Theoretical investigation on the reactivity of sulfur-centered heterocumulenes as dienophiles in Diels-Alder reactions and endo-lone-pair effect

Abstract

AM1 and PM3 computations show that thiocarbonyl S-oxide and thiocarbonyl S,S-Dioxide undergo Diels–Alder cycloadditions with cyclopentadiene and anthracene to form thiabicyclic adducts through asynchronous transition structures (TSs) with C(DOUBLE BOND)S π* involving in the reaction at a very early stage. Calculated activation barriers indicate that the dienophilicity of these heterocumulenes decrease gradually with progressive addition of oxygen atom on thiocarbonyl sulfur, in reasonable agreement with experimental observations. Frontier Molecular Orbital (FMO) and deformation energy analyses reveal that the above trend is due to gradual destabilization of lowest unoccupied molecular orbital (LUMO) of the dienophile and increase of deformation energy of both diene and dienophile with increase of oxygen atoms around sulfur. Analysis of bond orders and TS geometries show that the TSs are neither “early” nor “late.” Chlorine substitution on these heterocumulenes does not seem to increase their reactivity contrary to expectations. The reactions of monosubstituted sulfines with cyclopentadiene pass through four very closely lying TSs and stereoselectively form four stereoisomeric products. For the above reason, the computed barriers show a mixed trend although the relative exothermicity of these reactions are in order. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 66: 309–322, 1998