Radiation generation of radical cations of amides. An electron spin resonance study

Abstract

Exposure of dilute solutions of a range of fully alkylated amides in dichlorofluoromethane (CFCl₃) to ⁶⁰Co γ-rays at 77 K gave good yields of the corresponding radical cations, characterised by their e.s.r. spectra. Dimethyl-formamide and -acetamide gave the parent cations, the spin density being largely localised on nitrogen. Proton hyperfine coupling to the two N-methyl protons was surprisingly high (ca. 33 G). These cations were stable up to ca. 160 K, in marked contrast with the corresponding ester cations such as methylacetate, which probably undergo internal rearrangement even at ca. 4 K.

Two cyclic amides (N-methylpyrolidone and N-formylpiperidine) also gave the parent π-cations. The former exhibited the expected large hyperfine couplings to the two β-ring protons, whereas the latter gave relatively small coupling constants. This decrease is explained in terms of a switch in conformation on going from the five- to the six-membered ring systems. In contrast, N-methyldiacetamide, MeCO–N(Me)–CoMe, failed to give the expected π-cation with high spin density on nitrogen and the N-methyl group. However, a well defined spectrum was obtained for a species exhibiting relatively large g-shifts and a small coupling to ¹⁴N. The results are tentatively interpreted in terms of a SOMO confined to one, or probably, to both in-plane carbonyl oxygen atoms, which may become weakly σ-bonded as a result of electron loss. Tetramethylurea failed to give the expected π-cation with high spin densities on the two nitrogen atoms. The e.s.r. results established the presence of only one nitrogen and two strongly coupled methyl groups. The e.s.r. parameters are close to those for the dimethylamino radical, so we suggest that fragmentation to give ˙NMe₂ and Me₂NCO⁺ is facile. The latter diamagnetic product is isoelectronic with dimethylcyanamide and is expected to be reasonably stable. A similar reaction gave ˙NEt₂ from the tetraethyl derivative.