Abstract

Photochemical denitrogenation reactions of bicyclic azoalkanes produce strained bicyclic compounds of interest to synthetic organic chemists. We report a computational study on the mechanism of diazabicyclo[2.2.1]heptenes to address long standing mechanistic questions. Indeed, the mechanism of these reactions have been disputed for over six decades. We employed non-adiabatic molecular dynamics (NAMD) simulations combined with state-of-the-art multireference quantum mechanical calculations to understand the photophysical properties and mechanisms of these diazabicyclo[2.2.1]heptenes. The energetically accessible lowest excitations are n_NN(σ_CN) → π* and range from 3.94 – 3.97 eV. From the >292 trajectories, the reaction proceeds through a dynamically concerted but asynchronous denitrogenation reactions. One σ_CN bond breaks along the S₁-surface; the other σ_CN breaks after hopping to the S₀. We identified two clusters of S₁/S₀ surface hopping points from these trajectories. In the first cluster, the methylene bridge is fully inverted relative to the reactant geometry. In the second cluster, the inversion is only partial, with one of the carbon atoms in the methylene bridge inverted relative to the reactant. We identified each cluster's corresponding minimum energy conical intersection (MECI), indicating at least two possible S₁/S₀-MECIs.  Our dynamics simulations illustrate that inversion begins in the excited state immediately after the first σ_CN bond breaks. This inversion is driven by the atomic momenta acquired after the bond breaks. These dynamical effects promote the formation of the inverted housane, thereby explaining the observed selectivities. A minority of trajectories undergo thermal conversion in the ground state, producing the minor retained housane product from inverted housane.

Keywords: Photochemistry; non-adiabatic molecular dynamics; quantum mechanical calculations; stereoselectivity

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Adao Gomes, L.; Lopez, S. A. Beilstein Arch. 2025, 202531. doi:10.3762/bxiv.2025.31.v1

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