Posts Tagged ‘pericyclic’

The thermal reactions … took precisely the opposite stereochemical course to that which we had predicted. A non-covalent-interaction view of the model.

Wednesday, February 3rd, 2021

Another foray into one of the more famous anecdotal chemistry “models”, the analysis of which led directly to the formulation of the WoodWard-Hoffmann (stereochemical) rules for pericyclic reactions. Previously, I tried to produce a modern computer model of what Woodward might have had to hand when discovering that the stereochemical outcome of a key reaction in his vitamin B12 synthesis was opposite to that predicted using his best model of the reaction.

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The thermal reactions … took precisely the opposite stereochemical course to that which we had predicted

Wednesday, January 20th, 2021

The quote of the post title comes from R. B. Woodward explaining the genesis of the discovery of what are now known as the Woodward-Hoffmann rules for pericyclic reactions.[1] I first wrote about this in 2012, noting that “for (that) blog, I do not want to investigate the transition states”. Here I take a closer look at this aspect.

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References

  1. R.B. Woodward, and R. Hoffmann, "Stereochemistry of Electrocyclic Reactions", Journal of the American Chemical Society, vol. 87, pp. 395-397, 1965. http://dx.doi.org/10.1021/ja01080a054

Trimerous pericyclic reactions: what is the effect of changing the electron count by two?

Monday, November 2nd, 2020

In an earlier post, I pondered on how the “arrow pushing” for the thermal pericyclic reactions of some annulenes (cyclic conjugated hydrocarbons) could be represented in terms of either two separate electrocyclic reactions or of one cycloaddition reaction. Each reaction is governed by selection rules which can be stated in terms of the anticipated aromaticity of the pericyclic transition state as belonging to a 4n or a 4n+2 class. This in turn determines whether the topology of the transition state belongs to a class of aromatic species known as either Hückel or Möbius. Here I play with the observation that by adding or removing two electrons from the molecule, the two classes 4n and 4n+2 can be swapped. What happens to the aromaticities of the transition states if that is done?

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Trimerous pericyclic reactions.

Thursday, October 8th, 2020

I occasionally spot an old blog that emerges, if only briefly, as “trending”. In this instance, only the second blog I ever wrote here, way back in 2009 as a follow up to this article.[1] With something of that age, its always worth revisiting to see if any aspect needs updating or expanding, given the uptick in interest. It related to the observation that there can be more than one way of expressing the “curly arrows” for some pericyclic reactions. These alternatives may each represent different types of such reactions, hence leading to a conundrum for students of how to label the mechanism. I had noted in that blog that I intended to revisit the topic and so a mere eleven years later here it is!

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References

  1. H.S. Rzepa, "The Aromaticity of Pericyclic Reaction Transition States", Journal of Chemical Education, vol. 84, pp. 1535, 2007. http://dx.doi.org/10.1021/ed084p1535

Three-for-one: a pericyclic brain teaser.

Sunday, January 12th, 2014

A game one can play with pericyclic reactions is to ask students to identify what type a given example is. So take for example the reaction below.

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A simple pericyclic reaction encapsulating the four thermal selection rules.

Thursday, January 2nd, 2014

As my previous post hints, I am performing my annual spring-clean of lecture notes on pericyclic reactions. Such reactions, and their stereochemistry, are described by a set of selection rules. I am always on the lookout for a simple example which can most concisely summarise these rules. The (hypothetical) one shown below I think nicely achieves this, and raises some interesting issues in the process.14vs12

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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", Zeitschrift f�r Physik, vol. 9, pp. 1-67, 1922. http://dx.doi.org/10.1007/BF01326955

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", Zeitschrift f�r 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", Journal of the Chemical Society, Perkin Transactions 1, pp. 299, 1993. http://dx.doi.org/10.1039/P19930000299