Woodward and Hoffmann. Hoffmann and Woodward. A Close Collaboration Had Yet to Begin

Abstract

In 1965, R. B. Woodward and Roald Hoffmann published five communications in the Journal of the American Chemical Society in which they outlined the mechanisms of electrocyclizations, cycloadditions, and sigmatropic reactions - today known as the Woodward-Hoffmann rules. Over the next several years, the organic chemistry community rushed to test the validity of the W-H rules and expand the range of reactions covered by them. Meanwhile, Woodward and Hoffmann were besieged with invitations to lecture and write expositions on these concepts. In this publication, I present an analysis of Woodward and Hoffmann's next publications in 1966 and 1967 on the W-H rules. Two of these publications were based on lectures Woodward or Hoffmann presented in late 1965 and 1966. I also discuss their own continuing research on the topic in this time period (all by Hoffmann; none by Woodward). I conclude that the assumed intimate collaboration of Woodward and Hoffmann had actually not yet begun.

Keywords: History of chemistry; Woodward-Hoffmann rules; collaborations; pericyclic reactions.

Figure 1

(Top) Graphics I–VI are reproduced from Hoffmann's first draft of a manuscript entitled “Some Further Selection Rules for Cycloadditions” which was intended by him to be the sixth W−H communication of 1965. The manuscript, as such, was never submitted for publication. (Bottom) Hoffmann was not going to waste his ideas or his writings. Being a “salvager,” Hoffmann was able to use most of the chemistry including that shown in the (Top) in later articles. For example, the three structures shown as VII are derived from VIa and appeared in Hoffmann's 1971 JACS publication with his Cornell colleague Melvin J. Goldstein entitled “Symmetry, Topology, and Aromaticity.” Structures 286 and 287 appeared in W−H's 1969 treatise and 1970 monograph; compare these with structures shown by VIb.

Figure 2

Hoffmann's cover letter to Woodward on August 19, 1965, in which he provided Woodward with two “recently written” manuscripts, one with Roy Olofson on “The Dependence of Conformational and Isomer Stability on the Number of Electrons in Extended π Systems,” submitted to JACS on September 21, 1965. The other is likely to have been “Some Theoretical Observations on Cyclopropane,” submitted to JACS on August 21, 1965. It is amusing to note that Hoffmann signed his letter “Roald Hoffmann,” this after Woodward and he had already published three breakthrough JACS communications and had submitted two others. Furthermore, how many individuals named “Roald” might Woodward have known, for Hoffmann to include his last name, especially those having Hoffmann's trademark penmanship?

Figure 3

Hoffmann's letter to Woodward dated October 21, 1965. In this letter, Hoffmann congratulated Woodward on his upcoming Nobel Prize and joked with him about teaching Woodward Swedish (as Hoffmann's wife, Eva, is Swedish by birth, and he had become fluent in Swedish by then). Hoffmann also inquired hopefully about the fate of the draft of their sixth (never published) communication that he had sent to Woodward previously.

Figure 4

Page 5 from a draft manuscript of W‐H 6NP written by Hoffmann and provided to Woodward. On this page, Woodward made note of several chemists whose research related to Hoffmann's text, likely as pointers to possible citations. The only other handwritten marks in the draft was a reference on page 3, not shown in this publication.

Figure 5

Excerpt from page 476 of Hoffmann's 1966 publication in the Transactions of the New York Academy of Sciences. This is the sixth Woodward‐Hoffmann publication, numbered W−H 7 herein (see Table 1). Compare the structures on this page with those shown in Figure 1. Reprinted with permission from the New York Academy of Sciences.

Figure 6

An excerpt from page 109 of Hoffmann's Laboratory Notebook 17, ca. June 1, 1965. Notice Hoffmann's extensive use of symmetry and group theory to predict the reactivity of some of these molecules. The structures at the left and top right appear in W−H 6NP in Figure 1 (structure VI) and Figure 5 (bottom).

Figure 7

Excerpt from Hoffmann's Laboratory Notebook 16, page 33, ca. February 3, 1965. Hoffmann analyzed several eight‐electron and 10‐electron cycloadditions (top correlation diagrams). At the bottom left, he provided the first steps toward a generalized cycloaddition selection rule that appeared in Woodward and Hoffmann's 1969 treatise. Note the visual similarity for the two correlation diagrams for the eight‐electron transformations. And the dramatic difference between the correlation diagrams for the eight‐electron (W‐H forbidden) versus the 10‐electron (W‐H allowed) transformations.

Figure 8

Table 1 from Hoffmann and Woodward's second communication on cycloadditions comprises their selection rule for allowed cycloadditions, submitted on March 22, 1965, and published in the May 5, 1965, issue of JACS. At this time, all cycloadditions were considered to be suprafacial, and not because Woodward and Hoffmann had ruled out antarafacial cycloadditions. Rather, they had yet to envision antarafacial cycloadditions that require a very special orientation, i. e., a perpendicular approach of the reactants. Reprinted with permission from R. Hoffmann, R. B. Woodward, J. Am. Chem. Soc. 1965, 87, 2046‐2048. Copyright 1965 American Chemical Society.

Figure 9

Excerpt from Hoffmann's Laboratory Notebook 16, page 143, ca. March 14, 1965. This page records four electron, six electron, and eight electron pericyclic reactions – including group transfers and eliminations, and a Cope‐like transformation. Several notations of “2+2 no” and “4+2 yes” led to Hoffmann's statement, the “4 n+2 rule seems to hold again.” He did not specify the alternation effect in pericyclic reactions, namely “4 n”.

Figure 10

Cover letter from Hoffmann to Woodward in which Hoffmann attaches “an enumeration of all concerted processes with less than 10 electrons taking place on the periphery of a circle,” i. e., pericyclic reactions. The letter is erroneously dated January 21, 1965. Hoffmann surely meant January 21, 1966; see, for example, the citation to “Tokura et al … J.O.C. 31, 399 (1966)” could not have been recorded in 1965! Furthermore, Hoffmann arrived at Cornell in mid‐June 1965. As this letter has several important statements from a historical perspective that are discussed in the text, unambiguous dating is a crucial historical datapoint.

Figure 11

Page 1 of Hoffmann's 18‐page report entitled “A Catalogue of Concerted Reactions,” Ithaca, NY, version produced after the summer of 1966. Note Hoffmann's acknowledgment of Warren J. Hehre and John B. Lisle, undergraduate students who worked with him during the summers of 1965 and 1966.

Figure 12

Page 3 of Hoffmann's 18‐page report entitled “A Catalogue of Concerted Reactions,” Ithaca, NY, after the summer of 1966. The first entry of “Six Electrons” on page 2 is 1,3‐cyclohexadiene ⇋ 1,3,5‐hexatriene, a prototypical electrocyclization. On page 3 are examples of cycloadditions, sigmatropic reactions, and group transfers and eliminations. Some of the bonds have been darkened by Seeman for improved visibility.

Scheme 1

The ene reaction.

Figure 13

Woodward's partial outline of his Aromaticity lecture and essay.

Figure 14

First page of Woodward's typewritten lecture given on July 8, 1967. It is addressed to “Professor [William David] Ollis, Ladies, and Gentlemen” and begins, “Particularly after the remarks of the Lord Mayor of Sheffield … ” Woodward then used the lecture to derive a first draft of what would become his chapter in the Aromaticity monograph. At the very top is the first documented use of the term “The Conservation of Orbital Symmetry.”

Figure 15

Graphical reproduction from Woodward's autobiographical discussion of the origins of the Woodward‐Hoffmann rules published by Woodward in 1967.[ ¹⁰⁴ , ¹⁶⁷ ] Woodward described Subramania Ranganathan's 1963 – early 1964 set of reactions that were central to Woodward's ultimately unsuccessful route to the total synthesis of vitamin B₁₂. The several sets of stereospecific reactions in this figure became Woodward's fifth mysterious reaction that ultimately led to his collaboration with Hoffmann in May 1964 (and his collaboration with Eschenmoser in 1965 on the synthesis of vitamin B₁₂). Woodward's proposed synthetic path to vitamin B₁₂ included the transformation of 1 into 4 via an electrocyclization but 3 was formed instead. Woodward's inclusion of this scheme in his Aromaticity chapter (W−H 9) was his first and only publication of Ranganathan's chemistry by Woodward, though it has been discussed and reprinted several times thereafter by others[ ⁴⁴ , ¹⁶⁷ , ¹⁶⁸ , ¹⁶⁹ , ¹⁷⁰ ] (typically without the “plus sign” between 3 and 4).

Scheme 2

A graphical excerpt from Woodward's Aromaticity publication illustrating the disrotatory electrocyclization shown in Figure 15. An early beautiful illustration of the role of conformation in these reactions, i. e., the 1,3‐cyclohexadiene XX is not planar as drawn, was provided by Baldwin and Krueger in 1969.

Scheme 3

A graphical excerpt from Woodward's Aromaticity publication illustrating a disrotatory electrocyclization with the HOMOs shown. Each oval in XXIV is a shorthand representation of two adjacent atomic orbitals having the same phase. This shorthand can be confusing, and while used by others than Woodward in the mid‐1960s, it is not recommended and rarely, if ever, used today.

Figure 16

(A) Symmetry element used here is the plane of symmetry shown.. (B) In this graphic taken from Woodward's Aromaticity chapter, Woodward cleaved the σ‐bonds as shown by the red squiggly lines to form the HOMO of ethylene and the HOMO of 1,3‐butadiene. This is essentially a correlation diagram, absent the other key MOs, symmetry elements, and relative energies of the MOs, analogous to that produced by Zimmerman and Zweig in 1961 for a sigmatropic reaction. (C) The correlation diagram that plots the MOs of the reactants with the MOs the products, as specified in (B). The green correlation lines for the four symmetric (S) molecular orbitals differ from those in Hoffmann and Woodward's 1965 JACS publication on cycloadditions (see Figure 17). The MOs identified in red and blue font correlate (B) and (C). The non‐crossing rule[ ¹⁸⁰ , ¹⁸¹ , ¹⁸² ] for MOs having the same symmetry would result in a correlation diagram in which the green lines in (C) match the correlation diagram MO levels in Figure 17B. Note: All the text and lines in color in this figure were added by Seeman. The chemical pictograph in (A) and the correlation diagram in (C) are reproduced in the former and adapted in the latter with permission from R. Hoffmann, R. B. Woodward, J. Am. Chem. Soc. 1965, 87, 2046‐2048. Copyright 1965 American Chemical Society.

Figure 17

(A) Symmetry element used for [4+2] cycloadditions. (B) Correlation diagram for the [4+2] Diels‐Alder cycloaddition taken from Woodward and Hoffmann's second 1965 JACS communication. Reprinted with permission from R. Hoffmann, R. B. Woodward, J. Am. Chem. Soc. 1965, 87, 2046–2048. Copyright 1965 American Chemical Society. This is also the correlation diagram that appears as Figure 11 in their 1969 treatise (W−H 11).[ ¹² , ¹³ ]

Figure 18

Examples from Woodward's Aromaticity publication of alternations in pericyclic reactions within the W−H allowed and forbidden schema: (A) electrocyclizations; (B) cycloadditions; (C) sigmatropic reactions; and (D) the ene reaction. These graphics illustrate the range of chemistry governed by the principle of conservation of orbital symmetry, as Woodward envisioned it in 1967. Note that all of these reactions involved hydrocarbons though Woodward included a number of reactions with hetero‐substituted compounds. Adapted from: R. B. Woodward, “The Conservation of Orbital Symmetry” in Aromaticity: An International Symposium Held at Sheffield on 6th‐8th July 1966, Special Publication No. 21, pp. 217–249, The Chemical Society (London), London, 1967.

Figure 19

The first Cope Awardees, Roald Hoffmann (far left) and R. B. Woodward (far right) with Mrs. Arthur C. (Harriet) Cope and Herman Bloch, then chairman of the board of the American Chemical Society, State Ballroom, Palmer House, Chicago, IL, August 28, 1973. Photograph courtesy of Harvard University Archives.