Post-Transition-State Dynamics Induced Product Diversity in the Thermal Rearrangement of Cyclopropenyl Azides

Cyclopropenyl azides (<b>1</b>) are known to undergo three major thermal rearrangements, namely, 3,3-sigmatropic rearrangement to form 1,2,3-triazine (<b>2</b>), allylic arrangement to form the degenerate product (<b>3</b>), and N₂ elimination to form azete (<b>4</b>). Previous experiments have reported conflicting product distribution that is contingent upon the nature of the substituents on the cyclopropenyl ring and also the reaction temperature. The present article describes quasi-classical trajectory (QCT) calculations on the density-functional theory (DFT) potential energy surfaces for a variety of substituted cyclopropenyl azides (<b>1a</b>-<b>1j</b>). Electron-donating substituents not only produce the degenerate product as the kinetically controlled product (KCP) but also assist post-transition state bifurcation (PTSB) between 1,2,3-triazine and azete. For the electron-withdrawing F, Cl, and Br substituents, 1,2,3-triazine is the KCP and abrogates dynamical effects. Post-transition-state bifurcation from the triazine-forming transition state allows direct N₂ elimination without the formation of 1,2,3-triazine. Trajectories produce azete either via a triazabenzvalene-type intermediate (<b>INT</b>) or by skipping <b>INT</b> altogether. For R = <i>t</i>-Bu, at low and ambient temperatures, the degenerate product is formed kinetically, but at high temperature, 1,2,3-triazine and azete are found to be dynamically entangled. This corroborates azete as the major product and 1,2,3-triazine as the byproduct at <i>T</i> = 125 °C in experiments. This work highlights the critical role of nonstatistical dynamics beyond the intrinsic reaction coordinate (IRC)-based minimum energy paths to understand multiple product channels from common reactants or intermediates.