Archive for the ‘Interesting chemistry’ Category

WATOC2014 Conference report. Emergent themes.

Thursday, October 9th, 2014

This second report highlights two “themes”, or common ideas that seem to emerge spontaneously from diversely different talks. Most conferences do have them.

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WATOC2014 Conference report. Concepts for Organizing Chemical Knowledge

Monday, October 6th, 2014

I am attending a conference. Plenaries at such events can sometimes provide interesting pointers on things to come (and sometimes they simply point to things past). At WATOC2014 in Santiago Chile, the first plenary was by Paul Ayers with the impressive title “Concepts for organising chemical knowledge” which certainly sounds as if it is pointing forward!

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Using a polar bond to flip the (stereochemical) outcome of a pericyclic reaction.

Monday, August 4th, 2014

The outcome of pericyclic reactions con depend most simply on three conditions, any two of which determine the third. Whether the catalyst is Δ or hν (heat or light), the topology determining any stereochemistry and the participating electron count (4n+2/4n). It is always neat to conjure up a simple switch to toggle these; heat or light is simple, but what are the options for toggling the electron count? Here is one I have contrived by playing a game with the periodic table. divinylketon The ring closure of a divinylketone is called the Nazarov reaction, it being promoted thermodynamically by coordination of a Lewis acid to atom X. Divinyl ketone can be regarded as a hidden pentadienyl cation, since the C=O bond is polarised Cδ+Oδ- in the time-honoured manner of organic chemistry. In this (formal) resonance form, it becomes part of a pentadienyl cation and can electrocyclise via a 4-electron reaction involving a stereochemical process known as conrotation. The new bond is formed antarafacially (from opposite faces) at the termini of the pentadienyl cation (ωB97XD/6-311G(d,p)/SCRF=dichloromethane.[1]). Note that for the uncatalysed reaction, the barrier is high and the reaction is endothermic but adding a BF3 to the oxygen lowers the barrier and removes the endothermicity.[2] nazarov-Oa nazarov-Oa nazarov-OBF3 So, one can play a game and ask what would happen if the polarity of the C=X bond were to be reversed. This means going left of oxygen in the periodic table, ending at Be.[3] The reaction has a high barrier, but it is strongly exothermic. However the most noteworthy aspect is that the stereochemistry of the electrocyclisation is now disrotatory, with suprafacial bond formation (from the bottom face in the animation below). The stereochemical outcome of this reaction has been flipped by reversing the polarity of the CX bond. nazarov-Beanazarov-Bea This little example shows how a thought game played using the periodic table can then be reality tested by solving appropriate quantum mechanical equations. In this instance, one is not going to rush into the laboratory to try to replicate the experiment, but it might help catalyse new thoughts amongst the readers of this blog.

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References

  1. Henry S. Rzepa., "Gaussian Job Archive for C5H6O", 2014. http://dx.doi.org/10.6084/m9.figshare.1125721
  2. Henry S. Rzepa., "Gaussian Job Archive for C5H6BF3O", 2014. http://dx.doi.org/10.6084/m9.figshare.1125724
  3. Henry S. Rzepa., "Gaussian Job Archive for C5H6Be", 2014. http://dx.doi.org/10.6084/m9.figshare.1125792

Data nightmares: B40 and counting its π-electrons

Saturday, July 19th, 2014

Whilst clusters of carbon atoms are well-known, my eye was caught by a recent article describing the detection of a cluster of boron atoms, B40 to be specific.[1] My interest was in how the σ and π-electrons were partitioned. In a C40, one can reliably predict that each carbon would contribute precisely one π-electron. But boron, being more electropositive, does not always play like that. Having one electron less per atom, one might imagine that a fullerene-like boron cluster would have no π-electrons. But the element has a propensity[2] to promote its σ-electrons into the π-manifold, leaving a σ-hole. So how many π-electrons does B40 have? These sorts of clusters are difficult to build using regular structure editors, and so coordinates are essential. The starting point for a set of coordinates with which to compute a wavefunction was the supporting information. Here is the relevant page: B401 The coordinates are certainly there (that is not always the case), but you have to know a few tricks to make them usable.

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References

  1. H. Zhai, Y. Zhao, W. Li, Q. Chen, H. Bai, H. Hu, Z.A. Piazza, W. Tian, H. Lu, Y. Wu, Y. Mu, G. Wei, Z. Liu, J. Li, S. Li, and L. Wang, "Observation of an all-boron fullerene", Nature Chem, 2014. http://dx.doi.org/10.1038/nchem.1999
  2. H.S. Rzepa, "The distortivity of π-electrons in conjugated boron rings", Phys. Chem. Chem. Phys., vol. 11, pp. 10042, 2009. http://dx.doi.org/10.1039/B911817A

Amides and inverting the electronics of the Bürgi–Dunitz trajectory.

Thursday, June 26th, 2014

The Bürgi–Dunitz angle describes the trajectory of an approaching nucleophile towards the carbon atom of a carbonyl group. A colleague recently came to my office to ask about the inverse, that is what angle would an electrophile approach (an amide)? Thus it might approach either syn or anti with respect to the nitrogen, which is a feature not found with nucleophilic attack. amide My first thought was to calculate the wavefunction and identify the location and energy (= electrophilicity) of the lone pairs (the presumed attractor of an electrophile). But a better more direct approach soon dawned. A search of the crystal structure database. Here is the search definition, with the C=O-E angle, the O-E distance and the N-C=O-E torsion defined (also specified for R factor < 5%, no errors and no disorder). search   The first plot is of the torsion vs the distance, for E = H-X (X=O,F, Cl) amides

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Kekulé’s vibration: A modern example of its use.

Friday, June 6th, 2014

Following the discussion here of Kekulé’s suggestion of what we now call a vibrational mode (and which in fact now bears his name), I thought I might apply the concept to a recent molecule known as [2.2]paracyclophane. The idea was sparked by Steve Bachrach’s latest post, where the “zero-point” structure of the molecule has recently been clarified as having D2 symmetry.[1]

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References

  1. H. Wolf, D. Leusser, . Mads R. V. Jørgensen, R. Herbst-Irmer, Y. Chen, E. Scheidt, W. Scherer, B.B. Iversen, and D. Stalke, "Phase Transition of [2,2]-Paracyclophane - An End to an Apparently Endless Story", Chemistry - A European Journal, vol. 20, pp. 7048-7053, 2014. http://dx.doi.org/10.1002/chem.201304972

Ribulose-1,5-bisphosphate + carbon dioxide → carbon fixation!

Sunday, April 20th, 2014

Ribulose-1,5-bisphosphate reacts with carbon dioxide to produce 3-keto-2-carboxyarabinitol 1,5-bisphosphate as the first step in the biochemical process of carbon fixation. It needs an enzyme to do this (Ribulose-1,5-bisphosphate carboxylase/oxygenase, or RuBisCO) and lots of ATP (adenosine triphosphate, produced by photosynthesis). Here I ask what the nature of the uncatalysed transition state is, and hence the task that might be facing the catalyst in reducing the activation barrier to that of a facile thermal reaction. I present my process in the order it was done.

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More (blog) connections spotted. Something new about diphenyl magnesium?

Thursday, April 17th, 2014

I have just noticed unexpected links between two old posts, one about benzene, one about diphenyl magnesium and a link to August Kekulé.

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Enantioselective epoxidation of alkenes using the Shi Fructose-based catalyst. An undergraduate experiment.

Tuesday, April 15th, 2014

The journal of chemical education can be a fertile source of ideas for undergraduate student experiments. Take this procedure for asymmetric epoxidation of an alkene.[1] When I first spotted it, I thought not only would it be interesting to do in the lab, but could be extended by incorporating some modern computational aspects as well. 

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References

  1. A. Burke, P. Dillon, K. Martin, and T.W. Hanks, "Catalytic Asymmetric Epoxidation Using a Fructose-Derived Catalyst", J. Chem. Educ., vol. 77, pp. 271, 2000. http://dx.doi.org/10.1021/ed077p271

Artemisinin: are stereo-electronics at the core of its (re)activity?

Sunday, April 13th, 2014

Around 100 tons of the potent antimalarial artemisinin is produced annually; a remarkable quantity given its very unusual and fragile looking molecular structure (below). When I looked at this, I was immediately struck by a thought: surely this is a classic molecule for analyzing stereoelectronic effects (anomeric and gauche). Here this aspect is explored.

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