Quasiphosphonium intermediates. Part 3. Preparation, structure, and reactivity of alkoxyphosphonium halides in the reactions of neopentyl diphenylphosphinite, dineopentyl phenylphosphonite, and trineopentyl phosphite with halogenomethanes and the effect of phenoxy-substituents on the mechanism of alkyl–oxygen fission in Michaelis–Arbuzov reactions

Abstract

The reactions of neopentyl diphenylphosphinite with chloro-, bromo-, or iodo-methane and of dineopentyl phenylphosphonite with bromo- or iodo-methane yield crystalline alkoxyphosphonium halides. In deuteriochloroform these intermediates decompose by a first-order process which involves rate-determining collapse of the phosphonium halide ion-pair with S_N2-type fission of the alkyl–oxygen bond. Rates for chloride, bromide, and iodide are similar. In a more ionising medium (deuterioacetonitrile) dissociation leads to stabilisation of the intermediates and to deviation from first-order decomposition. Previously determined X-ray diffraction data for the bromides, together with relative rates of decomposition in deuteriochloroform for intermediates in the series Ph_n(RO)_3–n[graphic omitted]MeX^–(n= 0, 1, or 2) suggest that the stability and reactivity of alkoxyphosphonium intermediates are determined largely by inductive rather than mesomeric effects of ligands. The presence of phenoxy-substituents on phosphorus may cause a tendency towards S_N1-type fission of the alkyl–oxygen bond in certain circumstances.