Collins reagent complex

The double bond transposition could also be achieved by the conversion of an intermediate for PGA2 synthesis into a 1,3-diene iron tricarbonyl complex from which PGC2 was synthesized in four steps. The Fe(CO)3 diene complex which survived the Wittig reaction was cleanly removed by Collins reagent in the subsequent step (Ref. 10). 

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... 

Aldehydes are further oxidized to carboxylic acids, RCOOH. To get aldehydes, milder reagents, such as the Jones (diluted chromic acid in acetone) or Collins reagent (a complex of CrOj with 2 mol of pyridine), are used. 

Many other reagents and procedures have been developed for oxidizing alcohols. Some are simply modifications of the procedures we have seen. For example, the Collins reagent is a complex of chromium trioxide and pyridine, the original version of PCC. The Jones reagent is a milder form of chromic acid a solution of diluted chromic acid in acetone. 

The complex formed by 2,2 -bipyridine with chromium(VI) oxide is a milder oxidant than die Collins reagent. as indicated by the need for long reaction times (up to 48 h) and a larger excess of oxidant (8 equiv.). 

Oxidation of carbohydrates The oxidation of isolated secondary hydroxyl groups of carbohydrates with CrOs complexed with pyridine is often unsatisfactory. This oxidation can be effected generally in 70-85% yield with PCC reaction in CH2CI2 is very slow, but proceeds readily in refluxing benzene. The various oxidants based on DMSO are not useful. PCC is also effective for oxidation of primary hydroxyl groups of carbohydrates Collins reagent is also effective in this case. 

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]. 

The mode of interaction of the oxidant with the acetoxypalladation adduct is not certain. The oxidant could be removing electrons from Pd as the Pd(II)—C bond is broken and Pd(0) is never formed, or the Pd(II) could be oxidized to Pd(IV) which would leave much more easily than Pd(II). Another possibility is that the organic radical is transferred to the oxidant followed by decomposition. It would be difficult to distinguish between the various possibilities. Related reactions are the cleavage of a-bonded palladium complexes with Collins reagent (280), decomposition of rr-allyls with oxidants (164), and the decomposition of oxypalladation adducts of diolefins with oxidants (Section IV, B). 

All attempts to convert 40 or its hydroxyl protected derivatives to the enone needed to complete the functionalization of the A-ring were unsuccessful. Most often, treatment of benzoate 50 with a variety of oxidizing agents (e.g., with Collins reagent) led to complex reaction mixtures wherein, in addition to small amounts of the desired enone 51, enone 52 was the major product. 

In contrast to the Collins reagent, the complex formed by 3,S-dimethylpyrazoIe with chiomium(VI) oxide (2) is very soluble in dichloromethane. Hence, reactions (up to 0.1 mol scale) can be carried out in the minimum amount of solvent. Generally, 2.5 equiv. of complex, generated in situ, gives good yields of aldehydes and ketones. In addition, upon work-up most of the chromium salts may be precipitated by dilution with diethyl ether. 

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