With metrics in science publishing controversial to say the least, I pondered whether to write about the impact/influence a science-based blog might have (never mind whether it constitutes any measure of esteem). These are all terms that feature large when an (academic) organisation undertakes a survey of its researchers’ effectiveness.‡ WordPress (the organisation that provides the software used for this blog) recently enhanced the stats it offers for its users, and one of these caught my eye.
The demographics of a blog readership.
January 20th, 2013Aromaticity in the benzidine-like π-complex formed from PhNHOPh.
January 19th, 2013The transient π-complex formed during the “[5,5]” sigmatropic rearrangement of protonated N,O-diphenyl hydroxylamine can be (formally) represented as below, namely the interaction of a six-π-electron aromatic ring (the phenoxide anion 2) with a four-π-electron phenyl dication-anion pair 1. Can one analyse this interaction in terms of aromaticity?
The π-complex in the benzidine rearrangement: a molecular orbital analysis.
January 18th, 2013Michael Dewar[1] famously implicated a so-called π-complex in the benzidine rearrangement, back in the days when quantum mechanical calculations could not yet provide a quantitatively accurate reality check. Because this π-complex actually remains a relatively unusual species to encounter in day-to-day chemistry, I thought I would try to show in a simple way how it forms.
References
- M. Dewar, and H. McNicoll, "Mechanism of the benzidine rearrangement", Tetrahedron Letters, vol. 1, pp. 22-23, 1959. https://doi.org/10.1016/s0040-4039(01)82765-9
The strangely attractive conformation of C17H36.
January 13th, 2013We 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.
References
- N.O.B. Lüttschwager, T.N. Wassermann, R.A. Mata, and M.A. Suhm, "The Last Globally Stable Extended Alkane", Angewandte Chemie International Edition, vol. 52, pp. 463-466, 2012. https://doi.org/10.1002/anie.201202894
The Benzidine rearrangement. Computed kinetic isotope effects.
January 11th, 2013Kinetic isotope effects have become something of a lost art when it comes to exploring reaction mechanisms. But in their heyday they were absolutely critical for establishing the mechanism of the benzidine rearrangement[1]. This classic mechanism proceeds via bisprotonation of diphenyl hydrazine, but what happens next was the crux. Does this species rearrange directly to the C-C coupled intermediate (a concerted [5,5] sigmatropic reaction) or does it instead form a π-complex, as famously first suggested by Michael Dewar[2] [via TS(NN] and only then in a second step [via TS(CC)] form the C-C bond? Here I explore the isotope effects measured and calculated for this exact system.
References
- H.J. Shine, H. Zmuda, K.H. Park, H. Kwart, A.G. Horgan, and M. Brechbiel, "Benzidine rearrangements. 16. The use of heavy-atom kinetic isotope effects in solving the mechanism of the acid-catalyzed rearrangement of hydrazobenzene. The concerted pathway to benzidine and the nonconcerted pathway to diphenyline", Journal of the American Chemical Society, vol. 104, pp. 2501-2509, 1982. https://doi.org/10.1021/ja00373a028
- M. Dewar, and H. McNicoll, "Mechanism of the benzidine rearrangement", Tetrahedron Letters, vol. 1, pp. 22-23, 1959. https://doi.org/10.1016/s0040-4039(01)82765-9
A conflation of concepts: Conformation and pericyclic.
January 10th, 2013This is an interesting result I got when studying the [1,4] sigmatropic rearrangement of heptamethylbicyclo-[3.1.0]hexenyl cations. It fits into the last lecture of a series on pericyclic mechanisms, and just before the first lecture on conformational analysis. This is how they join.
NCI (non-covalent-interaction) analysis for some π-hydrogen bonded systems.
January 8th, 2013In this post, I looked at some hydrogen bonds formed by interaction of a π-system with an acidic hydrogen. Unlike normal lone pair donors, π-systems can involve more than two electrons, most commonly four or six. Here I look at examples of both these higher-order donors.
The mechanism of the Benzidine rearrangement.
January 6th, 2013The benzidine rearrangement is claimed to be an example of the quite rare [5,5] sigmatropic migration[1], which is a ten-electron homologation of the very common [3,3] sigmatropic reaction (e.g. the Cope or Claisen). Some benzidine rearrangements are indeed thought to go through the [3,3] route[2]. The topic has been reviewed here[3].
References
- H.J. Shine, K.H. Park, M.L. Brownawell, and J. San Filippo, "Benzidine rearrangements. 19. The concerted nature of the one-proton rearrangement of 2,2'-dimethoxyhydrazobenzene", Journal of the American Chemical Society, vol. 106, pp. 7077-7082, 1984. https://doi.org/10.1021/ja00335a035
- H.J. Shine, L. Kupczyk-Subotkowska, and W. Subotkowski, "Heavy-atom kinetic isotope effects in the acid-catalyzed rearrangement of N-2-naphthyl-N'-phenylhydrazine. Rearrangement is shown to be a concerted process", Journal of the American Chemical Society, vol. 107, pp. 6674-6678, 1985. https://doi.org/10.1021/ja00309a041
- H.J. Shine, "Reflections on the π‐complex theory of benzidine rearrangements", Journal of Physical Organic Chemistry, vol. 2, pp. 491-506, 1989. https://doi.org/10.1002/poc.610020702