Modern Heck Reactions - New Trends in Cross-Coupling: Theory and Applications - page 377

media in the presence of the phase-transfer and colloid protector agent
Aliquat-336 was reported. 142
The catalytic systems of the second type are serviced, similarly to the first
type, by Pd species with ill-defined coordination shells, without distinct
stable ancillaries, usually under rather harsh conditions, and are often
helped by additives that prolong the lifetime of Pd(0) species by labile li-
gation. The best known of such additives are quaternary ammonium salts,
particularly Bu 4 NBr. The role of such additives is likely to be simple - by
forming labile anionic Pd(0) complexes the aggregation is delayed and thus
the margin for entering the narrow entry gate of the catalytic cycle is
widened. The other known role is the well-established Amatore-Jutand ef-
fect, the increase in reactivity towards oxidative addition, 143,144 equivalent to
widening the entry gate. Both effects thus act in the same direction.
Although the sources of palladium can be as diverse as in the type 1
catalytic systems, here the role of the precatalyst is more critical than just to
supply some Pd(0) species. Indeed, the need to match the rate of pre-
activation with the rate of Pd(0) consumption would be best met by pre-
catalysts able to release Pd(0) species at a controllable rate, so that by
optimization of the conditions, adjusting the temperature and supplying
additives this rate can be tuned to the requirements of a particular catalytic
cycle. This need is best suited by stable palladium complexes of various sorts
capable of slow decomposition under the conditions of the Heck reaction.
Such palladium-containing materials can be regarded as slow-release
precatalysts. 124
The third type of catalytic system is processes that rely on specific stable
ancillary ligands to increase the reactivity of Pd(0) species towards oxidative
addition and simultaneously to suppress altogether the aggregation of Pd(0)
species and deactivation through the formation of metallic particles. This
type of Heck reaction was discovered by Heck himself, who noted that the
addition of Ph 3 P substantially increased the yields of reactions with less
reactive aryl bromides. Soon afterwards, he discovered the second classical
ligand - tris(o-tolyl)phosphine, revealing that increasing the bulk of the
ligand is a favourable factor, probably because the coordination equilibria
shift to species bearing less stable ligands per metal atom and we need no
more than two of such species for active Pd particles. Hence the classical
Heck protocols - Pd acetate or dba complex plus Ph 3 Por(o-tol) 3 P plus Et 3 N
or Bu 3 N base in a polar solvent (DMF, DMA, NMP, MeCN) - were established
and these protocols with minor changes still dominate Heck chemistry with
aryl bromides and also aryl iodides, when reliable operation and yields are
more important than economical value or very high turnover number (TON).
The systems of this type can be called ligand-accelerated systems, bearing in
mind that the effects of the stable ancillary are not only an increase in rate
and the use of less reactive substrates, but also the improvement of stability
and convenience, most important in product-oriented studies when the
target compound of a complex synthesis is what matters and optimization of
the catalytic system is a waste of time and labour.