Collins reagent solvents

In contrast to the Collins reagent, the complex formed by 3,S-dimethylpyrazoIe 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 work-up most of the chromium salts may be precipitated by dilution with diethyl ether. 

A modification was introduced by Collins et al., and when applied to the oxidation of alcohols it has come to be known as Collins oxidation. This modification was developed to circumvent the danger inherent in preparing the reagent, deal with the problem of poor yields in the oxidation of primary alcohols to aldehydes, and facilitate isolation of the carbonyl products. The Sisler-Sarett reagent formed by reaction of chromium trioxide and pyridine was first removed from the pyridine solvent and added to dichloromethane, and this mixture was then treated with the alcohol. The oxidation typically required a 5 1 or 6 1 ratio of complex/alcohol, and reaction occurred at ambient temperatures. Cyclohexanol was oxidized to... 

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