Asymmetric hydrolytic kinetic resolution with recyclable polymeric Co(iii)–salen complexes: a practical strategy in the preparation of (S)-metoprolol, (S)-toliprolol and (S)-alprenolol: computational rationale for enantioselectivity

Abstract

A series of chiral polymeric Co(III)–salen complexes based on a number of achiral and chiral linkers were synthesized and their catalytic performances were assessed in the asymmetric hydrolytic kinetic resolution of terminal epoxides. The effects of the linker were judiciously studied and it was found that in the case of the chiral BINOL-based polymeric salen complex 1, there was an enrichment in catalyst reactivity and enantioselectivity of the unreacted epoxide, particularly in the case of short as well as long chain aliphatic epoxides. Good isolated yields of the unreacted epoxide (up to 46% compared to 50% theoretical yield) along with high enantioselectivity (up to 99%) were obtained in most cases using catalyst 1. Further studies showed that catalyst 1 could retain its catalytic activity for six cycles under the present reaction conditions without any significant loss in activity or enantioselectivity. To show the practical applicability of the above synthesized catalyst we have synthesised some potent chiral β-blockers in moderate yield and high enantioselectivity using complex 1. The DFT (M06-L/6-31+G**//ONIOM(B3LYP/6-31G*:STO-3G)) calculations revealed that the chiral BINOL linker influences the enantioselectivity achieved with Co(III)–salen complexes. Further, the transition state calculations show that the R-BINOL linker with the (S,S)-Co(III)–salen complex is energetically preferred over the corresponding S-BINOL linker with the (S,S)-Co(III)–salen complex for the HKR of 1,2-epoxyhexane. The role of non-covalent C–H⋯π interactions and steric effects has been discussed to control the HKR reaction of 1,2-epoxyhexane.