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Mechanism of the Heck Reaction Using a Phosphapalladacycle as the Catalyst: Classical versus Palladium( IV ) Intermediates
Author(s) -
Böhm Volker P. W.,
Herrmann Wolfgang A.
Publication year - 2001
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/1521-3765(20011001)7:19<4191::aid-chem4191>3.0.co;2-1
Subject(s) - palladium , catalytic cycle , catalysis , chemistry , styrene , heck reaction , oxidative addition , aryl , medicinal chemistry , organic chemistry , alkyl , copolymer , polymer
The mechanism of the (Mizoroki–)Heck vinylation of aryl bromides catalyzed by the phosphapalladacycle [{PdCH 2 C 6 H 4 P( 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.