NC-palladacycles as highly effective cheap precursors for the phosphine-free Heck reactions

Eight cyclopalladated complexes of the formula [Pd2(μ-L)2(NC)2] (L=OAc, Cl; NC=cyclometalled N donor: o-(2-pyridyl)phenyl, o-(2-pyridyloxy)phenyl, o-(2-pyridylmethyl)phenyl, o-(N,N-dimethylaminomethyl)phenyl, 8-quinolylmethyl and others) and a six-membered palladacycle with OC coordination (ligand related to 2-acetoamido-4-nitrophenyl), are highly efficient catalysts for the Heck arylation of olefins (styrene, ethyl acrylate) by aryl halides (iodobenzene, bromobenzene, 4-bromoacetophenone). These catalysts are air stable, easy to obtain from a vast number of readily available nitrogen containing molecules, are generally much cheaper than phosphine-ligated palladacycles, but as or more efficient than the latter. Turnover numbers (ton) of up to 4 100 000 and turnover frequencies (tof) up to 530 000 are achieved in the reaction of iodobenzene with ethyl acrylate. Bromobenzene undergoes the Heck reaction (ton=400–700; tof=5–30) in the presence of the promoter additive Bu4NBr. The palladacycles are likely to operate in a common phosphine-free Pd(0)/Pd(II) catalytic cycle, while the differences between various types of palladacycle precursors are accounted for by the kinetics of the catalyst preactivation step.