Phosphonium ketene acetals, reactions

The sulphur atom of alkyl(thioalkyl)phosphonium salts forms a new onium centre on triethyloxonium tetrafluoroborate alkylation in nitromethane848,849 (thiocetals see above). Phosphonium ketene acetals are potential alkylating agents for phosphorus dithioic acid anions in non-aqueous, aprotic and aqueous media and in phase-transfer catalysis conditions296 (reaction 263). It is suggested that onium ketene acetals react by nucleophilic attack on the methyl group of the acetal. 

By 1989 Mukaiyama had already explored the behaviour of phosphonium salts as Lewis acid catalysts. It was possible to show that the aldol-type reaction of aldehydes or acetals with several nucleophiles and the Michael reaction of a,j3-unsatu-rated ketones or acetals with silyl nucleophiles gave the products in good yields with a phosphonium salt catalyst [116]. In addition, the same group applied bisphosphonium salts as shown in Scheme 45 in the synthesis of ]3-aminoesters [117]. High yields up to 98% were obtained in the reaction of A-benzylideneaniline and the ketene silyl acetal of methyl isobutyrate. Various analogues of the reaction parteers gave similar results. The bisphosphonium salt was found to be superior to Lewis acids like TiCl and SnCl, which are deactivated by the resulting amines. 

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