Diels-Alder reactivities of five sets of 1-aza, 2-aza, and unsubstituted heterocyclopentadienes with ethylene have been studied computationally using density functional theory (DFT) and the distortion/interaction (D/I) model. The 1-aza substituted heterocyclopentadienes have disfavorable activation and reaction energies relative to corresponding 2-aza substituted and unsubstituted cycles. Using D/I analysis, the increased activation energy of 1-aza compounds was found to be associated with increased distortion energies (Ed) and decreased interaction energies (Ei). The Diels-Alder reaction can be modeled as hydrogenation of a double bond in the 1,3-diene reactant. Hydrogenation of a C=N bond has been calculated to be 7.0 kcal/mol less favorable than C=C hydrogenation, accounting for the increased distortion energy in D-A reactions of 1-aza cycles. The smaller difference in reaction and activation energies between acyclic 2-aza and 1-aza molecules relative to their cyclic counterparts is explained by the increased stability of the Diels-Alder product of acyclic 1-aza molecules and ethylene due to conjugation of the lone pair of nitrogen to the C=C double bond, and the lack of conjugation in the product of 2-aza cycles and ethylene.

Why 1-aza heterocyclopentadienes are generally less reactive than 2-aza heterocyclopentadienes in Diels-Alder cycloadditions

248th National Meeting (2014)

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