Posts Tagged ‘pericyclic’

Woodward’s symmetry considerations applied to electrocyclic reactions.

Monday, May 20th, 2013

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", Z. Physik, vol. 9, pp. 1-67, 1922. http://dx.doi.org/10.1007/BF01326955

Lithiation of heteroaromatic rings: analogy to electrophilic substitution?

Saturday, March 16th, 2013

Functionalisation of a (hetero)aromatic ring by selectively (directedly) removing protons using the metal lithium is a relative mechanistic newcomer, compared to the pantheon of knowledge on aromatic electrophilic substitution. Investigating the mechanism using quantum calculations poses some interesting challenges, ones I have not previously discussed on this blog.

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Secrets of a university tutor: unravelling a mechanism using spectroscopy.

Thursday, January 31st, 2013

It is always rewarding when one comes across a problem in chemistry that can be solved using a continuous stream of rules and logical inferences from them. The example below[1] is one I have been using as a tutor in organic chemistry for a few years now, and I share it here. It takes around 50 minutes to unravel with students.

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References

  1. K. Harano, M. Eto, K. Ono, K. Misaka, and T. Hisano, "Sequential pericyclic reactions of unsaturated xanthates. One-pot synthesis of hydrobenzo[c]thiophenes", J. Chem. Soc., Perkin Trans. 1, pp. 299, 1993. http://dx.doi.org/10.1039/P19930000299

Why is N,O-diphenyl hydroxylamine (PhNHOPh) unknown?

Wednesday, January 16th, 2013

If you search e.g. Scifinder for N,O-diphenyl hydroxylamine (RN 24928-98-1) there is just one literature citation, to a 1962 patent. Nothing else; not even a calculation (an increasing proportion of the molecules reported in Chemical Abstracts have now only ever been subjected to calculation, not synthesis). A search of Reaxys also offers only one hit[1] reporting one unsuccessful attempt in 1963 to prepare this compound. Again, nothing else. Yet show this structure to most organic chemists, and I venture to suggest few would immediately predict this (unless they are experts on benzidine rearrangements).

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References

  1. J.R. Cox, and M.F. Dunn, "The chemistry of O,N-diarylhydroxlamines - I", Tetrahedron Letters, vol. 4, pp. 985-989, 1963. http://dx.doi.org/10.1016/S0040-4039(01)90757-9

A conflation of concepts: Conformation and pericyclic.

Thursday, January 10th, 2013

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.

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The mechanism of the Benzidine rearrangement.

Sunday, January 6th, 2013

The 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].

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References

  1. 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", J. Am. Chem. Soc., vol. 106, pp. 7077-7082, 1984. http://dx.doi.org/10.1021/ja00335a035
  2. 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", J. Am. Chem. Soc., vol. 107, pp. 6674-6678, 1985. http://dx.doi.org/10.1021/ja00309a041
  3. H.J. Shine, "Reflections on the ?-complex theory of benzidine rearrangements", Journal of Physical Organic Chemistry, vol. 2, pp. 491-506, 1989. http://dx.doi.org/10.1002/poc.610020702

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

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

  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

A pericyclic dichotomy.

Friday, November 30th, 2012

A dichotomy is a division into two mutually exclusive, opposed, or contradictory groups. Consider the reaction below. The bicyclic pentadiene on the left could in principle open on heating to give the monocyclic [12]-annulene (blue or red) via what is called an electrocyclic reaction as either a six (red) or eight (blue) electron process. These two possibilities represent our dichotomy; according to the Woodward-Hoffmann (WH) pericyclic selection rules, they represent contradictory groups. Depending on the (relative) stereochemistry at the ring junctions, if one reaction is allowed by the WH rules, the other must be forbidden, and of course vice-versa. It is a nice challenge to ask students to see if the dichotomy can be reconciled.

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Di-imide reduction with a twist: A Möbius version.

Monday, November 26th, 2012

I was intrigued by one aspect of the calculated transition state for di-imide reduction of an alkene; the calculated NMR shieldings indicated an diatropic ring current at the centre of the ring, but very deshielded shifts for the hydrogen atoms being transferred. This indicated, like most thermal pericyclic reactions, an aromatic transition state. Well, one game one can play with this sort of reaction is to add a double bond. This adds quite a twist to this classical reaction!

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The regiospecificity of di-imide reduction of an alkene.

Sunday, November 25th, 2012

Not a few posts on this blog dissect the mechanisms of well known text-book reactions. But one reaction type where there are few examples on these pages are reductions. These come in three types; using electrons, using a hydride anion and using di-hydrogen. Here I first take a closer look at the third type, and in particular di-hydrogen as delivered from di-imide.

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