Semibullvalene is a molecule which undergoes a facile [3,3] sigmatropic shift. So facile that it appears this equilibrium can be frozen out at the transition state if suitable substituents are used. This is a six-electron process, which leads to one of those homologous questions; what happens with ten electrons?
Archive for the ‘Interesting chemistry’ Category
The ten-electron homologue of semibullvalene.
Friday, September 21st, 2012Predicted properties of a candidate for a frozen semibullvalene.
Monday, September 17th, 2012I am following up on one unfinished thread in my previous post; a candidate was proposed in which the transition state for [3,3] sigmatropic rearrangement in a semibullvalene might be frozen out to become instead a stable minimum.
Frozen Semibullvalene: a holy grail (and a bis-homoaromatic molecule).
Saturday, September 15th, 2012Semibullvalene is an unsettling molecule. Whilst it has a classical structure describable by a combination of Lewis-style two electron and four electron bonds, its NMR behaviour reveals it to be highly fluxional. This means that even at low temperatures, the position of these two-electron bonds rapidly shifts in the equilibrium shown below. Nevertheless, this dynamic behaviour can be frozen out at sufficiently low temperatures. But the barrier was sufficiently low that a challenge was set; could one achieve a system in which the barrier was removed entirely, to freeze out the coordinates of the molecule into a structure where the transition state (shown at the top) became instead a true minimum (bottom)? A similar challenge had been set for freezing out the transition state for the Sn2 reaction into a minimum, the topic also of a more recent post here. Here I explore how close we might be to achieving inversion of the semibullvalene [3,3] sigmatropic potential.
What is the range of values for a (sp3)C-C(sp3) single bond length?
Wednesday, September 12th, 2012Here is a challenge: what is the longest C-C bond actually determined (in which both carbon termini are sp3 hybridised)? I pose this question since Steve Bachrach has posted on how to stabilize long bonds by attractive dispersive interactions, and more recently commenting on what the longest straight chain alkane might be before dispersive interaction start to fold it (the answer appears to be C17).
The Sn2 reaction and the anomaly of carbon.
Thursday, September 6th, 2012It was three years ago that I first blogged on the topic of the Sn2 reaction. Matthias Bickelhaupt had suggested that the Sn2 reaction involving displacement at a carbon atom was an anomaly; the true behaviour was in fact exhibited by the next element down in the series, silicon. The pentacoordinate species shown below (X=Si) is naturally a minimum, and the fact that for carbon (X=C) one gets instead a transition state resulting in a significant thermal barrier (~ 20 kcal/mol) was a manifestation of abnormal behaviour.
Dynamic effects in nucleophilic substitution at trigonal carbon.
Monday, July 16th, 2012Singleton and co-workers have produced some wonderful work showing how dynamic effects and not just transition states can control the outcome of reactions. Steve Bachrach’s blog contains many examples, including this recent one.
More joining up of pieces. Stereocontrol in the ring opening of cyclopropenes.
Thursday, July 12th, 2012Years ago, I was travelling from Cambridge to London on a train. I found myself sitting next to a chemist, and (as chemists do), he scribbled the following on a piece of paper. When I got to work the next day Vera (my student) was unleashed on the problem, and our thoughts were published[1]. That was then.
References
- M.S. Baird, J.R. Al Dulayymi, H.S. Rzepa, and V. Thoss, "An unusual example of stereoelectronic control in the ring opening of 3,3-disubstituted 1,2-dichlorocyclopropenes", Journal of the Chemical Society, Chemical Communications, pp. 1323, 1992. https://doi.org/10.1039/c39920001323
Connections in chemistry. Anti-malaria drug ↔ organocatalysis.
Thursday, July 5th, 2012Back in 1994, we published the crystal structure of the molecule below (X=H), a putative anti-malarial drug called halofantrine. Little did we realise that a whole area of organo catalysis based on a thiourea catalyst with a similar motif would emerge a little later. Here is how the two are connected.
Streptomycin: a case study in the progress of science.
Monday, May 28th, 2012Streptomycin is an antibiotic active against tuberculosis, and its discovery has become something of a cause célèbre. It was first isolated on October 19, 1943 by a graduate student Albert Schatz in the laboratory of Selman Waksman at Rutgers University. I want to concentrate in this post on its molecular structure. Its initial isolation was followed by an extraordinarily concentrated period of about three years devoted to identifying that structure, culminating in a review of this chemistry in 1948 by Lemieux and Wolfram.[1] This review presents the structure as shown below (left). The modern rendering on the right is based on a crystal structure done in 1978.[2]‡
References
- R. Lemieux, and M. Wolfrom, "The Chemistry of Streptomycin", Advances in Carbohydrate Chemistry, pp. 337-384, 1948. https://doi.org/10.1016/s0096-5332(08)60034-x
- S. Neidle, D. Rogers, and M.B. Hursthouse, "The crystal and molecular structure of streptomycin oxime selenate tetrahydrate", Proceedings of the Royal Society of London. A. Mathematical and Physical Sciences, vol. 359, pp. 365-388, 1978. https://doi.org/10.1098/rspa.1978.0047