Experimental and theoretical studies on the thermal decomposition of heterocyclic nitrosimines

A series of substituted 2-nitrosiminobenzothiazolines (2) were synthesized by the nitrosation of the corresponding 2-iminobenzothiazolines (6). Thermal decomposition of 2a-f and of the seleno analogue 7 in methanol and of 3-methyl-2-nitrosobenzothiazoline (2a) in acetonitrile, 1,4-dioxane, and cyclohexane followed first-order kinetics. The activation parameters for thermal deazetization of 2a were measured in cyclohexane (ΔH‡ = 25.3 ± 0.5 kcal/mol, ΔS‡ = 1.3 ± 1.5 eu) and in methanol (ΔH‡ = 22.5 ± 0.7 kcal/mol, ΔS‡ = -12.9 ± 2.1 eu). These results indicate a unimolecular decomposition and are consistent with a proposed stepwise mechanism involving cyclization of the nitrosimine followed by loss of N₂. The ground-state conformations of the parent nitrosiminothiazoline (9a) and transition states for rotation around the exocyclic C=N bond, electrocyclic ring closure, and loss of N₂ were calculated using ab initio molecular orbital theory at the MP2/6-31G* level. The calculated gas-phase barrier height for the loss of N₂ from 9a (25.2 kcal/mol, MP4(SDQ, FC)/6-31G*//MP2/6-31G* + ZPE) compares favorably with the experimental barrier for 2a of 25.3 kcal/mol in cyclohexane. The potential energy surface is unusual; the rotational transition state 9a-rot-ts connects directly to the orthogonal transition state for ring-closure 9aTS. The decoupling of rotational and pseudopericyclic bond-forming transition states is contrasted with the single pericyclic transition state (15TS) for the electrocyclic ring-opening of oxetene (15) to acrolein (16). For comparison, the calculated homolytic strength of the N-NO bond is 40.0 kcal/mol (MP4(SDQ, FC)/6-31G* + ZPE).