Effective enantiodifferentiation of spirochalcogenuranes by the dirhodium method: Towards the determination of absolute configurations?

Adamantanes as spherical nanosondes in adducts with a chiral dirhodium complex-discriminating enantiomers and probing spatial proximities

Three different kinds of substituted chiral adamantane molecules-adamantanones, dioxolanoadamantanes and dithiolano-adamantanes-were studied in the dirhodium experiment (NMR measurement with 1:1 molar mixtures with Rh((II))(2)[(R)-(+)-MTPA](4) in CDCl(3)). Their different behavior in adduct formation is described, and the possibility of determining enantiomeric purities and absolute configurations is explored. Detailed inspection of one- and two-dimensional NMR experiments allowed for an interpretation of steric and electronic intra-adduct interaction showing that the phenyl groups of Rh* tend to enwrap the bound adamantane ligand so that through-space effects over a range of 6-7 Å away from the binding rhodium atom can be observed. Even slight differences in the relative orientation of phenyl groups can be monitored when comparing diastereomeric adducts via NMR signal dispersion.

A new method proposed for the determination of absolute configurations of alpha-amino acids

Enantiopure alpha-amino acids were converted to 4-substituted 2-aryl- and 2-alkyl-5(4H)-oxazolones under partial racemization. These nonracemic mixtures were dissolved in CDCl(3), an equimolar amount of the chiral dirhodium complex Rh(II)(2)[(R)-(+)-MTPA](4) (MTPA-H = Mosher's acid) was added, and the (1)H NMR spectra of the resulting samples were recorded (dirhodium method). The relative intensities of (1)H signals dispersed by the formation of diastereomeric adducts allow to determine the absolute configuration (AC) of the starting alpha-amino acids. Binding atoms in the adducts were identified by comparing the (1)H and (13)C chemical shifts of the oxazolones in the absence and presence of Rh(II)(2)[(R)-(+)-MTPA](4). Thereby, information about the scope and limits of this method can be extracted. A protocol how to use this method is presented.

Atropisomeric 3-aryl-2-oxo-4-oxazolidinones and some thione analogues—Enantiodifferentiation and ligand competition in applying the dirhodium method

The atropisomeric 2-oxo-4-oxazolidinones 1Z bind weakly to the rhodium atoms in the complex Rh(II)2 [(R)-(+)-MTPA]4 (Rh*, MTPA-H = methoxytrifluoromethylphenylacetic acid identical with Mosher's acid), presumably via the C-2 carbonyl oxygen atom. There are some 1H and 13C NMR signals in these compounds which show small dispersion effects suitable for enantiodifferentiation. In contrast, the thiocarbonyl sulfur atoms in 2Z and 3Z bind strongly so that significant complexation shifts (Delta delta) and diastereomeric dispersion effects (Delta nu) can be observed, and chiral discrimination and the determination of enantiomeric ratios of these thiocarbonyl compounds is easy. So, it is shown that--as expected--C=S is a much better binding site when competing with C=O. In compounds of Series 2 a "syn-methyl effect" was discovered which describes the dependence of dispersion effects of syn-oriented methyl groups 6 on the nature of the substituents Z. A mechanism of combined steric and electronic interaction influencing the conformational equilibria inside the adducts is proposed. Determination of absolute configurations by correlation fails, at least on the basis of the data available.

Formation of a spirodiazaselenurane and its corresponding azaselenonium derivatives from the oxidation of 2,2′-selenobis(benzamide). Structure, properties and glutathione peroxidase activity

The oxidation of 2,2'-selenobis(benzamide) with N-chlorosuccinimide or hydrogen peroxide afforded the corresponding stable azaselenonium chloride and hydroxide, respectively. Both structures were characterized by spectroscopic and X-ray crystallographic methods. Each contains a covalent N-Se bond, as well as a noncovalent interaction between the selenium atom and the carbonyl oxygen atom of the other amide moiety. The treatment of the azaselenonium chloride with an excess of potassium hydride in DMSO-d(6) afforded the corresponding spirodiazaselenurane species, which proved hydrolytically unstable, but was characterized by NMR spectroscopy. The azaselenonium chloride displayed significant glutathione peroxidase-like catalytic activity in an assay with benzyl thiol and either hydrogen peroxide or tert-butyl hydroperoxide.

The dirhodium-method in the determination of absolute configurations of phospholene chalcogenides

The 1H, 13C, and 31P NMR signals of six chiral phospholene chalcogenides (X = O, S, Se) are duplicated in the presence of one mole equivalent of the chiral auxiliary Rh2[(R)-MTPA]4 (diastereomeric dispersion Deltanu; in Hz). The samples were investigated as nonracemic mixtures of enantiomers with known absolute configurations so that signs can be attributed to the Deltanu-values and each signal set can be assigned to the respective enantiomer. The signs are uniform--in particular those of 1H nuclei--and nearly independent of the nature of the chalcogen atom. Thus, if the absolute configuration of one compound is known, it is possible to derive absolute configurations in the whole series (correlation method).

Absolute Configuration ofC2-Symmetric Spiroselenurane: 3,3,3′,3′-Tetramethyl-1,1′-spirobi[3H,2,1]Benzoxaselenole

The enantiomers of 3,3,3',3'-tetramethyl-1,1'-spirobi[3 H,2,1]benzoxaselenole have been separated on a chiral preparative chromatographic column. The experimental vibrational circular dichroism (VCD) spectra have been obtained for both enantiomers in CH(2)Cl(2). The theoretical VCD spectra have been obtained by means of density functional theoretical calculations with the B3 LYP density functional. From a comparison of experimental and theoretical VCD spectra, the absolute configuration of an enantiomer with positive specific rotation in CH(2)Cl(2) at 589 nm is determined to be R. This conclusion has been verified by comparing results of experimental optical rotatory dispersion (ORD) and electronic circular dichroism (ECD) to predictions of the same properties using the B3 LYP functional for the title compound.

Rh2[MTPA]4, a dirhodium complex as NMR auxiliary for chiral recognition

The dirhodium complex Rh(+II)2[(+)-MTPA]4 is introduced as an NMR auxiliary for enantiodifferentiation. As a soft Lewis acid, it is particularly useful for the recognition of soft Lewis base molecules. The donor properties of ligands can be divided into several categories and are exemplified by various classes of phosphorus-containing functionalities. In addition, the thermodynamics of the underlying ligand-complex equilibria are discussed and stereochemical implications demonstrated. The extension of the dirhodium method to absolute configuration determination is evaluated. The outlook for future applications with further non-phosphorus ligands is presented.