Spiroketal-Based Phosphorus Ligands for Highly Regioselective Hydroformylation of Terminal and Internal Olefins

A new class of bidentate phosphoramidite ligands, based on a spiroketal backbone, has been developed for the rhodium-catalyzed hydroformylation reactions. A range of short- and long-chain olefins, were found amenable to the protocol, affording high catalytic activity and excellent regioselectivity for the linear aldehydes. Under the optimized reaction conditions, a turnover number (TON) of up to 2.3x104 and linear to branched ratio (l/b) of up to 174.4 were obtained in the RhI-catalyzed hydroformylation of terminal olefins. Remarkably, the catalysts were also found to be efficient in the isomerizationhydroformylation of some internal olefins, to regioselectively afford the linear aldehydes with TON values of up to 2.0x104 and l/b ratios in the range of 23.430.6. X-ray crystallographic analysis revealed the cis coordination of the ligand in the precatalyst [Rh(3d)(acac)], whereas NMR and IR studies on the catalytically active hydride complex [HRh(CO)(2)(3d)] suggested an eqeq coordination of the ligand in the species.