Understanding the origin of stereoselectivity in the photochemical denitrogenation of 2,3-diazabicyclo[2.2.1]heptene and its derivatives with non-adiabatic molecular dynamics

TL;DR

This paper uses advanced simulations to explain how a specific chemical reaction produces different products based on how bonds break and atoms move.

Contribution

The study reveals the mechanism of stereoselectivity in the photochemical denitrogenation of diazabicyclo[2.2.1]heptenes using non-adiabatic molecular dynamics.

Findings

The reaction proceeds through a dynamically concerted but asynchronous denitrogenation process.

Two clusters of surface hopping points were identified, leading to different product geometries.

Dynamical effects after bond breaking drive the formation of the inverted housane product.

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 has 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 nNN(σCN) → π* and range from 3.94–3.97 eV. From the >292 trajectories, the reaction proceeds through a dynamically concerted but asynchronous denitrogenation reaction. One σCN bond breaks along the S1 surface; the other σCN 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 σCN 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/diradical.

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