Chromium optical resolution

Optically active cathinone has been obtained through the optical resolution of racemic norephedrine using 0,C>-dibenzoyl-D-tartaric acid (26). Each enantiomer was formylated (9) and oxidized with chromium trioxide in pyridine, and the product (13) was then deformylated by heating with 20% hydrochloric acid at 40 C (Scheme 2). Estimation of the optical purity by formation of a urea (14) with (—)-l-phenylethyl isocyanate and HPLC scrutiny, showed an optical purity exceeding 98% for each enantiomer. Cathinone as the free base racemizes and dimerizes readily in hydroxylic media, and similar behavior, at a somewhat reduced rate, is also observed for solutions of the oxalate salt sometimes used for isolation purposes. Cathinone base is fairly stable in dilute solution in nonhydroxy lie, nonpolar media. It readily decomposes during drying of the leaf, hence the desire to use fresh material for hedonistic purposes 14,27). 

The optical resolution of racemicp-hydroxypheny Iglycine with 3-bromocamphor-8-sulfonic acid has also been achieved. This resolving agent has also been widely used in the preparation of optically pure chromium and cobalt complexes. ... 

In the field of arene tricarbonyl chromium complexes, racemic aldehydes can be resolved quantitatively by chromatographic separation of the corresponding semioxamazones. The first optical resolution was carried out on the semioxamazone made from the chromium tricarbonyl complex of o-anisaldehyde. - The separation of the diastereomers was done by silica gel... 

On oxidation of the chromium complex 111 with CuCl2, Wittig and Riimpler (110) obtained a mixture of two rigid conformers 112 and 113 of o-hexaphen-ylene (3% yield). The D2 symmetry of 113, m.p. 345-346°C, was demonstrated by its spontaneous optical resolution (up to [a] d 11°) as well as by chromatographic resolution employing Si02-(-)-TAPA columns (up to [a] 36S +110°). The chiral nature of 113 was further confirmed by X-ray crystallography. 

Chromate, tetrakis(dioxygen)-stereochemistry, 94 Chromate, tricyanodiperoxy-structure, 78 Chromate, tris(oxalato)-racemization solid state, 466 strychnine salt racemization, 466 Chromatography optical resolution, 26 Chrome azurol S metallochromic indicator, 556 Chromium... 

Due to the inherent unsymmetric arene substitution pattern the benzannulation reaction creates a plane of chirality in the resulting tricarbonyl chromium complex, and - under achiral conditions - produces a racemic mixture of arene Cr(CO)3 complexes. Since the resolution of planar chiral arene chromium complexes can be rather tedious, diastereoselective benzannulation approaches towards optically pure planar chiral products appear highly attractive. This strategy requires the incorporation of chiral information into the starting materials which may be based on one of three options a stereogenic element can be introduced in the alkyne side chain, in the carbene carbon side chain or - most general and most attractive - in the heteroatom carbene side chain (Scheme 20). 

Equilibrium studies have shown that the first formation constant of the chromium(iii)-ethylenediamine system is < 10, over 10 -fold smaller than the value (10 ) previously reported. [Cr(en)3 (tn) ] (x = 0—3 and tn = tri-methylenediamine) complexes have been prepared and resolved using nitro-(-f )D-camphor. These mixed complexes have the same absolute configuration, A, as the pure [Cr(en)3] and [Cr(tn)3] species. Selective intervention of an optically active counterion in the relaxation processes of excited enantiomeric complexes can lead to partial resolution. This has been achieved for [Cr(phen)3] using D-tartrate. ... 

Enantiomer-differentiating co-polymerization of terminal epoxides is achieved by chiral chromium and cobalt complexes. Jacobsen etal. reported the co-polymerization of 1-hexene oxide with GO2 by using complex 35a. The reaction proceeds with kinetic resolution at 90% conversion, the unreacted epoxide is found to be enriched in the (i )-enantiomer of 90% ee. Detailed information about the resultant polymer, however, is not described. As discussed in the previous section, chiral cobalt-salen complex 34c co-polymerizes PO and GO2 (Table 3). When 34c with /r<3 / j--(li ,2i )-diaminocyclohexane backbone is applied to the co-polymerization, (A)-PO is consumed preferentially over (i )-enantiomer with a of 2.8 to give optically active PPG (Equation (8)). In a similar manner, a binary catalyst system, 34d/Bu4NGl, preferentially consumes (A)-PO over R)-PO with = 2.8-3.5. ... 

After the resolution of 1-2-chloro-ammino-diethylenediamino-cobaltie chloride many analogous resolutions of optically active compounds of octahedral symmetry were carried out, and active isomers of substances containing central cobalt, chromium, platinum, rhodium, iron atoms are known. The asymmetry is not confined to ammines alone, but is found in salts of complex type for example, potassium tri-oxalato-chromium, [Cr(Ca04)3]K3, exists in two optically active forms. These forms were separated by Werner2 by means of the base strychnine. More than forty series of compounds possessing octahedral symmetry have been proved to exist in optically active forms, so that the spatial configuration for co-ordination number six is firmly established. 

The electronic301 and magnetic properties of mononuclear chromium(III) complexes are quite well understood however there is a distinct tendency for octahedral symmetry to be invoked in cases where the true symmetry is much lower. Chromium(III) is a hard Lewis acid and many stable complexes are formed with oxygen donors. In particular hydroxide complexes are readily formed in aqueous solution, and this may be a problem in synthesis. Substitution at chromium(III) centres is slow302,303 and may well have some associative character in many cases. The kinetic inertness of chromium(III) has led to the resolution of many optically active complexes this work has been extensively reviewed.304... 

Sievers and his coworkers have used the volatility of the metal diketonates in the separation and analysis of heavy metals and have done interesting stereochemical work on a few of them. They have achieved at least partial resolution of the optical isomers of tris(hexafluoroacetyl-acetonato)chromium(III) by passing the vapor through a column of quartz powder at 55 °C, and have separated the facial and meridional isomers of tris(trifluoroacetylacetonato)chromium(III) by gas chromatography at 115 °C.40... 

The simple model complex, tris(cateeholato)chromate(III) has been prepared, and complete resolution of the optical isomers was achieved at pH 13. The known crystal structure of [Cr(cat)3]3- and arguments similar to those for the hydroxamate chromium complexes lead to the assignments of absolute configuration of the CD spectra. It was found that the CD spectra of A-[Cr(cat)3f and [Cr(ent)3f " are essentially identical, and the mirror image of chromic desferriferrichrome (Fig. 28), which shows that enterobactin has a predominant A-cis absolute configuration 147). Unfortunately the usual oxidation sensitivity of the catechol dianion is substantially increased in the chromium complexes, which precludes their use as biological probes l47). 

A molecule is optically active when it cannot be superimposed on its mirror image. Although this condition is met by an octahedral complex such as MLaLbLgLdLgLf it is rare indeed to be able to resolve such a complex. In practice, optical activity is largely confined to octahedral complexes of chelating ligands. Optical activity has also been observed for chelated tetrahedral and square planar complexes but only rarely. It is necessary for the chelated complex to be stable kinetically to permit resolution, it must retain its configuration for at least a matter of minutes. This confines attention to complexes of a few ions, of which cobalt(III), chromium(III)... 

Figure 6.13 Synchrotron radiation microprobe mapping of potassium and chromium oxidation states in a single human epithelial cell exposed to particulate chromate, and corresponding optical microscopy view. Beam spatial resolution (VxH) 0.5x1 pm, color scale in counts per pixel. Cr(VI) distribution shows a perinuclear localization. 2005 American Chemical Society.

---