Structural characterisation in solution of intermediates in rhodium-catalysed hydroformylation and their interconversion pathways

Abstract

The reaction of HRh(CO)(PPh₃)₃(1) with CO has been studied by ¹H, ¹³C, and ³¹P n.m.r. The main species present under ambient conditions is HRh(CO)₂(PPh₃)₂(2) which exists as two rapidly equilibrating trigonal bipyramidal isomers. Complexes (1) and (2) are in rapid equilibrium via CO and PPh₃ dissociation steps and the square-planar complexes HRh(CO)(PPh₃)₂(3) and HRh(CO)₂PPh₃(4) are likely transient intermediates. The chemistry of these PPh₃ complexes is compared with that of closely related 5-phenyl-5H-dibenzophosphole and 1,3-bis(diphenylphosphino)propane analogues. Complex (1) catalyses the isomerisation of (Z)-[1,2-²H₂]styrene, effectively suppressed by CO or PPh₃. HRh(CO)₂P₂ complexes trap methylenecyclopropane. In the presence of styrene and CO, complex (1) is converted into a branched acyl derivative which readily equilibrates with its linear isomer; the stereochemistry of these acyl derivatives (CO)₂(PPh₃)₂RhCOR is determined by low-temperature n.m.r.; at higher temperatures rapid inter- and intra-molecular exchange processes occur. The relevance of these observations to rhodium-catalysed hydroformylation is discussed and it is proposed that the regiochemistry of reaction is largely controlled by competitive olefin trapping involving complexes (3) and (4).