This 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.
Posts Tagged ‘Reaction Mechanism’
A conflation of concepts: Conformation and pericyclic.
Thursday, January 10th, 2013The mechanism of the Benzidine rearrangement.
Sunday, 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. http://dx.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. http://dx.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. http://dx.doi.org/10.1002/poc.610020702
Sharpless epoxidation, enantioselectivity and conformational analysis.
Thursday, January 3rd, 2013I return to this reaction one more time. Trying to explain why it is enantioselective for the epoxide product poses peculiar difficulties. Most of the substituents can adopt one of several conformations, and some exploration of this conformational space is needed.
Non covalent interactions in the Sharpless transition state for asymmetric epoxidation.
Wednesday, December 19th, 2012The Sharpless epoxidation of an allylic alcohol had a big impact on synthetic chemistry when it was introduced in the 1980s, and led the way for the discovery (design?) of many new asymmetric catalytic systems. Each achieves its chiral magic by control of the geometry at the transition state for the reaction, and the stabilizations (or destabilizations) that occur at that geometry. These in turn can originate from factors such as stereoelectronic control or simply by the overall sum of many small attractions and repulsions we call dispersion interactions. Here I take an initial look at these for the binuclear transition state shown schematically below.
Why is the Sharpless epoxidation enantioselective? Part 1: a simple model.
Sunday, December 9th, 2012Sharpless epoxidation converts a prochiral allylic alcohol into the corresponding chiral epoxide with > 90% enantiomeric excess[1],[2]. Here is the first step in trying to explain how this magic is achieved.
References
- J.M. Klunder, S.Y. Ko, and K.B. Sharpless, "Asymmetric epoxidation of allyl alcohol: efficient routes to homochiral .beta.-adrenergic blocking agents", The Journal of Organic Chemistry, vol. 51, pp. 3710-3712, 1986. http://dx.doi.org/10.1021/jo00369a032
- R.M. Hanson, and K.B. Sharpless, "Procedure for the catalytic asymmetric epoxidation of allylic alcohols in the presence of molecular sieves", The Journal of Organic Chemistry, vol. 51, pp. 1922-1925, 1986. http://dx.doi.org/10.1021/jo00360a058
The mechanism of the Birch reduction. Sequel to benzene reduction.
Wednesday, December 5th, 2012I noted briefly in discussing why Birch reduction of benzene gives 1,4-cyclohexadiene (diagram below) that the geometry of the end-stage pentadienyl anion was distorted in the presence of the sodium cation to favour this product. This distortion actually has some pedagogic value, and so I elaborate this here.
The mechanism of the Birch reduction. Part 3: reduction of benzene
Tuesday, December 4th, 2012Birch reduction of benzene itself results in 1,4-cyclohexadiene rather than the more stable (conjugated) 1,3-cyclohexadiene. Why is this?