Chapter 6: Reactions of Alkenes: Addition Reactions

Hydroboration / Oxidation of Alkenes

Summary

• Overall transformation :  C=C to H-C-C-OH
• Reagents (two steps) 1. BH₃ or B₂H₆ then 2) aq. NaOH / H₂O₂
• Electrophile : B atom
• Regioselectivity : Anti-Markovnikov, since the B in ultimately converted to the -OH.
• 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 it is stereospecific.
• More sterically hindered dialkyl boranes (i.e. R₂BH) such as 9-BBN can also be used, typically with increased regioselectivity due to increased steric effects.

The image shows the electrostatic potential map 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. Electronegativities: B = 2.0, H = 2.1 Therefore : d- H-B d+ , the B is the electrophilic center and the H is the nucleophilic center. QUESTION : How many electrons are there around the B in borane ? ANSWER

Borane (and realated systems) react with alkenes via a concerted mechanism. This requires simultaneous making of both the C-B and C-H bonds and breaking of the C=C and B-H bonds.  Electrophilic B atom adds at the least substituted end of the alkene due to a combination of electronic and steric effects (larger B atom/group at the less hindered 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 (think of d+ developing on the C that best stabilises the charge). A JSMOL image of the transition state for the reaction of BH3 with propene is shown to the right. The image shows the reaction key features.... concerted formation of B-C and C-H bonds and breaking of C=C and B-H, regiochemistry (B (pink) at the least substituted end of the alkene). On a more advanced level, the process is asynchronous, meaning that the extent of the bonding changes are slightly different. Here it shows the C-B bond formation ahead of the C-H bond formation (indicated by the bond lengths).

Related reactions

• Hydration of alkenes

MECHANISM FOR REACTION OF ALKENES WITH BH3
Step 1: A concerted reaction. The π 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.

© Dr. Ian Hunt, Department of Chemistry