Pyridinium chromate

Pyridinium chromate on silica ° is also a good general oxidant, even in the presence of acid labile groups. In contrast to PCC on alumina this oxidant may be stored at room temperature for one year, with no loss of activity. It is convenient to use it in the form of a column when performing small scale oxidations the substrate is allowed to stand on the column for a few hours, and tiien the carbonyl compound is obtained simply by eluting the column with a suitable solvent. 

Gelbard et al. have prepared a numbor of neutral and acidic supported ammonium chromates. Neutral resins woe found to be generally more effective dian acidic resins. The large differences in leactiviQr between different unbound, soluble alkylammonium chromates (Section 2.7.6.1) are not so pronounced with the polymer-supported oxidants. Some commoits about the reactivity of these supported oxidants woe made (i) quaternary ammonium salts are more reactive than the tertiary pyridinium chromates (ii) the reactivity of a conqilex chromate XCtOr was found to increase as the basicity of X decreases. Best... 

The procedure is commendable for its sinq>licity, reduced toxicity (chromium in all its oxidation states is carcinogenic) and achieves good yields of ketones from alcohol, for example, octan-2-ol is oxidized into octan-2-one (92%), cyclohexanol into cyclohexanone (90%) and menthol into menthone (98%). Pyridinium chromate is also a well-known oxidant for allylic oxidations. As a silica gel supported reagent, this is turned into an efficient alcohol oxidant that will leave acid-labile functions unscathed. Another advantage of the reagent is the long shelf-life of more than a year. These solid-supported oxidants also greatly facilitate pr uct work-up, when compared with their solution counterparts. 

Hexavalent chromium compounds soluble in organic solvents are pyr-idinium chromate [597] and pyridinium dichromate [603], Pyridinium chromate is prepared by adding 2 mol of pyridine to 1 mol of chromium trioxide adsorbed on silica gel. 

To 4-(dimethylamino)pyridinium chromate (5 mole), obtained from... 

Several references have appeared on the use of solid-phase oxidants. Solid potassium permanganate-copper sulphate mixtures oxidize secondary alcohols to ketones in high yield, and pyridinium chromate or chromic acid on silica gel are described as convenient off-the-shelf reagents for oxidation of both primary and secondary alcohols. Anhydrous chromium trioxide-celite effects similar transformations only when ether is present as co-solvent. An excellent review, with over 400 references, on supported oxidants covers the use of silver carbonate-celite, chromium trioxide-pyridine-celite, ozone-silica, chromyl chloride-silica, chromium trioxide-graphite, manganese dioxide-carbon, and potassium permanganate-molecular sieve. 

The same type of reactions can be conducted with methyl 2,3,6-tri-O-benzoyl-4-0-trifyl a-D-glucopyranoside to give modified D-galactoses. The free 4-hy-droxyl-carbon can be oxidized with pyridinium chromate to produce the 4-keto derivative, which, in turn, can be fluorinated with DAST to provide the 4-g m-di-fluoride (reaction 4.113). 

Sucrose can be selectively oxidized at C-6 by reacting 2,3,4,l, 3, 4, 6 -hepta-6>-benzoyl sucrose with pyridinium chromate to convert C-6 into an aldehyde. The aldehyde group can be reduced with sodium borohydride to create specifically labeled 6-pH]sucrose (reaction 4.135). 

By Oxidation of Alcohols.—Barium manganate is readily available and stable, and has been recommended for the oxidation of primary and secondary alcohols to aldehydes and ketones respectively. Chromic acid adsorbed on to silica geP and the recyclable poly[vinyl(pyridinium chromate)] effect the same changes, while acid-stable primary and secondary alcohols are oxidized rapidly using potassium dichromate, sulphuric acid, methylene chloride, and a phase-transfer catalyst." Benzeneseleninic anhydride is an alternative reagent for the oxidation of alcohols under essentially neutral conditions. ... 

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