The alkaline peroxide oxidation of 1′,2′-dihydroxy-3,4-benzotropolones

Abstract

The alkaline peroxide oxidation of 1′,2′-dihydroxy-3,4-benzotropolones proceeds via the corresponding 1′,2′-quinone anions, which undergo two types of peroxide decomposition. (a) At low temperature and alkali concentration, peroxide attack on the 3′- and 4′-positions of the cyclohexadiene ring predominates, and results in the formation of unstable hydroperoxides which decompose to hydroxyquinones, also formed directly by solvolysis. The greater reactivity of the 3′-position is ascribed to its conjugation with both quinonoid and tropolone carbonyl groups. The 2′-hydroxy-1′,4′-quinones formed from 3′-substituted substrates are further oxidised to tropolone-3,4-dicarboxylic anhydride. (b) At high temperature and alkali concentration, muconic acids are formed via tetrahydroxybenzotropolones and dihydroxy-2′,6′-quinones arising from peroxide attack on the 5- and 6- and 7-positions of the tropolone ring. These tetrahydroxybenzotroplones (also obtained by hydrolysis of the abnormal Thiele penta-acetates of the 1′,2′-quinones) are not formed when the substrate bears a carboxy-group at the 6-position; in this case peroxide attack on the 1′- and 2′-positions of the cyclohexadiene ring predominates.

These results are discussed in terms of steric and charge effects, and mechanisms for each type of reaction path are suggested.