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.