Modern Heck Reactions - New Trends in Cross-Coupling: Theory and Applications - page 357

The possibility of using the reaction for heterocyclization to afford a flexible
route to various heterocyclic systems was realized. 37,38 Basic principles of
what were later to become known as domino reactions, in which organo-
palladium intermediates formed via carbopalladation are captured by vari-
ous follow-up reactions, e.g. intramolecular carbopalladation and allylic
substitution, were discovered. 39-43 In these studies, Heck and co-workers
revealed that the modification of basic triphenylphosphine ligand gave new
ligands with improved performance and greater effects on catalytic activity.
Tris(o-tolylphosphine) was identified as a ligand useful in performing
reactions with the least reactive, electron-rich aryl bromides, such as
p-bromo-N,N-diethylaniline or p-bromophenol, and the effect of the ligand
was traced to catalyst inactivation mechanisms. 44 This ligand, reintroduced
later by Spencer, 45,46 became one of the key indispensable ancillaries for the
Mizoroki-Heck reaction. The formation of a palladacycle from tris(o-tolyl-
phosphine) and Pd(OAc) 2 was observed and its structure established. 47 This
palladacycle would be rediscovered a decade later by Herrmann and
co-workers and serve as a model precatalyst in further intense efforts to
develop superior catalyst precursors. 48,49
Thus, the chemistry rapidly grew into a fully fledged synthetic strategy and
in 1979 an impressive account of the newly born chemistry, explicitly
showing its enormous preparative potential, versatility and promise for
further development, was written by Heck, 50 followed by a chapter in Organic
Reactions. 51
Heck and co-workers in fact discovered (alone or simultaneously with the
Mizoroki and Fujiwara-Moritani teams in Japan) not a single reaction, but a
general type of transition metal-mediated arylation/alkenylation of un-
saturated bonds, and its realization in no less than three different catalytic
processes: (a) the oxidative process, stoichiometric in palladium in the ab-
sence of an external oxidant, which should be given the name Heck-
Fujiwara-Moritani reaction; (b) the catalytic process taking place in the
presence of Pd complexes not requiring specific stable ligands (often re-
ferred to as phosphine-free or even ligand-free, both being misnomers); and
(c) the catalytic reaction taking place in the presence of Pd complexes in-
volving stable ligands (almost always phosphines), which exert a critically
important influence on the performance (the activity or selectivity) of the
catalysts.
In a general sense, the Heck reaction can be defined as a process involving
the addition of a s-organylpalladium complex (carbopalladation) of un-
saturated (usually double bonds, but triple bonds cannot be discarded from
consideration). In fact, this chemistry is not necessarily restricted to pal-
ladium, as was shown by Heck himself - it can be performed with other
transition metals and this story is under investigation, albeit much more
slowly than Heck's chemistry of palladium.
Heck's chemistry of today includes catalytic and stoichiometric reactions,
and about a dozen named processes (Mizoroki-Heck, Fujiwara-Moritani,
Blaser, Myers, Matsuda-Heck, etc.) are included in this context. Further