Conformational selectivity in the Diels-Alder cycloaddition: Predictions for reactions of s-trans-1,3-butadiene

Diels-Alder cycloaddition of s-trans-1,3-butadiene (1) should yield trans-c yclohexene (7), just as reaction of the s-cis conformer gives cis-cyclohexe ne (9). Investigation of this long-overlooked process with Hartree-Fock, Mo ller-Plesset, CASSCF, and DFT methods yielded in every case a C-2-symmetric concerted transition state. At the B3LYP/6-31G* (+ZPVE) level, this struct ure is predicted to be 42.6 kcal/mol above reactants, while the overall rea ction is endothermic by 16.7 kcal/mol. A stepwise diradical process has bee n studied by UBLYP/6-31G* theory and found to have barriers of 35.5 and 17. 7 kcal/mol for the two steps. Spin correction lowers these values to 30.1 a nd 13.0 kcal/mol. The barrier to ct-bond rotation in cis-cyclohexene (9) is predicted (B3LYP theory) to be 62.4 kcal/mol, with trans-cyclohexene (7) l ying 53.3 kcal/mol above cis isomer 9. Results suggest that pi -bond isomer ization and concerted reaction may provide competitive routes for Diels-Ald er cycloreversion. It is concluded that full understanding of the Diels-Ald er reaction requires consideration of both conformers of 1,3-butadiene.