Modern Heck Reactions - New Trends in Cross-Coupling: Theory and Applications - page 387

chlorides and actually the results with typical aryl bromides are not much
different from those obtained in ''phosphine-free'' systems, thus obviously
falling behind in activity (see below), although undoubtedly being superior
in scope. Thus, on the one hand the system works very similarly to slow-
release precatalyst (SRPC)-based systems (activation at high temperature,
peak performance with reactive substrates), but on the other hand is un-
precedentedly versatile towards substrates of different reactivity. Therefore,
it could be hypothesized that the resting stable chelate, which maintains a
pool of palladium protected against decomposition and deactivation, is
thermally activated to release the monophosphine complex (Scheme 9.46),
very stable and active to support the reaction of deactivated aryl bromides. In
this respect, Tedicyp-based systems provide an interesting and special case
of phosphine-assisted catalysis, well supported by ample evidence.
Heterocyclic carbene ligands are apparently the most important major
group of non-phosphine ligands introduced so far into transition metal
catalysis. A large series of carbene ligands including mono- and bidentate,
normal and sterically hindered ligands giving homo- and heteroleptic
complexes, and also pincer-type structures, were obtained and tested in
palladium-catalysed reactions. Bulky electron-rich carbenes effectively
mimic the new-generation phosphine ligands, but also have their own
unique features that find them a special and important place in transition
metal catalysis. 178 Indeed, in C-C and C-heteroatom cross-coupling re-
actions, and also in some other important Pd-catalysed reactions, many new,
effective protocols involving carbene ligands and preformed carbene com-
plexes have been introduced to demonstrate that the expectations are in
general satisfied and a serious competitor to the phosphine family does
exist. 179-181 The ability of heterocyclic carbene ligands, particularly the most
often used ones, IMes, IPr, SIMes and SIPr (Schemes 9.47 and 9.48), to
catalyse cross-coupling reactions with the least reactive electron-rich and
bulky aryl chlorides and bromides under mild conditions at room tem-
perature or gentle heating below 60 1C is well documented.

Pd

Pd

Pd Pd

active

inactive

Scheme 9.46 Probable coordination modes of the tetraphosphine Tedicyp.

R

R

NN

R'

R'

R R

Scheme 9.47 Heterocyclic carbene ligands IPr (3)(R ¼ i Pr, R 0 ¼ H) and IMes (4)
(R, R 0 ¼ Me).