Archive for December, 2012
Monday, December 24th, 2012
tpap[1], as it is affectionately known, is a ruthenium-based oxidant of primary alcohols to aldehydes discovered by Griffith and Ley. Whereas ruthenium tetroxide (RuO4) is a voracious oxidant[2], its radical anion countered by a tetra-propylammonium cation is considered a more moderate animal[3]. In this post, I want to try to use quantum mechanically derived energies as a pathfinder for exploring what might be going on (or a reality-check if you like).
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References
- S.V. Ley, J. Norman, W.P. Griffith, and S.P. Marsden, "Tetrapropylammonium Perruthenate, Pr<sub>4</sub>N<sup>+</sup>RuO<sub>4</sub>
<sup>-</sup>, TPAP: A Catalytic Oxidant for Organic Synthesis", Synthesis, vol. 1994, pp. 639-666, 1994. https://doi.org/10.1055/s-1994-25538
- D.G. Lee, U.A. Spitzer, J. Cleland, and M.E. Olson, "The oxidation of cyclobutanol by ruthenium tetroxide and sodium ruthenate", Canadian Journal of Chemistry, vol. 54, pp. 2124-2126, 1976. https://doi.org/10.1139/v76-304
- D.G. Lee, Z. Wang, and W.D. Chandler, "Autocatalysis during the reduction of tetra-n-propylammonium perruthenate by 2-propanol", The Journal of Organic Chemistry, vol. 57, pp. 3276-3277, 1992. https://doi.org/10.1021/jo00038a009
Tags:catalysis, energy, free energy, low energy elimination, metal, react freq, Reaction Mechanism, RuO4+ ethanol, triplet state energy, Tutorial material
Posted in Interesting chemistry | 2 Comments »
Thursday, December 20th, 2012
I have written earlier about dihydrocostunolide, and how in 1963 Corey missed spotting the electronic origins of a key step in its synthesis.[1]. A nice juxtaposition to this failed opportunity relates to Woodward’s project at around the same time to synthesize vitamin B12. The step in the synthesis that caused him to ponder is shown below.
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References
- E.J. Corey, and A.G. Hortmann, "The Total Synthesis of Dihydrocostunolide", Journal of the American Chemical Society, vol. 87, pp. 5736-5742, 1965. https://doi.org/10.1021/ja00952a037
Tags:dispersion energy, free energy, Historical, pericyclic
Posted in Interesting chemistry, pericyclic, reaction mechanism | No Comments »
Wednesday, December 19th, 2012
The 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.
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Tags:asymmetric catalytic systems, Bob Hanson, catalysis, Julia Contreras-Garcia, little web site, NCI, non-covalent-analysis, Reaction Mechanism, terminal =CH
Posted in Interesting chemistry | No Comments »
Monday, December 17th, 2012
Part one on this topic showed how a quantum mechanical model employing just one titanium centre was not successful in predicting the stereochemical outcome of the Sharpless asymmetric epoxidation. Here in part 2, I investigate whether a binuclear model might have more success. The new model is constructed using two units of Ti(OiPr)4, which are likely to assemble into a dimer such as that shown below (in this crystal structure, some of the iPr groups are perfluorinated).
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Tags:catalysis, catalytic metal centre, Enantioselective, free energy difference, Reaction Mechanism, Sharpless epoxidation, t-butyl, This free energy difference, Tutorial material
Posted in Interesting chemistry | 2 Comments »
Sunday, December 9th, 2012
Sharpless 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.
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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. https://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. https://doi.org/10.1021/jo00360a058
Tags:animation, asymmetric epoxidation, catalysis, Enantioselective, free energy, lower energy conformations, Reaction Mechanism, Tutorial material
Posted in Interesting chemistry | 1 Comment »
Monday, December 3rd, 2012
I promised that the follow-up to on the topic of Birch reduction would focus on the proton transfer reaction between the radical anion of anisole and a proton source, as part of analysing whether the mechanistic pathway proceeds O or M.
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Tags:Birch reduction, dielectric, energy, free energy, free energy barrier, Hammonds postulate, proton transfer, Reaction Mechanism, Tutorial material
Posted in Uncategorised | 2 Comments »
Saturday, December 1st, 2012
The Birch reduction is a classic method for partially reducing e.g. aryl ethers using electrons (from sodium dissolved in ammonia) as the reductant rather than e.g. dihydrogen. As happens occasionally in chemistry, a long debate broke out over the two alternative mechanisms labelled O (for ortho protonation of the initial radical anion intermediate) or M (for meta protonation). Text books seem to have settled down of late in favour of O. Here I take a look at the issue myself.
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Tags:dielectric, energy, free energy, Reaction Mechanism, Tutorial material
Posted in Uncategorised | 2 Comments »