STRAIN-FREE TRANSITION-STATES IN THE FORMATION OF STRAINED RINGS - AN AB-INITIO STUDY OF THIIRANE, THIETAN, AND TETRAHYDROTHIOPHENE

Ab initio calculations (MP2/6-31+G**//MP2/6-31+G*) have been used to probe the potential energy surfaces of the ring closures of HSCH2CH2S-, HSCH2CH2CH2S-, and HSCH2CH2CH2CH2S- to give thiirane, thietan, and tetrahydrothiophene. The calculated activation barriers (19.2, 29.4, and 23.1 kcal/mol, respectively) indicate that formation of the 3-membered ring has the smallest barrier. Moreover, the barrier to thiirane formation is smaller than that of the corresponding S(N)2 reaction of an acyclic analog (25.0 kcal/mol for CH3S- + CH3CH2SH). The surprisingly small barrier to forming thiirane is the result of two factors. First, the nature of the bonding in the transition state limits the angular strain. Second, geometric constraints force the nucleophilic sulfur to be near the electrophilic carbon in HSCH2CH2S-. This proximity effect destabilizes the ring-opened form (part of the typical S(N)2 barrier is built into the ground state) and consequently lowers the barrier to ring formation.