A stable bromoalkoxyphosphorane, a model for the postulated five-co-ordinate intermediate in the Arbuzov and related reactions

Abstract

A stable model for the postulated intermediate in Arbuzov and related reactions, bromoalkoxyphosphorane (6), is obtained from the reaction of bromine and the phosphine ether (8). The analogous phosphine (13), however, gives benzyl bromide and the ionic alkoxyphosphonium bromide (17). The latter when heated produces a further equivalent of benzyl bromide and a new dioxyphosphorane (18). The phosphorane (18) is also formed upon spontaneous cyclodehydration of the product obtained in the reaction of methylmagnesium bromide and the diester (19). The methiodide (21) shows chemical shift nonequivalence of every benzylic methyl and benzylic proton. Rotation around the appropriate C–P bonds is presumed to be strictly hindered and proceeds with ΔG^‡ of ca.18 kcal/mol at 80 °C. Surprisingly, bromoalkoxyphosphorane (6) is not very sensitive to water and is also prepared from the phosphine oxide (10) and 48% HBr.