Chromate ester, oxidation with

The initially formed allylic chromate ester equilibrates with an isomeric chromate ester Both allylic chromate esters produce the epoxidation of the alkene. The resulting epoxy alcohols are oxidized to epoxy ketones A and B in a 5 3 ratio. Starting from an equatorial alcohol instead of an axial one, an uneventful oxidation to enone occurs without transposition. 

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). 

Note 3. Although in principle the chromate ester can be formed directly from the 18-iodo-l 8,20-ether with silver chromate, hydrolysis and oxidation with aqueous chromic acid sulfurc acid is equally efficient. 

The aldehyde hydrate can then react with HCrOzf (and H+) to form a chromate ester, and this can then be oxidized to the carboxylic acid. 

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. ... 

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. 

It is also possible to oxidize o-allylphenols to chromenes with potassium dichromate. The oxidant may be supported on an anionic exchange resin, but it is preferable to dissolve the dichromate in benzene using Adogen 464, a mixture of methyltrialkylammonium chlorides (77TL4167). The oxidation is assumed to proceed through the chromate ester (107) which yields the quinoneallide (79CC836). 

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... 

A naive look at the product suggests an oxidation to a ketone followed by a Baeyer-Villiger like reaction. The product is best explained by a fragmentation from an intermediate chromate ester, resulting on an aldehyde and a stabilized tertiary carbocation that is transformed into a tertiary alcohol by reaction with water. The hydroxyaldehyde so obtained may evolve to the final lactone either via a lactol or a hydroxyacid. 

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). 

The expected enone is obtained in 40% yield. A 15% yield of the product, resulting from hydroxy-directed epoxidation followed by oxidation to ketone, is obtained. A third product, obtained in 30% yield, can be explained by the equilibration of the initially formed allylic chromate ester with an isomeric chromate ester that directs the epoxidation of an alkene, giving an epoxy alcohol that is further oxidized to an... 

The result of this oxidative degradation can be explained by an initial fragmentation leading to formaldehyde, and a cation that can be trapped by reaction with dichromate, resulting in a chromate ester that yields a lactone. The authors of this reaction pursued as much fragmentation as possible, and found that best yields of fragmented product were obtained by the use of AC2O as accelerant. 

PCC reacts with tertiary allylic alcohols, forming an intermediate chromate ester that evolves giving a conjugated enone or enal. Sometimes, the isomeric chromate ester produces the epoxidation of the alkene, giving an epoxy alcohol that can be further oxidized to an epoxy ketone. 

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... 

The authors of this book are not aware of any case, in which a primary allylic alcohol suffers an oxidative transposition with PCC. Such case would be most unlikely, because it would involve an equilibrating pair of allylic chromate ester, in which the less stable minor one would evolve to a carbonyl compound. 

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. 

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. 

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... 

It is important to note that the relative velocity of an uneventful oxidation of an alcohol with PCC versus a carbon-carbon bond breakage from a chromate ester, driven by the generation of a stable carbocation, is substantially substrate-dependent, and may change according to stereoelec-tronic factors, which may be difficult to predict. Thus, many alcohols are successfully oxidized to aldehydes and ketones, regardless of an apparently potential carbon-carbon bond breakage leading to stabilized carboca-tions.315 Consequently, failure to try an alcohol oxidation with PCC, because of fear of this side reaction is not recommended. 

A ketone, resulting from the normal oxidation of a secondary alcohol, is obtained along with an alkene, resulting from an opening of the cyclopropane. The secondary product can be explained by the intermediacy of either a chromate ester, or a protonated alcohol. Treatment of the starting alcohol with 10% HC1 leads to a 87% yield of the secondary product, suggesting a mechanism involving PCC as a proton donor. 

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. 

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 oxidation of substituted /3-benzoylpropionic acids by PFC follows the Hammett relation with a negative reaction constant. A possible mechanism for the oxidation has been discussed.5 The oxidation of maleic, fumaric, crotonic, and cinnamic acids by PCC is of first order with respect to PCC and the acid. The oxidation rate in 19 organic solvents has been analysed by Kamlet s and Swain s multiparametric equations. A mechanism involving a three-centre transition state has been postulated.6 The relative reactivity of bishomoallylic tertiary alcohols toward PCC, to yield substituted THF products via the tethered chromate ester, is dependent only on the number of alkyl groups. This observation suggests a symmetrical transition state in this intramolecular Cr(VI)-alkene reaction.7 Mechanisms have been proposed for the oxidation of 2-nitrobenzaldehyde with PBC8 and of crotonaldehyde with tetraethylammonium chlorochromate.9... 

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