Three competitive transition states in the benzoin condensation compared to the clear rate-determining step in the Cannizzaro reaction.
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(-)CH[double bond, length as m-dash]O attraction. In RX B, seven TSs were obtained. The CC 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 (DeltaG(double dagger)s). The result DeltaG(double dagger)(TS4(B)) > or = DeltaG(double dagger)(TS7(B)) approximately DeltaG(double dagger)(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.
Yamabe
Shinichi
S
Department of Chemistry, Nara University of Education, Takabatake-cho, Nara 630-8528, Japan. yamabes@nara-edu.ac.jp
Yamazaki
Shoko
S