Subtilisin-Catalyzed Transesterifications in the Presence of Iron Oxide Nanoparticles in Organic Solvent: Dramatic Catalytic Improvement

Abstract

Enzymatic transformations in organic media provide a useful methodology for the synthesis of a wide range of enantiomeric compounds. 1 In spite of remarkable advances in this field, the enzymes still suffer from a limitation that they display significantly reduced activities in organic media, which are often several orders of magnitude lower than aqueous counterparts. In the past decades, intensive studies have been done to develop methods for enhancing enzymatic activity in organic media. A useful approach is the lyophilization of enzyme in the presence of excipients such as lyoprotectants, polymers, and salts. 2 For example, subtilisin lyophilized in the presence of KCl salts (98%, W/ W) displayed three orders of magnitude higher activity than its salt-free counterpart in organic solvent. 3 The salt– activated subtilisin as the catalyst for synthetic applications, however, has some disadvantages that a relatively large volume of enzyme–salt mixture should be used and its recovery for reuse is troublesome. We herein wish to report a practical approach using iron oxide nanoparticles (IONs) as the additive for high activity and easy recovery of subtilisin. Subtilisin Carlsberg (SC, Bacillus licheniformis) displays (S)–enantioselectivity toward simple secondary alcohols, 4 which is complementary to the (R)-enantioselectivity of lipase. Recently, our group and others demonstrated that SC has great potential as the catalyst for the dynamic kinetic resolution (DKR) of secondary alcohols 5a,b and primary amines. 5c DKR is a powerful tool for the transformations of racemates to single enantiomers because it can provide high yields and excellent enantiopurities, both approaching 100%. For its wider applications in DKR, SC should be readily activated and recovered. During our testing of IONs as a magnetically recoverable supporter for SC, 6