Organic Chemistry

Section 4.9 Addition of Borane: Hydroboration–Oxidation 165

carbon for the same reason: to form the more stable carbocation or carbocation-
like transition state.

The alkylborane formed in the first step of the reaction reacts with another mole-
cule of alkene to form a dialkylborane, which then reacts with yet another molecule of
alkene to form a trialkylborane. In each of these reactions, boron adds to the car-
bon bonded to the greater number of hydrogens and the nucleophilic hydride ion adds
to the other carbon.

The alkylborane is a bulkier molecule than because R is a larger sub-
stituent than H. The dialkylborane with two R groups is even bulkier than the
alkylborane. Thus, there are now two reasons for the alkylborane and the dialkylbo-
rane to add to the carbon that is bonded to the greater number of hydrogens: first,
to achieve the most stable carbocation-like transition state, and second, because there
is more roomat this carbon for the bulky group to attach itself. Steric effectsare
space-filling effects. Steric hindrancerefers to bulky groups at the site of the reaction
that make it difficult for the reactants to approach each other. Steric hindrance associ-
ated with the alkylborane—and particularly with the dialkylborane—causes the addi-
tion to occur at the carbon that is bonded to the greater number of hydrogens
because that is the least sterically hindered of the two carbons. Therefore, in each
of the three successive additions to the alkene, boron adds to the carbon that is
bonded to the greater number of hydrogens and adds to the other carbon.
When the hydroboration reaction is over, aqueous sodium hydroxide and hydrogen
peroxide are added to the reaction mixture. Notice that both hydroxide ion and hy-
droperoxide ion are reagents in the reaction:

The end result is replacement of boron by an OH group. Because replacing boron by
an OH group is an oxidation reaction, the overall reaction is called hydroboration–
oxidation. An oxidation reactionincreases the number of C O, C N, or C X
bonds in a compound (where X denotes a halogen), or it decreases the number of
C H bonds.

HO−, H 2 O 2 , H 2 O

ROH BO 3

R

R

R

B

3 −

3 +

¬

¬ ¬ ¬

HOOH+HO-∆HOO-+H 2 O

H- sp^2

sp^2

sp^2

sp^2

sp^2

(R 2 BH)

(RBH 2 ) BH 3

an alkylborane

a dialkylborane

CH 3 CH CH 2 + BH 2

a dialkylborane a trialkylborane

CH 3 CH CH 2 BH

CH 3 CH

+

CH 2 BH R

H

CH 3 CH CH 2 BR

R H R

R

R

sp^2

sp^2

sp^2

CH 3 CCH

H H

BH 2

δ+

CH 3 CCH

H H

H

less stable

transition state

δ+

more stable

transition state

addition of BH 3 addition of HBr

δ−

CH 3 CCH

H H

Br H

δ+

more stable

transition state

δ−

CH 3 CCH

H H

H Br

δ+

less stable

transition state

H H (^2) δ−B δ−
Oxidation decreases the number of
C H bonds or increases the number of
C¬O, C¬N, or C¬X bonds.
¬