G2(+) Investigation on the α‐Effect in the S_N2 Reactions at Saturated Carbon

Abstract

As a continuing theoretical study on the alpha-effect in the S(N)2 reactions at saturated carbon centers, 28 gas-phase reactions have been examined computationally by using the high-level G2(+) method. The reactions include: Nu(-)+CH(3)X-->CH(3)Nu+X(-) (X=F and Cl; Nu(-)=HO(-), HS(-), CH(3)O(-), Cl(-), Br(-), HOO(-), HSO(-), FO(-), ClO(-), BrO(-), NH(2)O(-), and HC(==O)OO(-)). It was found that all alpha-nucleophiles examined exhibit downward deviations from the correlation line between the overall barriers and proton affinities for normal nucleophiles, indicating the existence of the alpha-effect in the gas phase. The transition states (TS) for the alpha-nucleophiles are characterized by less advanced C--X bond cleavages than the normal nucleophiles, leading to smaller deformation energies and overall barriers. The size of the alpha-effect is related to the electron density on the alpha-atom, and increases when the position of alpha-atom is changed from left to right and from bottom to top in the periodic table. The reaction with CH(3)F exhibits a larger alpha-effect than that with CH(3)Cl, which can be explained by a later TS and a more positively charged methyl group at the TS for CH(3)F, [NuCH(3)F](- not equal). Thus, a higher electron density on the alpha-atom and a more positive methyl moiety at the TS result in a larger alpha-effect.

The α-effect exhibited in gas-phase SN2@N and SN2@C reactions

“…Compared with the correlation, Δ H def vs. Δ H ≠ ovr , previously found in the S N 2@C reactions, the current results reveal a different situation. In fact, no Δ G ≠ ovr vs. Δ G def correlation is found for the S N 2@N reactions.…”

“…Previous studies suggested that the α‐effect was not manifested in the gas phase, but it arose from solvent effect . Our research interest in the S N 2 and E2 reactions has led us to conduct several theoretical investigations on the α‐effect in recent years . Our results indicate that the α‐effect does exist in gas‐phase reactions and suggest that α‐effect is measured by the downward deviation from the plot of overall enthalpy of activation, Δ H ≠ ovr , with respect to separated reactants vs. proton affinity (PA) of n ‐Nu (or base).…”

“…It was found that three anions, HOO − , MeOO − , and N 3 − each with an adjacent electron donating group, react particularly fast. The α‐effect has been studied for more than 50 years by experimental and theoretical chemists, and its origin has been rationalized by many theories, but still without a definitive answer. Theories that have been put forward for this effect include: (1) transition state (TS) stabilization, (2) destabilization of the ground state, (3) different solvent effects for normal and α‐Nu, and (4) thermodynamic stability of the product .…”

Nucleophilic Substitution (SN2): Dependence on Nucleophile, Leaving Group, Central Atom, Substituents, and Solvent

“…Ren and Yamataka studied the reaction between several other anionic nucleophiles (HO − , HS − , CH 3 O − , HOO − , HSO − , FO − , ClO − , BrO − , NH 2 O − , HC(=O)OO − ) with CH 3 FCl and CH 3 F at the G2(+) level in the gas‐phase. Their computations indicate that nucleophiles with an adjacent lone pair ( α ‐nucleophiles) display enhanced nucleophilicity, a phenomenon known as the α ‐effect .…”

Dual bifunctional catalysis and the α-effect in the reaction of hydroxylamine with phenylacetate

“…The high and unexpected reactivity of these species is called the a-effect, and the origin of this effect has attracted the attention of many researchers. [3][4][5][6][7][8][9][10][11] In fact, it remains controversial whether the a-effect is intrinsic to the nucleophile or has its origin in the solvent effect for anionic nucleophiles. Although recent high-level theoretical calculations have provided evidence that the a-effect is intrinsic to these nucleophiles, [6][7][8] the solvent effect is very important for these reactions and also contributes for the a-effect.…”