Diels–Alder addition of butadiene to various thiocarbonyl(R 2 CSO n , n =0–2) heterodienophiles and endo ‐lone pair effect in heterocumulene

Diels–Alder reactions of various thiocarbonyl S ‐oxides and thiocarbonyl S , S ‐dioxides with buta‐1,3‐diene were studied by semiempirical AM1 and PM3 methods. Calculations show that the reactions of such unsymmetrical heterodienophiles pass through asynchronous transition states (TSs) with C=S π* involved in the reaction. Calculated activation energies indicate that the reactivity of these heterodienophiles, viz. R 2 C=S, R 2 C=SO and R 2 C=SO 2 , decrease gradually with successive addition of oxygen atoms to the thiocarbonyl sulfur. This is in good agreement with the experimental observations. The predictions based on LUMO and deformation energies show that the above reactivity trend corresponds to a gradual destabilization of LUMO of the dienophile and increase in deformation energy of both diene and dienophile with increase in the number of oxygen atoms around sulfur. Thiophosgene and their S ‐oxides in Diels–Alder reactions are found to be less reactive than the parent analogues. Monosubstituted ( Z/E )‐sulfines react with buta‐1,3‐diene to form cis and trans products through closely lying TSs. Calculations predict that ( E )‐sulfine has a higher reactivity than the Z ‐isomer, in reasonable agreement with experimental results. © 1997 John Wiley & Sons, Ltd.