Modern Heck Reactions - New Trends in Cross-Coupling: Theory and Applications - page 372

although the Heck process itself is the attack of acylpalladium at the double
bond and is therefore not much different from other carbonylative Heck
reactions.
9.2 The Mizoroki-Heck Reaction
9.2.1 Ancillary Ligands in Heck Chemistry
From the very beginning of Heck chemistry, from the work of the Heck,
Mizoroki, Fujiwara and Moritani teams it became evident that this chemistry
in general is not dependent on special ancillary ligands - all starting catalytic
systems relied on a random choice of catalyst precursors, the role of which
was excellently performed by the first available Pd(II) salts. Meanwhile, the
mechanism of these processes contains many distinct steps involving the
preactivation of the precatalyst [reduction to Pd(0) in the Mizoroki-Heck
reaction, formation of monomeric Pd complexes from normally oligomeric
PdCl 2 , Pd(OAc) 2 , etc., in oxidative arylation], oxidative addition or (trans)-
metallation, binding of olefin, migratory insertion, reductive elimination,
regeneration of active Pd species - and none of these steps depends on
specific stable ancillaries (detailed discussions of the mechanism can be
found elsewhere 87,114-116 ), in cases when specific tasks (reactions of less
reactive substrates, controlling regio- or stereoselectivity, etc.) are not pur-
sued. It should be stated that from this viewpoint, Heck chemistry is dif-
ferent from the majority of other important transition-metal-catalysed
reactions critically dependent on the correct choice of ancillaries - an
archetypical example is the Buchwald-Hartwig C-N and C-O, C-C (carba-
nion arylation) cross-coupling reactions in which the effect of specific stable
ancillaries is critical in the nucleophile binding and reductive elimination
steps, and also for suppression of side reactions.
Although it is not commonly realized, the opposite seems to be true in
Heck chemistry. Most Heck reactions - both oxidative and regular types - are
performed without specific ancillary ligands. It should be stressed that this
concerns only those ligands for which the binding mode is considered to be
stable - as opposed to labile and hemi-labile ligands. Stable ligands survive in
the coordination shell of the metal centre throughout all the reactions in-
volved in the transformation and in the catalytic process they survive for
many turns of the cycle. The processes performed in the absence of stable
ancillary ligands are commonly called phosphine-free or even ligand-free. The
latter term is an inappropriate misnomer, and the former is also a misnomer
(Heck chemistry can involve non-phosphine stable ancillaries, such as
arsines, carbenes, N,N-chelators and others) but it tacitly reflects the com-
mon impression that phosphines are by far the most important archetypical
ligands for palladium catalysis as a whole and Heck chemistry in particular.
Therefore, in further discussion we reserve the use of the term ''phosphine-
free'' to designate the reactions served by palladium complexes lacking
definite stable ancillary ligands.