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Intrinsic asymmetry and diastereotopism

par Reisse, Jacques ;Ottinger, Robert ;Bickart, Paul;Mislow, Kurt
Référence Journal of the American Chemical Society, 100, 3, page (911-915)
Publication Publié, 1978

Article révisé par les pairs

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Résumé :

The term "intrinsic asymmetry", as applied to the time-averaged anisochrony of geminal groups in systems of the type [R*]CU2V (where R* is a chiral perturbing group), is critically discussed against the background of symmetry criteria for chemical shift nonequivalence. It is concluded that continued use of the term is no longer justified. The symmetry requirements for time-averaged anisochrony under conditions of equipopulation of conformers are extended to the general case of an n-fold barrier to internal rotation. Chemical shift nonequivalence of geminal methyl groups in a compound of the type XYZC-C=C-CH(CH3)2 is demonstrated and the adequacy of the free rotation (windmilling) model for such a system is discussed. It is noted that for NMR purposes, the distinction between groups diastereotopic by internal and external comparison is more a matter of convenience than of principle. © 1978 American Chemical Society.