Preparation of carbonyl rhodium polyether guanidinium ionic liquids and application in asymmetric hydroformylation based on homogeneous catalysis-biphasic separation system

Abstract

Four carbonyl rhodium polyether guanidinium ionic liquids (Rh(CO)4-PolyGILs) were firstly synthesized by utilizing the ion exchange strategy. This protocol was achieved by introducing polyether guanidinium ionic liquids (PolyGILs) into K[Rh(CO)4]. Asymmetric hydroformylation (AHF) of styrene was employed as probe reaction to investigate the catalytic performance of the above obtained four functional ionic liquids. The AHF of styrene provided 79.00% conversion, 73.30% yield and 54% enantiomeric excess for 2-phenylpropionaldehyde ([Me(EO)16TMG][Rh(CO)4] served as catalyst, (R)-BINAP served as chiral ligand at 60°C and 2.0 MPa for 4 h). In addition, based on the unique solubility properties of the above obtained four ionic liquids, a highly effective homogeneous catalysis-biphasic separation (HCBS) system was established for the AHF. HCBS system with [Me(EO)16TMG][Rh[(R)-BINAP](CO)2] could be reused four times without significantly decrease in the activity. The structure of [Me(EO)16TMG][Rh[(R)-BINAP](CO)2] was further studied by using the density functional theory (DFT) method, which revealed that the final energy of the molecule was tending to -9460.83 eV and the presence of Ph substituent provided a chiral pocket. The presence of closed rigid chiral pocket could lead to better enantioselectivities.