The mechanism of the reaction of alkenes known as ozonolysis was first set out in its modern form by Criegee. The crucial steps, (a), (b) and (d), are all pericyclic cycloaddition/eliminations. The last step (e) is known as reductive ozonolysis, and this step is often treated as an afterthought, part of the work-up of the reaction if you like (it is not illustrated in Criegee’s review for example). Here, I will attempt to show that it is actually a very interesting mechanistic step.
Posts Tagged ‘Historical’
Reductive ozonolysis: the interesting step.
Monday, May 7th, 2012The mechanism of the Baeyer-Villiger rearrangement.
Monday, May 7th, 2012The Baeyer-Villiger rearrangement was named after its discoverers, who in 1899 described the transformation of menthone into the corresponding lactone using Caro’s acid (peroxysulfuric acid). The mechanism is described in all text books of organic chemistry as involving an alkyl migration. Here I take a look at the scheme described by Alvarez-Idaboy, Reyes and Mora-Diez[1], and which may well not yet have made it to all the text books!
References
- J.R. Alvarez-Idaboy, L. Reyes, and N. Mora-Diez, "The mechanism of the Baeyer–Villiger rearrangement: quantum chemistry and TST study supported by experimental kinetic data", Organic & Biomolecular Chemistry, vol. 5, pp. 3682, 2007. http://dx.doi.org/10.1039/b712608e
The Dieneone-phenol controversies.
Monday, April 30th, 2012During the 1960s, a holy grail of synthetic chemists was to devise an efficient route to steroids. R. B. Woodward was one the chemists who undertook this challenge, starting from compounds known as dienones (e.g. 1) and their mysterious conversion to phenols (e.g. 2 or 3) under acidic conditions. This was also the golden era of mechanistic exploration, which coupled with an abundance of radioactive isotopes from the war effort had ignited the great dienone-phenol debates of that time (now largely forgotten). In a classic recording from the late 1970s, Woodward muses how chemistry had changed since he started in the early 1940s. In particular he notes how crystallography had revolutionised the reliability and speed of molecular structure determination. Here I speculate what he might have made of modern computational chemistry, and in particular whether it might cast new light on those mechanistic controversies of the past.
Stereoselectivities of Proline-Catalyzed Asymmetric Intermolecular Aldol Reactions.
Sunday, April 22nd, 2012Astronomers who discover an asteroid get to name it, mathematicians have theorems named after them. Synthetic chemists get to name molecules (Hector’s base and Meldrum’s acid spring to mind) and reactions between them. What do computational chemists get to name? Transition states! One of the most famous of recent years is the Houk-List.
A golden age for (computational) spectroscopy.
Monday, April 2nd, 2012I mentioned in my last post an unjustly neglected paper from that golden age of 1951-1953 by Kirkwood and co. They had shown that Fischer’s famous guess for the absolute configurations of organic chiral molecules was correct. The two molecules used to infer this are shown below.
Violations. There are none!
Sunday, December 11th, 2011Thus famously wrote Woodward and Hoffmann (WH) in their classic monograph about the conservation of orbital symmetry in pericyclic reactions. But they also note that the “fantastic” hydrocarbon (number 85 in their review) shown below presents a situation of great interest in having a half life of ~30 minutes at 353K (a free energy barrier of ~ 26.2 kcal/mol). Here I investigate if it might actually be such a violation. (more…)
So near and yet so far. The story of the electrocyclic ring opening of a cyclohexadiene.
Tuesday, December 6th, 2011My previous three posts set out my take on three principle categories of pericyclic reaction. Here I tell a prequel to the understanding of these reactions. In 1965, Woodward and Hoffmann[1] in their theoretical analysis (submitted Nov 30, 1964) for which the Nobel prize (to Hoffmann only of the pair, Woodward having died) was later awarded. But in the same year, Elias Corey[2] reported the conclusion of a project started several years earlier (first reported (DOI: 10.1021/ja00907a030, Nov 1, 1963) to synthesize the sesquiterpene dihydrocostunolide.
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
- E.J. Corey, and A.G. Hortmann, "The Total Synthesis of Dihydrocostunolide", Journal of the American Chemical Society, vol. 87, pp. 5736-5742, 1965. http://dx.doi.org/10.1021/ja00952a037
A modern take on the pericyclic electrocyclic ring opening of cyclobutene.
Saturday, November 26th, 2011Woodward and Hoffmann published their milestone article “Stereochemistry of Electrocyclic Reactions” in 1965. This brought maturity to the electronic theory of organic chemistry, arguably started by the proto-theory of Armstrong some 75 years earlier. Here, I take a modern look at the archetypal carrier of this insight, the ring opening of dimethylcyclobutene.
The dawn of organic reaction mechanism: the prequel.
Sunday, November 13th, 2011Following on from Armstrong’s almost electronic theory of chemistry in 1887-1890, and Beckmann’s radical idea around the same time that molecules undergoing transformations might do so via a reaction mechanism involving unseen intermediates (in his case, a transient enol of a ketone) I here describe how these concepts underwent further evolution in the early 1920s. My focus is on Edith Hilda Usherwood, who was then a PhD student at Imperial College working under the supervision of Martha Whitely.1