Organic Chemistry

BH 3

disiamylborane

248 CHAPTER 6 Reactions of Alkynes • Introduction to Multistep Synthesis

PROBLEM 12

Draw all the enol tautomers for each of the ketones in Problem 11.

6.7 Addition of Borane: Hydroboration–Oxidation

Borane adds to alkynes in the same way it adds to alkenes. That is, one mole of

reacts with three moles of alkyne to form one mole of boron-substituted alkene

(Section 4.9). When the addition reaction is over, aqueous sodium hydroxide and hy-

drogen peroxide are added to the reaction mixture. The end result, as in the case of

alkenes, is replacement of the boron by an OH group. The enol product immediately

rearranges to a ketone.

In order to obtain the enol as the product of the addition reaction, only one equiva-

lent of can be allowed to add to the alkyne. In other words, the reaction must stop

at the alkene stage. In the case of internal alkynes, the substituents on the boron-

substituted alkene prevent the second addition from occurring. However, there is less

steric hindrance in a terminal alkyne, so it is harder to stop the addition reaction at the

alkene stage. A special reagent called disiamylborane has been developed for use with

terminal alkynes (“siamyl”stands for secondary iso amyl; amyl is a common name for

a five-carbon fragment). The bulky alkyl groups of disiamylborane prevent a second

addition to the boron-substituted alkene. So borane can be used to hydrate internal

alkynes, but disiamylborane is preferred for the hydration of terminal alkynes.

The addition of borane (or disiamylborane) to a terminal alkyne exhibits the same re-

gioselectivity seen in borane addition to an alkene. Boron, with its electron-seeking

empty orbital, adds preferentially to the spcarbon bonded to the hydrogen. In the sec-

ond step of the hydroboration–oxidation reaction, boron is replaced by an OH group.

The overall reaction is anti-Markovnikovaddition because the hydrogen does notadd to

the spcarbon bonded to the greater number of hydrogens. In hydroboration–oxidation,

is not the electrophile, is the nucleophile. The reaction, however,doesfollow

the general rule for electrophilic addition reactions: The electrophile adds to the

spcarbon bonded to the greater number of hydrogens. Consequently, mercuric-ion-

catalyzed addition of water to a terminal alkyne produces a ketone(the carbonyl group

(BH 3 )

H+ H≠-

BH 3

R

R

3 CH 3 C CCH 3 + BH (^3) H
THF 2 O
HO−, H 2 O 2
B
3
OH
O
3 CH 3 CH 2 CCH 3
boron-substituted alkene
an enol
H 3 C
H
CH 3
CC
H 3 C
H
CH 3
CC
BH 3
Addition of water to a terminal alkyne
forms a ketone.
3-D Molecule:
Disiamylborane
an enol
CH 3 CH 3
CH CHCH 3
O
CH 3 CH 2 CH 2 CH
CH 3 CH 2 CCH+ BH H
2 O
HO−, H 2 O 2
bis(1,2-dimethylpropyl)borane
disiamylborane
CH 3
2
CH 3
CH 3 CH CH
B 2
CH 3 CH 2
H
H
CC
OH
CH 3 CH 2
H
H
CC
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