Palladacycles: Efficient New Catalysts for the Heck Vinylation of Aryl Halides

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. The versatility of cyclometallated palladium phosphine complexes (right: X = OAc, Br; R = o‐tolyl, mesityl; R′ = H, Me) as single‐component catalyst precursors is demonstrated inthe Heck reaction. A comparison of several catalyst systems under optimized conditions reveals the unexpected efficiency of standard triphenylphosphine‐based catalysts and shows the superior activity of palladacycles in Heck reactions with donor‐ and acceptor‐substituted olefins.