8.8 : アルケンのヒドロホウ素化酸化

In addition to oxymercuration-reduction, hydroboration-oxidation is another significant reaction that converts an alkene into an alcohol. However, unlike oxymercuration-reduction, hydroboration-oxidation proceeds with anti-Markovnikov's regioselectivity.

This is a two-step transformation. The first step is hydroboration, in which borane dissolved in tetrahydrofuran adds to an alkene to form an organoborane intermediate, followed by the oxidation of the intermediate using hydrogen peroxide to yield an alcohol.

The boron atom in borane has a vacant 2p orbital and an incomplete octet, thereby making it electron-deficient and electrophilic.

Monomeric borane is unstable and readily dimerizes into diborane. However, it can be stabilized in a solvent, such as tetrahydrofuran, which donates an electron pair into the vacant 2p boron orbital, forming a stable boron-ether complex.

The hydroboration mechanism proceeds with the attack on borane by the alkene π electrons in a concerted manner. This leads to a transition state where boron is partially bonded to the less substituted and less sterically hindered carbon.

The net result is a syn-addition of BH₂ and hydrogen across the alkene double bond, forming an alkylborane.

Successive addition of two alkene molecules produces a trialkylborane.

The second part of the reaction is oxidation. Here, a hydroxide ion deprotonates the hydrogen peroxide to form a hydroperoxide, which further acts as a nucleophile and attacks the trialkylborane to yield an unstable intermediate.

Next, the migration of an alkyl group from boron to the adjacent oxygen atom results in the expulsion of a hydroxide ion to form an alkoxyborane. These steps are repeated to convert the trialkylborane into a trialkoxyborane.

The boron atom of the trialkoxyborane further undergoes a nucleophilic attack by another hydroxide ion. Lastly, the departure of the alkoxide ion followed by its protonation produces the target alcohol.

Thus, the hydroboration-oxidation of 2-methyl-2-butene, being syn stereoselective, forms a pair of enantiomeric 3-methyl-2-butanol molecules with anti-Markovnikov's regioselectivity. In contrast, the lack of stereoselectivity in oxymercuration-reduction yields 2-methyl-2-butanol in syn and anti configuration as per Markovnikov's regioselectivity.