Organophosphorus compounds. IV. The reaction of Alk-1-ynes with phosphorus pentachloride, and a new route to Alka-1,2-dienes

The addition of phosphorus pentachloride to four representative linear alk-1-ynes is accompanied by an allylic shift of a hydrogen atom. Hydrolysis of the resulting complexes yields 2-chloroalk-2-enylphosphonic acids rather than 2-chloroalk-1-enylphosphonic acids as reported in the literature. The 2-chloroalk-2-enylphosphonic acids undergo fragmentation in hot alkali yielding 1,2-alkadienes, chloride, and phosphate ions. The sequence alk-1-yne ? 2-chloroalk-2-enylphosphonic acid ? l,2-alkadiene constitutes a new and mild method for effecting this isomerization.