Modern Heck Reactions - New Trends in Cross-Coupling: Theory and Applications - page 358

generalizations can involve the addition of other s-bonded complexes of
palladium, formally similar to what happens in carbopalladation, an already
well-established technique, e.g. in the aza-Heck (Narasaka-Heck cyclization)
reaction based on intramolecular aza-palladation of the double bond. 52-56
In Scheme 9.1, a minimalistic overview of the Heck chemistry manifold is
shown to demonstrate the main stages involved in the majority of Heck
reactions. The process should involve the generation of a reactive but rela-
tively stable (isolable or detectable) s-organylpalladium complex by (i) oxi-
dative addition from organic electrophile with an appropriate nucleofugic
group or (ii) transmetallation or electrophilic palladation of C-H or C-metal
bonds or (iii) various addition reactions to triple bonds. This complex next
adds to a C ¼ C double bond either inter- or intramolecularly. The resulting
adduct is either labile, if there is a vicinal H-atom, which can rotate into the
syn-position relative to the Pd centre, to undergo syn elimination of Pd hy-
dride giving the product and the palladium hydride species is recycled via
deprotonation or oxidation. Such a simple scheme describes both regular
and oxidative Heck reactions. Each of the stages allows for multiple alter-
natives, which accounts for the amazing diversity of Heck reactions.

9.1.1 The Diversity of Heck Reactions
Regarding the alternatives to syn-PdH elimination, the most common
product-forming step, we can obtain an interesting classification based on
net stoichiometry. In the cases when there is a nucleofugic group at the
adjacent atom, the alternative termination may involve the elimination of
Pd 21 species and this nucleofuge. If we take into account this possibility,
four distinct stoichiometries can be considered, two of which are redox
processes involving Pd(0)/Pd(II) and Pd(II)/Pd(0) pairs and the other two are
isohypsic involving the same oxidation states at the termini (Scheme 9.2). All
four are known and are pairwise complementary.
The elimination of nucleofuge was initially regarded as a minor pathway,
occurring only in specific cases. Such an elementary step is actually
equivalent to a retro-Wacker reaction (Scheme 9.3).

subsrates

next cycl e Pd(0)

RX

Pd(0 )

Y R'

by base or
spontaneously

Y R '

R R'

R"

H

Pd(II)

H R" syn- eliminatio n

RH or RM RPd(X) + PdH(X)

R Pd (X) Y R"

oxidation o f hydride

PdH(X) or PdXX '

hydropalladation,
hal opal lad ati

next cycle Pd(II)

o n, etc.

Scheme 9.1 The core of the Heck reaction paradigm.