REACTIONS OF CONJUGATED DIENES WITH NITROGEN MONOXIDE AND DIOXIDE

Addition of NO to dilute (ca. 10(-4) mol l(-1)) solutions of 2,3-dimet hylbuta-1,3-diene gave at least two nitroxyl radicals. The major radic al, with a half-life of ca. 30 min, had an EPR spectrum with H-1 hyper fine splitting of ca. 10 G from four equivalent protons. Similarly, th e major nitroxyl radical formed from 2,5-dimethylhexa-2,4-diene gave a major triplet showing no resolved H-1 hyperfine coupling, but satelli te features from C-13 (a = 10 G) from two equivalent nuclei. A minor r adical with slightly different N-14 coupling and a C-13 Splitting of 6 .5 G was also detected. These radicals were very stable. On dilution o f the diene, the second species was favoured over the former, and beca me the major component at less than or equal to 10(-5) mol l(-1). Thes e reactions occurred in the complete absence of (NO2)-N-.. No radicals were detected using pure (NO2)-N-., but mixtures gave strong signals from nitroxyl radicals. Addition of (NO)-N-. to dilute solutions of ph orone gave a long-lived nitroxyl radical with a(C-13)= 6.5 G from four equivalent carbon nuclei. However, in this case, (NO2)-N-. gave a 30 G triplet assigned to an alkoxy-nitroxyl radical, and mixtures of (NO) -N-. + (NO2)-N-. gave both signals in about the same proportion. It is suggested that for the butadienes the (NO)-N-. adds reversibly to giv e allylic radicals. These can cyclise, dimerise or add a second (NO)-N -.. The resulting nitroso derivatives can then add another radical to give nitroxyl radicals. For 2,3-dimethylbuta-1,3-diene, the cyclic nit roxide has a very short lifetime and hence was not detected. However, for the hexadiene, although the species with a(C-13) = 10 G cannot be the simple cyclic derivative, the second species with a(C-13)= 6.5 G p robably has a cyclic structure. The results for phorone can all be und erstood in terms of a cyclic 1:1 adduct, both for (NO2)-N-. and (NO2)- N-. and we favour this assignment. Hence, it is suggested that phorone , or derivatives thereof, may be useful for EPR detection of (NO)-N-. radicals, and possibly of both (NO)-N-. and (NO2)-N-. in mixtures.