Hydroboration Mechanism And Transition State

In organic chemistry, hydroboration refers to the addition of a hydrogen-boron bond to certain double and triple bonds involving carbon (, , , and ). This chemical reaction is useful in the organic synthesis of organic compounds.

Hydroboration produces organoborane compounds that react with a variety of reagents to produce useful compounds, such as alcohols, amines, or alkyl halides. The most widely known reaction of the organoboranes is oxidation to produce alcohols from alkenes.

The development of this technology and the underlying concepts were recognized by the Nobel Prize in Chemistry to Herbert C. Brown.

Borane adducts

Much of the original work on hydroboration employed diborane as a source of BH₃. Usually however, borane dimethylsulfide complex BH₃S(CH₃)₂ (BMS) is used instead. It can be obtained in highly concentrated forms.

The adduct BH₃(THF) is also commercially available as THF solutions. Its shelf life is less than BMS.

In terms of synthetic results, diborane or the more conveniently handle BMS and borane-THF are equivalent.

Hydroboration of alkenes

The stoichiometry and idealized regiochemistry of hydroboration of terminal alkenes follows:
:
In reality, each hydroboration step follows 1,2-addition but ca. 4% gives the 2,1 addition (affording the B(CH(CH3)R isomer).
In extreme cases, such as risubstituted alkenes, hydroboration affords. This significant rate difference in producing di- and tri-alkyl boranes is useful in the synthesis of bulky boranes that can enhance regioselectivity.

In terms of regiochemistry, hydroboration is typically anti-Markovnikov, i.e. the hydrogen adds to the most substituted carbon of the double bond. That the regiochemistry is reverse of a typical HX addition reflects the polarity of the B^δ+-H^δ− bonds. Hydroboration proceeds via a four-membered transition state: the hydrogen and the boron atoms added on the same face of the double bond. Granted that the mechanism is concerted, the formation of the C-B bond proceeds slightly faster than the formation of the C-H bond. As a result, in the transition state, boron develops a partially negative charge while the more substituted carbon bears a partially positive charge. This partial positive charge is better supported by the more substituted carbon. Formally, the reaction is an example of a group transfer reaction. However, an analysis of the orbitals involved reveals that the reaction is 'pseudopericyclic' and not subject to the Woodward–Hoffmann rules for pericyclic reactivity.

Hydroboration of internal alkenes

Hydroboration of trisubstituted alkenes places boron on the less substituted carbon.

Hydroboration of 1,2-disubstituted alkenes, such as a ''cis'' or ''trans'' olefin, produces generally a mixture of the two organoboranes of comparable amounts, even if the steric properties of the substituents are very different. For such 1,2-disubstituted olefins, regioselectivity can be observed only when one of the two substituents is a phenyl ring. In such cases, such as ''trans''-1-phenylpropene, the boron atom is placed on the carbon adjacent to the phenyl ring. The observations above indicate that the addition of H-B bond to olefins is under electronic control rather than steric control.

Hydroboration of alkynes

Hydroboration of alkynes gives alkenylboranes. The stereochemistry is cis-addition. With terminal alkynes, both and are formed. Often the hydroboration of alkynes use bulky boranes such as 9-BBN to give monoalkenylborane products. The alkenylboranes are susceptible to many reactions such as protonolysis to give the alkene and oxidation to give the aldehyde or ketone.

Reactions of organoboranes

As honored by the Nobel Prize to Brown, hydroboration is widely practiced because the alkylboranes are susceptible to many reactions.

Oxidation

Treatment of alkylboranes with base and hydrogen peroxide gives alcohols:

The net reaction is hydration.

Because the addition of H-B to olefins is stereospecific, this oxidation reaction will be diastereoselective when the alkene is trisubstituted. Hydroboration-oxidation is thus an excellent way of producing alcohols in a stereospecific and anti-Markovnikov fashion.

Other C-heteroatom bond forming reactions

Hydroboration can also lead to amines by treating the intermediate organoboranes with monochloramine or O-hydroxylaminesulfonic acid (HSA).

Terminal olefins are converted to the corresponding alkyl bromides and alkyl iodides by treating the organoborane intermediates with bromine or iodine. Such reactions have not however proven very popular, because succinimide based reagents such as NIS and NBS are more versatile and do not require rigorous conditions as do organoboranes.
etc.

Carbonylations

Trialkylboranes react with carbon monoxide to afford homologated products such as 2-bora-1,3-dioxolanes. When the addition of CO is conducted in the presence of a hydride reducing agent, the primary alcohol is produced.

Specialty boranes for hydroboration

One example of a monoalkylborane is thexylborane (ThxBH₂), produced by the hydroboration of tetramethylethylene:
:B₂H₆ + 2 Me₂C=CMe₂ → e₂CHCMe₂BH₂sub>2
A chiral example is monoisopinocampheylborane. Although often written as IpcBH₂, it is a dimer pcBH₂sub>2. It is obtained by hydroboration of (−)‐α‐pinene with borane dimethyl sulfide.

Monobromo- and monochloro-borane can be prepared from BMS and the corresponding boron trihalides. The stable complex of monochloroborane and 1,4-dioxane effects hydroboration of terminal alkenes.

Prominent among hindered dialkylboranes is disiamylborane, abbreviated Sia₂BH. It also is a dimer. Owing to its steric bulk, it selectively hydroborates less hindered, usually terminal alkenes in the presence of more substituted alkenes. Disiamylborane must be freshly prepared as its solutions can only be stored at 0 °C for a few hours. Dicyclohexylborane Chx₂BH exhibits improved thermal stability than Sia₂BH.

A versatile dialkylborane is 9-BBN. Also called "banana borane", it exists as a dimer. Reactions with 9-BBN typically occur at 60–80 °C, with most alkenes reacting within one hour. Tetrasubstituted alkenes add 9-BBN at elevated temperature. Hydroboration of alkenes with 9-BBN proceeds with excellent regioselectivity. It is more sensitive to steric differences than Sia₂BH, perhaps because of it rigid C₈ backbone. 9-BBN is more reactive towards alkenes than alkynes.

For catalytic hydroboration, pinacolborane and catecholborane are widely used. They also exhibit higher reactivity toward alkynes. Pinacolborane is also widely used in a catalyst-free hydroborations.

See also

* Carboboration
* metal-catalysed hydroboration

References

{{Alkenes
Organic reactions