Modular metal-carbon stabilized palladium nanoparticles for the catalytic hydrogenation of N-heterocycles

Article abstract of DOI:10.1016/j.tetlet.2015.12.008

We report here the first modular metal-carbon stabilized palladium nanoparticles based on binaphthyl scaffolds, which can be prepared from palladium salts and substituted binaphthyl diazonium salts in homogeneous system through direct reduction using sodium borohydride. The resulting palladium nanoparticles subjected to the electron density of modular moieties are found to be novel and efficient catalysts for the catalytic hydrogenation of N-heterocycles, affording the corresponding adducts in good to excellent yields under mild conditions.

Full text of DOI:10.1016/j.tetlet.2015.12.008

Tetrahedron Letters 57 (2016) 329–332
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Modular metal–carbon stabilized palladium nanoparticles for the
catalytic hydrogenation of N-heterocycles
Yu Zhang a, Mao Mao a, Yi-Gang Ji a,b, Jie Zhu a, Lei Wu a,
⇑
a Jiangsu Key Laboratory of Pesticide Science and Department of Chemistry, College of Sciences, Nanjing Agricultural University, Nanjing 210095, China
b Department of Life Sciences and Chemistry, Jiangsu Second Normal University, Nanjing 210013, China
a r t i c l e i n f o
a b s t r a c t
Article history:
We report here the first modular metal–carbon stabilized palladium nanoparticles based on binaphthyl
scaffolds, which can be prepared from palladium salts and substituted binaphthyl diazonium salts in
homogeneous system through direct reduction using sodium borohydride. The resulting palladium
nanoparticles subjected to the electron density of modular moieties are found to be novel and efficient
catalysts for the catalytic hydrogenation of N-heterocycles, affording the corresponding adducts in good
to excellent yields under mild conditions.
Received 31 October 2015
Revised 28 November 2015
Accepted 2 December 2015
Available online 7 December 2015
Keywords:
Ó 2015 Elsevier Ltd. All rights reserved.
Metal–carbon stabilization
Palladium nanoparticles
Hydrogenation
N-heterocycles
Introduction
the bonding energy of Pd–C bond to be about 436 kJ/mol, much lar-
ger than that of metal–S and metal–N bonds.13 This remarkable
Nanocatalysis has become a frontier domain in recent decades
owing to the high catalytic efficiency offered by their large
surface-to-volume ratio.1–3 It is well-known that the surface
energy increases with the decreasing size of the metal nanoparti-
cles. Metal nanoparticles of smaller size, however, are known to
aggregate more easily and seriously, which would further lead to
the decrease/loss of catalytic activity and selectivity. The growth
dynamics of the particles has been supposed to be controlled by
two competing processes, nucleation of zero valence metal atoms
to form the cores and passivation of the surfactant ligands to limit
the growth of the cores.4 Besides, the charge also affected the reac-
tivity of supported transition metal catalysts.5 Thus, the choice of
the organic capping ligands plays a vital role in the regulation of
nanoparticle structure and hence their chemical/physical
properties and functional applications. So far, various stabilizers
have been employed, including micelles,6 microemulsions,7,8
surfactants9 and coordinated ligands where polymers10 and den-
drimers11 with binding heteroatoms are extensively explored in
catalysis. Until recently, covalent stabilization of nanoparticles
via carbon–metal bonds is emerging as novel type of surface func-
tionalities. In 2006, Mirkhalaf and co-workers firstly synthesized
the metal–carbon stabilized gold nanoparticles and platinum
nanoparticles.12 Subsequently, Ghosh and co-workers determined
bond strength differentiates the catalytic performance from
metal–carbon nanoparticles to traditional coordinative stabilized
nanoparticles.
However, only limited investigations on the catalytic properties
of the carbon–metal stabilized nanoparticles have been reported.
In Gopidas’s work, for an instance, the palladium nanoparticles-
cored G1-dendrimer stabilized by Pd–carbon bonds was firstly
synthesized and applied to reduction of carbon–carbon multiple
bonds (Fig. 1a).14 Recently, Sekar et al. reported the efficiency
and reusability of palladium nanoparticles stabilized by Pd–C
(binaphthyl) covalent bonds in the Heck, Suzuki–Miyaura and
Sonogashira cross-coupling reactions,15 lately in polysubstituted
olefin synthesis (Fig. 1b).16 To be noted, the above-mentioned
reports focused on modification of palladium nanoparticles surface
utilizing ordinary aryl scaffolds, where no modular position was
considered in the precursor of nanocatalyst. We envisioned that
binaphthyl scaffolds with modular moiety could regulate the elec-
tron density on metal surface via conjugation effect, towards tun-
able catalytic efficiency, which might expand the potential
applications of metal–carbon stabilized nanoparticles in organic
transformations. In addition, although palladium has been well-
known to hydrogenate the N-heterocycles, they presented clear
drawbacks of harsh conditions (high hydrogen pressure or elevated
temperature).17 Thus, the development of novel nanocatalyst for
hydrogenation is still in a great demand. As part of our continuing
interests in the synthesis and application of nanocatalysts,18
⇑
Corresponding author. Tel./fax: +86 25 84396716.
E-mail address: rickywu@njau.edu.cn (L. Wu).
http://dx.doi.org/10.1016/j.tetlet.2015.12.008
0040-4039/Ó 2015 Elsevier Ltd. All rights reserved.

330
Y. Zhang et al. / Tetrahedron Letters 57 (2016) 329–332
Previous work
a. Gopidas:
Dendron
Pd
Reduction of Carbon-Carbon multiple bonds
Tetrahedron Lett. 2011, 52, 3102-3105
b. Sekar:
Carbon-Carbon formations
Pd
Org. Lett. 2014, 16, 3856-3859;
Cat. Commun. 2013, 39, 50-54.
This work
Modular Moiety
Hydrogenation of N-heterocycles
R
Pd
Figure 1. Current metal–carbon stabilized PdNPs in catalysis.
herein, we report the first preparation of a series of modular palla-
dium-carbon stabilized nanoparticles based on binaphthyl scaf-
folds and their unprecedented applications in the catalytic
hydrogenation of N-heterocycles (Fig. 1).14–16
initial result, we were curious to investigate the impact of electron
density on catalytic efficiency as mentioned above. Thus, nanocat-
alysts bearing electron-deficient and electron-rich groups (C2–C5)
were introduced in this reaction. The hydrogenation took place
smoothly depending on the structure of modular moiety, and the
results are summarized in Table 1. Strong electron-deficient groups
on binaphthyl led to lowered conversion; moreover, high steric
groups seriously impaired the catalytic efficiency (entry 5). The
catalyst C-2 with acetyl group was found to enable the hydrogena-
tion with highest conversion up to 80% in THF. To improve the
yield and efficiency of this reaction, the effects of solvents and
pressure were further studied (entries 8–22), albeit the catalyst
was also active for heterogeneous system in water and alcoholic
solvents. Eventually, the optimized hydrogenation reaction was
achieved with quantitative conversion in dichloromethane under
1.0 MPa H2 pressure for 12 h (entry 21).
Finally, the substrate scopes of the catalytic hydrogenation of N-
heterocycles including quinoline, quinoxane and derivatives were
investigated under the conditions of ambient temperature and
pressure of 1.0 MPa H2 using 3 mol % of the nano-sized palladium
catalyst C-2 for 12 h. The results are listed in Table 2. In general,
all of the quinoline derivates were hydrogenated smoothly with
good selectivity and excellent conversion (70–99%) with 3 mol %
catalyst, except for entry 6, 9 and 12 with 10 mol %, 10 mol % and
5 mol %, respectively. The catalytic activity was relatively insensi-
tive to the position of substituents on aryl or heteroaryl moiety
of quinolines (entries 1–12); moreover, the catalyst was quite
stable for substrates with multi-coordination groups (entries 5, 6,
10). Notably, hydrogenation of quinoline-2-carbaldehyde led to
thereduction of both heteroaryl and aldehyde group (entries 11).
Excellent results were also achieved with quinoxaline, phenazine
and their derivatives (entries 13–16).
Results and discussion
The diazonium precursors (B) were readily synthesized from
various mono-protected 1,10-binaphthyl-2,20-diamine (BINAM)
and the substitution groups are listed in Figure 2. Initial attempt
to synthesize palladium nanoparticles via two-phase reduction15
led to ‘palladium black’ unfortunately. The preparation process
was finally succeeded when ‘one-phase reduction’ method was
tried with palladium salts and diazonium precursors in the mix-
ture solvent of THF and MeOH, affording a homogeneous black
solution. The resulting serial palladium nanoparticles (C), obtained
as black powders, were freely soluble in aprotic solvents, such as
THF, toluene and dichloromethane; but insoluble in hexanes,
MeOH and water. The palladium nanoparticle was further charac-
terized by transition electron microscopy (TEM) and Inductively
Coupled Plasma (ICP). As illustrated in Figure 2, the TEM analysis
indicated that palladium nanoparticles were highly dispersed well
with an average particle size of 2 0.5 nm (e.g., catalyst C-2), and
the palladium contents were determined as 8.14 wt % (C1),
4.61 wt % (C2), 11.7 wt % (C3), 1.14 wt % (C4), and 0.43 wt % (C5),
respectively. IR and 1H NMR spectra of the catalyst C2 witnessed
the existence of organic shell on the nanoparticles (S2 and S24 in
Supporting Information).
With the synthesized catalysts in hand, we studied their cat-
alytic performance in the hydrogenation of quinoline. This choice
was based on the fact that such a reaction provides a powerful
approach for the synthesis of tetraquinoline skeleton, which was
found in many natural and synthetic products.19 Furthermore,
reports on catalytic hydrogenation of quinolines using nanocata-
lysts were rare,20 let alone metal–carbon stabilized nanoparticles.
Our preliminary study started with hydrogenation of quinoline
under 2.5 MPa H2, in the presence of 3 mol % of palladium
nanoparticles (C1) at room temperature. The reaction afforded
58% of 1,2,3,4-tetrahydroquinoline after 24 h. Encouraged by this
Conclusion
In summary, we have synthesized and characterized a novel
class of modular metal–carbon stabilized palladium nanoparticles
based on binaphthyl scaffolds. The nanocatalysts performed effi-
ciently when applied to the hydrogenation of N-heterocycles,

Y. Zhang et al. / Tetrahedron Letters 57 (2016) 329–332
331
-
N2+BF4
X
X
Pd
NH2
X
NaNO2
HBF4
Pd(OAc)2, NaBH4
X
X
THF/MeOH
X
B
A
C
X= NHTs, NHAc, etc.
O
NH
NH PPh2
NH2
NHTs
NH2
NHAc
NH2
N
NH2
H
O
A-1
A-2
A-4
A-3
CF3
S
NH
NH2
NHEt
NH2
NH
NH2
NH
CF3
A-7
A-5
A-6
Figure 2. The preparation of modular metal–carbon stabilized PdNPs (top), and TEM images of the palladium nanoparticles with distributions (inset (b) is the corresponding
morphology distribution of the PdNPs derived from 50 of PdNPs in image (a)).
Table 1
Optimization of reaction conditions
Pd NPs C (3 mol%)
Solvent, r.t.
+
H2
N
N
H
Entry
Catalyst
Solvent
Time (h)
H2 Pressure (MPa)
Conv. (%)a
1
2
C-1
C-2
THF
THF
24
24
2.5
2.5
58
80
(continued on next page)

332
Y. Zhang et al. / Tetrahedron Letters 57 (2016) 329–332
Table 1 (continued)
Entry
Catalyst
Solvent
Time (h)
H2 Pressure (MPa)
Conv. (%)a
3
4
5
6
7
C-3
C-4
C-5
C-6
C-7
C-2
C-2
C-2
C-2
C-2
C-2
C-2
C-2
C-2
C-2
C-2
C-2
C-2
C-2
C-2
THF
THF
THF
THF
THF
DMF
CH2Cl2
Toluene
1,4-Dioxane
MeOH
i-propanol
H2O
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
24
24
24
24
24
24
24
24
24
24
24
24
6
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
0.1
0.5
1.0
1.5
39
40
Trace
12
46
10
>99
16
17
60
19
37
57
73
>99
>99
65
74
>99
>99
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
9
12
15
12
12
12
12
a
The conversions were determined by 1H NMR.
Table 2
BK20141359), ‘333 High Level Talent Project’, ‘QinLan Project’ of
JiangSu Province, Foundation of the Jiangsu Education Committee,
China (No. 14KJD150002), and Jiangsu Key Laboratory of Biofuction
Molecule for financial support.
Nanocatalyst C-1 catalyzed hydrogenation of quinoline derivatives
X
X
R1
R2
Pd NPs C-2 (3-10 mol%)
R1
R2
+
H2
1.0 MPa
DCM 12h, rt
N
H
N
Supplementary data
Entry
X
R1
H
6-MeO
6-OH
8-Me
8-OH
8-NH2
H
H
H
H
H
R2
Catalyst (mol %)
Conv. (%)a
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2015.12.
008.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
C
C
C
C
C
C
C
C
C
C
C
C
N
N
N
N
H
H
H
H
H
H
2-Me
3-Me
4-Me
2-NH2
3-CHO
2-Me
Quinoxane
2-Methyl-quinoxane
Quinoxane
Phenazine
3
3
3
3
3
10
3
3
10
3
3
>99
>99(92)
70
>99
>99
94
>99(90)
>99
References and notes
1. Astruc, D. Nanoparticle and Catalysis; VCH: Weinheim, 2007.
2. Li, C.; Liu, Y. Bridging Heterogeneous and Homogeneous Catalysis: Concepts,
Strategies, and Applications; John Wiley & Sons, 2014.
3. Ji, Y. G.; Wu, L.; Fan, Q. H. Acta. Chimica. Sinica. 2014, 72, 798–808.
4. Chen, S.; Templeton, A. C.; Murray, R. W. Langmuir 2000, 16, 3543–3548.
5. Liang, H.; Bo, H.; Yongjie, X.; Robert, C. F.; Hansong, C. ACS Catal. 2015, 5, 4592–
4597.
6. Kocak, G.; Bütün, V. Colloid Polym. Sci. 2015, 1.
7. Du, Y.; Wang, W.; Li, X.; Zhao, J.; Ma, J.; Liu, Y.; Lu, G. Mater. Lett. 2012, 68, 168–
170.
8. Wani, I. A.; Khatoon, S.; Ganguly, A.; Ahmed, J.; Ahmad, T. Colloids Surf., A 2013,
101, 243–250.
9. Santra, S.; Tapec, R.; Theodoropoulou, N.; Dobson, J.; Hebard, A.; Tan, W.
Langmuir 2001, 17, 2900–2906.
10. Rao, J. P.; Geckeler, K. E. Prog. Polym. Sci. 2011, 36, 887–913.
11. Myers, V. S.; Weir, M. G.; Carino, E. V.; Yancey, D. F.; Pande, S.; Crooks, R. M.
Chem. Sci. 2011, 2, 1632–1646.
12. Mirkhalaf, F.; Paprotny, J.; Schiffrin, D. J. J. Am. Chem. Soc. 2006, 128, 7400–
7401.
13. Ghosh, D.; Chen, S. J. Mater. Chem. 2008, 18, 755–762.
14. Kumar, V. K. R.; Gopidas, K. R. Tetrahedron Lett. 2011, 52, 3102–3105.
15. Ganapathy, D.; Sekar, G. Catal. Commun. 2013, 39, 50–54.
16. Ganapathy, D.; Sekar, G. Org. Lett. 2014, 16, 3856–3859.
17. Dell’Anna, M. M.; Romanazzi, G.; Intini, S.; Rizzuti, A.; Leonelli, C.; Piccinni, A.
F.; Mastrorilli, P. J. Mol. Catal. A: Chem. 2015, 402, 83–91. and references cited
herein.
18. (a) Wu, L.; Ling, J.; Wu, Z. Q. Adv. Synth. Catal. 2011, 353, 1452–1456; (b) Wu, L.;
He, Y. M.; Fan, Q. H. Adv. Synth. Catal. 2011, 353, 2915–2919; (c) Wu, L.; Zhang,
X.; Tao, Z. M. Catal. Sci. Tech. 2012, 2, 707–710; (d) Wu, L.; Zhang, Y.; Ji, Y. G.
Curr. Org. Chem. 2013, 17, 1288–1302.
>99(88)
>99
98b
6-Me
H
H
5-Me
H
5
3
3
3
>99
>99
>99
>99
3
>99
The conversions were determined by 1H NMR. Isolated yields are shown in
parentheses.
a
b
The aldehyde group was reduced.
affording the corresponding adducts in good to excellent yields
under mild conditions. Notably, the rationally designed modular
moiety bearing electron-rich and electron-deficient groups regu-
lated the catalytic performance for hydrogenation efficiently. In
this context, we are designing and exploring the potential applica-
tion of chiral metal–carbon stabilized palladium nanoparticles for
asymmetric catalysis. These studies are underway in our
laboratory.
Acknowledgments
19. Girault, S.; Grellier, P.; Berecibar, A.; Maes, L.; Lemière, P.; Mouray, E.; Davioud-
Charvet, E.; Sergheraert, C. J. Med. Chem. 2001, 44, 1658–1665.
20. Yan, M.; Jin, T.; Chen, Q.; Ho, H. E.; Fujita, T.; Chen, L.-Y.; Bao, M.; Chen, M.-W.;
Asao, N.; Yamamoto, Y. Org. Lett. 2013, 15, 1484–1487.
The authors thank the National Natural Science Foundation of
China (NSFC, Grant No. 21372118), the Foundation Research Pro-
ject of Jiangsu Province (The Natural Science Foundation No.