A kinetic study was carried out for the reaction of benzaldehyde and borane (BH₃) in tetrahydrofuran. The effect of BH₃ concentration on the rate constant showed that the reaction order with respect to BH₃ was 1.6. Substituent effects gave a linear Hammett plot with a ρ value of −0.51. It was concluded that the reaction proceeds through a rate-determining hydride-transfer transition state with two BH₃ molecules, in which one molecule of BH₃ acts as a reducing agent and the other serves as a catalyst. Copyright © 2011 John Wiley & Sons, Ltd.

The reaction of PhCOCH₂Cl with OH^– gave the expected α-substituted alcohol (PhCOCH₂OH) in addition to three dimer products. To clarify whether the substitution product is formed by direct S_N2 or via carbonyl addition, the reaction of PhCOCH₂Cl and OMe^– was examined. The reaction gave two products, PhCOCH₂OH as the major product after acid hydrolysis and PhCOCH₂OMe as the minor product. An electron-withdrawing substituent on the phenyl ring enhanced the overall reactivity and gave more alcohol than ether. It was concluded that the alcohol was formed via carbonyl addition-epoxidation route, whereas the ether was formed by the direct substitution route. Copyright © 2012 John Wiley & Sons, Ltd.

The mechanistic dichotomy between concerted E2 and stepwise E1cb of the base-promoted elimination of 2-aryl-3-chloro-2-R-propanols was examined computationally at the HF, M05-2X, and MP2 levels of theory. Optimizations of transition states (TSs) and reaction intermediates, and intrinsic reaction coordinates (IRC) calculations showed that there was a single reaction route for each substrate, and that the mechanism could be changed from E2 to E1cb by making a carbanion intermediate more stable through the introduction of electron-withdrawing substituents. Molecular dynamics simulations revealed that trajectories started at a single TS led directly to two product regions; the carbanion intermediate region in the E1cb mechanism, and the alkene product region in the E2 mechanism, through path bifurcation after the TS. The present system is a new example of bifurcation in reactions of closed-shell molecules. The overall reaction mechanism changes dynamically from E2 to E1cb by a gradual change in the ratio of E2 and E1cb trajectories, rather than a path switch in concurrent pathways.

Pinacol rearrangement is often written to proceed via 1,2-Me migration to the tertiary cationic center, followed by deprotonation to give pinacolone. Computational study was carried out for model reactions to clarify why the migration of the OH group is not involved in the mechanistic scheme despite the fact that OH is a better migrating group than Me. It was found that the migratory aptitude of X in both XCMe₂-CH₂Cl and XCMe₂-CMe₂Cl is in the order, NH₂ > OMe > Ph > Me, indicating that a migrating group with n-electrons has a larger aptitude than a π- or σ-electron group. However, the reactivity differences became much smaller for XCMe₂-CMe₂OMe, a model compound for aliphatic pinacol rearrangement. Calculations of MeOCMe₂-CMe₂OMe revealed that three initial ionization steps, CO heterolysis, concerted OMe migration and concerted Me migration, compete with each other. On the other hand, the ring-opening step of the epoxide-type intermediate formed via OMe migration was shown to have quite a large activation barrier. It was suggested that aliphatic pinacol rearrangement proceeds via the concerted Me migration route or the CO heterolysis-Me migration-deprotonation route. Epoxide may form by the concerted MeO migration, but it would not be an important intermediate of pinacol rearrangement. Copyright © 2010 John Wiley & Sons, Ltd.