Organic Chemistry

Section 5.19 Stereochemistry of Electrophilic Addition Reactions of Alkenes 225

PROBLEM 43

a. What stereoisomers are formed in the following reaction?

b. What stereoisomer is formed in greater yield?

3. Stereochemistry of Hydroboration–Oxidation
   The addition of borane to an alkene is a concerted reaction. The boron and the hydride
   ion add to the two carbons of the double bond at the same time (Section 4.9). Be-
   cause the two species add simultaneously, they must add to the same side of the dou-
   ble bond. So the addition of borane to an alkene, like the addition of hydrogen, is a syn
   addition.

When the alkylborane is oxidized by reaction with hydrogen peroxide and hydrox-
ide ion, the OH group ends up in the same position as the boron group it replaces. Con-
sequently, the overall hydroboration–oxidation reaction amounts to a syn addition of
water to a carbon–carbon double bond.

Because only syn addition occurs, hydroboration–oxidation is stereoselective—only
two of the four possible stereoisomers are formed. As we saw when we looked at the
addition of to a cycloalkene, syn addition results in the formation of only the pair
of enantiomers that has the added groups on the same side of the ring.

PROBLEM 44

What stereoisomers would be obtained from hydroboration–oxidation of the following

compounds?

a. cyclohexene c. 1,2-dimethylcyclopentene

b. 1-ethylcyclohexene d.cis-2-butene

1. BH 3 /THF


2. HO−, H 2 O 2 , H 2 O
   H 3 C CH 3 H 3 C

H OH

+

CH 3

H 3 C

H OH

CH 3

H 2

HO−

H 2 O 2

H 2 O

an alkyl borane

hydroboration–oxidation is

a syn addition of water

an alcohol

HOH

C C

HBH 2

C C

BH 3

HBH 2

C C

H B

H

H

C C

syn addition of borane

CC

sp^2

CH 3

CH 2 + H 2

Pd/C