Formate-mediated reductive cross-couplings
of vinyl halides with
aryl iodides via palladium(I) catalysis occur with highly uncommon cine-substitution. The active dianionic palladium(I) catalyst,
[Pd2I4][NBu4]2, is generated in situ from Pd(OAc)2, Bu4NI, and
formate. Oxidative addition of aryl iodide followed by dissociation
of the dimer provides the monomeric anionic T-shaped arylpalladium(II)
species, [Pd(Ar)(I)2(NBu4)], which, upon vinyl
halide carbopalladation, forms products of cine-substitution
by way of palladium(IV) carbenes, as corroborated by deuterium-labeling
experiments.