The reactivity of organophosphorus compounds. Part 31. On the formation of 1,4-diphenylnaphthalene from o-bromophenol and 1,2,5-triphenylphosphole, and related reactions

Abstract

Attempted synthesis of the 1,2,5-triphenylphospholium betaine (2)via thermolysis at 250 °C of o-bromophenol in the presence of 1,2,5-triphenylphosphole failed, the reaction giving instead bromobenzene (8%), diphenyl phenylphosphonate [(PhO)₂P(O)Ph, 7.5%] copious amounts of hydrogen bromide, and 1,4-diphenylnaphthalene (34%). The last-named has been shown to arise via intramolecular rearrangement of 1,2,5-triphenylphosphole, catalysed by hydrogen bromide produced by the thermolysis of o-bromophenol. Diphenyl phenylphosphonate is also formed on thermolysis of 1-phenoxy-1,2,5-triphenylphospholium bromide (8), which may therefore be an intermediate in the reaction of o-bromophenol with 1,2,5-triphenylphosphole. 1-Phenylnaphthalene is also formed in the former reaction and in the hydrogen bromide-catalysed decomposition of 1,2,5-triphenylphosphole oxide. These reactions are interpreted in terms of ring expansion of strained phospholium species.