Organoboranes for synthesis: oxidation of organoboranes with alkaline hydrogen peroxide as a convenient route for the cis-hydration of alkenes via hydroboration

Abstract

Aqueous hydrogen peroxide in the presence of dilute alkali effects the oxidation of organoboranes. The conditions necessary for a clean and quantitative transformation of organoboranes into the corresponding alcohols have been established. Thus, one mole of trialkylborane reacts with three moles of hydrogen peroxide in the presence of one mole of sodium hydroxide to provide three moles of the corresponding alcohol. The concentrations of these reagents or the reaction temperature can be varied widely without affecting the yield significantly. Oxidation proceeds well in water-miscible solvents, such as diglyme and THF. However, the reaction is slow and incomplete in diethyl ether. The addition of ethanol as a cosolvent circumvents this difficulty. Wide variations in the structure of organoboranes do not affect the reaction greatly. A variety of common organic functional groups, such as alkenes, alkynes, esters, ketones, nitriles, etc., are unaffected under the normal oxidation conditions. However, aldehydes are somewhat unstable under these conditions, although they do not interfere with the oxidation of organoboranes.