Page 544 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

R′CH CH 2 525
R 2 BX + 0.25 LiAlH 4 R 2 BH R 2 BCH 2 CH 2 R′
R′CH CH 2
+ 0.50 LiAlH 4 RBHCH 2 CH 2 R′ SECTION 5.7
RBCl 2 RBH 2
Synthesis and Reactions
of Alkylboranes
Catecholborane 201 and pinacolborane, 202 in which the boron has two oxygen
substituents,aremuchlessreactivehydroboratingreagentsthanalkylorhaloboranes.The
boronelectrondeficiencyisattenuatedbytheoxygenatoms.Nevertheless,theyareuseful
reagents for certain applications. The reactivity of catecholborane has been found to be
substantially enhanced by addition of 10–20% of N N-dimethylacetamide to CH Cl . 203
2
2
CH 3
O
CH 3
O B H
B H
CH 3 O
O
CH 3
catechol borane pinacol borane

Hydroboration is thermally reversible. At 160 C and above, B−H moieties are
eliminated from alkylboranes, but the equilibrium is still in favor of the addition
products. This provides a mechanism for migration of the boron group along the carbon
chain by a series of eliminations and additions.
R H R H R H R H
R C CCH 2 CH 3 R C CCH 2 CH 3 R C C CHCH 3 R C CCH CH 2
H BR' 2 H H H BR' 2 H H
R' 2 B H R' B H
2
R H
R C CCH CH BR'
2 2 2
H H
Migration cannot occur past a quaternary carbon, however, since the required elimi-
nation is blocked. At equilibrium, the major trialkylborane is the least-substituted
terminal isomer that is accessible, because it is the one that minimizes unfavorable
steric interactions. The availability of the isomerization provides a means for thermo-
dynamic control of the hydroboration reaction.

CH 3 CH 3
CH 3 CH 3
160°C
H
CH 3
B CH 2 B
Ref. 204
1) B 2 H 6
CH 3 (CH 2 ) 13 CH CH(CH 2 ) 13 CH 3 [CH 3 (CH 2 ) 28 CH 2 ] 3 B
2) 80 °C, 14 hr
Ref. 205
201 H. C. Brown and S. K. Gupta, J. Am. Chem. Soc., 97, 5249 (1975); H. C. Brown and J. Chandrasekharan,
J. Org. Chem., 48, 5080 (1983).
202
C. E. Tucker, J. Davidson, and P. Knochel, J. Org. Chem., 57, 3482 (1992).
203 C. E. Garrett and G. C. Fu, J. Org. Chem., 61, 3224 (1996).
204 G. Zweifel and H. C. Brown, J. Am. Chem. Soc., 86, 393 (1964).
205
K. Maruyama, K. Terada, and Y. Yamamoto, J. Org. Chem., 45, 737 (1980).