Advices For Studying Organic Chemistry

boron hydride adds successively to the double bonds of three molecules of the

alkene to form a trialkylborane:

CH 3 CH CH 2

+

H B

B BH

B

H 2

More substituted Less substituted

CH 3 CHCH 2

H

H 2 (CH 3 CH 2 CH 2 ) 2

(CH 3 CH 2 CH 2 ) 2

Tripropylborane

CH 3 CH CH 2

CH 3 CH CH 2

2. The boron atom becomes attached to the less substituted carbon atom of the
   double bond.

1. Hydroboration is regioselective and is anti-Markovnikov.

CH 3 CH 2 CCH 2

1 % 99 %

CH 3 Less substituted

CH 3 CCHCH 3

CH 3

2% 98%

Less substituted

2) The observed regioselectivity of hydroboration results in part from steric

factors –– the bulky boron-containing group can approach the less substituted

carbon atom more easily.

3. Mechanism of hydroboration:
   
   
   1. In the first step, the π electrons of the double bond adds to the vacant p orbital of
      BH 3.
   
   
   2. In the second step, the π complex becomes the addition product by passing
      through a four-center transition state in which the boron atom is partially bonded
      to the less substituted carbon atom of the double bond.
      i) Electrons shift in the direction of the boron atom and away from the more
      substituted carbon atom of the double bond.
      ii) This makes the more substituted carbon atom develop a partial positive charge,
      and because it bears an electron-releasing alkyl group, it is better able to
      accommodate this positive charge.