Hyperconjugation effect in substituted methyl boranes: an orbital deletion procedure analysis
Yirong Mo1, Haijun Jiao, Paul von Ragué Schleyer
1Department of Chemistry, Western Michigan University, Kalamazoo, Michigan 49008, USA. ymo@wmich.edu
Hyperconjugation in methyl boranes was quantified using orbital deletion. Stabilization energies ranged from 3.4 to 6.8 kcal/mol, influenced by substituent electronegativity and bonding competition.
Area of Science:
- Computational Chemistry
- Quantum Chemistry
- Organoboron Chemistry
Background:
- Hyperconjugation is a key electronic effect influencing molecular structure and stability.
- Substituted methyl boranes offer a model system to study hyperconjugation due to the vacant p-orbital on boron.
Purpose of the Study:
- To quantitatively evaluate the hyperconjugation effect in various substituted methyl boranes (XCH2BH2).
- To investigate the relationship between substituent properties and the strength of hyperconjugative interactions.
- To understand the structural consequences of hyperconjugation in these systems.
Main Methods:
- Utilized the orbital deletion procedure (ODP) to isolate and quantify hyperconjugation.
- Systematically varied the substituent (X) on the methyl group, including H, CH3, NH2, PH2, OH, SH, F, Cl, and Br.
- Analyzed structural parameters such as B-C bond length and bond angles.
Main Results:
- Hyperconjugative stabilization energies were determined, ranging from 3.4 to 6.8 kcal/mol for most compounds.
- Significant structural changes observed include B-C bond shortening and reduction in XCB/HCB bond angles.
- The NH2 substituent uniquely formed a three-membered ring, deviating from typical hyperconjugation.
Conclusions:
- Hyperconjugation strength correlates with the electronegativity of the substituent X.
- Competition between sigma(C-X) and sigma(C-H) donation influences conformational preferences (staggered vs. eclipsed).
- Electron correlation and steric effects become important when sigma-bond donation abilities are similar.
More Related Videos
07:50Efficient Synthesis of All-Carbon Quaternary Centers via the Conjugate Addition of Functionalized Monoorganozinc Bromides
Published on: May 26, 2019
08:56Synthesis of a Borylated Ibuprofen Derivative Through Suzuki Cross-Coupling and Alkene Boracarboxylation Reactions
Published on: November 30, 2022
Related Concept Videos
Hybridization of Atomic Orbitals I
Hybridization of Atomic Orbitals II
Molecular Orbital Theory II
Hydroboration-Oxidation of Alkenes
Regioselectivity and Stereochemistry of Hydroboration
Hydroboration proceeds in a concerted fashion with the attack of borane on the π bond, giving a cyclic four-centered transition state. The –BH2 group is bonded to the less substituted carbon and –H to the more substituted carbon. The concerted nature requires the simultaneous addition of –H and –BH2 across the same face of the alkene giving syn stereochemistry.
π Molecular Orbitals of the Allyl Cation and Anion
