Modern Heck Reactions - New Trends in Cross-Coupling: Theory and Applications - page 359

A

Pd 0 PdH(X)

Y Y

RX + +HX

R

+2e

Pd 2+

Pd 0

B

Y

+X - +Y - RX +

R

-2e

Pd 0

Pd 2+

C

Y Y

+M + +H + RM +

R

Pd 2+ Pd 2+

D

Y

RM + +MY

R

Scheme 9.2 Basic stoichiometric types of Heck reactions (X, Y ¼ nucleofugic groups,
M ¼ electrofugic group). (A) Common Mizoroki-Heck reaction, an
isohypsic process requiring base for regeneration of catalyst; a spon-
taneous base-free version is also possible. (B) Reductive Heck reaction,
requires reductant for regeneration of Pd(0) catalyst. (C) Oxidative Heck
reaction (Fujiwara-Moritani reaction if M ¼ H), requires oxidant for
regeneration of catalyst. D A rare type of Heck reaction, isohypsic and
not requiring any auxiliary reagents for regeneration of catalyst, pre-
paratively identical with cross-coupling but catalysed by Pd(II) and not
by Pd(0).

Pd II

Pd II

Pd II Y -

Y

Scheme 9.3 The b-eliminaition of Pd(II) and nucleofuge viewed as the retro-Wacker
reaction.

Historically, the process was revealed as early as 1977 by Kasahara et al. 57
The importance of this chemistry in the Heck reaction was demonstrated by
Cabri et al., 58 who actually showed that this can be realized as a special
version of the Heck reaction, which corresponds to the stoichiometric type B
in Scheme 9.2. In recent studies, this termination was revealed as a common
phenomenon occurring in many reactions involving carbopalladation. Suf-
ficient cases were accumulated to make possible a thorough review of factors
governing the selectivity of elimination and the competition between the
normal PdH elimination and the Cabri-Kasahara termination. 59
Such a process occurs, for example, in the arylation of vinyl acetate leading
to styrenes, discovered by Cabri et al. 58 As the process ends with Pd(II), an
external reducing agent is required to make the process catalytic. In reality
this is not a problem, as both olefin and Et 3 N commonly used in such
protocols do reduce Pd(II), but nevertheless the participation of a reducing
agent should be explicitly taken into consideration in the stoichiometry and
it should not be forgotten that the reductant is actually consumed in the
reaction in equivalent quantities (1 equiv. of two-electron reductant per