Thermolysis of Alkyldioxetanes: Effect of 3,3-Cyclic Substituents and Conformation on the Activation Parameters

Article abstract of DOI:10.1021/jo00197a034

3,3-Tetramethylene-1,2-dioxetane (1), 3,3-pentamethylene-1,2-dioxetane (2), 3,3-hexamethylene-1,2-dioxetane (3), cis-4-tert-butylcyclohexanespiro-3'-(1',2'-dioxetane) (4), and 4-methylcyclohexanespiro-3'-(1',2'-dioxetanes) 5a and 5e were synthesized in ca. 10percent yield by closure of the β-bromo hydroperoxides with base at low temperature.The dioxetanes were purified by low-temperature (-78 deg C) column chromatography on silica gel.The configurations of 4 and 5a were shown to have oxygen-2 in an axial position while that of 5e was found to have oxygen-2 in an equatorial position (relative to the equatorial 4-alkyl groups).Thermal decomposition of 1-5 produced the expected cleavage products.Thermolysis of the compounds produced (directly) high yields of excited triplet carbonyl products.The activation parameters for the thermal decomposition of 1-5 in xylenes were determined by the Arrhenius method: for 1, E_a = 23.4 kcal/mol, log A = 13.1, k_{60 deg C} = 5.8*10^-3s^-1; for 2, E_a = 24.6 kcal/mol, log A = 13.2, k_{60 deg C} = 1.2*10^-3s^-1; for 3, E_a = 24.7 kcal/mol, log A = 13.2, k_{60 deg C} = 1.3*10^-3s^-1; for 4, E_a = 24.5 kcal/mol, log A = 13.1, k_{60 deg C} = 1.0*10^-3s^-1; for 5a, E_a = 24.9 kcal/mol, log A = 13.3, k_{60 deg C} = 1.1*10^-3s^-1; and for 5e, E_a = 24.1 kcal/mol, log A = 13.0, k_{60 deg C} = 1.4*10^-3s^-1.The data for 2-4 and 5a are similar to those for 3,3-diethyl-1,2-dioxetane.The data for 5e show that the "equatorial" dioxetane is less stable than the "axial" dioxetane due to lower steric interactions.An interpretation of the data for 1 suggests that the lowest steric interactions occur in 1, resulting in the lowest E_a.The results are consistent with a diradical mechanism of dioxetane thermolysis.