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

Cyclopropenyl azides (1) are known to undergo three major thermal rearrangements, namely, 3,3-sigmatropic rearrangement to form 1,2,3-triazine (2), allylic arrangement to form the degenerate product (3), and N₂ elimination to form azete (4). 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 (1a-1j). 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 (INT) or by skipping INT altogether. For R = t-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 T = 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.