Page 819 - Advanced Organic Chemistry Part B - Reactions & Synthesis

Treatment of alkenyldialkylboranes with iodine results in the formation of the Z-alkene 795
with migration of one boron substituent. 26
SECTION 9.1
– I R I Organoboron
R B H H I R H R B H H Compounds
2
I 2 RB R H C C
C C C C RB C C C C R
H R H I R H I H I R R
+ R
Similarly, alkenyllithium reagents add to dimethyl boronate to give adducts that
decompose to Z-alkenes on treatment with iodine. 27

Li H OCH 3 R R ′
I 2
RB(OCH ) + C C RB – CH CHR ′ C C
3 2
H R ′ OCH 3 H H

The synthesis of Z-alkenes can also be carried out starting with an alkylbromoborane, in
which case migration presumably follows replacement of the bromide by methoxide. 28

R′BBr H H H
MeO –
R′BHBr + HC CR C C I C C
H R 2 R′ R

The stereoselectivity of these reactions arises from a base-induced anti elimination
after the migration. The elimination is induced by addition of methoxide to the boron,
generating an anionic center.

R′
MeO MeO – H H
H (MeO) B – H R′ H (MeO) 2 B H
2
R′ B – (MeO) B R H R R′
2
R I R
H + R H I R′
MeO I
H
E-Alkenes can be prepared by several related reactions. 29 Hydroboration of
a bromoalkyne generates an -bromoalkenylborane. On treatment with methoxide
ion these intermediates undergo B → C migration to give an alkyl alkenylborinate.
Protonolysis generates an E-alkene.
OCH 3
R Br – R BR′ R H
2
3
RC CBr + R′ BH OMe CH CO H
2
H BR′ 2 H R' H R′
26 G. Zweifel, H. Arzoumanian, and C. C. Whitney, J. Am. Chem. Soc., 89, 3652 (1967); G. Zweifel,
R. P. Fisher, J. T. Snow, and C. C. Whitney, J. Am. Chem. Soc., 93, 6309 (1971).
27
D. A. Evans, T. C. Crawford, R. C. Thomas, and J. A. Walker, J. Org. Chem., 41, 3947 (1976).
28 H. C. Brown, D. Basavaiah, S. U. Kulkarni, N. G. Bhat, and J. V. N. Vara Prasad, J. Org. Chem., 53,
239 (1988).
29
H. C. Brown, D. Basavaiah, S. U. Kulkarni, H. P. Lee, E. Negishi, and J.-J. Katz, J. Org. Chem., 51,
5270 (1986).