A Direct Dynamics Study of Protonated Alcohol Dehydration and the Diels-Alder Reaction

The dynamics of dehydration of the protonated (R)-3,3-dimethylbutan-2-ol ( pinacolyl alcohol), [(CH3)(3)C-CH(OH2)CH3](+), and of ethene + 1,3-butadiene cycloaddition were studied with the Born-Oppenheimer molecular dynamics (BOMD) technique for direct dynamics using the AM1 method. More than 10,000 trajectories were generated, most of them related to the unexplored simulated annealing/fragmentation approach. The AM1 potential energy surface ( PES) for the protonated pinacolyl alcohol presents two transition states related to the [(CH3)(3)C-CHCH3](+)center dot center dot center dot OH2 intermediate complex and to CH3 migration leading to the [(CH3)(2)C-CH(CH3)(2)](+) center dot center dot center dot OH2 product complex. Direct dynamics yielded negligible trajectories involving these complexes, since the momentum acquired by the H2O fragment led to a complete dissociation. Thus, rearrangement of the secondary carbocation [(CH3)(3)C-CHCH3](+) was practically inexistent during the dynamics. Despite the concerted path (H2O dissociation and CH3 migration) not being an IRC (intrinsic reaction coordinate) path in AM1-PES, a statistically significant number of trajectories involved this path. As for the Diels-Alder reaction, even when started from a symmetric transition state using the spin restricted AM1 wavefunction, the dynamics yielded a significant number of trajectories that followed asymmetric, i.e. non-IRC, paths toward cyclohexene, independent of the initialization approach. It is noteworthy that all these asymmetric path trajectories led to a concerted reaction mechanism.