ABSTRACT
Heterogenization of the asynunetric hydrogenation catalyst [(R,R)-Me-(DuPHOS)Rh(COD)]OTf on the mesoporous silica MCM-41 is reported. Immobilization is accomplished by slurrying the organometallic reagent with MCM-41 in methylene chloride. Weak interactions immobilize the catalyst on the mesoporous surface which allows the catalyst to be recovered and recycled by simple filtration. As evidenced by XRD, pore volume, surface area, elemental analysis, TGA and NMR, hydrogen bonding between the triflate anion and SiO-H groups on the silica surface fixes [(R,R)-Me-(DuPHOS)Rh(COD)]+ inside the pores of MCM-41. Deactivation due to mesopore fouling by the product of hydrogenation occurs after several cycles, and is avoided by decanting the solvent product mixture from the catalyst followed re-suspension in fresh solvent. Enantioselectivities and relative rates for the reaction of several enarnide esters in hexane with the heterogeneous [(R,R)-Me-(DuPHOS)Rh(COD)]OTf/MCM-41 material show it to be a rare example of a heterogenized catalyst which is as, and in some cases more, active and seleative as the homogenous catalyst under optimized conditions.
