One-pot synthesis of triphenylphosphine oxide derivatives

Protonation of the highly reactive 1:1 intermediates (3), produced in the reaction between tri(o-tolyl)phosphine (1) and alkyl acetylenecarboxylates (2) by water leads to vinyltriphenylphophonium salts (4), which undergo coupling reaction with conjugate base to produce corresponding pentavalent phosphorus intermediates (5). The pentavalent phosphorus intermediates (5) are unstable and fragment to tri(o-tolyl)phosphine oxide (6) and alkenes (7) under reaction conditions.