Chronology of CH···O Hydrogen Bonding from Molecular Dynamics Studies of the Phosphoric Acid-Catalyzed Allylboration of Benzaldehyde.

CH···O hydrogen bonds involving formyl groups have been invoked as a crucial factor controlling many asymmetric transformations. We conducted quasi-classical direct molecular dynamics simulations on the phosphoric acid-catalyzed allylboration of benzaldehyde to understand the synergy between the phosphoric acid OH···O hydrogen bond and the secondary CH···O formyl hydrogen bond as the reaction occurs. In the gas phase, both the CH···O and OH···O hydrogen bonds are enhanced from reactants to transition states. In toluene, the trend of H-bond enhancement is observed with a smaller magnitude because of solvent caging. The strength of the formyl hydrogen bond in the TS, a second CH···O interaction between the P═O oxygen and ortho-hydrogen of the phenyl ring and the OH···O hydrogen bond were determined using quantum mechanical calculations (4.6, 1.0, and 14.5 kcal mol-1, respectively).