Archive for the ‘Interesting chemistry’ Category
Sunday, October 28th, 2012
The reaction described in the previous post (below) is an unusual example of nucleophilic attack at an sp2-carbon centre, reportedly resulting in inversion of configuration[1]. One can break it down to a sequence of up to eight individual steps, which makes teaching it far easier. But how real is that sequence?
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References
- T.C. Clarke, and R.G. Bergman, "Olefinic cyclization at a vinyl cation center. Inversion preference for intramolecular nucleophilic substitution by a double bond", Journal of the American Chemical Society, vol. 94, pp. 3627-3629, 1972. https://doi.org/10.1021/ja00765a062
Tags:immediate product, inversion, Reaction Mechanism, triflate leaving, Tutorial material, vinyl carbocation
Posted in Interesting chemistry | 1 Comment »
Wednesday, October 17th, 2012
Every once in a while, one encounters a molecule which instantly makes an interesting point. Thus Ruthenium is ten electrons short of completing an 18-electron shell, and it can form a complex with benzene on one face and a ligand known as trimethylenemethane on the other[1].
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References
- G.E. Herberich, and T.P. Spaniol, "Trimethylenemethane complexes of ruthenium via the trimethylenemethane dianion", Journal of the Chemical Society, Chemical Communications, pp. 1457, 1991. https://doi.org/10.1039/c39910001457
Tags:Iron complex, metal, Postscript
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Monday, October 8th, 2012
Metathesis reactions are a series of catalysed transformations which transpose the atoms in alkenes or alkynes. Alkyne metathesis is closely related to the same reaction for alkenes, and one catalyst that is specific to alkynes was introduced by Schrock (who with Grubbs won the Nobel prize for these discoveries) and is based on tungsten (M=W(OR)3).
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Tags:Reaction Mechanism, X-ray
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Monday, September 17th, 2012
I 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.
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Tags:pericyclic
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Saturday, September 15th, 2012
Semibullvalene 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.
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Tags:candidate for synthesis perhaps, energy, pericyclic, Postscript, Reaction Mechanism
Posted in Interesting chemistry | 3 Comments »
Wednesday, September 12th, 2012
Here 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).
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Tags:Cambridge, Steve Bachrach
Posted in Interesting chemistry | 8 Comments »