A computational study of the reactions of thiiranes with ammonia and amines

The relative rates of reaction of thiirane and thiirane derivatives with NH3, a series of secondary amines including aziridine, and trimethylamine were determined in the gas phase by means of B3LYP/6-31+G(d)//HF/6-31+G(d) computations and transition state theory. Convergence of the results was selectively tested using the 6-311++G(d,p) basis set. Comparison with MP2/6-31 + G(d)//MP2/6-31G(d) computations was made in model cases. These results are significant in that they supplement the only reported gas-phase experimental study of this type of reaction. The reaction rates of thiirane with secondary amines can best be rationalized by means of an interplay of steric and polarizability effects. While beta -halo substituents retard S(N)2 reactions in solution, both 2-fluorothiirane and its acyclic model react more than 10(6) times faster with NH3 than the unsubstituted compounds in the gas phase. 2-Fluorothiirane was calculated to react with NH3 at C-2 by a factor of 0.142 with respect to thiirane itself, attack at C-3 was found to be 3.42 x 10(6) times faster than the parent compound. 2-Methylthirane reacts with NH3 at 0.230 the rate of thiirane with a 12.8-fold regioselectivity for C-3. In the reaction of 2,2-dimethylthirane and NH3, this preference for C-3 increases to a factor of 124. Ground-state destabilization of cis-2,3-dimethylthiirane is sufficient to account for its calculated rate acceleration with respect to the trans isomer.