Wiley InterScience Backfile Collection 1832-2000
Chemistry and Pharmacology
Cyclopalladated complexes of the general formula [Pd2(μ-L)2(P—C)2] (L = bridging ligand, e.g. OAc, Cl, Br, I; P—C = cyclometallated P donor, e.g. o-CH2C6H4P(o-Tol)2 or o-CH2C6H2(CH3)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] +nPPh3), and also exhibit a higher stability towards air than conventional Pd0-based systems (e.g. [Pd(PPh3)4]). Turnover numbers (TON) of up to 1000000 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 ((NBu4)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] + nPPh3 system. The scope of the new C—C 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.
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