8.8: Hydroboration-Oxidation of Alkenes
In addition to the oxymercuration–demercuration method, which converts the alkenes to alcohols with Markovnikov orientation, a complementary hydroboration-oxidation method yields the anti-Markovnikov product. The hydroboration reaction, discovered in 1959 by H.C. Brown, involves the addition of a B–H bond of borane to an alkene giving an organoborane intermediate. The oxidation of this intermediate with basic hydrogen peroxide forms an alcohol.
Borane as a reagent is very reactive, as the boron atom has only six electrons in its valence shell. The unoccupied 2p orbital of the boron is perpendicular to the plane, which is occupied by the boron and the three other hydrogens oriented at an angle of 120°. Thus, borane is electrophilic with its structure resembling a carbocation without any charge.
Due to high reactivity, two borane molecules dimerize such that two hydrogen atoms are partially bonded to two boron atoms with a total of two electrons. Hence, they are called three-center, two-electron bonds. Diborane co-exists in equilibrium with a small amount of borane.
The electron-deficient borane easily accepts an electron pair from tetrahydrofuran to complete its octet forming a stable borane-ether complex. This is used as a reagent in hydroboration reactions under an inert atmosphere to avoid spontaneous ignition in the air.
The mechanism starts with a borane attacking the π bond at the less substituted and sterically less hindered site of an alkene forming a cyclic transition state. The overall result is a syn-addition of BH2 and hydrogen across the alkene double bond, producing an alkylborane. The reaction of a second alkene with the alkylborane yields a dialkylborane followed by the addition of a third alkene to produce a trialkylborane.
The oxidation begins with the deprotonation of hydrogen peroxide by a hydroxide ion forming a hydroperoxide. The hydroperoxide acts as a nucleophile and attacks the trialkylborane, resulting in an unstable intermediate. This is followed by the migration of an alkyl group from boron to the adjacent oxygen atom, releasing a hydroxide ion. This series of three steps is repeated to convert the remaining trialkylborane into a trialkoxyborane.
The boron atom of the trialkoxyborane is attacked by the nucleophilic hydroxide ion with the subsequent departure of the alkoxide ion neutralizing the formal charge on the boron atom. Finally, protonation of the alkoxide ion gives an alcohol as the final product.