Final answer:
The hydroboration reaction mechanism features the nucleophilic attack of an alkene on boron from borane, leading to the simultaneous addition of hydrogen to the alkene and the formation of an organoborane intermediate with syn stereochemistry. It is regioselective for the anti-Markovnikov addition of hydrogen to the less substituted carbon.
Step-by-step explanation:
Hydroboration Mechanism
Hydroboration is a reaction that involves the addition of borane (BH3) or a borane derivative to alkenes, which results in the formation of an organoborane intermediate. The mechanism is considered to be regioselective and stereospecific. In this reaction, the boron adds to the least substituted carbon due to its partially positive charge, while the hydrogen adds to the more substituted carbon. Due to the nature of the transition state, this addition occurs with syn stereochemistry, meaning that the hydrogen and boron add to the same side of the alkene.
In the first step, the π electrons of the alkene attack the electrophilic boron atom, forming a four-membered transition state. Subsequently, the boron atom bonds to the least substituted carbon, while a hydrogen atom bonds to the more substituted carbon atom. This addition is concerted and proceeds without the formation of a carbocation, therefore there is neither rearrangement nor tautomerism involved. As a result of the alkene's steric and electronic properties, this reaction is regioselective and leads to the anti-Markovnikov addition of hydrogen. Moreover, since boron is less electronegative than hydrogen, the resulting organoborane is stable enough to be isolated if desired.