Alcohol oxidation with chromium Collins reagent

The most widely used chromium(VI)-oxo reagents for synthetic use in organic chemistry are the Collins reagent, Cr03(py)2,273 and the Corey reagent, Cr03CrpyH+,297 with the latter being superior for the oxidation of primary alcohols to aldehydes.275... 

Deacetoxyhtion. In a synlhesis of I l-desoxy-IOz-hydroxyprostaglandins, Crabbe et al. oxidized the alcohol (1) with Collins reagent (dipyridine chromium(Vl) oxide, this volume) to give the diacetoxy aldehyde (2) treatment of this with pyridine for a few... 

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]. 

Pyridinium Chlorochromate. The need for improved oxidation of primary alcohols and greater ease for isolation of products prompted further research into the nature of Cr(VI) reagents. Corey found that addition of pyridine to a solution of chromium trioxide in aqueous HCl allowed crystallization of a solid reagent characterized as 31, pyridinium chlorochromate (PCC). This reagent was superior for the conversion of primary alcohols to aldehydes in dichloromethane but less efficient than the Collins oxidation when applied to allylic alcohols. Oxidation of 1-heptanol with PCC in dichloromethane gave 78% of heptanal, for example. As stated by Corey, PCC is an effective oxidant in dichloromethane although aqueous chlorochromate species are not very effective oxidants. Oxidation of secondary alcohols to ketones is straightforward, as in Banwell s synthesis of y-lycorane, in which 32 was oxidized by PCC to the ketone (33). ... 

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

Oxidation Adjacent to Oxygen of Alcohols by Chromium Reagents Table 2 Oxidation of Alcohols with Ci03 py2 (Collins Oxidation)... 

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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