Woodward-Hoffmann rules



The Woodward-Hoffmann rules devised by Kenichi 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 photochemical reactions). In the original publication in 1965 [1] three rules are stated as:

  • In a 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:

disrotatory
Cyclopropyl conrotatory
Cyclopropyl anion → allyl anion: conrotatory
conrotatory

The stated rules are supported by theoretical calculations using the conrotatory 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 [2]

Controversy

It has been stated that the chemist Journal of Organic Chemistry [3] 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 [4] 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 [5] and 1965 [6] Corey described a total synthesis of the compound dihydrocostunolide and although in it an electrocyclic reaction is described Corey has nothing to offer with respect to explaining its stereospecifity.

This photochemical reaction involving 4*1+2 electrons is now recognized as conrotatory.

See also

  • Woodward's rules for calculating UV absorptions

References

  1. ^ Stereochemistry of Electrocyclic Reactions R. B. Woodward, Roald Hoffmann J. Am. Chem. Soc.; 1965; 87(2); 395-397. doi:10.1021/ja01080a054
  2. ^ "Biasing Reaction Pathways with Mechanical Force. Nature (2007) 446:423-427" (See also the corresponding "News and Views" in the same issue of Nature)
  3. ^ Impossible Dreams E. J. Corey J. Org. Chem.; 2004; 69(9) pp 2917 - 2919; (Perspective) doi:10.1021/jo049925d
  4. ^ 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
  5. ^ Total Synthesis of Dihydrocostunolide E. J. Corey and Alfred G. Hortmann J. Am. Chem. Soc. 85 1963 pp 4033 - 4034; doi:10.1021/ja00907a030
  6. ^ The Total Synthesis of Dihydrocostunolide E. J. Corey, Alfred G. Hortmann J. Am. Chem. Soc.; 1965; 87(24); 5736-5742. doi:10.1021/ja00952a037
 
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