Posts Tagged ‘Steve Bachrach’

To blog or to publish. That is the question.

Tuesday, February 9th, 2010

Scientists write blogs for a variety of reasons. But these do probably not include getting tenure (or grants). For that one has to publish. And I will argue here that a blog is not currently accepted as a scientific publication (for more discussion on this point, see this article by Maureen Pennock and Richard Davis). For chemists, publication means in a relatively small number of high-impact journals. Anything more than five articles a year in such journals, and your tenure is (probably) secure (if not your funding).

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The nature of the C≡S triple bond

Tuesday, December 1st, 2009

Steve Bachrach has just blogged on a recent article (DOI: 10.1002/anie.200903969) claiming the isolation of a compound with a C≡S triple bond;

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Mechanistic Ménage à trois

Wednesday, November 18th, 2009

Curly arrow pushing is one of the essential tools of a mechanistic chemist. Many a published article will speculate about the arrow pushing in a mechanism, although it is becoming increasingly common for these speculations to be backed up by quantitative quantum mechanical and dynamical calculations. These have the potential of exposing the underlying choreography of the electronic dance (the order in which the steps take place). The basic grammar of describing that choreography tends to be the full-headed curly arrow for closed shell systems and its half-barbed equivalent for open shell systems. An effectively unstated and hence implicit rule for closed shell systems is that only one curly arrow is used per breaking or forming bond, i.e. electrons move around bonds in pairs. So consider the following reaction (inspired by a posting on  Steve Bachrach’s blog)

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Capturing penta-coordinate carbon! (Part 1).

Tuesday, September 22nd, 2009

The bimolecular nucleophilic substitution reaction at saturated carbon is an icon of organic chemistry, and is better known by its mechanistic label, SN2. It is normally a slow reaction, with half lives often measured in hours. This implies a significant barrier to reaction (~15-20 kcal/mol) for the transition state, shown below (X is normally both a good nucleophile and a good nucleofuge/leaving group, such as halide, cyanide, etc.  Y can have a wide variety of forms).

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A molecule with an identity crisis: Aromatic or anti-aromatic?

Monday, April 13th, 2009

In 1988, Wilke[1] reported molecule 1

A [24] annulene. Click on image for model.

A 24-annulene. Click for 3D.


It was a highly unexpected outcome of a nickel-catalyzed reaction and was described as a 24-annulene with an unusual 3D shape. Little attention has been paid to this molecule since its original report, but the focus has now returned! The reason is that a 24- annulene belongs formally to a class of molecule with 4n (n=6) π-electrons, and which makes it antiaromatic according to the (extended) Hückel rule. This is a select class of molecule, of which the first two members are cyclobutadiene and cyclo-octatetraene. The first of these is exceptionally reactive and unstable and is the archetypal anti-aromatic molecule. The second is not actually unstable, but it is reactive and conventional wisdom has it that it avoids the undesirable antiaromaticity by adopting a highly non-planar tub shape and hence instead adopts reactive non-aromaticity. Both these examples have localized double bonds, a great contrast with the molecule which sandwiches them, cyclo-hexatriene (i.e. benzene). The reason for the resurgent interest is that a number of crystalline, apparently stable, antiaromatic molecules have recently been discovered, and ostensibly, molecule 1 belongs to this select class!

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References

  1. G. Wilke, "Contributions to Organo-Nickel Chemistry", Angewandte Chemie International Edition in English, vol. 27, pp. 185-206, 1988. http://dx.doi.org/10.1002/anie.198801851

A Disrotatory 4n+2 electron anti-aromatic Möbius transition state for a thermal electrocyclic reaction.

Thursday, April 2nd, 2009

Mauksch and Tsogoeva have recently published an article illustrating how a thermal electrocyclic reaction can proceed with distoratory ring closure, whilst simultaneously also exhibiting 4n electron Möbius-aromatic character[1]. Why is this remarkable? Because the simple Woodward-Hoffmann rules state that a disrotatory thermal electrocyclic reaction should proceed via a Hückel-aromatic 4n+2 electron transition state. Famously, Woodward and Hoffmann stated there were no exceptions to this rule. Yet here we apparently have one! So what is the more fundamental? The disrotatory character, or the 4n/Möbius character in the example above? Mauksch and Tsogoeva are in no doubt; it is the former that gives, and the latter is correct.

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References

  1. M. Mauksch, and S. Tsogoeva, "A Preferred Disrotatory 4n Electron Möbius Aromatic Transition State for a Thermal Electrocyclic Reaction", Angewandte Chemie International Edition, vol. 48, pp. 2959-2963, 2009. http://dx.doi.org/10.1002/anie.200806009