Scheme 3
Table 1. Hydroboration of 1,1-Disubstituted Fluorinated
Olefins with Boranes of Varying Electronic and Steric
Environments
olefin
reagent
solvent
time, h
yield, % 3°/1°-ola
1a
1a
1a
1a
1a
1b
1b
BH3•THF
HBCl2
n-HexBHCl
ChxBHCl
ThxBHCl
HBCl2
THF
5
82
90
72
73
65
86
78
14/86
15/85
7/93
5/95
1/99
8/92
1/99
hexane
hexane
hexane
hexane
hexane
hexane
inst.
4
6
6
1
6
(Scheme 3). Such regioselective hydroborations at a tertiary
position are rare.7 The generality of this observation was
demonstrated by hydroborating 1i-k. In all of these cases,
we obtained >99% of the 3°-alcohol.
ThxBHCl
a Determined by a combination of 1H NMR, 19F NMR, and GC analyses.
Clearly, boron is placed on the carbon with the perfluo-
roalkyl substituent as long as the difference in the degree of
substitution is e 1 as in a 2° vs 1°, 2° vs 2°, or 3° vs 2°
carbon. When the selection is between a 3° and a 1° carbon,
the sterics surrounding the carbon overtake the electronics
of the perfluoroalkyl substituent.
provided the R-fluoroalkyl(aryl) alcohols (2c-g)4 almost
exclusively in 76-89% isolated yields. Even the presence
of a phenyl group (1d, 1g) did not alter the regioselectivity
(Scheme 2).
It is known that the difference in sterics between a 3° and
a 2° carbon is considerably more than the difference between
a 2° and a 1° carbon.8 Yet, the perfluoroakyl substituent
controls the regioselectivity of hydroboration. Brown and
Chandrasekharan have reported that haloboranes are very
sensitive to minor electronic differences.9 The hydroboration
of fluoroolefins with dichloroborane provides another ex-
ample of this phenomenon.
Scheme 2
In conclusion, we have examined the hydroboration of
substituted fluoroolefins and encountered a rare example of
the formation of 3°-alcohols by stoichiometric hydrobora-
tion-oxidation.
The hydroboration of 1,1,2-trisubstituted perfluoroalkyl
olefins revealed an exceptional reaction! On the basis of the
results in Schemes 1 and 2, we expected that a combination
of the steric and electronic effects in trisubstituted olefins
would direct the boron to reside on both of the carbons. There
have been reports that the stereoelectronic requirements of
perfluoroalkyl groups are quite large in the various reactions
studied.5 Accordingly, we anticipated a slight preference for
the â-carbon with respect to the fluoroalkyl moiety. However,
the hydroboration of 6,6,7,7,8,8,9,9,9-nonafluoro-5-n-pro-
pylnon-4-ene (1h) with HBCl2 furnished exclusively, by 1H
NMR analysis, the 3°-alcohol (2h)6 in 82% isolated yield
Acknowledgment. We thank the Herbert C. Brown
Center for Borane Research10 and the Eastman Kodak Co.
for their generous financial support.
Supporting Information Available: A typical experi-
mental procedure and spectral data for compounds 1a-b,
1d-h, 1j-k, 2a-d, 2f-h, and 2j. This material is available
OL016779E
(6) (a) Thurkauf, A.; Costa, B.; Yamaguchi, S.; Mattson, M. V.; Jacobson,
A. E.; Rice, K. C.; Rogowski, M. A. J. Med. Chem. 1990, 33, 1452. (b)
Rong, G.; Keese, R. Tetrahedron Lett. 1990, 31, 5617.
(7) The hydroboration of R-methylstyrene with catecholborane catalyzed
by [Rh(acac)(DPPB)] providing 95:5 regioselectivity in favor of the 3°-
derivative has been reported. Westcott, S. A.; Marder, T. B.; Baker, R. T.
Organometallics 1993, 12, 975.
(4) (a) Sibille, S.; Mcharek, S.; Perichon, J. Tetrahedron 1989, 45, 1423.
(b) Hayase, T.; Sugiyama, T.; Suzuki, M.; Shibata, T.; Soai, K. J. Fluorine
Chem. 1997, 84, 1.
(5) (a) Bott, G.; Field, L. D. Sternhell, S. J. Am. Chem. Soc. 1980, 102,
5618. (b) Mosher, H. S.; Stevenot, J. E.; Kimble, D. O. J. Am. Chem. Soc.
1956, 78, 4374. (c) Ramachandran, P. V. Teodorovic, A. V.; Brown, H. C.
Tetrahedron 1993, 49, 1725.
(8) Brown, H. C.; Barbaras, G. K. J. Chem. Phys. 1946, 14, 114.
(9) Brown, H. C.; Chandrasekharan, J. J. Org. Chem. 1983, 48, 644.
(10) Contribution no.14 from the Herbert C. Brown Center for Borane
Research
3790
Org. Lett., Vol. 3, No. 23, 2001