CIP rewrite

26 December 2011 - Chirality

point chirality by photoisomerisation  The well-known CIP system perhaps needs a footnote added in order to be able to incorporate the novel molecule constructed by Hashim & Kamaoki. They report (DOI) the conversion of prochiral A - a quarternary carbon atom with a methyl, a phenyl and two azobenzene groups to chiral compound B with one of the azobenzene groups photoisomerized from the trans to the cis configuration. Because the central carbon atom now has 4 different substituents it is an asymmetric carbon and now has point chirality.
However please note that the new compound is still racemic. The enantiomers could only be detected individually and isolated by chiral column chromatography. Synthesis is also transient because a second isomerization step will produce the achiral double Z,Z isomer.
And is this truly "the first example of the induction of point chirality in which two of the substituents around an sp3 carbon atom are geometric isomers."? Surely in the huge CAS database there must be countless of compounds with a quaternary carbon atom and two isomeric substituents except that then the researchers did not bother to mention it. Did Hashim & Kamaoki look hard enough or exactly how do you search for such a compound in a database? In any event, as Hashim & Kamaoki are now pushing towards synthesis of the enantiopure compounds via asymmetric synthesis, the rest of the world can decide which one to call the S and which one the R. To be continued.



Update: Reader S comments the comments are not on! He is right. Still needs to be coded into the application. But here is his comment: CIP takes care of such questions - I'm pretty sure Z beats E. And also, R beats S if you have two mirror-image substituents on a chiral carbon that only differ based on the R or S configuration of the chiral centre in them - from the Wikipedia page you cite: "It is possible in rare cases that two substituents on an atom differ only in their absolute configuration (R or S). If the relative priorities of these substituents need to be established, R takes priority over S. When this happens, the descriptor of the stereocenter is a lowercase letter (r or s) instead of the uppercase letter normally used". It's an oversight that the same situation with E/Z is not mentioned.