Electroenzymatic reduction system

Scheme 8.3 Schematic representation of an electroenzymatic reduction system.

The successful synthetic application of this electroenzymatic system has first been shown for the in-situ electroenzymatic reduction of pyruvate to D-lactate using the NADH-dependent D-lactate dehydrogenase. Electrolysis at — 0.6 V vs a Ag/AgCl-reference electrode of 50 mL of a 0.1 M tris-HCL buffer of pH 7.5 containing pentamethylcyclopentadienyl-2,2 -bipyridinechloro-rhodium(III) (1 x 10 3 M), NAD+ (2 x 10 3 M), pyruvate (2 x 10 2 M), 1300 units D-lactate dehydrogenase (divided cell, carbon foil electrode) after 3 h resulted in the formation of D-lactate (1.4 x 10 2 M) with an enantiomeric excess of 93.5%. This means that the reaction occurred at a rate of 5 turnovers per hour with respect to the mediator with a 70% turnover of the starting material. The current efficiency was 67% [67],... 

The same authors proposed a complex system for the electrochemically driven enzymatic reduction of carbon dioxide to form methanol. In this case, methyl viologen or the cofactor PQQ were used as mediators for the electroenzymatic reduction of carbon dioxide to formic acid catalyzed by formate dehydrogenase followed by the electrochemically driven enzymatic reduction of formate to methanol catalyzed by a PQQ-dependent alcohol dehydrogenase. With methyl viologen as mediator, the reaction goes through the intermediate formation of formaldehyde while with PQQ, methanol is formed directly [77],... 

This system has been successfully applied to the in-situ electroenzymatic reduction of pyruvate to D-lactate using the NADH-dependent D-lactate dehydrogenase or the reduction of 4-phenyl-2-butanone to (5)-4-phenyl-2-butanol using the NADH-dependent horse liver alcohol dehydrogenase (HLADH) with high enantioselectivity (Fig. 22.4) [65]. 

This system has been efficiently applied in the in situ electroenzymatic reduction of pyruvate to D-lactate by means of the NADH-dependent D-lactate dehydrogenase (Fig. 23). Using pentamethylcyclopentadienyl-2-2 -bipyridinechloro-rhodium(III) ([Cp Rh(bpy)Cl]Cl) as redox catalyst, D-lactate was formed with an ee value of 93.5% after 3 h at a rate of five turnovers per hour [112]. 

A common class of mediators, which fulfill these requirements, are rhodium complexes (e.g., tris(2,2 -bipyridyl)- and substituted or nonsubstituted (2,2 -bipyridyl) (pentamethylcy-clopentadienyl)-rhodium complexes). This regeneration system has been efficiently applied in electroenzymatic reduction of pyruvate to D-lactate and the reduction of 4-phenyl-2-butanone to (S)-4-phenyl-2-butanol [1]. In an electrochemical membrane reactor, NADH was... 

Table 8.2 gives a representative overview over some electroenzymatic reduction reactions. It is obvious that the state of development of this promising approach significantly falls back behind the other regeneration systems discussed later in this chapter. EspedaDy, the total turnover numbers for mediators/regeneration catalysts/ NAD(P)+ achieved are several orders of magnitude too low for economic feasibflity. Possibly, this can be attributed to the spedalized equipment necessary for this kind of reactions, which is available in few laboratories only. 

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