Thermal Decomposition of Triphenylphosphonium Alkyl Ester Salts

Abstract

In thermolyses of molten triphenylphosphonium alkyl ester bromides and chlorides, alkyl = methyl, ethyl, isopropyl, at 130 and 225°C, initial attack of the halide ion on the methyl group gives the methyl halide and ylid 1 , Ph 3 P = CH 2 , which can be methylated, or is protonated by the phosphonium salt with transylidation giving Ph 3 P+-CH 3 X−, X = Br, Cl. The initial reactions of the ethyl or isopropyl esters are with the halide ion, X−, as a base giving ylid, 1 , which can be protonated by HX or by transylidation. The t-butyl ester generates Ph3P+-CH 3 X−but no products of transylidation. The first-formed ylid 1 , can be trapped by reactive alkyl and acyl halides, and the transient ylidic esters decompose thermally to triphenyl phosphine oxide, Ph 3 P = O, react further with unreacted phosphonium ester, or are trapped by added aldehyde in a Wittig reaction. The final product compositions are affected by a decrease in pressure, due to escape of volatile intermediates, and by replacement of the X− halide ion by the less nucleophilic and basic tosylate ion. Reactions under reflux, in solution in chloroform, or in suspension in benzene, are similar to those of the molten salts, but yields are generally lower at the lower temperatures.