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-CO
2
ArCOO
-
+CuF
2
(ArCOO )Cu F
2
ArCuF
2
Pd(OAc)
2
Heck catalytic cycle
ArPd(OAc)
Scheme 9.167 Hypothetical mechanism explaining the particular role of CuF
2
in the
decarboxylative arylation by o-nitrobenzoates.
O
CO
2
F
F
F
O
O
Pd
Ar
L
Pd
O
F
F
F
L
Ar
Pd
L
Ar
Scheme 9.168 The palladodecarboxylation transition state.
the well-known decarbonylation pathway, with the only essential difference
being that CO
2
unlike CO is a very poor ligand immediately leaving the co-
ordination shell) (Scheme 9.168).
This reasoning shows that in general, the transformation of an arene-
carboxylate palladium complex into an arylpalladium complex with extru-
sion of carbon dioxide is a concerted process - a continuous rearrangement
taking place via a four-centred cyclic transition state. As was noted above for
other oxidative palladation processes, the concerted pathways often involve
what is known as a variable transition state in which the degree of bond
rupture/bond formation may take place in a non-synchronous fashion, thus
adjusting in each particular case to the electronic requirements of the par-
ticular reagents. Concerning decarboxylative palladation, two boundary
cases should be considered. With electron-rich benzoic acids, the process
resembles electrophilic substitution with Pd-C bond formation taking the
lead over rupture of the C-carboxylate bond. This case is favoured by the
enhanced electrophilicity of the Pd centre in the absence of strongly bonded
donor ligands and additionally is apparently guided by an ortho-directing
effect from weakly coordinating ortho substituents, such as MeO.
In the other boundary case, cleavage of carboxylate leads in the process,
with C-Pd bond formation lagging behind, hence in the transition state a
carbanion-like configuration emerges. This case is favoured by a Pd centre
the electrophilicity of which is suppressed by donor ligands, such as phos-
phines and electron-deficient arenecarboxylic acids.
Theoretical modelling of reaction pathways give clues as to why ortho
substituents, even such simple ones as methyl, apparently favour the re-
action. In addition to an obvious suggestion that o-methoxy and similar
groups simply direct ipso-ortho metallation, ortho substituents for steric
reasons favour intramolecular concerted attack with the four-centred tran-
sition state cycle being perpendicular to the aryl ring - thus ortho substitu-
ents push the Pd centre towards the correct position for the attack and
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