A Catalytic and Mechanistic Investigation of Palladium-Phosphine Catalysed Aqueous Hydroxycarbonylation. Steric and Electronic Properties of the Ligands

The steric and electronic properties of eight water-soluble phosphines (1-8) were investigated by different methods. Phosphines 1-6 were tested as ligands in palladium catalysed hydroxycarbonylation of styrene. In line with the proposal that a ligand with low basicity should facilitate nucleophilic attack of water on the acyl complex, the electron rich {4 - [Bis(2 - diethylaminoethyl)] - aminobenzyl} diphenylphosphine, N3P, (1), has lower catalytic activity than the electron poor tris(3-sodiumsulfonatophenyl)phosphine, TPPTS, (4). N3P derivates {4-[Bis(2-diethylaminoethyl)] aminobenzyl}- Bis(4-methoxyphenyl)phosphine (2) and {4-[Bis(2-diethylaminoethyl)] aminobenzyl}-Bis(2-methylphenyl)phosphine (3), which are more electron rich than N3P, show an even lower activity. The extremely bulky TPPTS derivatives tris(2-methoxy-3-sodiumsulfonatophenyl)phosphine (5) and tris(2-methyl-3-sodiumsulfonatophenyl) phosphine (6) are inactive in the hydroxycarbonylation of styrene.

Mechanistic investigations employing N3P (1) and TPPTS (4) as ligands and styrene as substrate were performed and a range of reaction intermediates were identified by NMR spectroscopy. A high CO pressure gives a kinetic preference for the iso-acyl and at moderate temperatures the hydrolysis of this 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 in this system is fast even at low temperature.

The hydroxycarbonylation of ?-hydroxyolefins using ligands 1 and 4 was investigated and the main reaction in this case was the isomerisation of the substrate to the corresponding aldehyde. The alkoxycarbonylation products - the lactones - are formed via an intramolecular nucleophilic attack, where the hydroxyl group coordinates first to the palladium. Mechanistic investigations by NMR and IR, employing 3-buten-1-ol as substrate, led to the identification of the chelated alkyl complex with both ligands and the iso-acyl complex with N3P. The aldehyde is formed via a chain walking process. The 2,1-insertion is strongly favoured over 1,2-insertion in the TPPTS system, while for N3P, this preference is less pronounced.