Thermolysis of 3-Methyl-3-alkyl-1,2-dioxetanes: Steric Effects on the Activation Parameters

Article abstract of DOI:10.1021/jo00169a020

A series of 3-methyl-3-alkyl-1,2-dioxetanes (1-5) was synthesized by the bromo hydroperoxide method.The activation parameters for the thermal decomposition of 1-5 were determined by the chemiluminescence method a = 24.5 kcal/mol, log A = 13.1, k_{60 deg C} = 1.0E-4 s^-1; for 3-methyl-3-(1-propyl)-1,2-dioxetane (2), E_a = 24.6 kcal/mol, log A = 13.1, k_{60 deg C} = 1.0E-4 s_-1; for 3-methyl-3-(1-butyl)-1,2-dioxetane (3), E_a = 24.4 kcal/mol, log A = 13.3, k_{60 deg C} = 9.6E-5 s^-1; for 3-methyl-3-(2-propyl)-1,2-dioxetane (4), E_a = 25.0 kcal/mol, log A = 13.2, k_{60 deg C} = 5.8E-5 s^-1; and for 3-methyl-3-tert-butyl-1,2-dioxetane (5), E_a = 25.8 kcal/mol, log A = 13.3, k_{60 deg C} = 2.6E-5 s^-1>.Thermal decomposition of 1-5 produced the expected cleavage products in all cases.As expected for alkyl dioxetanes, thermolysis of 1-5 resulted in high yields (ca. 10percent) of triplet excited products and low yields (<0.01percent) of singlet excited products as determined by the 9,10-dibromoanthracene/9,10-diphenylanthracene method.The data showed that increased steric interactions due to branched substituents produced increased activation energies with little or no effect observable in the ΔS(excit.) terms.The results are in agreement with a diradical mechanism of dioxetane thermolysis.