Modern Heck Reactions - New Trends in Cross-Coupling: Theory and Applications - page 460

best results when the bidentate phosphine dppe is used, which under an
oxygen atmosphere is probably rapidly oxidized to monodentate dppe-
monoxide ligand. The main problem in this reaction is the high nucleo-
philicity of sulfinates, which competitively form Michael adducts with
acrylates and similar electronegative olefins. Further, in the absence of a
phosphine ancillary, the second sulfinate ligand may enter the coordin-
ation sphere of Pd(II) to inhibit the Heck reaction at the expense of side
reactions. However, overall, the new process is impressive in scope, being
applicable to various olefins and sulfinates bearing both electron-donating
and electron-withdrawing substituents in the aryl ring (Scheme 9.176). As
the ligand exchange with nucleophilic sulfinate ion is indeed likely to be
weakly sensitive to the nature of the arene substituents and the extrusion
of SO 2 is more facile than that of CO 2 from formally analogous carb-
oxylates, the reaction shows relatively lower sensitivity to electronic effects
among the oxidative Heck reactions. The process definitely invites further
research and refinement of the protocol, e.g. to find a more convenient
solvent and to establish whether the phosphine ancillaries are indeed
needed, as the beneficial effect of addition of the ligand reported 453

is

actually rather marginal.
Arylation of enones by sodium arenesulfinates gives the products of
conjugate addition, similarly to what is observed in other oxidative Heck
reactions (cf. Scheme 9.150). The reactions take place in an unusual reaction
system using Pd(OH) 2 precatalyst, DABCO and an excess of trifluoroacetic
acid, which probably facilitates the protolysis of the palladium enolate
intermediate, in aqueous dioxane. Being an isohypsic reaction, this process
avoids the use of reoxidants. 454 Further, Tian and co-workers introduced an
interesting modification of this reaction by applying readily available are-
nesulfonylhydrazides in the reaction with Pd(II) catalyst. 455 Deng and co-
workers further showed that in the reaction with enones these substrates
gave conjugate addition products in high yields and in a very simple catalytic
system (Scheme 9.177). 456 Apparently, the hydrazide is first oxidized to some

Z
PdCl 2 (5 mol% )
O 2, DPEpho s
PhOMe, 85 °C, 24 h

SO 2 Na CO 2 Bu

49-8 5%

R R

R=Me,OMe, t -Bu , F, B r, CF 3, NO 2

Scheme 9.176 Oxidative Heck reaction using arenesulfinates.

Ph

O O

PhSO 2 NHNH 2
Pd(acac) 2 (5 mol% )
O 2, dioxane-H 2 O, 80°C, 24-32 h

59-81%

R R

Scheme 9.177 Conjugate arylation of enones by arenesulfonylhydrazides.