Chromium oxide-3,5-dimethylpyrazole

Allylic oxidations may also be carried out with the related Chromium(Vl) Oxide-3,5-Dimethylpyrazole (DMP) complex.3... 

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. 

Although chromium(VI) oxide-(3,5-dimethylpyrazole) is also a good oxidant for allylic C—bonds, it is surprising that this reagent has not been more widely adopted. 

Two independent syntheses of quadrone eiiq>loyed an allylic oxidation with rearrangement, as shown in equation (3), where the chromium trioxide-(3,5-dimethylpyrazole) reagent (CrOs DMP) was used. In some cases, the success of the reaction strongly depends on the nature of the oxidant, as shown in an iq>proach to (-)-upial (equation 4). Here the chi ium trioxide-heterocycle reagents, which are weaker oxidants, are quite inferior compared to the Fieser reagent ... 

In contrast to the Collins reagoit, the complex formed by 3,5-dimethylpyrazole with chiomium(VI) oxide (2) is very soluble in dichloromethane. Hence, reactions (up to 0.1 mol scale) can be carried out in the minimum amount of solvent. Generally, 2.5 equiv. of complex, generated in situ, gives good yields of aldehydes and ketones. In addition, upon woric-up most of the chromium salts may be precipitated by dilution with diethyl ether. 

Chromium trioxide-3,5-dimethylpyrazole is a mild and selective reagent for oxidation of a-methylenecyclopropanes into cyclopropylketones <93CC954>. 

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>. 

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]. 

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. 

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. 

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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