monodentate and bidentate phosphine ligands have
been reported.3 Among them, Sodeoka reported impressive
work on the stereoselective conjugate reduction of β,β-
disubstituted R,β-unsaturated ketones utilizing the com-
plexes of Pd-BINAP and Pd-DUPHOS.3f,g However, the
research in this field is inadequate and a more efficient
catalytic system is still greatly desired to achieve higher
chemoselectivity and activity.
Compared with the transition metal complexes of
monodentate and bidentate ligands, individual tridentate
pincer-metal complexes4 (I in Figure 1) are considered to
be more structurally stable and to possess fewer catalytic
intermediate conformations. This usually improves selec-
tivity and activity leading to excellent catalytic behavior
in reactions involving metal-hydride intermediates5 and
others.6 Pd-hydride mediated transfer hydrogenations
have been extensively studied for the reduction of various
unsaturated compounds.7 Recently, pincer-Pd-hydride
complexes have attracted increasing attention for their
application in O2 and CO2 insertion reactions.8 However,
there are no reports concerning the insertion of other un-
saturated compounds into pincer-Pd-hydride complexes.
Herein we present the first example of chemoselective
transfer hydrogenations of R,β-unsaturated ketones cat-
alyzed by PCP pincer-Pd complexes (II in Figure 1),
employing the easily accessible and inexpensive n-BuOH
as a hydrogen donor and solvent.9 We envisioned that the
pincer-Pd-hydride complex generated in this reaction
system may act as a key intermediate in the catalytic cycle.
Figure 1. Pincer-metal complexes and PCP pincer-Pd complexes.
(3) For a Ru complex with a monodentate phosphine ligand, see: (a)
Sasson, Y.; Blum, J. Tetrahedron Lett. 1971, 2167. (b) Sasson, Y.; Cohen, M.;
Blum, J. Synthesis 1973, 359. (c) Sasson, Y.; Blum, J. J. Org. Chem. 1975, 40,
1887. (d) Doi, T.; Fukuyama, T.; Horiguchi, J.; Okamura, T.; Tyu, I. Synlett
2006, 721. For an Ir complex with bidentate phosphine ligands, see: (e)
Sakaguchi, S.; Yamaga, T.; Ishii, Y. J. Org. Chem. 2001, 66, 4710. For a Pd
complex with bidentate phosphine ligands, see: (f) Tsuchiya, Y.; Hamashima,
Y.; Sodeoka, M. Org. Lett. 2006, 8, 4851. (g) Mogushi, D.; Beemelmanns, C.;
Hashizume, D.; Hamashima, Y.; Sodeoka, M. J. Organomet. Chem. 2008,
693, 867. For a Ni complex with bidentate phosphine ligands, see: (h)
Castellanos-Blanco, N.; Flores-Alamo, M.; Garcıa, J. J. Organometallics
2012, 31, 680.
The PCP pincer ligand in II was readily synthesized
from 1,5-bis(chloromethyl)-2,4-dimethylbenzene using a
modified procedure10 involving nucleophilic substitution
by diphenylphosphine oxide and reduction by HSiCl3.
The ligand was coordinated with Pd salts to form the PCP
pincer-Pd complexes II possessing different anions, and
the structures of complexes A and C were confirmed by
single crystal X-ray diffraction (for the details, see Sup-
porting Information (SI)).
(4) For reviews, see: (a) Albrecht, M.; van Koten, G. Angew. Chem.,
Int. Ed. 2001, 40, 3750. (b) der Boom, M. E.; Milstein, D. Chem. Rev.
2003, 103, 1759. (c) Singleton, J. T. Tetrahedron 2003, 59, 1837. (d) Slagt,
M. Q.; van Zwieten, D. A. P.; Moerkerk, A. J. C. M.; Gebbink, R. J. M.
The prepared PCP pincer-Pd complex A was tested in
the conjugate reduction of chalcone 1a to investigate the
ꢁ
K.; van Koten, G. Coord. Chem. Rev. 2004, 248, 2275. (e) Szabo, K. J.
Synlett 2006, 6, 811. (f) Morales-Morales, D., Jensen, C. M., Eds. The
Chemistry of Pincer Compounds; Elsevier: Amsterdam, 2007. (g) Morales-
Morales, D. Mini-Rev. Org. Chem. 2008, 5, 141. (h) Choi, J.; MacArthur,
A. H. R.; Brookhart, M.; Goldman, A. S. Chem. Rev. 2011, 111, 1761. (i)
(8) For the synthesis of pincer-Pd-hydride: (a) Fan, L.; Foxman,
B. M.; Ozerov, O. V. Organometallics 2004, 23, 326. (b) Koridze, A. A.;
Kuklin, S. A.; Sheloumov, A. M.; Dolgushin, F. M.; Lagunova, Y. V.;
Petukhova, I. I.; Ezernitskaya, M. G.; Peregudov, A. S.; Petrovskii,
P. V.; Vorontsov, E. V.; Baya, M.; Poli, R. Organometallics 2004, 23,
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R. A.; Goldberg, K. I. J. Am. Chem. Soc. 2009, 131, 1346. (e) Melero, C.;
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Selander, N.; Szabo, K. J. Chem. Rev. 2011, 111, 2048. (j) Niu, J.; Hao,
X.; Gong, J.; Song, M. Dalton Trans. 2011, 40, 5135. (k) Schneider, S.;
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€
1999, 2443. (b) Gottker-Schnetmann, I.; Brookhart, M. J. Am. Chem.
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Milstein, D. Angew. Chem., Int. Ed. 2011, 50, 2120. (f) Balaraman, E.;
Ben-David, Y.; Milstein, D. Angew. Chem., Int. Ed. 2011, 50, 11702. (g)
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ꢁ
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Martınez-Prieto, L. M.; Plama, P.; del Rio, D.; Alvarez, E.; Campora, J.
Chem. Commun. 2010, 46, 8851. (f) Gerber, R.; Fox, T.; French, C. M.
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Campora, J.; Alvarez, E. Organometallics 2012, 31, 1425. For the
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Szabo, K. J. Angew. Chem., Int. Ed. 2010, 49, 4051. (d) Vechorkin, O.;
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tion using rhodium as a metal and silicon hydride as a hydrogen source;
see: (a) Kanazawa, Y.; Tsuchiya, Y.; Kobayashi, K.; Shiomi, T.; Itoh, J.;
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Chen, T.; Zhao, Y.; Qiu, R.; Zhou, Y.; Yin, S.; Wang, X.; Goto, M.;
Han, L. J. Am. Chem. Soc. 2011, 133, 17037. (d) Shen, K.; Han, X.; Lu,
X. Org. Lett. 2013, 15, 1732.
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