- Woodward–Hoffmann rules
The Woodward–Hoffmann rules devised by
Robert Burns Woodward andRoald Hoffmann are a set of rules inorganic chemistry predicting thestereochemistry ofpericyclic reaction s based onorbital symmetry . These includeelectrocyclic reaction s,cycloaddition s, andsigmatropic reaction s. Hoffmann was awarded the1981 Nobel Prize in Chemistry for this work, shared withKenichi Fukui who developed a similar model, while Woodward had died two years before he could win a second Nobel Prize for Chemistry.Electrocyclic reaction
The rules apply to the observed
stereospecificity of electrocyclic ring-opening andring-closing reaction s at the termini of open chain conjugatedpolyene s either by application of heat (thermal reactions) or application of light (photochemical reactions). In the original publication in 1965 ["Stereochemistry of Electrocyclic Reactions" R. B. Woodward, Roald HoffmannJ. Am. Chem. Soc. ; 1965; 87(2); 395-397. DOI|10.1021/ja01080a054] 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 aconrotatory process. An example of such reaction type is theNazarov 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 theHOMO of the reactant is promoted to anexcited 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 :Cyclopropylanion → 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 ofbutadiene can be calculated as a function of the C-C-Cbond angle s keeping the other variables constant. Angles larger than 117° show a slight preference for adisrotatory reaction but with smaller angles aconrotatory reaction mode is preferred.A recent paper describes how
mechanical stress can be used to reshape chemical reactions' reaction pathways to lead to products that apparently violate Woodward–Hoffman rules ["Biasing Reaction Pathways with Mechanical Force". "Nature" (2007) 446:423-427" (See also the corresponding "News and Views" in the same issue of Nature)]Controversy
It has been stated that the chemist
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. In a 2004 memoir published in theJournal of Organic Chemistry ["Impossible Dreams"E. J. CoreyJ. Org. Chem. ; 2004; 69(9) pp 2917 - 2919; (Perspective) DOI|10.1021/jo049925d] Corey makes his claim to fame with the single sentence: "On May 4, 1964, I suggested to my colleague R. B. Woodward a simple explanation involving the symmetry of the perturbed (HOMO) molecular orbitals for the stereoselective cyclobutene to 1,3-butadiene and 1,3,5-hexatriene to cyclohexadiene conversions that provided the basis for the further development of these ideas into what became known as the Woodward–Hoffmann rules".In a 2004 rebuttal published in the
Angewandte Chemie Roald Hoffmann ["A Claim on the Development of the Frontier Orbital Explanation of Electrocyclic Reactions" Roald Hoffman;Angew. Chem. Int. Ed. ; 2004; 43; 6586-6590. DOI|10.1002/anie.200461440] denied the claim: he quotes Woodward from a lecture given in 1966 saying: "I REMEMBER very clearly—and it still surprises me somewhat—that the crucial flash of enlightenment came to me in algebraic, rather than in pictorial or geometric form. Out of the blue, it occurred to me that the coefficients of the terminal terms in the mathematical expression representing the highest occupied molecular orbital of butadiene were of opposite sign, while those of the corresponding expression for hexatriene possessed the same sign. From here it was but a short step to the geometric, and more obviously chemically relevant, view that in the internal cyclisation of a diene, the top face of one terminal atom should attack the bottom face of the other, while in the triene case, the formation of a new bond should involve the top (or pari passu, the bottom) faces of both terminal atoms."In addition, Hoffmann points out that in 2 publications from 1963 ["Total Synthesis of Dihydrocostunolide" E. J. Corey and Alfred G. Hortmann
J. Am. Chem. Soc. 85 1963 pp 4033 - 4034; DOI|10.1021/ja00907a030] and 1965 ["The Total Synthesis of Dihydrocostunolide" E. J. Corey, Alfred G. HortmannJ. Am. Chem. Soc. ; 1965; 87(24); 5736-5742. DOI|10.1021/ja00952a037] Corey described atotal synthesis of the compound "dihydrocostunolide" and although in it an electrocyclic reaction is described Corey has nothing to offer with respect to explaining itsstereospecifity .:
This
photochemical reaction involving 4*1+2 electrons is now recognized as conrotatory.ee also
*
Woodward's rules for calculating UV absorptionsExternal links
* "Symmetry rules!", Sophie Wilkinson
Chemical & Engineering News January 27, 2003 Volume 81, Number 04 CENEAR 81 04 pp. 59 ISSN 0009-2347 [http://pubs.acs.org/cen/topstory/8104/8104sci4.html Article]
* "SHMO calculator", Simple Huckel molecular orbital theory calculator [http://www.chem.ucalgary.ca/SHMO/ Link]References
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