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6597410 
Journal Article 
Mechanism of the Heck reaction using a phosphapalladacycle as the catalyst: classical versus palladium(IV) intermediates 
Böhm, VP; Herrmann, WA; , 
2001 
Yes 
Chemistry: A European Journal
ISSN: 0947-6539
EISSN: 1521-3765 
WILEY-V C H VERLAG GMBH 
WEINHEIM 
7 
19 
4191-4197 
English 
11686598 
WOS:000171452800015 
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. 
Heck reaction; Homogeneous catalysis; Metallacycles; Palladium; Reaction mechanisms