Modern Heck Reactions - New Trends in Cross-Coupling: Theory and Applications - page 459

vinyl chlorides, tosylates, etc.) under mild conditions (see above). Here, its
role is apparently associated with decarboxylative palladation. The other
interesting feature of this reaction is that under the conditions developed,
no regular arylation of cinnamic acid by aryl iodide takes place

(Scheme 9.174). 449
Of further interest with respect to the use of various carboxylic acids in
palladium-catalysed decarboxylative processes is the cross-coupling of
2-pyridylacetates with aryl bromides, in which effective decarboxylative pal-
ladation is ensured by the use of Xantphos or similar wide bite angle
bidentate ligands (DPEPhos, BINAP, TolBINAP) and similarly harsh con-
ditions [up to 2 mol% Pd 2 (dba) 3 ,3L(L ¼ Xantphos), diglyme, 150 1C]. 450 The
process can be viewed as a Heck-type arylation of enamine equivalent.

9.3.6 Arylation by Arenesulfinates
The use of chlorosulfonylarenes in the regular Mizoroki-Heck reaction was
described as early as in 1989 by Miura and co-workers. 451,452 The reaction
involves extrusion of SO 2 from an arenesulfonylpalladium complex. The
evident alternative approach to such complexes - by ligand exchange from
Pd(II) species, which allows an alternative approach to the same goal
(Scheme 167) - had to wait until 2011 to be added to the Heck family of
reactions. Deng and co-workers discovered a catalytic system that permits
the use of arenesulfinates in the oxidative Heck reaction. 453 Formal com-
parison of two pathways - regular Mizoroki-Heck reaction initiated by oxi-
dative addition of Pd(0) and oxidative Heck reaction initiated by nucleophilic
substitution (ligand exchange) at Pd(II) - shows that these two pathways are
perfectly complementary (Scheme 9.175).
Nevertheless, unlike the reaction of chlorosulfonylarenes which is per-
formed in a ''phosphine-free'' reaction, its oxidative counterpart gives the

PdCl 2, CyJohnPhos
Ag 2 CO 3, DMA , 150°C

COOH Ph

+PhI

Ph Ph

Ph

COOH

PCy 2
CyJohnPhos

Ph

Scheme 9.174 Decarboxylative cross-coupling preparatively equivalent to the

Mizoroki-Heck reaction.

Ar SO 2 Cl

Pd(0)

SO 2

Heck
reaction

Ar SO 2 PdCl Ar PdCl

PdCl 2

A rSO 2 Na Z

Scheme 9.175 Complementary pathways in Heck reactions via SO 2 extrusion.