Reductions

5.05.3.1 Cytochrome P450

5.05.3.1.1 Reductive dehalogenation

Under certain conditions, particularly anaerobic conditions, cytochrome P450 can function as a reductase. The most well-recognized reaction in this regard is probably reductive dehalogenation. In a series of papers in the early 1980s, Ullrich and co-workers213-215 established that cytochrome P450 could catalyze the reductive removal of halogens from polyhalogenated alkanes such as hexachloroethane (188), to yield the corresponding carbon-based radical (Scheme 20). The radical would then undergo a second one-electron addition, to yield the carbanion. Elimination of a chloride ion or the addition of a proton yielded the observed products, tetrachloroethylene (189) and pentachloroethane (190), respectively.

Carbon tetrachloride (191) is a solvent that is chemically inert and highly resistant to oxidation, but biologically toxic. Despite its chemical stability, cytochrome P450 is able to convert 191 to a reactive species (Figure 9). Reduced cytochrome P450 transfers an electron to 191, to form a chloride anion and the reactive trichloromethyl radical 192,216 in accordance with the mechanism proposed by Ullrich.213-215 The trichloromethyl radical has a number of

Scheme 20

Cl Cl

Cl/VH Cl Cl

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