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

Abstract

Two different homogeneous catalyst precursor complexes, i.e. [Rh(COD) 2 ]BF 4 and [Rh(COD)Cl] 2 , were immobilized on phosphotungstic acid-modified alumina to form γ-Al 2 O 3 /PTA/ Rh(COD) 2 BF 4 (1b) and γ-Al 2 O 3 /PTA/[Rh(COD)Cl] 2 (2b), respectively. These immobilized complexes have been modified with (R,R)-MeDuPHOS to form the immobilized chiral catalysts γ-Al 2 O 3 /PTA/Rh((R,R)-MeDuPHOS)(COD)BF 4 (1c) and γ-Al 2 O 3 / 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)]BF 4 (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 H 2 pressure the activity and selectivity of 1c are comparable to those of its homogeneous analogue. Complex 1b was also used to prepare γ-Al 2 O 3 /PTA/Rh(DiPFc)-(COD)BF 4 (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.

Immobilization Techniques

“…Thus, these systems are probably analogous to the SHB catalysts, which are described above. Researchers at Engelhard Corporation have actively investigated this new type of immobilized catalysts in cooperation with Chirotech Technology Ltd. [32][33][34][35], while Johnson Matthey -another catalyst producer -has obtained a license for this Cataxa technology from Seton Hall University [36]. 42.…”

Recyclable Stereoselective Catalysts

“…The chiral ligands ( R ) -2,2 ′ -bis(diphenylphosphino) -1,1 ′ -binaphthyl (BINAP) [15] , ( R,R ) -Me -DuPhos [16] and (2 S ,4 S ) -BDPP [17] , and carbon -activated [18] supports were studied under Augustine et al ' s immobilization methodology [19] . Generally, the heterogeneous catalysts were found to be less active than their homogeneous partners under batchwise conditions but provided comparable levels of enantioselectivities (generally above 90% ee).…”

Immobilized Complexes for Enantioselective Catalysis: When Will They Be Used in Industry?

“…However, for larger multifunctional substrates entrapment is not feasible since the required small pores will usually not allow easy access to the catalyst. For these reasons, the adsorption approach will be the method of choice since it can be applied with unmodified commercial ligands and last but not least since there are already several promising examples described in the literature (See e.g., [24][25][26]). …”