Modern Heck Reactions - New Trends in Cross-Coupling: Theory and Applications - page 392

and mixing of reagents, e.g. microwave and microflow techniques, further
facilitate such reactions. 206,207
In some cases, even with aryl iodides, the net rate of the catalytic cycle can
be slow owing to other factors, 208 e.g. weak binding of electron-deficient
olefins and/or low solubility of aryl iodides in the reaction medium. In this
case, the TOF would be low and withdrawal of palladium from the catalytic
process occurs. The addition of an extra amount of precatalyst was required
to ensure high yields, probably because the generation of a reactive olefin by
elimination lags behind the rate of the Mizoroki-Heck catalytic process,
hence the Pd(0) concentration increases up to levels where the nucleation
and growth of inactive metal withdraws the catalyst. Such systems never-
theless can be useful even in challenging cases. 209
Studies on new catalysts meanwhile continued and hundreds of different
complexes, including simple and pincer palladacycles, carbene complexes,
various chelates and hybrid complexes involving palladacyclic, carbene and
phosphine ligands, were obtained and assayed for catalytic activity in model
Mizoroki-Heck reactions (for a comparative compilation of results, see
Ref. 124). Almost all of the results, except for a very few, 124 could be fitted
within the concept that disregarding the initial form used, during the pre-
activation stage (marked by induction periods of variable length) all specific
ligands were stripped and the catalytic cycles were led by ill-defined Pd
species with coordination shells filled by solvent molecules and ions from
the respective reaction media. Hence the sophisticated ligands behaved like
''disposable wrappers'' for delivery of Pd, and therefore the simplest possible
structures based on very cheap ligands (''junk ligands'') can behave as well
as new, sophisticated, specially synthesized ligands. 210 The role of the
precatalyst is just to supply Pd species at a rate compatible with the
consumption rate (see above), thus avoiding deactivation through aggre-
gation to clusters, then nanoparticles and their death via Ostwald ripening.
To describe this behaviour, it is convenient to use the concept of slow-
release precatalysts (SRPCs), 124 a notion actually inspired by agriculture,
where slow-release fertilizers are becoming popular, as they continuously
and slowly feed the crops when administered just once at the start of the
season. The key idea is to make the rate of release compatible with the rate of
consumption. In contrast, common (fast-release) fertilizers give a sudden
large overdosage of nutrients, which cannot be consumed and the un-
consumed excess fails to feed the roots but instead poisons the environ-
ment. A similar thing happens in catalysis by palladium in the catalytic
cycles involving less reactive substrates, as simple Pd precatalysts (salts, dba
complex, etc.) supply Pd(0) at such a rate that it cannot be consumed owing
to limitations of the net rate of the catalytic reaction, whereas the rate of
release from an SRPC can be compatible with the rate of consumption by the
catalytic cycle.
The mechanisms of release involve common reactions at the Pd centre
invoking reductive elimination and sequential rupture of chelate bonds. The
mechanisms of release have nothing to do with the inherent thermal