Bridging Group Effects in Chelating Bis(2,5-diphenylphospholane) Ligands for Rhodium-Catalyzed Asymmetric Hydroformylation

A series of seven bis(2,5-diphenylphospholane) ligands has been evaluated for utility in rhodium-catalyzed asymmetric hydroformylation. The ligands differ in the nature of the moiety that bridges the two phospholane rings. Hydroformylation of styrene, vinyl acetate, and allyl cyanide was performed in tandem using parallel pressure vessels. Significant differences in rate, regioselectivity, and enantioselectivity were observed between catalysts. Ligands with two-carbon bridges exhibited selectivities comparable to the ethylene-bridged (S,S)-Ph-BPE. Electron-deficient heterocyclic bridges (pyrazine and quinoxaline) gave increased rates over (S,S)-Ph-BPE. Bis(2,5-diphenylphospholane) ligands with -CH2-, -CH2CH2CH2-, and 1,1'-ferrocenyl bridges led to significantly lower selectivities and rates than (S,S)-Ph-BPE. X-ray crystal structures of [(SS`)-Ph-Quinoxaline]Rh(acac), [(R,R)-Ph-BPM]Rh(acac), and [(SS)-Ph-5-Fc]Rh(acac) are reported. The P-Rh-P bite angles in these complexes are 87.46(3)degrees, 74.10(5)degrees, and 99.06(3)degrees, respectively. As with asymmetric olefin hydrogenation using bis-phospholane ligands, the maximum enantioselectivity in asymmetric hydroformylation was found with bis-phospholanes that adopt P-Rh-P bite angles near 85 degrees.