Chromium complexes reactivity

Control of chromium penetration, essential to permit tannage of the center of the hide, is accompHshed by pH adjustment. At a pH > 3.0 the reactivity of the hide to the chromium complex is greatiy increased. The pH is therefore raised gradually to the desired point by addition of a mild alkah, usually sodium bicarbonate. The chemistry of chrome tanning involves competing reactions that must be controlled for satisfactory results. 

Mixed cobalt/chromium complexes of the symmetrical 1 2 type in which each ligand contains a low-reactivity system linked to the coupling component (section 5.5.2) have been widely used for continuous dyeing and printing. They offer high light fastness and the... 

The enantioselective lithiation of anisolechromium tricarbonyl was used by Schmalz and Schellhaas in a route towards the natural product (+)-ptilocaulin . In situ hthi-ation and silylation of 410 with ent-h M gave ewf-411 in an optimized 91% ee (reaction carried ont at — 100°C over 10 min, see Scheme 169). A second, substrate-directed lithiation with BuLi alone, formation of the copper derivative and a quench with crotyl bromide gave 420. The planar chirality and reactivity of the chromium complex was then exploited in a nucleophilic addition of dithiane, which generated ptilocaulin precnrsor 421 (Scheme 172). The stereochemistry of componnd 421 has also been used to direct dearomatizing additions, yielding other classes of enones. ... 

Although slightly outside the scope of this review, an interesting case of stereoselection should be presented here. It has been observed by Gibson (nee Thomas) and coworkers during the deprotonation of tricarbonylchromium complexes of benzyl alkyl ethers by means of the chiral bis(lithiumamide) base 234 (equation 54) . The base removes the benzylic pro-R-H atom in 233 from the most reactive conformation to form the planary chiral intermediate 235. The attack of the electrophile forming 236 proceeds exclusively from the upper face in 235, because the bulky chromium moiety shields the lower face. Simpkins and coworkers extended the method to the enantioselective substitution of the chromium complexes of 1,3-dihydroisobenzofurans . 

According to Widdowson, [(methoxymethoxy)benzene]tricarbonylchromium (448) was deprotonated with enantiotopos differentiation by n-BuLi/(—)-sparteine (11), and the lithium intermediate 449 was trapped by various electrophiles to give the products 451 with ee values up to 97% (equation 122) . Surprisingly, opposite enantiomers are formed when stoichiometric or excess amounts of base are applied. The authors presume that in the dilithium intermediate 450 the C—Li bond (in the rear) has a higher reactivity than the other one (pointed to the front). The deprotonation procedure was also applied to a couple of 1,4-disubstituted chromium complexes . 

Cyclopropylalkoxycarbene complexes are much less reactive towards nucleophilic reactions. The methoxy chromium complex does, however, react with iodine to give directly methyl 4-iodobutyrate, derived by hydrolysis of the expected diiodovinyl ether (equation 78)139. 

Tridentate Schiff base chromium(III) complexes were identified as the optimal catalysts for the enantioselective ring opening of meso-aziridines by TMSN3.51 Indeed, preliminary studies have shown that, although the (salen)chromium complexes catalyzed the reaction to some extent, they consistently led to low enantioselectivities (<14% ee). It was rationalized that the diminished reactivity and selectivity of the salen complexes with aziridines compared to epoxides was a result of the steric hindrance created by the /V-substituent of the coordinated aziridine. As expected, improved results were observed using tridentate ligands on the chromium center because they offer a less-hindered coordination environment (Figure 17.7).51... 

The malonato complexes of chromium(III) are analogous to the oxalate complexes of chromium(III). Since malonic acid is a weaker acid than oxalic acid, the malonato complexes are expected to be more labile than the oxalato complexes. The dicarboxylate complexes of chromium(III) form a group of anionic complexes which are suitable for the study of octahedral complex reactivity. 

Mayer, L. M., Schick, L. L., and Chang, C. A. (1983). Kinetics of trivalent chromium complex-ation and adsorption at the river-estuary interface. Implications for reactive pollutant transport. In Proceedings of the International Conference on Heavy Metals in the Environment, CEP Consultants, pp. 1112-1115. 

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