Enantiodifferentiation analysis

Enantiodifferentiation analysis through the SAPS-HSQC spectrum has been shown to be superior than the conventional ID the conventional or even the broadband homodecoupled ID ZS spectra. SAPS-HSQC data allow the detection of enantiodifferentiated signals even in the case that AAd( H) or AAd( C) is close to 0, whenever one of these two values is sufficiendy dispersed. In a previous work, ID pure shift H NMR already demonstrated its practical usefulness in enantiodifferentiation studies to distinct signals separated by more than... 

In an attempt to rationalize the factors that control selectivity in the Rh- and Ir-catalyzed hydroboration reactions, Fernandez and Bo [35] carried out experimental and theoretical studies on the H—B addition of catecholborane to vinylarenes with [M(C0D)(R-QUINAP)]BF4, (QUINAP = l-(2-diphenylphosphino-l-naphthyl) isoquinoHne). A considerable difference was found in the stability of the isomers when the substrate was coordinated to the iridium(I) or rhodium(I) complexes. In particular, the difference between pro-R B1 and pro-S B2 isomers was not so great when the metal center was iridium and not rhodium (Figure 7.1), which explains the low ee-values observed experimentally when asymmetric iridium-catalyzed hydroboration was performed. Structurally, the energy analysis of the n2 and Tti interactions [36] seems to be responsible for the extra stabilization of the B2 isomer in the iridium intermediates (Figure 7.1). The coordination and insertion of alkenes, then, could be considered key steps in the enantiodifferentiation pathway. 

By carrying out the chromatographic analysis at different temperatures a van t Hoff analysis is possible which demonstrates that enantiodifferentiation is enthalpic in nature with the all-5 enantiomer of the host binding more strongly by ca. 0.5 kcal mol-1 (2 kj mol-1)... 

It has been long appreciated that a chiral environment may differentiate any physical property of enantiomeric molecules. NMR spectroscopy is a sensitive probe for the occurrence of interactions between chiral molecules [4]. NMR spectra of enantiomers in an achiral medium are identical because enantiotopic groups display the same values of NMR parameters. Enantiodifferentiation of the spectral parameters (chemical shifts, spin-spin coupling constants, relaxation rates) requires the use of a chiral medium, such as CyDs, that converts the mixture of enantiomers into a mixture of diastereomeric complexes. Other types of chiral systems used in NMR spectroscopy include chiral lanthanide chemical shift reagents [61, 62] and chiral liquid crystals [63, 64). These approaches can be combined. For example, CyD as a chiral solvating medium was used for chiral recognition in the analysis of residual quadrupolar splittings in an achiral lyotropic liquid crystal [65]. 

SAPS-HSQC) [72] method has proved to be a fast and very efficient tool for the detection and accurate differentiation and quantification of very small A Ad values simultaneously for and (Fig. 6). The method combines the complementary features of the pure shift approach that enhances signal resolution in the alternate dimension with those related to spectral aliasing in the dimension. This approach has been successfully applied to enantiodifferentiation studies and it can found interest in the analysis of complex mixtures or the distinction of isomers with very similar NMR spectra. 

---