Chirality, conformational transmission

The origin of the observed 1,4-asymmetric induction in Michael reactions of chiral imines (Scheme 28) has been rationalized by conformational transmission of chirality. Thus, the phenethylamine auxiliary forces the cyclohexene part of the intermediate enamine into a half-chair conformation (140) that is 0.8 kcal mol-1 lower in energy than (141). Axial attack as shown then leads to the major product the energy difference between (140) and (141) roughly correlates with the observed diastereoselectivity ( 9 l).105... 

Variations in the magnitudes of Cotton effects associated with the 4-en-3-one system in a series of C(17)-substituted oestr-4-en-3-ones have been attributed to conformational transmission the C(17)-substituents are thought to influence the geometry of the molecule, and as a consequence the chirality of the enone chromophore. In view of the discussion above (see also below), it seems probable that the electronic character of C(17)-substituents should also be taken into account in assessing their effects, but the separation of electronic and conformational effects in such a situation is notoriously difficult. 

Lucero, M. J., Houk, K. N. Conformational Transmission of Chirality The Origin of 1,4-Asymmetric Induction in Michael Reactions of Chiral Imines. J. Am. Chem. Soc. 1997, 119, 826-827. 

That such conformational and configurational effects do actually occur in the transmission of chirality in molecules is demonstrated by corresponding findings and conceptual models from the field of polymer chemistry The sergeant-and-soldier principle and the majority rules model [38]. 

Self-irnmolative chirality transfer for rr-sLabilized [2,3] Wittig systems derived from nonracemic secondary alcohols follows a stereochemical course consistent with rearrangement through the conformation (vide supra). For bisallylic ethers 46, the fidelity of chiral transmission is larger than 90%34. 

The host is not required to adopt chiral forms in the absence of a chiral guest. In a complexed state, (a) the chiral guest biases the equilibrium between enantiomeric conformations of the host and (b) the chiral guest induces the host to change in structure from achiral to chiral upon complexation and biases the newly generated equilibrium through the supramolecular transmission of chirality. 

The mirror image was induced by the addition of (S,S)-5. The observation is considered a result of conformational switching from a non-helical anti form to a helical syn form, followed by biasing the newly generated equilibrium to a particular sense in the syn-formed complex through supramolecular transmission of guest chirality to double-arm-crossing dynamic helicity (case (b) in Scheme 14.1). 

---