Chiral Rhodium(II)Catalysts Extensions of this chemistry wherein an anionic polyethylene ligand is used to complex and recover a transition-metal catalyst led to one of the earliest and most successful examples of a recoverable ho-mogeneous asymmetric catalyst that produced prod-uct with practical ee values.This work coupled the successful development by Doyle’s group of chiral catalysts for asymmetric cyclopropanation and C-H insertion with the use of polyethylene carboxylates as ligands for recoverable Rh(II)carboxylate cata-lysts.53In this chemistry,the polyethylene oligomers containing terminal carboxyl groups that had been used in the formation of the[(PE Olig CO2)2Rh]2above were converted into hydroxymethyl-terminated poly-ethylene oligomers and coupled to2-pyrrolidone-5(S)-carboxylic acid(PYCA)via an ester bond.A Rh(II) catalyst,PE Olig Rh2(5(S)-PYCA)4(53),was then pre-pared by ligand exchange(eq33)at elevated tem-perature.Cooling precipitated the polyethylene and led to isolation of the resulting catalyst.The catalyst so formed was successfully used in asymmetric C-H insertion reactions and in intramolecular cyclopro-panations.Good enantioselectivity and isolated yields were seen in repeated runs with seven successful cycles of eq34.Some erosion of the initially high enantioselectivity(98%ee)was seen,with the percent ee gradually decreasing to61%by run7.

The entropically favored presence of at least one polyethylene ligand per rhodium catalyst even in the presence of some of the low molecular weight ligand was advantageously used in this case to increase enantioselectivity in the insertion chemistry(eq34). This addition of a small amount(ca.2.7%)of the low molecular weight ligand as an additive in this case did not compromise catalyst recovery.On the basis of the earlier studies of Ru3cluster chemistry and the studies of competitive complexation and recovery of europium(III)carboxylates by PE Olig CO2H,the catalyst53was expected to always have at least one polyethylene ligand,so rhodium leaching did not occur even if small amounts of additives were added.

10.Polyethylene-Bound Phase-Transfer Catalysts

Ionic catalysts in the form of phosphonium salts are widely used as phase-transfer catalysts.93Poly-ethylene ligands have been successfully used to bind phosphonium salts,ammonium salts,and crown ethers for use as recoverable,recyclable phase-transfer catalysts.77,78Kinetic studies of polyethylene-bound phase-transfer catalysts prepared from anion-ically oligomerized ethylene show that the polyeth-ylene-bound catalysts’activities in these reactions are comparable to those of their low molecular weight analogues.77An important consideration in this case is that kinetic studies showed that the polymer-bound catalyst was quite active.The reported rates were such that reaction half-lives of1-8h were readily attained at oligomeric catalyst concentrations of3×10-3M.Catalyst recovery was effected by cooling,precipitation,and filtration.As was true in the other catalytic chemistry described above,these catalysts could be used in multiple sequential reac-tions to form different products without cross-contamination of one product by another as measured by GC analysis.

The most convenient way to use these oligomeric polyethylene-bound phase-transfer catalysts proved to be in solid/liquid phase-transfer reactions.In these cases,0.5mol%polyethylene-bound crown ether catalyst54was dissolved in a solvent such as xylene along with the substrate.The nucleophile in the form of a sodium salt was present as a suspension.Simple

heating to110°C and magnetic stirring then effected

complete conversion of the substrate to product over the course of16h(eq35).Hot filtration or decanta-tion separated the excess sodium salt of the nucleo-phile from the organic solution.Cooling and filtration then produced a precipitate of the polyethylene-bound crown ether54that was recovered by https://www.doczj.com/doc/0613817246.html,ing this protocol,three successive preparative reactions on a50mmol scale were carried out using 1-bromooctane and sodium iodide,octanal and so-dium borohydride,and1-bromooctane and sodium cyanide with no contamination of the products from one reaction to the next.Isolated yields in these

reactions were90%,93%,and85%,respectively. Soluble polyethylene-bound phase-transfer cata-lysts have also been prepared using commercially available oxidized polyethylene as a support.78In this case,bifunctional catalysts containing ammonium groups and a polyether were prepared by amidation of the carboxylic acid groups of oxidized polyethylene using commercially available R,ω-diaminopoly(alkene oxide)s.By using an excess of this reagent,a poly-ethylene-bound poly(alkene oxide)amine could be prepared and isolated.Subsequent quaternization with butyl bromide yielded a tetraalkylammonium phase-transfer catalyst that has polyethylene-like temperature-dependent solubility and activity similar to that of phase-transfer catalysts in nucleophilic substitutions of primary alkyl halides with NaCN.

11.Polyethylene-Bound Tin Catalysts

Polyethylene oligomers can also be used to support other organic catalysts.A comparison of the utility of soluble polystyrene-and polyethylene-supported tin halides55and56as catalysts for reductions of aryl and alkyl bromides and iodides using sodium borohydride as the penultimate reductant has been reported.70In this chemistry,the soluble polymer-bound tin halides were prepared by anionic oligo-merization of ethylene or styrene followed by treat-ment of the reactive organolithium oligomers so formed with excess Bu2SnCl2(eqs36and37).The products were characterized by both1H and

119Sn NMR spectroscopy.Loadings were also esti-mated by integrating the119Sn NMR peaks versus a soluble tin standard(Bu4Sn).Typical loadings esti-mated by NMR spectroscopy were in the0.1-0.4 mmol of tin/g of oligomer range,values that were in rough agreement with the results of elemental analy-ses for tin that showed loadings were in the range of 0.3-0.6mmol of tin/g of oligomer.

Catalysts55and56were successfully used in alkyl and aryl bromide reductions(eq38).In these reac-tions,the polyethylene-bound tin catalyst was recov-ered and reused following the protocol described in Figure6.The polystyrene-bound catalyst had reac-tivity analogous to that of the polyethylene-bound catalyst in bromide reductions.However,the cool-ing-precipitation process used to recover the poly-ethylene-bound catalyst was more convenient than solvent precipitation that was required for recovery/ reuse of the polystyrene-bound catalyst.Recycling experiments consequently mainly used the polyeth-ylene-bound catalyst.Kinetic studies showed that catalysts55and56had comparable reactivity. 12.Polyethylene-Bound Polymerization Catalysts Polymerization catalysts are often not separated from the product polymer;their presence is often at very low levels or is inconsequential for the eventual polymer end use.Ziegler-Natta and metallocene olefin polymerization catalysts provide a widely used example of efficient catalysts that are not recovered or reused.However,in some cases where the catalyst residue can affect a polymer’s stability or properties, there is interest in recovering and reusing polymer-ization catalysts.

a.Polyethylene-Bound Diene Polymerization https://www.doczj.com/doc/0613817246.html,nthanide catalysts for stereoselective polymerization of butadiene were one of the first examples of a polyethylene-bound polymerization catalyst that was successfully recovered and reused.68 In this case,a(PE Olig CO2)3Nd catalyst was used with Et3Al or EtAlCl2as a reducing agent to stereospe-cifically polymerize butadiene(eq39).While the resulting catalysts had activity equivalent to that of a divinylbenzene-cross-linked-polystyrene-bound neo-dymium carboxylate,separation of the catalyst was a problem.When this polymerization was carried out to produce a concentrated solution of poly(cis-1,4-butadiene)product,the cooled solution was too viscous for filtration or centrifugation to recover the polyethylene-bound catalyst.Catalyst filtration(or centrifugation)to recover the polyethylene-bound catalyst57was only possible if the reaction mixture was first diluted with excess solvent.This

require-

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;

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