Rhodium(I) N-Heterocyclic Carbene Complexes as Highly Selective Catalysts for 1-Hexene Hydroformylation

Rhodium(I) carbene complexes of the type [Rh(NHC)(cod)X] (where NHC = N-heterocyclic carbene obtained from dialkylimidazolium cation; cod = 1,5-cyclooctadiene; X = Cl, Br, I, SCN) with P(OPh)3 as a modifying ligand have been found to be very active catalysts for 1-hexene hydroformylation under mild conditions (80 °C, 10 atm of H2/CO, 4 h). Even at a low rhodium concentration ([1-hexene]/[Rh] = 2000), aldehydes have been obtained in yields of up to 90% with a high n/iso ratio of ca. 7. During the catalytic process, rhodium(I) carbene complexes, [Rh(NHC)(P(OPh)3)2X], reacted with H2/CO, giving a catalytically active rhodium(I) hydrido complex containing an N-heterocyclic carbene ligand. The presence of [HRh(CO)(P(OPh)3)3] and [Rh(NHC)(P(OPh)3)(CO)X] complexes in the catalytic system has been confirmed. When P(OCH2CF3)3 was used as modifying ligand with [Rh(NHC)(cod)Br] as catalyst precursor, formation of [HRh(CO)(NHC)(P(OCH2CF3)3)2] with a square-pyramidal structure was evidenced by 1H and 31P NMR.