Regioselectivity in aqueous palladium catalysed hydroxycarbonylation of styrene: a catalytic and mechanistic study

Regioselectivity control was studied in palladium catalysed hydroxycarbonylation of styrene in neat water with water-soluble phosphines, mostly trisulfonated triphenylphosphine, TPPTS, but also N-bis(N', N'-diethyl-2-aminoethyl)-4-aminomethylphenyl-diphenylphosphine, N3P. The factor giving the highest changes in regioselectivity in the TPPTS system, under similar reaction conditions, is the temperature. In the N3P case, only a minor variation in the n/i ratio as a function of temperature is observed. In situ normal- and high-pressure NMR experiments were performed to obtain further information about the catalytic cycle and the reaction intermediates. Two palladium hydride intermediates, a palladium eta(3)-benzylic complex and both the branched and linear palladium acyl complexes were identified in the HP NMR experiments. The hydroxycarbonylation in water using styrene as a substrate operates using a hydride mechanism for pathways leading to both linear and branched product. Insertion of styrene in the palladium - hydride bond gives an eta(3)-benzyl compound. A high CO pressure gives a kinetic preference for the iso-acyl in the next step. In the TPPTS system, at moderate temperatures, the hydrolysis of the iso-acyl is faster than its conversion to the thermodynamically more stable n-acyl. A low n/i therefore requires high pressures and reasonably low temperatures. The N3P ligand always favours the linear product since isomerisation of the iso-acyl to the n-acyl in this system is fast under all conditions investigated.