Field of Science

Science as a messy human endeavor: The origin of the Woodward-Hoffmann rules

A 1973 slide from Roald Hoffmann displaying the 'Woodward
Challenge' - four mysterious reactions which spurred the
Woodward-Hoffmann rules
There is a remarkable and unique article written by my friend and noted historian of chemistry Jeff Seeman that has just come out in the Journal of Organic Chemistry. The paper deals with seven pivotal months in 1964 when Robert Burns Woodward and Roald Hoffmann worked out the basic structure of what we call the Woodward-Hoffmann rules

Organic chemists need no introduction to these seminal rules, but for non-chemists it might suffice to say that they opened the door to an entire world of key chemical reactions - both in nature and in the chemist's test tube - whose essential details had hitherto stayed mysterious. These details include the probability of such reactions occurring in the first place and the stereochemistry (geometric disposition) of their molecular constituents. The rules were probably the first significant meld between theoretical and organic chemistry - ten commandments carried down from a mountain by Woodward and Hoffmann, pointing to the discovery of the promised land.The recognition of their importance was relatively quick; In 1980 Hoffmann shared a Nobel Prize for his contributions, and Woodward would have shared it too (it would have been his second) had he not suddenly passed away in 1979.

The first paper on these rules was submitted in November, 1964 and it came out in January, 1965. Jeff's piece essentially traces the conception of the rules in the previous six months or so. The article is very valuable for the light it sheds not just on the human aspect of scientific discovery but on its meandering, haphazard nature. It is one of the best testaments to science as a process of fits and starts that I have recently seen. Even from a strictly historical perspective Jeff's article is wholly unique. He had unprecedented access to Hoffmann in the form of daylong interviews at Cornell as well as unfettered access to Hoffmann's office. He has also interviewed many other important historical figures such as Andrew Streitweiser, George Whitesides and Jack Roberts who were working in physical organic chemistry at the time: insightful and amusing quotes from all these people (such as Whitesides's reference to the demise of a computer at MIT implying that he would now have to perform calculations using an abacus or his toes) litter the account. And there are copious and fascinating images of scores of notebook pages from Hoffmann's research as well as amusing and interesting letters to editors, lists of publications, scribblings in margins and other correspondence between friends and colleagues. Anyone who knows Jeff and has worked with him will be nodding their heads when they see how thorough the job here is.

The story begins when Woodward was already the world's most acclaimed organic chemist and Hoffmann was an upcoming theoretical chemistry postdoc at Harvard. Then as now, Hoffmann was the quintessential fox whose interests knew no bounds and who was eager to apply theoretical knowledge to almost any problem in chemistry that suited his interests. By then he had already developed Extended Hückel Theory (EHT), a method for calculating energies and orbitals of molecules which was the poster child for a model: imprecise, inaccurate, semiquantitative and yet pitched at the right level so that it could explain a variety of facts in chemistry. Woodward had already been interested in theory for a while and had worked on some theoretical constructs like the octant rule. It was a marriage made in heaven.

The most striking thing that emerges from Jeff's exhaustive and meticulous work is how relatively laid back Woodward and Hoffmann's research was in a sense. Hoffmann became aware of what was called the 'Woodward challenge' early in 1964 during an important meeting; this challenge involved the then mysterious stereochemical disposition during some well-known four and six electron reactions, reactions whose jargon ("electrocyclization", conrotatory") has now turned into household banter for organic chemists. The conventional story would then have had both Woodward and Hoffmann burning the midnight oil and persisting doggedly for the next few months until they cracked the puzzle like warriors on a quest. This was far from the case. Both pursued other interests, often ended up traveling and only occasionally touching base. Why they did this is unclear, but then it's no more unclear than why humans do anything else for that matter. Once they realized that could crack the puzzle however they kicked the door open. The paper that emerged in early 1965 was so long and comprehensive that they worried about its suitability for JACS in a cover letter to the editor.

Jeff's story also touches on a tantalizing conundrum whose solution many readers would have loved to know - E. J. Corey's potential role or the lack thereof in the conception of the rules, a role Corey unambiguously acknowledged in his 2004 Priestley Medal address, setting off a firestorm. Unfortunately Corey declined to talk to Jeff about this article (although he does dispute the timing of Woodward and Hoffmann's first meeting). His side of the story may never be known.

There is a lot of good stuff in the 45-page article that is worth reading about which I can only mention in passing here. Many of the actual mechanistic and technical details would be of interest only to organic chemists. But the more general message should not be lost upon more general readers: science is a messy, almost always unheroic, haphazard process. In addition, its real story is often warped by malleable memory, shifting egos, mundane oversights and blind alleys. For a long time science was described in bestselling books and newspaper articles as a determined, heroic march to the truth. These days there is an increasing number of books aimed at uncovering science's massive storehouses of failure and ignorance. But there is a third view of science - that of a journey to the truth which is more mundane, more complex, perpetually puzzling because of its mystery and perpetually comforting because of its human nature.

In this case even Jeff's exhaustive research leaves us with kaleidoscopic questions, questions that may likely remain unanswered. These pertain to Woodward and Hoffmann's occasional indifference to what was clearly a pivotal piece of research, to Corey's claim about the reactions, to the potential cross-fertilization between whatever else Woodward and Hoffmann were doing during this time and the project in question, and to the number of insights they might have imbibed from the community at large. Jeff conjectures answers to these questions, but even his probing mind provides no comforting conclusions, probably because there are none. The quote from Roald Hoffmann with which the piece ends captures the humanity quite well.
"Life is messy. Science is not all straight logic. And all scientists are not always logical. We're just scrabbler for knowledge and understanding."
Here's to the messy scrabblers.

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