Three competitive transition states in the benzoin condensation compared to the clear rate-determining step in the Cannizzaro reaction

Abstract

The Cannizzaro reaction (“RX C”) and the benzoin reaction (“RX B”) were investigated by density functional theory calculations. Reaction models (benzaldehyde)₂ + X⁻ + (H₂O)_n [for X⁻ = OH⁻ (RX C) n = 8, and for X⁻ = CN⁻ (RX B) n = 8 or n = 14] were adopted. Three transition states (TSs) were obtained for RX C, and the rate-determining step was confirmed to be the hydride shift. The single electron transfer path was also obtained, which is supported by the C–O⁻⋯CHO attraction. In RX B, seven TSs were obtained. The C⋯C bond formation TS, TS4(B), was found to be the rate-determining step. However, the carbanion-formation TS (TS3(B)) and the CN⁻ release TS (TS7(B)) are also of large activation free energies (ΔG^‡s). The result ΔG^‡(TS4(B)) ≥ΔG^‡(TS7(B)) ∼ΔG^‡(TS3(B)) was obtained with both n = 8 and n = 14 models. Proton relays along the linear hydrogen bonds are concerned with bond interchanges and promote well arranged and successive elementary processes in RXs C and B.