Modern Heck Reactions - New Trends in Cross-Coupling: Theory and Applications - page 440

control by intramolecular chelation as are known for other versions of
regular and oxidative Heck reactions. In deborylative reactions, palladium
acetate is often used, which, at the first glance, would not favour the polar
pathway, particularly when copper acetate is used as terminal oxidant to
supply a good excess of acetate ligands for Pd species. However, it is very
likely that the polar pathway is nevertheless enforced by using such neutral
labile ligands as sulfoxides (the addition of DMSO to the solvent or special
sulfoxide ligands), which may be expected to effect acetate-sulfoxide ligand
exchange, which, given the coordination lability of sulfoxide ligands, can be
considered as an effective means of triggering the polar pathway

(Scheme 9.137).
Bidentate disulfoxide can perform this task even without being used in
huge excess because of the chelating effect. Intramolecular chelation of
cationic palladium intermediates is likely to exert an excellent degree of
control over regioselectivity in the arylation of various functionalized olefins
by arylboronic acids 399 (Scheme 9.138).
This interesting and highly fruitful approach implicitly exploits directed
carbopalladation (various O, N and S centres do the task; for a review on
chelation-controlled Heck reactions, see Ref. 400), in which case steric rea-
sons dictate (a) the attack of the Pd centre at the proximal atom of the double
bond and (b) fixation of the chelate ring restricting PdH elimination from an
inner atom, thus suppressing the double-bond shift. Regular Mizoroki-Heck
reactions involving neutral arylpalladium intermediates in such cases are
non-selective, giving variable regioselectivity and double-bond shifts (as
shown by pathways marked with dashed arrows in Scheme 9.139).
The retention of the double-bond position in the arylation of allylic
acetates was reported by Su and Jiao to require a sophisticated catalytic
system including both silver and copper salts and also KHF 2 additive. 401 The
effect was attributed to chelation control (Scheme 9.140).

Pd(O Ac) 2 RR'SO

ArM ArPdO Ac

ArPd(RR'SO)

OAc -

neutral pathway polar pathway

Scheme 9.137 Multiplicity of pathways in deborylative (and other oxidative) Heck
reactions in the presence of (hemi)labile sulfoxide ligands.

P hB(OH) 2
P d(OAc) 2 · L (10 mol% Pd)
AcOH, benzoquinone
dioxane, r.t., 24 h

S Bu S Bu

Ph

56%

Ph Ph

S +

S +

L=

- O

O -

Scheme 9.138 Regioselective deborylative arylation of remote double bonds in
chelation-enabled alkenes.