HYDROBORATION / OXIDATION

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.