ORGANIC
LETTERS
2
005
Vol. 7, No. 23
167-5170
Benzylic Oxidation Catalyzed by
Dirhodium(II,III) Caprolactamate
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Arthur J. Catino, Jason M. Nichols, Hojae Choi, Sidhartha Gottipamula, and
Michael P. Doyle*
Department of Chemistry and Biochemistry, UniVersity of Maryland,
College Park, Maryland 20742
Received August 18, 2005
ABSTRACT
Dirhodium caprolactamate [Rh
Sodium bicarbonate is the optimal base additive for substrate conversion. Benzylic carbonyl compounds are readily obtained, and a formal
synthesis of palmarumycin CP using this methodology is described.
2 4
(cap) ] is an effective catalyst for benzylic oxidation with tert-butyl hydroperoxide (TBHP) under mild conditions.
2
Selective oxidative functionalization of hydrocarbons using
transition metal catalysis is a long-standing goal in organic
process development. Because of the stabilization offered
2 4
We have recently reported that Rh (cap) catalyzes the
allylic oxidation of olefins in conjunction with TBHP. In
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to reaction intermediates, allylic and benzylic oxidations have
been preferred targets, and peroxide-based oxidants have
(4) For Cr-catalyzed procedures, see: (a) Pearson, A. J.; Han, G. R. J.
Org. Chem. 1985, 50, 2791. (b) Muzart, J. Tetrahedron Lett. 1986, 27,
2
3
139. (e) Rathore, R.; Saxena, N.; Chandrasekaran, S. Synth. Commun. 1986,
been the reagents of choice.3 Several transition-metal-
catalyzed processes in conjunction with TBHP have been
reported for benzylic oxidation,4 which are generally less
facile than allylic oxidations. However, reported benzylic
oxidations have utilized only a limited number of substrates,
and a general catalytic protocol capable of oxidizing a wide
16, 1493. (c) Muzart, J. Tetrahedron Lett. 1987, 28, 2131. (d) Muzart, J.;
Ajjou, A. N. A. J. Mol. Catal. 1991, 66, 155. (e) Choudary, B. M.; Prasad,
A. D.; Bhuma, V.; Swapna, V. J. Org. Chem. 1992, 57, 5841. (f) Das, T.
K.; Chaudhari, K.; Nandanan, E.; Chandwadkar, A. J.; Sudalai, A.;
Ravindranathan, T.; Sivasanker, S. Tetrahedron Lett. 1997, 38, 3631. (g)
Rothenberg, G.; Wiener, H.; Sasson, Y. J. Mol. Catal. A: Chem. 1998, 136,
-7
2
53.
(
5) For Co-catalyzed procedures, see: (a) Modica, E.; Bombieri, G.;
8
Colombo, D.; Marchini, N.; Ronchetti, F.; Scala, A.; Toma, L. Eur. J. Org.
Chem. 2003, 2964. (b) Jurado-Gonzalez, M.; Sullivan, A. C.; Wilson, J. R.
H. Tetrahedron Lett. 2003, 44, 4283.
(6) For Ru-catalyzed procedures, see: (a) Murahashi, S.; Oda, Y.; Naota,
T.; Kuwabara, T. Tetrahedron Lett. 1993, 34, 1299. (b) Nikalje, M. D.;
Sudalai, A. Tetrahedron 1999, 55, 5903.
(7) For Mn-catalyzed procedures, see: (a) Blay, G.; Fern a´ ndez, I.;
Gim e´ nez, T.; Pedro, J. R.; Ruiz, R.; Pardo, E.; Lloret, F.; Mu n˜ oz, M. C.
Chem. Commun. 2001, 2102. (b) Pan, J. F.; Chen, K. M. J. Mol. Catal. A:
Chem. 2001, 176, 19.
(8) For recent work, see: (a) Ishii, Y.; Nakayama, K.; Takeno, M.;
Sakaguchi, S.; Iwahama, T.; Nishiyama, Y. J. Org. Chem. 1995, 60, 3934.
(b) Yamazaki, S. Org. Lett. 1999, 1, 2129. (c) Nicolaou, K. C.; Baran, P.
S.; Zhong, Y. L. J. Am. Chem. Soc. 2001, 123, 3183. (d) Lee, S.; Fuchs, P.
L. J. Am. Chem. Soc. 2002, 124, 13978 (e) Yang, G.; Zhang, Q.; Miao, H.;
Tong, X.; Xu, J. Org. Lett. 2005, 7, 263.
range of substrates has not been achieved. Herein, we
describe a mild, efficient, regio- and chemoselective benzylic
oxidation using TBHP made possible through the catalytic
intervention of mixed-valent dirhodium(II,III) tetrakis-
9
(caprolactamate).
(1) For transition-metal-catalyzed oxidations, see: (a) Modern Oxidation
Methods; B a¨ ckvall, J.-E., Ed.; Wiley: Weinheim, 2004. (b) Mimoun, H.
In ComprehensiVe Coordination Chemistry; Wilkinson, G., Gillard, R. D.,
McCleverty, J. A., Eds.; Pergamon: Oxford, 1987; Vol. 6, p 317. (c)
Sheldon, R. A.; Kochi, J. K., Metal-Catalyzed Oxidations of Organic
Compounds; Academic Press: New York, 1981.
(
2) (a) Bulman Page, P. C.; McCarthy, T. J. In ComprehensiVe Organic
Synthesis; Trost, B. M., Ed.; Pergamon: Oxford, UK, 1991; Vol. 7, p 83.
b) Olah, G. A.; Moln a´ r, AÄ . Oxidation-Oxygenation. In Hydrocarbon
Chemistry, 2nd ed; Wiley: Hoboken; 2003; p 427.
3) Sharpless, K. B.; Verhoeven, T. R. Aldrichimica Acta 1979, 12, 63.
(
(9) For mixed-valent dirhodium(II,III) tetrakis(caprolactamate), see:
Catino, A. J.; Nichols, J. M.; Forslund, R. E.; Doyle, M. P. Org. Lett. 2005,
7, 2787.
(
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0.1021/ol0520020 CCC: $30.25
© 2005 American Chemical Society
Published on Web 10/19/2005