Coupled chemoenzymatic transfer hydrogenation catalysis for enantioselective reduction and oxidation reactions

Abstract

Stereoselective reductions of prochiral ketones were performed using a new thermophilic, NAD-dependent alcohol dehydrogenase from Thermus sp. (TADH). The enzyme was produced on 2L-scale from recombinant Escherichia coli and purified by a simple, one-step heat treatment procedure yielding 220 mg of pure enzyme. Regeneration of NADH was catalyzed by the organometallic complex [Cp*Rh(bpy)(H 2O)] 2+ using formate as a reducing agent. The catalytic performance of [Cp*Rh(bpy)(H 2O)] 2+ in terms of total number of catalytic cycles and number of catalytic cycles per hour achieved herein (up to 1500 and more than 400 h −1, respectively), are the highest reported for a non-enzymatic nicotinamide regeneration system so far. Chemoenzymatic reduction reactions in a two liquid phase setup were performed on a gramme-scale, for example, 1.3 g of enantiopure (1 S,3 S)-3-methylcyclohexanol was obtained after purification. The volumetric productivity reached up to 3.9 mM h −1 with an average of 2.6 mM h −1 (5.3 g L −1 d −1) over 10 h. In addition, chemoenzymatic oxidations utilizing the same catalyst set and molecular oxygen as a terminal electron acceptor were performed. Thus, the preparative value of chemoenzymatic transfer hydrogenations with [Cp*Rh(bpy)(H 2O)] 2+ as a regeneration catalyst coupled especially to thermophilic ADHs was demonstrated.