Homolytic abstraction of enolic hydrogen

Abstract

The e.s.r. spectrum of the homoallylic radical (C) is observed when a mixture of di-t-butyl peroxide and a cyclopropylcarbinol (A; R = H or alkyl) is photolysed at low temperature, but near room temperature the spectrum shows that the enolic hydrogen has been transferred to the alkyl radical to give the enoxyl radical (D). Studies of the thermolysis of di-t-butyl peroxide or of di-t-butyl hyponitrite in the presence of 1-cyclopropylethan[²H]ol confirmed that deuterium was located in the 5-position of the pentan-2-one which was formed. The same sequence [graphic omitted] of radicals (B)→(C)→(D)(R = Me) occurs when cyclopropyl methyl ketone is photolysed in the presence of 1-cyclopropylethanol. Aryl(cyclopropyl)methanols (A; R = Ar) yield only the radical (B), and no ring opening could be detected. 2-(Hydroxymethyl)oxiran shows the formation of the 3-hydroxyprop-2-enoxyl radical, consistent with the occurrence of ring opening and hydrogen transfer from enolic oxygen to oxygen, but no evidence could be found for transfer of hydrogen from lactim oxygen to carbon, or from enolic oxygen to nitrogen, in the reactions of 1- or 2-hydroxyalkylaziridines.