Alcohol oxidation with chromium pyridinium dichromate

Chromium-based oxidants tend to react quicker with unsaturated alcohols, although the difference of oxidation speed with saturated alcohols is normally not sufficient for synthetic purposes. Nevertheless, the chromium-based reagent pyridinium dichromate (PDC) possesses a mildness and, therefore, a relative greater selectivity that allows its occasional employment for selective oxidations of allylic and benzylic alcohols.134... 

Reaction of the C-0 and O-H Bonds Primary alcohols oxidize to carboxylic acids secondary alcohols oxidize to ketones with chromium trioxide or sodium dichromate. Tertiary alcohols do not oxidize under mild conditions. With pyridinium chlorochromate (PCC) the oxidation of primary alcohols can be stopped at aldehydes. 

As the oxidation of the alcohols with chromium trioxide, or pyridinium dichromate in DMF gave unsatisfactory yields of the target dienoic acids, we tried Swern-oxidation to the aldehyde followed by further oxidation with sodium chlorite. This sequence proved to be highly successful as the Swern-oxidation provided an 80% yield. Subsequent oxidation of the aldehyde using sodium chlorite and 2,3-dimethyl-butene as the scavenger was almost quantitative to produce highly pure tmns-, cis-13-octadecadienoic acid (29). 

This complex, formerly called pyridine perchromate and now finding application as a powerful and selective oxidant, is violently explosive when dry [1], Use while moist on the day of preparation and destroy any surplus with dilute alkali [2], Preparation and use of the reagent have been detailed further [3], The analogous complexes with aniline, piperidine and quinoline may be similarly hazardous [4], The damage caused by a 1 g sample of the pyridine complex exploding during desiccation on a warm day was extensive. Desiccation of the aniline complex had to be at ice temperature to avoid violent explosion [4]. Pyridinium chlorochromate is commercially available as a safer alternative oxidant of alcohols to aldehydes [5], See Chromium trioxide Pyridine Dipyridinium dichromate See Other AMMINECHROMIUM PEROXOCOMPLEXES... 

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