GAS-PHASE REACTIONS OF METHYLOXIRANE WITH HO- AND METHYLTHIIRANE WITH HO- AND HS- - AN AB-INITIO STUDY OF ADDITION AND ELIMINATION

For the title reactions, high-level ab initio calculations (MP2/6-31+G**parallel to MP2/6-31f+G*) were used to characterize the potential energy surfaces for nucleophilic addition at carbon (C-1 or C-2) and E2 elimination (anti or syn). In accord with gas phase experiments, elimination is slightly favored over addition when HO- is the nucleophile. In all the HO--induced eliminations, the constraints of the 3-membered ring do not allow for periplanar transition states. When HS- is used, the barrier to elimination is high and addition is the only viable pathway. Attack at the sulfur of thiirane (desulfurization) was also investigated for HO- and HS-. Asynchronous reactions with relatively large barriers are observed, and surprisingly, a stable, hypervalent sulfur intermediate is present in the reaction of HO- with thiirane. To assess the effects of ring strain, calculations were also completed on the corresponding reactions of acyclic analogs (CH3CH2OCH3 and CH3CH2SCH3). Although the additions of HO- involve early transition states, the calculations suggest that similar to 75% of the ring strain is released at the transition state. Because the HO--induced eliminations are E1cb-like, much less ring strain (similar to 30%) is released at the transition state. In contrast, the HS--induced eliminations have relatively late transition states and a larger portion of the ring strain is released.