I left the story of the molecule below on the precipice of a cliff. I had shaved off the four benzo groups (blue) in the time honoured computational tradition of clearing away distractions. Unfortunately, it became clear as the story unfolded that the benzo groups had a distractingly critical role to play, and so its time to start adding them back again, but in stages.
To recapitulate, this reaction (with four benzo groups) has a half life of ~30 minutes at 353K, giving it a a free energy barrier of ~26.2 kcal/mol. According to Woodward and Hoffmann, it is a “forbidden” reaction (4n, n=2, electrons require an antarafacial component from somewhere). Shorn of the benzo groups, such a component developed in one of the forming phenyl rings (forming a Möbius benzene). But, by adding two benzo groups back to this phenyl ring (turning it into an anthracene), we will stop this process in its tracks. Read the rest of this entry »
Violations. There are none! Part 2.
December 26th, 2011Quadruple antarafacial delight.
December 18th, 2011A feature of many a classic review article is that not only does it organise and rationalise existing literature, but it will predict new chemistry as well. I have already noted Woodward and Hoffmann’s (WH) review as achieving the former, and here I take a (sideways) look at one of their predictions. Read the rest of this entry »
Molecular gymnastics in 2+2 cycloadditions. Two different moves compared.
December 15th, 2011The previous post showed how the 2+2 cycloaddition of an alkene could occur by a sort of sideways insinuation of the bonds. I have also shown how the same reaction can occur with a dramatic rotation of one of the double bonds. This post compares the two moves side by side.
Molecular gymnastics in 2+2 cycloadditions.
December 14th, 2011In this earlier post, I described how the stereochemistry of π2+π2 cycloadditions occurs suprafacially if induced by light, and how one antarafacial component appears if the reaction is induced by heat alone. I also noted how Woodward and Hoffmann (WH) explained that violations to their rules were avoided by mandating a change in mechanism requiring stepwise pathways with intermediates along the route. Here I illustrate how the stereochemistry of a thermal π2+π2 cycloaddition can indeed avoid an antarafacial component by performing appropriate gymnastic contortions instead of a mechanistic change (a WH violation certainly in the letter of their law, if not their spirit).
Read the rest of this entry »
Mechanistic morphemes. Perisolvolysis of a cyclopropyl chloride.
December 13th, 2011There are many treasures in Woodward and Hoffmann’s (WH) classic monograph. One such is acetolysis of the endo chloride (green), which is much much faster than that of the exo isomer (red). The explanation given in their article (p 805) confines itself to succinctly stating that only loss of the endo halogen can be concerted with a required disrotatory ring opening of the cyclopropane. Demonstrating the truth of this statement by computational modelling turns out to be an interesting challenge. Read the rest of this entry »
Violations. There are none!
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. Read the rest of this entry »
Validating the chemical literature heritage. Eudesma-1,3-dien-6,13-olide.
December 8th, 2011Previously, I had noted that Corey reported in 1963/65 the total synthesis of the sesquiterpene dihydrocostunolide. Compound 16, known as Eudesma-1,3-dien-6,13-olide was represented as shown below in black; the hydrogen shown in red was implicit in Corey’s representation, as was its stereochemistry. As of this instant, this compound is just one of 64,688,893 molecules recorded by Chemical Abstracts. How can we, in 2011, validate this particular entry, and resolve the stereochemical ambiguity? Here I discuss one approach (a vision if you like of the semantic web). Read the rest of this entry »
So near and yet so far. The story of the electrocyclic ring opening of a cyclohexadiene.
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. https://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. https://doi.org/10.1021/ja00952a037
A modern take on pericyclic sigmatropic migrations.
November 29th, 2011Another common type of pericyclic reaction is the migration of hydrogen or carbon along a conjugated chain, as in the [1,3] migration of a carbon as shown below. As before, I explore the stereochemistry of the thermal and photochemical reactions.