Nanoparticle-supported and magnetically recoverable ruthenium hydroxide catalyst: Efficient hydration of nitriles to amides in aqueous medium

Article abstract of DOI:10.1002/chem.200802264

A simple and efficient synthesis of nanoferrite-supported, magnetically recyclable ruthenium hydroxide [Ru(OH)_x] catalyst and its applications in the hydration of nitriles in a benign aqueous medium, was reported. The catalyst was prepared by sonicating nanoferrites with dopamine in water for 2 hr followed by addition of ruthenium (Ru) chloride at a basic pH. The reaction temperature increased to 130°C results in the reaction proceeded with 65% conversion within 20 min. Various benzonitriles derivatives and aliphatic nitriles are smoothly hydrated to corresponding amides in excellent yield. The nanoferrite-supported ruthenium hydroxide catalyst could be used at least 3 times without any change in the activity. The Ru concentration of the catalyst is found to be 3.22% before the reaction and 3.16% after the reaction.

Full text of DOI:10.1002/chem.200802264

DOI: 10.1002/chem.200802264
Nanoparticle-Supported and Magnetically Recoverable Ruthenium
Hydroxide Catalyst: Efficient Hydration of Nitriles to Amides in Aqueous
Medium
Vivek Polshettiwar and Rajender S. Varma*[a]
The hydration of nitriles is one of the most imperative
technologies for the large-scale synthesis of amides, which
are a very significant group of compounds in the chemical
and pharmaceutical industry.[1,2] Conventionally, amides
have been synthesized by the hydration of nitriles, catalyzed
by strong acids[3] and bases.[4] However, under these condi-
tions, various by-products, such as carboxylic acids, are
formed by hydrolysis of the starting nitriles and as well as
amides. Also, many sensitive functional groups do not
endure such harsh conditions, which consequently decrease
the selectivity of reaction protocol. Therefore, the develop-
ment of efficient procedures for the synthesis of amides that
circumvent the extravagant use of stoichiometric reagents
and/or acidic and basic media is highly desirable.
aqueous medium, the protocol still needs traditional
workup, using organic solvents to isolate the product and
uses exotic ruthenium complexes as catalysts. Thus, the mild
and efficient hydration of nitriles in eco-friendly conditions,
a challenging research area, still far from being achieved by
synthetic chemists.
Magnetic nanoparticles have emerged as realistic substi-
tutes for conventional materials such as silica, polymer, etc.
and as a robust, high-surface-area heterogeneous catalyst
support.[13] Magnetic recoverability, which eliminates the ne-
cessity of catalyst filtration after completion of the reaction,
is an additional attribute of these materials. In quest to ex-
ploit the diverse catalytic applications of magnetic nanoma-
terials with special emphasis on the development of sustain-
able organic transformations[14] and nanomaterials,[15] herein,
we report a simple and efficient synthesis of nanoferrite-sup-
ported, magnetically recyclable ruthenium hydroxide
[Ru(OH)x] catalyst and its application in the hydration of ni-
triles in a benign aqueous medium, which circumvents the
use of organic solvents , even in the reaction workup stage.
The primary step of this objective was achieved by the
synthesis and functionalization of magnetic nanoparticles
(Scheme 1). The catalyst was prepared by sonicating nano-
ferrites with dopamine (which acts as a robust anchor and
avoid [Ru(OH)]x-leaching) in water for 2 h, followed by ad-
dition of ruthenium (Ru) chloride at a basic pH. Material
with [Ru(OH)]x on the amine-functionalized nanoferrites
was obtained in excellent yield.
Catalyst characterization by X-ray diffraction (XRD)
(Figure 1b,d) and transmission electron microscopy (TEM)
(Figure 1a,c) confirm the formation of the single-phase
[Fe3O4] nanoparticles, with spherical morphology and a size
range of 11–16 nm, which is comparable with the crystallite
size calculated from X-ray spectrum by using the Scherer
formula (11.52 nm). Analysis of the FT-IR spectra confirms
the anchoring of dopamine on ferrite surfaces (Figure S3 in
the Supporting Information). The signals of Ru and
[Ru(OH)]x were not detected in XRD, owing to the highly
dispersed low percentage of Ru in the sample. The weight
Several protocols, which predominantly use homogeneous
metal complexes, have been reported for hydration of ni-
triles.[5] These methods suffer various drawbacks, especially
the difficulty in separation of product and catalyst from the
reaction mixture, as well as the use of inert atmosphere for
handling air-sensitive metal catalysts. Heterogeneous sys-
tems have also been reported, such as alumina,[6] potassium
[7]
fluoride doped Al2O3 and phosphates,[8] silica supported
manganese oxides,[9] modified hydroxyapatite,[10] and ruthe-
nium hydroxide coated on alumina and ferrites.[11] However,
turnover numbers of these protocols are still very low and
reusability of the catalyst is difficult. Pioneers in this field,
Cadierno et al. recently developed excellent hydration pro-
tocol in pure water under neutral conditions.[12] Although
this work led the way to advance the hydration reaction in
[a] Dr. V. Polshettiwar, Prof. Dr. R. S. Varma
Sustainable Technology Division
National Risk Management Research Laboratory
U. S. Environmental Protection Agency
MS 443, Cincinnati, Ohio 45268 (USA)
Fax : (+1)513-569-7677
E-mail: varma.rajender@epa.gov
polshettiwar.vivek@epa.gov
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200802264.
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COMMUNICATION
polar nanocatalysts as well as water molecules with micro-
AHCTUNGTRENNUNG
waves[13a,17] and the reaction mixture is rapidly heated to
requisite temperatures under MW irradiation with the pre-
cise control of the reaction temperature.[18]
Initially, experiments were performed to optimize reaction
conditions for hydration of benzonitrile as a substrate, in
aqueous medium (Table 1). First, the reaction was conduct-
Table 1. Optimization of reaction conditions.[a]
Entry Catalyst
T [8C][b] t [min][c] Conversion [%]
1
2
3
4
5
nano
nano
nano
nano
nano
N
100
130
30
30
20
20
30
0
0
30
65
85
À
À
À
[a] Reactions were carried out with 1 mmol of benzonitrile, 100 mg nano-
catalyst, in water under MW irradiation. [b] Reaction temperature. [c]
Reaction time.
Scheme 1. Synthesis of nanoferrite–[Ru(OH)]x.
ed using nanoferrites with particle sizes from 10–15 nm (Fig-
ure 1a,b) and hydration did not proceed at 1008C as well as
at 1308C even after 30 min of MW exposure (Table 1, en-
tries 1 & 2). We then tested nanoferrite–[Ru(OH)]x with
particle sizes from 11–16 nm (Figure 1c,d) as catalyst at
1008C for 20 min under MW irradiation and the low conver-
sion of nitrile to amide (30%) was observed (Table 1,
entry 3). However, when the reaction temperature was in-
creased to 1308C, the reaction proceeded expeditiously with
65% conversion within 20 min (Table 1, entry 4), which was
further increased to 85% by extending the reaction time by
another 10 min (Table 1, entry 5).
Using the above optimized conditions, the scope of the
present nanoferrite–[Ru(OH)]x catalyst was then explored
for hydration of a variety of nitriles (Table 2). Nanoferrite–
[Ru(OH)]x has shown high catalytic activity for hydration of
activated, inactivated, and heterocyclic nitriles in pure
water. A variety of benzonitriles derivatives (Table 1, en-
tries 1 & 10) as well as aliphatic nitriles (Table 1, entries 11
& 12) were smoothly hydrated to corresponding amides in
excellent yield. The rates were barely influenced by the elec-
tronic effects of the substituentꢁs on the aromatic ring of
benzonitriles. Interestingly, the hydration of m- and p-nitro
benzonitriles (Table 2, entries 5 & 6) as well as 3- and 4-
cyano pyridine (Table 2, entries 7 & 8) proceeded with simi-
lar rate, without any difference in reactivity, which shows
negligible influence of substituentꢁs position on reaction
rate. Heterocyclic nitriles (Table 2, entries 7 to 10), exten-
sively used building-blocks in drug discovery underwent hy-
dration reaction with high yield, proving the suitability of
this protocol for assembly of bio-molecules. However, hy-
dration of isonicotinonitrile N-oxide does not yield corre-
sponding isonicotinoamide N-oxide, instead isonicotino-
À
Figure 1. a) TEM of [Fe3O4], b) XRD of [Fe3O4], c) TEM of [Fe3O4]
À
[Ru(OH)]x, and d) XRD of [Fe3O4] [Ru(OH)]x.
percentage of Ru was found to be 3.22% by inductively
coupled plasma-atomic emission spectroscopy (ICP-AES)
analysis.
This ruthenium hydroxide coated nanomaterial was then
explored as a heterogeneous catalyst for hydration of nitriles
in aqueous medium as a benign solvent,[16] under microwave
(MW) irradiation conditions (Scheme 2). Use of MW-assist-
ed chemistry is due to the efficiency of the interaction of
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
group to afford the corresponding amide, while keeping the
dioxole ring intact (Table 2, entry 10) This chemoselective
Scheme 2. Nanoferrite–[Ru(OH)]x-catalyzed hydration of nitriles. X=Cl,
OMe, NO2, NMe2, heterocycles.
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R. S. Varma and V. Polshettiwar
Table 2. Hydration of nitriles using nanoferrite–[Ru(OH)]x.[a]
Entry
Substrate
Product
Yield [%][b]
85
1
2
3
4
5
88
Scheme 3. Nanoferrite–[Ru(OH)]x-catalyzed oxygenation of benzylamine.
84
61[c]
88
the reaction time was further increased to 60 min, the reac-
tion proceeded expeditiously with 71% benzylidenebenzyl-
AHCTUNGTREGaNNUN mine (II). These results open up a benign route for the
conversion of benzylamine to benzylidenebenzylamine and
prove the versatility of this catalyst system.
Separation and recovery of the catalyst is an important
aspect for the synthesis of fine chemicals; which is generally
performed by filtration with reduced efficiency. In our cata-
lytic system, because of the super-paramagnetic nature of
À
6
85
7
8
9
88
85
75
the nanoACTHNUGRTNE[UNG Fe3O4] [Ru(OH)]x, it can be recovered by simply
using external magnets, with high efficiency and up to 95%
recovery of catalyst (Figure 2). After completion of the reac-
10
80
11
12
86
76
13
14
82
85
Figure 2. Hydration of benzonitrile to benzamide using nanoferrite–
[Ru(OH)]x catalyst. a) During reaction under stirring, b) after completion
of reaction without stirring, c) catalyst removal by external magnet, and
d) product crystals appeared after cooling the aqueous solution.
[a] Reactions were carried out with 1 mmol of nitrile, 100 mg nanocata-
lyst at 1308C for 30 min, in water under MW irradiation. [b] Isolated
yield. [c] Reaction time 45 min.
tion, the reaction mixture turned clear and the catalyst was
deposited on the magnetic bar within 30–45 sec (Figure 2b),
which was easily removed from reaction mixture using an
external magnet (Figure 2c). After separation of catalyst,
the clear liquid was cooled slowly and analytically pure crys-
tals of benzamides appeared (Figure 2d), which can be iso-
lated from water medium by simple decantation. Thus,
whole process was carried out in benign aqueous medium
and no organic solvents were used, even in the workup steps
of the reaction.
aspect of the protocol bodes well for total synthesis of drug
molecules, for which it is required that a nitrile group be se-
lectively hydrated to its amide counterpart without affecting
other functional groups. We evaluated this catalyst for hy-
dration of syn-benzaldoxime (Table 2, entry 13) and cyclo-
hexanone oxime (Table 2, entry 14); which produced the
corresponding benzaldehyde and cyclohexanone in good
yield.
The scope of this catalyst was also examined for the oxi-
dation of amines to amides (Scheme 3). First the reaction
was conducted using 1 mmol of benzyl amine and 100 mg of
catalyst at 1408C under MW irradiation for 30 min and for-
mation of corresponding amide (I) and by-product benzyli-
denebenzylamine (II) were observed, with poor conversions
and as the reaction time increased up to 45 min, II was ob-
tained with moderate (32%) conversion. However, when
For practical applications of heterogeneous systems, the
lifetime of the catalyst and its level of reusability are very
important factors. To clarify this issue, we established a set
of experiments for the hydration of benzonitrile using the
nanoferrite–[Ru(OH)]x catalyst. After the completion of the
first reaction to afford the corresponding benzamide, the
catalyst was recovered magnetically, washed with methanol,
and finally dried at 508C. A new reaction was then per-
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Nanoparticle-Supported Ruthenium Catalyst
COMMUNICATION
Surface modification of nanoferrites: NanoACTHNURGTNEG[UN Fe3O4] (2 g) was dispersed in
formed with fresh benzonitrile under the same conditions.
The nanoferrite-supported ruthenium hydroxide catalyst
could be used at least 3 times without any change in the ac-
tivity.
25 mL water by sonication for 30 min. Dopamine hydrochloride (2 g) dis-
solved in 5 mL of water was added to this solution and again sonicated
for 2 h. The amine-functionalized nanomaterial was then precipitated
using acetone, isolated by using an external magnet, and dried under
vacuum at 508C for 2 h. Analysis of the FT-IR spectra confirms the an-
choring of dopamine on ferrite surfaces (Figure S3 in the Supporting In-
formation).
Heterogeneity and metal leaching of this catalyst for the
hydration of benzonitrile was examined by the modified
ꢂhot filtrationꢁ test. The reaction was stopped at 25% con-
version (10 min reaction time) and after 30 sec, the reaction
mixture turned clear liquid and solid catalyst was deposited
on the magnetic bar. Half portion of the liquid reaction mix-
ture was taken into another reaction tube under hot condi-
tions. After an additional 20 min MW exposure at 1308C,
the portion containing the nanocatalyst had proceeded to
85% conversion, while the catalyst-free portion reacted only
upto 32%, evidently proving the heterogeneity of catalyst.
Metal leaching was studied by ICP-AES analysis of the cata-
lyst before and after the three reaction cycles. The Ru con-
centration of the catalyst was found to be 3.22% before the
reaction and 3.16% after the reaction and there was no Ru
detected in the final hydration product, which confirmed
negligible Ru leaching. This is owed to a well defined
amine-binding site located on the surface of nanoferrite
(Scheme 1), which acts as a pseudo-ligand by non-covalent
binding with [Ru(OH)]x through metal-ligand interaction.
This non-covalent anchoring minimizes deterioration and
metal leaching and allows efficient catalyst recycling. The
most important criterion in choosing a catalyst is the metal
recovery. It would be preferable to use a magnetically recov-
erable nanoferrite–[Ru(OH)]x catalyst, provided that the re-
action proceeds at high turnover number (TON) and turn-
over frequencies (TOF) (Figure S4 in the Supporting Infor-
mation) and that the catalyst leaves no remnants of metal in
the end product, since metal contamination is highly regulat-
ed by the chemical industries. All above conditions were
well satisfied by our recyclable nanoferrite-supported
[Ru(OH)]x catalyst.
Synthesis of nanoferrite–[Ru(OH)]x catalyst: Amine-functionalized nano-
AHCTNUGTREN[GUNN Fe3O4] (2 g) was dispersed in water and RuCl3 solution in water (60 mL,
8.3ꢃ10À3 m) was added to it and stirred for 20 min. Aqueous solution of
sodium hydroxide (1m) was added dropwise to bring the pH of this mix-
ture to 13, and the resulting slurry was stirred for 36 h at room tempera-
ture. The product was separated magnetically, washed several times with
water and methanol, and dried under vacuum at 508C for 2 h. The
weight percentage of Ru in the catalyst was found to be 3.22% by ICP-
AES analysis.
Hydration of nitriles using nanoferrite–[Ru(OH)]x catalyst: 1 mmol of ni-
trile and 100 mg of nanoferrite–[Ru(OH)]x (0.003 mole% of Ru) catalyst
were placed in a crimp-sealed thick-walled glass tube equipped with a
pressure sensor and a magnetic stirrer and 5 mL of water was then added
to the reaction mixture. The reaction tube was placed inside the cavity of
a CEM Discover focused microwave synthesis system, operated at 130Æ
58C (temperature monitored by a built-in infrared sensor), power 50 to
140 Watt and pressure 10–60 psi for 30 min (Table 2). After completion
of the reaction, the reaction mixture turned clear and catalyst was depos-
ited on the magnetic bar within 30–45 sec (Figure 2b), which was easily
removed from reaction mixture using an external magnet (Figure 2c).
After separation of catalyst, the clear liquid was cooled slowly and ana-
lytically pure crystals of benzamides were obtained (Figure 2d), which
can be isolated from water medium by simple decantation/filtration.
Acknowledgements
V.P. is a research associate at the National Risk Management Research
Laboratory administered by the Oak Ridge Institute for Science and
Education.
Keywords: amides · green chemistry · nanotechnology ·
ruthenium · supported catalysts
In summary, we have developed a convenient synthesis of
nanoferrite-supported ruthenium hydroxide catalyst, which
can be readily prepared from inexpensive starting materials
in water. This nanomaterial then catalyzed the hydration of
nitriles with high yield and excellent selectivity, which pro-
ceed exclusively in aqueous medium without using any or-
ganic solvents even in the workup stage. Also, being mag-
netically separable eliminated the requirement of catalyst
filtration after completion of the reaction, which is an addi-
tional sustainable attribute of this oxidation protocol.
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Received: October 30, 2008
Published online: January 2, 2009
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