One-Bond-Nucleophilicity and -Electrophilicity Parameters: An Efficient Ordering System for 1,3-Dipolar
Le Li1, Robert J Mayer2, Armin R Ofial1
1Department Chemie, Ludwig-Maximilians-Universität München, Butenandtstr. 5-13, 81377 München, Germany.
Journal of the American Chemical Society
|March 23, 2023
Summary
This study introduces a new linear free energy relationship to precisely predict reaction rates in 1,3-dipolar cycloadditions, offering a more accurate alternative to Frontier Molecular Orbital theory for understanding diazoalkane reactivity.
Area of Science:
- Organic Chemistry
- Chemical Kinetics
- Computational Chemistry
Background:
- Diazoalkanes are versatile 1,3-dipoles reacting via Huisgen cycloadditions.
- Frontier Molecular Orbital (FMO) theory is traditionally used to explain their reactivity patterns.
- A previously identified low-lying unoccupied molecular orbital can alter reaction mechanisms.
Purpose of the Study:
- To propose and validate a new linear free energy relationship for analyzing 1,3-dipolar cycloaddition reactions.
- To offer a more accurate predictive model for reaction rates compared to FMO theory.
- To quantify the energy of concert in transition states.
Main Methods:
- Measurement of rate constants for diazoalkane cycloadditions.
- Application of a linear free energy relationship (lg k2 = sN(N + E)) with solvent-dependent parameters.
- Comparison of experimental and calculated Gibbs energies of activation.
- Density Functional Theory (DFT) calculations with the SMD solvent model.
Main Results:
- The proposed linear free energy relationship accurately predicts reaction rates and mechanism changes.
- V-shaped plots derived from the relationship provide absolute rate constants for borderline stepwise reactions.
- This new approach offers more precise predictions of dipolarophile reactivity than FMO theory or ionization potentials.
- DFT calculations support the mechanistic interpretations.
Conclusions:
- The new linear free energy relationship provides a robust framework for understanding 1,3-dipolar cycloadditions.
- It offers superior predictive power for reaction rates and mechanisms, especially for highly asynchronous concerted cycloadditions.
- This work refines the understanding of transition state stabilization and reaction pathways in cycloaddition chemistry.
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