Chromium VI reagents

Primary and secondary amines, double bonds, aldehydes, sulfides and certain aromatic and dihydroaroraatic systems are also oxidized by chromium VI reagents under standard hydroxyl oxidizing conditions. Amines are commonly protected by salt formation or by conversion to amides. Aldehydes and... 

A variety of chromium (VI) oxidizing systems have been developed which allow for the oxidation of a wide range of sensitive compounds. One of the most widely used chromium (VI) reagents is the Jones reagent (/), whose use is detailed in the procedure. A related system employs acetic acid as the solvent, and an example of this reagent is also given. 

A photo-induced dihydroxylation of methacryamide by chromium (VI) reagent in aqueous solution was recently reported and may have potential synthetic applications in the syn-dihydroxylation of electron-deficient olefins.63 Recently, Minato et al. demonstrated that K3Fe(CN)6 in the presence of K2C03 in aqueous rm-butyl alcohol provides a powerful system for the osmium-catalyzed dihydroxylation of olefins.64 This combination overcomes the disadvantages of overoxidation and low reactivity on hindered olefins related to previous processes (Eq. 3.14). 

J. Herscovici, M.-J. Egron, and K. Antonakis, New oxidative systems for alcohols Molecular sieves with chromium-(VI) reagents, /. Chem. Soc. Perkin Trans. I (1982) 1967-1973. 

The deployment of chromium(VI) reagents in the oxidative transformation is compromised due to inherent toxicity, involved preparation of its various complex forms (with pyridine or acetic acid) and cumbersome workup procedures. Chromium trioxide (Cr03) immobilized on premoistened alumina enables efficient oxidation of ben -... 

III. Chromium VI Reagents / 227 Chromium Vi-acetic acid / 227 Chromium Vl-acetone / 228 Chromium trioxide-pyridine complex / 229 Chromic acid-dimethylformamide / 231 Two-phase oxidations / 232... 

Oxidation of sec-alcohols.1 In the presence of CH3C03H, this chromium(VI) reagent can be used in catalytic amount (2 mole %) for oxidation of sec-alcohols to ketones in high yield. Under these conditions cholesterol is oxidized to A5-3-cholestanone (84% yield). Primary alcohols can be oxidized to aldehydes (—80% yield). f-Butyl hydroperoxide or anhydrous H202 cannot replace the peracetic acid as the reoxidant. 

Oxidation of n-allylpalladiam chloride complexes. Snatzke s reagent is superior lo other chromium(VI) reagents for oxidation of steroidal tt-allylpalladium chloride complexes to a,/l-enones.1 Examples ... 

Aluminophosphates (A1P04) were discovered in 198248 and a large amount of research has been directed towards the incorporation of various elements into the framework of these molecular sieves 49 A particular area of study is the oxidation of primary and secondary alcohols to the corresponding carbonyl compounds, which are useful synthetic intermediates. Traditionally, alcohol transformations are performed with stoichiometric chromium(VI) reagents.50 However, due to environmental problems associated with chromium-containing effluent, attention has focused on the use of chromium in conjunction with oxidizing agents such as tert-butyl hydroperoxide.51 Sheldon and co-workers... 

The oxidation of primary and secondary alcohols into the corresponding carbonyl compounds plays a central role in organic synthesis [1, 139, 140]. Traditional methods for performing such transformations generally involve the use of stoichiometric quantities of inorganic oxidants, notably chromium(VI) reagents [141]. However, from both an economic and environmental viewpoint, atom efficient, catalytic methods that employ clean oxidants such as 02 and H202 are more desirable. 

Other chromium(VI) reagents are known to attack at tertiary C—bonds. For example, CrOa/AcOH appears to be a general reagent for the introduction of an OH group at the 14-position of steroids (equation 63).Yields depend critically upon the amount of water present, 1-2% being best CrOzXz (X Q or OAc) also reacts with alkanes to give oxidized products. 

Examples of the use of chromium(VI) reagents to effect the allylic oxidation of alkenes to give a,3-unsaturated carbonyl compounds are very common in the literature. The reaction was first reported by Treibs and Schmidt for the allylic oxidations of a-pinene to verbenone and verbenol, of dipentene to carvone and caiveol, and of cyclohexene to cyclohexenol and cyclohexenone, using a solution of chromium trioxide in a mixture of acetic anhydride and carbon tetrachloride. However, yields were low and no synthetic use of this observation was made. 

Cyclobutanols oxidize with ring cleavage to 4-hydroxy ketones, 4-hydroxy acids, or 1,4-diones under the influence of chromium(VI) reagents (Scheme 11). The first formed product is a 4-hydroxycarbonyI compound (31) which exists as the five-membered ring hemiacetal (32). This form will persist in the absence of excess reagent under nonforcing conditions otherwise further oxidation takes place to give a... 

Oxidative cyclization of selected 5,6-dihydroxyalkenes to ci.v-2,5-disubstituted tetrahydrofurans has been realized with chromium(VI) reagents. In fact, when Collins reagent or pyridinium chlorochromate are employed at 20 °C for 3 min, the cyclization affords the cis-2,5-products in moderate yield. The resulting (tetrahydrofuranyl)diols are diastereomerically pure by H- and i5C-NMR spectroscopy. Moreover, a comparison of the reaction mixture with authentic 2,5-trans samples indicates a stereoselectivity of 99.5%47. 

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