Chromium trioxide-pyridine complex Collins reagent

An alternate reagent is the chromium trioxide(pyridine)2 complex, QO3 (py)2 (where py = pyridine), often referred to as the Collins reagent ... 

Many other reagents and procedures have been developed for oxidizing alcohols. Some are simply modifications of the procedures we have seen. For example, the Collins reagent is a complex of chromium trioxide and pyridine, the original version of PCC. The Jones reagent is a milder form of chromic acid a solution of diluted chromic acid in acetone. 

Another pyridine-chromium trioxide complex, pyridinium chloro-chromate, CsHsNHCrOjCl (PCC), is prepared by adding pyridine to a solution of chromium trioxide in 6 M hydrochloric acid [605. This complex is superior to Collins reagent in that much a smaller excess is needed, with the ratio of the substrate to the oxidant being 1 1.5-2 (equation 211). 

A chromium(VI) oxidant that is applicable to oxidations of acid-sensitive substrates is the complex of chromium trioxide with two molecules of pyridine (Collins reagent). As described on pages 22 and 274, its preparation requires the portionwise addition of chromium trioxide to dry pyridine at 15-20 C (addition of pyridine to chromium oxide could cause ignition) [592, 595, 599]. Up to 6 mol of the complex is used to oxidize alcohols in dichloromethane solutions at 25 °C, and the reaction is finished in 5-15 min [595]. Alternatively, the oxidation can be carried out in pyridine cooled with an ice bath and is finished at room temperature within 15-22 h [592, 599]. 

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... 

Collins reagent (Cr03 2 pyridine) A complex of chromium trioxide with pyridine, used to oxidize primary alcohols selectively to aldehydes, (p. 465)... 

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