THERMAL-DECOMPOSITION OF 3,3,5-TRISUBSTITUTED-4,4-DIMETHYL-4,5-DIHYDRO-5-HYDROPEROXY-3H-PYRAZOLES - ROUTE TO BETA,GAMMA-UNSATURATED KETONES

The thermal decomposition of a series of cyclic alpha-azo hydroperoxides (3,3,5-R1, R2, R3-4,4-dimethyl-4,5-dihydro-5-?? hydroperoxy-3H-pyrazoles; 2a R1 = R2 = R3 = Ph; 2b R1 = R3 = Ph, R2 = Me; 2c R1 = R3 = p-Anisyl, R2 = Me; 2d R1 = R2 = Me, R3 = Ph; 2e R1 = R3 = Me, R2 = Ph), synthesized by oxidation of the corresponding 3,4-dihydro-2H-pyrazoles, proceeded smoothly with evolution of nitrogen. The relative stability series was found to be 2a > 2c almost-equal-to 2b > 2d > 2e. For 2a, the products were 1,4,4-triphenyl-2,2-dimethyl-l-propanone and 1,1-dimethyl-2,2-diphenylethylene. For 2b-e, beta, gamma-unsaturated ketones [R1-C(= CH2)-CMe2-C(=O)R3, 5a-d] were obtained as the major products in approximately 60% yield from the thermolyses. The products are consistent with a free-radical mechanism involving initial homolysis of the O-O bond followed by loss of nitrogen to yield a free-radical beta to the carbonyl group. For 2a, beta-scission and hydrogen-atom abstraction of the hydroperoxy proton by the beta-keto radical (induced decomposition) are the major pathways leading to products. For 2b-c, abstraction of a gamma-hydrogen atom of the beta-keto radicals by hydroxy radical accounts for the formation of the beta,gamma-unsaturated compounds as the major product.