Page 371 - Advanced Organic Chemistry Part B - Reactions & Synthesis

This provides a mechanism for migration of the boron group along the carbon chain 343
by a series of eliminations and additions.
SECTION 4.5
R R R H R Addition at Double
Bonds via Organoborane
R C CH CH 3 R C CH CH 3 + R C C CH 2 R C CH 2 CH 2 B Intermediates
H B H H
H B H B

Migration cannot occur past a quaternary carbon, however, since the required elimi-
nation is blocked. At equilibrium the major trialkyl borane is the least-substituted
terminal isomer that is accessible, since this isomer minimizes unfavorable steric
interactions.

H C H
3
H CH
160°C 2
B B
3 3 Ref. 173
H
1) B 2 6
CH (CH ) CH CH(CH ) CH 3 [CH (CH ) ] B
2 13
3
2 13
3
2 29 3
2) 80°C, 14 h Ref. 174

Migrations are more facile for tetra-substituted alkenes and occur at 50 –60 C. 175
Bulky substituents on boron facilitate the migration. bis-Bicyclo[2.2.2]octanylboranes,
in which there are no complications from migrations in the bicyclic substituent, were
found to be particularly useful.
Δ
2
3 2
2
B H+ CH CH C(CH ) BCH CH CH(CH )
3 2
3
Ref. 176
There is evidence that boron migration occurs intramolecularly. 177 A TS involving an
electron-deficient
complex about 20–25 kcal above the trialkylborane that describes
the migration has been located computationally. 178

B H B H H B
C C H C C H C C H
H H H

173
G. Zweifel and H. C. Brown, J. Am. Chem. Soc., 86, 393 (1964).
174 K. Maruyama, K. Terada, and Y. Yamamoto, J. Org. Chem., 45, 737 (1980).
175
L. O. Bromm, H. Laaziri, F. Lhermitte, K. Harms, and P. Knochel, J. Am. Chem. Soc., 122, 10218
(2000).
176
H. C. Brown and U. S. Racherla, J. Am. Chem. Soc., 105, 6506 (1983).
177 S. E. Wood and B. Rickborn, J. Org. Chem., 48, 555 (1983).
178
N. J. R. van Eikema Hommes and P. v. R. Schleyer, J. Org. Chem., 56, 4074 (1991).