Chromium complexes isomers

As expected, under a hydrogen atmosphere in the presence of Pd/C in ethanol, the benzannulated pyrrolizine 48 leads to the dihydropyrrolizine derivative 49. However, semireduction of the pyrrole ring could be performed via the tricarbonyl chromium complex of 49 with various hydrides. Use of cyanoborohydride in trifluoroacetic acid (TFA) gave the best results for compound 50, both in terms of chemical yield (92%) and diastereoselectivity (90% of the trans-isomer) <2000TL1123>. 

In 1941 two different isomers of the 1 2 chromium complex of an o-carboxy-o -hydroxyazo dye were isolated. It was concluded that the three donor ligand atoms of each... 

The geometry of the two ConI-isomers is shown in Figure 88.131,132 As already discussed for the chromium complexes, the trans-isomer affords an octahedral geometry, whereas the cis-isomer gives rise to a trigonal prismatic geometry. 

Unlike desferrioxamine analogs designed for specific therapeutic purposes described above, chiral DFO analogs that form conformationally unique complexes with iron(lll) were designed to serve as chemical probes of microbial iron(lll) uptake processes. As mentioned above, ferrioxamine B can form a total of five isomers when binding trivalent metal ions, each as a racemic mixture. Muller and Raymond studied three separate, kinetically inert chromium complexes of desferrioxamine B (N-cis,cis, C-cis,cis and trans isomers), which showed the same inhibition of Fe-ferrioxamine B uptake by Streptomyces pilosus. This result may indicate either that (i) ferrioxamine B receptor in this microorganism does not discriminate between geometrical isomers, or that (ii) ferrioxamine B complexes are conformationally poorly defined and are not optimal to serve as probes. 

Chromium(III) complexes of a number of polyhydroxamic acids, microbial iron sequestering and transport agents (siderochromes) have been reported.797,798 The kinetic inertness of the chromium(III) complexes allows the facile separation of isomers for the model complex tris(iV-methyl-( - )-methoxyacetylhydroxamato)chromium(III), D-cis, L-cis and the l/d-trans isomers have been separated.798 The chromium complexes of desferrioxamine B (191) have been investigated the possible isomers are illustrated below (192-196). The cis isomer was isolated in relatively pure form.799 Thiohydroxamate800 and dihydroxamate (rhodotorulic acid) complexes have also been studied.801... 

In 1939, Drew83 proposed structures for 2 1 chromium complexes of o,octahedrally coordinated to the central chromium ion (81). Pfitzner71 independently arrived at the same conclusion and 2 1 chromium complexes having this equatorial or meridional (mer) configuration have been described84 as Drew—Pfitzner types. This configuration permits only an enantiomeric pair of isomers. 

The processes depend on the formation of the cyclohexadienyl anion intermediates in a favorable equilibrium (carbon nucleophiles from carbon acids with pKt > 22 or so), protonation (which can occur at low temperature with even weak acids, such as acetic acid) and hydrogen shifts in the proposed diene-chromium intermediates (25) and (26). Hydrogen shifts lead to an isomer (26), which allows elimination of HX and regeneration of an arene-chromium complex (27), now with the carbanion unit indirectly substituted for X (Scheme 9). 

For example, by chromatographic techniques it can be shown that the 1 2 chromium complex 15 exhibits only one isomer (a,a form), the mixed 1 2 chromium complex 16 two (a,a and a,(3 forms), and the symmetric 1 2 chromium complex 17 three isomers (a,a, a,(3 and (3,(3 forms), assuming that these dyes exist only in the mer configuration. 

The spiro carbon is a stereogenic center in spiropyrans, but because of the achiral structure of the open merocyanine form, the photochromic process will always lead to racemization unless additional chiral moieties are present. When a chiral substituent was introduced, remote from the spiro center, it was possible to isolate diastereo-isomers of the spiropyrans, but rapid epimerization at the spiro center occurred.1441 Diastereoselective switching was successful with 28, in which a stereogenic center was present close to the spiro carbon (Scheme 15).[45] Distinct changes in CD absorption at 250 nm were monitored upon irradiation with UV (250 nm) and with visible light (>530 nm) and a diastereomeric ratio of 1.6 1.0 was calculated for the closed form 28. Furthermore, a temperature-dependent CD effect was observed with this system it was attributed to an inversion of the diastereomeric composition at low temperatures. It might be possible to exploit such effects in dual-mode chiral response systems. A diastereoselective ring-closure was also recently observed in a photochromic N6-spirobenzopyran tricarbonyl chromium complex. 451 ... 

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). 

For the ferric siderophore complexes, comparison of the CD spectra of the chromium complexes of ferrichrome and enterobactin with the CD spectra of their iron complexes [and the separation of optical isomers of even ferric(benzhydroxamate)3 complexes in nonaqueous solution 192)] have shown that the same rule applied to the CD spectra for chromium complexes can be used for iron siderophore complexes as well iron(III) complexes will have a predominant A configuration in solution if the CD band in... 

Fischer-type carbenes are known as potential carbene transfer reagents to electron-rich and electron-deficient alkenes. Little is known about the chemistry of carbene complexes with silicon substituents at the carbene C-atom, whereas complexes with germanium, tin, or lead have not yet been prepared. The tungsten-carbene complexes 6 react with an excess of ethyl vinyl ether to give l,2-diethoxy-l-(trialkylsilyl)cyclopropanes 7." Only the f-isomers were formed and similar results can be achieved by using the corresponding molybdenum or chromium complexes. On the other hand, no reaction takes place with 2,3-dihydrofuran or ethyl ( )-but-2-enoate. ... 

The synthesis is similar to that for the chromium complex, using 1,3,5-trimethyl-1,3,5-triazacyclohexane (2.1 g, 16mmol), molybdenum hexacarbonyl (1.86g, 7.0 mmol), and xylenes (30 mL, mixture of isomers). The mixture is heated so that the solvent refluxes vigorously for 3 h. The product is obtained as a yellow powder (1.9 g, 88%). 

When the benzene ring is functionalized, ortho, meta, or para isomers are possible upon reaction of the chromium complex with nucleophiles. In general, the meta substitution product predominates 14,615 but all... 

This chromium complex yields a mixture of the diastereomeric pyrazolines 6.83 and 6.84 in a ratio 92 8. The latter isomer corresponds to the pyrazoline formed with the uncomplexed methyl 1,2-dihydro-2-methylnaphthalene-3-carboxylate (6.80, R = methyl), as major product. The corresponding chromium complex of the methyl... 

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