Electron spin resonance studies of reduction by solvated electrons in liquid ammonia. Part 6.—α,β-Unsaturated ketones

Abstract

The reaction of e^–_amm with α,β-unsaturated ketones yields the corresponding ketyl radical-anion when solutions of the reactants are mixed in the cavity of an electron spin resonance spectrometer at temperatures near 200 K utilizing the equipment described in Part 5. The spectra of certain ketyls, such as that of methyl vinyl ketone, are composite, indicating production of both s-cis- and s-trans-conformations, whilst mesityl oxide produces a single species, i.e. exclusively the s-cis-conformer. Assignment of groups of coupling constants to particular conformations has been effected by invoking a degree of twisting about the single bond of the π-system in the s-cis-conformation of the type suggested by i.r. and u.v. studies in the parent ketones. INDO calculations have been performed on all but the most structurally complex of the radical-anions characterized in these experiments.

Spectra obtained from cyclic α,β-unsaturated ketones are generally those corresponding to simple electron attachment; in the case of cyclohex-2-enone itself, however, an alternating line-width affect was apparent in the temperature range 202–229 K which is characteristic of time-modulation of the hyperfine coupling constants by a ring-flipping process. Analysis of the line-broadening yielded rate constants for this process together with the corresponding activation parameters. The magnitude of the coupling constants indicate that the radical-anions are basically planar, except that in the cyclohex-2-enone series the C-5 atom is out-of-plane and the methylene protons at the C-4 and C-6 positions are in almost purely axial and equatorial positions.

In the presence of a proton source, 3,5,5-trimethylcyclohex-2-enone gave the acidic form of the ketyl radical on reduction. Prolonged reaction in a “static” system gave different spectra in all cases, which could be assigned in some instances to semidione radicals.