Mechanism of the vinylcyclopropane-cyclopentene rearrangement studied by quasiclassical direct dynamics

The stereochemical course of the thermal rearrangement of vinylcyclopropane to cyclopentene is computed using quasiclassical trajectories run on a modified AM1 potential parametrized to fit ab initio calculations. At 573 K, 34000 trajectories are initialized quasiclassically with a Boltzmann distribution at 3 transition state (TS) structures, trideuterated for a total of 8 diastereomeric TSs. The computed product ratio is si:sr:ar:ai = 42:30:10:18 (exptl 40:23:13:24), where s,a refers to suprafacial or antarafacial allylic participation and r,i refers to retention or inversion of the migrating methylene. Initialization at each TS leads to all 4 products-that is, the product distribution is entirely under dynamical control. The temperature dependence of the product ratio over 400-1000 K is small. At 573 K, 83% of the trajectories have lifetimes under 400 fs. For times t ( 400 fs the product ratio is strongly time-dependent; for t > 400 fs the product ratio is approximately constant and closer to statistical. However, trajectories initialized at the 3 TS structures do not give identical product distributions under any circumstances. Root-mean-square angular velocities of the torsions are approximately constant over nearly the entire course of the reaction. These data demonstrate nonstatistical dynamics and are inconsistent with a mechanism involving a statistical intermediate. Instead, the mechanism consists of 4 competing direct reactions.