Nitroxides (aminyl oxides) - XLI. Formation and dimerization of β-sulfonyl vinyl nitroxides. CC-versus OC-dimerization

Abstract

Upon oxidation of hydroxylamines 1a–c only nitrones 2a–c could be isolated although the nitroxides 3a–c were detected by ESR. Oxidation of 1f yields compound 5, the CC-dimer of 4. Whereas oxidation of 1d affords only 7d which arises from 3d by CC-dimerization and elimination of propylsulfinic acid, from vinyl nitroxide 3e products 7e, 8 and 9 are formed by CC-dimerization and subsequent reaction steps, as well as 11 which is formed via the intermediate OC-dimer 10. The reaction steps leading to the different products are discussed. The reactivity of vinyl nitroxides can be attributed to their tendency to undergo bond formation at the β-C-atom. Thus, in principle they can form CC-bonded dimers or OC-bonded dimers. 3 For N-tert-butyl substituted vinyl nitroxides CC-dimerization was found to be the preferred reaction pathway at room temperature. It has been assumed that OC-dimers can be formed in a reversible reaction. However, if there is no possibility to form a more stable product in a subsequent reaction step, OC-dimerization should be only an unproductive reaction pathway. 4 To verify this assumption we have now studied the dimerization of some β-sulfonyl substituted vinyl nitroxides 3 generated by oxidation of the corresponding hydroxylamines 1. 1