Hydroperoxides, alkyl trialkylborane

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). 

Whereas the use of 1.5 equiv. of oxygen to 1 of trialkylborane leads to quantitative yield of alcohol, as in equation (29), use of 2 equiv. of oxygen followed by addition of hydrogen peroxide (to prevent internal redox reactions) proceeds according to equation (35) to give 2 mol equiv. of alkyl hydroperoxide per mol of trialkylborane. ... 

A very mild means of oxidation of trialkylboranes to the corresponding alcohols is provided in the stoicheiometrically controlled reaction with oxygen at least a portion of the reaction is free-radical in nature, iodine acting as an inhibitor this leads to some loss of stereospecificity in cases leading to chiral alcohols. If the oxidation is performed at low temperature in dilute solution, alkyl hydroperoxides are obtained. ... 

It is known that organoboranes readily undergo autoxidation in the presence of oxygen and this reaction has been used to prepare alcohols and alkyl hydroperoxides as well as to induce free radical reactions. Organoboranes can also be used to alkylate a,P-unsaturated carbonyl compounds through a free radical 1,4-addition reaction in the presence of oxygen. However, organoboranes do not normally react with saturated carbonyl compounds except for the reaction of formaldehyde with trialkylboranes in the presence of air. ... 

The trialkylborane is next attacked by the hydroperoxide anion (it is still an electrophile with an empty p orbital), and the alkyl group migrates from boron to oxygen. Finally, the trialkoxybo rane is hydrolyzed by water. 

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