Reaction of methoxide ion with the Z‐ and E‐isomers of O‐methylbenzohydroximoyl cyanide. Theoretical calculations on the conformations of the tetrahedral intermediate

Abstract

Theoretical calculations (HF/6–31 + G*, MP2/6–31 + G*//HF/6–31 + G*, B3LYP/6–31 + G*//HF/6–31 + G*, HF-SCIPCM/6–31 + G*//HF/6–31 + G*, and B3LYP-SCIPCM/6–31 + G*//HF/6–31 + G*) were carried out on the tetrahedral intermediate RC(CN)(OCH3)—NOCH3 −, where R = C6H5, CH2CH or H, generated by nucleophilic attack of methoxide ion on (Z)- or (E)-O-methylhydroximoyl cyanide [RC(CN)NOCH3]. These calculations indicate that the staggered conformation 4C, which would lead to an E-substitution product, is approximately 7 (R = H) or 8 kcal mol−1 (R = CH2CH) less stable than conformation 4A, which leads to a Z-substitution product. Furthermore, the lowest energy transition state leading to a Z-substitution product is approximately 5 kcal mol−1 lower in energy than the lowest energy transition state leading to an E-substitution product. These results provide an explanation for the experimental results reported recently that (Z)-O-methylbenzohydroximoyl cyanide gives only the less thermodynamically stable Z-substitution product under conditions where the purported tetrahedral intermediate should have a long enough lifetime to establish an equilibrium between all the staggered conformations. Copyright © 1999 John Wiley & Sons, Ltd.