Thermal Decomposition of Triphenylphosphonium Alkyl Ester Salts Fernando Casta ˜ neda, 1 Christian Aliaga, 1 Cristina Acu ˜ na, 1 Paul Silva, 1 and Clifford A. Bunton 2 1 University of Chile, Santiago, Chile 2 University of California, Santa Barbara, California, USA 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 Ph 3 P + -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. Keywords Carboalkoxy phosphonium salts; thermolyses; solvent free INTRODUCTION Thermolyses of carboxylic and xanthate esters give the alkene through a spontaneous syn elimination. 1,2 The phosphonium salts, R 3 P + -CH 2 CO 2 R ′ X − , decompose at temperatures between 130 and 225 ◦ C to give the alkene derived from R ′ . 3−5 The course of these and sim- ilar reactions is understandable in terms of mechanistic treatments of reactions in solution. The corresponding reactions of carboalkoxy phos- phonium salts, shown as triphenyl derivatives, are complex, because the Thanks are due to the CEPEDEQ of the Faculty of Chemical and Pharmaceutical Sciences, University of Chile, for instrumental facilities. Address correspondence to Fernando Casta ˜ neda, Departamento de Qu´ ımica Org ´ anica y Fisicoqu´ ımica, Facultad de Ciencias Qu´ ımicas y Farmac´ euticas, Universidad de Chile, Casilla 233, Santiago, Chile. E-mail: fcastane@ciq.uchile.cl 1188