Hüdroboratsioonoksüdatsioon

Hydroboration-oxidation is a two-step reaction in organic chemistry used to convert alkenes and alkynes into alcohols. It proceeds with anti-Markovnikov regioselectivity and syn stereoselectivity. The first step, hydroboration, involves the addition of a borane reagent (such as BH3 or its derivatives) across the double or triple bond. Boron, being the more electronegative element in the B-H bond, acts as the electrophile and attacks the less substituted carbon of the alkene/alkyne, while hydrogen attacks the more substituted carbon. This results in the formation of an organoborane intermediate. This step is stereoselective, with the boron and hydrogen adding to the same face of the pi system, hence a syn addition. The regioselectivity is anti-Markovnikov, meaning the boron atom attaches to the less substituted carbon, due to steric and electronic factors.

The second step, oxidation, typically uses hydrogen peroxide (H2O2) in the presence of a base (like NaOH). The peroxide anion attacks the boron atom, leading to the displacement of the alkyl group from boron and the formation of an alkyl hydroperoxide. This intermediate then rearranges, with the alkyl group migrating from boron to oxygen, displacing the hydroxide ion. Finally, the resulting borate ester is hydrolyzed by the base to yield the alcohol and borate salts. The overall transformation results in the hydration of the alkene or alkyne with the hydroxyl group ending up on the less substituted carbon and the hydrogen on the more substituted carbon. This method is valuable for synthesizing alcohols where Markovnikov addition is not desired or is difficult to achieve through other means.