Alkyl radical generation using cyclohexa-1,4-diene-3-carboxylates and 2,5-dihydrofuran-2-carboxylates

Abstract

3-Methylcyclohexa-1,4-diene-3-carboxylic acid and 2-methyl-2,5-dihydrofuran-2-carboxylic acid were prepared by Birch reduction and alkylation of benzoic and furoic acid respectively and converted to alkyl esters. Cyclohexadienyl and 2,5-dihydrofuranyl radicals were generated by hydrogen abstraction and characterised by EPR spectroscopy. The esters decomposed thermally in the presence of a radical initiator to generate alkyl radicals which could be trapped with moderate efficiency by halogen donors or alkenes. Loss of methyl to afford an alkyl benzoate was an important side reaction at higher temperatures. From the thermal reaction of hex-5-enyl 3-methylcyclohexa-1,4-diene-3-carboxylate the rate constant for hydrogen abstraction from the ester by hexenyl radicals was determined to be 0.82 × 10⁵ dm³ mol^-1 s^-1 at 140 °C.