Page 380 - Advanced Organic Chemistry Part B - Reactions & Synthesis

352 4.5.4. Hydroboration of Alkynes

CHAPTER 4 Alkynes are reactive toward hydroboration reagents. The most useful procedures
Electrophilic Additions involve addition of a disubstituted borane to the alkyne, which avoids complications
to Carbon-Carbon that occur with borane and lead to polymeric structures. Catechol borane is a partic-
Multiple Bonds
ularly useful reagent for hydroboration of alkynes. 212 Protonolysis of the adduct with
acetic acid results in reduction of the alkyne to the corresponding cis-alkene. Oxidative
workup with hydrogen peroxide gives ketones via enol intermediates.
D H

CH CO D R R′
3
2
O
–
O O H O , OH HO H
2
2
BH + RC CR′ RCCH R′
2
O O B H R R′
R R′ Br 2
Br R′
– OCH 3
R H
Treatment of the vinylborane with bromine and base leads to vinyl bromides. The
reaction occurs with net anti addition, and the stereoselectivity is explained on the
basis of anti addition of bromine followed by a second anti elimination of bromide
and boron.
L B H Br Br H L B H R H
2
2
2
R R R
L B
R R 2 Br Br Br Br R
R
Exceptions to this stereoselectivity have been noted. 213
The adducts derived from catechol borane are hydrolyzed by water to vinylboronic
acids. These materials are useful intermediates for the preparation of terminal vinyl
iodides. Since the hydroboration is a syn addition and the iodinolysis occurs with
retention of the alkene geometry, the iodides have the E-configuration. 214

O H O (HO )B H I 2 I H
2
2
O B H H R H R
H R
The dimethyl sulfide complex of dibromoborane 215 and pinacolborane 216 are also
useful for synthesis of E-vinyl iodides from terminal alkynes.
–
– + Br 2 B H 1) OH, H O I H
Br BH S(CH ) + HC CR 2
2
3 2
–
H R 2) OH, I 2 H R
212 H. C. Brown, T. Hamaoka, and N. Ravindran, J. Am. Chem. Soc., 95, 6456 (1973); C. F. Lane and
G. W. Kabalka, Tetrahedron, 32, 981 (1976).
213
J. R. Wiersig, N. Waespe-Sarcevic, and C. Djerassi, J. Org. Chem., 44, 3374 (1979).
214 H. C. Brown, T. Hamaoka, and N. Ravindran, J. Am. Chem. Soc., 95, 5786 (1973).
215 H. C. Brown and J. B. Campbell, Jr., J. Org. Chem., 45, 389 (1980); H. C. Brown, T. Hamaoka,
N. Ravindran, C. Subrahmanyam, V. Somayaji, and N. G. Bhat, J. Org. Chem., 54, 6075 (1989).
216
C. E. Tucker, J. Davidson, and P. Knochel, J. Org. Chem., 57, 3482 (1992).