Archive for September, 2022

A new type of bispericyclic reaction: Cyclopropanone + butadiene.

Friday, September 30th, 2022

The term bispericyclic reaction was famously coined by Caramella et al in 2002[1] to describe the unusual features of the apparently innocuous dimerisation of cyclopentadiene. It shows features of two paths for different pericyclic reactions, comprising a 2+4 cycloaddition in the early stages, but evolving into a (degenerate) pair of [3,3] sigmatropic reactions in the latter stages. Houk (who also uses the term ambimodal) has in recent years extended the number of examples of such pericyclic sequences to trispericyclic[2] (see here) and even an ambimodel tetrapericyclic reaction, as reported at the recent WATOC event. Here I show an example of a new type of bispericyclic reaction, comprising a 2+4 cycloaddition combined with a electrocyclic ring opening.

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References

  1. P. Caramella, P. Quadrelli, and L. Toma, "An Unexpected Bispericyclic Transition Structure Leading to 4+2 and 2+4 Cycloadducts in the Endo Dimerization of Cyclopentadiene", Journal of the American Chemical Society, vol. 124, pp. 1130-1131, 2002. http://dx.doi.org/10.1021/ja016622h
  2. X. Xue, C.S. Jamieson, M. Garcia-Borràs, X. Dong, Z. Yang, and K.N. Houk, "Ambimodal Trispericyclic Transition State and Dynamic Control of Periselectivity", Journal of the American Chemical Society, vol. 141, pp. 1217-1221, 2019. http://dx.doi.org/10.1021/jacs.8b12674

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Examples of inverted or hemispherical carbon?

Thursday, September 15th, 2022

In previously asking what the largest angle subtended at four-coordinate carbon might be, I noted that as the angle increases beyond 180°, the carbon becomes inverted, or hemispherical (all four ligands in one hemisphere). So what does a search for this situation reveal in the CSD? The query can be formulated as below, in which the distance from the centroid of the four ligands to the central carbon is specified to be in e.g. the range 0.8 to 1.1Å. For tetrahedral carbon surrounded by four carbon ligands, the value would be close to zero, so any value larger than say 0.8Å is worth inspecting.

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What is the largest angle possible at 4-coordinate carbon – 180°?

Sunday, September 11th, 2022

Four-coordinate carbon normally adopts a tetrahedral shape, where the four angles at the carbon are all 109.47°. But how large can that angle get, and can it even get to be 180°?

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Why does octafluorocubane have such a high sublimation point?

Thursday, September 8th, 2022

The recently reported synthesis[1] of octafluorocubane established a sublimation point as 168.1–177.1°C (a melting point was not observed). In contrast, the heavier perfluoro-octane has an m.p. of -25°C. Why the difference? Firstly, the crystal structure is shown below, albeit as a dimer rather than a periodic lattice (click on image to obtain 3D coordinates).

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References

  1. M. Sugiyama, M. Akiyama, Y. Yonezawa, K. Komaguchi, M. Higashi, K. Nozaki, and T. Okazoe, "Electron in a cube: Synthesis and characterization of perfluorocubane as an electron acceptor", Science, vol. 377, pp. 756-759, 2022. http://dx.doi.org/10.1126/science.abq0516

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