Chromate esters chromium oxidation

Chromium compounds decompose primary and secondary hydroperoxides to the corresponding carbonyl compounds, both homogeneously and heterogeneously (187—191). The mechanism of chromium catalyst interaction with hydroperoxides may involve generation of hexavalent chromium in the form of an alkyl chromate, which decomposes heterolyticaHy to give ketone (192). The oxidation of alcohol intermediates may also proceed through chromate ester intermediates (193). Therefore, chromium catalysis tends to increase the ketone alcohol ratio in the product (194,195). 

Mechanism. Chromic acid reacts with isopropanol to produce a chromate ester intermediate. An elimination reaction occurs by removal of a hydrogen atom from the alcohol carbon, and departure of the chromium group with a pair of electrons. The Cr is reduced from Cr (VI) to Cr (IV), and the alcohol is oxidized. 

Any anhydrous chromium solution will work. Cr03 in acetonitrile or chromate esters in hexane are good candidates. Lower valent compounds, like diarenechromium in hexane, can also be used because they are oxidized to the same Cr(VI) surface species during the second calcining in air. Even chromyl chloride vapor can be used if enough surface hydroxyls are left to... 

It is now generally admitted that this reaction involves both one-electron and two-electron transfer reactions. Carbonyl compounds are directly produced from the two-electron oxidation of alcohols by both Crvl- and Crv-oxo species, respectively transformed into CrIV and Crm species. Chromium(IV) species generate radicals by one-electron oxidation of alcohols and are responsible for the formation of cleavage by-products, e.g. benzyl alcohol and benzaldehyde from the oxidation of 1,2-diphenyl ethanol.294,295 The key step for carbonyl compound formation is the decomposition of the chromate ester resulting from the reaction of the alcohol with the Crvl-oxo reagent (equation 97).296... 

Sometimes, an alcohol via the corresponding chromate ester may direct a chromium-promoted epoxidation of an aJkcne. This side reaction, which can happen with other chromium-based oxidants,83 depends on very exacting stereoelectronic factors to occur. 

Tertiary allylic alcohols form a chromate ester that, as it lacks a hydrogen on a to the alcohol, instead of suffering a normal oxidation to ketone rearranges to an enone. This transformation, which can be brought about by other chromium-based reagents, is normally carried out with PCC when it is purposefully sought at (see page 55). 

Mechanistic evidences show that PDC, similar to other chromium-based oxidants, operates via an intermediate chromate ester that evolves to a carbonyl compound in the rate-determining step.125... 

Similar to other chromium-based reagents, kinetic evidence shows that oxidation of alcohols by PCC operates via a chromate ester intermediate that evolves to an aldehyde or ketone in the rate-determining step.194... 

Chromium coordinates selectively with the 1,2-diol, forming a stable cyclic chromate ester that evolves producing the formation of a tetrahydrofuran. Observe that no formation of tetrahydrofuran from the alcohol on the left occurs, for this would involve the intermediacy of a less stable simple chromate ester (vide infra). The experimental conditions are so mild that no direct oxidation of the secondary alcohol to ketone is observed, either on the starting compound or in the product. 

This mechanistically fascinating product can be explained by the initial formation of a cyclic chromate ester, facilitated by the formation of a five-membered ling and the (cis) relationship in the 1,2-diol. Interestingly, this stable chromate does not evolve resulting in the oxidation of the secondary alcohol, but it suffers elimination producing a very electron-rich benzyloxy alkene that is easily epoxidized intramolecularly by chromium. Observe that the epoxide oxygen enters from the same face than the secondary alcohol. 

These same alkoxy compounds are also the primary products in the oxidation of alcohols with high oxidation state metal oxo complexes. In a typical process, the reaction of an alcohol with the chromium(vi) compound [HCr04] is shown in Fig. 9-16. The intermediate is often described as a chromate ester, but it is in all respects identical to the alkoxide complexes that we described earlier. 

The probable mechanism of oxidation of alcohols by chromium (VI) species involves the formation of chromate esters and their subsequent decomposition to ketones (57). As a rule, in the absence of competing side reactions, the more hindered alcohols react faster than the less hindered compounds. It has also been... 

Chromic acid oxidation of an alcohol (Section 11-2A) occurs in two steps formation of the chromate ester, followed by an elimination of H+ and chromium. Which step do you expect to be rate-limiting Careful kinetic studies have shown that Compound A undergoes chromic acid oxidation over 10 times as fast as Compound B. Explain this large difference in rates. 

Mechanism The mechanism of the Cr( VI) oxidation of aldehydes has been studied in detail in Scheme 7.14. A hydrate of aldehyde A is formed first, which reacts with chromium species to form a chromate ester B. A base abstracts a proton from the chromate ester B and Cr species leaves (E2 elimination) to give carboxylic acid. 

Chromium(VI) oxidations of alcohols involve the decomposition of chromate ester 8.3, which proceeds via cyclic transition states (see Scheme 7.1). 

Chromium(VI) oxide in various solvent systems provides an excellent oxidizing agent for alcohols, since it rapidly forms chromate esters which are intermediates in the oxidation of alcohols to aldehydes and ketones. The oxidation of [2- H]propan-2-ol showed a significant isotope effect when compared to propan-2-ol. Hence the abstraction of a proton by a base in the fragmentation of these esters is the rate-determining step in the reaction (Scheme 2.19). 

Chromate ester (Section 12.12A) An intermediate in the chromium-mediated oxidation of an alcohol having the general structure R-0-Cr03H. 

The oxidation of a secondary alcohol to a ketone with chromium (VI) is a complex reaction. With unhindered alcohols, oxidation proceeds via initial rapid formation of the chromate ester followed by a rate-determining E2-type elimination of HCr03 as the leaving group. 

Cr produced by reaction of lower-valence states with hydroperoxides, can react with alcohols to produce chromate esters. These esters will, of course, decompose heterolytically, in the manner described above, to produce carbonyls and, for example, H2C1O3 [79]. A similar but somewhat altered mechanism of het-erolytic chromate ester decomposition has been proposed to explain differences noted among several alcohols in the oxidation of alcohols with Cr [81]. The use of chromium-containing catalysts to decompose hydroperoxides to carbonyls in reactions conducted without autoxidation has been frequently noted [82-85]. 

Chromium trioxide and chromic acid, H2C1O4, are commonly used to oxidize alcohols. The alcohol adds to the chromium species to give a chromate ester, which undergoes elimination by an E2 process. 

Activity. Although pure CrO begins to decompose above 200 C into Oo and eventually Cr203,l a certain amount (0.4 Cr/nm ) is stabilized on silica up to 900 C in 02. And lower valent salts of chromium are easily oxidized on silica to the hexavalent form. This is probably due to the formation of a stable chromate ester on the silica surface as in (3). 

The mechanism involves the formation of a chromate ester, and at the end of the reaction, the oxidation state of the chromium changes from +6 (orange colour) to +3 (green colour), i.e. the chromium is reduced. 

Unfortunately, the catalyst can also become deactivated during the calcination, by several processes. Bulk hexavalent chromium oxide, CrC>3, or chromic acid, is unstable at temperatures above approximately 200 °C and begins to decompose into the trivalent oxide Cr2C>3 [39,40,42], On the catalyst, it is only the esterification with silica that stabilizes chromium in the hexavalent form at temperatures up to 900 °C. However, the chromate or dichromate ester can be hydrolyzed by water vapor present in the air used for the catalyst activation, as shown in Scheme 53. When this happens at elevated temperatures, decomposition to Cr(III) occurs. In the presence of water vapor and traces of Cr(VI), large crystallites of a-chromia are formed [74,75,134,135,731-733], which can be very difficult to reoxidize and disperse. 

Chromic acid and the other chromium-containing oxidizing reagents oxidize an alcohol by first forming a chromate ester. The carbonyl compound is formed when the chromate ester undergoes an E2 elimination (Section 11.1). 

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