Posts Tagged ‘Chemistry’

Organocatalytic cyclopropanation of an enal: (computational) assignment of absolute configurations.

Saturday, September 1st, 2018
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I am exploring the fascinating diverse facets of a recently published laboratory experiment for undergraduate students.[1] Previously I looked at a possible mechanistic route for the reaction between an enal (a conjugated aldehyde-alkene) and benzyl chloride catalysed by base and a chiral amine, followed by the use of NMR coupling constants to assign relative stereochemistries. Here I take a look at some chiroptical techniques which can be used to assign absolute stereochemistries (configurations).

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References

  1. M. Meazza, A. Kowalczuk, S. Watkins, S. Holland, T.A. Logothetis, and R. Rios, "Organocatalytic Cyclopropanation of (E)-Dec-2-enal: Synthesis, Spectral Analysis and Mechanistic Understanding", Journal of Chemical Education, vol. 95, pp. 1832-1839, 2018. http://dx.doi.org/10.1021/acs.jchemed.7b00566

Organocatalytic cyclopropanation of an enal: (computational) product stereochemical assignments.

Sunday, August 26th, 2018
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In the previous post, I investigated the mechanism of cyclopropanation of an enal using a benzylic chloride using a quantum chemistry based procedure. Here I take a look at the NMR spectra of the resulting cyclopropane products, with an evaluation of the original stereochemical assignments.[1]

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References

  1. M. Meazza, A. Kowalczuk, S. Watkins, S. Holland, T.A. Logothetis, and R. Rios, "Organocatalytic Cyclopropanation of (E)-Dec-2-enal: Synthesis, Spectral Analysis and Mechanistic Understanding", Journal of Chemical Education, vol. 95, pp. 1832-1839, 2018. http://dx.doi.org/10.1021/acs.jchemed.7b00566

Organocatalytic cyclopropanation of an enal: (computational) mechanistic understanding.

Saturday, August 25th, 2018
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Symbiosis between computation and experiment is increasingly evident in pedagogic journals such as J. Chemical Education. Thus an example of original laboratory experiments[1],[2] that later became twinned with a computational counterpart.[3] So when I spotted this recent lab experiment[4] I felt another twinning approaching.

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References

  1. A. Burke, P. Dillon, K. Martin, and T.W. Hanks, "Catalytic Asymmetric Epoxidation Using a Fructose-Derived Catalyst", Journal of Chemical Education, vol. 77, pp. 271, 2000. http://dx.doi.org/10.1021/ed077p271
  2. J. Hanson, "Synthesis and Use of Jacobsen's Catalyst: Enantioselective Epoxidation in the Introductory Organic Laboratory", Journal of Chemical Education, vol. 78, pp. 1266, 2001. http://dx.doi.org/10.1021/ed078p1266
  3. K.K.(. Hii, H.S. Rzepa, and E.H. Smith, "Asymmetric Epoxidation: A Twinned Laboratory and Molecular Modeling Experiment for Upper-Level Organic Chemistry Students", Journal of Chemical Education, vol. 92, pp. 1385-1389, 2015. http://dx.doi.org/10.1021/ed500398e
  4. M. Meazza, A. Kowalczuk, S. Watkins, S. Holland, T.A. Logothetis, and R. Rios, "Organocatalytic Cyclopropanation of (E)-Dec-2-enal: Synthesis, Spectral Analysis and Mechanistic Understanding", Journal of Chemical Education, vol. 95, pp. 1832-1839, 2018. http://dx.doi.org/10.1021/acs.jchemed.7b00566

Tetrahedral carbon and cyclohexane.

Wednesday, August 22nd, 2018
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Following the general recognition of carbon as being tetrahedrally tetravalent in 1869 (Paterno) and 1874 (Van’t Hoff and Le Bell), an early seminal exploitation of this to the conformation of cyclohexane was by Hermann Sachse in 1890.[1] This was verified when the Braggs in 1913[2], followed by an oft-cited article by Mohr in 1918,[3] established the crystal structure of diamond as comprising repeating rings in the chair conformation. So by 1926, you might imagine that the shape (or conformation as we would now call it) of cyclohexane would be well-known. No quite so for everyone!

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References

  1. H. Sachse, "Ueber die geometrischen Isomerien der Hexamethylenderivate", Berichte der deutschen chemischen Gesellschaft, vol. 23, pp. 1363-1370, 1890. http://dx.doi.org/10.1002/cber.189002301216
  2. W.H. Bragg, and W.L. Bragg, "The Structure of the Diamond", Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences, vol. 89, pp. 277-291, 1913. http://dx.doi.org/10.1098/rspa.1913.0084
  3. E. Mohr, "Die Baeyersche Spannungstheorie und die Struktur des Diamanten", Journal f�r Praktische Chemie, vol. 98, pp. 315-353, 1918. http://dx.doi.org/10.1002/prac.19180980123

Early “curly” (reaction) arrows. Those of Ingold in 1926.

Wednesday, August 22nd, 2018
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In 2012, I wrote a story of the first ever reaction curly arrows, attributed to Robert Robinson in 1924. At the time there was a great rivalry between him and another UK chemist, Christopher Ingold, with the latter also asserting his claim for their use. As part of the move to White City a lot of bookshelves were cleared out from the old buildings in South Kensington, with the result that yesterday a colleague brought me a slim volume they had found entitled The Journal of the Imperial College Chemical Society (Volume 6). 

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The “White City Trio” – The formation of an amide from an acid and an amine in non-polar solution (updated).

Wednesday, August 8th, 2018
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White City is a small area in west london created as an exhibition site in 1908, morphing over the years into an Olympic games venue, a greyhound track, the home nearby of the BBC (British Broadcasting Corporation) and most recently the new western campus for Imperial College London. The first Imperial department to move into the MSRH (Molecular Sciences Research Hub) building is chemistry. As a personal celebration of this occasion, I here dedicate three transition states located during my first week of occupancy there, naming them the White City trio following earlier inspiration by a string trio and their own instruments.

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A Theoretical Method for Distinguishing X‐H Bond Activation Mechanisms.

Wednesday, July 25th, 2018
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Consider the four reactions. The first two are taught in introductory organic chemistry as (a) a proton transfer, often abbreviated PT, from X to B (a base) and (b) a hydride transfer from X to A (an acid). The third example is taught as a hydrogen atom transfer or HAT from X to (in this example) O. Recently an article has appeared[1] citing an example of a fourth fundamental type (d), which is given the acronym cPCET which I will expand later. Here I explore this last type a bit further, in the context that X-H bond activations are currently a very active area of research.

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References

  1. J.E.M.N. Klein, and G. Knizia, "cPCET versus HAT: A Direct Theoretical Method for Distinguishing X-H Bond-Activation Mechanisms", Angewandte Chemie International Edition, vol. 57, pp. 11913-11917, 2018. http://dx.doi.org/10.1002/anie.201805511

Why do flowers such as roses, peonies, dahlias, delphiniums (etc), exhibit so many shades of colours?

Monday, June 18th, 2018
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It was about a year ago that I came across a profusion of colour in my local Park. Although colour in fact was the topic that sparked my interest in chemistry many years ago (the fantastic reds produced by diazocoupling reactions), I had never really tracked down the origin of colours in many flowers. It is of course a vast field. Here I take a look at just one class of molecule responsible for many flower colours, anthocyanidin, this being the sugar-free counterpart of the anthocyanins found in nature.

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Aromaticity-induced basicity.

Wednesday, April 18th, 2018
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The molecules below were discussed in the previous post as examples of highly polar but formally neutral molecules, a property induced by aromatisation of up to three rings. Since e.g. compound 3 is known only in its protonated phenolic form, here I take a look at the basicity of the oxygen in these systems to see if deprotonation of the ionic phenol form to the neutral polar form is viable.

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A record polarity for a neutral compound?

Friday, April 13th, 2018
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In several posts a year or so ago I considered various suggestions for the most polar neutral molecules, as measured by the dipole moment. A record had been claimed[1] for a synthesized molecule of ~14.1±0.7D. I pushed this to a calculated 21.7D for an admittedly hypothetical and unsynthesized molecule. Here I propose a new family of compounds which have the potential to extend the dipole moment for a formally neutral molecule up still further.

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References

  1. J. Wudarczyk, G. Papamokos, V. Margaritis, D. Schollmeyer, F. Hinkel, M. Baumgarten, G. Floudas, and K. Müllen, "Hexasubstituted Benzenes with Ultrastrong Dipole Moments", Angewandte Chemie International Edition, vol. 55, pp. 3220-3223, 2016. http://dx.doi.org/10.1002/anie.201508249