NMR, chirality

As already mentioned, chiral cations are involved in many areas of chemistry and, unfortunately, only few simple methods are available to determine their optical purity with precision. In the last decades, NMR has evolved as one of the methods of choice for the measurement of the enantiomeric purity of chiral species [ 110,111 ]. Anionic substances have an advantage over neutral reagents to behave as NMR chiral shift agents for chiral cations. They can form dia-stereomeric contact pairs directly and the short-range interactions that result can lead to clear differences in the NMR spectra of the diastereomeric salts. 

An overall efficiency of TRISPHAT 8 and BINPHAT 15 anions as NMR chiral shift agents for chiral cations has been demonstrated over the last few years. Additions of ammonium salts of the A or A enantiomers of 8 and 15 to solutions of racemic or enantioenriched chiral cationic substrates have generally led to efficient NMR enantiodifferentiations [112-121]. Well-separated signals are usually observed on the spectra of the diastereomeric salts generated in situ. 

Unfortunately, in most of the previous examples, the extent of the asymmetry-induction was determined by chiroptical measurements (ORD, CD) that gave qualitative and not quantitative information. The NMR chiral shift efficiency of TRISPHAT 8 and other hexacoordinated phosphate anions was therefore considered as an excellent analytical tool to provide accurate measurement of the induced selectivity by NMR spectroscopy. 

Both these optically active betweenanenes exhibited a (—)-Cotton effect47 at 195 nm, indicating their (R)-configuration. Hydroboration-oxidation of these specimens gave the ( )-fused alcohols 106 whose respective optical purities (7.6 and 6.0%) were estimated by means of the NMR chiral shift reagent method. 

For reviews of nmr chiral solvating agents, see Weisman, in Morrison, Ref. 88, vol. I. pp. 153-171 Pirkle Hoover Top. Stereochem. 1982,13, 263-331. For literature references, see Sweeting Anet Org. Magn. Resort. 1984, 22, 539. Sec also Pirkle Tsipouras Tetrahedron Lett. 1985, 26, 2989 Parker Taylor Tetrahedron 1987, 43, 5451. 

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