Bimolecular formation of radicals by hydrogen transfer. 14. The uncatalyzed Transfer Hydrogenation of ?-Methylstyrene by 2,6-disubstituted 9,10-Dihydroanthracenes

Abstract

2,6-Dimethoxy-(4a), 2,6-bis(dimethylamino)-(4b), 2,6-dichloro- (4c) and 2,6-dimethoxycarbonyl-9,10-dihydroanthracene (4d) were prepared by conventional methods and used as hydrogen transfer donors to α-methylstyrene (5) between 290-350 °C. The mechanism followed second order kinetics and the rate constants were only slightly influenced by the solvent polarity and the type of substituents introduced. The activation parameters are also closely similar in the series with ΔS≠ values between −21 and −28 cal/mol K. These results, together with the observation of a large isotope effect (kH/kD = 1.4–2.0 at 310–350 °C), suggest that the mechanism involves a primary kinetic H-atom-transfer from the donors to α-methylstyrene (5) in the rate determining step. The compounds 4a–4d constitute a new probe for investigating polar effects on H-transfer reactions.