Hydroboration–oxidation reaction is a two-step
hydration reaction
In chemistry, a hydration reaction is a chemical reaction in which a substance combines with water. In organic chemistry, water is added to an unsaturated substrate, which is usually an alkene or an alkyne. This type of reaction is employed indust ...
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 ...
into an
alcohol
Alcohol most commonly refers to:
* Alcohol (chemistry), an organic compound in which a hydroxyl group is bound to a carbon atom
* Alcohol (drug), an intoxicant found in alcoholic drinks
Alcohol may also refer to:
Chemicals
* Ethanol, one of sev ...
. The process results in the
syn addition
In organic chemistry, syn- and anti-addition are different ways in which substituent molecules can be added to an alkene or alkyne. The concepts of syn and anti addition are used to characterize the different reactions of organic chemistry by refl ...
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 hydrox ...
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
In chemistry, stereospecificity is the property of a reaction mechanism that leads to different stereoisomeric reaction products from different stereoisomeric reactants, or which operates on only one (or a subset) of the stereoisomers."Overlap Con ...
and complementary
regiochemical alternative to other hydration reactions such as
acid-catalyzed
In acid catalysis and base catalysis, a chemical reaction is catalyzed by an acid or a base. By Brønsted–Lowry acid–base theory, the acid is the proton (hydrogen ion, H+) donor and the base is the proton acceptor. Typical reactions 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:
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 (BH
3) adds to 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 ...
, 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 t ...
. This
hydroboration is repeated two additional times, successively reacting each B–H bond so that three alkenes add to each BH
3. 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. The reaction was originally described by
H.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 1957 for the conversion of
1-hexene into
1-hexanol.
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
In chemistry, stereospecificity is the property of a reaction mechanism that leads to different stereoisomeric reaction products from different stereoisomeric reactants, or which operates on only one (or a subset) of the stereoisomers."Overlap Con ...
, giving
syn addition
In organic chemistry, syn- and anti-addition are different ways in which substituent molecules can be added to an alkene or alkyne. The concepts of syn and anti addition are used to characterize the different reactions of organic chemistry by refl ...
(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 BH
2 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 R
2BH are commonly used, where R can represents the remainder of the molecule. Such modified hydroboration reagents include
9-BBN,
catecholborane
Catecholborane (abbreviated HBcat) is an organoboron compound that is useful in organic synthesis. This colourless liquid is a derivative of catechol and a borane, having the formula C6H4O2BH.
Synthesis and structure
Traditionally catecholborane ...
, and
disiamylborane.
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 B(OR)
3, rather than the monoalkyl borinic ester BH
2OR).
The 'H' atom in the reaction comes from B
2H
6, the 'O' atom comes from hydrogen peroxide (H
2O
2) whereas the O attached 'H' atom comes from the solvent (refer mechanism).
Alkyne hydroboration
A hydroboration reaction also takes place on
alkyne
\ce
\ce
Acetylene
\ce
\ce
\ce
Propyne
\ce
\ce
\ce
\ce
1-Butyne
In organic chemistry, an alkyne is an unsaturated hydrocarbon containing at least one carbon—carbon triple bond. The simplest acyclic alkynes with only one triple bond and n ...
s. 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 gro ...
s from terminal alkynes and
ketone
In organic chemistry, a ketone is a functional group with the structure R–C(=O)–R', where R and R' can be a variety of carbon-containing substituents. Ketones contain a carbonyl group –C(=O)– (which contains a carbon-oxygen double b ...
s 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 converts the alkylborane into a carbonyl, thus a ketone or
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 gro ...
product depending on what other groups were attached to that carbon in the original alkene. Various
dichromates or related
chromium
Chromium is a chemical element with the symbol Cr and atomic number 24. It is the first element in group 6. It is a steely-grey, lustrous, hard, and brittle transition metal.
Chromium metal is valued for its high corrosion resistance and hard ...
(VI) reagents give ketones as well, but give
carboxylic acid
In organic chemistry, a carboxylic acid is an organic acid that contains a carboxyl group () attached to an R-group. The general formula of a carboxylic acid is or , with R referring to the alkyl, alkenyl, aryl, or other group. Carboxyli ...
s 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