Inorganic chemistry,hydroboration refers to the addition of ahydrogen-boron bond to certaindouble andtriple bonds involvingcarbon (C=C,C=N,C=O, andC≡C). Thischemical reaction is useful in theorganic synthesis oforganic compounds.[1]
Hydroboration producesorganoborane compounds that react with a variety of reagents to produce useful compounds, such asalcohols,amines, oralkyl halides. The most widely known reaction of the organoboranes isoxidation to produce alcohols from alkenes.
The development of this technology and the underlying concepts were recognized by theNobel Prize in Chemistry toHerbert C. Brown.[2][3]
Much of the original work on hydroboration employeddiborane as a source of BH3. Usually however,borane dimethylsulfide complex BH3S(CH3)2 (BMS) is used instead.[5] It can be obtained in highly concentrated forms.[6]
Theadduct BH3(THF) is also commercially available as THF solutions. Its shelf life is less than BMS.[7]
In terms of synthetic results, diborane or the more conveniently handle BMS and borane-THF are equivalent.
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).[1]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 typicallyanti-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 Bδ+-Hδ− 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 agroup transfer reaction. However, an analysis of the orbitals involved reveals that the reaction is 'pseudopericyclic' and not subject to theWoodward–Hoffmann rules forpericyclic reactivity.
Hydroboration of trisubstitutedalkenes places boron on the less substituted carbon.[8]
Hydroboration of 1,2-disubstituted alkenes, such as acis ortrans 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 astrans-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 gives alkenylboranes. The stereochemistry is cis-addition. With terminal alkynes, bothH2BCH=HR andHB(CH=CHR)2 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.[9]
As honored by the Nobel Prize to Brown, hydroboration is widely practiced because the alkylboranes are susceptible to many reactions.
Treatment of alkylboranes with base andhydrogen peroxide gives alcohols:
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The net reaction is hydration.
Because the addition of H-B to olefins is stereospecific, this oxidation reaction will bediastereoselective when the alkene is trisubstituted.[10] Hydroboration-oxidation is thus an excellent way of producing alcohols in a stereospecific and anti-Markovnikov fashion.
Hydroboration can also lead to amines by treating the intermediate organoboranes withmonochloramine or O-hydroxylaminesulfonic acid (HSA).[11]
Terminal olefins are converted to the correspondingalkyl bromides andalkyl iodides by treating the organoborane intermediates withbromine[12] or iodine.[13] Such reactions have not however proven very popular, becausesuccinimide based reagents such as NIS and NBS are more versatile and do not require rigorous conditions as do organoboranes.etc.
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.
One example of a monoalkylborane isthexylborane (ThxBH2), produced by the hydroboration oftetramethylethylene:[14]
A chiral example is monoisopinocampheylborane. Although often written as IpcBH2, it is a dimer [IpcBH2]2. It is obtained by hydroboration of (−)‐α‐pinene withborane dimethyl sulfide.[15]
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.[16]
Prominent among hindered dialkylboranes isdisiamylborane, 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.[17] 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 is9-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.[18]
For catalytic hydroboration,pinacolborane andcatecholborane are widely used. They also exhibit higher reactivity toward alkynes.[19] Pinacolborane is also widely used in a catalyst-free hydroborations.