Electron transfer-II: Accumulation of 5-ethyl-3-methyllumiflavin radical by spontaneous conversions of 5-ethyl-3- methyllumiflavinium salts

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5-Ethyl-3-methyllumiflavinium salts 3 (Scheme 1 ; 5-EtFl + ox +, A - in Scheme 2) may arise in situ on adding an acid (HA) to solutions of the 4 a-flavin adducts 5 in low polar solvents. The acidified solutions were kept under N 2 at 25° in the dark to give spontaneous accumulations of the 5-ethyl-3-methyllumiflavin radical 6 (5-EtFl· and/or 5-EtFlH +·) and of some 3-methyllumiflavin 10 (Scheme 3) in dependence on the nature of the solvent and, on the nature and the concentration of the acid. The use of TFA;TCA;AcOH;α-ketoglutaric and salicyclic aci (Table 1) gave 6 and 10 in yields of 60–90% and 6–21%, respectively. The anaerobic production of 10 limits the formation of 6 to a theoretical yield of 66.7%. On suppressing the limiting pathway (eqn 3) the formation of 6 is increased which, however, will not always be revealed by an increased accumulation of 6. In a radical termination, 6 could react with another radical to give a 4 a-flavin adduct. The use of TCA in MeCN gave a decrease of 10 coupled with the increased occurrence of Cl 3CCOO· and Cl 3C· radicals as appeared from the spontaneous generation of CO 2 (eqns 3+7). 5-EtFl· was probably trapped by Cl 3,C· to give 5-ethyl-3-methyl-4 a-trichloromethyllumiflavin (eqn 8). In contrast, the use of HCOOH promised the achievements of quantitative accumulations of 6 which was indeed realized (Table 2 ; Figs 2 and 3).