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Palladacycles: Efficient New Catalysts for the Heck Vinylation of Aryl Halides
Author(s) -
Herrmann Wolfgang A.,
Brossmer Christoph,
Reisinger ClausPeter,
Riermeier Thomas H.,
Öfele Karl,
Beller Matthias
Publication year - 1997
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/chem.19970030823
Subject(s) - aryl , chemistry , catalysis , heck reaction , medicinal chemistry , halide , palladium , ligand (biochemistry) , organic chemistry , receptor , biochemistry , alkyl
Cyclopalladated complexes of the general formula [Pd 2 (μ‐L) 2 (PC) 2 ] (L = bridging ligand, e.g. OAc, Cl, Br, I; PC = cyclometallated P donor, e.g. o ‐CH 2 C 6 H 4 P( o ‐Tol) 2 or o ‐CH 2 C 6 H 2 (CH 3 ) 2 ‐P(Mes) 2 ) are highly efficient catalysts for the Heck vinylation of aryl halides. The isolated complexes are easily accessible from palladium(II) acetate by spontaneous metallation of ortho ‐methyl‐substituted arylphosphines. They display improved activity and stability compared to conventional catalyst mixtures (e.g. [Pd(OAc) 2 ] + n PPh 3 ), and also exhibit a higher stability towards air than conventional Pd 0 ‐based systems (e.g. [Pd(PPh 3 ) 4 ]). Turnover numbers (TON) of up to 1 and turnover frequencies (TOF) in the range of 5000‐20000 are achieved in catalytic coupling reactions of aryl bromides. Even technically interesting aryl chlorides undergo the Heck reaction (TON = 600‐40000) if promoting salts are added to the catalyst ((NBu 4 )Br, LiBr). The new structural type for catalysts is compared to palladacycles formed in situ from mixtures of [Pd(OAc) 2 ] + P( o ‐tolyl) 3 and the established [Pd(OAc) 2 ] + n PPh 3 system. The scope of the new CC coupling catalysts is outlined for the vinylation of aryl halides by the use of different mono‐ and disubstituted olefins. Mechanistic consequences for the Heck reaction in general are discussed.