Thermolysis of 3-alkyl-3-methyl-1,2-dioxetanes: Activation parameters and chemiexcitation yields

3-Methyl-3-(3-pentyl)-1,2-dioxetane 1 and 3-methyl-3-(2,2-dimethyl-1-propyl )-1,2-dioxetane 2 were synthesized in low yield by the alpha -bromohydroper oxide method. The activation parameters were determined by the chemilumines cence method (for 1 DeltaH double dagger = 25.0 +/- 0.3 kcal/mol, DeltaS do uble dagger = -1.0 entropy unit (e.u.), DeltaG double dagger = 25.3 kcal/mo l, k(1) (60 degreesC) = 4.6 x 10(-4)s(-1); for 2 DeltaG double dagger = 24. 2 +/- 0.2 kcal/mol, DeltaS double dagger = -2.0 e.u., DeltaG double dagger = 24.9 kcal/mol, k(1) (60 degreesC) = 9.2 x 10(-4)s(-1). Thermolysis of 1-2 produced excited carbonyl fragments (direct production of high yields of t riplets relative to excited singlets) (chemiexcitation yields for 1: phi (T ) = 0.02, phi (S) less than or equal to 0.0005; for 2: phi (T) = 0.02, phi (S) less than or equal to 0.0004). The results are discussed in relation to a diradical-like mechanism. (C) 2001 John Wiley & Sons, Inc.