Origin of the α‐Effect in S_N2 Reactions

Abstract

The α‐effect is a term used to explain the dramatically enhanced reactivity of α‐nucleophiles (R−Y−X:−) compared to their parent normal nucleophile (R−X:−) by deviating from the classical Brønsted‐type reactivity‐basicity relationship. The exact origin of this effect is, however, still heavily under debate. In this work, we have quantum chemically analyzed the α‐effect of a set of anionic nucleophiles, including O‐, N‐ and S‐based normal and α‐nucleophiles, participating in an SN2 reaction with ethyl chloride using relativistic density functional theory at ZORA‐OLYP/QZ4P. Our activation strain and Kohn–Sham molecular orbital analyses identified two criteria an α‐nucleophile needs to fulfill in order to show α‐effect: (i) a small HOMO lobe on the nucleophilic center, pointing towards the substrate, to reduce the repulsive occupied–occupied orbital overlap and hence (steric) Pauli repulsion with the substrate; and (ii) a sufficiently high energy HOMO to overcome the loss of favorable HOMO–LUMO orbital overlap with the substrate, as a consequence of the first criterion, by reducing the HOMO–LUMO orbital energy gap. If one of these two criteria is not fulfilled, one can expect no α‐effect or inverse α‐effect.

C(spn)−X (n=1–3) Bond Activation by Palladium

“…Note that these EDA plots should be interpreted with caution. We previously found,[ 7c , 7d , 7e ] as one would expect, that the EDA results are strongly dependent on the distance between the reactants, in this case, the catalyst and substrate. The Pd⋅⋅⋅C distance is more than 0.2 Å longer for the Pd+H 3 C−CH 2 −H than Pd+HC≡C−H, i.e., 2.14 Å and 1.92 Å, respectively, at the same consistent TS‐like geometries (C⋅⋅⋅H bond stretch of 0.50 Å), as obtained from the IRC.…”

How Solvation Influences the SN2 versus E2 Competition

“…This critical reaction coordinate undergoes a well-defined change during the reaction from the reactant complex via the transition state to the product and is shown to be a valid reaction coordinate for studying S N 2/E2 reactions. 3h , 7 …”

“…This methodological approach enables the investigation of the potential energy surface and the activation barrier by decomposing the system’s total energy into chemically intuitive terms, proving to be valuable for understanding chemical reactivity. 3h , 7 …”

Stronger Together! Mechanistic Investigation into Synergistic Effects during Homogeneous Carbon Dioxide Hydrogenation Using a Heterobimetallic Catalyst

“…With [Mo 0 (CO) 3 ;Ir III H] and [W 0 (CO) 3 ;Ir III H], this is not the case, leading to a Pauli repulsion-lowering and hence reducing the energy of hydride transfer. This effect of Pauli repulsion-lowering was found to play a large role in catalysis and the lowering of the reaction barrier. − Thus, it is reasonable to assume a lowering of [TS2] in the catalytic cycle of C1 and C2 compared to C3 , hence leading to the overall better TON. It should further be noted that while C2 shows HOMO and LUMO arrangements similar to those of C1 , the molecular orbitals in the monometallic M1 can be compared to those of C3 because the HOMO is located at Ir as well (see chapter 9.2).…”