Page 373 - Advanced Organic Chemistry Part B - Reactions & Synthesis

large scale. 180 Conditions that permit oxidation of organoboranes to alcohols using 345
molecular oxygen, 181 sodium peroxycarbonate 182 or amine oxides 183 as oxidants have
SECTION 4.5
also been developed. The reaction with molecular oxygen is particularly effective in
perfluoroalkane solvents. 184 Addition at Double
Bonds via Organoborane
Intermediates
H )
1) HB(C 2 5 2
OH
2) O , Br(CF ) CF 3
2 7
2
82%
More vigorous oxidants such as Cr(VI) reagents effect replacement of boron and
oxidation to the carbonyl level. 185

Ph Ph
1) B H O
2 6
2) K Cr O 7
2
2
An alternative procedure for oxidation to ketones involves treatment of the alkylborane
with a quaternary ammonium perruthenate salt and an amine oxide 186 (see Entry 6 in
Scheme 4.9). Use of dibromoborane-dimethyl sulfide for hydroboration of terminal
alkenes, followed by hydrolysis and Cr(VI) oxidation gives carboxylic acids. 187

1) BHBr 2 S(CH ) Cr(VI)
3 2
RCH CH 2 RCH CH B(OH) 2 RCH CO H
2
2
2
2
2) H O HOAc, H 2 O
2
The boron atom can also be replaced by an amino group. 188 The reagents that effect
this conversion are chloramine or hydroxylamine-O-sulfonic acid, and the mechanism
of these reactions is very similar to that of the hydrogen peroxide oxidation of organo-
boranes. The nitrogen-containing reagent initially reacts as a nucleophile by adding at
boron andaBtoN rearrangement with expulsion of chloride or sulfate ion follows.
Usually only two of the three alkyl groups migrate. As in the oxidation, the migration
step occurs with retention of configuration. The amine is freed by hydrolysis.
NH X – NH X H O
2
2
2
R B R B NH X R B NH RB(NHR) 2 2 RNH 2
2
2
3
R R
X = Cl or OSO 3
180
D. H. B. Ripin, W. Cai, and S. T. Brenek, Tetrahedron Lett., 41, 5817 (2000).
181 H. C. Brown, M. M. Midland, and G. W. Kabalka, J. Am. Chem. Soc., 93, 1024 (1971).
182 G. W. Kabalka, P. P. Wadgaonkar, and T. M. Shoup, Tetrahedron Lett., 30, 5103 (1989).
183
G. W. Kabalka and H. C. Hedgecock, Jr., J. Org. Chem., 40, 1776 (1975); R. Koster and Y. Monta,
Liebigs Ann. Chem., 704, 70 (1967).
184 I. Klement and P. Knochel, Synlett, 1004 (1996).
185
H. C. Brown and C. P. Garg, J. Am. Chem. Soc., 83, 2951 (1961); H. C. Brown, C. Rao, and S. Kulkarni,
J. Organomet. Chem., 172, C20 (1979).
186
M. H. Yates, Tetrahedron Lett., 38, 2813 (1997).
187 H. C. Brown, S. V. Kulkarni, V. V. Khanna, V. D. Patil, and U. S. Racherla, J. Org. Chem., 57, 6173
(1992).
188
M. W. Rathke, N. Inoue, K. R. Varma, and H. C. Brown, J. Am. Chem. Soc., 88, 2870 (1966);
G. W. Kabalka, K. A. R. Sastry, G. W. McCollum, and H. Yoshioka, J. Org. Chem., 46, 4296 (1981).