ALDEHYDES FROM PRIMARY ALCOHOLS BY OXIDATION WITH CHROMIUM TRIOXIDE

ALDEHYDES FROM PRIMARY ALCOHOLS BY OXIDATION WITH CHROMIUM TRIOXIDE 1-HEPTANAL, 52, 5 ALDEHYDES FROM sym-TRITHIANE n-PENTADECANAL, 51, 39 Aldehydes, acetylenic, 54, 45 Aldehydes, aromatic, 54, 45 Aldehydes, benzyl, 54, 45 Aldehydes, olefinic, 54, 45... 

ALDEHYDES FROM PRIMARY ALCOHOLS BY OXIDATION WITH CHROMIUM TRIOXIDE 1-HEPTANAL... 

Jones s oxidation is a powerful oxidizing medium for the conversion of alcohols to ketones. Unfortunately, this is such a powerful oxidizing medium that unwanted products are possible due to overoxidation. When a primary alcohol such as 1-pentanol (15) reacts with chromium trioxide and aqueous sulfuric acid, it follows the same mechanistic pathway as 9, with formation of chromate ester 16. However, experiments show that the yields of aldehyde from primary alcohols can be very low. 1-Propanol is oxidized to propanal, for example, in only 49% yield, and to obtain the product requires a short reaction time. Very often, a carboxylic acid is formed as a second product or even the major oxidation product rather than the aldehyde. It is known that aldehydes are easily oxidized to carboxyhc acids, even by oxygen in the air. If a sample of butanal were spilled, for example, it is rapidly oxidized to butanoic acid by air. This oxidation is easily detected as the sharp butanal smell is replaced by the pungent butanoic acid smell. Butanoic acid is found in rancid butter and in dirty feet, for example. 

The oxidation of primary alcohols to aldehydes also suffers from the problem of overoxidation of the aldehyde to a carboxylic acid. Mild methods capable of stopping die oxidation at the aldehyde oxidation level are required if aldehydes are to be obtained. The most common and effective reagent for this purpose is pyridinium chlorochromate (PCC), produced by the reaction of pyridinium hydrochloride with chromium trioxide. This reagent is soluble in dichloromethane and smoothly oxidizes primary alcohols to aldehydes in high yields. Because of die mild, neutral reaction conditions and the use of stoichiomettic amounts of oxidant, the aldehyde product is not oxidized further. 

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... 

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