Mechanism of the Heck reaction using a phosphapalladacycle as the catalyst: classical versus palladium(IV)...

The mechanism of the (Mizoroki-)Heck vinylation of aryl bromides catalyzed by the phosphapalladacycle [[PdCH2C6H4P(o-Tol)2(OAc)]2] (1) was investigated in order to determine whether palladium(IV) intermediates play a significant role in the catalytic cycle. The phosphapalladacycle 1 was compared to the related palladium(0) catalyst [Pd[P(o-Tol)3]2] (2) and an intermediate of the classical catalytic cycle [[PdAr[P(o-Tol)3]Br]2] (3) obtained by oxidative addition of an aryl bromide to 2. Small, yet measurable deviations of the phosphapalladacycle 1 from the other catalysts 2 and 3 in the product distribution of styrene, in competition experiments of styrene and n-butyl acrylate, in the isotope effect on styrene, and the Hammett correlation on aryl bromides led to the conclusion that a different catalytic cycle is operative. Nevertheless, due to only small differences in these competition experiments a modified classical catalytic cycle with a novel, cyclometallated, anionic palladium(0) species 6 is proposed rather than palladium(IV) intermediates. Species 6 accounts for the observed high activity and stability of palladacycle catalysts in the Heck reaction. Palladium(IV) intermediates can be ruled out by taking into account the experiments performed here and recently published by others.