ORGANIC
LETTERS
2003
Vol. 5, No. 8
1221-1223
Samarium(II)-Mediated Pinacol Coupling
in Water: Occurrence of Unexpected
Disproportionation and Action of
Low-Valent Samarium as an Active
Species
Satoru Matsukawa* and Yumi Hinakubo
Department of Science, Faculty of Education, Ibaraki UniVersity,
Mito, Ibaraki 310-8512, Japan
Received January 24, 2003
ABSTRACT
Mechanistic studies of one-electron reduction in water using samarium were carried out. Unexpected disproportionation in water was observed
via UV−vis spectroscopic analysis. This fact indicates that low-valent samarium species can exist in water. Furthermore, the SmCl3−Sm and
SmCl3−Mg systems were found to act as good one-electron reducing agents in water.
The use of samarium as a single electron transfer reagent in
organic synthesis has been developed, and numerous ex-
amples, such as coupling, reduction, cyclization, and the
Barbier reaction, have been widely studied and utilized.1,2
On the other hand, the use of water as a solvent for organic
reactions has been attracting attention because of its advan-
tages with regard to environmental concerns, and numerous
successful reactions have been reported.3,4 Samarium-medi-
ated reactions in aqueous media have also been reported;
however, the mechanism underlying these reactions remains
unclear.5 To develop this environmentally benign method,
we investigated the mechanism of these samarium-mediated
reactions. A low-valent samarium species was found to exist
in water and act as a one-electron reducing agent.
Initially, we conducted UV-vis spectroscopic analysis to
obtain information on the reaction intermediates.6 UV-vis
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Press: London, Great Britain, 1994. (d) Molander, G. A.; Harris, C. R.
Chem. ReV. 1996, 96, 307.
(2) Reviews on the use of Sm powder in organic synthesis: (a) Banik,
B. K. Eur. J. Org. Chem. 2002, 2421. Also see: (b) Yanada, R.; Bessyo,
K.; Yanada, K. Chem. Lett. 1994. 1279. (c) Yanada, R.; Negoro, N.; Yanada,
K.; Fujita, T. Tetrahedron Lett. 1996, 37, 9313. (d) Banik, B. K.;
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1998, 39, 7243. (e) Ogawa, A.; Takeuchi, H.; Hirao, T. Tetrahedron Lett.
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Tetrahedron 1999, 55, 13947.
(3) For reviews on organic reactions in water, see: (a) Li, C.-J. Chem.
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1994, 741. (c) Li, C.-J.; Chen, T.-H. Organic Reactions in Aqueous Media;
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Grieco, P. A., Ed.; Blackie Academic & Professional: Glasgow, 1998. (e)
Li, C.-J.; Chen, T.-H. Tetrahedron. 1999, 55, 11149.
(4) For recent reports, see: (a) Wei, C.; Li, C.-J. J. Am. Chem. Soc.
2002, 124, 5639. (b) Miura, K.; Nakagawa, T.; Hosomi, A. J. Am. Chem.
Soc. 2002, 124, 537. (c) Kobayashi, S.; Hamada, T.; Manabe, K. J. Am.
Chem. Soc. 2002, 124, 5641. (d) Miyabe, H.; Ueda, M.; Nishimura, A.;
Naito, T. Org. Lett. 2002, 4, 131. (e) Lautens, M.; Mancuso, J. Org. Lett.
2002, 4, 2105. (f) Leadbeater, N. E.; Marco, M. Org. Lett. 2002, 4, 2973.
(g) Zhang, J.; Li, C.-J. J. Org. Chem. 2002, 67, 3969.
(5) (a) Wang, L.; Zhang, Y. Tetrahedron Lett. 1998, 39, 5257. (b) Wang,
L.; Zhang, Y. Tetrahedron 1999, 55, 10695. (c) Talukdar, S.; Fang, J.-M.
J. Org. Chem. 2001, 66, 330. Reaction in MeOH, see: (d) Ghatak, A.;
Becker, F. F.; Banik, B. K. Tetrahedron Lett. 2000, 41, 3793. (e) Yu, C.;
Liu, B.; Hu, L. J. Org. Chem. 2001, 66, 919.
(6) Flowers reported mechanistic studies of Sm(II) species including
UV-vis analysis of SmCl2, SmBr2, and SmI2. (a) Miller, R. S.; Sealy, J.
M.; Shabangi, M.; Kuhlman, M. L.; Fuchs, J. R.; Flowers, R. A., II. J. Am.
Chem. Soc. 2000, 122, 7718. Also see: (b) Shabangi, M.; Sealy, J. M.;
Fuchs, J. R.; Flowers, R. A., II. Tetrahedron Lett. 1998, 39, 4429. (c)
Kuhlman, M. L.; Flowers, IR. A., II. Tetrahedron Lett. 2000, 41, 8049. (d)
Knetlle, B. W.; Flowers, R. A., II. Org. Lett. 2001, 3, 2321.
10.1021/ol0341344 CCC: $25.00 © 2003 American Chemical Society
Published on Web 03/27/2003