Chiral diamagnetism

Since the first report of the nonequivalence phenomenon, approximately 40 chiral substances have been reported to induce enantiomeric nonequivalence in the NMR spectra of a host of solutes. These CSAs are encountered in subsequent discussions. Two qualities considered to be essential in the design of the first reported experiment (3) are evident in nearly all CSA-solute combinations. In all cases, the CSA and the solute have the common feature of complementary functionality, which permits their interaction. Both are in general hydrogen bond donors or acceptors the CSAs are acids, amines, alcohols, sulfoxides, or cyclic compounds such as cyclodextiins, crown ethers, or peptides, which attractively interact with appropriate enantiomeric solutes, engendering different spatial environments for their nuclei. In nearly every case the CSA contains a group of high diamagnetic anisotropy near its asymmetric center, a feature... 

Chrominm(III) and Cobalt(III) Nucleotide Complexes. Coordination isomers of Cr(III)- and Co(III)-nucleotides can be separated and used to test for specificity. Since Cr(III) is diamagnetic, EPR and NMR experiments can supplement the chirality probes . ... 

Exchange-inert complexes of Co(III) with nucleotides that have proven to be extremely useful as chirality probes because the different coordination isomers are stable and can be prepared and separated In addition, these nucleotides can be used as dead-end inhibitors of enzyme-catalyzed reactions and, since Co(III) is diamagnetic, a number of spectroscopic protocols can be utilized. See Exchange-Inert Complexes Chromium-Nucleotide Complexes Metal Ion-Nucleotide Interactions... 

The cobalt complexes are all diamagnetic and behave as 1 1 electrolytes in methanol.7 The infrared spectra of the complexes contain sharp, strong N—H stretching absorptions (Table I). The electronic spectral data are also shown in Table I. Isomers I and II differ in the chiralities of nitrogen atoms in the macro-cyclic ligands.7 Both chemical and physical properties are dependent on ring... 

Addition of chiral LSR such as (+)tris[3-heptafluoropropyl hydroxymethylene]-D-[cam-phorato]Eu(III), Eu[(+)fpc]3 to a racemic mixture of diamagnetic Ni complex with propylenediamine backbone results in a composite of two overlapping spectra [92], The changes in the methyl doublet are shown in Fig. 10.27. Combined with spectral integration, this technique may be used for the analysis of mixtures of optically active complexes. 

CSA (Chiral solvating agent) A diamagnetic additive of known enantiomeric purity used to induce anisochrony in enantiomers of a racemate for NMR analysis. See Section 2.3.4. 

As was the case with chiral shift reagents, preferential population of one diastereomer over the other (Kr Ks) is not a prerequisite for induced anisochrony of enantiotopic groups. Additionally, since the CSA is diamagnetic, it may be used in excess over the analyte. A five-fold excess is usually sufficient to drive the equilibria of Equation 2.16 to the outside, such that the solute is present only as its two diastereomeric solvates. Since the observed spectra are time-averages of all the species in solution, this chemical trick simplifies analysis of absolute configuration by focussing on the diastereomeric solvates alone. 

Isolation of enantiomerically pure helicates. One disadvantage of Zn is its relatively large lability with respect to transmetallation, so that successful attempts have been made to replace it with Cr which, in addition to be optically active (see point 4 below), is kinetically inert, opening the way to designing chiral helical luminescent probes (Cantuel et al., 2004). Another possibility to produce inert 3d-4f helicates is to insert Co ions into the self-assembled edifices, followed by a mild and selective oxidation into the low-spin diamagnetic, and kinetically inert Co ion (Rigault et al., 1998). 

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