PALLADACYCLES - EFFICIENT NEW CATALYSTS FOR THE HECK VINYLATION OF ARYL HALIDES

Cyclopalladated complexes of the general formula [Pd-2(mu-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 h ighly efficient catalysts for the Heck vinylation of aryl halides. The isolated complexes are easily accessible from palladium(II) acetate b y spontaneous metallation of ortho-methyl-substituted arylphosphines, They display improved activity and stability compared to conventional catalyst mixtures (e.g. [Pd(OAc)(2)] +nPPh(3)), and also exhibit a hig her stability towards air than conventional Pd-0-based systems (e.g. [ Pd(PPh3)(4)]). Turnover numbers (TON) of up to 1000000 and turnover fr equencies (TOF) in the range of 5000-20000 are achieved in catalytic c oupling reactions of aryl bromides, Even technically interesting aryl chlorides undergo the Heck reaction (TON = 600-40000) if promoting sal ts are added to the catalyst ((NBu4)Br, LiBr), The new structural type for catalysts is compared to palladacycles formed in situ from mixtur es of [Pd(OAc)(2)] +P(o-tolyl)(3) and the established [Pd(OAc)(2)] +nP Ph(3) 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 d isubstituted olefins. Mechanistic consequences for the Heck reaction i n general are discussed.