Tutorial Review on Modeling Nonstatistical Reactivity: an Example of Light and Heat in the Garratt–Braverman/[1,5]-H Shift of Ene-diallenes

Broad terms in the literature, such as nonstatistical reactivity or nontraditional luminescence, emerge when standard theories fail to explain experimental results. In the case of nonstatistical and dynamic effects, reaction rates and product ratios may vary wildly from transition state theory (TST) predictions and are commonly accompanied by a lack of temperature dependence. In this tutorial, we explain how to use modern and freely available computational chemistry tools to model a reported nonstatistical reaction of a relatively large organic molecule, the thermal Garratt–Braverman/[1,5]-H shift of an ene-diallene, such that a non-expert can easily replicate our approach by following our steps. As a team of synthetic organic chemists and computational chemists, we also hope to encourage the use of preparatory computational work that may aid in reaction design during the experimental process, not just as complementary data to finished experimental studies. Through this approach, we discover that the thermal Garratt–Braverman/[1,5]-H shift exhibits a parallel light-enabled reaction that bypasses the rate-limiting first step. Additionally, when tunneling effects are accounted for, TST predictions return to realistic values, only to be disproved again by careful variable temperature experiments. As the motifs of reactive π−π∗ absorptions, hydrogen transfers, and diradical intermediates are quite common, the points presented in this paper are broadly applicable and indicative of the underlying complexity behind many chemical reactions that exhibit unexpected rates and ratios. The failure of TST also underscores a critical limitation of massive reaction network discovery schemes that heavily rely on calculated ground state activation energies and the potential pitfalls of the conventional free energy diagram.