A Mechanistic Study of the Thermal Disproportionation Reaction of Quinone Monoketals

Article abstract of DOI:10.1021/jo00233a010

Thermolysis of quinone monoketals 1a-d at 180 deg C leads to disproportionation to the p-alkoxyphenols 2a-d and the carbonyl compounds derived from the alcohol moiety of the ketal.The thermolysis of 4,4-dimethoxy-2,5-cyclohexadienone (1a) followed first-order kinetics, and the rate of reaction decreased by a factor of 3.6 when the methoxy groups were replaced with methoxy-d3 groups.The suggested mechanism for the reaction involves reversible dissociation of 1a into a p-methoxyphenoxy-methoxy radical pair, followed by rate-controlling hydrogen atom transfer to give p-methoxyphenol and formaldehyde.Although 1a has a half-life of about 4 h at 180 deg C, the monoethylene glycol ketal of benzoquinone was recovered in > 90percent yield after heating for 24 h at the same temperature.Finally, acid-catalysed exchange of 3,3,6,6-tetramethoxy-1,4-cyclohexadiene with alcohols furnished a convenient route to quinone bisketals not obtainable in good yield via anodic oxidation.