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

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)-1,2-2H2styrene, 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).