Modern Heck Reactions - New Trends in Cross-Coupling: Theory and Applications - page 451

Ar SnBu 3

HPdL +

R

LP d 2+

Ar

Pd L +

ArPdL

R R R

Ar Ar

PdL +

ArS nBu 3 ArS nBu 3

R Ar

Ar

Pd L

PdL

R

Ar Ar

R Ar

Ar

R

Ar Ar

Scheme 9.158 Transformations of carbopalladation adduct in the destannylative
Heck reaction.

chance for the formation of Heck product in the base-free system, where the
PdH intermediate is likely to be a long-lived species, waiting for spontaneous
deprotonation or oxidation by O 2 . Under such conditions, the isomeric ad-
duct is given the chance to define the predominant pathway. In order to do
so, this adduct should be relatively the most stable of all the intermediates
involved, hence it can be trapped in the cascaded Stille cross-coupling.
Moreover, such a mechanism involves the reversible elimination-addition
of PdH. What is unusual (although not without precedent - cf. the similar
chemistry in the deborylative reactions, e.g. Scheme 9.142) is that these
elimination and addition events seem to stay bound to the initial reaction
site and do not lead to shifts of the double bond. The experiment with a
substrate containing a chiral centre next to the reaction centre shows that
elimination in this direction does not take place.
This result certainly can be explained by the suppression of PdH syn
elimination by the coordinating group, which is a common feature of Heck
reactions. However, in other substrates without such groups, the double-
bond shift would be manifested by the formation of 1,3-, 1,4-, etc. arylation
products, which were not observed. Thus, all the evidence indeed demon-
strates that the isomeric adduct is somehow stabilized. This feature was
explained by appending an Z 3 -benzylpalladium complex to the pathway,
being in equilibrium with the normal Z 1 -benzylpalladium complex
(Scheme 9.159).
The idea is probably relevant for other protocols in Heck chemistry, where
the double-bond migrations are suppressed. The formation of relatively
stable Z 3 -benzyl complexes is probably favoured by cationic palladium
centres possessing ''free'' coordination sites, but more research efforts are
required to clarify the factors that favour such a coordination mode.