Modern Heck Reactions - New Trends in Cross-Coupling: Theory and Applications - page 452

Pd L +

PdL +

R R

Scheme 9.159 Equilibrium between Z 1 - and Z 3 -benzylpalladium complexes.

9.3.5 Decarboxylative Heck Reaction
The idea that carboxylate ions can be regarded as masked carbanions is a
matter of common knowledge going back to the distant past. The use of
carboxylates as equivalents of organometallic compounds is, however, a
rather late addition to the domain of palladium catalysis, but rapidly gaining
in popularity. An arylpalladium intermediate is formed by decarboxylation
presumably of a Pd carboxylate complex 429 (for recent reviews on transition
metal decarboxylative reactions, see Refs 430 and 431). The decarboxylative
Heck reaction was first described by Myers and co-workers, who used an
excess of silver carbonate both as base and as terminal oxidant. 429,432,433
Experimental mechanistic studies 429 revealed distinct steps involved in
the catalytic cycle: (a) ligand exchange in which arenecarboxylate enters the
coordination sphere; (b) extrusion of carbon dioxide from the thus formed
Pd carboxylate; and (c) the rest faithfully follows the standard Heck catalytic
cycle with oxidative regeneration of Pd(II) catalyst (Scheme 9.160).
It should be noted that from the beginning, palladodecarboxylation was
regarded as an electrophilic ipso-substitution reaction. The course of such a
reaction would critically depend on maintaining an adequate level of elec-
trophilicity of the Pd centre, which requires that the system does not contain
strongly bonding ligands, such as donor solvents, halide ions, amines,
phosphines, etc. These limitations made the formulation of the reaction
systems (choice of solvents and base) rather tricky. The presence of DMSO in
the reaction medium is dictated by the finding of Myers and co-workers 429
that this solvent serves as a labile ligand adequately protecting the Pd centre
from deactivation. Besides, DMSO can serve as a labile ligand ensuring the
polar pathway mechanism of subsequent Heck transformations by keeping
anionic ligands (acetate, hydrocarbonate, etc., often being major constitu-
ents of the catalytic systems through Pd and Cu acetates, silver salts, etc.),
similarly to what we commented upon in the discussion of the deborylative
Heck reaction (see above).
The system can be as simple as just some Pd(OAc) 2 under an oxygen at-
mosphere without any extra additives. Such a system works well both for
standard Heck olefins (acrylates and styrenes) and for more challenging
olefins with internal double bonds, in which case complete inversion of
configuration takes place as in other Heck reactions with such olefins
(Scheme 9.161). 434
Su and co-workers developed an alternative protocol involving Pd tri-
fluoroacetate in the presence of benzoquinone and 1-adamantanecarboxylic
acid in a complex mixture of solvents, applicable to electron-rich