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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.

Preparation of anti-Markovnikov alcohols from trialkylboranes

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.

Oxidation and hydrolysis of tris-(2-methylbutyl)borane.

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.

Exercise 1

Exercise 2

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