Bimolecular formation of radicals by hydrogen transfer.: 14 -: The uncatalyzed transfer hydrogenation of α-methylstyrene by 2,6-disubstituted 9,10-dihydroanthracenes

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 alpha-methylstyrene (5) between 290-350 degrees 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 Delta S-not equal values between -21 and -28 cal/mol K. These results, together with the observation of a large isotope effect (k(H)/k(D) = 1.4- 2.0 at 310-350 degrees C), suggest that the mechanism involves a primary kinetic H-atom-transfer from the donors to alpha-methylstyrene (5) in the rate determining step. The compounds 4a-4d constitute a new probe for investigating polar effects on H-transfer reactions.