Modern Heck Reactions - New Trends in Cross-Coupling: Theory and Applications - page 401

in reactions of reactive aryl bromides and iodides with standard olefins
(0.0001-1 mol% Pd, KOAc or DIPEA, NMP, 120-160 1C, 5-40 h). 232
A pincer Se-N-Se complex behaved as an SRPC with modest activity,
showing high TON values for aryl iodides and bromides, but only incomplete
conversions and low yields with electron-rich aryl bromides. 233 Several other
systems, including phenethylamine palladacycle 234 and amidoamine

dendrimer-Pd complex, 235 were tested as typical SRPCs for standard

Mizoroki-Heck reactions.

9.2.4 Pd(II)/Pd(IV) Mechanism
The Pd(II)/Pd(IV) mechanism for Mizoroki-Heck reactions has been in-
tensely discussed in connection with palladacyclic precatalysts, but in the
absence of decisive evidence the hypothesis was dropped, but recently re-
vived again. Recently, the arguments were partially revived because of in-
creasing interest in Pd(IV) chemistry (see recent detailed reviews 236-238 ). The
existing manifestations of Pd(IV) complexes in organic reactions explicitly
state that so far no evidence on b-elimination from Pd(IV) complexes has
been documented, which alone makes the interpretation of Heck reactions
with Pd(IV)/Pd(II) catalytic cycles highly speculative.
Concerning the other key step in Heck chemistry, oxidative addition, some
Pd(II) chelates, 239 and also P-C-P and N-C-N pincer complexes, were shown
to undergo oxidative addition of hypervalent iodine compounds, leading to
Pd(IV) intermediates bearing aryl or alkynyl ligands. 240 An aryl-Pd(IV)
intermediate thus indeed can form in the interaction of diaryliodonium with
Pd(II) pincers, while further reaction of this intermediate with olefins
leading to Mizoroki-Heck products has not so far been documented ex-
perimentally, although the potential feasibility of such process was con-
firmed by computational modelling of the Pd(II)/Pd(IV) Heck pathway 241,242
Diaryliodonium salts are among the most reactive compounds in Pd-
catalysed reactions, 243 similarly to arenediazonium salts, 244 but more con-
venient in handling as they are not so prone to uncontrolled decomposition
and side reactions. In simple cases, extremely fast reactions can be per-
formed using neat water as solvent. 245 However, as these substrates are
the least atom economical, expensive and severely limited in scope, their
application lags behind the chemistry of arenediazonium compounds and
the gap is swiftly widening. Both types of substrates are used also because
both enforce a polar pathway. The mechanisms of such Mizoroki-Heck re-
actions nevertheless almost certainly conform to the common Pd(0)/Pd(II)
pathways.
Pincer complexes are able to catalyse Mizoroki-Heck reactions of various
olefins with diaryliodonium salts under mild conditions. Although the
catalytic activity (both TON and TOF) is rather modest, a broad scope of
olefins forming normal Mizoroki-Heck products without the usual double
bond shifts and other complications makes the protocol useful for complex
synthetic tasks (Scheme 9.68). Such behaviour may be a starting point for the