Modern Heck Reactions - New Trends in Cross-Coupling: Theory and Applications - page 394

''phosphine-free'' Mizoroki-Heck reactions can be catalysed by very small
amounts of active palladium (the so-called ''homeopathic'' catalysis 213 ).
Palladium complexes of tetraphenylporphyrin and phthalocyanine were
shown to sustain the Mizoroki-Heck reaction of aryl bromides and even
chlorides with standard olefins, giving incomplete but in some cases fairly
high conversions. 214 However, it is known that the stability of Pd complexes
of porphyrins and phthalocyanines is enormous; the former can slowly re-
lease Pd when heated with concentrated sulfuric acid 215 and the latter is
even more robust. Both complexes, however, because of the very harsh
conditions of synthesis and if improperly purified, may contain Pd-con-
taining impurities, which can release active species. The results obtained
with highly stable Pd complexes should be treated with due caution.
Several phosphine-free systems have been reported that gave very good
results not only for electron-rich bromoarenes, but also chloroarenes. Elec-
tron-rich chloroarenes were reported earlier from time to time to be reactive
under the conditions of ''phosphine-free'' catalysis, but with low TON values
and incomplete conversions (for the analysis of data obtained before 2008
and predictions on the perspectives of SRPCs for less reactive chloroarenes,
see Ref. 124). Several recent studies showed that by further optimization and
development of new precatalysts, this task can be solved and electron-rich
chloroarenes can be made to react at least with standard olefins with high
conversions and yields. The conditions, however, are much more important
than the nature of the precatalyst and spectacular results can be obtained
using common Pd sources.
Thus, molten mixtures of Bu 4 NBr (TBAB) and Bu 4 NAc (TBAA) were shown
by Nacci and co-workers to serve as excellent ionic liquid media for phos-
phine-free reactions. 216,217 In such media, the halide effect and support of
Pd(0) species (see above) should be manifested most strongly due to the very
high concentration of ions. These media were used in the protocol, allowing
the reaction of chloroarenes, including electron-rich compounds, with a
wide range of olefins, including not only standard styrenes and acrylates, but
also terminal alkenes, vinyl ethers, vic- and gem-disubstituted olefins such as
methacrylate, crotonate, benzalacetone, cinnamate and stilbene. 218 Pd
acetate (1.5 mol%) was used as a precatalyst, which under the conditions
applied was rapidly reduced to a fine Pd sol, maintaining an active pool of
SRPC for effective processing even of the most stubborn and demanding
substrate pairs (Scheme 9.55).
The performance of this system is truly impressive in clearly showing the
huge reserves of phosphine-free catalyst systems. Furthermore, a sharp
discrimination of reactivity between chloro and bromo substituents can be
achieved by simply changing the temperature and reaction time, hence a
selective

possible 218

stepwise one-pot transformation becomes

(Scheme 9.56).
Another system, reported by R¨hlich and K¨hler, 214,219 takes advantage of
a highly stable Pd chelate apparently used as an SRPC. Optimization of the
conditions gave a system in which a typical electron-rich chloroarene reacts