Electrochemically Induced Diels-Alder Reactions of Some Substituted o-quinones with 1,3-cyclopentadiene: An Interesting Finding in the Diels-Alder Reactions

The electrochemical oxidation of 4-methylcatechol (1a), 4-tert-butylcatechol (1b), and 3-methylcatechol (5) has been studied in the presence of 1,3-cyclopentadiene (3) in an ethanol/water (60:40 v/v) mixture using cyclic voltammetry, controlled-potential coulometry, and preparative electrolysis. The electrochemically generated benzoquinones took part, as dienophiles, in a Diels-Alder reaction with 3 to form methanotetrahydronaphtoquinones through an EC mechanism. The cycloaddition of 3-methyl-o-benzoquinone (5a) to 3 is highly regioselective, affording product 6, a novel regioisomer. It is proposed that the cycloaddition would occur with the benzoquinones adsorbed on the carbon electrode surface. The controlled potential electrolyses were carried out in a one-compartment cell.