Chromium hydrolytic reactions

The hydrolysis reaction is very slow at ambient temperatures and is accelerated by boiling chromium salt solutions (5). The hydrolysis reaction is characterized by the transformation of the deep blue colored CrtHgOJg to green colored hydrolyzed olates. Another indication is tnatan aged or boiled Cr(III) salt solution has a higher neutralization equivalent than a fresh one due to the hydrolytically produced protons. One way to establish hydrolytic equilibria quickly is to add appropriate equivalents of bases such as NaOH to Cr(III) salt solutions. 

Rhodium acetylacetonate differed considerably from the other metal chelates in the acetylation reaction (26). Under the same conditions that had given extensive acetylation of the cobalt and chromium acetylacetonates, the rhodium chelate reacted very slowly and formed only a small amount of the monoacetylated compound (XX). Fortunately, the hydrolytic stability of rhodium acetylacetonate is such that the Friedel-Crafts reaction can be carried out under vigorous conditions that would rapidly degrade the chromium and cobalt chelates. Thus treatment of rhodium acetylacetonate with acetyl chloride and aluminum chloride in dichloroethane afforded the mono- and diacetylated chelates (XX and XXI). No triacetylated chelate was isolated from this reaction. In a similar manner butyryl-and benzoyl-substituted rhodium chelates (XXIII and XXIV) have been prepared. These and other experiments indicate that the rhodium acetylacetonate ring is less reactive than the cobalt or chromium rings. 

The synthesis is a catalogue of modern asymmetric catalytic methods. The epoxide 25 was resolved by a hydrolytic kinetic resolution (chapter 28) using a synthetic asymmetric cobalt complex. The asymmetric Diels-Alder reaction (chapter 26) was catalysed by a synthetic chromium... 

Marked colour changes occur during the reaction, the dark purple Cr"(en) + species converting to an orange intermediate with a gradual change to the red coloration of the chromic species on evolution of the alkane. The intermediate is identified as an alkyl(ethylenediamine)-chromium(iii) species which is hydrolytically unstable, e.g. in the reaction of n-butyl bromide ... 

Besides biomimetic complexes, Jacobsen described particularly efficient bis (chromium-salen) catalyst 9 for the asymmetric ring-opening reaction of epoxides with azide (Scheme 9) [42]. The efficiency of this class of catalysts is attributed to a cooperative mechanism, both substrates being activated toward each other by their respective chromium atom. Of note, a less pronounced cooperative effect was initially demonstrated in an intermolecular manner using monomeric Cr(N3)-salen catalyst [43]. Jacobsen also showed that an analogous cooperative mechanism takes place using polymer-supported chiral Co(salen) complexes for the hydrolytic kinetic resolution of terminal epoxides [44, 45]. 

The toxicological impact of chromium is dependent upon its oxidation state (chromium (III) or chromium (VI)) which, in turn, is controlled by the oxidation-reduction potential and pH. Within this framework, the hydrolytic speciation of chromium(III) is important since contamination of water by chromium is widespread (Ball and Nordstrom, 1998). Solution conditions, including ionic strength, affect the speciation of chromium and its adsorp-tion/desorption and precipitation/dissolution reactions (Ball and Nordstrom, 1998). 

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