Heterolytic Splitting of Allylic Alcohols with Palladium(0)−TPPTS in Water. Stabilities of the Allylphosphonium Salt of TPPTS and of the Ionic Complex [Pd(η3-allyl)(TPPTS)2]+

Abstract

The Pd(TPPTS)3 complex (TPPTS is the sodium salt of tris(m-sulfophenyl)phosphine) easily ionizes allyl alcohol in water over a wide range of pH: OH− and TPPTS are released, and [Pd(η3-allyl)(TPPTS)2]+ is formed. The released TPPTS further reacts with the palladium cationic complex to reversibly produce both the allylphosphonium salt of TPPTS [(allyl)Ar3P]+ and Pd(TPPTS)3, the latter acting as the catalyst of the allylation of TPPTS by allyl alcohol. Primary allylic alcohols, such as butenol (trans-2-buten-1-ol), prenol (3-methyl-2-buten-1-ol), geraniol, and cinnamyl alcohol, react with Pd(TPPTS)3 to produce hydroxide ion, the corresponding hydrosoluble cationic palladium complex, and allylic phosphonium salts. At room temperature, [Pd(η3-allyl)(TPPTS)2]+ is stable up to pH 12, but beyond this value, palladium precipitates. The temperature has an adverse effect on the complex stability: palladium precipitates at 80 °C, even at pH 7, with the formation of a small amount of propylene. The addition of [(allyl)Ar3P]+ increases the stability of [Pd(η3-allyl)(TPPTS)2]+. Above pH 10, [(allyl)Ar3P]+ decomposes into OTPPTS and propylene by reaction with OH−. At lower pH, [(allyl)Ar3P]+ is slowly isomerized into [(propenyl)Ar3P]+, which further reduces its stability toward pH and temperature. These consecutive reactions of the TPPTS ligand could explain most of the catalyst instability. This study outlines the basis for a better understanding of the instability phenomenon of the catalytic system Pd(0)−TPPTS in reactions with allylic intermediates, e.g. the Tsuji−Trost reaction, and in the reaction of dienes in aqueous media in which palladium often precipitates.