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The α-effect exhibited in gas-phase S(N)2@N and S(N)2@C reactions
Yi Ren1, Xi-Guang Wei, Si-Jia Ren
1College of Chemistry and Key State Laboratory of Biotherapy, Sichuan University, Chengdu 610064, China. renyi@scu.edu.cn
Journal of Computational Chemistry
|June 21, 2013
Summary
This study investigates the alpha-effect in gas-phase S(N)2@N reactions, finding enhanced reactivity due to the alpha-atom. The alpha-effect in these nitrogen-centered reactions originates from transition state stabilization.
Area of Science:
- Computational Chemistry
- Reaction Mechanisms
- Quantum Chemistry
Background:
- The alpha-effect describes enhanced reactivity of nucleophiles with an atom adjacent to a heteroatom bearing a lone pair.
- Understanding the alpha-effect in S(N)2@N reactions is crucial for comparing it with well-studied S(N)2@C systems.
Purpose of the Study:
- To explore the alpha-effect in gas-phase S(N)2@N reactions.
- To compare the characteristics of the alpha-effect in S(N)2@N reactions with those in S(N)2@C reactions.
- To elucidate the origin of the alpha-effect in S(N)2@N reactions.
Main Methods:
- Theoretical study using G2(+)M computational method.
- Investigation of six alpha-oxy-nucleophiles (FO(-), ClO(-), BrO(-), HOO(-), HSO(-), H2NO(-)).
- Reactions with nitrogen-based electrophiles (NR2Cl) and carbon-based electrophiles (RCl).
Main Results:
- An enhanced reactivity, or alpha-effect, was observed in all studied S(N)2@N systems.
- The magnitude of the alpha-effect in S(N)2@N reactions was generally smaller than in analogous S(N)2@C reactions, but trends were similar.
- Activation strain and thermodynamic analyses indicated transition state stabilization as the primary source of the alpha-effect in S(N)2@N reactions.
- Negative hyperconjugation (n(N) → σ*(O-Y)) contributed to the enhanced thermodynamic stability of products.
Conclusions:
- The alpha-effect exists in gas-phase S(N)2@N reactions, primarily due to transition state stabilization.
- Similarities and differences in the alpha-effect were identified between S(N)2@N and S(N)2@C reactions.
- Reactivity trends with different electrophiles (NMe2Cl vs. i-PrCl) in S(N)2 reactions were observed, contrasting with some experimental findings.
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