Modern Heck Reactions - New Trends in Cross-Coupling: Theory and Applications - page 356

Arylpalladium complexes, generated from organomercuric compounds
(organotin and organolead compounds were also tried 2 ) were found to ary-
late olefins. 3,4 These studies set the range of olefins that later became the
main targets of research for over 40 years - styrenes, acrylates, allylic alco-
hols and enol ethers. Reoxidation of Pd(0) formed by cupric salts was
introduced to make the oxidative arylation catalytic in palladium.
The classical mechanism of arylation involving three major steps - oxi-
dative addition (transmetallation in oxidative cases), migratory insertion and
reductive elimination to generate an XPdH intermediate - was postulated
and experimentally substantiated to become the main mechanistic para-
digm, which has remained practically unchanged except in minor details. 5,6
Heck discovered and interpreted double-bond shifts, intramolecular che-
lation in heteroatom-substituted olefins and other detailed effects, which
later became subjects of numerous studies. 6
Simultaneously and independently, Fujiwara, Moritani and co-workers
started to exploit the well-known ability of simple Pd(II) salts to metallate
benzene and other arenes, with the thus generated arylpalladium inter-
mediates engaged in reactions with olefins. Reactions were conducted only
with stoichiometric amounts of palladium salts, without attempts to recycle
Pd(II) with cheaper reoxidants and, except in a few special cases, were fairly
unselective and gave poor to modest yields. 7-20
Thus, up to the end of the 1960s, a thorough understanding of the
chemistry involved in the reaction of arylpalladium intermediates, generated
via stoichiometric transmetallation, with olefins was accumulated, primarily
due to the masterful in-depth studies of Heck. Simultaneously, the gener-
ation of arylpalladium intermediates by oxidative addition of Pd(0) to aryl
iodides using stable phosphine complexes of Pd(0) was described by Fitton
and co-workers. 21-23 The stage was thus set for a major breakthrough.
The story of a true catalytic process began in 1971 when Mizoroki and co-
workers published a one-page preliminary communication (the full paper
appeared in 1973) 24,25 on the arylation of olefins (ethylene, propylene, styr-
ene, methyl acrylate) by iodobenzene in the presence of 1 mol% of PdCl 2
with KOAc as base in methanol at 120 1C in an autoclave. Palladium black,
formed by prereduction of PdCl 2 , was shown to deliver the same level of
catalytic activity as the soluble precatalyst.
In 1972, a new series of papers by Heck and co-workers describing the
palladium-catalysed reaction of olefins with aryl and vinyl halides began to
appear. In the first paper, a system using Pd(OAc) 2 in the presence of tertiary
amine base in a polar aprotic solvent was elaborated by Heck and Nolley to
become, with minor variations, the classical entry-level catalytic system for
generations of researchers to follow. 26 In subsequent studies, the effect of
phosphine ligands was discovered by Dieck and Heck, which permitted the
reaction with less reactive and readily available aryl bromides 27 and vinyl
bromides. 28,29 Further papers described the arylation and vinylation of
allylic alcohols, 30 acetals of unsaturated aldehydes and ketones, 31 ethyl-
ene, 32 heteroaryl-substituted alkenes, 33 N-vinylpyrrolidone 34 and dienes, 35,36