Solid-supported ruthenium(0): An efficient heterogeneous catalyst for hydration of nitriles to amides under microwave irradiation

Article abstract of DOI:10.1039/c3nj00493g

Solid-supported ruthenium(0) was synthesized by the reduction deposition method and used as a heterogeneous catalyst for the hydration of nitriles to amides under microwave irradiation. A wide range of aromatic, α,β-unsaturated and aliphatic nitriles were efficiently converted to their corresponding primary amides under milder conditions. The catalyst was found to be very stable under moisture and microwave irradiation, easily separable from the reaction mixture, to cause negligible metal contamination of the product and was recyclable up to ten times without significant loss of catalytic activity.

Full text of DOI:10.1039/c3nj00493g

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Solid-supported ruthenium(0): an efficient
heterogeneous catalyst for hydration of
Cite this: DOI: 10.1039/c3nj00493g
nitriles to amides under microwave irradiation†‡
Received (in Porto Alegre, Brazil)
1
0th May 2013,
Sandeep Kumar and Pralay Das*
Accepted 7th August 2013
DOI: 10.1039/c3nj00493g
www.rsc.org/njc
Solid-supported ruthenium(0) was synthesized by the reduction Although the hydration reactions have been carried out in
deposition method and used as a heterogeneous catalyst for the aqueous media under neutral conditions, the role of catalytically
À
hydration of nitriles to amides under microwave irradiation. A wide bound OH in Ru(OH)
x
cannot be ignored. So far, the totally base
range of aromatic, a,b-unsaturated and aliphatic nitriles were efficiently free hydration of organonitriles has been difficult except for the
1
4
converted to their corresponding primary amides under milder condi- use of ligands with ruthenium metal. Recently, a Nafion–Ru
tions. The catalyst was found to be very stable under moisture and nanoparticle system has been developed that catalyzes the reac-
microwave irradiation, easily separable from the reaction mixture, tion of aromatic and heterocyclic organonitriles under base free
1
5
to cause negligible metal contamination of the product and conditions, but the hydration of less reactive aliphatic nitriles
was recyclable up to ten times without significant loss of cata- has not been attempted. Further, the long reaction time (12 h)
lytic activity.
and high temperature (175 1C) also limits this methodology.
In continuation of our previous work on solid supported
1
16
The hydration of organonitriles to amides in water is one of the transition metal nanoparticles, herein we report solid supported
most important functional group transformations in academia and ruthenium(0) nanoparticles that efficiently catalyze the hydration
industry because of their usefulness as chemical intermediates of aromatic, heterocyclic, a,b-unsaturated and aliphatic nitriles in
in organic synthesis, pharmaceutical molecules and functional base free conditions under microwave irradiation.
Solid supported nanoparticles of ruthenium(0) (SS-Ru) were
and selective than traditional acid base approaches. A variety synthesized by a reduction deposition approach. Amberlite IRA
2
3
4
polymers. Transition metal catalyzed processes are more prevalent
5
6
of homogeneous transition-metal complexes of ruthenium,
900 chloride form resin, selected as the solid support, was
7
8
9
10
rhodium, palladium, platinum and gold have been developed treated with a catalytic quantity of NaBH
4
to partially exchange
ÁH O in
À
for this purpose. But the special handling of sensitive metal chloride ions for BH
4
ions and then treated with RuCl
3
2
complexes, difficulty of product–catalyst separation and no DMF at 100 1C for 1 h. The white solid surface of the resin was
reusability in most cases limits the use of these homogeneous soon found to be turned grey/blackish after complete impreg-
metal complexes. In this context, heterogeneous transition nation of Ru(0) (Fig. 1).
metal catalysts have gained enormous interest due to easy
After complete immobilization of Ru over the solid surface,
handling, excellent stability, easy recovery and reusability com- the resin was washed with water, then with acetone and dried
pared to homogeneous catalysts. Much effort has been given to
11
the synthesis of Ru(OH)
x
based catalysts, such as Ru(OH)
x
/Al
synthesized
under strong basic conditions using NaOH or NH OH. The
catalyst was also recharged in NaOH for its reuse.
2 3
O ,
12
13
nanoferrite-Ru(OH)
x
2 x
and nano-Fe@SiO Ru(OH)
4
11
x 2 3
Ru(OH) /Al O
Natural Plant Products Division, CSIR-Institute of Himalayan Bioresource Technology,
Palampur-176061, H.P., India. E-mail: pdas@ihbt.res.in, pdas_nbu@yahoo.com;
Fax: +91-1894-230433
†
‡
IHBT Communication No. 3492.
1
Electronic supplementary information (ESI) available: Experimental details, H,
C NMR and HRMS data. See DOI: 10.1039/c3nj00493g
Fig. 1 (A) The polymeric polystyrene resin as a solid support; (B) the solid
1
3
supported Ru(0) catalyst.
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Table 2 SS-Ru catalyzed hydration of substituted nitriles under microwave
irradiation
a
Time
(min) Product
Yield
(%)
Entry Substrate
1
60
120
120
60
86 (>99)
83 (>99)
84 (>99)
88 (>99)
79 (>99)
b
2
Fig. 2 A transmission electron microscopy (TEM) image of SS-Ru nanoparticles
and a histogram representing particle size distribution.
b
3
under reduced pressure. The conversion of Ru(III) to Ru(0) was
4
5
1
7
monitored by UV-Vis spectroscopy and confirmed by energy
dispersive X-ray (EDX) and X-ray diffraction (XRD) analysis, as
1
8
60
mentioned in our earlier report.
The impregnation and distribution of nanoparticles of Ru(0)
over the solid matrix was determined by transmission electron
microscopy (TEM) and represented by a histogram (Fig. 2).
In an attempt to find the best reaction conditions, the hydration
of p-bromobenzonitrile as a model substrate with homogeneous
Ru(III) and heterogeneous SS-Ru catalysts in different solvents was
6
60
85 (>99)
7
8
9
60
60
60
30
84
monitored by GCMS. The use of homogeneous RuCl
3
ÁH
2
O catalyst
83 (>99)
86 (>99)
77
afforded 76% of product 2a under the reaction conditions but
the catalyst is not recoverable and recyclable. The reaction
under conventional heating and unpressurized conditions
using SS-Ru (2–3 mol%) gave poor conversion of 1a (Table 1,
entries 4 and 5). Among different catalysts and conditions,
SS-Ru (2 mol% Ru) in water at 130 1C (80 psi, 100 W) yielded the
best results (Table 1, entry 3) and was selected as the optimum
10
conditions for the hydration of substituted nitriles to the 11
corresponding amides in water.
30
120
120
73
80
82
The less soluble amides, like p-bromobenzamide, crystallized
in the water and can be separated by decanting. In the cases of
c
12
c
13
Table 1 Optimization of reaction conditions for the microwave assisted hydration
of p-bromobenzonitrile
1
1
4
5
60
60
60
60
84 (>99)
88 (>99)
80 (>97)
86 (>99)
a
Entry
Catalyst (mol%)
Solvent
Temperature (1C)
Yield (%)
1
2
3
SS-Ru (3)
SS-Ru (3)
SS-Ru (2)
SS-Ru (3)
SS-Ru (2)
SS-Ru (1)
H
H
H
H
H
H
H
H
2
2
2
2
2
2
2
2
O
O
O
O
O
O
O
O
100
130
130
130
130
130
130
130
130
130
130
35
>99
>99 (85)
10
16
17
b
4
b
5
8
60
76
nr
nr
nr
nr
6
7
8
9
1
1
RuCl
—
3
ÁH
2
O (2)
1
8
120
120
72 (>80)
60
SS-Ru (2)
SS-Ru (2)
SS-Ru (2)
DMF
THF
EtOH
0
1
1
9
a
b
a
b
Isolated yield in parenthesis; nr = no reaction. GCMS yield. Reaction
carried out under conventional heating for 3 h.
Isolated yields. GCMS yield in parentheses. SS-Ru (3 mol%) was
c
used. Reactions carried out at 150 1C.
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products that are not crystallized, the crude reaction mixture nitrogen atom and then the nucleophilic attack of water on the
was extracted with ethyl acetate and further purified by column electrophilic carbon to yield the amide.
chromatography.
The catalyst is easily recoverable and recyclable. The recycla-
The scope of this transformation was explored using a variety bility experiments of SS-Ru were conducted by using 4-cyano-
of substrates, including aromatic, heterocyclic, a,b-unsaturated pyridine as a test substrate. After completion of the reaction,
and aliphatic nitriles. The reaction of benzonitrile 1b under the the product was extracted using ethyl acetate. The catalyst was
optimized reaction conditions afforded benzamide in good yield. washed with water and acetone then dried. The catalyst was used
Different aromatic nitriles containing electron donating substi- ten times without significant loss of catalytic activity (Fig. 3).
tuents, like methyl, methoxy 1c and 1d, and electron withdraw-
ing nitro, acetyl and trifluoromethyl 1e–g groups gave the
desired products in good yields (Table 2, entries 2–9). In the
Conclusions
case of electron donating aromatic nitriles, the reaction pro-
ceeded slowly at the optimized catalyst concentration. Increasing
the catalyst loading to 3 mol% and reaction time to 2 h afforded
the corresponding products in good yields. The slow reaction
rate observed might be due to the fact that electron donating
groups deactivate the eletrophilicity of nitrile carbon atom,
making it less susceptible to attack from the nucleophilic water
molecule. The halo substituted benzonitriles, such as iodo,
chloro, fluoro 1h–j, smoothly yielded the corresponding products
in good yields without any dehalogenation. Further, 1,3-benzo-
In conclusion, we have developed air and moisture stable solid
supported Ru(0) nanoparticles as a heterogeneous catalyst that
efficiently catalyzed the base free hydration of a wide range of
nitriles to their corresponding amides under microwave irradia-
tion. The catalyst was easy to handle, recoverable and reusable
up to several runs and no significant loss of catalytic activity was
observed.
Acknowledgements
dinitriles and 1,4-benzodinitrile, 1k–n, were selectively hydrated We are grateful to Director CSIR-IHBT for providing necessary
to mono- and di-amides by controlling the reaction parameters, facilities during the course of work. We gratefully acknowledge
such as time and temperature. The mono-amides were selec- financial assistance from CSIR project ORIGIN, CSC0108 and the
tively obtained in 73–77% yields by shortening the reaction time Department of Science & Technology (Nano Mission), New Delhi
to 30 minutes, while diamides were obtained by increasing the (Grant No. SR/NM/NS-95/2009). S.K. thanks UGC, New Delhi for
reaction time to 2 hours at 150 1C. Heterocyclic nitriles were also awarding junior research fellowship.
efficiently converted to nicotinamide, a part of the vitamin B
group and isonicotinamide from 3-cyanopyridine and 4-cyano-
pyridine, respectively 1o–p. The a,b-unsaturated cinnamonitrile
Notes and references
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