Modern Heck Reactions - New Trends in Cross-Coupling: Theory and Applications - page 457

accordingly sterically destabilize all other configurations along the pathway
(Scheme 9.169). 441 Moreover, this hypothesis predicts that two ortho sub-
stituents are required, but their nature is not important (indeed, 2,6-dime-
thoxy, difluoro, dimethyl, 2-fluoro-6-trifluoromethyl, 2-chloro-6-fluoro, etc.
patterns can be found in papers dealing with decarboxylative reactions giv-
ing good results, whereas only one ortho substituent rarely suces, except,
probably for a nitro group (see below).
The protocols described so far for decarboxylative reactions can be
grouped into two types - those useful for electron-rich acids and those useful
for electron-deficient acids. Additional clues as to the difference between
decarboxylative reactions of electron-rich and electron-deficient benzoic
acids can be drawn from decarboxylative cross-coupling reactions. In such
reactions, by analogy with regular cross-coupling, benzoate is a nucleophilic
coupling partner, serving the role of organometallic reagent, a carbanion
equivalent. If this is true, the decarboxylative step should come after oxi-
dative addition to aryl halide or triflate. The latter requires phosphine lig-
ands and therefore it can be expected that electron-deficient benzoic acids
would be preferred over electron-rich acids - and this is indeed the case, as
was shown by Liu and co-workers (Scheme 9.170). 442
Heck-like carbopalladation of C=O or C=N bonds was also recently de-
scribed to involve a typical Myers-style Pd-Ag system, although in the ab-
sence of base and with catalytic amounts of both PdCl 2 (10 mol%) and AgOTf
(20 mol%) (Scheme 9.171). 443 This process does not need a terminal oxidant,
because it is fully serviced by Pd(II), as unlike the carbopalladation of a C=C
bond here PdH is not generated and the product is released from the car-
bopalladation adduct by protolysis (Scheme 9.172) and not by reductive
elimination.
Similar deborylative reactions were observed by Yamamoto and co-
workers to take place in the presence of ruthenium and rhodium cata-
lysts, 444,445 and palladium- and copper-catalysed versions by Wu and
co-workers 90,446 and platinum- and palladium-catalysed versions by Hu and

Pd

Pd Pd

O

O

O

O O

O

Scheme 9.169 Effect of ortho substituents in decarboxylative palladation - steering
the Pd centre away from alternative coordination modes.

ArX (X = Cl, Br, O Tf)
Pd(O Ac) 2 (1-4 mol%)
(o-tol) 3 PorCy 3 P
diglyme, 130-1 60°C

COOK Ar

F n F n

Scheme 9.170 Decarboxylative cross-coupling using electron-deficient

polyfluorobenzoates.