| Abstract
| - The chalcogeno-Diels−Alder reactions of H2CX (X = S, Se, Te) with butadiene, with trans,trans-and cis,trans-2,4-hexadiene, as well as of ethylene with thio-, seleno-, and telluroacrolein andreactions of thioformaldehyde with thioacrolein are examined theoretically. The B3LYP exchange-correlation functional with the 6-31G(d) and LanL2DZ(d) basis sets is employed. Stepwise diradicaland concerted pathways are considered for all reactants. A modified concerted mechanism via apre-reaction complex followed by a concerted transition state is studied for thioformaldehyde reactingwith thioacrolein. The stepwise diradical pathways are predicted to be energetically less favorablethan the concerted pathways for all cases considered. Even the sterically hindered reaction betweenselenoformaldehyde and cis,trans-2,4-hexadiene prefers a concerted path. It is a considerablechallenge to reverse this energy preference for the concerted reaction given that both electronicand steric factors act to increase or decrease the activation energies of the concerted and diradicalstepwise paths in the same way. A modified concerted mechanism operates for reagents with verysmall HOMO−LUMO gaps such as thioformaldehyde and thioacrolein. This mechanism iscompletely synchronous, with a vanishingly small barrier.
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