Hydroboration

Hydroboration general reaction

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Hydroboration orbitals and transition state 01

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Hydroboration mechanism and transition state

Regiospecific hydroboration with borane-(1)

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Hydroboration-Oxidation of (E)-prop-1-en-1-ylbenzene

Hydroboration is a chemical reaction that involves the addition of borane (BH₃) or its derivatives to alkenes and alkynes to form organoboranes, compounds in which a carbon-boron bond is formed. This reaction is significant in organic chemistry for the synthesis of organoboranes, which can be further converted into a variety of functional groups. The hydroboration process is stereospecific, typically resulting in the addition of boron to the less substituted carbon in the double bond due to its mechanism.

Mechanism

The mechanism of hydroboration involves the concerted addition of a boron atom and a hydrogen atom across the carbon-carbon double bond of alkenes or the triple bond of alkynes. This process occurs in a syn-addition manner, meaning that the boron and hydrogen add to the same side of the double or triple bond. The mechanism can be divided into three steps:

1. The alkene or alkyne approaches the borane.
2. A four-center transition state is formed, where the electrons from the carbon-carbon multiple bond are used to form a bond with the boron atom and one of the boron's hydrogen atoms.
3. The organoborane product is formed with the addition of the boron atom to the less substituted carbon due to steric and electronic reasons.

Applications

Hydroboration has several applications in organic synthesis. One of the most important is the conversion of the resulting organoboranes to alcohols, a process known as hydroboration-oxidation. This reaction involves two steps: the hydroboration of the alkene or alkyne, followed by oxidation with hydrogen peroxide (H₂O₂) in the presence of a base, typically sodium hydroxide (NaOH). The outcome is the formation of an alcohol where the boron was attached, with the regiochemistry of the reaction being anti-Markovnikov.

Another application is in the synthesis of amines through the amination of organoboranes, as well as in the preparation of various other functional groups. Hydroboration is also used in the homologation of carbonyl compounds and in the stereoselective synthesis of compounds.

Variants

Several variants of hydroboration exist, utilizing different boron-containing reagents. These include:

• Disiamylborane (Sia₂BH), used for the selective hydroboration of terminal alkenes.
• 9-Borabicyclo[3.3.1]nonane (9-BBN), which offers high selectivity and stability.
• Catalytic hydroboration, which uses a catalyst to facilitate the reaction under milder conditions or with substrates that are less reactive.

Safety and Environmental Considerations

Handling boranes requires caution due to their toxicity and flammability. Proper safety measures, including the use of appropriate personal protective equipment and working in a well-ventilated area or under an inert atmosphere, are essential. The environmental impact of boranes and their derivatives should also be considered, with efforts made to minimize waste and exposure.

See Also

• Organic synthesis
• Addition reaction
• Stereochemistry
• Regiochemistry