Electro-organic reactions. Part 54. Quinodimethane chemistry. Part 2. Electrogeneration and reactivity of o-quinodimethanes

The electrochemical generation and characterisation of a variety of o-quino dimethanes (o-QDMs) are described together with the outcome of preparative experiments in which they are key intermediates. The quinodimethanes are co nveniently formed, in DMF, by both direct and redox-catalysed electroreduct ion of 1,2-bis(halomethyl)arenes. Their predominant reaction is polymerisat ion to poly(o-xylylene) (o-PX) polymers. In the presence of dienophiles the electrogenerated o-QDMs may undergo efficient cycloaddition reaction and d istinctions between the possible mechanisms have been attempted on the basi s of voltammetric, preparative and stereochemical experiments. Contrary to the precedent of the corresponding methyl ester, diphenyl maleate radical-a nion isomerises only slowly to the fumarate radical-anion, yet co-electroly sis of 2,3-bis(bromomethyl)-1,4-dimethoxybenzene and diphenyl maleate or di phenyl fumarate gives exclusively the corresponding trans-adduct. Co-electr olysis of dimethyl maleate with either 1,2-bis(bromomethyl)benzene (more ea sily reduced) or 2,3-bis(bromomethyl)-1,4-dimethoxybenzene (less easily red uced) gave only o-PX polymer. The results are rationalised in terms of a do uble nucleophilic substitution mechanism where electron transfer between di enophile radical-anion and dihalide is relatively slow. Where electron tran sfer from maleate or fumarate radical-anions is likely to be fast o-quinodi methanes are formed by redox-catalysis and they polymerise rather than unde rgo Diels-Alder reaction. Dimerisation of the dienophile radical-anions, wi th k(2)=10(4) to 10(5) M-1 s(-1), does not apparently compete with nucleoph ilic substitution or, where relevant, electron transfer.