Sarett oxidation chromium oxide/pyridine complex

The Collins/Sarett oxidation (chromium trioxide-pyridine complex), and Corey s PCC (pyridinium chlorochromate) and PDC (pyridinium dichromate) oxidations follow a similar pathway as the Jones oxidation. All these oxidants have a chromium (VI), normally yellow, which is reduced to Cr(IV), often green. 

B.i. Chromium Trioxide—Pyridine. In 1948, Sisler and co-workers isolated and characterized a stable complex from the reaction of chromium trioxide and pyridine. Sisler did not use this reagent for the oxidation of organic molecules but Sarett and co-workers recognized its utility in the synthesis of steroids. In this connection, alcohol 24 was oxidized to 26 in 89% yield. The reagent, which probably has the trigonal bipyramidal structure shown in 25, proved useful for the general oxidation of primary and secondary alcohols, even in the presence of double bonds and thioethers,. The oxidation usually requires pyridine as a solvent and... 

Dipyridiue-chromium(VI) oxide2 was introduced as an oxidant for the conversion of acid-sensitive alcohols to carbonyl compounds by Poos, Arth, Beyler, and Sarett.3 The complex, dispersed in pyridine, smoothly converts secondary alcohols to ketones, but oxidations of primary alcohols to aldehydes are capricious.4 In 1968, Collins, Hess, and Frank found that anhydrous dipyridine-chromium(VI) oxide is moderately soluble in chlorinated hydrocarbons and chose dichloro-methane as the solvent.5 By this modification, primary and secondary alcohols were oxidized to aldehydes and ketones in yields of 87-98%. Subsequently Dauben, Lorber, and Fullerton showed that dichloro-methane solutions of the complex are also useful for accomplishing allylic oxidations.6... 

Sarett and coworkers discovered that the complex (1) prepared by the addition of chromium(VI) oxide to pyridine (CAUTION—reverse order of addition may cause the mixture to inflame) is an efficient oxidizing agent for the preparation of ketones from secondary alcohols. The reagent, as prepared by Sarett, is moderately soluble in pyridine, but is only sparingly soluble in standard organic solvents. Thus the normal procedure is to add a solution of the dcohol in pyridine to three equivalents of the complex, also in pyrictoe. This procedure is also useful for the preparation of aromatic and a, -unsaturated aldehydes, but the use of pyridine as solvent prohibits the oxidation of volatile, saturated primary alcohols. ... 

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... 

Precursors of imidazoquinoxalines-compounds 109a, b and 112-have been synthesized from chloroquinoxalines 128 and corresponding amines in the presence of EtsN. To oxidize aminoalcohols 112 to ketones 114, different oxidative systems have been used a complex of trimethylamine and sulfuric anhydride in DMSO, chromium(VI) oxide in pyridine (Sarett method) (Luzzio 1998 Caamano et al. 2000), and dichloroxalate in DMSO (Swern method) (Scheme 4.58) (Ohmori et al. 1997 Parra et al. 2001 Deleuze-Masqudfa et al. 2004). 

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