Modern Heck Reactions - New Trends in Cross-Coupling: Theory and Applications - page 376

occur in the reactions of aryl iodides and other reactive substrates. The
formation of such particles is often observed in these reactions, but in these
cases this only means that too much simple and readily reducible Pd pre-
catalyst is loaded or that the reactions are near completion. In such cases,
the formation of metal particles is not as important. This can only be re-
garded as a signal that the composition of the catalytic system is far from
optimal.
As soon as the rate of oxidative addition is low to very low for the typical
substrates in this category, the precatalyst loading and the rate of pre-
activation and generation of Pd(0) species should not be higher than the rate
of the first step of the catalytic cycle, as in this case Pd(0) would be accu-
mulated, the aggregation begins and Ostwald ripening contributes to with-
drawing Pd from the catalytic cycle until too little is left to support the cycle
running at an appreciable rate. On the other hand, if the loading or pre-
activation rate is too low, the rate of the catalytic process would be limited by
it and the overall rate again would be inappropriate.
Even with the most careful tuning of such systems and achieving a perfect
match between release and consumption rates, the metal particles would
appear near the reaction completion when the Pd(0) consumption rate
naturally decreases and aggregation begins. The freshly formed nano-
particles would serve as alternative precatalysts and maintain the pool of
accessible palladium until their quality worsens irreversibly due to Ostwald
ripening. Two things should be borne in mind:
1. palladium nanoparticles are never true catalysts, but rather function as
secondary precatalysts (cf. the detailed discussion in Ref. 124);
2. if they work (as the precatalysts) they are subject to Ostwald ripening
and therefore to gradual deterioration of performance because of un-
favourable changes in morphology (a similar effect was diagnosed in
other Pd-catalysed reactions accompanied by the formation of
nanoparticles 128 ).
The recyclable systems for the Mizoroki-Heck reaction (and the very
closely related, with respect to the design and operation of phosphine-free
catalytic systems, Suzuki-Miyaura reaction) can be built around Pd nano-
particles only if some mechanism of continuous counter-Ostwald redisper-
sion is implemented and, as far as we know, this trick remains
unaccomplished, although efforts have been made, to realize such systems,
e.g. via the use of nanoparticles supported on various sophisticated supports
including biopolymers, nanocomposites and structured materials, 129-141 in
which case the surface or the pores of the support to a certain extent regulate
the size of the dispersions. The success of such systems has so far been
disappointing, as the Ostwald ripening cannot be switched off when needed
(a discussion of earlier results on supported nanoparticles and similar tricks
in Mizoroki-Heck reactions can be found elsewhere 124,125 ). Among other
attempts to control dispersion, the use of ultrasonic stimulation in aqueous