STEREOSPECIFICITY OF THE S(3PJ) + BUTENE-2 REACTION AND THE NMR-SPECTRA OF THE 1,2-DIMETHYLTHIIRANES - AN EXPERIMENTAL AND THEORETICAL-STUDY

The gas-phase addition of S(3PJ) atoms to cis- and trans-butene-2 has been investigated and both reactions have been shown to be 100% stereospecific, yielding the lowest excited triplet state of the 2,3-dimethylthiirane end product. The triplet thiirane has a long collisional lifetime and is capable of undergoing facile, reversible addition to the butenes causing their geometrical isomerization, while retaining its own original geometry. The cause of the gradual loss of stereospecificity in the 2,3-dimethylthiirane product with increasing conversion has been identified as this geometrical isomerization of the butene-2 substrate by the primary triplet thiirane and those produced in the secondary photolysis of the thiirane product. Ab initio SCF-MO with 6-31G* basis set and limited CI calculations predict the lowest triplet ring opened A″ state of thiirane to lie 54 kcal mol-1 above the ground state with a rotational energy barrier for the terminal methylene of 5.4 kcal mol-1. The total computed reaction energy for the model reaction S(3P) + C2H4 is thus approximately the same as the change in internal energy of the reaction, 58.0 kcal mol-1, estimated from thermochemical data, and therefore isomerization will be slow relative to collisional deactivation. This gives a plausible account of the origin of the complete stereospecificity of triplet sulfur addition to olefins. The reaction initially proceeds in a concerted least-motion path on the lowest 3B1 (C2v) repulsive surface which intersects the lowest attractive 3A″ surface 1.9 kcal mol-1 above the reactant state. This intersection is identified as the transition state of the reaction for which the reported experimental activation energy is 1.6 kcal mol-1. Both the 3A″ and the next higher lying 3A′ surfaces feature a significant intrinsic activation energy barrier and therefore they are effectively eliminated as adiabatic reaction surfaces. The NMR spectra of the isomerically pure cis- and trans-2,3 dimethylthiiranes have been obtained and assigned by computer simulation. © 1990, American Chemical Society. All rights reserved.