Molecular Dynamics of the Diels-Alder Reactions of Tetrazines with Alkenes and N-2 Extrusions from Adducts

The cycloadditions of tetrazines with cyclopropenes and other strained alkenes have become among the most valuable bioorthogonal reactions. These reactions lead to bicyclic Diels-Alder adducts that spontaneously lose N-2. We report quantum mechanical (QM) and quasiclassical trajectory simulations on a number of these reactions, with special attention to stereoelectronic and dynamic effects on spontaneous N-2 loss from these adducts. QM calculations show that the barrier to N-2 loss is low, and molecular dynamics calculations show that the intermediate is frequently bypassed dynamically. There is a large preference for N-2 loss anti to the cyclopropane moiety rather than syn from adducts formed from reactions with cyclopropenes. This is explained by the interactions of the Walsh orbitals of the cyclopropane group with the breaking C-N bonds in N-2 loss. Dynamical effects opposing the QM preferences have also been discovered involving the coupling of vibrations associated with the formation of the new C-C bonds in the cycloaddition step, and those of the breaking C-N bonds during subsequent N-2 loss. This dynamic matching leads to pronounced nonstatistical effects on the lifetimes of Diels-Alder intermediates. An unusual oscillatory behavior of the intermediate decay rate has been identified and attributed to specific vibrational coupling.