Return to Contents Chapter 6: Reactions of Alkenes: Addition Reactions Ch 6 contents

Hydroboration / Oxidation of Alkenes

hydroboration of C=C

Reaction type: Electrophilic Addition

Summary

Reactivity of Borane (and related reagents):
electrostatic potential of borane 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

 
mechanism for addition of BH3
  • 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 due to a combination of electronic and steric effects (larger B atom at the less hindered end).
  • 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)

Related reactions

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.
addition of HBr to C=C
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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organic chemistry © Dr. Ian Hunt, Department of Chemistry University of Calgary