Hydroboration–oxidation reaction is a two-step hydration reaction that converts an

alkene In organic chemistry, an alkene is a hydrocarbon containing a carbon–carbon double bond. Alkene is often used as synonym of olefin, that is, any hydrocarbon containing one or more double bonds.H. Stephen Stoker (2015): General, Organic, an ...

into an alcohol. The process results in the syn addition of a hydrogen and a

hydroxyl In chemistry, a hydroxy or hydroxyl group is a functional group with the chemical formula and composed of one oxygen atom covalently bonded to one hydrogen atom. In organic chemistry, alcohols and carboxylic acids contain one or more hydroxy ...

group where the

double bond In chemistry, a double bond is a covalent bond between two atoms involving four bonding electrons as opposed to two in a single bond. Double bonds occur most commonly between two carbon atoms, for example in alkenes. Many double bonds exist betwee ...

had been. Hydroboration–oxidation is an anti-Markovnikov reaction, with the hydroxyl group attaching to the less-substituted carbon. The reaction thus provides a more stereospecific and complementary regiochemical alternative to other hydration reactions such as acid-catalyzed addition and the oxymercuration–reduction process. The reaction was first reported by

Herbert C. Brown Herbert Charles Brown (May 22, 1912 – December 19, 2004) was an American chemist and recipient of the 1979 Nobel Prize in Chemistry for his work with organoboranes. Life and career Brown was born Herbert Brovarnik in London, to Ukrainian Jewis ...

in the late 1950s and it was recognized in his receiving the

Nobel Prize in Chemistry ) , image = Nobel Prize.png , alt = A golden medallion with an embossed image of a bearded man facing left in profile. To the left of the man is the text "ALFR•" then "NOBEL", and on the right, the text (smaller) "NAT•" then "M ...

in 1979. The general form of the reaction is as follows: Hydroboration-oxidation reaction

Tetrahydrofuran Tetrahydrofuran (THF), or oxolane, is an organic compound with the formula (CH2)4O. The compound is classified as heterocyclic compound, specifically a cyclic ether. It is a colorless, water- miscible organic liquid with low viscosity. It is ...

(THF) is the archetypal

solvent A solvent (s) (from the Latin '' solvō'', "loosen, untie, solve") is a substance that dissolves a solute, resulting in a solution. A solvent is usually a liquid but can also be a solid, a gas, or a supercritical fluid. Water is a solvent for ...

used for hydroboration.

Mechanism and scope

Hydroboration step

In the first step,

borane Trihydridoboron, also known as borane or borine, is an unstable and highly reactive molecule with the chemical formula . The preparation of borane carbonyl, BH3(CO), played an important role in exploring the chemistry of boranes, as it indicated ...

(BH₃) adds to the

, transferring one of the hydrogen atoms to the carbon adjacent to the one that becomes bonded to the

boron Boron is a chemical element with the symbol B and atomic number 5. In its crystalline form it is a brittle, dark, lustrous metalloid; in its amorphous form it is a brown powder. As the lightest element of the ''boron group'' it has th ...

. This

hydroboration 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 p ...

is repeated two additional times, successively reacting each B–H bond so that three alkenes add to each BH₃. The resulting trialkylborane is treated with hydrogen peroxide in the second step. This process replaces the B-C bonds with HO-C bonds. The boron reagent is converted to

boric acid Boric acid, more specifically orthoboric acid, is a compound of boron, oxygen, and hydrogen with formula . It may also be called hydrogen borate or boracic acid. It is usually encountered as colorless crystals or a white powder, that dissolve ...

. The reaction was originally described by H.C. Brown in 1957 for the conversion of 1-hexene into 1-hexanol. Hydroboration-oxidation-Brown-1957

Knowing that the group containing the boron will be replaced by a hydroxyl group, it can be seen that the initial hydroboration step determines the regioselectivity. Hydroboration proceeds in an anti-Markovnikov manner. The reaction sequence is also stereospecific, giving syn addition (on the same face of the alkene): the hydroboration is syn-selective and the oxidation replaces the boron with hydroxyl having the same geometric position. Thus 1-methylcyclopentene reacts with diborane predominantly to give ''trans''-1-hydroxy-2-methylcyclopentane—the newly added H and OH are ''cis'' to each other. Until all hydrogens attached to boron have been transferred away, the boron group BH₂ will continue adding to more alkenes. This means that one mole of hydroborane will undergo the reaction with three moles of alkene. Furthermore, it is not necessary for the hydroborane to have more than one hydrogen. For example, reagents of the type R₂BH are commonly used, where R can represents the remainder of the molecule. Such modified hydroboration reagents include

9-BBN 9-Borabicyclo .3.1onane or 9-BBN is an organoborane compound. This colourless solid is used in organic chemistry as a hydroboration reagent. The compound exists as a hydride-bridged dimer, which easily cleaves in the presence of reducible substr ...

, catecholborane, and

disiamylborane Disiamylborane (bis(1,2-dimethylpropyl)borane, Sia2BH) is an organoborane used in organic synthesis. It is used for hydroboration–oxidation reactions of terminal alkynes, giving aldehydes via anti-Markovnikov hydration followed by tautomerizati ...

Oxidation step

In the second step of the reaction sequence, the nucleophilic

hydroperoxide Hydroperoxides or peroxols are compounds containing the hydroperoxide functional group (ROOH). If the R is organic, the compounds are called organic hydroperoxides. Such compounds are a subset of organic peroxides, which have the formula ROOR. ...

anion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conve ...

attacks the boron atom. Alkyl migration to oxygen gives the alkyl borane with retention of stereochemistry (in reality, the reaction occurs via the trialkyl

borate A borate is any of several boron oxyanions, negative ions consisting of boron and oxygen, such as orthoborate , metaborate , or tetraborate ; or any salt with such anions, such as sodium metaborate, and disodium tetraborate . The name also re ...

B(OR)₃, rather than the monoalkyl borinic ester BH₂OR). Hydroboration-oxidation-mechanism

The 'H' atom in the reaction comes from B₂H₆, the 'O' atom comes from hydrogen peroxide (H₂O₂) whereas the O attached 'H' atom comes from the solvent (refer mechanism).

Alkyne hydroboration

A hydroboration reaction also takes place on alkynes. Again the mode of action is ''syn'' and secondary reaction products are

aldehyde In organic chemistry, an aldehyde () is an organic compound containing a functional group with the structure . The functional group itself (without the "R" side chain) can be referred to as an aldehyde but can also be classified as a formyl grou ...

s from terminal alkynes and ketones from internal alkynes. In order to prevent hydroboration across both the pi-bonds, a bulky borane like disiamyl (di-sec-iso-amyl) borane is used.

Alternative oxidations

Use of other oxidants instead of hydrogen peroxide can lead to carbonyl products rather than alcohols from alkenes. ''N''-Methylmorpholine ''N''-oxide with catalytic

tetrapropylammonium perruthenate Tetrapropylammonium perruthenate (TPAP or TPAPR) is the chemical compound described by the formula N(C3H7)4RuO4. Sometimes known as the Ley–Griffith reagent, this ruthenium compound is used as a reagent in organic synthesis. This salt consists ...

converts the alkylborane into a carbonyl, thus a ketone or

product depending on what other groups were attached to that carbon in the original alkene. Various dichromates or related chromium(VI) reagents give ketones as well, but give carboxylic acids instead of aldehydes for terminal alkenes.

References

External links

* Organic Chemistry Portal. Hydroboration (including recent literature)
https://www.organic-chemistry.org/namedreactions/brown-hydroboration.shtm
{{DEFAULTSORT:Hydroboration-oxidation reaction Addition reactions