Reactions of conjugated dienes with nitrogen monoxide and dioxide

Abstract

Addition of ˙NO to dilute (ca. 10^–4 mol l^–1) solutions of 2,3-dimethylbuta-1,3-diene gave at least two nitroxyl radicals. The major radical, with a half-life of ca. 30 min, had an EPR spectrum with ¹H hyperfine splitting of ca. 10 G from four equivalent protons. Similarly, the major nitroxyl radical formed from 2,5-dimethylhexa-2,4-diene gave a major triplet showing no resolved ¹H hyperfine coupling, but satellite features from ¹³C (a= 10 G) from two equivalent nuclei. A minor radical with slightly different ¹⁴N coupling and a ¹³C splitting of 6.5 G was also detected. These radicals were very stable. On dilution of the diene, the second species was favoured over the former, and became the major component at ⩽ 10^–5 mol l^–1. These reactions occurred in the complete absence of ˙NO₂. No radicals were detected using pure ˙NO₂, but mixtures gave strong signals from nitroxyl radicals.

Addition of ˙NO to dilute solutions of phorone gave a long-lived nitroxyl radical with a(¹³C)= 6.5 G from four equivalent carbon nuclei. However, in this case, ˙NO₂ gave a 30 G triplet assigned to an alkoxy-nitroxyl radical, and mixtures of ˙NO +˙NO₂ gave both signals in about the same proportion.

It is suggested that for the butadienes the ˙NO adds reversibly to give allylic radicals. These can cyclise, dimerise or add a second ˙NO. The resulting nitroso derivatives can then add another radical to give nitroxyl radicals. For 2,3-dimethylbuta-1,3-diene, the cyclic nitroxide has a very short lifetime and hence was not detected. However, for the hexadiene, although the species with a(¹³C)= 10 G cannot be the simple cyclic derivative, the second species with a(¹³C)= 6.5 G probably has a cyclic structure. The results for phorone can all be understood in terms of a cyclic 1:1 adduct, both for ˙NO and ˙NO₂, and we favour this assignment. Hence, it is suggested that phorone, or derivatives thereof, may be useful for EPR detection of ˙NO radicals, and possibly of both ˙NO and ˙NO₂ in mixtures.