On Gas Phase α-Effects. 1. The Gas-Phase Manifestation and Potential SET Character

Abstract

The possibility of a gas-phase alpha-effect has been explored for the methyl transfer from methyl formate to hydroxide, hydroperoxide, and ethoxide by computing barrier heights at the HF/6-311++G(2df,2p) level of theory. The alpha-nucleophile (hydroperoxide) is found to have a lower barrier than the gas-phase-acidity-matched normal nucleophile (ethoxide) by 3.6 kcal/mol, offering evidence for a gas phase alpha-effect. A Shi-Boyd analysis for these reactions indicates that there is more single-electron-transfer character in the hydroperoxide transition state than for either hydroxide or ethoxide, further bolstering the existence of a gas-phase alpha-effect and the appropriateness of the Hoz model for the alpha-effect.

Origin of the α-Effect in Nucleophilic Substitution Reactions of Y-Substituted Phenyl Benzoates with Butane-2,3-dione Monoximate and Z-Substituted Phenoxides: Ground-State Destabilization vs. Transition-State Stabilization

“…[1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] Thus, the enhanced reactivity shown by these nucleophiles was termed the α-effect. 1 Numerous studies have been performed to investigate the cause of the α-effect.…”

“…[2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] Many theories have been advanced to explain the α-effect, e.g., destabilization of the ground state (GS) due to electronic repulsion between the nonbonding electron pairs, transition state (TS) stabilization including general acid/base catalysis, thermodynamic stability of products, solvent effects. [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] However, none of these theories is conclusive. Particularly, solvent effect on the α-effect remains controversial.…”

“…Particularly, solvent effect on the α-effect remains controversial. [8][9][10][11][12][13][14][15][16][17][18][19] Solvent effect was suggested to be unimportant since the magnitude of the α-effects was found to be similar for reactions performed in H 2 O and in organic solvents such as MeCN and toluene. 8 Besides, it has been reported that α-nucleophiles are intrinsically more reactive than normal nucleophiles of similar basicity in gas-phase reactions.…”

“…8 Besides, it has been reported that α-nucleophiles are intrinsically more reactive than normal nucleophiles of similar basicity in gas-phase reactions. 9,10 High-level theoretical calculations have also shown that α-nucleophiles (e.g., HOO -, H2NO -, FO -and ClO -) exhibit lower activation energies than normal nucleophiles of similar basicity in gas-phase S N 2 reactions.…”

“…[8][9][10][11] In contrast, DePuy et al concluded that solvent effect is responsible for the α-effect shown by HOO -in H2O, since the α-nucleophile did not exhibit the α-effect in the gas-phase reaction with methyl formate. 12 Furthermore, from recent gas-phase ion-molecule studies, Bierbaum et al have found that α-nucleophiles such as HOO -, BrO -and ClO -do not exhibit enhanced reactivity in gas-phase reactions with alkyl chlorides.…”