|JACS AT 125
Will a proposed reaction proceed readily under thermal conditions? If not, will photochemical excitation enable the reaction to progress easily? In either case, what will be the stereochemistry of the products?
In the mid-1960s, Robert Burns (R. B.) Woodward, Donner Professor of Science at Harvard University, and Roald Hoffmann, a junior fellow in the university's Society of Fellows, developed rules that answer those questions for pericyclic reactions. These reactions--in which a closed loop of orbitals forms in the transition state--include electrocyclic, cycloaddition, and sigmatropic reactions such as the ring opening of cyclobutene, the Diels-Alder reaction, and the Cope rearrangement.
|Woodward and Hoffmann's 1965 paper set rules for outcome of many organic reactions
|PARTNERS Woodward (left) and Hoffmann around the time their paper was published.
In five communications in the Journal of the American Chemical Society--beginning with a paper [87, 395 (1965)] that ranks among the 125 most cited in JACS history--Woodward and Hoffmann illustrated how the feasibility and stereochemical outcome of pericyclic reactions are governed by the symmetry properties of the molecular orbitals of the reactants and products.
Woodward had intuited this concept based on his synthetic experience, his legendary familiarity with the literature of organic chemistry, and his knowledge of molecular orbital theory. Woodward and Hoffmann's collaboration began when the elder scientist asked the younger his opinion of Woodward's orbital symmetry hypothesis, says Hoffmann, who is now the Frank H. T. Rhodes Professor of Humane Letters at Cornell University. Hoffmann then applied his extended Hückel method--a way to calculate the electronic structure of molecules--to confirm Woodward's hunch.
The rules "brought order to a whole area of organic chemistry that previously seemed to be a collection of unrelated reactions, and gave predictions of new types of reactions that no one had thought to study previously," says Kendall N. Houk, a chemistry professor at the University of California, Los Angeles, and former graduate student of Woodward's. "This paper and its successors ushered in a new era in organic chemistry."
In addition, Houk says, Woodward and Hoffmann's insight permitted chemists to "apply molecular orbital theory to understand reactivities and stereoselectivities without doing elaborate calculations."
Jerome A. Berson, professor emeritus of chemistry at Yale University, says the Woodward-Hoffmann rules "permitted us for the first time to recognize which concerted reaction pathways would be the electronically favorable ones and which would not." Berson published a book in 1999 titled "Chemical Creativity: Ideas from the Work of Woodward, Meerwein, and Hückel."
|PREDICTIVE POWERS The Woodward-Hoffmann rules are based on conservation of orbital symmetry. Because thermal and photochemical reactions involve orbitals of different symmetry, they favor different directions of rotation and lead to different stereochemical outcomes.
When Woodward and Hoffmann first published their hypothesis, Berson says, "it was the talk of the town." But this was no short-lived fad. "It was the central focus of attention for a large fraction of the organic community for at least 15 or 20 years," he recalls.
According to Weston T. Borden, a University of Washington professor who studies synthetic, mechanistic, and theoretical organic chemistry, the Woodward-Hoffmann rules "made such clear predictions as to what would happen in organic reactions that testing and confirming the rules became a cottage industry for physical organic chemists for many years."
Hoffmann ultimately won the Nobel Prize in Chemistry for the work in 1981, when he was just 44. Woodward, who had already received a chemistry Nobel in 1965 for his synthetic work with natural products, would surely have shared in this second prize if he hadn't died in 1979 at the age of 62.
Hoffmann instead shared the 1981 Nobel with Kyoto University's Kenichi Fukui, who had independently worked out theories concerning the influence of molecular orbitals on chemical reactions.
Other key figures who helped to flesh out the rules relating the orbital symmetry of transition states to reaction outcomes include H. C. Longuet-Higgins at the University of Cambridge; Howard E. Zimmerman at the University of Wisconsin, Madison; and Michael J. S. Dewar at the University of Texas, Austin.
C&EN is celebrating the 125th volume of the Journal of the American Chemical Society by featuring selected papers from among its 125 most cited. This paper was ranked 88th.