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stability of the complexes, which is often very high, particularly for palla-
dacycles and pincer-type chelates, which in individual form withstand
heating above 200 1C. In the reaction medium, the disassembly of such
''stable'' complexes takes place at modest temperatures, as was recently
shown by Campora and co-workers
211
for the well-known Milstein's pincer
complexes,
212
which made a great impact on Heck chemistry, initially pre-
sumed to serve as the long-awaited well-defined catalysts operating via the
Pd(IV)/Pd(II) catalytic cycle (cf. Scheme 58 in Ref. 87). The disassembly oc-
curs under mild conditions at 50 1C to involve the PdH intermediate, readily
formed by b-hydride elimination, e.g. from an alcohol (or similarly from an
amine or a Wacker-type reaction with an olefin) (Scheme 9.54).
Until recently, the scope of ''phosphine-free'' procedures was commonly
believed to be limited to reactive substrates, mainly aryl iodides and reactive
bromides. However, low to modest yields were occasionally recorded for
electron-rich aryl bromides and even chlorides, so it was only a matter of
time before the stability of catalytic systems would be tuned to afford pre-
paratively meaningful processing of less reactive substrates. Indeed, a
number of impressive protocols have appeared in recent years. Such proto-
cols are an excellent demonstration of the capabilities of ill-defined cata-
lysts, but the enthusiasm in their favour should be moderated. Systems of
this sort cannot get rid of inherent deficiencies of the approach, the main
one of which is poor robustness of the protocols - any variation from the
published procedures, such as the choice of substrates, olefins, additives,
concentrations, temperatures, etc., would probably be deleterious or even
lead to complete failure. All users of the ''phosphine-free'' approach should
take into consideration that it is an art, and therefore before complaining
about the irreproducibility, the original protocol should first be tried exactly
as published. Due care should be exercised with new precatalysts and their
coordination chemistry should be verified in sources not associated with
catalysis. The role of impurities should also be taken into consideration, as
PR
2
R
2
P
Pd
Pd
R
2
P
Pd
PR
2
R
2
P
PR
2
PR
2
R
2
P
Pd
H
(8)
R=
i
-P r
PR
2
R
2
P
Pd
(9)
n
Scheme 9.54 Mechanism of release of Pd(0) from Milstein's pincer complex. Di-
meric (8) and polymeric (9) Pd(0) complexes were isolated and
characterized.
211
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