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Home , Kenichi Fukui, Robert Burns Woodward

Correspondence

Woodward–Hoffmann Rules A Claim on the Development of the Frontier Orbital Explanation of Electrocyclic Reactions *

Keywords: history of science · molecular orbitals · pericyclic reactions · reaction mechanisms · theoretical

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 ” or “orbital symmetry con- perimentally the consequences of the report on it.But in my opinion, the part trol.” The 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 in chemistry was not right in its characterization of an their molecular orbitals. awarded to 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- to Kenichi detailed 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 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 .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 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. 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, 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. may not deal leniently in this matter, Woodward describes in two lectures taking seriously the possibility not only To set the stage for my recollections the chemical and historical background of Bobs dishonesty, but of your own not of the period, let me recount the story of of the advent of the orbital symmetry unwitting participation in the extension my relationship with E.J. Corey and rules.In the first, his 1966 Sheffield of fraud.” R.B.Woodward. lecture,[8a] he uses the generic “we”; in In the spring of 1984, I visited I received my Ph.D. in 1962 with his second account, the 1973 Cope Harvard for a semester.During that W.N. Lipscomb, Jr. and M.P. Gouter- Award Lecture (published long after, time Corey and I spoke for a couple of man, and began a Junior Fellowship in from a copy of a handwritten text by hours on the issue.He then sent me the summer of 1962.I was given an R.B. Woodward), [8b] there is a direct another letter on April 29, 1984[6] in office in the basement of Conant Hall, a first person account, as follows: which he gives as the reason that he did few doors down from Coreys office in not publish his claim in Woodwards Converse Hall.And I began to learn “I REMEMBER [capitals in the lifetime as follows: organic chemistry, applying the extend- original] very clearly—and it still sur-

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prises me somewhat—that the crucial 1964 (Figure 1) the day after the Corey it.I have no written record of that early flash of enlightenment came to me in and Woodward conversation (as report- conversation.It must be said, however, algebraic, rather than in pictorial or ed by Corey). that Applequist does not think he had geometric [all underlining is in original The May 5 entry (p.80; the entire such a conversation with me prior to text] form. Out of the blue, it occurred to notebook, covering roughly the period May 5, 1964.[9] me that the coefficients of the terminal March to June 1964, is in the Cornell Returning to my notebook, after terms in the mathematical expression University Library, collection cited[6]) some thoughts on cyclopropyl cation, representing the highest occupied molec- says “Talk with Woodward and Apple- two pages later I sketch out a plan for ular orbital of butadiene were of opposite quist,” and below it contains the stereo- studying what would later be called con- sign, while those of the corresponding chemical essence of the four- and six- and disrotatory motion.These preoccu- expression for hexatriene possessed the electron electrocyclic reaction, without py me for quite a few pages/days (no same sign. From here it was but a short orbitals.But clearly orbitals and their further dates appear in the notebook step to the geometric, and more obvious- phases are in the discussion, because until June 9, 1964); the discussion does ly chemically relevant, view that in the below these reactions I sketch the cyclo- not focus on orbitals but is “computa- internal cyclisation of a diene, the top propyl to allyl opening and draw the tional”.It is pretty well summarized in face of one terminal atom should attack lowest allyl orbital as presumably con- our first paper.At this point I was still a the bottom face of the other, while in the trolling the disrotatory motion in the computational , and even triene case, the formation of a new bond cation. though I used orbitals and perturbation should involve the top (or pari passu, the This is the first mention of electro- theory, explanations constructed in fron- bottom) faces of both terminal atoms.” cyclic reactions in my notebook.But I tier orbital language were not yet a believe I heard of the HOMO symmetry common part of my work and thought. In this period I kept a hard-bound argument earlier (and prior to May 4, They quickly became so.In fact, this—a notebook.But I did not keep it carefully, the date of the Corey–Woodward meet- recognition of the force of simple bond- with only occasional dates written in. ing) in—I think—a conversation I had ing arguments, and their utility in a There is in the notebook an April 17 with D.E.Applequist, who recounted it theory–experiment dialogue—was on a date for a seminar by George Olah, and as an explanation that Woodward sug- personal level the main consequence for the next date in the notebook is May 5, gested and asked me for my reaction to me of the orbital symmetry story. Heres how Woodward described the beginning of our interaction, in his Cope Award Lecture: “AFTER a brief false start in ex- tending these ideas, attributable clearly to my gaucherie in the details of quan- tum chemistry, I very soon realized that I needed more help than was available in my immediate circle, and I sought out Roald Hoffmann… I told him my story, and then, essentially, put to him the question Can you make this respectable in more sophisticated theoretical terms?”[8b] Woodward and I began to talk in that period.An important conversation I recall took place after a seminar by W.von.E.Doering.By that time I had done the calculations and could con- firm the ground- and excited-state ster- eochemical preferences.The electrocy- clic reaction paper was submitted on November 30, 1964, and published in the January 20, 1965 issue of the J. Am. Chem. Soc.[6a] Then R.B.Woodward and I went on to the rest of our orbital symmetry work.There was a lot to do…

Figure 1. May 5, 1964, entry in Roald Hoffmann’s notebook. The year in the date was written in some time later.

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Corey’s Role in the Orbital My Perception of What My strong feeling is that Woodward Symmetry Story and Woodward’s Happened had the frontier orbital idea before he Denial of It spoke to Corey.I think that Coreys I have no reason to doubt Coreys coming up with a similar way of thinking In the letter to me of November 16, statement that there was a conversation about the reaction in that May 4 con- 1981, E.J.Corey says that he told me in between Corey and Woodward on versation perhaps might have prompted May 1964 that he had suggested to R.B. May 4, 1964, even though we have only Woodward to push on, and to ask for my Woodward the HOMO explanation for Coreys account of it. assistance with calculations supporting the stereochemistry of electrocyclic re- But, I also do not think that the the frontier orbital approach.For some actions, and that Bob had taken over the conversation “provided the basis for the reason, he was unsure of himself (not a idea from him.I spoke to Corey often in further development of these ideas into characteristic one normally associated that period, but I have no recollection of what became known as the Woodward– with Woodward…) and needed the sup- such a conversation; I think Coreys Hoffmann rules,” as Corey writes.Here port of a theoretical chemist. statement would have been so striking are some factors that go into my reason- Going back to the Corey–Woodward (especially the part about Woodward ing: meeting of May 4, 1964, it would not be taking the idea from Corey) that I would a) Woodward had a habit of posing to the first time that two people came away have remembered it. people problems of great interest. from a conversation with diametrically Nevertheless, there was an idea in And also of posing such problems as opposed ideas of what was said. my mind then that Corey had a stake in puzzles, even if he had the solution. the electrocyclic reaction story; my Specifically, we know that he talked reconstruction today is that in the late about the electrocyclic conun- Crediting Suggestions: Then and spring and summer of 1964 Corey kept drum—to H.C. Longuet-Higgins, Now talking to me as if he did have such a and Lionel Salem, for instance—as role, as if the electrocyclic reaction it was taking shape experimentally in Assuming that the contents of the research were joint work between Cor- the work of others and in the vita- May 4, 1964 conversation were as E.J. ey, Woodward, and me.Eventually, I min B12 synthesis he was pursuing in Corey reports them, then I think that at was sufficiently puzzled by this that I did collaboration with A.Eschenmos- best Corey has a claim to a rediscovery speak to Woodward.I do not remember ers group. of the Oosterhoff orbital argument of when that was; I suspect it was later, in b) I think Woodward was perfectly 1961.In a paper by E.Havinga and the fall of 1964, when we wrote our first capable of coming up with the fron- J.L.M.A. Schlatmann, the authors paper. tier orbital explanation by himself. write: When Woodward and I spoke of the Though he began his career with VB As Prof. Oosterhoff pointed out, matter—again, I cannot remember arguments, I found Woodward in another factor that possibly contributes when it was—I asked him directly if 1964 pretty well-versed in MO ideas. to this stereochemical difference between Corey had a role in this work.He said Woodward had learned a lot from the thermal and photoinduced ring clo- “no”—this was our (R.B.W. and R.H.) William Moffitt, around the ferro- sure may be found in the symmetry work. cene and octant rule stories.Orbital characteristics of the highest occupied I did not question Woodward fur- ideas were in the air, the classic p orbital of the conjugated hexatriene ther.In retrospect, I regret not doing so, Streitwieser and Roberts books system.[11] and I regret not asking Corey more widely read, the success of the directly to tell me his role in the work. Hückel rule amply demonstrated. The Oosterhoff “forerunner” to the R.B. Woodward was 47, the top syn- There was also the L.J.Oosterhoff orbital symmetry control ideas is men- thetic organic chemist in the world.E.J. “forerunner” (see below), which tioned in a footnote in our first 1965 Corey was 36, a full Professor at Har- Woodward knew about. paper; its role has been discussed by vard.In May 1964 I was 26, a new Ph.D., One interesting piece of evidence for J.A. Berson in his book “Chemical a Junior Fellow to be sure, but just Woodwards knowledge of molecular Creativity.”[12] starting out in my career. orbitals is in a published comment he In 1963–1964, E.J.Corey and A.G. In the more than a year that I made after a lecture by Rolf Huisgen in Hortmann accomplished the total syn- remained at Harvard (I left in June a Welch Foundation Conference in thesis of dihydrocostunolide.A crucial 1965), I kept on talking to Corey and 1961.[10] Woodward draws quite explic- step involved a stereospecific electro- learned a lot from him.I do not remem- itly the orbitals of a vinyl carbene (albeit cyclic reaction.The two papers on the ber any cooling of our relationship, without phases), and asks “Do we have work[13] do not have any reference to an though I certainly spent more and more specific orbital geometric requirements electronic factor in the stereoselectivity. time with Woodward.Corey, in his [for Huisgens reaction] and would they Neither is the Havinga and Schlatmann correspondence with me, disagrees, say- in this case preclude the operation of the paper mentioned.The full Corey and ing that our previously frequent conver- mechanism which is so general in many Hortmann paper was submitted on Au- sations stopped after I began working of the cases?” gust 5, 1965, some months after publi- with Woodward. cation of the first paper by Woodward and me.

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The full paper on the dihydrocostu- without publishing his account, wanted [3] R.B.Woodward, R.Hoffmann, Angew. nolide synthesis provided a place for me to tell others that his claim was valid. Chem. 1969, 81, 797 – 870; Angew. Corey to state his electronic explana- People will form their own opinions Chem. Int. Ed. Engl. 1969, 8, 781 – 853 tion.It was an opportunity not taken. of Coreys explanation of why he didnt (reprinted in book form by Verlag Chemie and Academic Press). Should Coreys suggestion of the press a public claim (that it would have [4] E.J. Corey, Chem. Eng. News 2004, frontier orbital explanation for the ster- hurt Harvard, that Woodward would 82(13), 42 – 44. eochemistry of electrocyclic reactions eventually give him credit).Too bad he [5] E.J. Corey, J. Org. Chem. 2004, 69, have been credited in our first paper and didnt speak out; had Corey done so in 2917 – 2919. subsequent ones? I can only speak for 1964, or in the period 1964–1979, we [6] R.Hoffmann, “Scientific Papers”, [Col- myself, and from todays perspective. could have had Woodwards viewpoint. lection No.14/8/3360] Division of Rare We all make decisions, some easy, some I believe that E.J. Coreys percep- and Manuscript Collections, Cornell hard, of what to reference/credit in our tion of the consequences of what he told University Library. [7] “ Woodward”: E.Blout, papers.If the substance of what E.J. R.B. Woodward is just that—what he, Biographical Memoirs, Vol. 80, The Na- Corey reports was said in that conversa- Corey, believes.Based on the arguments tional Academy Press, Washington, tion with Woodward is true, then I I made above, and on what I remember, D.C., 2001, pp.2 – 23. believe it should have been credited. I dont think that the conversation in [8] a) R.B.Woodward, “The Conservation As Oosterhoffs suggestion was.I would question influenced in any significant of Orbital Symmetry, ” in Aromaticity, be ready today to regret the omission way the beginning of the orbital sym- special publication no.21, London: The and apologize to Corey for it.But, I am metry control story.And given that Chemical Society, London, 1967, torn… between the conviction with Coreys claim was (and is being) made pp.217 – 249; b) O.T. Benfey and which Corey tells his story in 1981 and after Woodwards death—when the P.J.T.Morris, : Architect and Artist in the World of today, torn between that and the impos- claim could have been made for 15 years Molecules, Chemical Heritage Founda- sibility of Woodward commenting on in Woodwards lifetime—that claim is tion, , 2001, pp.415 – 452; the decision today, as well as Wood- also deeply and fundamentally unfair. this book also reprints reference [8a]. wards 1964 denial to me of a role for [9] D.E.Applequist, personal communica- && && Corey.With Woodward not alive, and no Published Online: , 2004 tion to P.Glitz, 2004. record except Coreys memory for the [10] R.Woodward in Welch Foundation essence of that conversation, I just Conferences on Chemical Research IV. cannot say today that the conversation Molecular Structure and Organic Reac- [1] a) R.B. Woodward, R. Hoffmann, J. tions, (Ed.: W.O. Milligan), Welch should have been credited. Am. Chem. Soc. 1965, 87, 395 – 397; Foundation, Houston, 1961, p.87. b) R.Hoffmann, R.B.Woodward, J. [11] E.Havinga, J.L.M.A.Schlatmann, Tet- Am. Chem. Soc. 1965, 87, 2046 – 2048; rahedron 1961, 146 – 152. A Claim Unfairly Made c) R.B. Woodward, R. Hoffmann, J. [12] J.A.Berson, Chemical Creativity, Wiley- Am. Chem. Soc. 1965, 87, 2511 – 2513; VCH, Weinheim, 1999, pp.26 – 31. The matter should have been argued d) R.Hoffmann, R.B.Woodward, J. [13] E.J.Corey, A.J.Hortmann, J. Am. Am. Chem. Soc. 1965, 87, 4388 – 4389; out in 1964, by Corey persisting in Chem. Soc. 1963, 85, 4033 – 4034; E.J. e) R.Hoffmann, R.B.Woodward, J. Corey, A.J. Hortmann, J. Am. Chem. telling Woodward of his (Coreys) per- Am. Chem. Soc. , 87, 4389 – 4390. 1965 Soc. 1965, 87, 5736 – 5742. ception of his contributions.Seventeen [2] R.Hoffmann, R.B.Woodward, Acc. years later, Woodward gone, Corey, Chem. Res. 1968, 1, 17 – 22.

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