The photoreduction of 8-substituted 5-deazaflavins and the 5-deazaflavin catalyzed photoreduction of methyl viologen

Prolonged illumination of 8-X-5-deazaflavins (X = C1, N(CH₃)₂, NH₂, p-NH₂-C₆H₄) in the presence of an electron donor leads to the formation of a 5,5′-dimer and/or a 6,7-dihydro compound. The course and rate of these photoreductions were established and discussed in terms of electronic and steric effects, exerted by the substituent at position 8 and the electron donor. Pseudo first-order kinetics were found to apply to the photoreduction of 8-X-5-deazaflavins (X = Cl, NH₂, p-NH₂-C₆H₄) while the rate of the photoreduction of 8-X-5-deazaflavin (X = N(CH₃)₂) appeared to contain an autocatalytic element. The catalytic effect of 8-X-5-deazaflavins in the photoreduction of methyl viologen by EDTA was investigated. The substituent effect on the rate of the 8-X-5-deazaflavin mediated photoreduction of methyl viologen by EDTA was found to be comparable with that on the photoreduction rate of 8-X-5-deazaflavin in the presence of EDTA with the exception of 8-X-5-deazaflavin (X = N(CH₃)₂), which showed a remarkable relative enhancement of the reactivity towards methyl viologen photoreduction.