HYDROBORATION / OXIDATION
Hydroboration-Oxidation
is a two step pathway used to produce alcohols.
The
reaction proceeds in an Anti-Markovnikov manner, where the hydrogen
(from BH3 or BHR2) attaches to the
more substituted carbon and the
boron attaches to the least
substituted carbon in the double bond.
Borane acts as a lewis acid by accepting two electrons in its empty
p orbital from an alkene that is electron rich.
This process allows boron to have an
electron octet.
The
Hydroboration mechanism has the elements of both hydrogenation
and electrophilic addition and it is a
stereospecific (syn addition), meaning
that
the hydroboration takes place on the same face of the double bond,
this leads cis stereochemistry.
THE OVERALL REACTION OF HYDROBORATION
AND OXIDATION IS SHOWN BELOW:
refer to my post on hydration reacction an electrophillic addition for further clarity url: http://chemistryjee.blogspot.in/2014/01/hydration-of-alkenes.html
The reagent is usually
borane, BH3, but an organoborane can also be used, as long as at
least one B-H bond is present.
In the laboratory, the borane-THF complex,
dissoved in tetrahydrofuran (THF) is often used.
All three hydrogens of borane
are usable.
For simplicity, we will usually designate the
borane as R2BH, where R can be alkyl or hydrogen.
The initial product of
addition of borane or an organoborane across a carbon-carbon pi bond is an
organoborane, where a new B-C bond has been made, along with a new C-H bond.
These two bonds are formed and the B-H and C-C pi bonds are broken, all in
concert, i.e., in a single reaction step with no intermediates being involved.
These organoboranes
are not stable in air, reacting rather rapidly with oxygen.
Instead of
isolating them, they are normally treated in situ (i.e., in
place) with alkaline hydrogen peroxide, a treatment which converts the B-C bond
to a C-O bond (and a B-O bond).
But we will note that the overall result of these two
steps (hydroboration plus oxidation) is to convert an alkene to an alcohol.
This reaction was discovered by Professor H.C. Brown of Purdue University and
is an important enough synthetic conversion that he was awarded the Nobel Prize
for Chemistry primarily based upon this work.
We should note that
the net addition of water which occurs during hydroboration/oxidation is in
the anti-Markovnikov regiochemical sense, with propene giving 1-propanol, rather
than the 2-propanol which is generated by the acid catalyzed, electrophilic
hydration mechanism.
The second step (oxidation of the borane to the alcohol) takes
place with retention of configuration.
Hydroperoxide ion adds to
the borane, causing the alkyl group to
migrate from boron to oxygen.
The alkyl group migrates with retention of configuration because it moves with its electron pair and does not alter the
tetrahedral structure of the migrating carbon atom.
Hydrolysis of the borate ester gives the alcohol.
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