Another foray into one of the more famous anecdotal chemistry “models”, the analysis of which led directly to the formulation of the WoodWard-Hoffmann (stereochemical) rules for pericyclic reactions. Previously, I tried to produce a modern computer model of what Woodward might have had to hand when discovering that the stereochemical outcome of a key reaction in his vitamin B12 synthesis was opposite to that predicted using his best model of the reaction.
Archive for the ‘Historical’ Category
The thermal reactions … took precisely the opposite stereochemical course to that which we had predicted. A non-covalent-interaction view of the model.
Wednesday, February 3rd, 2021The Stevens rearrangement: how history gives us new insights.
Friday, January 29th, 2021In a recent post, I told the story of how in the early 1960s, Robert Woodward had encountered an unexpected stereochemical outcome to the reaction of a hexatriene, part of his grand synthesis of vitamin B12. He had constructed a model of the reaction he wanted to undertake, perhaps with the help of a physical model, concluding that the most favourable of the two he had built was not matched by the actual outcome of the reaction. He was thus driven to systematise such (Pericyclic) reactions by developing rules for them with Roald Hoffmann. This involved a classification scheme of “allowed” and “forbidden” pericyclic reactions and his original favoured model in fact corresponded to the latter type. When physical model building in the 1960s was gradually replaced by models based on quantum mechanical calculations from the 1970s onwards, the term “allowed” morphed into “a relatively low energy transition state for the reaction can be located” and very often “no transition state exists for a forbidden reaction”. The famous quote “there are no exceptions” (to this rule) was often interpreted that if a “forbidden reaction” did apparently proceed, its mechanism was NOT that of a pericyclic reaction. Inspired by all of this, I recollected a famous “exception” to the rules which is often explained by such non-pericyclic character, the Stevens rearrangement[1],[2],[3] by a 1,2-shift. 
References
- T.S. Stevens, E.M. Creighton, A.B. Gordon, and M. MacNicol, "CCCCXXIII.—Degradation of quaternary ammonium salts. Part I", J. Chem. Soc., vol. 0, pp. 3193-3197, 1928. http://dx.doi.org/10.1039/JR9280003193
- T.S. Stevens, "CCLXX.—Degradation of quaternary ammonium salts. Part II", J. Chem. Soc., vol. 0, pp. 2107-2119, 1930. http://dx.doi.org/10.1039/JR9300002107
- T.S. Stevens, W.W. Snedden, E.T. Stiller, and T. Thomson, "CCLXXI.—Degradation of quaternary ammonium salts. Part III", J. Chem. Soc., vol. 0, pp. 2119-2125, 1930. http://dx.doi.org/10.1039/JR9300002119
The thermal reactions … took precisely the opposite stereochemical course to that which we had predicted
Wednesday, January 20th, 2021The quote of the post title comes from R. B. Woodward explaining the genesis of the discovery of what are now known as the Woodward-Hoffmann rules for pericyclic reactions.[1] I first wrote about this in 2012, noting that “for (that) blog, I do not want to investigate the transition states”. Here I take a closer look at this aspect.
References
- R.B. Woodward, and R. Hoffmann, "Stereochemistry of Electrocyclic Reactions", Journal of the American Chemical Society, vol. 87, pp. 395-397, 1965. http://dx.doi.org/10.1021/ja01080a054
Internet Archeology: an example of a revitalised molecular resource with a new activity now built in.
Thursday, November 5th, 2020In Internet terms, 23 years ago is verging on pre-history. Much of what was happening around 1997 on the Web was still highly experimental and so its worth taking a look at some of this to see how it has survived or whether it can be “curated” into a form that would still be useful. I had noted in my earlier comment a site which early on had become non-functional and then speculated whether any volunteers might have suggestions for how to best rescue it.
Blasts from the past: a snapshot of online content in chemistry, ~1994-1998.
Monday, September 28th, 2020With universities around the world having to very rapidly transition to blended learning (a mixture of virtual and face-2-face experiences) with a very large component based on online materials, I thought it might be interesting to try to give one snapshot of when the online experience started to happen in chemistry.
The (+) in D-(+)-glyceraldehyde means it has a positive optical rotation? Wrong!
Friday, December 6th, 2019Text books often show the following diagram, famously consolidated over many years by Emil Fischer from 1891 onwards. At the top sits D-(+)-glyceraldehyde, to which all the monosaccharides below are connected by painstaking chemical transformations.
Prediction preceding experiment in chemistry – how unlucky was John Kirkwood?
Saturday, November 30th, 2019Some areas of science progressed via very famous predictions that were subsequently verified by experiments. Think of Einstein and gravitational waves or of Dirac and the positron. There are fewer well-known examples in chemistry; perhaps Watson and Crick’s prediction of the structure of DNA, albeit based on the interpretation of an existing experimental result. Here I take a look at a what if, that of John Kirkwood’s prediction of the absolute configuration of a small molecule based entirely on matching up the sign of a measured optical rotation with that predicted by (his) theory.
The history of Alizarin (and madder).
Thursday, October 18th, 2018The Royal Society of Chemistry historical group (of which I am a member) organises two or three one day meetings a year. Yesterday the October meeting covered (amongst other themes) the fascinating history of madder and its approximately synthetic equivalent alizarin. Here I add a little to the talk given by Alan Dronsfield on the synthesis of alizarin and the impact this had on the entire industry.
Octet expansion and hypervalence in dimethylidyne-λ6-sulfane.
Tuesday, November 28th, 2017I started this story by looking at octet expansion and hypervalence in non-polar hypercoordinate species such as S(-CH3)6, then moved on to S(=CH2)3. Finally now its the turn of S(≡CH)2.‡
Twenty one years of chemistry-related Java apps: RIP Java?
Saturday, June 10th, 2017In an earlier post, I lamented the modern difficulties in running old instances of Jmol, an example of an application program written in the Java programming language. When I wrote that, I had quite forgotten a treasure trove of links to old Java that I had collected in 1996-7 and then abandoned. Here I browse through a few of the things I found.