Solution Conformations of CaCl2 and Ca(NO3)2 Complexes of Chiral Tetramethyl 18-Crown-6 Macrocycles: A 1D and 2D¹ H and¹³ C NMR Investigation

One- and two-dimensional NMR techniques have been used to investigate the solution structures of (2S,6S,11S,15S)-2,6,11,15-tetramethyl-1,4,7,10,13,16-hexaoxacyclooctadecane and (2R,3R, 11R,12R)-2,3,l 1,12-tetramethyl-1,4,7,10,13,16-hexaoxacyclooctadecane (II) macrocycle complexes of CaCl2 and Ca(NO3)2 in CDC13. Previous chiroptical studies of these and similar crown complexes by circularly polarized luminescence (CPL), total luminescence (TL), and circular dichroism (CD) spectroscopy have shown that the macrocycle asymmetric carbons in these complexes constrain the ring such that the sense of the ring twist in the S chiral ring complexes is opposite to that in the R chiral ring complexes. These studies have also shown that there is an added chirality element in the di- and trivalent (alkaline earth and lanthanide metal, respectively) nitrate complexes of I and II associated with the twist of the nitrate anions relative to one another as they sterically interact with the ring methyl groups. It has been proposed that channels are created on each face of the macrocycle ring by the axially positioned methyl groups in the solution structures of the I and II complexes and that the planar nitrate anions are constrained to fit in these channels. In this study, we show that the NMR data do indeed support the prominent axial positioning of the methyl groups in the CaCl2 and Ca(NO3)2 complexes of I and II in solution. Furthermore, the NMR data are consistent with the assertion that the S chiral ring twists in an opposite sense to the R chiral ring in these complexes. The data for this conformational analysis were obtained by exploiting coupling constant information and the nuclear Overhauser effect, observed by using both one- and two-dimensional NMR techniques. © 1989, American Chemical Society. All rights reserved.

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Author Richard A. Palmer Chemistry