Summary.

• Overall transformation :  C=C to H-C-C-OH
• Reagents (two steps) 1. BH₃ or B₂H₆ then 2) NaOH/ H₂O₂
• Regioselectivity : Anti-Markovnikov, since the B is the electrophile.
• Stereoselectivity : Syn since the C-B and C-H bonds form simultaneously from the BH₃.
• The alcohol is formed over a series of steps involving the B center (see below), with retention of configuration at the C.
• Compliments simple hydration with opposite regiochemistry and stereospecificity.

The image shows the electrostatic potential for borane, BH3.  The more red an area is, the higher the electron density and the more blue an area is, the lower the electron density. The low electron density (blue) is on B atom, after all, it has an incomplete octet. The high electron density (red) is on the H atoms. Therefore : d- H-B d+ , the B is the electrophile.

Borane reacts with alkenes via a concerted mechanism Simultaneous making of C-B and C-H bonds as C=C and B-H break.  Electrophilic B atom adds at the least substituted end of the alkene. Although a carbocation is not involved, the reaction occurs as if the electrophilic B atom were making the more stable carbocation. A CHIME image of the transition state for the reaction with propene is shown to the right.

MECHANISM FOR REACTION OF ALKENES WITH BH3
Step 1: A concerted reaction. The p electrons act as the nucleophile with the electrophilic B and the H is transferred to the C with syn stereochemistry.
Step 2: First step repeats twice more so that all of the B-H bonds react with C=C
Step 3: Peroxide ion reacts as the nucleophile with the electrophilic B atom.
Step 4: Migration of C-B bond to form a C-O bond and displace hydroxide. Stereochemistry of C center is retained.
Step 5: Attack of hydroxide as a nucleophile with the electrophilic B displacing the alkoxide.
Step 6: An acid / base reaction to form the alcohol.

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