15N nuclear polarisation in nitration and related reactions. Part 7. The mechanisms of rearrangement of 4-methyl-4-nitrocyclohexa-2,5-dienones

Abstract

The rearrangement of 4-methyl-4-nitrocyclohexa-2,5-dienone to 4-methyl-2-nitrophenol has been studied in acetic anhydride at 22–37.5°C in the presence of varying concentrations of sulfuric acid. Similar studies have been carried out on the ¹⁵N labelled compound and on the effects of some substituents (2-Me, 3-Me, 2-NO₂). During reaction, the thermal rearrangement of the labelled compound gives very strongly enhanced ¹⁵N NMR absorption signals (enhancement coefficient ca. 1000) for both the substrate and the product. For the acid catalysed reaction, the enhancement coefficient of the signals is less (ca. 130) but still sufficient to indicate a homolytic reaction path. This interpretation is shown to be consistent with the properties of the radicals involved and with the substituent effects observed.