Palladacycles: Efficient new catalysts for the heck vinylation of aryl halides

Article abstract of DOI:10.1002/chem.19970030823

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 Pd(o)-based systems (e.g. [Pd(P-Ph₃)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 Hock 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 Hack reaction in general are discussed.