ORGANIC
LETTERS
2
007
Vol. 9, No. 22
427-4429
Expanding the Scope of Trialkylborane/
Water-Mediated Radical Reactions
4
†
‡
Matthew R. Medeiros, Laura N. Schacherer, David A. Spiegel, and
John L. Wood*
Department of Chemistry, Colorado State UniVersity, Fort Collins, Colorado 80523
Received July 13, 2007
ABSTRACT
The dehalogenation of alkyl iodides, as well as the chemoselective deoxygenation of a secondary alcohol in the presence of various alkyl and
aryl halides, can be accomplished employing a trialkylborane/air/water system.
Functional group interconversions involving deamination,
dehalogenation, deoxygenation, and other methods are
ubiquitous in synthetic chemistry, and many useful proce-
dures to accomplish these transformations have been devel-
oped. Recently, we, and others, described the application
of Lewis acid activated water as a hydrogen atom transfer
reagent. Efforts to expand the scope of our previously
disclosed radical deoxygenation reaction to dehalogenation
and the fortuitous manifestation of a chemoselective deoxy-
genation are presented herein.
were unsuccessful (Table 1, entries 2 and 3) but not
unexpected. Typical reactions of triethylborane with alkyl
4a
bromides involve R-bromoesters, where a stabilized R-car-
bonyl radical is formed after halogen atom abstraction.
However, given that examples of iodine atom abstraction
from simple alkyl iodides are prevalent in the literature,10
we next turned to the deiodination of alkyl iodides. In the
event, attempts to dehalogenate alkyl iodides using tribu-
tylborane and water proved fruitful. The deiodination of
1
2
3
(4) (a) Usugi, S.; Yorimitsu, H.; Shinokubo, H.; Oshima, K. Bull. Chem.
Triethylborane has been applied to halogen atom abstrac-
tion reactions to mediate intermolecular radical additions to
Soc. Jpn. 2002, 75, 2049-2052. (b) Usugi, S.; Yorimitsu, H.; Shinokubo,
H.; Oshima, K. Bull. Chem. Soc. Jpn. 2002, 75, 2687-2690. (c) Baciocchi,
E.; Muraglia, E. Tetrahedron Lett. 1993, 34, 5015-5018. (d) Baciocchi,
E.; Muraglia, E. Tetrahedron Lett. 1994, 35, 2763-2766. (e) Miura, K.;
Takeyama, Y.; Oshima, K.; Utimoto, K. Bull. Chem. Soc. Jpn. 1991, 64,
1542-1553. (f) Nakamura, T.; Yorimitsu, H.; Shinokubo, H.; Oshima, K.
Synlett 1998, 1351-1352. (g) Suzuki, A.; Nozawa, S.; Harada, M.; Itoh,
M.; Brown, H. C.; Midland, M. M. J. Am. Chem. Soc. 1971, 93, 1508-
4
alkenes and alkynes, oxygenation of R-iodocarbonyl com-
5
6
pounds, carboazidation of olefins, as well as intramolecular
7
cyclization/atom transfer reactions. This precedent, coupled
with our desire to expand the application of organoboranes
8
to radical reactions, encouraged us to attempt the dehalo-
1
509.
(5) Kihara, N.; Ollivier, C.; Renaud, P. Org. Lett. 1999, 1, 1419-
1422.
genation of alkyl halides.
Our initial deoxygenation studies using trialkylborane9/
water mixtures to reduce simple alkyl bromide substrates
(
6) Panchaud, P.; Renaud, P. J. Org. Chem. 2004, 69, 3205-3207.
(7) Yorimitsu, H.; Shinokubo, H.; Oshima, K. Synlett 2002, 674-686.
(8) For a comprehensive discussion of organoborane involvement in
†
Department of Chemistry, Columbia University, New York, NY, 10027.
Department of Chemistry, Yale University, New Haven, CT, 06520.
radical reactions, see: (a) Ollivier, C.; Renaud, P. Chem. ReV. 2001, 101,
3415-3434. (b) Renaud, P.; Beauseigneur, A.; Brecht-Forster, A.; Becattini,
B.; Darmency, V.; Kandhasamy, S.; Montermini, F.; Ollivier, C.; Panchaud,
P.; Pozzi, D.; Scanlan, E. M.; Schaffner, A.-P.; Weber, V. Pure Appl. Chem.
2007, 79, 223-233.
(9) Tributylborane was used in the experiments described herein due to
its reduced pyrophoricity compared to triethylborane. Trimethylborane was
employed due to our prior success using it for deoxygenation of alcohols
(ref 2a).
‡
(
1) Imamoto, T.; McCombie, S. W.; Fry, A. J. In ComprehensiVe Organic
Synthesis; Trost, B. M., Fleming, I., Eds.; Pergamon Press: Oxford, 1991.
2) Spiegel, D. A.; Schacherer, L. N.; Medeiros, M. R.; Wood, J. L. J.
Am. Chem. Soc. 2005, 127, 11056.
3) (a) For evidence of a â-alkylcatecholborane/alcohol complex, see:
Pozzi, D.; Scanlan, E. M.; Renaud, P J. Am. Chem. Soc. 2005, 127, 12067.
b) For evidence of the activation of water for hydrogen atom transfer by
(
(
(
other Lewis acids, see: Cuerva, J. M.; Campa n˜ a, A. G.; Justicia, J.; Rosales,
A.; Oller-L o´ pez, J. L.; Robles, R.; C a´ rdenas, D. J.; Bu n˜ uel, E.; Oltra, J. E.
Angew. Chem., Int. Ed. 2006, 45, 5522-5526.
(10) (a) Miyabe, H.; Yamaoka, Y.; Takemoto, Y. J. Org. Chem. 2005,
70, 3324-3327. (b) Liu, J.-Y.; Jang, Y.-J.; Lin, W.-W.; Liu, J.-T.; Yao,
C.-F. J. Org. Chem. 2003, 68, 4030-4038.
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0.1021/ol7016609 CCC: $37.00
© 2007 American Chemical Society
Published on Web 09/28/2007