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Topic: Is this a chiral centre??  (Read 4249 times)

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Offline mirkoleone

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Is this a chiral centre??
« on: June 22, 2016, 02:06:34 PM »
Hi, i'm a new forum member, my name is Mirko and i study Industrial Chemistry in Milan.
I have a doubt about this particular molecule:


Other than the 2 chiral centre (red*), is also the blue* a chiral centre too?
I always thought NO, because i assume that the molecule has a simmetry centre in that chiral centre, but my teacher told me that it is NOT a chiral centre only if the 2 chiral centre in red are both R or S. So he told me that in this cause, because one centre is R and the other S, the blue* is a chiral centre.
Is it true??And how do i assign the priorities and the configuration of this centre?
Thanks for the attention and sorry for the "not so special" english

Offline lb2

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Re: Is this a chiral centre??
« Reply #1 on: June 22, 2016, 03:02:30 PM »
This kind of problem is quite notorious in the history of stereochemistry. The confusion mostly stems from the fact that chiral, asymmetric, stereogenic centers and so on are all taken to be equivalent, but they all refer to different aspects. For this particular case, it's important to understand that "stereogenicity" and "chirotopicity" are two separate concepts (although in most simple organic chemistry situations the difference can be irrelevant for practical purposes.)

Centers such as the center in blue in your compound for a long time (and older textbooks/professors may still use this language) were referred to as "pseudoasymmetric" centers, mostly because no one knew what to make of them. The definitive answer came in the classic (at least for stereochemistry enthusiasts) paper by Mislow and Siegel J. Am. Chem. Soc. 1984, 106, 3319.

From that paper (and the discussion about compounds 11-14 starting on p. 3322), you'll find that the blue center in your compound is a "stereogenic and achirotopic" center. If you invert the stereochemistry of one of the red centers, the center in blue becomes "nonsterogenic and chirotopic". Simply put, you cannot designate the center in blue with one such term as "chiral center".
« Last Edit: June 22, 2016, 03:39:54 PM by lb2 »

Offline mirkoleone

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Re: Is this a chiral centre??
« Reply #2 on: June 22, 2016, 03:22:47 PM »
Well, first of all thank you for this really interesting and complete answer. I just thought it was a waay simpler problem and i think i will deepen the difference between a/chirotopic centre and stereogenic centre.
Thanks again!!

Offline orgopete

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Re: Is this a chiral centre??
« Reply #3 on: June 24, 2016, 03:28:36 PM »

Other than the 2 chiral centre (red*), is also the blue* a chiral centre too?
I always thought NO, because i assume that the molecule has a simmetry centre in that chiral centre, but my teacher told me that it is NOT a chiral centre only if the 2 chiral centre in red are both R or S. So he told me that in this cause, because one centre is R and the other S, the blue* is a chiral centre.
Is it true??And how do i assign the priorities and the configuration of this centre?

I can understand the potential this compound could have and for which Mislow and Siegel may have provided a solution. However, I'm going to assume this is a misinterpretation of the instructor. This molecule is symmetrical, its mirror image can be superimposed on it and get the same molecule, hence not chiral. If the red centers "are both R or S", meaning RR or SS, then it would be chiral, the mirror images cannot be superimposed, and the situation of Mislow and Siegel presumably would apply.
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Offline Dan

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Re: Is this a chiral centre??
« Reply #4 on: June 25, 2016, 03:04:12 AM »
To clarify:
My research: Google Scholar and Researchgate

Offline lb2

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Re: Is this a chiral centre??
« Reply #5 on: June 25, 2016, 04:17:49 PM »
@Dan

Well, part of the point of the Mislow/Siegel paper is to say that "pseudoasymmetric center" shouldn't be used at all. It's old unofficial nomenclature that is better left forgotten. Much like taking "sterospecific" to mean "100% stereoselective".

According to them (and most modern stereochemistry textbook have adopted this view), you have to call such a center "stereogenic and achirotopic". For the purpose of this discussion, only the fact that it is a stereogenic (tetrahedral with four different substituents leading to local asymmetry) center is relevant (the fact that it is in an achiral environment, and thus achirotopic, is not really relevant here.) I don't know but I would think that if one wanted (I don't!), one could dig in the CIP nomenclature rules to potentially find a way to determine the R/S configuration of the blue center. If one wanted. As you mention, I think the more common treatment is to instead go for r/s.

Indeed, if you leave the original meso compound and go to the (R,R) or (S,S) compounds, the blue center becomes non-stereogenic (and as you've shown, "inverting" it doesn't lead to a different molecule) and you couldn't assign R or S. It also becomes chirotopic.


@orgopete

I wouldn't give too much thought as to the phrasing in the OP's post as he states that English is not his forte and also because a sentence such as "it is NOT a chiral centre only if the 2 chiral centre in red are both R or S" is a bit confusing. But yeah, (R,R) and (S,S) makes it a chiral molecule. However, the discussion in the OP's post was really about if the center was chiral or not, not the molecule.

Cheers!
« Last Edit: June 25, 2016, 04:33:32 PM by lb2 »

Offline Dan

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Re: Is this a chiral centre??
« Reply #6 on: June 25, 2016, 06:22:42 PM »
I don't know but I would think that if one wanted (I don't!), one could dig in the CIP nomenclature rules to potentially find a way to determine the R/S configuration of the blue center.

No, you can't use R/S. Pseudoasymmetric centres (as defined in the IUPAC gold book), or stereogenic achirotropic centres if you prefer (I take no issue with either term), are invariant upon reflection, so we use r/s instead.
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Offline lb2

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Re: Is this a chiral centre??
« Reply #7 on: June 25, 2016, 06:24:59 PM »
Oh cool, thanks for the info. Much appreciated.

Offline NewmanProj

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Re: Is this a chiral centre??
« Reply #8 on: August 19, 2016, 11:53:17 AM »
Hi, i'm a new forum member, my name is Mirko and i study Industrial Chemistry in Milan.
I have a doubt about this particular molecule:


Other than the 2 chiral centre (red*), is also the blue* a chiral centre too?
I always thought NO, because i assume that the molecule has a simmetry centre in that chiral centre, but my teacher told me that it is NOT a chiral centre only if the 2 chiral centre in red are both R or S. So he told me that in this cause, because one centre is R and the other S, the blue* is a chiral centre.
Is it true??And how do i assign the priorities and the configuration of this centre?
Thanks for the attention and sorry for the "not so special" english

Mirkoleone, with the type of problem you've suggested, realize you can draw a plane of symmetry directly through the carbon with the blue star. If the plane of symmetry reflects one side with the other, then that center is not chiral.

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