THEORETICAL INVESTIGATION ON THE REACTIVITY OF SULFUR-CENTERED HETEROCUMULENES AS DIENOPHILES IN DIELS-ALDER REACTIONS AND ENDO-LONE-PAIR EFFECT

AM1 and PM3 computations show that thiocarbonyl S-oxide and thiocarbon yl S,S-Dioxide undergo Diels-Alder cycloadditions with cyclopentadiene and anthracene to form thiabicyclic adducts through asynchronous tran sition structures (TSs) with C=S pi involving in the reaction at a ve ry early stage. Calculated activation barriers indicate that the dieno philicity of these heterocumulenes decrease gradually with progressive addition of oxygen atom on thiocarbonyl sulfur, in reasonable agreeme nt with experimental observations. Frontier Molecular Orbital (FMO) an d deformation energy analyses reveal that the above trend is due to gr adual destabilization of lowest unoccupied molecular orbital (LUMO) of the dienophile and increase of deformation energy of both diene and d ienophile with increase of oxygen atoms around sulfur. Analysis of bon d orders and TS geometries show that the TSs are neither ''early'' nor ''late.'' Chlorine substitution on these heterocumulenes does not see m to increase their reactivity contrary to expectations. The reactions of monosubstituted sulfines with cyclopentadiene pass through four ve ry closely lying TSs and stereoselectively form four stereoisomeric pr oducts. For the above reason, the computed barriers show a mixed trend although the relative exothermicity of these reactions are in order. (C) 1998 John Wiley & Sons, Inc.