Hydration of Alkenes
Oxidation and Hydrolysis of Alkylboranes
Alkylboranes, the products of hydroboration of alkenes, serve as starting materials for various reactions. One of the most important applications involves a reaction sequence of oxidation and hydrolysis, resulting in an alcohol with anti-Markovnikov orientation. This reaction sequence is carried out as a one-pot reaction by treating, for example, the alkylborane with an aqueous solution of hydrogen peroxide (H2O2) and sodium hydroxide.
Deprotonation of hydrogen peroxide by the hydroxide anion of sodium hydroxide yields the hydroperoxide anion which adds to boron in nucleophilic fashion. Subsequently, a 1,2-alkyl shift from boron to the boron-bonded oxygen occurs with concomitant loss of hydroxide anion, i.e. a B-alkyl bond is replaced by a B-O(alkyl) bond. If the starting material is a trialkylborane, all alkyl groups attached to boron are oxidized in the same manner yielding a boric acid ester. Under the reaction conditions, the spontaneous hydrolysis of the ester produces the alcohol with anti-Markovnikov orientation. The boric acid (B(OH)3) remains as sodium tetrahydridoborate (Na+(B(OH)4)-) in solution.
The overall result of the reaction sequence is the addition of water to a double bond. Contrary to the acid-catalyzed hydration, alcohols with anti-Markovnikov orientation are produced by hydroboration providing a very important regiochemical alternative to other alkene hydrations. In addition, hydroborations show very high regio- (anti-Markovnikov) and stereoselectivity, quickly proceed under mild conditions and because of the absence of a carbenium intermediate rarely produce any rearrangement products.
