Woodward-Hoffmann rules
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The Woodward-Hoffmann rules devised by Robert Burns Woodward and Roald Hoffmann are a set of rules in organic chemistry predicting the stereochemistry of pericyclic reactions based on orbital symmetry. These include electrocyclic reactions, cycloadditions, and sigmatropic reactions. Hoffmann was awarded the 1981 Nobel Prize in Chemistry for this work, shared with Kenichi Fukui who developed a similar model, while Woodward had died two years before he could win a second Nobel Prize for Chemistry.
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[edit] Electrocyclic reaction
The rules apply to the observed stereospecificity of electrocyclic ring-opening and ring closing reactions at the termini of open chain conjugated polyenes either by application of heat (thermal reactions) or application of light ( photochemical reactions). In the original publication in 1965 three rules are stated as:
- In an open-chain system containing 4n-electrons, the orbital symmetry of the highest occupied ground-state orbital is such that a bonding interaction between the termini must involve overlap between orbital envelopes on opposite faces of the system and this can only be achieved in a conrotatory process. An example of such reaction type is the Nazarov cyclization reaction of divinylketones.
- In open systems containing 4n + 2 electrons, terminal bonding interaction within ground-state molecules requires overlap of orbital envelopes on the same face of the system, attainable only by disrotatory displacements
- In a photochemical reaction an electron in the HOMO of the reactant is promoted to an excited state leading to a reversal of terminal symmetry relationships and reversal of stereospecificity.
Organic reactions that obey these rules are said to be symmetry allowed. Reactions that take the opposite course are symmetry forbidden and require a lot more energy to take place if they take place at all.
The rules predict the outcome of several ground-state reactions:
- Cyclopropyl cation → allyl cation: disrotatory
- Cyclopropyl radical → allyl radical: conrotatory
- Cyclopropyl anion → allyl anion: conrotatory
- Cyclopentenyl cation → pentadienyl cation: conrotatory
The stated rules are supported by theoretical calculations using the extended Hückel theory. For example, the activation energy required for thermal ring closing reaction of butadiene can be calculated as a function of the C-C-C bond angles keeping the other variables constant. Angles larger than 117° show a slight preference for a disrotatory reaction but with smaller angles a conrotatory reaction mode is preferred.
[edit] Controversy
It has been stated that E.J. Corey feels he is responsible for the ideas that laid the foundation for this research, and that Woodward unfairly neglected to credit him in the discovery.
[edit] External links
- Symmetry rules!, Sophie Wilkinson Chemical & Engineering News January 27, 2003 Volume 81, Number 04 CENEAR 81 04 pp. 59 ISSN 0009-2347 Article
[edit] References
- ↑ Stereochemistry of Electrocyclic Reactions R. B. Woodward, Roald Hoffmann; J. Am. Chem.Soc.; 1965; 87(2); 395-397. Article
- ↑ A Claim on the Development of the Frontier Orbital Explanation of Electrocyclic Reactions Roald Hoffman; Angew. Chem. Int. Ed.; 2004; 43; 6586-6590.[3]
