Thermal decomposition of 3,3,5-trisubstituted-4,4-dimethyl-4,5-dihydro-5-hydroperoxy-3H-pyrazoles: Route to β,γ-unsaturated ketones

The thermal decomposition of a series of cyclic α-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 ≈? 2b > 2d > 2e . For 2a , the products were 1,4,4-triphenyl-2,2-dimethyl-1-propanone and 1,1-dimethyl-2,2-diphenylethylene. For 2b-e , β,γ-unsaturated ketones R₁-C(= CH₂)-CMe₂-C(= O)R₃, 5a-d] were obtained as the major products in ～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, β -scission and hydrogen-atom abstraction of the hydroperoxy proton by the β-keto radical (induced decomposition) are the major pathways leading to products. For 2b-c , abstraction of a γ-hydrogen atom of the β-keto radicals by hydroxy radical accounts for the formation of the β,γ-unsaturated compounds as the major product.