Advices For Studying Organic Chemistry

11.6 HYDROBORATION: SYNTHESIS OF ORGANOBORANES

1. Hydroboration, discovered by Herbert C. Brown of Purdue University
   (co-winner of the Nobel Prize for Chemistry in 1979), involves an addition of a
   H–B bond (a boron hydride) to an alkene.

CC H B

B

CC

H

Alkene

+ hydroboration

Boron hydride Organoborane

2. Hydroboration can be carried out by using the boron hydride (B 2 H 6 ) called
   diborane.

1. It is much more convenient to use a THF solution of diborane.

B 2 H 6 O (^2) O+ BH− 3
THF

• 2
  BH 3 is a Lewis acid (because
  the boron has only six electrons
  in its valence shell). It accepts
  an electron pair from the
  Diborane THF : BH 3 oxygen atom of THF.

2. Solutions containing the THF:BH 3 complex is commercially available.


3. Hydroboration reactions are usually carried out in ether: either in (C 2 H 5 ) 2 O, or in
   some higher molecular weight ether such as “diglyme” [(CH 3 OCH 2 CH 2 ) 2 O,
   diethylene glycol dimethyl ether].

3. Great care must be used in handling diborane and alkylboranes because they
   ignite spontaneously in air (with a green flame). The solution of THF:BH 3 is
   considerably less prone to spontaneous ignition but still must be used in an
   inert atmosphere and with care.

11.6A MECHANISM OF HYDROBORATION

1. When an 1-alkene is treated with a solution containing the THF:BH 3 complex, the