Posts Tagged ‘free energy’

Woodward’s symmetry considerations applied to electrocyclic reactions.

Monday, May 20th, 2013
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Sometimes the originators of seminal theories in chemistry write a personal and anecdotal account of their work. Niels Bohr[1] was one such and four decades later Robert Woodward wrote “The conservation of orbital symmetry” (Chem. Soc. Special Publications (Aromaticity), 1967, 21, 217-249; it is not online and so no doi can be given). Much interesting chemistry is described there, but (like Bohr in his article), Woodward lists no citations at the end, merely giving attributions by name. Thus the following chemistry (p 236 of this article) is attributed to a Professor Fonken, and goes as follows (excluding the structure in red):

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References

  1. N. Bohr, "Der Bau der Atome und die physikalischen und chemischen Eigenschaften der Elemente", Zeitschrift f�r Physik, vol. 9, pp. 1-67, 1922. http://dx.doi.org/10.1007/BF01326955

Au and Pt π-complexes of cyclobutadiene.

Wednesday, May 15th, 2013
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In the preceding post, I introduced Dewar’s π-complex theory for alkene-metal compounds, outlining the molecular orbital analysis he presented, in which the filled π-MO of the alkene donates into a Ag+ empty metal orbital and back-donation occurs from a filled metal orbital into the alkene π* MO. Here I play a little “what if” game with this scenario to see what one can learn from doing so.

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Hidden intermediates in the (acid catalysed) ring opening of propene epoxide.

Monday, May 6th, 2013
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In a previous post on the topic, I remarked how the regiospecific ethanolysis of propene epoxide[1] could be quickly and simply rationalised by inspecting the localized NBO orbital calculated for either the neutral or the protonated epoxide. This is an application of Hammond’s postulate[[2] in extrapolating the properties of a reactant to its reaction transition state. This approach implies that for acid-catalysed hydrolysis, the fully protonated epoxide is a good model for the subsequent transition state. But is this true? Can this postulate be tested? Here goes.

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References

  1. H.C. Chitwood, and B.T. Freure, "The Reaction of Propylene Oxide with Alcohols", J. Am. Chem. Soc., vol. 68, pp. 680-683, 1946. http://dx.doi.org/10.1021/ja01208a047
  2. G.S. Hammond, "A Correlation of Reaction Rates", J. Am. Chem. Soc., vol. 77, pp. 334-338, 1955. http://dx.doi.org/10.1021/ja01607a027

A sideways look at the mechanism of ester hydrolysis.

Friday, March 29th, 2013
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The mechanism of ester hydrolysis is a staple of examination questions in organic chemistry. To get a good grade, one might have to reproduce something like the below. Here, I subject that answer to a reality check.

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Kinetic vs Thermodynamic control. Subversive thoughts for electrophilic substitution of Indole.

Sunday, March 10th, 2013
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I mentioned in the last post that one can try to predict the outcome of electrophilic aromatic substitution by approximating the properties of the transition state from those of either the reactant or the (presumed Wheland) intermediate by invoking Hammond’s postulate[1]. A third option is readily available nowadays; calculate the transition state directly. Here are the results of exploring this third variation.

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References

  1. G.S. Hammond, "A Correlation of Reaction Rates", J. Am. Chem. Soc., vol. 77, pp. 334-338, 1955. http://dx.doi.org/10.1021/ja01607a027

The conformation of acetaldehyde: a simple molecule, a complex explanation?

Friday, February 8th, 2013
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Consider acetaldehyde (ethanal for progressive nomenclaturists). What conformation does it adopt, and why? This question was posed of me by a student at the end of a recent lecture of mine. Surely, an easy answer to give? Read on …

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Sharpless epoxidation, enantioselectivity and conformational analysis.

Thursday, January 3rd, 2013
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I 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.

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How to tame an oxidant: the mysteries of “tpap” (tetra-n-propylammonium perruthenate).

Monday, December 24th, 2012
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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

  1. S.V. Ley, J. Norman, W.P. Griffith, and S.P. Marsden, " Tetrapropylammonium Perruthenate, Pr 4 N + RuO 4 - , TPAP: A Catalytic Oxidant for Organic Synthesis ", Synthesis, vol. 1994, pp. 639-666, 1994. http://dx.doi.org/10.1055/s-1994-25538
  2. 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. http://dx.doi.org/10.1139/v76-304
  3. 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. http://dx.doi.org/10.1021/jo00038a009

Vitamin B12 and the genesis of a new theory of chemistry.

Thursday, December 20th, 2012
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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

  1. E.J. Corey, and A.G. Hortmann, "The Total Synthesis of Dihydrocostunolide", J. Am. Chem. Soc., vol. 87, pp. 5736-5742, 1965. http://dx.doi.org/10.1021/ja00952a037

Why is the Sharpless epoxidation enantioselective? Part 1: a simple model.

Sunday, December 9th, 2012
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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

  1. 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
  2. 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