Fluorinated semiquinones

Radical flavin species have not been detected during turnover of CS therefore, these species must be short lived. However, a radical flavin species was observed using the EPSP analogue (6R)-6-fluoro-EPSP. Phosphate is eliminated from this analogue however, due to the replacement of the C6 pro-R hydrogen with fluorine, the reaction cannot proceed further, resulting in the formation of a substrate-derived radical and neutral flavin semiquinone. 

Ketyls, semidiones, semiquinones, and azo radicals with fluorine atoms have been studied in several laboratories. Certain of the more prominent features of these investigations are discussed in this section. 

Early work established that for semiquinone LVn was modest compared to the constants for the corresponding hydrogen and carbon nuclei in LV and LVI (123). These results infer that the 8-hydrogen atom Is orbital spin population and the 3-carbon atom 25 orbital spin populations are about an order of magnitude greater than the j3-fluorine atom spin population of 0.002. The and constants observed for fluoroalkyl radicals and nitrobenzene anion radicals also indicate, in general, that p" is much greater than p2s-... 

Semiquinone LVIil was investigated in another attempt to establish the value for a /3-fluorine atom constrained unambiguously to the nodal plane of the pi electron system (124). The observed ajg constant, 0.85 G, is only about fourfold smaller than the constant for the trifluoromethylsemiquinone, 2.7 G. This result contrasts sharply with the observations for the nitrobenzene and aniline derivatives discussed in Section IV.E.4. There are two viable interpretations. On the one hand, the result is compatible with an important role for p p interactions between the carbon atom p orbital and the nonbonding p orbitals of the fluorine atom (63,64). On the other hand, the fluorine and oxygen atoms are virtually in contact in semiquinone LVIII, Accordingly, a direct interaction between the p orbitals of these atoms may contribute to the delocalization of spin density to the fluorine atom (64). This interpretation is especially viable because pl for benzosemiquinone is 0.21, more than twice the value of Pc2 (125). 

A recent experimental observation suggests that the direct interaction is more important 55b). The epr spectra of several benzosemiquinones and naphthosemiquinones were analyzed. The analysis is based on the fact that the Po and pc2 values of benzosemiquinone, LIX, and naphthosemiquinone, LX, differ appreciably (125). Under the assumption that the same spin density distribution prevail in the triptycene derivatives, LXI-LXIV, it is expected that 00 will exhibit a 40% increase if the spin density pc2 is the dominant consideration but a modest decrease if po is the critical factor. The results for these semiquinones and the unsubstituted derivatives, LXI-LXII, establish that the fluorine has a noticeable influence on the spin density distribution (55b). Nevertheless, the fact that the Op parameters are comparable for LXIII and LXIV strongly infers that the spin density at the oxygen atom rather than at the carbon... 

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