Correspondence Woodward–Hoffmann Rules A Claim on the Development of the Frontier Orbital Explanation of Electrocyclic Reactions Roald Hoffmann* Keywords: history of science · molecular orbitals · pericyclic reactions · reaction mechanisms · theoretical chemistry An important discovery took place in puzzling, body of chemistry, but also speak about the contents of the con- the period 1964–1969—it was the elab- were able to make predictions that were versation because I was unaware of it at oration of the complex of ideas known verifiable by a community eminently the time; given that Woodward is no variously as “the conservation of orbital capable and interested in probing ex- longer alive, we only have Coreys symmetry” or “orbital symmetry con- perimentally the consequences of the report on it.But in my opinion, the part trol.” The stereochemistry and ease (or theory.Orbital symmetry considerations of E.J.Coreys published statement that difficulty) of the elementary steps of were expeditiously tested; they were reads “…that provided the basis for the many organic reactions found an ex- found to be portable and productive. further development of these ideas”is planation in the phase relationships of The 1981 Nobel Prize in chemistry was not right in its characterization of an their molecular orbitals. awarded to Kenichi Fukui and me for episode in an important discovery. It was my fortune to collaborate with this work; had R.B. Woodward lived, R.B. Woodward in this story. Our there isnt the least doubt that he would published work consisted of a series of have shared in this recognition. Corey’s Claim, Made in Prior five communications to the Editor of J. Conversations or Exchanges with Am. Chem. Soc. in 1965,[1] a paper in Me on the Subject Accounts of Chemical Research,[2] and, E. J. Corey’s Public Claim importantly, a final exposition of the In a visit to Cornell in the 1970s, ideas in an article of some length in In his 2004 Priestley Medal address, Corey told me that he made my career Angewandte Chemie.[3] published in Chemical and Engineering by telling R.B.Woodward the HOMO Woodward and I used a variety of News,[4] E.J.Corey writes, explanation in 1964.To my knowledge, theoretical approaches in our work— “On May 4, 1964, I suggested to my this was the first time he made this direct from simple frontier orbital (HOMO/ colleague R.B. Woodward a simple ex- claim to me.I was puzzled, and I think I LUMO) arguments, through orbital cor- planation involving the symmetry of the ignored the comment. relation diagrams, interaction diagrams, perturbed (HOMO) molecular orbitals On November 2, 1981, after the and perturbation theory arguments, to for the stereoselective cyclobutene/1,3- Nobel Prize in Chemistry to Kenichi detailed molecular orbital calculations. butadiene and 1,3,5-hexatriene/cyclohex- Fukui and me was announced, Corey There were good reasons not to use a adiene conversions that provided the wrote me a letter stating his claim in single approach—symmetry was a way basis for the further development of these substantial detail.The original of this in to the solution, but its unthinking use ideas into what became known as the letter is deposited in my papers in the would have imprisoned us.Still, there Woodward–Hoffmann rules.” Cornell University Library (available to was a leitmotif in our work: orbitals and the public).[6] In the letter Corey re- their symmetries mattered, and so did The same sentence appears in a 2004 counts a conversation he had with the number of electrons. Journal of Organic Chemistry perspec- Woodward on the evening of May 4, Woodward and I were lucky—we tive by E.J. Corey, “Impossible 1964, in which he told Woodward, who not only rationalized a large, piecewise Dreams”.[5] This terse reminiscence of posed the problem of the electrocyclic a conversation, and a claim of its sig- reactions to him (and who seemed, nificance has given rise to considerable according to Corey, more interested in [*] Prof. Dr. R. Hoffmann discussion and speculation in the com- geometrical explanations), that the ex- Department of Chemistry and Chemical munity. planation might be found in the symme- Biology Let me recount what I know, re- try properties of the orbitals involved. Cornell University, Baker Laboratory Ithaca, NY 14853-1301 (USA) member (and dont remember), and Corey continues his account by relating Fax : (+1)607-255-3419 what I infer about the events 40 years how the next day Woodward appeared E-mail: [email protected] ago surrounding this claim.I cannot in Coreys office and told the same 2 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim DOI: 10.1002/anie.200461440 Angew. Chem. Int. Ed. 2004, 43,2–6 Angewandte Chemie explanation briefly to Douglas Apple- “I am amazed that you would think it ed Hückel method that a group of us quist (a visitor to Harvard that semes- possible that I would consider doing developed in the Lipscomb group to ter) in Coreys presence, as his (Wood- anything against Harvard, to which I organic molecules.When I finished the wards) “new idea.” Corey goes on in his was and am so devoted.” first extended Hückel paper on hydro- letter to say: carbons I sent a preprint (this was early In his first letter to me on Novem- 1963) to the organic chemists at Har- “…the fact is I conceived of the idea, ber 2, 1981, Corey gave another reason vard—Woodward, Corey, Bartlett, and not Bob. In a manner of which few would for not writing himself of the way he Westheimer—and then set up appoint- be capable he pirated the idea, evidently perceived things had happened: ments to see all of them.This was the preferring that over my good will. Even first time (after five years at Harvard) more incredible than what Bob did was “I had hoped that Bob himself would that I spoke to Woodward about science. how he did it.” do it as he grew older, more considerate, Of the people I saw, Corey respond- and more sensitive to his own con- In his November 2, 1981 letter Corey ed most positively to my growing inter- science.” expressed concern that I would tell the est in organic chemistry.I was down the orbital symmetry story in my Nobel To the best of my knowledge, Corey hall from him, and as busy as he was, he Lecture (I had already chosen to talk never made an attempt to set the record always welcomed me to talk.By telling about something new, the isolobal anal- straight (according to his own percep- me of hot subjects—nonclassical carbo- ogy).And he wanted me to tell people of tions) by speaking of it to Woodward in nium ions and organic photochemis- his, Coreys, contribution. the 15 years between 1964 and 1979, try—he helped draw me into organic This letter, coming at the time that it when Woodward died.Or by publishing chemistry. did, and with such claims, disturbed me his claim.Corey made no public claims, I began to go to organic seminars greatly.I answered it on November 8, as far as I know, until the statement in and went to one of the legendary 1981.[6] I said that I would not tell people his Priestley Medal address in 2004. evening Woodward meetings.In the of Coreys claim—he chose not to do so Corey did talk to others at Harvard spring of 1964, one of Coreys colleagues himself in public (or to Woodward); it and elsewhere of the matter.In his from Illinois, Douglas Applequist, visited would be unfair to Woodward for me to November 2, 1981 letter to me, Corey Harvard for the semester.He gave a do so then, in 1981, after his death in writes that he told his story to E.B. course on strained molecules which I 1979. Wilson in 1964.And I know he discussed listened to with great interest.Apple- Corey then wrote another letter to the matter subsequently with Jeremy quist and I talked to each other a lot.I me (November 16, 1981)[6] in which he Knowles.Frank Westheimer and Elkan was in the midst of discovering for myself reiterated and escalated his claims, in a Blout knew about Coreys claim and the full beauty of organic chemistry. manner that I felt as intimidating to agonized over the question for years threatening.Here are two excerpts from while composing a memorial notice on that letter, which convey its tone: Woodward.In the end, despite my My Recollections of the Specific “You cannot deny that despite the objections, Elkan Blout included in his Period in 1964 possibility of appalling dishonesty at the 2001 biographical memoir for Wood- ward the following sentence: “In 1964, The simple frontier orbital explan- roots of your collaboration with Bob, after a brief discussion with E. J. Corey ation of the stereochemistry of electro- you elected to close your mind. The two and others of his thoughts and his of you in effect ”stonewalled“ my con- cyclic reactions did not originate with chemical results, Woodward was able to tribution so completely that you chose me.It was brought to me by Woodward. formulate his important ideas in this not even to make a perfunctory acknowl- Let me outline what I have a record of, area [7] edgement in your papers.” .” what I remember, and what I dont recall, concerning my introduction to and that frontier orbital explanation.And Scientific and Personal how my collaboration with Woodward “Roald, please consider that history Background to 1964 Events began.