N-15 NUCLEAR-POLARIZATION IN NITRATION AND RELATED REACTIONS .8. THE MECHANISMS OF REARRANGEMENT OF 2-METHYL-2-NITROCYCLOHEXA-3,5-DIENONES

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-degrees-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 N-15-labelled compounds. The rearrangement of the unsubstituted dienone gives no significant N-15 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 N-15 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.