title: Why Do Enolate Anions Favor O-Alkylation over C-Alkylation in the Gas Phase? The Roles of Resonance and Inductive Effects in the Gas-Phase SN2 Reaction between the Acetaldehyde Enolate Anion and Methyl Fluoride.
abstract: Contributions by resonance and inductive effects toward the net activation barrier were determined computationally for the gas-phase SN2 reaction between the acetaldehyde enolate anion and methyl fluoride, for both O-methylation and C-methylation, in order to understand why this reaction favors O-methylation. With the use of the vinylogue extrapolation methodology, resonance effects were determined to contribute toward increasing the size of the barrier by about 9.5 kcal/mol for O-methylation and by about 21.2 kcal/mol for C-methylation. Inductive effects were determined to contribute toward increasing the size of the barrier by about 1.7 kcal/mol for O-methylation and 4.2 kcal/mol for C-methylation. Employing our block-localized wave function methodology, we determined the contributions by resonance to be 12.8 kcal/mol for O-methylation and 22.3 kcal/mol for C-methylation. Thus, whereas inductive effects have significant contributions, resonance is the dominant factor that leads to O-methylation being favored. More specifically, resonance serves to increase the size the barrier for C-methylation significantly more than it does for O-methylation.
authors.affiliation: Department of Chemistry, Elon University , Elon, North Carolina 27244, United States.
authors.forename: Christian G
authors.initials: CG
authors.lastname: Seitz
authors.affiliation: Department of Chemistry, Western Michigan University , Kalamazoo, Michigan 49008, United States.
authors.forename: Huaiyu
authors.initials: H
authors.lastname: Zhang
authors.affiliation: Department of Chemistry, Western Michigan University , Kalamazoo, Michigan 49008, United States.
authors.forename: Yirong
authors.initials: Y
authors.lastname: Mo
authors.affiliation: Department of Chemistry, Elon University , Elon, North Carolina 27244, United States.
authors.forename: Joel M
authors.initials: JM
authors.lastname: Karty