Woodward–Hoffmann rules

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 was not eligible to win what would have been his second Nobel Prize for Chemistry.

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,^[1] 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.

A recent paper describes how mechanical stress can be used to reshape chemical reaction pathways to lead to products that apparently violate Woodward–Hoffman rules.^[2]

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 the 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.

Corey, then 35, was working into the evening on Monday, May 4, as he and the other driven chemists often did. At about 8:30 p.m., he dropped by Woodward's office, and Woodward posed a question about how to predict the type of ring a chain of atoms would form. After some discussion, Corey proposed that the configuration of electrons governed the course of the reaction. Woodward insisted the solution wouldn't work, but Corey left drawings in the office, sure that he was on to something.^[4]

I felt that this was going to be a really interesting development and was looking forward to some sort of joint undertaking," he wrote. But the next day, Woodward flew into Corey's office as he and a colleague were leaving for lunch and presented Corey's idea as his own -- and then left. Corey was stunned.

In a 2004 rebuttal published in the Angewandte Chemie,^[5] Roald Hoffmann 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^[6] and 1965,^[7] Corey described a total synthesis of the compound dihydrocostunolide. Although in it an electrocyclic reaction is described, Corey has nothing to offer with respect to explaining its stereospecifity. Further, it is noteworthy that Corey said nothing of his so-called idea for nearly 2 decades after Woodward's death.

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

See also

• Woodward's rules for calculating UV absorptions

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
• "SHMO calculator", Simple Huckel molecular orbital theory calculator Link

References