jo001719s_si_004.pdf (38.07 kB)
A Computational Study of the Reactions of Thiiranes with Ammonia and Amines
journal contribution
posted on 2001-08-10, 00:00 authored by Harold D. Banks, William E. WhiteThe 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 β-halo substituents retard SN2 reactions in solution, both
2-fluorothiirane and its acyclic model react more than l06 times faster with NH3 than the
unsubstituted compounds in the gas phase. 2-Fluorothiirane was calculated to react with NH3 at
C2 by a factor of 0.142 with respect to thiirane itself; attack at C3 was found to be 3.42 × 106 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 C3. In the reaction of 2,2-dimethylthirane and NH3, this
preference for C3 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.