Science of Synthesis

Schobert, R.; Gordon, G. J., Science of Synthesis, (2004) 27, 1000.

Treatment of trimethylphosphine with (chloromethyl)(trimethyl)silane generates a silylated phosphonium salt ‌51‌, which at a temperature of 180°C gives [(trimethylsilyl)methylene]trimethylphosphorane ‌52‌ (‌Scheme 18‌); this product can be obtained salt-free and in excellent yield by distillation under reduced pressure.[‌342‌] Mechanistically, the chloride ion of the salt ‌51‌ is thought to attack the C—Si bond of the counter cation at the elevated temperature to generate chlorotrimethylsilane and (methylene)trimethylphosphorane. The latter acts as a base, undergoing transylidation with the remaining silylated phosphonium salt to give the ylide ‌52‌ and tetramethylphosphonium chloride. A number of silyl ylides have been prepared by this method,[‌28‌] and the intermediate methylenephosphoranes have been trapped with ketones.[‌343‌] In the latter approach a phosphine, chlorotrimethylsilane, and a ketone when heated together at 150°C first generate the corresponding methylenephosphorane and this then reacts with the ketone, before deprotonation of any residual salt can occur. Pyrolysis of (1-alkoxycarbonylalkyl)triphenylphosphonium salts ‌53‌[‌343‌] results in a similar attack of the halide ion on the C—O bond. This causes the expulsion of an alkyl halide and carbon dioxide from the salt, leaving an alkylphosphorane ‌54‌ (‌Scheme 18‌).

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References

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• 8.Meeeee-Meee, (8888) M 8, 888.
• 8.Meeeee-Meee, (8888) M 8-8, 8888.