201001474_ftp[1]

MICROREVIEW

DOI:10.1002/ejoc.201001474 Direct Nucleophilic S N1-Type Reactions of Alcohols Enrico Emer,[a]Riccardo Sinisi,[a]Montse Guiteras Capdevila,[a]Diego Petruzziello,[a]

Francesco De Vincentiis,[a]and Pier Giorgio Cozzi*[a]

Keywords:Alcohols/Lewis acids/Br?nsted acids/Nucleophilic substitution/Green chemistry

In2005,the ACS Green Chemistry Institute(GCI)and the global pharmaceutical corporations developed the ACS GCI Pharmaceutical Roundtable to encourage the development of green chemistry and green engineering in the pharmaceuti-cal industry.The Roundtable has established a list of key re-search areas including the direct nucleophilic reactions of alcohols.The substitution of activated alcohols is a frequently used approach for the preparation of active pharmaceutical ingredients.Alcohols are transformed into the reactive ha-lides or sulfonate esters,thereby allowing their reaction with 1.Introduction

Redeveloping chemical transformations to be more con-scientious with regard to the use of resources is becoming an increasingly important research theme.The consider-ation of atom economy and the use of more environmen-tally friendly reactions is becoming a key focus of many research groups.

Nucleophilic substitution of alcohols is a very important process,used widely to access a variety of derivatives.As shown herein,the practical catalytic nucleophilic direct sub-stitution of alcohols offers a potential solution to many en-vironmental problems.In the synthesis of pharmaceutical products,the substitution of activated alcohols is a fre-quently used approach for the production of active ingredi-ents,and this key transformation is realized through alcohol activation.In2005,the ACS Green Chemistry In-stitute and the global pharmaceutical corporations devel-oped the ACS GCI Pharmaceutical Roundtable to encour-age the integration of green chemistry into the pharmaceu-tical industry.[1]The Roundtable has developed a list of key research areas with OH activation for nucleophilic substitu-tion considered a central issue.Currently,activation is quite a wasteful process.Direct nucleophilic substitution of an alcohol is attractive,as it should produce water as the by-product.However,this reaction is difficult because hydrox-ide is such a poor leaving group and therefore activation is usually required.

[a]ALMA MATER STUDIORUM,Universitàdi Bologna,

Dipartimento di Chimica“G.Ciamician”,

via Selmi2,40126,Bologna,Italy

Fax:+39-051-2099456

E-mail:piergiorgio.cozzi@unibo.it nucleophiles.Although the direct nucleophilic substitution of an alcohol should be an attractive process,as one of the by-products from the reaction yields water,hydroxide is a poor leaving group that hinders the reaction.Recently,the direct substitution of allylic,benzylic,and tertiary alcohols has been achieved through an S N1reaction with catalytic amounts of Br?nsted or Lewis acids.In this review,the approaches lead-ing to a greener process are examined in detail,and the ad-vances achieved to date in this important transformation are presented.

Direct activation of benzylic,propargylic,and allylic alcohols may be achieved through S N1reactions by the use of Br?nsted or Lewis acids.In this review we will examine the recent developments in the direct nucleophilic substitu-tion of alcohols from the point of the view of the different acids used in the transformations.

2.S N1Nucleophilic Reaction of Alcohols by Generation of Carbocations

The addition of nucleophiles to the reactive center of a prostereogenic ketone or aldehyde is one the most widely studied and important reactions used in organic chemistry. However,aldehydes and ketones are weak electrophiles,and normally they are activated towards nucleophilic attack by the use of Lewis acids or Br?nsted acids.[2]S N1-Type reac-tions are frequently used in organic synthesis.The use of more electrophilic reagents has been considered,and imin-ium and oxonium[3]ions are generated and employed in the presence of Lewis acids.One of the most effective electro-philes is a carbenium ion.From pioneering studies of Olah, the theoretical knowledge of carbocations has greatly in-creased and raised the possibility of developing widely ap-plicable reactions.[4]The generation of a carbocation in a controlled way is also crucial in important industrial pro-cesses.[5]Generally,carbenium ions are believed to be un-stable species and highly reactive;however,there is a quan-titative approach to classify the stability and reactivity of carbocations.In a recent review on Friedel–Crafts direct reactions of alcohols,[6]the concept ofπ-activated alcohols was introduced.A more precise and quantitative definition of activated alcohols can be derived from the stability of the

P .G.Cozzi et al.

MICROREVIEW

carbocation generated by the alcohols.If the carbocation is very electrophilic and it reacts to the diffusion limits,a lim-ited class of nucleophiles is able to intercept the presence of the carbocation.Through a precise and quantitative defini-tion of nucleophilicity and electrophilicity given by the me-ticulous work of Mayr,[7]the S N 1-type reaction of alcohols can be interpreted,thereby enabling the development of many new reactions (Scheme 1).

Electrophilicity parameters were introduced by Mayr,and he has demonstrated that one parameter for the electro-phile (E )and two parameters for nucleophiles (s and N )are sufficient for a quantitative description of the rates of a large variety of electrophile–nucleophile combinations.

log k (20°C)=s (N +E )

(1)

The parameter E in the equation represents the electro-philicity parameter,defined by the use of a reference com-pound.The E parameter for many diarylcarbenium ions has been determined,and variation of the para and meta substituents modifies their electrophilicity by more than 16orders of magnitude.The more reactive

carbocations

Dr.Enrico Emer (bottom row,left)was born in Verona,Italy,in 1979.He received his BSc degree in Chemistry in 2005from the University of Bologna.In the same year he joined the group of Prof.A.Umani-Ronchi at the University of Bologna (C.I.N.M.P .I.S.research grant).In 2008,he worked in the group of Dr.F.Paradisi,University College Dublin (UCD),Ireland (Marco Polo research grant).In 2009,he received his PhD in Organic Chemistry under the supervision of Prof.D.Giacom-ini,and in the same year he joined the group of Dr.G.Varchi at the National Council of Research (CNR),Bologna.Since February 2010,he has been working in the group of Prof.P .G.Cozzi on the CATAFLU.OR project,at the University of Bologna.His main research focuses on the stereose-lective α-alkylation of aldehydes with stabilized carbocations catalyzed by chiral imidazolidinones and Lewis Acids.

Riccardo Sinisi (bottom row,right)was born in Terlizzi (Italy)in 1979.He received his laurea in

Chemistry and Pharmaceutical Technology in 2005.He was then awarded a one-year fellowship by Bologna University working with Prof.G.Cainelli.Successively,he spent five months as a temporary employee with Prof.M.R.Gagne at the University of Chapel Hill working on cross-coupling reactions.He then returned to Bologna,where he obtained his PhD in 2010working with Dr.M.Bandini and Prof.A.Umani-Ronchi.During his PhD he spent seven months as a visiting student in the group of Prof.G.C.Fu developing a novel phosphane-catalyzed reaction.In the same year he joined the research group of Prof.P .G.Cozzi as a postdoctoral associate.

Montserrat Guiteras Capdevila (top row,center)was born in 1980in Barcelona (Catalonia),Spain.She graduated from the University of Barcelona in 2006and started her work on the synthesis of new Semicorinne derivatives in the research training network IBBAC project under the supervision of Prof.P .G.Cozzi at the University of Bologna,Italy (2007).Before she started her PhD in the same group concerning the development of new organocatalytic stereoselective S N 1-type reactions,she studied new surfactants and amphotropic materials in Cologne,Germany (2008)under the supervision of Prof.D.Blunk.

Diego Petruzziello (top row,left)was born in Carpi (Italy)in 1984.In February 2009,he received his masters in Advanced Chemical Methodologies from the University of Bologna.In March 2009,he obtained a fellowship in Bologna working in the field of the synthesis of new hydroxyquinoline-derived fluorescent probes for biological application under the supervision of Prof.C.Trombini.In 2010,he started PhD research on the development of new ferrocenyl-based organocatalysts at the University of Bologna under the supervision of Prof.P .G.Cozzi.

Francesco De Vincentiis (top row,right)was born in Taranto (Italy)in 1982.He studied at the faculty of Industrial Chemistry at the University of Bologna,where he received his degree in 2009under the direction of Prof.G.Bartoli and Dr.P .Melchiorre.He is currently working under the supervision of Prof.P .G.Cozzi at the Department of Chemistry “G.Ciamician”of the University of Bologna.

Pier Giorgio Cozzi (bottom row,center)studied chemistry at the University of Milan where he received his Laurea degree in 1989.After spending four years as a research associate in Lausanne (Switzerland),he was appointed Assistant (1994)and then Associate Professor (2000)at the University of Bologna.The development of new,enantioselective catalytic reactions,the design of new chiral ligands,and the development of enantioselective variant of famous named reactions (Nozaki–Hiyama,Reformatsky)are his preeminent interests.Prof.Cozzi has been visiting professor in the University of Aachen (Germany),Neucha ?tel (Switzerland),Ottawa (Canada),Basel (Switzerland),Aarhus (Denmark),Hong Kong,and GuangZhou (China),and he has participated in two European Networks in the Sixth Framework Program Priority;the European LigBank,and the IBA 2C projects.He is coordinator of the program CATAFLU.OR in the Fp7and is participating in the ITN BioChemLig.

Scheme 1.S N 1-Type nucleophilic substitution of alcohols pro-moted by Lewis or Br?nsted acids.

ure 1).Several S N 1-type reactions can now be rationally de-signed through the use of the Mayr scale of reactivity.[8]An important

lesson that came from the work of Mayr is to consider not just the electrophilic partner of the reaction (i.e.,the carbocation generated by the alcohol)but also the nucleophile,the nucleophilic properties of the solvent,and also the generated byproduct.It is very important in the design of new reactions of carbocations to consider a com-bination (Figure 1)of the electrophiles and the nucleo-philes.

Figure 1.Selected electrophiles and nucleophiles from the Mayr scale.

In fact,Mayr has established a predictive “rule of thumb ”[9]for examining if a reaction between an electro-phile and a nucleophile will be successful.Of course,there are limitations that come from neglected steric effects.[10]In addition,as mentioned,the nucleophilic character of the solvent needs to be taken into account,as the nucleophilic-ity of the solvent can compete with the nucleophilicity of other reagents.[11]It is important to realize that for some nucleophile–electrophile combinations,the corresponding reaction cannot occur on the basis of their position on the scale developed by Mayr.In general,carbocations gener-ated by alcohols are derived from benzylic,propargylic,or allylic alcohols.Recently,an example of the use of a less-activated alcohol was reported.This study is very important and elegantly illustrates the fact that the formation of very reactive and electrophilic carbocations,from inactivated alcohols,needs to be investigated further.[12]

alcohols can the presence of acids,carbocations that are found in the electrophilicity scale of Mayr.[13]Strong electron-donating substituents enhance the stability of the carbocations.[14]Benzylic carbocations are of moderate stability and high reactivity (e.g.,mesitylbenzylic carbocation is positioned at +6on the Mayr scale)and are capable of reacting with a large variety of nucleophiles.Because of the low ability of the alkynyl group to stabilize a positive charge,reactions of propargyl cations are quite limited.Quite interestingly,the coordination of the triple bond with Co 2(CO)6reduces the electrophilicity of the carbocation generated by the alcohols.The well-studied and developed Nicholas reac-tions [15]can now be well interpreted by using the Mayr table of reactivity.[16]Finally,allylic alcohols,another type of substrate illustrated in this review,forms a carbocation that is electrophilic,and it is positioned above 1in the Mayr scale.[17]Consequently,in all the reactions illustrated in this review,where Br?nsted or Lewis acids are employed,a bet-ter understanding of the reactions can come from careful examination of the concepts established by the work of Mayr.

3.Reactions of Alcohols Promoted by

Stoichiometric Amounts of Lewis or Br?nsted Acids

In the total synthesis of natural products,the generation of carbocations has been used with considerable success.In examples reported,stoichiometric amounts of Br?nsted and Lewis acids were used to accomplish the direct substi-tution of alcohols.As the main focus of this review is the use of catalytic amounts of acids for the nucleophilic substi-tution of alcohols,we have selected only a few stoichiomet-ric variants from the literature.[18]

3.1.Stoichiometric Amounts of Br?nsted

and Lewis Acids

In 2002,Poli recognized the ability of carbocations gen-erated with stoichiometric amounts of BF 3from benzhy-drylic alcohol 1[19]to react with activate methylene com-pounds (Nu =diketone a ,keto ester c ).The authors recog-nized the potential developments in this field,as the alcohols gave considerable advantages compared with the more classical halide-based basic conditions (Scheme 2).

Scheme 2.Reaction of benzhydrylic alcohol 1with nucleophiles a and c in the presence of BF 3.

A more recent example of stoichiometric amounts of Lewis acids being used in the activation of alcohols,is a simple and efficient AlCl 3-mediated direct substitution of

P .G.Cozzi et al.

MICROREVIEW

the hydroxy group in hydroxyketene-S ,S -acetals 12(Scheme 3)and various arenes described by Zhao and Liu.

[20]

Scheme 3.AlCl 3-catalyzed C–C coupling reaction of hydroxyketene cyclic dithioacetals 12with arenes.

3,4-Disubstituted dihydrocoumarins were prepared in high yields by sequential Friedel–Crafts alkylation and in-tramolecular annulation of (R )-hydroxyketene acyclic-S ,S -acetals with phenols under mild conditions.The reaction was conducted in CH 2Cl 2at room temperature in the pres-ence of AlCl 3(1equiv.).Benzene,alkyl benzenes,and phe-nols were the nucleophiles used in the transformation.Liu and Zhang reported the BF 3·Et 2O-catalyzed coupling reac-tion between readily available ketene-(S ,S )-acetals 14and various alcohols through direct substitution of the hydroxy group in alcohols (alcohols 1–3and others).[21]Other Lewis acids,such as AlCl 3and FeCl 3,also effected the transfor-mation (Scheme 4)but only after a prolonged reaction

time.

Scheme 4.Reaction of ketene-(S ,S )-acetals 14with l in the presence of BF 3.

The stoichiometric reaction with Br?nsted or Lewis acids can be advantageous,particularly when strong coordinating groups are present in the alcohols that are subjected to S N 1-type reactions conditions.In fact,in the synthesis of Tolter-odine [16,N ,N -diisopropyl-3-(2-hydroxy-5-methylphenyl)-3-phenylpropanamine],the drug of choice for most patients for the treatment of urinary incontinence,Rhee and De Ca-sto described a Friedel–Crafts reaction of a phenyl propane amino alcohol with p -cresol performed with aqueous 70%hydroperchloric acid.

[22]

Alkoxide C–O bond cleavage occurs readily at room tem-perature in the presence of titanium(IV)halide.Capture of the resultant carbocation by alkynes provides an efficient route to trisubstituted (E )-alkenyl halides with high stereo-selectivity The reaction is conducted in DCM at 0°C in the presence of a stoichiometric amount of titanium halides.[23]

4.Catalytic Activation of Alcohols

4.1.S N 1-Type Nucleophilic Substitution of Alcohols in the Presence of Br?nsted Acids

Sanz described,in a series of papers,a simpler approach towards the nucleophilic direct substitution of alcohols by the use of a simple Br?nsted acid as the catalyst.Secondary and tertiary propargylic alcohols 6and 7were treated with dicarbonyl compounds (β-diketones a ,b ;β-keto esters c )in the presence of p -toluenesulfonic acid (PTSA,5mol-%)in CH 3CN at reflux.Internal alkynes with either an aromatic or heteroaromatic group at the propargylic position and bearing an aromatic,heteroaromatic,or alkyl group at the terminal position were used in the reaction [24](Scheme

5).

Scheme 5.Propargylation of alkynol 6in the presence of PTSA.

The Br?nsted acid catalyzed C3-selective tert -alkylation of indoles with tertiary propargylic alcohols 7and 8and benzylic alcohols 2were described by Sanz.Various C3-propargylated indole derivatives with a quaternary carbon at the propargylic position were synthesized in an efficient manner,with the reaction being performed in air with rea-gent-grade solvents.[25]The use of less-reactive nucleophiles such as alkynylsilanes with alcohols 7and 8gave a compli-cated mixture of products derived from rearrangements of the formed carbocation.[26]The use of PTSA was applied to the synthesis of bicyclo[3.1.0]hexanes.[27,28]In addition,an efficient one-pot propargylation/cycloisomerization tan-dem process catalyzed by PTSA was developed for the syn-thesis of substituted oxazole derivatives,starting from pro-pargylic alcohols and amides.[29]An interesting approach to performing the reaction in the presence of a Br?nsted acid involved the use of ionic liquids.In fact,most of the S N 1-type reactions of alcohols required hazardous or volatile solvents,such as nitromethane,dichloromethane,acetoni-trile,and toluene,and the recovery and reuse of the cata-lysts was very limited.Supported ionic liquid Br?nsted ac-ids have been used by Kunabiki in the S N 1-type reaction with alcohols.[30]The Br?nsted acid catalyzed direct reac-tion of benzylic alcohol 2,allylic alcohols 3and 4,and pro-pargylic alcohol 6with 1,3-dicarbonyl compounds pro-ceeded with various alcohols in an ionic liquid such as N -ethyl-N -methylimidazolium trifluoromethanesulfonate (EMIOTf)at 100°C.Tandem benzylation–cyclization–de-hydration of 1,3-dicarbonyl compounds to give function-alized 4H -chromenes was also reported.Mac Cubbin and co-workers described the use of pentafluorophenylboronic acid (18)in 10mol-%to catalyze a series of Friedel–Crafts

a variety of electron-rich aromatic and heteroaromatic sub-strates at room temperature.

The reaction was carried out in the presence of molecular sieves.In all examples reported,the products resulted from a formal conjugate nucleophilic addition to the allylic alcohol,suggesting the possibility that an S N 2?mechanism is involved.Phenols,indoles,and methoxy-substituted ben-zenes were the nucleophiles employed in the reaction.[31]The same authors reported that 18efficiently catalyzed the coupling of electron-rich arenes and heteroarenes with benzylic alcohol 2to

afford di-,tri-,and tetraarylme-thanes.[32]In this case,the reaction conditions required 10mol-%of the catalyst in dichloroethane at reflux.Shim-izu developed a new reaction system,which is based on the inverse phase-transfer catalysis of functionalized water-sol-uble calixarenes and water-soluble calix[4]resorcinarene sul-fonic acid 19.Acid 19was able to promote the metal-free dehydrative amination of alcohols in water (Scheme 6).

Scheme 6.Allylic amination of alcohol 3in the presence of catalytic amounts of catalyst 19.

The importance of examining other Br?nsted acids is il-lustrated by the fact that trans -1,3-diphenyl-2-propen-1-ol (3)did not react with tosyl-,CBz-,or Boc-amines in water in the presence of common Br?nsted acids such as AcOH,TFA,MsOH,TsOH,and TfOH.However,water-soluble 19used in a catalytic amount (10mol-%)was

found to be an efficient catalyst.[33]Barbero and co-workers have described acid-catalyzed organic reactions,such as etherification,es-terification,acetal synthesis,cleavage,and interconversion,and pinacol rearrangement,in the presence of catalytic amounts of o -benzenedisulfonimide (21,10mol-%)as a Br?nsted acid catalyst.[34]

Chan and Mothe [35]developed a highly efficient triflic acid catalyzed ring opening of a wide variety of 1-cyclopro-pyl-2-propyn-1-ols 22with alcohols,which are efficient syn-thetic precursors for conjugated enynes.The reaction was accomplished in very good yield by the use of 0.01mol-%of triflic acid (Scheme 7).

Scheme 7.Acid-catalyzed opening of cyclopropyl alcohols.

4.2.Solid Br?nsted Acids

Kaneda and co-workers [36]have developed an environ-mentally benign synthetic approach to direct nucleophilic substitution reactions,avoiding the activation of the alcohols.The transformation,which minimizes the forma-tion of by-products,is catalyzed by proton-and metal-ex-changed montmorillonites (H-

and M n +-mont).These Lewis acidic solids were easily prepared by treating Na+-mont with an aqueous solution of hydrogen chloride or metal salt.The H-mont possessed outstanding catalytic ac-tivity for nucleophilic substitution reactions of a variety of alcohols (Scheme 1,1–7)with a range of nucleophiles.The catalyst was used in 10mol-%in heptane at 150°C (Scheme 8).

Scheme 8.Nucleophilic substitution of cyclohexanol promoted by solid Lewis acid montmorillonite.

A remarkable result obtained by Kaneda was the use of aliphatic alcohols such as 24,which form less-stabilized carbocations.The active species for these nucleophilic ad-ditions seems to be a Br?nsted acid site,and the reaction involves a carbocation intermediate.The formation of the ether by reaction of the alcohol with itself,and the revers-ible regeneration of the carbocation promoted by water,seemed to be involved in the process.An efficient and oper-ationally simple method for C3-alkylation of 4-hydroxycou-marins was developed under acidic medium,giving good yields of the products.In this report,a reusable Amberlite IR-120(Htform)was used as an acid catalyst and second-ary benzyl alcohols were used as alkylating agents.[37]

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MICROREVIEW

Corma and Renz described the introduction of single,iso-lated Lewis acid sites into the framework of molecular si-eves (Scheme 9).The Lewis acidic properties of the solid catalyst obtained are quite interesting,as water-stable Lewis acid catalysts were obtained.[38]The catalyst was used to promote the etherification of benzyl alcohols 26by using the nucleophilic alcohol as the solvent,at 100°C,and these Lewis acid catalysts can be used in cascade-type reac-tions.

[39]

Scheme 9.Etherification of benzylic alcohols by solid Lewis acid catalyst Sn-beta.

4.3.Other Supported Br?nsted Acids

Heteropolyanionic acids were supported on SiO 2and used by Srihari and co-workers in the reaction of propar-gylic alcohols 6and 7with aromatic compounds (naph-thols,phenols,and indoles).The reaction was performed in CH 3CN or under neat conditions at room temperature in a few hours by using 1mol-%of the supported acid.[40]4.4.Catalytic Activation by Lewis Acids

The Lewis acid mediated processes for S N 1-type reac-tions with different nucleophiles are mediated by coordina-tion of the Lewis acid and consequent formation of the cor-responding carbocation.As we have already pointed out in this review,the nucleophilicity and electrophilicity scale of Mayr indicates the possibility of performing S N 1-type reac-tions.On the other hand,differences among the Lewis acids can be involved in the formation of the cation.The work of Sanderson and Brayse gives a theoretical insight into the type of Lewis acid to use.[41]The acidity of BiX 3(X =Cl,Br,I)was explored by using density functional theory (DFT)studies of simple complexes with various alcohols.The calculated relative energies of the complexes follow the trend of hardness for BiX 3(Cl ?Br ?I).The activation of an alcohol with a Lewis acid can in principle give different products (Scheme

10).

Scheme 10.Lewis acid reaction of alcohols:the effect of the hard-ness of BiX 3.

The trend in the Lewis activity of BiX 3was examined in terms of chemical hardness and energy of formation.All

three BiX 3molecules are predicted to form Lewis com-plexes with the model alcohol,with the strongest interac-tion for the chloride.The trend can be correlated with the measurements of hardness,which place BiCl 3as the hardest and BiI 3as the softest in this series.The differences in reac-tivity of the chloride and bromide with alcohols can be ex-plained in terms of relative hardness.The elimination of OH by the Lewis acid will be favored by similarity in hard-ness between the OH group and the counterion of the Lewis acid.This theoretical study is also crucial for the develop-ment of S N 1-type enantioselective direct substitution.The formation of an intimate ion pair preceded a carbocationic pathway that can give different products (Scheme 10).The OH group is coordinated by the Lewis acid in the intimate ion pair and the halide of the Lewis acid can react with the carbocation.

4.5.Bismuth

A highly efficient Bi(OTf)3-catalyzed benzylation of ar-enes and heteroarenes with alcohol 2was described by Rueping.The reaction was carried out in nitromethane or CH 2Cl 2at reflux in the presence of a catalyst (0.5mol-%).The reaction is particularly interesting,as not only nucleo-philic arenes (p-methoxyphenol p and dimethoxybenzene q )are reactive substrates,but also weak nucleophiles such as toluene or xylene (N =–4;N =–3,in the Mayr scale).The reaction was not restricted to arenes,and diketones a and b were also suitable nucleophiles (Scheme 11).

[42]

Scheme 11.Bismuth triflate catalyzed Friedel–Crafts alkylation of a benzylic alcohol.

In another reaction promoted by Bi(OTf)3,Bach de-scribed the direct alkylation of silyl enol ethers with p -meth-oxybenzylic alcohols with Bi(OTf)3(5mol-%)in CH 3NO 2as the solvent.The reaction provided the α-benzylated car-bonyl compounds in high yields after short reaction times.In addition,the alcohol can be reduced in the presence of Bi(OTf)3to the corresponding alkanes with Et 3SiH (l ).[43]A general and efficient BiCl 3-catalyzed substitution reac-tion of propargylic alcohols with carbon and heteroatom-centered nucleophiles such as allyl trimethylsilane,alcohols,aromatic compounds,thiols,and amides,leading to the construction of C–C,C–O,C–S,and C–N bonds,has been described by Zhan and co-workers.[44]The reaction is con-ducted at room temperature to 60°C in CH 3CN by the use of 10mol-%of the catalyst.The advantage of this system is the absence of the rearranged unsaturated ketones that are sometimes obtained with propargylic alcohols with other Lewis acidic catalysts.

tion of allylic alcohols 3and 4and propargylic alcohols 6(Scheme 12)with amides g –i in the presence of Bi(OTf)3/KPF 6(1mol-%).It is important to mention that although Bi(OTf)3alone promoted the reaction,the use of KPF 6ac-celerates the reaction.The scope of the amides was quite large,as carbamates,oxazolidinones,and unsaturated amides were suitable nucleophiles.The scope of the reaction with respect to the alcohol was quite interesting,as non-benzylic allylic alcohols were also useful electrophiles.Amides attacked selectively the sterically less-hindered car-bon atom of the allylic alcohol functionality.On the other hand,the propargylic

alcohols reacted regioselectively at the propargylic position.The reaction could be interpreted by a mechanism in which a carbenium intermediate is formed,by the observation that enantioenriched allylic alcohols react to give racemic products,with the reaction appearing to be reversible under the reaction conditions.An efficient substitution of the Baylis–Hillman adduct was promoted by using BiCl 3(10mol-%)in the reaction with different alcohols.[46]

Scheme 12.Amination of

an allylic alcohol promoted by activated bismuth triflate catalyst.

4.6.Boron

A variety of aryl nitriles have been prepared in excellent yield from the corresponding benzylic,benzhydrylic,pro-pargylic,and

allylic alcohols by the use of B(C 6F 5)3(29,3mol-%)as a Lewis acid catalyst in CH 3CN using TMSCN (n )as the cyanide source.[47]

Figure 2.Formation of indene products by intramolecular Friedel–Crafts reaction.

An efficient acid-catalyzed protection of alcohols as trityl ethers is described by using triphenylmethanol (10)in the presence of 29(3mol-%)in dichloromethane at room tem-perature.As the reaction occurs by an S N 1-type reaction,this method allows the selective protection of primary alcohols in the presence of secondary alcohols.[48]Catalytic amounts of BF 3were

used by Li and co-workers [49]in the catalyzed Friedel–Crafts intermolecular cyclization of io-dinated allylic alcohol 30(Figure 2).4.7.Cerium Lewis Acid

The reaction between 9H -xanthen-9-ol (32),pyrrole,and indoles in the presence of ceric ammonium nitrate (CAN)proceeded smoothly at room temperature to give the direct substitution of the OH groups with indole and pyrrole in 90%yield.[50]

4.8.Copper

Propargylic alcohols 6and 7undergo smooth deoxy-genative allylation with allylsilanes in the presence of a solution of copper(II)tetrafluoroborate (10mol-%)in ace-tonitrile to afford the corresponding 1,5-enynes in good to high yields under mild and neutral conditions.Scandium triflate was also found to catalyze efficiently the nucleo-philic substitution of propargylic alcohols with allylsi-lanes.[51]Huang and co-workers studied the copper(II)bro-

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MICROREVIEW

mide catalyzed substitution reaction of propargylic alcohols 6and 7with C,O,and S nucleophiles.[52]Quite interest-ingly,CuBr 2was the most effective Cu salt of the tested complexes and Cu II derivatives.

4.9.Indium

Baba reported the use of InCl 3in the direct substitution of allylic alcohol 3with nucleophiles (Scheme 13).

[53]

Scheme 13.Addition of malonate to allylic alcohol 3promoted by a catalytic amount of indium salt.

The reaction was studied with different Lewis acids (5mol-%)in toluene at 80°C.It is important to note that toluene,used as the reaction solvent,was inert under these conditions.The nucleophile investigated was malonate.BF 3,a palladium catalyst,and HCl all did not give the product,which was,however,present in a small amount (6–25%)when AlCl 3,GaCl 3,or Sc(OTf)3was used as the Lewis acid.InCl 3was found to act as a catalyst for the reaction as well as InBr 3,whereas other indium salts were ineffective [In(-OAc)3,In(OH)3,In(acac)3].The reaction of allylic alcohol was tested with active methylene compounds including dies-ters,keto esters,and diketones and gave the corresponding alkylated product.Benzylic alcohol 2and benzhydrylic alcohol 1were also reactive substrates.Mechanistic investi-gation showed that the ethers are obtained (Scheme 14)by the action of the In III

salts.

Scheme 14.Generation of a reactive benzhydrylic carbenium ion from ethers by indium salt.

Baba also reported the indium-catalyzed direct chlorina-tion of alcohols [54]and the reduction of alcohols by InCl 3in the presence of HClSiPh 2(35,1.1equiv.).[55]In the pres-ence of benzil (PhCOCOPh,36)as an additive,the course of the reaction significantly changed.The reaction was then optimized with the use of HSiClMe 2(37),AlCl 3,and Sc(OTf)3.This direct chlorination was investigated with a variety of alcohols.Whereas primary alcohols were not re-active,differently substituted benzhydrylic alcohol 1,benz-ylic alcohol 2,and propargylic alcohol 8gave the corre-sponding products in good isolated yields.In the presence of benzil,there was no formation of the silyl ether.Re-duction of benzil by 37with insertion of chlorine gave a species that is able to activate the alcohol,and through the generation of H 2,the Me 2SiCl fragment is transferred to the alcohol.Finally,the addition of chloride with concomitant elimination of oxygen promoted by InCl 3occurs.The inter-action among indium halides and R 3SiCl was used by Baba [56]to alter the Lewis acid properties of indiums salts.In fact,the combination of InCl 3and Me 3SiBr provided an enhanced Lewis acid system that was used in the direct reaction of silyl nucleophiles (allylsilane,propargyl and al-lenylsilane,silylalkynes)with benzhydrylic alcohol 1,benz-ylic alcohol 2,ferrocenylic alcohol 38,and propargylic alcohol 7(Scheme

15).

Scheme 15.Allylation of a ferrocenyl carbenium ion promoted by an indium salt.

The reaction was performed in hexane from 0°C.Me 3-SiBr was used in 10mol-%,whereas InCl 3was used in 5mol-%.The enhanced Lewis acidity of this system was rationalized by 13C NMR spectroscopy.In addition,the in-teraction between Me 3SiBr and In III was proven by 29Si NMR spectroscopy.Addition of dicarbonyl compounds to propargylic alcohols by using InCl 3as the catalyst was also reported.[57]It is important to note that the catalyst toler-ated the presence of aromatic and aliphatic primary amines.The catalyst was employed in 10mol-%in toluene at 60°C.The reaction of propargylic alcohols 6and 7with heteroar-omatic compounds promoted by InBr 3was described by Jadav.[58]Finally,the catalytic addition of TMSCN (Scheme 1,n )to a variety of benzylic alcohols promoted by InBr 3was reported recently by Ding.[59]Using 5–10mol-%of InBr 3as the catalyst,a variety of benzylic alcohols could be converted into the corresponding nitriles in 5–30min with yields of 46–99%.An interesting direct formation of the C–C bond between propargylic alcohol 6and allyl in-dium reagents has been reported.[60]The reaction is an S N 2?-type selective displacement of propargyl leading to an allene.The reaction appeared limited in scope,as no pro-pargylic substitution products through a S N -type reaction with cinnamyl bromide,crotyl bromide,cyclohexenyl bro-mide,and phenyl bromide was taking place.The authors reasoned that this may be due to the difficulty in obtaining the corresponding indium reagents in THF reaction sol-vents,but steric effects were not excluded.The highly α-regioselective N-nucleophilic substitution of Baylis–Hill-man adducts 40bearing five-or six-membered ring moieties with aromatic amines catalyzed by In(OTf)3(10mol-%)gave the α-products in good yield.[61]During the reaction,the allylic rearrangement from the γ-product to the α-prod-

product predominately (Scheme 16).[62]Different Lewis ac-ids,such as AgOTf,InBr 3,Zn(OTf)2,Sc(OTf)3,and molec-ular I 2,did not exhibit any catalytic activity.

Scheme 16.Catalytic amination of a Baylis–Hillman adduct pro-moted by an indium Lewis acid.

4.10.Iodine

Molecular iodine has many advantages due to its moder-ate Lewis acidity,its low cost,and its water tolerance,and it has received considerable attention as a catalyst for various organic transformations.[63]Liu and Chen [64]used I 2(10mol-%)for the addition of indole to Baylis–Hillman ad-duct 40.Different reaction conditions afforded

different products (Scheme 17),with THF being the optimum sol-vent for αattack,furnishing 24:1of the desired product with complete conversion.The γ-product is the kinetic re-sult of the reaction and can be transformed into the α-prod-uct by heating at reflux in THF in the presence of iodine.Electron-rich indoles (N -methylindole,2-methylindole,5-methoxyindole)were used in the reaction.

Scheme 17.Addition of 2-methylindole (v )to a Baylis–Hillman ad-duct promoted by a catalytic amount of iodine.

The authors suggested that the selectivity was dependent upon hydrogen-bonding interactions.With the weak I 2Lewis acid,THF acts as a strong hydrogen-bond acceptor,destroying the six-membered ring chelate between the OH group and the carbonyl group.In this case,the reaction proceeds through a Michael pathway,which is followed by an acid-catalyzed [1,3]sigmatropic carbon skeleton re-arrangement of the obtained allylindoles.Also,secondary benzylic alcohol 1can be activated by iodine in their reac-tion with 4-hydroxycumarines.Among the various substi-tuted coumarins,3-benzyl-substituted 4-hydroxycoumarins

are important clinical pharmaceutical structures:Warfarin (43),Coumatetralyl (44),Bromadialone (45),and Difenac-oum (46;Figure 3).[65]Chan and co-workers applied I 2as a catalyst in the nucleophilic addition of sulfonamides and carbamates to allylic alcohols.[66]

Figure 3.Pharmaceutical products containing the hydroxycouma-rin moiety.

The reaction is operationally straightforward and pro-ceeds under very mild conditions with 5mol-%of the cata-lyst at room temperature in good to excellent yields (up to 99%).Duan and Wu described the efficient benzylation of dicarbonyl compounds by the reaction with I 2operating in CH 3NO 2at 80°C.The reaction was also applied to ste-reogenic ferrocene derivatives,but only low diastereoselec-tion was observed.[67]Nucleophilic substitution reactions of benzylic alcohol 1and allylic alcohol 3with N,O,and S nucleophiles were described by other authors.[68]Hydride as a nucleophile was also examined in the reduction of benz-ylic alcohol with hypophosphorous acid.[69]

4.11.Iron

Among the different metal salt catalysts used,poorly toxic and inexpensive FeCl 3is known to activate allylic,propargylic,and benzylic alcohol derivatives toward the ad-dition of various

nucleophiles such as arenes,indoles,or active methylene compounds.Beller (Scheme 18)reported the addition of xylenes to benzylic alcohols 1and 2in a seminal paper.It is worthy to note the tolerance of a wide variety of functional groups,such as CHO,CO 2R,I,Br,Cl,F ,OH,and OMe,to the reaction conditions.In addition,thiophene and furan derivatives were employed.

Scheme 18.Iron-catalyzed Friedel–Crafts addition of xylenes to benzylic alcohol.

P.G.Cozzi et al. MICROREVIEW

The reaction was performed by using an excess amount of xylene and FeCl3(10mol-%)at80°C.[70]Beller also de-scribed the use of FeCl3in the addition of various1,3-di-carbonyl compounds with benzylic alcohols1and2at50–80°C.[71]The compounds are useful starting materials for the synthesis of functionalized triarylmethane.[72]The reac-tion of propargylic alcohols with a series of nucleophiles was reported by Jana.The use of FeCl3(5mol-%)in CH3CN allowed the direct substitution of propargylic alcohols with C,O,N,and S nucleophiles.

In particular,allylsilane,substituted alkyl and aryl alcohols,furan,pyrrole,and thiols were all suitable nucleo-philes.The reaction proceeded without exclusion of moist-ure or air from the reaction mixture.[73]The reaction with different alcohols capable of forming stabilized carbo-cations was also examined.Benzylic alcohols with amides in the presence of FeCl3were described by Jana[74] (Scheme19),who also reported the reaction of benzylic alcohols with indoles promoted by the same catalyst.[75]The reaction of indoles with allylic alcohols was reported by an-other group.[76]Jana recently reported the addition of benz-ylic alcohols to terminal aryl alkynes in the presence of FeCl3as the catalyst in nitromethane.The reaction provides rapid access to substituted aryl ketones.[77]Cossy and Re-ymond reported the Ritter reaction by the treatment of benzyl alcohols1and2with nitriles(PhCN,MeCN, CH2CH2CN)at150°C in the presence of FeCl3(10mol-%)and water(2equiv.).[78]The formation of indene by the intramolecular attack of an aromatic group in the allylic alcohols is sometimes observed as a side reaction in the nucleophilic direct substitution.Also,in the case of FeCl3 as a catalyst,allylic alcohols can form indenes,and this re-action was used to access indene derivatives.[79]The use of Morita–Baylis–Hillman alcohols(allylic alcohols)in the iron-catalyzed addition of nucleophiles was examined re-cently.A general and efficient direct method for theα-sub-stitution of Morita–Baylis–Hillman alcohols with carbon-and heteroatom-centered nucleophiles such as alcohols,ar-enes,1,3-dicarbonyl compounds,and thiols in the presence of FeCl3·6H2O as a catalyst has been developed.The reac-tion is operationally straightforward,proceeded in good to excellent product yields(40–99%),and with exclusiveα-re-gioselectivity under mild conditions that did not need an inert or moisture-free environment.[80]The same reaction was reported by another group.[81]Taking advantage of the different reactivities between gold and iron,and of their compatibility,Campagne developed an efficient gold–iron-catalyzed one-pot synthesis of2,3-dihydroisoxazole from propargylic alcohol6.The addition of Cbz-hydroxylamine to propargylic alcohol6was promoted by FeCl3(2.5mol-%)in DCM at reflux and NaAuCl4was used as the catalyst (in the presence of30mol-%of DMAP as the co-catalyst) in the one-pot procedure.The activation of the triple bond by using a Au catalyst allowed the intramolecular attack of the hydroxylamine to form the2,3-dihydrooxazoles.[82]A similar tandem reaction catalyzed by FeCl3was applied to the synthesis of furans.[83]Jana,pursuing the methodology of FeCl3in the S N1-type reaction further,presented a novel,simple,and straightforward one-pot reaction of alkynes with various alcohols in the presence of iron salts(FeCl3 and FeBr3)to yield the corresponding alkenyl halides with complete regioselectivity in favor of the E isomer.The use of40mol-%of iron salt afforded an isolated yield of70% with benzylic alcohol2,benzhydrylic alcohol1,and ter-minal and internal alkynes.[84]A mild,versatile,and ef-ficient method for the one-step synthesis of substituted di-hydro-and tetrahydroisoquinolines was described by Zhuo,[85]who used FeCl3to catalyze intramolecular allenyl-ation/cyclization reactions of benzylamino-substituted pro-pargylic alcohols49–51(Scheme

20).

Scheme19.Catalytic amination of benzhydrylic alcohols promoted by catalytic amounts of iron

trichloride.

Scheme20.Intramolecular allenylation/cyclization promoted by catalytic amounts of iron trichloride.

Various benzylamino-substituted propargylic alcohols were subjected to the reaction by using FeCl3(5mol-%)as the catalyst in CH3NO2.For less-reactive substrates,the re-action was performed at60°C.A similar strategy was used by Bandini and co-workers in the first example of a cata-lytic intramolecular Friedel–Crafts alkylation of substituted allylic alcohols by an arene bearing electron-withdrawing groups55–58(Scheme21).Tetrahydronaphthalene struc-tures59–62are available through this methodology,which is catalyzed efficiently by cheap FeCl3(10mol-%)in CH3NO2at80°C.

[86,87]

Scheme21.Intramolecular Friedel–Crafts reaction promoted by catalytic amounts of iron trichloride with deactivated arene nucleo-philes.

The reaction of propargylic alcohol6promoted by FeCl3 with carbon-and heteroatom-centered nucleophiles such as allyl trimethylsilane,alcohols,aromatic compounds,thiols,

curred with anhydrous FeCl 3(5mol-%)in CH 3CN at room temperature.The propargylation reaction did not proceed when acetamide,aniline,and piperidine were used as the nucleophiles.

4.12.Ytterbium,Scandium,and Lanthanum Lewis Acids Ishii,in 2003,reported the first use of M(OTf)n Lewis acids for the direct reaction of alcohols [88](Scheme 22).

Scheme 22.Friedel–Crafts alkylation of benzylic alcohol with sub-stituted β-naphthols.

The combination of secondary benzyl alcohol 2and a metal triflate (e.g.,La,Yb,Sc,and Hf triflate)in nitrometh-ane was investigated.Reactions of benzylic alcohols with carbon (aromatic compounds,olefins,an enol acetate),ni-trogen (amide derivatives),and alcohol nucleophiles were carried out in the presence of a metal triflate (0.01–1mol-%).Hf(OTf)4was the most active catalyst for this alky-lation,and trifluoromethanesulfonic

acid (triflic acid,TfOH)was also a good catalyst.The catalytic activity of metal triflates and TfOH increased in the order La(OTf)3?Yb(OTf)3?TfOH ?Sc(OTf)

3?Hf(OTf)4.A mechan-istic study was also performed.The reaction of optically pure (R )-2-naphthylethanol (63)with 3-COOMe-2-naph-thol (64)gave the desired product as a racemic mixture.

Quite interestingly,the reaction of phenylethanol (2)and 64was studied by GC analysis and Ishii found that 2is in equilibrium with ether 66(Scheme 23).

Scheme 23.Reversible formation of ethers by the action of La-(OTf)3on benzylic alcohols.

In many reaction systems in which benzylic alcohols 1and 2or allylic alcohols 3and 4are substituted in S N 1-type reactions,the formation of ether can be detected.The ether and the carbocation are in equilibrium in the system de-scribed by Ishii.The catalytic activities changed with dif-ferent acid catalysts,which may be explained by a different rate of the generation of the cation.The formation of by-products (elimination products,rearrangements)is derived by the different reactivity of the

nucleophile (according to

and Zhang scribed the use of Yb(OTf)3in a tandem reaction that starts with the Friedel–Crafts reaction of phenols to propargylic alcohol 6and 8(Scheme 24)[89]to give indenyl compounds of type 67.

Scheme 24.Tandem propargylation and Friedel–Crafts alkylation promoted by catalytic amounts of ytterbium triflate.

Although different Lewis acids were able to promote the reaction,higher yields were obtained operating in CH 3NO 2at 80°C with Yb(OTf)3(10mol-%).The product obtained was attacking the allenyl intermediate to form the resulting indenols.In addition,catalytic quantities of Yb(OTf)3can also effectively promote the propargylation and allylation of 4-hydroxycoumarins at the 3-position.By applying this reaction as the key step,a multisubstituted furocoumarin can easily be synthesized

in a one-pot procedure.[90]Y oshi-matsu described the scandium-catalyzed substitution reac-tions of phenylsulfanyl and phenylselenyl propargyl alcohols with nucleophiles (arene,heteroarene,enol silyl de-rivatives,allyl silane)to give the propargylated compounds in high yields.[91]

4.13.Mercury

A novel Hg(OTf)2-catalyzed arylene intramolecular cycli-zation of allylic alcohol 68(Scheme 25)was achieved with highly efficient catalytic turnover (up to 200times).

Scheme 25.Intramolecular cyclization of allylic alcohol promoted by catalytic amounts of Hg(OTf)2.

The author proposed that the reaction began with πcomplex 70,able to activate the double bond to the attack of the aromatic nucleophile.After stabilization of the carbocationic intermediate,the triflic acid formed in situ was able to protonate the organomercury intermediate,fav-oring the elimination of the HgOTf fragment that regener-ated the catalyst.The scope of the reaction was limited to indole or to methoxy-substituted benzene rings.[92]

P .G.Cozzi et al.

MICROREVIEW

4.14.Silver

The use of silver triflate (AgOTf,10mol-%)allowed the preparation of functionalized 1-vinyl-1,2,3,4-tetrahydro-naphthalenes 74–76through the direct activation of allylic alcohols [93](Scheme

26).

Scheme 26.Intramolecular cyclization of alcohols in the synthesis of dihydronaphthalenes.

AgOTf-catalyzed direct amination of primary alcohols with sulfonamides was described by Shreedar.This effective catalyst required no reactivation of the hydroxy group of the alcohol and was conducted in CH 3NO 2at 100°C,fur-nishing the aminated products in moderate to good yields.[94]4.15.Tin

Sn IV triflamide,obtained in a straightforward manner,was able to efficiently catalyze the nucleophilic direct substi-tution reaction of hydroxy groups of unprotected hydroxy N ,O -acetals 77(Scheme

27).

Scheme 27.Nucleophilic substitution of hydroxy N,O-acetals pro-moted by catalytic amounts of tin triflamide.

The reaction required a low catalyst loading (0.1–1mol-%)and it is carried out in CH 3CN at room temperature or at 50–60°C,depending on the nucleophiles.As again can be rationalized by Mayr’s nucleophilicity table,strongest nucleophiles require room temperature conditions (silyleno-lates),whereas dicarbonyl compounds,which attack the carbocation through the formation of the corresponding enol,required a higher temperature.[95]

5.Metal Complexes in the Activation of Alcohols

5.1.Ruthenium Complexes

Propargyl cations,extensively studied by Olah [96]can be efficiently described by the two resonance structures –pro-pargylium and allenylium structures (Figure 4,A and B).By the introduction of a metal in the γ-position of a propar-gyl ion (Figure 4,C)the stabilization of the positive charge can be enhanced.

Figure 4.Stabilization of a propargylic cation.

As a result of the stabilization,metal-stabilized propar-gylic cations have been employed in S N 1-type reactions.Ni-shibayashi and co-workers,in a series of interesting papers,exploited the reactivity of the chalcogeno-bridged diruthe-nium complex {Cp*RuCl(η2SMe)2}(79,Cp =η5-C 5Me 5)in propargylic alkylation in the presence of different nucleo-philes.[97]Nishibayashi described in 2000the first applica-tion of the diruthenium complexes in the addition of alcohols,anilines,thiols,and phosphane oxides to propar-gylic alcohols [98](Scheme

28).

Scheme 28.Propargylic amination promoted by ruthenium com-plex 79.

The reaction proceeded in the presence of 5mol-%of the catalyst and 10mol-%of NH 4BF 4in dichloroethane at 60°C.Several other catalytic transformations of propar-gylic alcohols were then explored by Nishibayashi and co-workers.[99]Acetones and various diketones were able to re-act through their enol form to the corresponding cationic propargylic compounds.The reaction was optimum with an excess amount of ketone at 60°C.The addition of silylenol-ate of ketones took place in a similar manner.The reaction also showed a moderate to good simple diastereoselection in the reaction of prostereogenic ketones.[100]Other dif-ferent nucleophiles,such as arenes and indoles were also reported.[101]A quite interesting reduction of propargylic alcohols 6by S N 1-type reactions was described by Nishi-bayashi [102](Scheme

29).

Scheme 29.Reduction of propargylic alcohol by catalytic amounts of cationic ruthenium complex 84.

Among all investigated silanes,the reduction took place by the use of Et 3SiH at 60–80°C in dichloroethane,with different substituted propargylic compounds.Active diru-

out with AgOTf.The mechanistic investigation supported the presence of a propargylic cation as a reactive intermedi-ate,in which the alkyne moiety may interact with the ruthe-nium centers of the thiolate-bridged diruthenium complex.Nucleophilic attack

of hydride from the triflate-activated silane on this ruthenium-coordinated allenylic compound gave the desired compound.In all cases,the propargylic alcohols formed a common intermediate,namely,a ruthe-nium–allenylidene,which was able to stabilize the carbo-cation.Nakamura carried out theoretical investigations of the reaction developed by Nishibayashi.[103,104]Thiolate-bridged diruthenium complexes produce,by the reaction with the propargylic alcohols,diruthenium–allenylidene complexes like 86(Figure 5).

Figure 5.Thiolate-bridged ruthenium complexes stabilizing pro-pargyl cations.

The flexible Ru–Ru core structure plays a critical role in the catalyst turnover step.The diruthenium core alters its strength as

a function of the coordinative unsaturation of the complexes.In addition,weaker back bonding interac-tions allow facile dissociation,allowing the turnover of the catalyst.The solvent is quite important in these reactions,as protic molecules (e.g.,MeOH)mediate the smooth pro-ton transfer in the propargyl alcohol–allenylidene transfor-mation.

Pregosin and co-workers (Scheme 30)used Ru IV complex 87in S N 1-type reactions.[105]

Scheme 30.Friedel–Crafts addition of indole to allylic alcohols promoted by cationic ruthenium complex 87.

Ru IV complexes were found to catalyze,when used in 5mol-%,the addition of indoles and pyrroles to various ArCH(OH)CH=CH 2alcohols (88and 89)in the presence of CSA (camphor sulfonic acid,5mol-%)at room tempera-ture.DFT calculations were performed on the Ru IV com-plexes containing no sulfonate and one or two sulfonate molecules coordinated to the ruthenium center.The coordi-nation of the sulfonate to ruthenium(IV)influences the LUMO of the allyl fragment coordinated to the ruthenium.

attack of nucleophile to the C1terminus of the coordinated allyl to yield the branched product appears to be determined by orbital control.Pregosin also reported the use of these interesting ruthenium complexes in other catalytic reac-

tions.[106]Lau and co-workers published an interesting re-port on the effective role played by Lewis acids in S N 1-type reactions.The perchlorate salt of the dicationic bipy–ruthe-nium complex cis -[Ru(6,6?-Cl 2bipy)(H 2O)2]2+(92)effec-tively catalyzes the addition of β-diketones to secondary alcohols 1and styrenes to yield the α-alkylated β-diketones.

It was proposed and confirmed by independent experi-ments that the catalytic addition of β-diketones to the sec-ondary alcohols was catalyzed by the Br?nsted acid HClO 4,which was generated by the reaction of the ruthenium com-plex with the β-diketone.[107]On the basis of this work,it must be noted that in S N 1-type reactions of alcohols pro-moted by Lewis

acidic metal catalysts,the true catalytic spe-cies cannot be the metal complex but the proton generated by the reaction of the metal with the substrate.

5.2.Gold Complexes

In 2005,Campagne reported the capability of gold salts in the direct activation of alcohols.[108]Propargylic alcohols 93and 94were treated with various nucleophiles (Scheme 31;allylsilane,alcohols,furan,dimethoxybenzene,thiols)with different gold(III)salts.

Scheme 31.Allylation of propargylic alcohols by gold salt.

The best results were observed with NaAuCl 4·2H 2O,which could be used in only 5mol-%in dichloromethane.Au I catalysts were less efficient,and no reaction occurred in the presence of PtCl 2and PdCl 2(PhCN)2.The reaction was promoted by the formation of the carbocation and oc-curs through an S N 1-type reaction.In fact when enantio-merically enriched propargylic alcohol (96%ee )was used in the presence of allylsilane,the corresponding allylated compound was obtained in racemic form.Gold(III)chlo-ride is a very mild and highly reactive Friedel–Crafts cata-lyst,and it is able to work at rather low concentrations for selective propargylation and benzylation reactions.Quite

P .G.Cozzi et al.

MICROREVIEW

interestingly,different selectivity is obtained as a function of the Lewis acid catalyst.[109]

Prim and Campagne described the direct amination of benzylic alcohols 1and 2with Boc-amine,tosylamine,ni-troaniline,and TMSN 3in the presence of NaAuCl 4(5mol-%)as catalyst in CH 2Cl 2at room temperature.Generally,the workup of the reaction comprised filtration through a silica pad to give the corresponding benzylic amine in ana-lytically pure form.[110]

The intramolecular coupling of an allylic alcohol with allylstannanes was described by Echavarren and used cat-ionic gold complex (Scheme 32)97.

[111]

Scheme 32.Intramolecular addition of allylstannane to allylic alcohol promoted by a catalytic amount of gold complex 97.

The use of NaAuCl 4in the preparation of ethers was reported by Asensio [112]in an efficient and broad-scoped method for the preparation of unsymmetrical ethers from alcohols.The procedure enables the etherification of benz-ylic alcohols 1and 2and tertiary alcohol 9with moderate to good yields under mild conditions with low catalyst load-ing,leading to the formation of unsymmetrical ethers.Starting form enantioenriched benzylic alcohols,the race-mic ether formed,suggesting that a carbocation was the intermediate in these reactions.The reaction was performed by using the alcohol as solvent and NaAuCl 4(5mol-%)at 100°C.Chan and Rao [113]reported an efficient synthetic route to pyrrolidines by employing the AuCl/AgOTf-cata-lyzed tandem amination/ring expansion of substituted cy-clopropyl methanols 100–103with sulfonamide g .In the convenient synthetic route to pyrrolidine derivatives exam-ined,the tandem amination/ring expansion of substituted cyclopropylmethanols and sulfonamides was catalyzed by AuCl in the presence of AgOTf as co-catalyst (Scheme

33).

Scheme 33.Tandem alkylation/ring expansion for the synthesis of pyrrolidines.

In this S N 1reaction,the formation of the cationic inter-mediate was followed by the rearrangement of a cyclopro-pyl group.The reaction proceeded at 100°C in toluene in the presence of 5mol-%of the catalyst.Among all the

Lewis acids examined for the reaction,only Cu(OTf)2was also effective,although it gave inferior yields.Also,TfOH as a Br?nsted acid permitted the desired transformation.The method was shown to be applicable to various cy-clopropylmethanols containing a variety of substituted aro-matic rings.Electron-withdrawing groups (Cl,F)or elec-tron-donating groups (t Bu,OMe)gave comparable results.Starting form optically enantioenriched cyclopropylaryl alcohol,the authors isolated a racemic pyrrolidine adduct,showing the involvement of carbocations in the reaction.5.3.Molybdenum and Niobium Lewis Acids Complexes The direct addition of nucleophiles to allylic alcohols was described by Koc ˇovskyet al.[114]The authors used Mo(acac)2(SbF 6)2(108)as the catalyst and performed the reaction with aromatic compounds (phenols and meth-oxyarene derivatives)and Me 3SiN 3

.

The catalyst was obtained by reducing Mo V in CH 3CN by adding the acac ligand and finally by exchanging the chlorides by adding AgSbF 6.It is noteworthy that the Mo IV complexes,which are mild Lewis acids,are able to induce the reaction with N nucleophiles,whereas Mo 0and Mo II are not active catalysts.Friedel–Crafts alkylations were de-scribed with the catalyst system formed by the molybde-num(II)complex [CpMoCl(CO)3](109)and o -chloranil.The reaction between the oxidant and chloranil generates a molybdenum catecholate able to undergo electrophilic reac-tion with the aromatic substrates (Scheme 34)before ulti-mately producing Br?nsted acid 110

.

Scheme 34.Molybdenum-catalyzed addition of methyl furan to benzhydrylic alcohols.

The reaction of exo -norborneol,benzylic alcohols 1and 2,allylic alcohols 3and 4,and propargylic alcohols 6and

out in the presence of 10mol-%of the catalyst at 80°C.At a lower temperature,methylfuran reacted with allylic and benzylic alcohols.[115]The use of MoCl 5in the reaction of benzylic alcohols with amides was reported by Reddy,[116]and the same author reported the use of MoCl 5in other C–C bond-forming reactions.[117]NbCl 5,a stable solid,which is easy to handle and soluble in many organic sol-vents,has also been well explored as a Lewis acid in pro-moting various organic transformations;[118]it was used in the nucleophilic addiction of C,N,O,and S nucleophiles to benzylic alcohols 1and 2.[119]The reaction required NbCl 5(5mol-%)in CH 3CN.Benzylic compounds bearing elec-tron-withdrawing groups on the aromatic ring are not reac-tive substrates for the reaction.In the reaction,many dif-ferent nucleophiles such as alcohols,indoles,thiols,naph-thols,and resorcinols were employed.Quite interestingly,direct azidation was obtained with NaN 3,although the re-action probably involves Nb–azide complexes.

5.4.Palladium Complexes

Pale and co-workers reported a method for the selective protection of alcohols by the synthesis of the di-phenylmethyl (DPM)or bis(methoxyphenyl)methyl (BMPM)ethers in good yield by using PdCl 2(CH 3CN)2as the catalyst in dichloroethane at 60or 20°C,respectively.The transformation occurs through an S N 1-type reaction promoted by palladium complexes.These conditions are compatible with other functional and protecting groups such as halides,esters,acetal,benzyl,para -methoxybenzyl,benzyloxycarbonyl,and tert -butyldiphenylsilyl.Deprotec-tion of diphenylmethyl or bis(4-methoxyphenyl)methyl ethers was efficiently achieved at room temperature by using PdCl 2(CH 3CN)2in the presence of EtOH,which in-tercepts the carbocation.[120]The direct substitution of al-lylic alcohols promoted by triphenyl phosphite–palladium complex through direct C–O bond cleavage,without adding any co-catalysts or bases,was also described by Ikariya.[121]The use of 5mol-%of the palladium complex at 80°C in the absence of solvents gave the desired allylic ethers in good to excellent yields.S N 1-Type reactions in the presence of a palladium catalyst was described in 2003by Abu-Omar.[122]He reported one of the first articles in this field involving the direct use of cationic palladium complexes [(R ,R )-DiopPd(OTf)2(112)obtained in situ by exchange with AgOTf].Although chiral ligands were employed in the reaction that occurred in nitromethane at 50°C,mixtures of diastereomers and no enantiomeric excess values were obtained consistently (Scheme 35).

Nonsymmetric ethers could also be obtained with good

selectivity by coupling two different alcohols.Direct amin-ation was observed with electron-deficient anilines and thi-ols,whereas the addition of H 3PO 2to benzylic alcohols by using Pd/xantphos in DMF at 110°C or t AmOH at reflux with a Dean–Stark trap were used.[123]

Scheme 35.Formation of ethers by using palladium complex 112.

5.5.Rhenium Complexes

Toste,on the basis of reports that metal–oxido complexes effect the rearrangement of propargyl alcohols to enones,investigated metal–oxido complexes for the conversion of propargyl alcohol to propargyl ethers.Rhenium(V)–oxido complex 115bearing the bidentate phosphane ligand dppm (dppm =diphenylphosphanylmethane)proved to be the most

effective catalyst for the desired transformation.[124]The reaction was conducted in CH 3CN at 60°C and dis-played a broad scope.Furthermore,displacement of the propargylic alcohol occurs preferentially even in the pres-ence of reactive electrophiles such as primary alkyl halides and conjugated esters.The same complex was used in the rhenium-promoted addition of substituted allylsilanes to propargylic alcohols (Scheme 36).[125]

Scheme 36.Rhenium-promoted allylation of propargylic alcohols.

The rhenium(V)–oxido dppm complex was also em-ployed in the addition of aromatic and heteroaromatic sub-strates to propargylic alcohols.[126]

5.6.Heteropolianionic Complexes

Heteropoly acids (HPAs)such as 12-phosphotungstic acid (PW A)and 12-phosphomolybdic acid (PMA)are promising solid acids,acting as catalysts under both homo-geneous and heterogeneous conditions,[127]and they exhibit high activities and selectivities in various synthetically use-ful https://www.doczj.com/doc/f42046215.html,d nucleophilic substitution reac-tions of benzhydrylic,benzylic,allylic,and simple aliphatic alcohols with sulfonamides,benzamide,and 4-nitroaniline in the presence of PW A were described by Wang.[128]PMA–silica gel has been utilized to catalyze efficiently the propar-gylation of aromatic compounds with arylpropargyl alcohol 6[129]in the absence of solvent under environmentally be-

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那一次我读懂了坚持作文

那一次我读懂了坚持作文

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32学时。 二. 课程内容及学时分配 章节内容学时 第一章数字音视频处理技术的产生与发展 2 第二章音频技术概述 2 第三章音频处理 8 第四章视频技术概述 2 第五章视频处理 12 第六章音视频处理技术综合应用 6 实验一音视频软件的安装与基本操作 2 实验二音频采集与编辑 4 实验三数字音频特效与合成 6 实验四视频采集与编辑 4 实验五数字视频特效 8 实验六音视频处理技术综合应用 8 合计 64 第一部分理论教学第一章数字音视频处理技术的产生与发展(2学时) 主要内容: 1. 数字音视频处理技术的基本概念; 2. 数字音视频处理技术的产生与发展过程; 3. 数字音视 频处理的主要研究内容;4. 数字音视频处理的软硬件环境。要求: 1. 了解数字音视频处理技术的基本概念、产生与发展过程; 2. 了解数字音视频处理的技术概况和主要研究内容; 3. 了解数字音视频处理的软硬件环境要求; 4. 了解常见的音视频处理软件及其功能特点。

那一次我读懂了父亲

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那一次我读懂了母爱作文

那一次我读懂了母爱作文 那一次我读懂了母爱作文（一） 人们一直说：父爱是山，母爱是海，但是我觉得母爱有时候更像是一杯味浓香甜的咖啡…… ;;题记 早晨的一缕阳光，温暖你的胸怀，使你整天充满活力；老师的一个目光，坚定你的信念，使你充满自信；朋友的一句问候，安慰你的心灵，使你找回自己。那么母亲一句关切的问候呢？它使我永远难忘母亲对我真挚、无私的爱。 在白色的灯光下，我的笔在作业本上沙沙作响，母亲在楼上楼下来回忙碌着，厨房里不时地传出水声。我不时地望着母亲繁忙、孤独的身影，我开始发呆。母亲见我停笔不写，就走过来轻声地问我：“困了吗？”我猛地惊醒，对着母亲歉意地笑笑，又继续做作业。当我再次抬头看母亲的背影时，发现面前已经多了一杯热气腾腾的咖啡，望着那徐徐上升，又悄然消失在空气中的热气，我陷入沉思。 在我的记忆里，母亲一直充当着我的良师益友，有时我们还会互相开玩笑，以此来增进我们母女之间的感情。但是昨天，我在跟母亲开玩笑的时候说错了话，母亲十分生气，到今天也不曾跟我说过一句话，但是她仍旧做着她每天的“任务”：为我烧饭。在这个时候，我意识到：也许真的是我错了。我不该无视她头上多出来的缕缕的白发，

不该无视她额头上渐渐增多的皱纹，不该无视她为我付出的艰辛。在这个宁静的夜晚，我突然领悟到：母爱有时就像是一杯咖啡，散发着苦涩的味道，香甜总是隐藏在最下面，只有细细品尝到最后，才会察觉到它的香甜。 一阵晚风吹来，我不禁打了个寒颤，母亲蹑手蹑脚地走过来，摸摸杯子，温和地说：“咖啡凉了，我再帮你换一杯。”是啊！茶凉了还可以换水，但是咖啡凉了却不能够换水，只能够再换一杯，母爱就是这样。母亲正在用她那无私、纯真的爱对待我，从来不换一滴水。 这个像咖啡一般的母爱，我品尝了十四年。在我十四岁之际，我终于体会到了它隐藏的深刻含义。此时，无论我是内疚还是感激，都无法弥补这十四年来我的无知和任性给母亲带来的伤害。但是母亲却从来不计较这些，她用她那博大的胸怀，包容着一切我的无知以及任性，和我的缺点。 十四年来，我喝过许多杯咖啡，但是这次我体会的最深。 在一个宁静的夜晚，母亲一个微笑的举动，使我体会到了母爱，让我读懂了母爱。 那一次我读懂了母爱作文（二） 盛夏，骄阳似火。中午妈妈不在家，我独自玩得不亦乐乎。待清醒过来，离上课只有十分钟了，我抓起书包，便冲出家门。我大汗淋漓的奔进教室，刚坐下，就习惯性的伸手去书包里搜寻。水……水杯？糟了！灌水后没装书包里，走时又太仓促……唉！都怪自己，一个人在家，不睡觉，书包也没空

数字音视频技术试卷

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我的父亲高考满分作文

我的父亲高考满分作文 我的父亲高考满分作文 我的父亲 王太华 父亲命苦！也有个性！ 他出生后三年，日军大举侵华。 爷爷奶奶在父亲幼时相继去世，五个姑妈皆已出嫁，年幼的他在姑妈家寄住。他不愿寄人篱下，从十几岁就独自在外晃荡。在外干过好多力气活，没有挣到钱，只是混饱了个肚子，父亲很是满足。 父亲二十几岁的时候，就有好事者给他做媒，先后介绍四五个没有成功。不是父亲长得丑陋，也不是他品行差。好多女娃第一次见到父亲就感兴趣，但是发展到要到他家看看时父亲就蔫了，因为他没有家，只有一个光刷刷的人。有哪个女娃愿意和他在树下成亲？好多女娃就那么含泪而去。 父亲明白了一个道理：要成家，必须要有一个住处。漂泊的他觉得老一个人晃也不是回事，凭着一斗高粱米在鸡不生蛋的老家屋场邀三喝六地建起了一间茅草屋。 茅草屋建起后，迫不及待的几个姑妈便老生常谈地张罗着给父亲说媳妇。二姑妈住在低山，天花乱坠地把母亲骗到父亲的茅草屋，冬天，母亲和家家翻山越岭走了一整天。空空的茅草屋只有借来的几把椅子，母亲心灰意冷，准备连夜返回。父亲变魔法似的拿出一个崭

新开水瓶。母亲和家家眼前一亮，亮闪闪的开水瓶在黑乎乎茅草屋中是那么扎眼，母亲终于决定留下来。 事后家家说：“从那个开水瓶就证实未来的女婿是个理事的人。”我不以为然，开水瓶算个什么好东西？幺姑妈恨恨地告诉我：“那时的开水瓶就相当于现在200英寸大彩电！” 200英寸？我没有见过，姑妈更是没有见过，她无非就是让我明白开水瓶在当时的价值。父亲后来和母亲吵架最后收尾总是一句：“你是个贪财的人，当时无非就是看起了我的一个开水瓶！”每到这时，母亲不说话，只笑。父亲这句话无从考证，我先后问过母亲和幺姑妈，她们都只笑。有一次姑妈被问烦了，扔下一句“千好万好，不如人好”算是对我的最终答复。 之后的数年，我们四弟兄陆续出生，从大哥降生到我降生，整整八年时间。母亲说，从大哥降生开始，父亲干活就不分日夜，白天在生产队里搞事，晚上放工后偷偷背煤炭和白炭。几十里山路背一趟五分钱，最怕让人知道，因为父亲的这种行为是搞资本主义。生产队里终于知道了，夜里召开社员大会批斗父亲。父亲相当难过，不是因为挨批了，而是他又耽误了挣五分钱的时间。父亲累教不改，有好心人给队长建议：“你看他不那样怎么行嘛，那么多娃子不能饿死啥。”生产队长长叹一声：“随他去，但是你们不能学他。” 我降生后没几年，家庭联产承包责任制开始了。父母天天在十几亩贫瘠的土地上忙碌，以此养家糊口。随着我们的长大，房子也在加宽，在一间茅草屋的基础上建了三间泥墙瓦房。

那一次我读懂了父爱短文

那一次我读懂了父爱短文 以下是为大家整理推荐的关于那一次我读懂了父爱短文，喜欢请收藏。 那一次我读懂了父爱短文篇1平时，我和爸爸很少说话，我总感觉他对我不够关心。 每次要钱的时候，他总舍不得，说：“我没有，找你妈要去吧。 可是就在一次为爸爸送行的过程中，我读懂了爸爸。 因为家里的花销越来越大，爸爸要到另一个城市打工，由于我们家离车站比较远，中间要转几次车不说，太晚了就赶不上火车了。 妈妈要照顾弟弟，爸妈经过商量，让我骑自行车送爸爸去车站。 虽然我有些不情愿，但看到妈妈紧锁的愁眉，我便答应了。 早春的寒气还有些让人伸不出手，出了门，我便紧了紧衣领，取了车，便让爸爸抱着包坐在车后。 上了路，已经晚上七点多钟了，街上的行人很少，显得很清静。 突然，我感觉到衣服一紧，原来是爸爸怕怀里的包掉下来，抓住了我的后背，想尽量坐稳一些。 寒风掠过，我回头看见爸爸的双手攥紧我的衣服，蜷着身，抱着包，身子斜靠到我身上。 看见这些，我觉得鼻子发酸，喉咙有些哽咽，感到凄凉。 这是一种信赖，他像一只饱经风霜的老麻雀，飞得很累，要找个小枝头小憩，作为他的归宿一样。

我现在感到爸爸老了，没有原来的冲劲了，是该休息了。 他需要别人的理解和关心，而我现在做到的也只是把车骑稳一些。 车站到了，和爸话别后，他提着包向前走，灯光下，他清瘦弱小的背影，在地面拉得老长。 我忍不住叫：“爸。 “什么?“天冷，多穿些衣服。 我这是怎么了?声音好像在颤，眼睛也有些湿。 “你快回去吧，明天还要上课，你妈让你转学的事别当真，有爸在，就有学费让你上最好的私立学校。 我点点头又摇摇头，不知道怎样表达自己的心情。 爸走了，我呆呆地站着，看着爸爸消失在人群中……为爸爸送行，让我读懂了父爱，我知道了父亲的艰辛。 原来，为了我能更好地接受教育，他付出了那么多，那么多。 不像别人家爸爸，成天和孩子有说有笑的，真让人羡慕。 可是自那件事后，我却改变了原先的看法，终于明白爸爸其实是很爱我的。 那是发生在暑假里的一件事。 爸爸终于抽空陪我和妈妈去北京旅游一次。 一路上，我们一家人有说有笑，共同游玩了许多著名的景点。 可是，不幸的事儿发生了，由于我吃不惯北方的食物，竟然拉起肚子来了。

那一次我懂得了 半命题作文

那一次我懂得了半命题作文 导读：篇一：那一次，我懂得了细心 细心对于每个人来说都是很重要的，如果不细心将出现预想不到的后果，也必将受到严重的惩罚。 记得有一次数学单元测试，我一开始都很紧张，但是等老师把试卷发下去后，我粗略浏览了一下，哈哈!原来都这么简单呀!我还以为多难呢，紧张的心情立刻放松了许多。 时间一到只听老师一声令下：“开始答卷!注意啊同学们!不要慌张认真做题!”大家就埋头写了起来。有的同学写得飞快;有的一边写一边抓耳挠腮;有的同学写了一会儿就呆坐在那不动了;还有的同学 干坐不写......而我也算是写得快的那一种吧，这卷子上的大部分题老师在课堂上都反复强调过，对我来说简直就是张飞吃豆芽——小菜一碟。我心里在暗暗高兴。 很快时间到了，我轻快的交上了卷，总觉得自己这次准能拿高分，在全班名列前茅。 终于成绩出来了。我信心满满地领走了试卷看了一下分数，天哪!我差点晕了过去!你猜我得了几分?猜不出来就告诉你——82分，我简直不相信自己的眼睛，我明明都做了呀!这18分扣在了哪儿呢?我认真的检查着：前面的题我全对了，这18分全扣在后面的解决问题上了，式子、步骤、得数都没错，但是都忘记写“答”了，按规定这样的题整个题都不得分。哎呀!真冤啊!100分泡汤了;全班第一泡汤

了;爸爸妈妈的礼物也泡汤了，一切都完了，能怨谁呢?这就是粗心的下场! 所以同学们，大家一定要细心，不管做什么事不能有半点马虎，特别是在考试的时候，只有细心做题才能拿高分。 篇二：那一次我懂得了真正的爱 今晚我补完课后，来到老师家楼下。“妈妈!”我一个箭步冲了过去抱住了妈妈。而妈妈却拉开我，朝我们还未装修好的新房子走去。“吱——”的一声门开了。这时妈妈转过身来，微笑着对我说：“敖柯，妈妈把这里打扫干净。你就在旁边玩吧!!!”我听了呆呆的点了点头。妈妈则去打扫卫生了....我在旁边没有事情做，就来回走动。看着妈妈辛苦的背影，我的心仿佛被什么揪了一下。此时，心里有两个小孩在议论着：小天使说：柯柯，你应该去帮你妈妈做一些事!小恶魔：才怪呢!好好地干嘛要去干活多累呀——小天使：别听他的，你想想你妈妈为你做的事吧——现在应该去帮你妈妈! 我使劲摇了摇头，再看看妈妈那忙碌的背影。心想道：嗯!我去帮帮忙!但看看周围好像无事可做。我顿时像泄了气的皮球。“撕——”我突然听见门口有些声音便跑去看：原来是邻居阿姨在撕门上的保护膜。对呀!我也可以帮着撕呀!我二话不说，拿起人字梯走到门口用心的撕起来。“敖柯!你在干嘛呀!”妈妈听见门口的声音急忙从厨房赶出来，“哦，你在撕这个——小心点啊——”“恩恩!”我答应道。