Collins reagent preparation

Allylic oxidation of some cyclopentenes and cyclohexenes to the corresponding enones, using Collins reagent prepared in situ, has been found to proceed satisfactorily if all the reagents are kept very dry. ... 

Collins reagent can be prepared and isolated or generated in. situ. Isolation of the reagent often leads to improved yields. 

The chief drawbacks to using the Collins reagent are the nuisance involved in preparing pure dipyridine chromium VI) oxide,6 its hygroscopic nature5 and its propensity to enflame during preparation.2 3 5 The present method avoids these difficulties by simply preparing diehloromethane solutions of the complex directly.7 In... 

Pyridinium chlorochromate (PCC) Corey and Suggs prepared PCC by mixing CrOs with pyridine in HCl. PCC is used for the oxidation of primary and secondary alcohols in CH2CI2. This reagent is less efficient than Collins reagent for the oxidation of allyl alcohols. 

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]. 

Understand that other choices are legitimate for example, Swem oxidation works about as well as PCC in the preparation of aldehydes, and Collins reagent or PCC will oxidize a 2° alcohol to a ketone as well as chromic acid. If you have a question about the appropriateness of a reagent you choose, consult the table in the text before Problem 11-2. 

Reagent prepared by the modification of Collins, Hess, and Frank (2, 74, ref. II a) was used by Ireland et al.e in the synthesis of 5,6-dimethyl-rra .v-5,9-decadienal (4), starting with the readily available Zran.v-dibromide (1). 

The preparation of MOM-protected lactaldehyde 658 parallels that of the MEM derivative. It can be obtained in a two-step sequence in which lactate 377 is initially reduced to the propanol 657 and then oxidized to the aldehyde under Swem conditions [199] or with Collins reagent [100]. Overall yields starting from ethyl L-lactate (2) average about 50%. Alternatively, ester 377 can be reduced directly to aldehyde 658 (52% yield) with diisobutyl-aluminum hydride at —78 °C [120,200]. 

For a recent synthesis of a-multistriatin (31) by Plaumann and Fraser-Reid (75) the starting material was the enone (83) which is readily prepared from glucose as outlined in Scheme 20 (26). However triacetyl glucal (81) is commercially available. The Ferrier reaction with ethanol gives (82 a) as a crystalline substance in excellent yield, and the derived diol (82 b) may be oxidized with manganese dioxide to the hydroxy enone (83 a). Alternatively the primary hydroxyl of the diol may be protected to allow for oxidation with Collins reagent. 

Various acid chlorides react with (stannylethynyl)amines to give aminoethynyl ketones and chlorostannanes." Trimethylsilylallenes are readily oxidized by atmospheric oxygen to acetylenic hydroperoxides." The preparative reaction was carried out in the presence of pyridine, presumably to reduce the hydroperoxide to the alcohol, oxidative work-up (Collins reagent) affording acetylenic ketones (Scheme 27). Acetylenic ketones may also be prepared by the carbonyla-... 

A study of the oxidation of /3-hydroxyketones has shown that Collins reagent and dimethylsulphoxide-oxalyl chloride are suitable oxidants for the preparation of l,3-diketones. Using these reagents, neither /3-elimination nor retro-aldol fragmentation were observed. a-Diazo-/3-hydroxyketones are transformed into diketones on treatment with acid, but the need for an acidic medium can limit the generality of the method. However, it has been reported that this transformation occurs rapidly under neutral conditions in the presence of a catalytic amount of rhodium(ii) acetate. ... 

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