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

A. Z. Bradley, Martin George Kociolek, R. P. Johnson

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30 Scopus citations

Abstract

Diels-Alder cycloaddition of s-trans-1,3-butadiene (1) should yield trans-cyclohexene (7), just as reaction of the s-cis conformer gives cis-cyclohexene (9). Investigation of this long-overlooked process with Hartree-Fock, Moller-Plesset, CASSCF, and DFT methods yielded in every case a C2-symmetric concerted transition state. At the B3LYP/6-31G(*) (+ZPVE) level, this structure is predicted to be 42.6 kcal/mol above reactants, while the overall reaction is endothermic by 16.7 kcal/mol. A stepwise diradical process has been 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 and 13.0 kcal/mol. The barrier to π-bond rotation in cis-cyclohexene (9) is predicted (B3LYP theory) to be 62.4 kcal/mol, with trans-cyclohexene (7) lying 53.3 kcal/mol above cis isomer 9. Results suggest that π-bond isomerization and concerted reaction may provide competitive routes for Diels-Alder cycloreversion. It is concluded that full understanding of the Diels-Alder reaction requires consideration of both conformers of 1,3-butadiene.

Original languageEnglish (US)
Pages (from-to)7134-7138
Number of pages5
JournalJournal of Organic Chemistry
Volume65
Issue number21
DOIs
StatePublished - Oct 20 2000

All Science Journal Classification (ASJC) codes

  • Organic Chemistry

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