Thermolysis of disubstituted 1,2‐dioxetanes: Activation parameters and chemiexcitation yields

Abstract trans‐3‐Methyl‐4‐(p‐anisyl)‐1,2‐dioxetane 1, trans‐3‐methyl‐4‐(o‐anisyl)‐1,2‐dioxetane 2, 3‐methyl‐3‐benzyl‐1,2‐dioxetane 3, and 3‐methyl‐3‐p‐methoxybenzyl‐1,2‐dioxetane 4 were synthesized in low yield by the β‐bromo hydroperoxide method. The activation parameters were determined by the chemiluminescence method (for 1 ΔG≠ = 22.8 ± 0.3 kcal/mol, Δ≠ = 22.2, ΔS≠ = −1.7 e.u., k60 = 7.6 × 10−3s−1; for 2 ΔG≠ + 23.6 ± 0.3 kcal/mol, ΔH≠ = 22.8, ΔS≠ = −2.2 e.u., k60 = 2.5 × 10−3S−1; for 3 ΔG≠ = 24.0 ± 0.4 kcal/mol, ΔH≠ = 23.1, ΔS≠ = −2.7 e.u., k60 = 1.2 × 10−3S−1; for 4 ΔG≠ = 24.0 ± 0.2 kcal/mol, ΔH≠, = 23.2, ΔS≠, = −2.4 e.u., k60 = 1.2 × 10−3s−1). Thermolysis of 1–4 produced excited carbonyl fragments (direct production of high yields of triplets relative to excited singlets) [chemiexcitation yields ϕT, ϕS, respectively: for 1 0.02, 0.0001; for 2 0.02, 0.0001; for 3 0.03, 0.0002; for 4 0.02, 0.0001]. The effect of paramethoxyaryl substitution was consistent with electronic effects. The ortho substitution in 2 resulted in an increase in stability of the dioxetane, opposite that observed for an electronic effect. The results are discussed in relation to a diradical‐like mechanism.