Zwitterions and unobserved intermediates in organocatalytic Diels-Alder reactions of linear and cross-conjugated trienamines.

The Diels-Alder reactions of cyclic linear and cross-conjugated trienamines with oxindoles have been studied with density functional theory [M06-2X/def2-TZVPP/IEFPCM//B97D/6-31+G(d,p)/IEFPCM]. These reactions are found to proceed in a stepwise fashion. Computations revealed that these transformations involve complex mechanisms including zwitterionic intermediates and several unstable alternate cycloadducts arising from (2 + 2) cycloadditions and hetero-Diels-Alder reactions. The observed regio- and stereochemistry can be rationalized by a combination of kinetic and thermodynamic control.