Optisch aktive, aromatische Spirane, 9. Mitt.: Überprüfung eines Näherungsansatzes für Chiralitätsfunktionen bei 5,5′,6′-trisubstituierten 2,2′-Spirobiindanen

Abstract

In order to test the semiempirical value of chirality functions in their mathematically most simple form 23 new optically active 5,5′-disubstituted 2,2′-spirobiindanes1 of known chirality and enantiomeric purity were prepared. Thus a set of about hundred compounds is now available with altogether sixteen types of ligands (i.e. substituents including hydrogen); the experimental molar rotation of fifteen compounds is used to determine the value of a ligandspecific parameter occuring in the used chirality polynomial. According to theory this polynomial is an approximation for the total rotation of derivatives with ligands of three different types and it approximates an experimentally separable part of the rotation as far as compounds with four different ligands are concerned.

The additional chirality component, occurring exclusively in the case of derivatives with four different types of ligands turns out to be relatively small but not vanishing. Accordingly, the molar rotation predicted by our method is very good for disubstituted spirobiindanes of type1 and rather good for others with three different types of ligands but is distinctly worse for those with four different ligands. The numerical trend, however, is clearly represented even in cases where our calculation in principle refers to a part of the phenomenon only and the predicted absolute configuration is in call cases in agreement with the experiment.

An adequate criterion to judge the quality of our approximation is introduced.