Dynamical Effects along the Bifurcation Pathway Control Semibullvalene Formation in Deazetization Reactions.

Post-transition-state dynamics during the deazetization of 3 resulting in two degenerate semibullvalenes (4 and 5) have been investigated with density functional theory (DFT) and quasi-classical trajectory (QCT) calculations. Removal of N2 from 3 occurs through a synchronous and concerted pathway through an ambimodal transition-state (TS1). In addition to TS2, the exclusively anticipated product from minimum energy pathway (MEP) calculations, trajectories initiated from TS1 produce 4, TS2, and 5 in 1:1:1 ratio. Isotopic substitutions (12C(13C/14C)-H(D) at 1-2 positions) result in purely Newtonian kinetic isotope effects (4:5 ≈ 1.4 for 13C1-13C2), an unequivocal evidence for dynamics controlled product formation.