INSERTION OF PALLADIUM(0) AND NICKEL(0) COMPLEXES INTO THE ALPHA-CARBON-BROMINE BONDS OF 2-BROMO-PHOSPHININE OR 2,6-DIBROMOPHOSPHININE - A VERSATILE ROUTE TO ALPHA-FUNCTIONALIZED PHOSPHININES

The reaction of 2,4,6-tribromo-3-methylphosphinine with a source of [P d(dppe)] [dppe = 1,2-bis(diphenylphosphino)ethane] leads exclusively t o the C-2-Pd(dppe)-Br insertion product whose geometry has been establ ished by X-ray crystal structure analysis. The Pd-C-2 bond has a norma l length at 2.072(6) Angstrom and the (dppe)Pd plane is orthogonal to the plane of the phosphinine ring, thus avoiding any steric hindrance from the methyl C-3-substituent. AM1 semi-empirical calculations sugge st that this selective palladium insertion into the C-2-Br bond result s from the initial formation of a Br-C-2-P-Pd chelate which is calcula ted to be more stable than the Br-C-6-P-Pd chelate by 1.3 kcal mol(-1) . Several new cross-coupling reactions at the C-2 position of 2,6-dibr omophosphinines have been devised using this selective palladium inser tion as the catalytic step. They include the arylation by arylzinc der ivatives, the synthesis of 2,2'-biphosphinines by reaction with 2-stan nylphosphinines, and the thiolation by tin thiolates. Various 2,6-difu nctional phosphinines with two different functionalities (PhC = C-, 2- furyl, 2-pyrrolyl, BuS-, Ph(2)P-) have also been synthesized. Prelimin ary experiments indicate that [Ni(dppe)] is also able to activate the C-2-Br bond of 2-bromophosphinines. Several 2,2'-biphosphinine chelate s of Ni(0), Cr(0), Mo(0), and W(0) have been thus prepared.