Insertion of Palladium(0) and Nickel(0) Complexes into the α-Carbon-Bromine Bonds of 2-Bromo- or 2,6-Dibromophosphinines: A Versatile Route to α-Functionalised Phosphinines

The reaction of 2,4,6-tribromo-3-methylphosphinine with a source of [Pd(dppe)] [dppe = 1,2-bis(diphenylphosphino)ethane] leads exclusively to the C₂-Pd(dppe)-Br insertion products whose geometry has been established by X-ray crystal structure analysis. The Pd-C₂ bond has a normal length at 2.072(6) Å and the (dppe)Pd plane is orthogonal to the plane of the phosphinine ring, thus avoiding any steric hindrance from the methyl C₃-substituent. AM1 semi-empirical calculations suggest that this selective palladium insertion into the C₂-Br bond results from the initial formation of a Br-C₂-P-Pd chelate which is calculated to be more stable than the Br-C₆-P-Pd chelate by 1.3 kcal mol^-1. Several new cross-coupling reactions at the C₂ position of 2,6-dibromophosphinines have been devised using this selective palladium insertion as the catalytic step. They include the arylation by arylzinc derivatives, the synthesis of 2,2’-biphosphinines by reaction with 2-stannylphosphinines, and the thiolation by tin thiolates. Various 2,6-difunctional phosphinines with two different functionalities (PhC ≡ C-, 2-furyl, 2-pyrrolyl, BuS-, Ph₂P-) have also been synthesized. Preliminary experiments indicate that [Ni(dppe)] is also able to activate the C₂-Br bond of 2-bromophosphinines. Several 2,2’-biphosphinine chelates of Ni(0), Cr(0), Mo(0), and W(0) have been thus prepared.

phosphinines - palladium - nickel - cross coupling - regioselectivity