Talk:Umpolung
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What are pericyclic reactions doing in in the Introduction ?
[edit]The Introduction currently mentions the Diels-Alder and Claisen rearrangement as examples of easy synthesis of 1,3 and 1,5 heterosubstituted skeletons resulting from ‘normal’ carbon polarity.
The Claisen rearrangement arguably does that, though not particularly as a result of polarity - it’s driven by C=O bond energy and the regioselectivity is baked in by starting material geometry, not polarity. But the canonical Diels-Alder doesn’t involve heteroatom substituents. Should pericyclic reactions be here at all? — Preceding unsigned comment added by Slrellison (talk • contribs) 11:34, 6 November 2022 (UTC)
Corey-Seebach Reaction
[edit]The dithiane umpolung is often referred to as the Corey-Seebach reaction; would anyone be against duplicating some of the content and extending it as part of a new article? — Preceding unsigned comment added by Mdlevin (talk • contribs) 02:43, 3 January 2011 (UTC)
Comments
[edit]Initial Comments: The article has a reasonable number of examples, but could use some more organization.
Suggestions:
- Have a global scheme that contrasts "normal" polarity reactions that result in 1,3- (aldol) and 1,5- (Michael) oxygenation with these "unnatural" polarity 1,2 (benzoin) and 1,4 (Stetter) reactions. It's mentioned in the text, but a graphic would be very useful.
- Showing carbenes as (+)(-) paired on a single, neutral center might be a useful way to show how they can invert polarity.
- The idea of alternating charges goes back a long way (Lapworth?) It might be useful to have a bit more discussion about the idea of alternating charges, and how newer methods override these natural tendencies.
- Some of the schemes are extremely crowded and "busy." Maybe break some of them up, add captions and labels, and stick to standard ACS settings and sizes.
- Phenolic coupling is an important reaction, and deserves some expanding on.
- Don't forget the Stetter reaction.
- I would put the NHC reactions together for organization. The scheme for the enone reaction shows 2a and 2b as resonance structures, but they are really tautomers.
Eugene Kwan (talk) 22:14, 24 October 2011 (UTC)
The NHC example is shown is more complicated to understand than a more general polarity inversion due to the elimination reaction with the bromine. It would be better to show NHC catalysed benzoin condensation so that the mechanism can be compared to the cyanide ion catalysed reaction. An example of an NHC catalysed benzoin reaction is in the book "New Discoveries in N-heterocyclic Carbene-catalyzed Homoenolate and Hydroacylation Reactions" by Audrey Chan on page 28, available on Google Books.
Stereochemistry
[edit]In Cyanide-type umpolung the OH should not show a stereochemistry (the product would be racemic). In Carbonyl umpolung structures 6 & 12 show stereochemistry at a carbon that isn't chiral - the other C of the epoxide should show a configuration. 69.72.27.110 (talk) 08:03, 31 October 2011 (UTC)