A theoretical study of some non-classical organic compounds with di-coordinated sulfur

Abstract

A series of di-coordinated sulfur organic compounds of non-classical structure containing the CSC, CSS or NSN group has been studied by post-Hartree-Fock (MP2, QCISD and QCISD(T)) and multiconfiguration methods (CASSCF, CASPT2). The calculated structures exhibit relatively short bonds to the central sulfur atom along with strong charge separations of these bonds (ylidic structures). The diradicaloid nature of the non-classical structures is indicated by low S 0 T 1 , energy gaps. The parent compound 2λ 4-thioallyl (thioformaldehyde S-methylide, 1) and the most simple thiosulfine (thioformaldehyde S-sulfide, 4) undergo conrotatory electrocyclic ring closure to the energetically favoured thiirane and dithiirane, respectively. The reaction is less exothermic and the energy barrier is higher for 4 than for 1. The 1,3-dipolar cycloaddition of 1 to ethylene is more exothermic than the reaction of butadiene to ethylene. The calculated energy of activation of the concerted cycloaddition of 1 amounts to only 8 kcal mol −1. Similar to the prototype Diels-Alder reaction, the concerted reaction of 1 is about 10 kcal mol −1 and is favoured over the non-concerted stepwise reaction.