Phosphorus lone pairs stabilization of carbocations: the synthesis and dynamics of unsymmetrical methylene phosphonium ions

Halide abstractions from P-chlorinated phosphorus ylides 9a–c either by AlCl3 or SnCl2 yield unsymmetrical methylene phosphonium ions 10a–c in which energy barriers > 83 kJ mol–1 for the rotation around the PC have been estimated by NMR techniques. The salts with AlCl4– counteranions are stable, but compounds with SnCl3– anions decompose stereoselectively to unsymmetrical methylene phosphanes, SnCl2, and ButCl.