Page 811 - Advanced Organic Chemistry Part B - Reactions & Synthesis

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sodium borohydride or lithium borohydride. The product of the first migration step 787
is reduced and subsequent hydrolysis gives a primary alcohol.
SECTION 9.1
B – C O + Organoboron
R 3
Compounds
100 –125°C
NaBH 4
100°C H 2 O
OH "O B CR "
3
OH
R B CHR H O , OH
2
2
2
RB CR 2
H O, OH
2
HO R 3 COH
RCH OH
2
H 2 O 2
O
RCR
In this synthesis of primary alcohols, only one of the three groups in the organo-
borane is converted to product. This disadvantage can be overcome by using a dialkyl-
borane, particularly 9-BBN, in the initial hydroboration. (See p. 338 to review the
abbreviations of some of the common boranes.) After carbonylation and B → C
migration, the reaction mixture can be processed to give an aldehyde, an alcohol, or
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the homologated 9-alkyl-BBN. The utility of 9-BBN in these procedures is the result
of the minimal tendency of the bicyclic ring to undergo migration.

– HOCH 2 CH 2 CH 2 R
OH
OH
H LiAIH B CH 2 CH 2 CH 2 R
B B CH 2 CH 2 R B CHCH 2 CH 2 R
RCH CH 2 KBH(O-i-Pr) 3 –20°C
CO
H 2 O 2
O CHCH 2 CH 2 R
Several alternative procedures have been developed in which other reagents
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replace carbon monoxide as the migration terminus. The most generally applicable of
these methods involves the use of cyanide ion and trifluoroacetic anhydride (TFAA).
In this reaction the borane initially forms an adduct with cyanide ion. The migration is
induced by N-acylation of the cyano group by TFAA. Oxidation and hydrolysis then
give a ketone.
O
2
(CF 3 CO) O – +
3
R B + – CN R 3 B – C N R B C N CCF 3
3
O O O CCF 3 H O
+ 2 2 O
–
R B C N CCF 3 R B C N CCF 3 R B N
3
2
C RCR
R
R R
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M. W. Rathke and H. C. Brown, J. Am. Chem. Soc., 89, 2740 (1967).
10 H. C. Brown, E. F. Knights, and R. A. Coleman, J. Am. Chem. Soc., 91, 2144 (1969); H. C. Brown,
T. M. Ford, and J. L. Hubbard, J. Org. Chem., 45, 4067 (1980).
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H. C. Brown and S. M. Singh, Organometallics, 5, 998 (1986).