Page 816 - Advanced Organic Chemistry Part B - Reactions & Synthesis

792 Higher enantiomeric purity can be obtained by a modified procedure in which the
monoalkylborane intermediate is prepared by reduction of a cyclic boronate. 18
CHAPTER 9
Carbon-Carbon H
Bond-Forming Reactions 1) CH CH O O CH R
of Compounds of Boron, B CH R 3 (HO) B CH 2 R HO(CH 2 ) 3 OH CH R 1) LiAlH 4 H B 2
2
2
Silicon, and Tin 2) NaOH 2 O B 2 H
H R H R 2) Me 3 SiCl R
H R
Subsequent steps involve introduction of a thexyl group and then the second ketone
substituent. Finally, the ketone is formed by the cyanide-TFAA method.

CH R′ 1) NaCN
(CH ) C C(CH ) CH 2 2 O
3 2 3 2 H R′CH CH 2 2) (CF CO) O
3
2
H B CH 2 R B CH R B CH 2 R R′CH CH CH 2 R
2
2
3) H O 2 2
H R H R H R 2 2 H R
By starting with enantiomerically enriched IpcBHCl, it is possible to construct
chiral cyclic ketones. For example, stepwise hydroboration of 1-allylcyclohexene and
ring construction provides trans-1-decalone in greater than 99% e.e. 19
H
1) CH CH O
1) IpcBHCl 3
2) Cl 2 CHOCH 3
BHIpc
– +
2) 0.25 equiv LiAIH 4 3) (CH ) CO K H
3 3
4) H O 2 O >99% e.e.
2

9.1.3. Homologation via -Haloenolates
Organoboranes can also be used to construct carbon-carbon bonds by several
other types of reactions that involve migration of a boron substituent to carbon. One
such reaction involves -halo carbonyl compounds. 20 For example, ethyl bromoac-
etate reacts with trialkylboranes in the presence of base to give alkylated acetic acid
derivatives in excellent yield. The reaction is most efficiently carried out with a 9-BBN
derivative. These reactions can also be effected with -alkenyl derivatives of 9-BBN
to give , -unsaturated esters. 21

B R – OC(CH )
3 3
+ BrCH CO R′ RCH CO R′
2
2
2
2
18 H. C. Brown, R. K. Bakshi, and B. Singaram, J. Am. Chem. Soc., 110, 1529 (1988); H. C. Brown,
M. Srebnik, R. K. Bakshi, and T. E. Cole, J. Am. Chem. Soc., 109, 5420 (1987).
19
H. C. Brown, V. K. Mahindroo, and U. P. Dhokte, J. Org. Chem., 61, 1906 (1996); U. P. Dhokte,
P. M. Pathare, V. K. Mahindroo, and H. C. Brown, J. Org. Chem., 63, 8276 (1998).
20 H. C. Brown, M. M. Rogic, M. W. Rathke, and G. W. Kabalka, J. Am. Chem. Soc., 90, 818 (1968);
H. C. Brown and M. M. Rogic, J. Am. Chem. Soc., 91, 2146 (1969).
21
H. C. Brown, N. G. Bhat, and J. B. Cambell, Jr., J. Org. Chem., 51, 3398 (1986).