The thermal rearrangements of azulenes to naphthalenes

Abstract

The rearrangements of azulenes to naphthalenes, which occur at 350–450°, are catalysed by methyl radicals and it is suggested that skeletal rearrangement takes place in radical adducts (e.g. RC₁₀H₈· from azulene itself). Detailed analysis is reported of the products of thermolyses of the five methylazulenes, of seven dimethylazulenes, of benz-[a]- and -[f]-azulenes, and of dimethyl azulene-4,5-dicarboxylate and the derived anhydride and N-methyl-imide. The majority of the products can be accounted for by proposing that rearrangement results from (i) attack of a radical (R·) on a carbon in the seven-membered ring, (ii) migration of the attacked carbon and its substituents (e.g. CHR or CRMe) into the five-membered ring, and (iii) loss of R, H, or Me to form the naphthalene products. However the final disposition of carbons and substituents in the former five-membered ring is not understood (2-methylazulene unexpectedly produces 1- as well as 2-methylnaphthalene).