Alcohols, primary with chromium trioxide-pyridine

Oxidation, of primary alcohols to aldehydes, 52, 5 of terminal olefins with chromyl chloride, 51, 6 of 2,4,4-trimethyl-1-pentene with chromyl chloride, 51, 4 with chromium trioxide-pyridine complex, 52, 5... 

If homolytic reaction conditions (heat and nonpolar solvents) can be avoided and if the reaction is conducted in the presence of a weak base, lead tetraacetate is an efficient oxidant for the conversion of primary and secondary alcohols to aldehydes and ketones. The yield of product is in many cases better than that obtained by oxidation with chromium trioxide. The reaction in pyridine is moderately slow the intial red pyridine complex turns to a yellow solution as the reaction progresses, the color change thus serving as an indicator. The method is surprisingly mild and free of side reactions. Thus 17a-ethinyl-17jS-hydroxy steroids are not attacked and 5a-hydroxy-3-ket-ones are not dehydrated. 

P. J. Garegg and B. Samuelsson, Oxidation of primary and secondary alcohols in partially protected sugars with the chromium trioxide-pyridine complex in the presence of acetic anhydride, Carbohydr. Res., 67 (1978) 267-270. 

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. 

Olefinic aldehydes have been synthesized by a variety of methods including oxidation of the corresponding primary alcohols with the chromium trioxide-pyridine complex 195—197) or N-chlorosuccinimide-dimethyl sulfide complex 198), heating a primary alken-l-yl mesylate with dimethylsulfoxide 199), or by alkylation of the lithium salt of 5,6-dihydro-2,4,4,6-tetramethyl-l,3-(4H)-oxazine with an alkynyl iodide followed by sodium borohydride reduction and acid hydrolysis (200). 

A better reagent for oxidation of primary alcohols to aldehydes in good yield is pyridinium chlorochromate (PCC), a complex of chromium trioxide with pyridine and HCl. 

A better reagent for the limited oxidation of primary alcohols to aldehydes is pyridinium chlorochromate (PCC), a complex of chromium trioxide with pyridine and HC1. PCC oxidizes most primary alcohols to aldehydes in excellent yields. Unlike most other oxidants, PCC is soluble in nonpolar solvents such as dichloromethane (CH2C12), which is an excellent solvent for most organic compounds. PCC can also serve as a mild reagent for oxidizing secondary alcohols to ketones. 

A complex of chromium trioxide with pyridine and HC1. PCC oxidizes primary alcohols to aldehydes without over-oxidizing them to carboxylic acids, (p. 471)... 

Attention may be drawn to a new reagent, chromium trioxide in pyridine, that is said to oxidize primary alcohols to aldehydes. The potential importance of its application to bicyclic diacetals with free terminal groups is clear, both as a preparative method for aldoses and as a means of assigning conformations. 

The primary alcohol 221 was oxidized to aldehyde 222 by chromium trioxide in pyridine in 85% yield. The alcohol 223 was obtained from compound 222 by treatment with Grignard reagent (225). The bromo compound (224) required for preparation of the Grignard reagent was prepared as outlined above in overall low yield. 

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... 

Pyridinium Chlorochromate. The need for improved oxidation of primary alcohols and greater ease for isolation of products prompted further research into the nature of Cr(VI) reagents. Corey found that addition of pyridine to a solution of chromium trioxide in aqueous HCl allowed crystallization of a solid reagent characterized as 31, pyridinium chlorochromate (PCC). This reagent was superior for the conversion of primary alcohols to aldehydes in dichloromethane but less efficient than the Collins oxidation when applied to allylic alcohols. Oxidation of 1-heptanol with PCC in dichloromethane gave 78% of heptanal, for example. As stated by Corey, PCC is an effective oxidant in dichloromethane although aqueous chlorochromate species are not very effective oxidants. Oxidation of secondary alcohols to ketones is straightforward, as in Banwell s synthesis of y-lycorane, in which 32 was oxidized by PCC to the ketone (33). ... 

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. 

Pyridinium chiorochromate (PCC) is a popular reagent for the selective oxidation of primary alcohols to aldehydes and secondary alcohols to ketones. PCC is prepared commercially by the reaction of pyridine with hydrochloric acid in the presence of chromium trioxide (CrOg). The chromium atom in both CrOg and also in PCC is in the 6-I- oxidation state (orange color). 

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