Rhodium Carbene Complexes:  Highly Selective Catalysts for the Hydroformylation of Styrene Derivatives

Two new carbene−rhodium complexes are reported that are active hydroformylation catalysts, giving very high selectivities for the branched isomer (>95:5) when vinyl arenes are used as substrates. The carbene analogue of Wilkinson's catalyst, [RhIMes(PPh 3 ) 2 Cl] (IMes = 1,3-bis(2,4,6-trimethylphenyl)-imidazol-2-ylidene), and the related carbonyl complex [RhIMes(PPh 3 )(CO)Cl] were both prepared, and the latter was fully characterized using spectroscopic and crystallographic means.