Use of ionic liquids to improve whole-cell biocatalytic asymmetric reduction of acetyltrimethylsilane for efficient synthesis of enantiopure (S)-1-trimethylsilylethanol

Abstract

The two typical ionic liquids (ILs), one hydrophobic (BMIM·PF₆) and one hydrophilic (BMIM·BF₄), were tested as solvents for use in the asymmetric reduction of acetyltrimethylsilane (ATMS) to enantiopure (S)-1-trimethylsilylethanol {(S)-1-TMSE} catalyzed by immobilized Saccharomyces cerevisiae cells. The results demonstrate that BMIM·PF₆ and BMIM·BF₄ can markedly boost the activity and the stability of the immobilized cells. To better understand the reaction performed in these IL-containing systems, various variables that influenced the performance of the reaction were examined. The optimal buffer pH, reaction temperature and substrate concentration were 7.3, 30 °C and 84 mM, respectively, for the BMIM·PF₆/buffer (1/6, v/v) biphasic system, and 7.5, 30 °C and 77 mM, respectively, for the 10% (v/v) BMIM·BF₄–buffer co-solvent system. Under the optimal conditions, the initial reaction rate, maximum yield and product e.e. were 63.4 mM h⁻¹, 99.9% and >99.9% with the former system, while those with the latter system were 74.5 mM h⁻¹, 99.2% and >99.9%, respectively, which were much higher than those achieved with either n-hexane/buffer (2/1, v/v) biphasic system or aqueous buffer. It was also found that the optimal pH and substrate concentration changed when n-hexane was replaced by BMIM·PF₆ or BMIM·BF₄. Although the optimal reaction temperature remained the same in the four kinds of reaction systems, the temperature profile of the reaction varied from case to case. Additionally, BMIM·BF₄ and especially BMIM·PF₆ exhibited greater biocompatibility with Saccharomyces cerevisiae than n-hexane, and could be used repeatedly for economically interesting whole-cell biocatalytic processes with in situ coenzyme regeneration.