Sarett oxidation alcohols

CrOs-pyridine complex The oxidation of alcohols to aldehydes and ketones with CrOs in the presence of pyridine solvent is known as the Sarett oxidation. Sarett used the CrOs-pyridine complex in the synthesis of steroids. Although primary alcohols give poor yield, benzylic, allylic and secondary alcohols give good yields with Sarett reagent. 

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. 

Sarett oxidation. Oxidation of primary and secondary alcohols to aldehydes and ketones by means of Cr03-pyridine complex. 

A second problem with Sarett oxidation is the difficulty in isolating the products from a pyridine solution. An advantage of the technique, as mentioned above, is that alkenes, ketals, sulfides, and tetrahydropyranyl ethers are oxidized much slower than alcohols and rarely give competitive side reactions. Oxidation of secondary alcohols proceeds in good yield, but oxidation of primary aliphatic alcohols often gives low yields of the aldehyde. 2 Benzylic and allylic alcohols give good yields, however. 

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. 

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... 

Similar to the Dakin-West procedure previously mentioned, the Henry nitro-aldol condensation reaction is most widely used to synthesize trifluoromethyl ketones, although there are many examples of a,a-difluoroalkyl ketones synthesized by this method (Table 6)JU 12271 The method for a,a-difluoroalkyl and trifluoromethyl ketone synthesis is identical except for the final oxidation although fluoroalkyl and a,a-difluoroalkyl ketones are easily oxidized by the Sarett method (Cr03/pyridine),[12 the corresponding trifluoromethyl ketones can only be oxidized under basic conditions (0.3 M NaOH) with KMn04Jul Also, in some of the syntheses of a,a-difluoroalkyl ketones, the nitro alcohol intermediate was protected by si-lylation with /ert-butylchlorodimethylsilane. The silyl group was later removed by TosOH prior to oxidation. The full details of this method are given in Section 15.1.4.3.2. 

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. ... 

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. 

Oxidation of alcohols to aldehydes or ketones with C1O3-H2SO4 in Me2CO (Jones) or C1O3 in pyridine (Sarett) (see 1st edition). 

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... 

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... 

Grignard reaction with formaldehyde closer to our desired product. Subsequent hydrolysis will yield 2,2-dimethylbutanol our desired product is 2,2-dimethylbutanal. Oxidation of the primary alcohol to the aldehyde can be accomplished with Sarett s reagent, a combination of chromium trioxide (CrOs) with pyridine. 

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