Chromate ester, formation

Isolated carbon-carbon multiple bonds are not normally attacked by Jones reagent, but some doublebond isomerization may occur during the preparation of a, -unsaturated aldehydes. Hydroxy-directed epoxidation (presumably via chromate ester formation, followed by oxygen transfer to the double bond) has also been observed in steroidal substrates for axial alcohols (equation 1). Equatorial alcohols undergo oxidation to give the expected enone. 

The first step in the oxidation of the alcohol is the formation of a chromate ester which probably decomposes unimolecularly to products, viz. 

The first step is the formation of a chromate ester of the alcohol. 

Chromate Oxidations Formation of the Chromate Ester Step 1... 

The formation of an intermediate with electron-deficient oxygen is also one of the possible paths for the oxidation of alcohols. An intermediate such as LVI, or the chromate ester LVII which might behave in similar fashion, could rearrange to give "abnormal products or lose a proton to give the usual, expected product.889-882... 

The pyridinium chlorochromate (PCC) oxidations of pentaamine cobalt(III)-bound and unbound mandelic and lactic acids have been studied and found to proceed at similar rates.Free-energy relationships in the oxidation of aromatic anils by PCC have been studied. Solvent effects in the oxidation of methionine by PCC and pyridinium bromochromate (PBC) have been investigated the reaction leads to the formation of the corresponding sulfoxide and mechanisms have been proposed. The major product of the acid-catalysed oxidation of a range of diols by PBC is the hydroxyaldehyde. The reaction is first order with respect to the diol and exhibits a substantial primary kinetic isotope effect. Proposed acid-dependent and acid-independent mechanisms involve the rapid formation of a chromate ester in a pre-equilibrium step, followed by rate-determining hydride ion transfer via a cyclic intermediate. PBC oxidation of thio acids has been studied. ... 

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, alcohols can direct the oxidation of alkenes, resulting in highly stereoselective formation of tetrahydrofurans by the action of Collins reagent. Thus, 1,2-diols can form cyclic chromate esters that can intramole-cularly oxidize alkenes, positioned so as to allow the operation of five-membered cyclic transition states.119... 

Fragmentation of chromate esters may be also driven by the formation... 

Although secondary allylic alcohols can suffer an oxidative transposition via the corresponding allylic chromate ester, in the same manner that the tertiary allylic alcohols normally, a direct oxidation to the corresponding enone with no transposition predominates.277 Nevertheless, minor amounts of enone, resulting from an oxidative transposition, can be formed.278 The formation of transposed enone may be minimized using the less transposing-prone PDC, instead of PCC.279... 

When the oxidative transposition of secondary allylic alcohols is purposefully looked after, it can be fostered by the addition of p-toluenesulfonic acid.280 Most probably, the added acid catalyzes the equilibration of the intermediate allylic chromate esters, allowing the major formation of transposed enone when the corresponding chromate ester is less hindered. This means that an oxidative transposition of a secondary allylic alcohol can only dominate when the thermodynamics of the equilibrating allylic chromate esters are favourable. 

Thanks to the addition of p-TsOH that catalyzes the equilibration of the intermediate allylic chromate ester the major product is the desired enal, resulting from an oxidative transposition. Failure to add p-TsOH leads to the major formation of the untransposed enone, ... 

PCC transforms 5,6-dihydroxyalkenes into tetrahydrofurans in a highly stereoselective manner284 (see Equation below). This transformation can be explained by the initial formation of a cyclic chromate ester by reaction with the diol moiety, followed by an intramolecular oxidative addition of the chromate ester on the alkene. 

Of course, the PCC-induced formation of tetrahydrofurans from 5, 6-dihydroxyalkenes fails when structural constrains prevent the approach of the intermediate cyclic chromate ester to the alkene.286... 

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. 

However, as the formation of an intermediate simple chromate ester is not as favorable as the generation of the cyclic chromate ester, involved in the oxidation of 5,6-dihydroxyalkenes, this reaction, demands harsher conditions. Therefore, only tertiary 5-hydroxyalkenes may be normally used as starting compounds, otherwise a direct oxidation of the alcohol to an aldehyde or ketone would occur.287 Because of the harsher conditions involved, very often the resulting 1-hydroxyalkyltetrahydrofuran is further oxidized to a y-lactone or to a ketone.288... 

The secondary alcohol is oxidized to a ketone that can be trapped intramolecularly as a cyclic hemiacetal. Alternatively, the tertiary alcohol can react with PCC forming a chromate ester that evolves by a carbon-carbon breakage, facilitated by the formation of a stable tertiary carbocation, and the release of annular tension resulting from the opening of a cyclobutane. The resulting carbocation produces an alkene... 

Formation of a chromate ester is followed by the opening of cyclopropane, driven by attack of chloride and elimination of chromate anion. 

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. 

An excellent method for the conversion of ether-soluble secondary alcohols to the corresponding ketones is by chromic acid oxidation in a two-phase ether-water system. The reaction is carried out at 25-30 °C with the stoichiometric quantity of chromic acid calculated on the basis of the above equation, and is exemplified by the preparation of octan-2-one and cyclohexanone (Expt 5.86). The success of this procedure is evidently due to the rapid formation of the chromate ester of the alcohol, which is then extracted into the aqueous phase, followed by formation of the ketone which is then extracted back into the ether phase and is thus protected from undesirable side reactions. 

Assuming that equilibrium formation of the chromate ester is fast, this rate expression maybe rewritten as Equation 4.22 ... 

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... 

The mechanism of chromic acid oxidation probably involves the formation of a chromate ester. Elimination of the chromate ester gives the ketone. In the elimination, the carbinol carbon retains its oxygen atom but loses its hydrogen and gains the second bond to oxygen. 

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. 

The mechanism starts with the formation of HCr04 ions, that is, Cr(VI), from dichromate ion in solution. In acid, these form chromate esters with alcohols. The esters (boxed in black) decompose by elimination of the Cr(IV) HCrO, which subsequently reacts with a Cr(VI) species to yield 2 x Cr(V). These Cr(V) species can oxidize alcohols in the same way, and are thereby reduced to Cr(III) (the final metal-containing by-product). Cr(VI) is orange and Cr(III) is green, so the progress of the reaction is easy to follow by colour change. 

Mechanism The mechanism of the alcohol oxidation with Cr(VI) is outlined in Scheme 7.1, and involves the formation of chromate ester. The base removes the proton and Cr species leaves in an intermolecular process (A) however, an intramolecular process (B) may also operate. The Cr(IV) ions in H2Cr03 or HCrOs" are converted back to Cr(III) ions. It is believed that part of alcohol molecules are oxidized by the free radical mechanism. 

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