Synthesis and reactions of 2,2′-biphenylylenebisdiethylphosphine. Formation of cyclic diquaternary dibromides with alkylene dibromides, and the nature and probable mechanism of their thermal decomposition

Abstract

The synthesis of the above diphosphine is described. It forms cyclic diquaternary dibromides by union with the alkylene dibromides, Br·[CH₂]_n·Br, where n= 1–4, and a similar salt with o-xylylene dibromide. These salts, and the dimethiodide of the diphosphine, when heated, undergo a novel degradation to give 9-alkyl-9-phosphafluorenes. These reactions are compared with those shown by the analogous cyclic tetramethyl diarsonium dibromides, in which the dibromides formed from Br·[CH₂]_n·Br where n= 2–4 undergo a similar degradation to give the 9-alkyl-9-arsafluorenes, but the other members lose alkyl bromide with retention of the heterocyclic ring as a ditertiary arsine. The strictly analogous tetraethyl diarsonium dibromides undergo thermal decomposition with degradation of the heterocyclic ring. It is suggested that the ring contraction reactions shown by all these series of salts when heated are examples of a general internal nucleophilic displacement mechanism; other examples of this are discussed.

Alkaline hydrolysis of certain of the above heterocyclic phosphonium salts occurs with ring-opening to give phosphine oxides.