Page 369 - Advanced Organic Chemistry Part B - Reactions & Synthesis

Catecholborane and pinacolborane are especially useful in hydroborations 341
catalyzed by transition metals. 163 Wilkinson’s catalyst Rh PPh Cl is among those
3 3
used frequently. 164 The general mechanism for catalysis is believed to be similar to SECTION 4.5
that for homogeneous hydrogenation and involves oxidative addition of the borane to Addition at Double
Bonds via Organoborane
the metal, generating a metal hydride. 165 Intermediates

C
Cl C C H O
O C C H C B
O
Rh B O L 2 RhCl + C C O
L 2 L Rh B
2
O 2 L Rh B
H H O O
Variation in catalyst and ligand can lead to changes in both regio- and enantio-
selectivity. For example, the hydroboration of vinyl arenes such as styrene and 6-
methoxy-2-vinylnaphthalene can be directed to the internal secondary borane by use
of Rh COD BF as a catalyst. 166 These reactions are enantioselective in the presence
4
2
of a chiral phosphorus ligand.
CH 3 CH 3
CH 3 CH 3 5 mol % Rh(COD) BF Ar CH CH ArCH CH OH
ArCH CH + O O 2 4 3 + 2 2
2
5 mol % Josiphos
B OH
Ar ratio yield e.e.
H
Phenyl 83:17 87% 84%
6-Methoxynaphthyl 95:5 83% 88%

On the other hand, iridium catalysts give very high selectivity for formation of the
primary borane. 167 Several other catalysts have been described, including, for example,
dimethyltitanocene. 168

catechol- O NaOH
borane RCH CH OH
RCH CH 2 RCH 2 CH 2 B H O 2 2
(Cp) Ti(CH ) O 2 2
3 2
2
5
(Cp = η – C H )
5 5
Catalyzed hydroboration has proven to be valuable in controlling the stereose-
lectivity of hydroboration of functionalized alkenes. 169 For example, allylic alcohols

163
I. Beletskaya and A. Pelter, Tetrahedron, 53, 4957 (1997); H. Wadepohl, Angew. Chem. Int. Ed. Engl.,
36, 2441 (1997); K. Burgess and M. J. Ohlmeyer, Chem. Rev., 91, 1179 (1991); C. M. Crudden and
D. Edwards, Eur. J. Org. Chem., 4695 (2003).
164 D. A. Evans, G. C. Fu, and A. H. Hoveyda, J. Am. Chem. Soc., 110, 6917 (1988); D. Maenning and
H. Noeth, Angew. Chem. Int. Ed. Engl., 24, 878 (1985).
165
D. A. Evans, G. C. Fu, and B. A. Anderson, J. Am. Chem. Soc., 114, 6679 (1992).
166
C. M. Crudden, Y. B. Hleba, and A. C. Chen, J. Am. Chem. Soc., 126, 9200 (2004).
167 Y. Yamamoto, R. Fujikawa, T. Unemoto, and N. Miyaura, Tetrahedron, 60, 10695 (2004).
168 X. He and J. F. Hartwig, J. Am. Chem. Soc., 118, 1696 (1996).
169
D. A. Evans, G. C. Fu, and A. H. Hoveyda, J. Am. Chem. Soc., 114, 6671 (1992).