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Symmetry-enthalpy correlations in Diels-Alder reactions
1Department of Natural Sciences, The Open University of Israel, 1 University Rd., Raanana 43107, Israel. inbaltu@openu.ac.il
Chemistry (Weinheim an Der Bergstrasse, Germany)
|July 12, 2012
Summary
Woodward-Hoffmann rules are not absolute; concerted reactions proceed even with less symmetry. Reactivity decreases as symmetry deviates from ideal, increasing activation enthalpy, showing symmetry is a primary factor.
Area of Science:
- Organic Chemistry
- Physical Chemistry
- Computational Chemistry
Background:
- Woodward-Hoffmann (WH) rules traditionally dictate reaction feasibility based on strict symmetry.
- Experimental evidence suggests concerted reactions can occur with less than perfect symmetry.
- A quantitative link between symmetry deviation and reactivity has been proposed.
Purpose of the Study:
- To quantitatively investigate the relationship between symmetry deviation and activation enthalpy in Diels-Alder reactions.
- To determine if reduced symmetry impedes concerted reactions and by how much.
- To assess the role of symmetry as a primary factor in reaction kinetics.
Main Methods:
- Calculated the continuous symmetry measure (CSM) for the [4+2] carbon skeleton transition state.
- Studied twelve Diels-Alder reactions across various dienes and dienophiles.
- Performed DFT calculations in seven solvents and the gas phase to analyze symmetry-enthalpy correlations.
Main Results:
- A strong correlation was observed between decreased symmetry and increased enthalpy of activation.
- Concerted reactions slow down as they deviate from ideal symmetry.
- Symmetry emerged as a dominant parameter influencing activation enthalpy, despite other factors.
Conclusions:
- The study provides quantitative evidence that symmetry is a primary determinant of reactivity in concerted reactions.
- Deviations from ideal symmetry lead to higher activation barriers, slowing reaction rates.
- Understanding symmetry's role is crucial for predicting and controlling chemical reactions.
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