Rolf Huisgen's Classic Studies of Cyclic Triene Diels-Alder Reactions Elaborated by Modern Computational Analysis.

: Rolf Huisgen explored the Diels-Alder reactions of 1,3,5-cycloheptatriene (CHT) and cyclooctatetraene (COT) with dienophiles maleic anhydride and 4-phenyl-1,2,4-triazoline-3,5-dione (PTAD) to determine the kinetics and mechanisms of various electrocyclizations and Diels-Alder reactions.  These reactions have been examined with density functional theory. Modern computational chemistry has provided information not previously available by experiment. Transition states for all the reactions have been identified, and their Gibbs energies are used to explain the experimental reactivities. Zwitterionic intermediates were not found in the [4+2] cycloadditions of both CHP or COT with PTAD and thus are not involved in these reactions. [2+2+2] cycloadditions as an alternative path to the Diels-Alder products are highly disfavored. Rapid double nitrogen inversion was found for the cycloaddition products with PTAD.