Modern Heck Reactions - New Trends in Cross-Coupling: Theory and Applications - page 375

collection of randomly accumulated data provides a unique, unbiased basis
for conclusions about real activity and catalytic eciency of palladium sys-
tems towards haloarenes of variable reactivity.
The first type of catalytic system is used with highly reactive substrates,
iodoarenes and, partially, the electron-deficient bromoarenes, although the
behaviour of the latter is on the borderline. With such substrates, the oxi-
dative addition is very fast and not rate limiting - Pd(0) species are rapidly
consumed, keeping the concentration of such species very low. Most prob-
ably none of the stages of the catalytic cycle is rate limiting and this role is
delegated to catalyst preactivation, delivering Pd in a zerovalent oxidation
state with a suciently labile coordination shell. The reactions run on ill-
defined (as opposed to well-defined) Pd species. Virtually any Pd compound
or Pd-containing material (including traces of Pd on laboratory glassware
and in the reagents used) can support such reactions, the composition of
some of such mixtures being so complex and obscure in functionality that
they deserve to be referred to as catalytic ''cocktails.'' 126,127
The second type is also phosphine free and probably is the most important
and abundant in the Mizoroki-Heck reactions. Unactivated bromoarenes
and even chloroarenes were from the very beginning established to react
under phosphine-free conditions, although usually giving incomplete con-
versions and modest to poor yields. The mainstream research placed such
substrates in the area requiring phosphine ligands and research to design
such ligands was launched. Meanwhile, more and more data showing that
unreactive substrates can be processed using ''phosphine-free'' systems ac-
cumulated. Indeed, if some reagent can take part in a few catalytic cycles,
why cannot it be made to react further? The answer is absolutely evident -
the problem is not the reactivity, but rather the stability of the catalyst. If the
stability of catalytic systems can be controlled, then bromo- and even
chloroarenes could be processed without special ligands.
The main factor governing the stability is also clear - this is the predis-
position of Pd(0) species lacking stable ancillaries to aggregate into clusters,
nanoparticles and then macroparticles. Therefore, a solid understanding of
a tight association between the phosphine-free reactions and the occurrence
and metamorphoses of the nanoparticles in such systems has been de-
veloped simultaneously by many researchers in the field.
Under the conditions of real Mizoroki-Heck reactions, such metallic
particles can partially redissolve, but the rate of such catalyst reactivation
falls rapidly with increasing size of the particles as an apparent consequence
of the decrease in available surface area. Thus, if the aggregation is left
uncontrolled, the result would be a decrease in net reaction rate and in-
complete conversions. Owing to the well-known effect, Ostwald ripening, the
size of metal particles under the conditions of reversible dissolution-
resedimentation tends to increase at the expense of actively dissolving small
particles.
To avoid misunderstanding, the critical importance of nanoparticles in
reactions with less-reactive substrates does not mean that Pd sols cannot