Stereospecificity of the sulfur(3PJ) + butene-2 reaction and the NMR spectra of the 1,2-dimethylthiiranes: an experimental and theoretical study

The gas-phase addition of S( 3 P J ) 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 at this geometrical isomerization of the butene-2 substrate by the primary triplet thiirane and those produced in the secondary photolysis of the thiirane product. The NMR spectra of the isomerically pure cis- and trans-2,3 dimethylthiiranes have been obtained and assigned by computer simulation