Dienone–phenol rearrangements of 3-acylcyclohexa-2,5-dienones: kinetics and mechanism

Abstract

The dienone-phenol rearrangements of 3-acetyl-4,4-dimethyl-, 3-ethoxycarbonyl-4,4-dimethyl-, and 3-acetyl-4,4,5-trimethylcyclohexa-2,5-dienones have been studied. The 3-acetyl and 3-ethoxycarbonyl compounds rearrange under acidic conditions to give the 3-acyl-4,5-dimethylphenols, vai a 4–5-methyl migration. Kinetic measurements were made which also allowed the protonation (basicity) behaviour of the 3-acetyl-4,4-dimethyl compound to be determined. The acetyl group considerably reduces the basicity of the dienone. At high acidities, where the dienone is largely protonated, it rearranges four times faster than the 4,4-dimethyl-, and 100 times faster than the 3,4,4-trimethyl dienone, showing the acetyl group to enhance the reactivity of the cation. At low acidities the amount of reactive cation, and thus the observed rate of rearrangement, is small. The effect of the acetyl group on the basicity and reactivity are attributed to its destabilising the dienone cation relative to the neutral dienone, and relative to the cation produced by rearrangement. The 3-ethoxycarbonyl group shows similar behaviour. The acceleration of rearrangement is also found for 3-acetyl-4,4,5-trimethylcyclohexa-2,5-dienone, the cation of which reacts slowly, but still at least 60 times faster than the 3,4,4,5-tetramethyl analogue.