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May 10[edit]

Atropisomerism[edit]

Atropisomers of 6,6'-dinitro-2,2'-diphenic acid, first experimentally described case, by Christie and Kenner (1922).

Are atropisomers always enantiomers of each other, or can they be diasteromers of each other. In my text book, Organic Chemistry 8th Edition, on page 148 there is a flowchart which indicates that atropisomers are a subgroup of conformational enantiomers, but in the study guide on page 162 it states "Enantiomers or diastereomers that do not readily interconvert by bond rotations are atropisomers". I'm trying to think of an example of a compound that would exhibit atropisomerism between two diasteromers, but I'm coming up with nothing. 61.247.39.121 (talk) 02:48, 10 May 2018 (UTC)[reply]

doi:10.1021/ol026817 was the first hit I got when googling [binap diastereomer] (BINAP itself being the first chemical that came to mind as an atropisomeric example). DMacks (talk) 03:31, 10 May 2018 (UTC)[reply]

I also found a bunch of refs regarding diastereomeric mixtures of the atropisomeris details of vancomycin (my go-to example of a natural product with this sort of stereochemistry). DMacks (talk) 03:37, 10 May 2018 (UTC)[reply]

According to the wikipage on BINAP it is composed of two enantiomers. When I manipulate the chemical structure given, it also seems pretty clear that it is a pair of enantiomers, not a pair of diastereomers [1]. 61.247.39.121 (talk) 04:19, 10 May 2018 (UTC)[reply]

Please read the ref I supplied...it is about derivatives of BINAP that have additional stereocenters. That's why I also mentioned the google search term. DMacks (talk) 04:27, 10 May 2018 (UTC)[reply]

Ah, ok I see what you're saying now. So adding another stereocenter to the two enantiomers, where the new stereocenters are both the same will preserve the atropisomerism while destroying the enantiomerism and changing it to diastereomerism. For example, for two atropisomeric enantiomers, R and S, we add an R enantiomerically pure group to them, then we get R,R and R,S products which are diastereomers with respect to each other and still atropismeric since they would be identical if rotation around the single bond wasn't hindered. 61.247.39.121 (talk) 04:42, 10 May 2018 (UTC)[reply]

This has some rather interesting consequences, since diastereomers often have very different physical properties. The two atropisomers could have, for example very different melting points, but would have the same melting point if only rotation around the single bond was possible. I guess that also makes sense from a thermodynamic perspective, since one of the atropisomers would be stuck in an unfavorable conformation, giving it a higher energy, and thus less stability in its solid crystals. 61.247.39.121 (talk) 04:46, 10 May 2018 (UTC)[reply]

Careful not to confuse intramolecular details (thermodynamics of one diastereomer vs another) with intermolecular details that govern melting point. A common problem in structural analysis is that the same molecule has different conformational preferences depending on the state of matter. Turning that idea around, what is most stable in the gas or dilute liquid phase (the "most stable atropisomer" in isolation) might not have the lowest energy packing possible. See for example the discussion at the end of the Atropisomer#Synthesis section. DMacks (talk) 05:11, 10 May 2018 (UTC)[reply]

Exactly. You can use M/P or R_a/S_a nomenclature to distinguish when you are talking about atropisomerism vs normal tetrahedral chirality. DMacks (talk) 05:11, 10 May 2018 (UTC)[reply]

Just to be clear, they certainly do not need to be enantiomers in every case. Consider the image from the top of the article, which I've reproduced at top here for clarity. Suppose we get rid of one double bond at the top of the upper ring (and get rid of a hydrogen) so that we can put a Cl at the top of the molecule pointing toward us and a Br on the same carbon atom pointing away. Then there will be one isomer with the Br and the bottom ring's CO2H on the same side and one with the Br and the bottom ring's NO2 on the same side, which clearly are not mirror images of one another. However, if the atropisomer site is the only chiral center in the molecule, I think then it will almost always generate enantiomers. The caveat is that such bonds might not always center on a 90 degree angle -- if you can make a cis-trans atropisomer, for example by having a single bond holding together two strands that form extensive hydrogen bonding with each other (like DNA, but reversible) then I suppose that would also not be enantiomeric. Wnt (talk) 11:42, 10 May 2018 (UTC)[reply]