Oxidation Collins-Sarett

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. 

Different from the Jones oxidation, die Collins-Sarett oxidation converts primary alcohols to the corresponding aldehydes. 

Different from the Jones oxidation, the Collins oxidation, also known as the Collins-Sarett oxidation, converts primary alcohols to the corresponding aldehydes. Cr03 2Pyr is known as the Collins reagent. 

A 62.5% yield of the desired ketone with no epimerization at the ot-position is obtained, employing catalytic TPAP as oxidant. Other oxidizing conditions, including Collins, Sarett, Oppenauer and Swern oxidations, as well as PCC, fail to deliver an acceptable yield of... 

The mechanism of the Sarett oxidation, Collins oxidation, with Corey s PCC and with PDC, follows a similar mechanism as shown in Scheme 7.1. The alcohol reacts with CrOs to give a chromate ester. Either a base (Py) removes a proton from the chromate ester to give an oxidized product (aldehyde or ketone) and HCrOs" or a proton is transferred by the intramolecular mechanism to give an aldehyde or ketone and H2Cr03. 

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... 

Because you can easily further oxidize aldehydes to carboxylic acids, you can only employ mild oxidizing agents and conditions in the formation of aldehydes. Typical mild oxidizing agents include manganese dioxide (MnO), Sarett-Collins reagent (CrO,—(CSHSN)2), and pyridinium chlorochromate (PCC),... 

Oxidation of akohois. The oxidation of alcohols with the reagent is conducted in methylene chloride with a sixfold molar excess of oxidant. The oxidation usually proceeds to completion in 3 -13 min. at 23°. The reagent is particularly recommended for oxidation of primary alcohols to aldehydes in this case use of the Sarett reagent (1,145 -146 2,74-75) usually gives low yields. Thus 1-heptanol can be oxidized by the Collins reagent in methylene chloride to l-heptanal in 70-84% yield. 

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... 

Ratcliffe has reported that 1-decanol is oxidized to 1-decanal in 63-66% yield, using dipyridine-chromium(VI) oxide prepared directly in methylene chloride at 20°C. This procedure has the advantage over the earlier reported procedure by Sarett and co-workers and Collins in that pure dipyridine-chromium(VI) oxide does not have to be separately prepared. The latter is troublesome since it is hygroscopic and has a propensity to enllame during preparations. 

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