Dynamics and the Failure of Transition State Theory in Alkene Hydroboration

Transition state theory fails to accurately predict the selectivity in an example where it is ubiquitously invoked, hydroboration. The hydroboration of terminal alkenes with BH3 is moderately regioselective, affording an 88:12-90:10 ratio of anti-Markovnikov/Markovnikov adducts. High-level ab initio calculations predict too large of an energy difference between anti-Markovnikov and Markovnikov transition structures to account for the observed product ratio, and consideration of calculational error, solvent, tunneling, and entropy effects does not resolve the discrepancy. Trajectory studies, however, predict well the experimental selectivity. The decreased selectivity versus transition state theory arises from the excess energy generated as the BH3 interacts with the alkene, and the observed selectivity is proposed to result from a combination of low selectivity in direct trajectories, moderate RRKM selectivity, and high selectivity after thermal equilibration.