Posts Tagged ‘Steve Bachrach’

Avoided (pericyclic) anti-aromaticity: Reactions of t-butyl-hydroxycarbene.

Wednesday, November 13th, 2013

Not long ago, I described a cyclic carbene in which elevating the carbene lone pair into a π-system transformed it from a formally 4n-antiaromatic π-cycle into a 4n+2 aromatic π-cycle. From an entirely different area of chemistry, another example of this behaviour emerges; Schreiner’s[1] trapping and reactions of t-butyl-hydroxycarbene, as described on Steve Bachrach’s blog. A point I often make is that chemistry is all about connections, and so here I will discuss such a connection.schreiner

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References

  1. D. Ley, D. Gerbig, and P.R. Schreiner, "Tunneling control of chemical reactions: C–H insertion versus H-tunneling in tert-butylhydroxycarbene", Chem. Sci., vol. 4, pp. 677, 2013. http://dx.doi.org/10.1039/C2SC21555A

The subtle effect of dispersion forces on the shapes of molecules: benzyl magnesium bromide.

Sunday, November 10th, 2013

In the previous post I mentioned in passing the Grignard reagent benzyl magnesium bromide as having tetrahedral coordination at Mg. But I have now noticed, largely through spotting Steve Bachrach’s post on “Acene dimers – open or closed?” another geometric effect perhaps worthy of note, certainly one not always noted in the past; that of dispersion forces.

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Six vs ten aromatic electrons?

Sunday, October 20th, 2013

Homoaromaticity is a special case of aromaticity in which π-conjugation is interrupted by a single sp3 hybridized carbon atom (it is sometimes referred to as a suspended π-bond with no underlying σ-foundation). But consider the carbene shown below. This example comes from a recently published article[1] which was highlighted on Steve Bachrach’s blog. Here aromaticity has resulted from a slightly different phenomenon, whereby a 4π-electron planar (and hence nominally anti-aromatic) molecule is elevated to aromatic peerage by promoting the two carbene σ-electrons to have π-status. 

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References

  1. B. Chen, A.Y. Rogachev, D.A. Hrovat, R. Hoffmann, and W.T. Borden, " How to Make the σ 0 π 2 Singlet the Ground State of Carbenes ", J. Am. Chem. Soc., vol. 135, pp. 13954-13964, 2013. http://dx.doi.org/10.1021/ja407116e

A short non-bonding H…H interaction (continued)

Wednesday, October 2nd, 2013

This is a continuation of the discussion started on Steve Bachrach’s blog about a molecule with a very short H…H interaction involving two Si-H groups with enforced proximity. It had been inferred from the X-ray structure[1] that the H…H distance was in the region of 1.50Å. It’s that cis-butene all over again! So is that H…H region a bond? Is it attractive or repulsive? Go read Steve’s blog first.

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References

  1. J. Zong, J.T. Mague, and R.A. Pascal, " Exceptional Steric Congestion in an in , in -Bis(hydrosilane) ", J. Am. Chem. Soc., vol. 135, pp. 13235-13237, 2013. http://dx.doi.org/10.1021/ja407398w

The strangely attractive conformation of C17H36.

Sunday, January 13th, 2013

We tend to think of simple hydrocarbons as relatively inert and un-interesting molecules. However, a recent article[1], which was in fact highlighted by Steve Bachrach on his blog , asks what “The Last Globally Stable Extended Alkane” might be. In other words, at what stage does a straight-chain hydrocarbon fold back upon itself, and no significant population of the linear form remain? The answer was suggested to be C17H36. I thought I might subject this conformation to an NCI (non-covalent-interaction) analysis.

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References

  1. N.O.B. Lüttschwager, T.N. Wassermann, R.A. Mata, and M.A. Suhm, "The Last Globally Stable Extended Alkane", Angew. Chem. Int. Ed., vol. 52, pp. 463-466, 2012. http://dx.doi.org/10.1002/anie.201202894

What is the range of values for a (sp3)C-C(sp3) single bond length?

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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Dynamic effects in nucleophilic substitution at trigonal carbon.

Monday, July 16th, 2012

Singleton 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.

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The importance of being complete.

Monday, September 26th, 2011

To (mis)quote Oscar Wilde again, ““To lose one methyl group may be regarded as a misfortune; to lose both looks like carelessness.” Here, I refer to the (past) tendency of molecular modellers to simplify molecular structures. Thus in 1977, quantum molecular modelling, even at the semi-empirical level, was beset by lost groups. One of my early efforts (DOI: 10.1021/ja00465a005) was selected for study because it had nothing left to lose; the mass spectrometric fragmentation of the radical cations of methane and ethane. Methyl, phenyl and other “large” groups were routinely replaced by hydrogen in order to enable the study. Cations indeed were always of interest to modellers; the relative lack of electrons almost always meant unusual or interesting structures and reactions (including this controversial species, DOI: 10.1021/ja00444a012). Inured to such functional loss, we modellers forgot that (unless in a mass spectrometer), cations have to have a counter anion. Here I explore one example of the model being complete(d).

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The stereochemistry of [8+2] pericyclic cycloadditions.

Sunday, July 10th, 2011

Steve Bachrach has blogged on the reaction shown below. If it were a pericyclic cycloaddition, both new bonds would form simultaneously, as shown with the indicated arrow pushing. Ten electrons would be involved, and in theory, the transition state would have 4n+2 aromaticity. In fact Fernandez, Sierra and Torres have reported that they can trap an intermediate zwitterion 2, and in this sense therefore, the reaction is not pericyclic but nucleophilic addition from the imine lone pair to the carbonyl of the ketene (it finds the half way stage convivial). But this got me thinking. Does this reaction have any pericyclic character at all? And if so, could it be enhanced by design?

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