and the reaction vessel was refilled with N2. This procedure was
repeated three times. The reaction mixture was then stirred under
an N2 balloon at 50 °C for 12 h. After the reaction was finished,
a 3-fold excess of EtOH and a 5-fold excess of EtOAc were
added to the reaction mixture. This mixture was left undisturbed
for 2 h to allow precipitation and then centrifuged at 6000 rpm
for 4 min. The decantate was poured off. The solid residue was
dried and used as the catalyst for the next round. The decantate
was evaporated with a rotary evaporator. The obtained mixture
was dissolved in EtOAc. The yield and conversion were deter-
mined by GC-MS using 1,3,5-trimethylbenzene as an internal
standard.
Acknowledgements
We are grateful to the National Nature Science Foundation of
China (20932002, 20972144, 90813008, 21172205, 20772188,
J1030412 and 973 program 2010CB912103).
Scheme 2 The proposed mechanism for the reaction.
synthesis of imines catalyzed by palladium nanoparticles with
the liberation of hydrogen. Moreover, taking the advantage of
the water-soluble reversibility of this catalyst, Pd/DNA could be
recovered and reused by a simple phase separation. By virtue of
the liberated H2, the nitroarenes could also be deoxidized in situ
into amines and a one-pot tandem synthesis of imines was rea-
lized from nitroarenes. All these transformations were performed
in water under very mild reaction conditions. As a result, these
environmentally benign and atom economical imines are in
accord with green chemistry. Additionally, a direct dehydrogena-
tion of various primary alcohols to aldehydes was also achieved
in water, providing an alternative synthesis for aldehydes.
Detailed mechanistic studies and other applications in organic
reactions of Pd/DNA catalyst are in progress.
Notes and references
1 (a) J. P. Adams, J. Chem. Soc., Perkin Trans. 1, 2000, 125;
(b) J. Gawronski, N. Wascinska and J. Gajewy, Chem. Rev., 2008, 108,
5227.
2 (a) K. Orito, T. Hatakeyama, M. Takeo, S. Uchiito, M. Tokuda and
H. Suginome, Tetrahedron, 1998, 54, 8403; (b) A. H. ÉII,
J. S. M. Samec, C. Brasse and J.-E. Bäckvall, Chem. Commun., 2002,
1144; (c) M. Largeron, A. Chiaroni and M.-B. Fleury, Chem.–Eur. J.,
2008, 14, 996; (d) C. S. Yi and D. W. Lee, Organometallics, 2009, 28,
947.
3 (a) J. S. M. Samec, A. H. ÉII and J.-E. Bäckvall, Chem.–Eur. J., 2005,
11, 2327; (b) S.-I. Murahashi, Y. Okano, H. Sato, T. Nakae and
N. Komiya, Synlett, 2007, 1675; (c) G. Jiang, J. Chen, J.-S. Huang and
C.-M. Che, Org. Lett., 2009, 11, 4568; (d) X.-Q. Gu, W. Chen,
D. Morales-Morales and C. M. Jensen, J. Mol. Catal. A: Chem., 2002,
189, 119.
4 L. Blackburn and R. J. K. Taylor, Org. Lett., 2001, 3, 1637.
5 (a) S. Sithambaram, R. Kumar, Y.-C. Son and S. L. Suib, J. Catal., 2008,
253, 269; (b) J. W. Kim, J. He, K. Yamaguchi and N. Mizono, Chem.
Lett., 2009, 38, 920; (c) S. Kegnæs, J. Mielby, U. V. Mentzel,
C. H. Christensen and A. Riisager, Green Chem., 2010, 12, 1437;
(d) L. Jiang, L. L. Jin, H. W. Tian, X. Q. Yuan, X. C. Yu and Q. Xu,
Chem. Commun., 2011, 47, 10833; (e) M. S. Kwon, S. Kim, S. Park,
W. Bosco, R. K. Chidrala and J. Park, J. Org. Chem., 2009, 74, 2877;
(f) H. Sun, F.-Z. Su, J. Ni, Y. Cao, H.-Y. He and K.-N. Fan, Angew.
Chem., Int. Ed., 2009, 48, 4390; (g) Q. Kang and Y.-G. Zhang, Green
Chem., 2012, 14, 1016.
Experimental
General procedure for the preparation of Pd/DNA catalyst
PdCl2 (0.20 mmol) and fish sperm DNA (20 mg) were dissolved
in Tris buffer (24 ml, 10 mM, pH = 7.4). The combined solution
was stirred for 24 h to ensure that the Pd2+ thoroughly bound to
the DNA. After this aging process, the solution was cooled to
0 °C and freshly dissolved NaBH4 (1.0 mmol) in Tris buffer
(16 ml) was added dropwise under N2 atmosphere. After
reduction, the solution was stirred for another 24 h in N2 from
0 °C to room temperature to obtain the resulting Pd/DNA nano-
hybrids (ca. 5 mM in Tris).
6 (a) R. R. Donthiri, R. D. Patil and S. Adimurthy, Eur. J. Org. Chem.,
2012, 4457; (b) J. Xu, R. Q. Zhuang, L. L. Bao, G. Tang and Y. F. Zhao,
Green Chem., 2012, 14, 2384.
7 (a) A. Friedrich and S. Schneider, ChemCatChem, 2009, 1, 72;
(b) T. C. Johnson, D. J. Morris and M. Wills, Chem. Soc. Rev., 2010, 39,
81; (c) G. E. Dobereiner and R. H. Crabtree, Chem. Rev., 2010, 110, 681.
8 (a) F. Gelman, J. Blum and D. Avnir, New J. Chem., 2003, 27, 205;
(b) A. Zanardi, J. A. Mata and E. Peris, Chem.–Eur. J., 2010, 16, 10502;
(c) C.-H. Tang, L. He, Y.-M. Liu, Y. Cao, H.-Y. He and K.-N. Fan,
Chem.–Eur. J., 2011, 17, 7172; (d) C.-C. Lee and S.-T. Liu, Chem.
Commun., 2011, 47, 6981; (e) Y. Liu, W. Chen, C. Feng and G. J. Deng,
Chem.–Asian J., 2011, 6, 1142; (f) K. Shimizu, K. Shimura,
M. Nishimura and A. Satsuma, ChemCatChem, 2011, 3, 1755;
(g) R. Cano, D. J. Ramón and M. Yus, J. Org. Chem., 2011, 76, 5547;
(h) F. H. X, Y. Liu, C. L. Tang and G. J. Deng, Org. Lett., 2012, 14, 984;
(i) X. J. Cui, C. M. Zhang, F. Shi and Y. Q. Deng, Chem. Commun.,
2012, 48, 9391.
General procedure for the synthesis of imines from alcohols and
amines with liberation of H2
The as-synthesized Pd/DNA nano-hybrids (2 ml) were precipi-
tated by adding a 3-fold excess of EtOH. After sitting for 1 h
and then centrifugation at 6000 rpm for 4 min, the decantate was
poured off. The obtained solid residue was dried by N2 flow and
redispersed in a water solution (1 ml) containing LiOH·H2O
(15.7 mg, 0.375 mmol). Benzyl alcohol (29.7 mg, 0.275 mmol)
and aniline (23.3 mg, 0.25 mmol) were then added to the solu-
tion. The air in the reaction mixture was removed under vacuum
9 B. Gnanaprakasam, J. Zhang and D. Milstein, Angew. Chem., Int. Ed.,
2010, 49, 1468.
10 M. A. Esteruelas, N. Honczek, M. Oliván, E. Onate and M. Valencia,
Organometallics, 2011, 30, 2468.
This journal is © The Royal Society of Chemistry 2012
Green Chem., 2012, 14, 3423–3428 | 3427