Esters chromate

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

Aldehydes can be oxidized to carboxylic acids by both Mn(VII) and Cr(VI). Fairly detailed mechanistic studies have been carried out for Cr(VI). A chromate ester of the aldehyde hydrate is believed to be formed, and this species decomposes in the rate-determining step by a mechanism similar to the one that operates in alcohol oxidations.209... 

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

The chromate ester is unstable and is not isolated. It transfers a proton to a base (usually water) and simultaneously eliminates an HCr03 ion. 

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

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 chromate ester from 3° alcohols does not bear a hydrogen that can be eliminated, and therefore no oxidation takes place. 

This chromate ester cannot undergo elimination of H2Cr03... 

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. ... 

Quinolinium dichromate (QDC) oxidations of primary and secondary alcohols both proceed via a cyclic chromate ester. Acrylonitrile polymerization was observed in the oxidation of several para- and meffl-substituted benzaldehydes to the corresponding benzoic acids by quinolinium chlorochromate (QCC). QCC oxidations of diphenacyl sulfide and of aromatic anils have been studied. 

The silica-supported chromate can be activated directly to a very efficient ethylene polymerization catalyst by ethylene itself or by reduction under CO, to yield active Cr(ll) bisiloxy species, ](=SiO)2Cr] [8]. While the silsesquioxane Cr derivative on its own does not lead to an active polymerization catalyst under ethylene (albeit only low ethylene pressure were tested), the silsesquioxane chromate ester can yield an active polymerization catalyst by addition of methyl-aluminoxane as co-catalyst. Comparison between the two catalytic systems is therefore possible but suffers from the lack of molecular definition of the active homogeneous species obtained after activation with the alkylating agent (Scheme 14.11). 

Similar mechanisms were postulated for the oxidation of glycols by periodate (32) and Ce(IV) (33, 34), and for the oxidation of glycerol by Ce(IV) (44). In these cases the existence of intermediate complexes was demonstrated. The oxidation of formaldehyde by Ce(IV) was also claimed to involve a pre-equilibrium of a Ce(IV)-formaldehyde complex (51). A similar complex was postulated in the formalde-hyde-Mn04 reaction (49, 87). The oxidation of isopropyl alcohol by chromate ions follows a similar mechanism, and a chromate ester was formed as intermediate (94). 

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). 

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... 

Dimethylpentane-2,4-diol chromate(VI) diester (1). The diol is prepared by reaction of diacetone alcohol with 2 equiv. of CH3Li (ether, -70-0°), 97% crude yield. The chromate ester (1) is obtained by reaction of the diol in CC14 with Cr03 after about 10 minutes P205 is added and then the clear solution is stored for use. 

The subsequent step is the slowest in the sequence and appears to involve attack of a base (water) at the alpha hydrogen of the chromate ester concurrent with elimination of the HCr03° group. There is an obvious analogy between this step and an E2 reaction (Section 8-8A) ... 

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... 

An alternative and more generally used oxidation method employs chromic acid. This process is an exception to our general theme, because here the alcohol is transformed to a carbonyl group by removal of electron density from oxygen rather than from carbon. The first step has been shown to be a rapid equilibrium between the alcohol and its chromate ester, followed by rate-determining decomposition of the ester in the manner shown in Scheme 7.42 It will be noted that the species eliminated from the carbon that becomes the carbonyl carbon is a Lewis acid, not a Lewis base. 

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). 

Sometimes, chromate esters from secondary a]]vi-transposition rather than direct oxidation, the rLI" ° l0ls Suffer chromate ester can either produce epoxidation of the iv transPosed oxidation yielding a transposed enone.84 a ene or suffer... 

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. 

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... 

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... 

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... 

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