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B values as a sensitive measure of steric effects
Renzo Ruzziconi1, Sara Spizzichino, Lodovico Lunazzi
1Chemistry Department, University, Via Elce di Sotto 10 06100 Perugia, Italy. ruzzchor@unipg.it
Chemistry (Weinheim an Der Bergstrasse, Germany)
|January 30, 2009
Summary
Biphenyl model-derived B values quantify steric repulsion in ortho-substituted aromatic compounds. These new parameters aid in predicting conformational behavior, complementing cyclohexane model-based A values.
Area of Science:
- Organic Chemistry
- Computational Chemistry
- Physical Chemistry
Background:
- Cyclohexane model-based A values are used to quantify steric repulsion.
- Understanding steric repulsion is crucial for predicting conformational behavior in aromatic compounds.
Purpose of the Study:
- To introduce biphenyl model-derived B values as a tool for quantifying steric repulsion.
- To determine torsional barriers and activation energies of aryl-aryl rotation in ortho-substituted biphenyls.
- To establish B values as predictive parameters for conformational analysis.
Main Methods:
- Computational analysis using the B3LYP density functional.
- Experimental determination via variable-temperature nuclear magnetic resonance (NMR).
- Utilizing a novel 3 -isopropyldimethylsilyl group as a diastereotopicity probe.
Main Results:
- Torsional barriers for 15 ortho-substituted biphenyls were determined.
- Activation energies for aryl-aryl rotation as low as 5 kcal mol(-1) were assessed.
- Derived 2-X/2 -H repulsion increments provide quantitative steric parameters.
Conclusions:
- Biphenyl model-derived B values are effective for quantifying steric repulsion.
- These B values are powerful for rationalizing and predicting conformational behavior.
- The study provides a novel method for assessing small activation energies in substituted biphenyls.
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