15N nuclear polarisation in nitration and related reactions. Part 8. The mechanisms of rearrangement of 2-methyl-2-nitrocyclohexa-3,5-dienones

Abstract

A kinetic study has been carried out on the regiospecific rearrangement of 2-methyl-2-nitrocyclohexa-3.5-dienone to 2-methyl-6-nitrophenol in acetic anhydride containing 0.2% sulfuric acid at –27.8 to –6.5 °C. Related studies have been carried out with other concentrations of sulfuric acid and on the effect of the substituents, 3-Me, 4-Me, 5-Me. 6-Me, 4-NO₂ and 6-NO₂. Some of these experiments have been repeated using ¹⁵N-labelled compounds. The rearrangement of the unsubstituted dienone gives no significant ¹⁵N nuclear polarisation on either the uncatalysed or acid-catalysed paths. However, when the 6-position is blocked by substituents, the nitro group migrates to the 4-position and these rearrangements show the strong ¹⁵N nuclear polarisation characteristic of reactions involving an ArO˙ NO₂˙ radical pair. Semi-empirical molecular orbital calculations are used to provide information on possible intermediates in reactions involving the radical pair.