JACS AT 125
SYMMETRY RULES! Woodward and Hoffmann's 1965 paper set rules for outcome of many organic reactions
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 WoodwardHoffmann 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."
seemed to be a collection of unrelated reILL A PROPOSED REACactions, and gave predictions of new types tion proceed readily unof reactions that no one had thought to der thermal conditions? If study previously," says Kendall N. Houk, a not, will photochemical excitation enable the re- chemistry professor at the University of California, Los action to progress easily? In either case, Angeles, and former graduwhat will be the stereochemistry of the ate student of Woodward's. products? "This paper and its succesIn the mid-1960s, Robert Burns (R. B.) sors ushered in a new era in Woodward, Donner Professor of Science organic chemistry" at Harvard University, and Roald Hoffmann, a junior fellow in the university's In addition, Houk says, Society of Fellows, developed rules that Woodward and Hoffmann's answer those questions for pericyclic reinsight permitted chemists actions. These reactions —in which a to "apply molecular orbital closed loop of orbitals forms in the transitheory to understand reaction state —include electrocyclic, cytivities and stereoselectivicloaddition, and sigmatropic reactions ties without doing elaborate such as the ring opening of cyclobutene, calculations." P A R T N E R S Woodward (left) and Hoffmann around the Diels-Alder reaction, and the Cope Jerome A. Berson, pro- the time their paper was published. rearrangement. fessor emeritus of chemistry Infivecommunications in theJournal of at Yale University, says the WoodwardHoffmann ultimately won the Nobel the American Chemical Society—beginning Hoffmann rules "permitted us for the first Prize in Chemistry for the work in 1981, time to recognize which concerted reacwith a paper {87, 395 (1965)1 that ranks when he was just 44. Woodward, who had among the 125 most cited in already received a chemistry JACS history—Woodward Nobel in 1965 for his synand Hoffmann illustrated thetic work with natural Heat how the feasibility and stereproducts, would surely have CH3 (conrotatory H ochemical outcome of perishared in this second prize if motion) CH, cyclic reactions are governed he hadn't died in 1979 at the trwM,tr>ns-2,4-H*xadt*iM frans-3(4-Diimthylcyclobut«n« by the symmetry properties age of 62. of the molecular orbitals of Hoffmann instead shared the reactants and products. the 1981 Nobel with Kyoto H Light H H-T^ CH H Woodward had intuited University's Kenichi Fukui, 3 (disrotatory this concept based on his who had independently motion) H3C CH3 synthetic experience, his legworked out theories conc/s-3,4-DimethylcyclobuUn« endary familiarity with the cerning the influence of moliterature of organic chemlecular orbitals on chemical istry, and his knowledge of P R E D I C T I V E P O W E R S The Woodward-Hoffmann rules are reactions. molecular orbital theory. based on conservation of orbital symmetry. Because t h e r m a l and Other key figures who Woodward and Hoffmann's photochemical reactions involve orbitals of different symmetry, helped to flesh out the rules collaboration began when they favor different directions of rotation and lead to different relating the orbital symmethe elder scientist asked the stereochemical outcomes. try of transition states to reyounger his opinion of action outcomes include H. Woodward's orbital symmetry hypotheC. Longuet-Higgins at the University of tion pathways would be the electronically sis, says Hoffmann, who is now the Frank Cambridge; Howard E. Zimmerman at the favorable ones and which would not." H. T. Rhodes Professor of Humane LetUniversity of Wisconsin, Madison; and Berson published a book in 1999 titled ters at Cornell University Hoffmann then "Chemical Creativity: Ideas from the Work Michael J. S. Dewar at the University of applied his extended Hiickel method—a of Woodward, Meerwein, and Hiickel." Texas, Austin.—SOPHIE WILKINSON way to calculate the electronic structure When Woodward and Hoffmann first of molecules —to confirm Woodward's published their hypothesis, Berson says, C&ENis celebrating the 125th volume ofthe hunch. "it was the talk of the town." But this was Journal of the American Chemical Socino short-lived fad. "It was the central focus The rules "brought order to a whole ety by featuring selectedpapers from among of attention for a large fraction of the area of organic chemistry that previously its 125 most cited. Thispaper was ranked 88 th
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