Electron spin resonance study of radicals photolytically generated from aromatic carbonyl compounds. Part III. Reduction of fluorenone and benzophenone in amines

Abstract

E.s.r. spectra of the 9-hydroxyfluorenyl radical, which is produced by the u.v. photolysis of fluorenone in tertiary amines, are presented and discussed. Tertiary amines appear to have a vital and unique role in the reduction process as the 9-hydroxyfluorenyl radical was not observed in photolysed solutions of fluorenone in other amines or in many non-amine solvents. Reduction probably occurs by electron transfer from the nitrogen atom to the excited carbonyl group to form the fluorenone radical anion which then rapidly abstracts a proton from the amine radical cation.

Amines can effect markedly the e.s.r. spectra of hydroxy-substituted radicals. Results on the Ph₂ĊOH radical in various amine solvents are presented and compared with results for the same radical in solvents in which the effects of solvation are likely to be minimal. Such solvents are cyclohexane and toluene. In the latter, the spectrum of the benzyl radical, PhĊH₂, is clearly resolved in addition to that for the Ph₂ĊOH radical. The observations can be explained in terms of solvation via hydrogen bonding between the hydroxy-group protons and the nitrogen atom of some amines. Triethylamine appears to be a particularly potent solvator and affects the coupling constants to the hydroxy-protons and, in the case of the 9-hydroxyfluorenyl radical, the ring proton coupling constants quite drastically.