Application of Immobilized Rhodium Catalyst Precursors in Enantio- and Chemoselective Hydrogenation Reactions

Abstract

Two different homogeneous catalyst precursor complexes, i.e. [Rh(COD)2]BF4 and [Rh(COD)Cl]2, were immobilized on phosphotungstic acid-modified alumina to form γ-Al2O3/PTA/Rh(COD)2BF4 (1b) and γ-Al2O3/PTA/[Rh(COD)Cl]2 (2b), respectively. These immobilized complexes have been modified with (R,R)-MeDuPHOS to form the immobilized chiral catalysts γ-Al2O3/PTA/Rh((R,R)-MeDuPHOS)(COD)BF4 (1c) and γ-Al2O3/PTA/Rh((R,R)-MeDuPHOS)(COD)Cl (2c). It is shown that immobilization and subsequent modification by ligand exchange reactions of general precursor complexes is a powerful method to prepare chiral and achiral anchored rhodium catalysts. Enantioselective hydrogenation reactions show that the activity and selectivity differences between the homogeneous catalysts [((R,R)-MeDuPHOS)Rh(COD)]BF4 (1a) and the in situ prepared [((R,R)-MeDuPHOS)Rh(COD)]Cl (2a) are larger than the differences between the immobilized analogues (1c, 2c). At elevated temperature and H2 pressure the activity and selectivity of 1c are comparable to those of its homogeneous analogue. Complex 1b was also used to prepare γ-Al2O3/PTA/Rh(DiPFc)(COD)BF4 (1d) via a ligand exchange reaction with 1,1‘-bis(diisopropylphosphino)ferrocene (DiPFc). This complex was used as a selective and heterogeneous hydrogenation catalyst with special chemoselective hydrogenation properties. The described immobilized rhodium catalysts prepared from the general precursor complexes 1b and 2b display hydrogenation activity and selectivity comparable to those of their homogeneous analogues. Moreover, it is demonstrated that these catalysts can be reused multiple times with neither activity nor selectivity loss and that leaching can be minimized by using optimized reaction parameters.