Modern Heck Reactions - New Trends in Cross-Coupling: Theory and Applications - page 391

R

R

O

P O

Pd

Pd

O P

O

R R

R=o-Tol

Scheme 9.53 The Herrmann-Beller palladacycle (trade name CataCXium C) (7).

Strong interest was drawn to these phenomena with the publication by
Herrmann, Beller and co-workers on the catalytic properties of the palla-
dacycle readily formed in the reaction of palladium acetate with (o-tol) 3 P
(Scheme 9.53). 198 This material had been known before, being discovered
first by Heck himself and then used by Spencer, but only in this last study 198
was it noted that the material is not just some interesting Pd complex, but a
potent source of catalytic activity.
The compound attracted intense interest and brought forward several new
concepts, including the following:

(a) The idea that at least some well-defined catalysts can operate not via
the standard Pd(0)/Pd(II) catalytic cycle, but via the Pd(II)/Pd(IV) route,
if the structure of the precatalyst (palladacycle or pincer palladabi-
cycle) is presumed to be conserved throughout the cycle.
(b) Monophosphine catalysis, if the palladacyclic structure is presumed to
be partially cleaved to release the very reactive PdL complex.
(c) Catalysis by nanoparticles, if the precatalyst is presumed to be cleaved
completely, giving rise to unstable Pd(0) species that rapidly form fine
and highly reactive Pd sols.

The study of each of these concepts, particularly that of monophosphine
catalysis, gave many interesting results in other areas. In reality, and this was
eventually directly demonstrated by de Vries and co-workers, 199-201 various
palladium precatalysts from simple salts to sophisticated palladacycles give
essentially similar results when reactions are carried out under comparable
conditions, which means that the reactions are actually catalysed by Pd
species bearing no specific ancillary ligands, but instead containing arbi-
trary ligands from the reaction media, of which the anions coming from
bases and salt additives used perform an important role in inhibiting early
aggregation and deactivation of the catalyst. Palladium sols (nanoparticles)
play only a subsidiary role, serving as pools of palladium species via
accretion-redissolution processes. Simple palladium precatalysts such as
Pd(OAc) 2 alone can serve as well as any sophisticated precatalyst if a suf-
ficiently high temperature is applied and the reaction conditions are care-
fully optimized. Sol-stabilizing additives, such as tetraalkylammonium salts,
are known to facilitate such reactions, making them less critically dependent
on specific conditions. This approach is well known as the Jeffery or Jeffery-
Larock method. 202-205 Various technical improvements allowing fast heating