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Migratory Insertion/De-insertion
•
• •
Migration of the one ligand to a neighboring unsaturated ligand (CO,
RNC, alkyne, alkene), generating a vacant site Usually reversible Vacant site is cis to the newly formed ligand
In situ reduction of Pd(II) to Pd(0)
LnPd0
Reduction by tertiary aliphatic amine
coordination LnPd
II
Beletskaya, Chem. Rev. 2000, 100, 3009
-hydride elimination
Overview of Heck Reaction
• • •
R1: Aryl, Vinyl, Benzylic, Allylic, Acyl X: N2+ > I > Br~OSO2CF3 >> Cl (relative rate of oxidative addition) PdX Pd(0) or Pd(II) Active Pd(0) species can be instantaneously made from Pd(II) in reaction media Base : Scavenger of HX PR3: Prevents Pd(0) from precipitation to make palladium mirror Solvent: Often coordinating solvents such as NMP, DMA, DMF, acetonitrile but sometimes toluene can be used Temperature: r.t. to 140 oC
Ni(0) or Pd(0) M = Al(i-Bu)2 Zr(Cl)Cp2 ZnX
Stille Reaction
Pd(0) M = SnR3
Hiyama Coupling
Pd(0) M = SiR3
Suzuki Reaction
Pd(0) M = BX2
Sonogashira
Pd(0) M = Cu (in situ)
Heck Reaction
General Reference about Pd-catalyzed Reactions Metal catalyzed Cross-coupling Reactions, Diederich, F. and Stang, P. J., Eds.; Wiley-VCH: New York, 1998 Heck Reaction • A.d.Meijere et al, Angew. Chem. Int. Ed. 1994, 33. 2379 • B. M. Trost et al, Angew. Chem. Int. Ed. 1995, 34. 259 • E. Negishi et al, Chem. Rev. 1996, 96, 365 • W. Cabri et al, Acc. Chem. Res. 1995, 28, 2
• • • •
Catalytic Cycle of Heck Reaction
Intramolecular Heck Cascade
Subsequent carbopalladation should be faster than termination process of the former carbopalladation intermediate (e.g. dehydropalldation)
Classifications based on the main group metal used to transfer R2 in the transmetalation event.
Kumada Coupling
Ni(0) or Pd(0) M = MgX, Li
Negishi Coupling
更多资源
Organo-transition Metal Chemistry
• Chemistry involves intermediates containing transition-metal carbon bonds
Electron Counting
• • Formal neutral ligand (L): PR3, NR3, CO, alkyne, Alkene Formal anionic ligand (X): R, Ar, H, X, CN, RCO
decomposition of alkylmetal complexes
General Mechanism
Cl PdII Cl R2 M R2 R 2 R1 R2 reductive elimination
II
Cl or Ni
II
Cl R = aryl, vinyl X = I, Br, OTf, Cl R1 X oxidative addition LnPd
• • • • •
R1, R2 : sp2-hybridized carbon X: I > OSO2CF3 > Br >> Cl Aptitude for tansmetalation: alkynyl> alkenyl> benzyl, allyl> alkyl Exceptional functional group tolerance High cost and toxicity of organotin reagent
• Two Heck reactions are achieved in one-pot
L. Tietze et al. J. Am. Chem. Soc. 1998, 120, 8971
Stable Neopentyl Organo-Pd-intermediate
•
Strained tricyclic system
Ligand Association and Ligand Dissociation
•
Ligand dissociation is usually the first step in pre-catalyst activation
Oxidative Addition/Reductive Elimination
Lecture: Palladium-Catalyzed Complexity Generating Reactions
Outline
• • • • • • • • • • • Organo-transition Metal Chemistry Transition metal-catalyzed Reaction Mechanisms Heck Reaction Heck Cascade Stille/Suzuki Coupling Reaction Allylic Alkylation Reaction Reference Mitchell. T.N. Synthesis, 1992, 803 Stille. J. K. ACIEE. 1986, 25, 508 Pure and applied chemistry 1994, 66, 213 Tetrahedron-Asymmetry 1992, 3, 1089
•
Ligand effect - electron donating ligands facilitate oxidative addition (e.g. PR3, R, H) - electron withdrawing ligands facilitate reductive elimination (e.g., CO, CN, olefins)
Tandem Heck Reaction
• • Intermolecular Heck reaction followed by intramolecular one Syn-Carbopalladation leads to only one geometric isomer
E. Negish. et al. J. Org. Chem. 1989, 54, 2507
X R2N(CH2)R X X-
LnPdII
R2 N
R
LnPdII R
X H
reductive elimination
LnPd0
H XR2 N
HX
Reduction by electron rich phosphines and bases
coordination LnPdII X X XPR3 L X PdII PR3 L Nu LnPd0 + NuPR3+Xphosphonium intermediate gets converted to phosphine oxide in the presence of atmospheric O2
L. E. Overman et al J. Am. Chem. Soc. 1999, 121, 5467
Stable Organo-Pd Intermediates
• Intermediate organo-palladium species are thermally stable due to the absence of hydrogen -syn to palladium
II
LnPd(0)
R1 R2
R1 X
LnPd
X M
transmetalation
R2 M
R2 = aryl, vinyl, alkyl
