Reduction biomimetic systems

Molybdenum has a marked predilection for OAT reactions and biomimetic systems have been reviewed. OAT is a concerted two-electron process that results in the oxidation of the oxygen atom acceptor and the reduction of the donor. The most common reactions for Mo are shown in equations (10) and (11). Catalysis results from the couphng of these reactions, leading to net oxidation of X by X O (equation 12, cf. equation 6). Transformations involving oxo-Mo(Vl) and desoxo-Mo(lV) complexes are recent and important developments (see Section 9). 

In this paper aspects of the carbon dioxide chemistry we have developed will be reviewed and discussed with reference to relevant natural processes. A few examples of utilisation of either enzymes or biomimetic systems will be presented, covering both the fixation and reduction reactions. 

Unique to this biomimetic system over that of the previously reported [FeyMoSs] cofactor systems is the inequivalence between the reduction of the first and second nitrogen atoms. The finding of the nitride atom in the new crystal structure of the FeyMoSg cluster may indicate that such a pathway may also occur in the nitrogenase system. 

The reduction of CO2 to CO occurs at the expense of the metal, which is oxidized in order to be able to repeat the cycle, the metal must be reduced to its original oxidation state. The utilization of PhSH as a source of protons and electrons has been demonstrated and is an interesting biomimetic system. Both diphenyldi-sulphide and the carbonyl species were detected [3, 86-93]. 

An intriguing use of a quaternary ammonium salt in a two-phase reaction is to be found with the regeneration of 1 -benzyl-1,4-dihydronicotinamide by sodium dithionite in a biomimetic reduction of thiones to thiols [12], The use of sodium dithionite in the presence of sodium carbonate for the 1,4-reduction of the pyri-dinium salts to 1,4-dihydropyridines is well established but, as both the dithionite and the pyridinium salts are soluble in water and the dihydropyridine and the thione are insoluble in the aqueous phase and totally soluble in the organic phase, it is difficult to identify the role of the quaternary ammonium salt in the reduction cycle. It is clear, however, that in the presence of benzyltriethylammonium chloride, the pyridine system is involved in as many as ten reduction cycles during the complete conversion of the thione into the thiol. In the absence of the catalyst, the thione is recovered quantitatively from the reaction mixture. As yet, the procedure does not appear to have any synthetic utility. 

Asymmetric reduction of ketones or aldehydes to chiral alcohols has received considerable attention. Methods to accomplish this include catalytic asymmetric hydrogenation, hydrosilylation, enzymatic reduction, reductions with biomimetic model systems, and chirally modified metal hydride and alkyl metal reagents. This chapter will be concerned with chiral aluminum-containing reducing re-... 

The catalytic, asymmetric hydrogenations of alkenes, ketones and imines are important transformations for the synthesis of chiral substrates. Organic dihydropyridine cofactors such as dihydronicotinamide adenine dinucleotide (NADH) are responsible for the enzyme-mediated asymmetric reductions of imines in living systems [86]. A biomimetic alternative to NADH is the Hantzsch dihydropyridine, 97. This simple compound has been an effective hydrogen source for the reductions of ketones and alkenes. A suitable catalyst is required to activate the substrate to hydride addition [87-89]. Recently, two groups have reported, independently, the use of 97 in the presence of a chiral phosphoric acid (68 or 98) catalyst for the asymmetric transfer hydrogenation of imines. 

Organomercuiial intermediates have also been utilized in the biomimetic conversion of communic acids to the pimarane system, during which the radical involved in the NaBH4-demeicuration step was captured by oxygen. Treatment of rrnn.r-communic acid with Hg(OAc)2 (2 equiv.), followed by reduction, led to (44) and (45) and other products. These results are consistent with the intervention of the radical formed from the dimercurial (46) and, indeed, separate reduction in the presence of oxygen provided the peroxy compound (45) directly (Scheme 36). 

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