Chromium trioxide-3,5-dimethylpyrazole complex

With the advent of the chromium trioxide-3,5-dimethylpyrazole complex as an oxidant - allylic oxidation has become far more valuable as a synthetic transfonnation. The reagent was applied by Sala-mond to the allylic oxidation of cholesteryl benzoate to give the correspondbg A -7-ketone (equation 35). However, a 20 molar excess of reagent was still requii to effect the reaction in less than 30 min at room temperature. 

Chromium trioxide-3,5-dimethylpyrazole complex, Cr03 CsHgN2, is... 

The chromium trioxide-3,5-dimethylpyrazol complex has been found to be a mild and selective reagent for the oxidation of cyclopropyl hydrocarbons (reaction proceeds in the dark at -20 °C) [67]. 

Corey, E. J. Fleet, G. W. J. Chromium Trioxide-3,5-Dimethylpyrazole complex as a Reagent for Oxidation of Alcohols to Carbonyl Compounds Tetrahedron Lett. 1973, 15, 4499-4501. 

Instead of pyridine, 3,5-dimethylpyrazole may be used for complexing chromium trioxide. It forms a l.T complex that can be generated in situ in dichloromethane at room temperature. The reaction is finished within 10 min. Addition of the alcohols and stirring of the mixture for 30 min at room temperature result in 78-100% yields of ketones (equation 252) 602. ... 

Pyrazole-tethered Schiff base ligands 62 promoted Suzuki cross-couplings of aryl bromides and chlorides with phenylboronic acid efficiently under mild conditions <05TL15>. Chromium-trioxide complex with 3,5-dimethylpyrazole 63 oxidation of cyclohexenecarbonitrile provided 3-oxocyclohex-1 -ene-1 -carbonitrile <05S3179>. 

Allylic Oxidation with 3,5-Dimethylpyrazole Chromium Trioxide Complex. 

The chief difficulty with this reagent is that the complex is highly hygroscopic. However, it can be prepared in situ, thus avoiding this major drawback. Pyridinium dichromate and chromium trioxide 3,5-dimethylpyrazole are also effective as selective oxidizing agents for these reactions. 

A 3,5-dimethylpyrazole-chromium trioxide complex, prepared at -20 °C by quickly adding 3,5-dimethylpyrazole to chromium trioxide in CH2CI2 has been shown to be an efficient reagent for allylic oxidation. A -Steroids (e.g. choles-teryl benzoate) are oxidized very rapidly in about 75% yield to the corresponding A -ketone, there being a rate increase of 100-fold compared with using a pyridine-chromium trioxide complex. This increased activity is thought to be partly due to the increased solubility of the complex and more importantly to the possibility of intramolecular acceleration due to the pyrazole nucleus. 

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