Stereoelectronic effects in Menshutkin‐type S_N2 reactions: theoretical study based on through‐space/bond orbital interaction analysis

Abstract

Through-space/bond orbital interaction analysis has been applied to investigate the stereoelectronic effects on stabilizing the transition state of Menshutkin-type S N 2 reactions. The mechanism of how the substituent effects work on accelerating the reactions has been demonstrated from orbital interaction perspective. The geometrical structures and Mulliken charge distributions have been compared to elucidate the substituent effects for the S N 2 reaction center. It is found that the substituents lower the activation energies by strengthening the orbital interactions in the S N 2 reaction process. When electron-donating and electron-accepting substituents (-C 6 H 5 and -CHO) are introduced to the same central carbon at the reaction center, the symmetry allows the π-π* interactions among the donor and acceptor in the transition state. It stabilizes the transition state much more than the reactant complex. And the π-π* interactions are estimated to decrease about 2.28 kcal/mol of the energy for transition state. The σ-like orbitals of the partial bond around the central carbon are reactive, and the σ-π* orbital interactions stabilize the reactant complex much more than the π-σ* interaction. When the σ-π* and π-σ* interactions are deleted from the system, the activation energy increases and turns close to the values of the systems which are without such substituents. It can be concluded that the π-π*, σ-π*, and π-σ* interactions cooperatively accelerates the S N 2 reaction by stabilizing its transition state.

Activation parameter changes as a mechanistic tool in SN2 reactions in solution

“…When electron-donating and electron-withdrawing substituents are introduced to the same central carbon at the reaction centre of Menschutkin-type SN2 reaction, the π -π * , σ -π * and π -σ * interactions among these substituents in the transition state cooperatively accelerate this reaction by stabilizing its transition state [105]. However, when electrondonating and electron-withdrawing substituents are varied in α-position to the reaction centre of neutral nucleophile (entries 76, 77 in Table 2), there is a curved Brønsted plot for the SN2 reactions (Scheme 7) [66] due to the variable TS-9 [20].…”

The Role of Hyperconjugation on the Structure and C–H Stretching Frequencies of 3,3′-Ethane-1,2-diyl- bis-1,3,5-triazabicyclo[3.2.1]octane (ETABOC): An X-Ray Structure and Vibrational Study

“…Additionally, it is well known that for diazabicycloalkanes and other polycyclic systems, the lone pair-lone pair interactions, either through space and/or sigma bonds, play an important role in stabilizing these polyamines, including their protonated or radical cation forms [6][7][8][9][10][11][12][13]. Moreover, such interactions depend on the spatial orientation of the lone pairs [12,13].…”