Chem. Pharm. Bull. 53(7) 820—825 (2005)

The abstraction of the active hydrogen of RCOCH2R by a base catalyst B M , and the subsequent formation of the enolate [RCOCHR ] M are described in Eq. 1 (deprotonation process). Secondly, the creation of a C–N bond, in which [RCOCHR ] M reacts with R ONO to form complex [RCOCHNO(OR )R ] M , is shown in Eq. 2 (C–N bond formation process). Finally, the reaction between [RCOCHNO(OR )R ] M with base catalyst B , then deprotonation and elimination of the [R O] group from [RCOCHNO(OR )R ] M to form the oxime anion [RCOC( NO)R ] M is presented in Eq. 3 (elimination process). Among the three processes, the rate-determining step in the nitrosation is the C–N bond formation process. The structure of the transition state (TS) of the C–N bond formation process is reflected as the ratio (E/Z) of the Eand Z-isomers of the resulting oxime. In a previous paper, two TS models, “metal-chelated pericyclic TS” (TSCHELATED) and “open-chain TS without metal” (TSOPEN), were presented to explain the change in the E/Z ratio as the reaction changed from aprotic solvent to protic solvent conditions. In this report, the stereochemical reaction mechanisms using two types of TS models during the nitrosation of 1 or 2 with 3 (shown in Eq. 4) were studied using an ab initio molecular orbital (MO) method. Differences in the calculations between the previous and present studies are attributable to: 1) tert-butyl nitrite being used instead of methyl nitrite, 2) following the reaction via TSOPEN during the C–N bond formation process, the subsequent elimination with the base proceeding without the participation of the counter-cation of the base, and 3) during the intrinsic reaction coordinate (IRC) calculations for the elimination process, not fixing the bond and dihedral angles in the geometric parameter of the hydrogen atom, which is attacked by the base catalyst, at 90 degrees.