Chromic acid organoboranes

Usually, organoboranes are sensitive to oxygen. Simple trialkylboranes are spontaneously flammable in contact with air. Nevertheless, under carefully controlled conditions the reaction of organoboranes with oxygen can be used for the preparation of alcohols or alkyl hydroperoxides (228,229). Aldehydes are produced by oxidation of primary alkylboranes with pyridinium chi orochrom ate (188). Chromic acid at pH < 3 transforms secondary alkyl and cycloalkylboranes into ketones pyridinium chi orochrom ate can also be used (230,231). A convenient procedure for the direct conversion of terminal alkenes into carboxyUc acids employs hydroboration with dibromoborane—dimethyl sulfide and oxidation of the intermediate alkyldibromoborane with chromium trioxide in 90% aqueous acetic acid (232,233). 

The conversion of an alkene into the corresponding ketone may be effected by means of a convenient sequence which involves hydroboration followed by oxidation with chromic acid of the resulting organoborane (cf. Section 5.4.3,... 

Pyridinium chlorochromate (PCC) is a very useful reagent for the oxidation of organoboranes to carbonyl compounds in mildly alkaline and anhydrous conditions. As well as oxidizing (secondary alkyl)boranes to ketones, PCC oxidizes (primary alkyl)boranes to aldehydes in excellent yields (equation 47). The latter transformation caimot be accomplished with chromic acid. The reagent tolerates the presence of alkene, ester and acetal groups. ... 

Oxidation of organohoranes. This oxidant is superior to chromic acid for oxidation of organoboranes derived from cycloalkenes to the corresponding ketones. For example, cyclohexanone is obtained in 81% yield from cyclohexene. 

Brown, H. C., Garg, C. P. 1961. Chromic acid oxidation of organoboranes. A convenient procedure for converting olefins into ketones via hydroboration. J. Am. Chem. Soc. 83 2951-2952. 

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