2-Diphenylphosphanyl-4-pyridyl(dimethyl)amine as an effective ligand for the ruthenium(II) complex catalyzed homogeneous hydration of nitriles under neutral conditions

Article abstract of DOI:10.1016/j.cattod.2010.11.050

New homogeneous catalyst comprised of [Ru(methallyl)₂(cod)] (cod = 1,5-cyclooctadiene) (1) and 2-diphenylphosphanyl-4-pyridyl(dimethyl)amine (2) is shown to efficiently catalyze the hydration of various nitriles under neutral conditions. The hydration proceeds in the presence of 0.5 mol% of the ruthenium catalyst at 80 °C in 1,2-dimethoxyethane solution and the corresponding amide is obtained within few hours without the formation of byproducts. Comparison of some phosphine ligands for the hydration reveals that the dimethylamino moiety of 2 improves the catalytic performance dramatically.

Full text of DOI:10.1016/j.cattod.2010.11.050

Catalysis Today 164 (2011) 552–555
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Catalysis Today
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2-Diphenylphosphanyl-4-pyridyl(dimethyl)amine as an effective ligand for the
ruthenium(II) complex catalyzed homogeneous hydration of nitriles under
neutral conditions
Makoto Muranaka, Isao Hyodo, Wataru Okumura, Toshiyuki Oshiki∗
Division of Chemistry and Biochemistry, Graduate School of Natural Science and Technology, Okayama University, 3-1-1 Tsushima-naka, Okayama 700-8530, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 1 July 2010
Received in revised form
22 November 2010
Accepted 23 November 2010
Available online 21 December 2010
New homogeneous catalyst comprised of [Ru(methallyl)2(cod)] (cod = 1,5-cyclooctadiene) (1) and 2-
diphenylphosphanyl-4-pyridyl(dimethyl)amine (2) is shown to efficiently catalyze the hydration of
various nitriles under neutral conditions. The hydration proceeds in the presence of 0.5 mol% of the
ruthenium catalyst at 80 ◦C in 1,2-dimethoxyethane solution and the corresponding amide is obtained
within few hours without the formation of byproducts. Comparison of some phosphine ligands for the
hydration reveals that the dimethylamino moiety of 2 improves the catalytic performance dramatically.
© 2010 Elsevier B.V. All rights reserved.
Keywords:
Hydration
Nitrile
Amides
Homogeneous catalysis
Ruthenium
Phosphine
1. Introduction
nitriles to amides under neutral conditions [17]. The high turnover
frequency of up to 20 900 (mol of amide)/((mol of catalyst) h)
The catalytic hydration of nitriles to amides is an ideal
atom-economical reaction and important reaction in commercial
production of amides [1] (Scheme 1). The reaction is catalyzed
by metalloenzymes [2], heterogeneous transition-metals [3] and
homogeneous transition-metals [4]. Some of metalloenzymes and
heterogeneous catalysts have been employed in the industrial pro-
duction of amides. For example, 5 × 105 tons of acrylamide per year
has been produced from acrylonitrile. In a laboratory, a variety of
homogeneous catalysts composed of transition-metal complexes
such as chromium [5], molybdenum [6], ruthenium [7], cobalt [8],
rhodium [9], iridium [10], nickel [11], palladium [12], platinum
[13], gold [14] and zinc [15] have been investigated.
A bifunctional catalysis is one of the recent trends in a
homogeneous catalytic hydration of an unsaturated bond
[7c–f,16]. A nitrogen-containing organophosphorus ligand (P,N
ligand) have been used in the hydrations. We have reported
that the ruthenium(II) complexes, [cis-Ru(acac)2(PPh2py)2]
probably caused by the promotion of nucleophilic addition of
water by the PPh2py ligand. The bifunctional ruthenium(II) cata-
lyst affords various nitriles in almost quantitative yields. However,
the reactions required high temperature (180 ◦C), and the yields
of amides were reduced significantly by decreasing the reaction
temperature. In the catalyst system, it is difficult to generate a
vacant coordination site for coordination of nitriles, because the
site would be generated by the dissociation of an oxygen atom of
the anionic bidentate acac ligand.
To achieve the catalytic hydration under more mild conditions,
we turned our attention to [Ru(methallyl)2(cod)] (cod = 1,5-
cyclooctadiene) (1) [18]. Complex 1 is a ruthenium(II) complex and
a fluxionality of the methallyl fragment forms ␩3- and ␩1-methallyl
species under mild conditions [19]. The generated ␩1-species is
a coordinatively unsaturated ruthenium(II) complexes and would
exhibit a high performance as a catalyst [20].
Here we describe a new ruthenium(II) catalyst comprised of 1
and 2-diphenylphosphanyl-4-pyridyl(dimethyl)amine (2). Prepa-
ration of pyridylphosphine 2 was first reported by Fort and
coworkers in 2002 [21]. They mentioned that electron-rich 2 is a
potentially interesting ligand for a transition-metal complex. How-
ever, there are no reports using 2 as a ligand in fields of coordination
and catalytic chemistry. Thus, here is a first example of that 2 acts as
(acac = acetylacetonate;
PPh2py = diphenyl-2-pyridilphosphine
(3)) was an excellent bifunctional catalysts for hydration of
∗
Corresponding author. Tel.: +81 86 286 8035; fax: +81 86 286 8035.
E-mail address: oshiki@cc.okayama-u.ac.jp (T. Oshiki).
0920-5861/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.cattod.2010.11.050

M. Muranaka et al. / Catalysis Today 164 (2011) 552–555
553
Table 2
Catalytic hydration of benzonitrile under various conditions.a
Entry
Solvent
Temp (◦C)
Time (h)
Yield (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
1,2-Dimethoxyethane
1,2-Dimethoxyethane
THF
THF
80
60
80
60
60
80
60
60
60
60
80
60
60
60
60
60
3.5
3.5
3.5
3.5
24
3.5
3.5
24
3.5
3.5
3.5
3.5
3.5
3.5
3.5
3.5
>99
30
90
2
37
>99
11
46
0
3
23
9
1
0
THF
Scheme 1. Catalytic hydration of a nitrile to an amide.
1,4-Dioxane
1,4-Dioxane
1,4-Dioxane
Triglyme
Cyclopentyl methyl ether
EtOH
EtOH
H2O
b
[BMIM]N(SO2CF3)2
[BMIM]BF4
b
0
9
None
a
Scheme 2. Complex 1 and schematic equation of its fluxional character.
Experimental conditions: benzonitrile (1.0 mmol), water (4.0 mmol),
1
(0.005 mmol, 0.5 mol% per benzonitrile), 2 (0.015 mmol), solvent (0.5 mL).
b
BMIM = 1-butyl-3-methyl-imidazorinium.
Table 1
Catalytic hydration of benzonitrile using 2–5 as a ligand.a
At 80 ◦C for 3.5 h, the yield of benzamide was very low using
3 as a ligand (entry 2). The combination of 1 and 2 dramatically
improved the rate of the reaction and benzamide was obtained
in 61% yield (entry 3). On the contrary, 4 [16b] and 5 were
slightly effective (entries 4 and 5). Theses results suggested that the
electron-donating dimethylamino group of 2 increased the electron
density of a nitrogen atom in the pyridine ring and water was more
activated via a hydrogen bond [17a] than in the case of using 3 as a
catalyst component. The rate of the reaction was accelerated with
an increasing the amount of 2 (entries 6–8). It was estimated that
the ratio of 1 to 2 was three for the catalytic preparation of ben-
zamide. Attempts to identify the ruthenium species generated by
Entry
Ligand
Amt of ligandb
Temp (◦C)
Time (h)
Yield (%)
1
2
3
4
5
6
7
8
3
3
2
4
5
2
2
2
2
2
2
2
2
3
4
0.5
180
80
80
80
80
80
80
80
0.17
3.5
3.5
3.5
3.5
3.5
3.5
3.5
>99
3
61
7
1
>99
91
10
a
Experimental conditions: benzonitrile (1.0 mmol), water (4.0 mmol),
1
(0.005 mmol, 0.5 mol% per benzonitrile), 1,2-dimethoxyethane (0.5 mL), 80 ◦C, 3.5 h.
b
Molar ratio between ligand and 1.
1
the reaction of 1 and 2 failed. The 31P{ H} NMR spectrum of the
an effective ligand for a homogeneous catalytic reaction. The hydra-
tion of various nitriles proceeded at 80 ◦C under neutral conditions
(Scheme 2).
mixture of 1 and 2 showed complex signals from −22 to 17 ppm
under various conditions.
Table 2 shows the results obtained for the hydration of ben-
zonitrile under different conditions. 1,2-Dimethoxyethane is the
best solvent for the hydration catalyzed by the mixture of 1 and
2. The hydration in 1,2-dimethoxyethane proceeded at 60 ◦C. After
48 h, benzamide was obtained in 99% isolated yield (Scheme 4).
THF and 1,4-dioxane were suitable solvent for the hydrations. No
reactions proceeded in highly polar ionic liquids (entries 14 and
15). The hydration under solvent-free conditions also examined.
Benzamide was only obtained in 9% yield (entry 10).
The results of the catalytic hydration of various kinds
of nitriles are summarized in Table 3. The para substituted
aromatic nitriles, 4-tolunitrile, 4-methoxybenzonitrile and 4-
chlorobenzonitrile were smoothly converted to the corresponding
amides in high yields (entries 1, 3 and 4). The electron with-
drawing Cl group did not interrupt the hydration. The reaction
of 2-tolunitrile is sluggish compare to that of 4-tolunitrile due to
the sterically crowded nitrile group (entry 2). The low reactiv-
ity of 2-tolunitrile is quite different from those observed using
2. Results and discussion
Initially catalytic hydration of nitriles was investigated using
benzonitrile as a substrate. Catalytic performances of the com-
bination of 1 and pyridylphosphine derivatives 2–4 and 5 were
summarized in Table 1 (Scheme 3).
Quantitative formation of benzamide was observed when using
a mixture of 1 and 3 at 180 ◦C for 10 min (entry 1). The effect of
ligand 3 was demonstrated by comparison with the hydration cat-
alyzed by 1 and PPh3. When using PPh3 as a ligand, no benzamide
was detected in the reaction. These preliminary results provide
the evidence that 1 is suitable ruthenium(II) complex and ligand
3 enhances the catalytic hydration. Thus, 3 behaves a effective P,N
ligand like our previously reported ruthenium(II) complexes for the
hydration [17a]. Interestingly, sole complex 1 afforded benzamide
in 8% yield.
Scheme 3. Structures of nitrogen containing phosphine ligands 2–5.

554
M. Muranaka et al. / Catalysis Today 164 (2011) 552–555
Scheme 4. Catalytic hydration at 60 ◦C.
Table 3
Catalytic hydration of various nitriles at 80 ◦C for 5 h.a
Entry
Nitrite
Yield (%)
Entry
Nitrile
Yield (%)
1
2
3
4
96
38
99
95
8b
9b
41
91
60
63
10b
11b
5
6
1
12
99
45
6
13b
7b
38
a
Experimental conditions: nitrile (1.0 mmol), water (4.0 mmol), 1 (0.01 mmol, 1.0 mol% per nitrile), 2 (0.03 mmol), 1,2-dimethoxyethane (0.5 mL).
0.02 mmol of 1 (2.0 mol% per nitrile) and 0.06 mmol of 2 were used.
b
our reported catalysts [17a]. The catalytic hydration proved to
can be prepared from readily available organic reagents. In addi-
tion, 2 is robust tertially phosphine ligand and can be stored
under ambient conditions. Owing to the strongly electron-donating
dimethylamino group of 2, the catalyst acted under 80 ◦C to give
various amides in high yield. The protocol using 2 as a ligand would
promises the broad application for a homogeneous catalytic reac-
tion in organic synthesis. Continuing studies on the effect of the
structure–efficiency relationships of ruthenium catalysts will allow
us to further the usefulness of homogeneous hydration catalyst
under neutral conditions.
be affected by the presence of carbonyl groups. For example, the
hydration of 4-cyanobenzaldehyde gave the corresponding amide
only 1% yield (entry 5). In spite of longer reaction time (12 h),
the yield of 4-cyanobenzoic acid methyl ester was very low (10%)
(entry 6). These carbonyl moieties of the starting materials did
not hydrolyzed in the reactions. Commercially useful nicotinamide
could be obtained from 3-cyanopyridine (entry 7). The catalyst
also hydrolyzed less reactive aliphatic nitriles. Phenylacetonitrile
was smoothly hydrated to give the desired amide in excellent
yield (entry 9). The catalytic hydration of phenoxyacetonitrile and
trans-cinnamonitrile afforded the corresponding amides in mod-
erate yields (entries 10 and 11). Nonanamide was obtained from
nonanonitrile in almost quantitative yield under conditions simi-
lar to those for aromatic nitriles as a substrate (entry 12). In this
type of aliphatic nitrile, the nitrile group is less electrophilic than
that of an aromatic nitrile. Contrary to the reported hydration cat-
alysts [3a,17a], the catalyst comprised of 1 and 2 is suitable for the
hydration of less reactive aliphatic nitriles. In all catalytic hydra-
tions examined, no byproduct was observed.
4. Experimental
All manipulations involving air- and moisture-sensitive com-
pounds were carried out using standard Schlenk techniques under
argon. Anhydrous hexane, THF, and toluene were purchased from
˚
Wako Pure Chemical Industries, Ltd. and stored on 4 A molecular
sieves under argon. Anhydrous 1,2-dimethoxyethane was pur-
chased from Kanto Chemicals and stored on 4 A molecular sieves
under argon. [Ru(methallyl)2(cod)] and 4 were purchased from
Strem and used as received. Diphenyl-2-pyridylphosphine (3) and
4-(dimethylamino)-phenyldiphenylphosphine (5) were purchased
3. Conclusion
from Aldrich and used as received. 1H, and 31P{ H} NMR spectra
1
In summary, we have presented new ruthenium(II) catalysts
for the hydration of nitriles. The combination of 1 and 2 exhib-
ited the best catalytic efficiency. Complex 1 is accessible and 2
were recorded at 399.65 and 161.70 MHz, respectively, on a JEOL
JNM-LA400 spectrometer. The 1H NMR chemical shift is relative to
tetramethylsilane; the resonance of the residual protons of CDCl3

M. Muranaka et al. / Catalysis Today 164 (2011) 552–555
555
was used as an internal standard (ı 7.26 ppm chloroform for 1H).
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described procedure [21] with some modifications as follows. A
solution of 2-(dimethylamino)ethanol (5 mL, 40 mmol) in hexane
(50 mL) was cooled to 0 ◦C, and n-BuLi (2 M hexane solution, 50 mL,
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1.5 h of stirring at 0 ◦C, 4-dimethylaminopyridine (2.4 g, 20 mmol)
was added as a solid. After 1 h of stirring at 0 ◦C, the reaction
mixture was cooled to −78 ◦C, and a hexane (100 mL) solution
of chlorodiphenylphosphine (9.0 mL, 50 mmol) was slowly added
as a droplet. After 1.5 h for stirring at −78 ◦C, the resulting reac-
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Complex 1 and ligand 2 were placed in a 5 mL test tube
equipped with a screw cap. 1,2-Dimethoxyethane (0.5 mL), ben-
zonitrile (102 ␮L, 1.0 mmol), and water (72 ␮L, 4.0 mmol) were
added, and then, the tube was sealed by the screw cap. The tube
was heated in an oil bath. After the reaction, the reaction mixture
was subjected to gas chromatography (Shimadzu GC-14A, column:
RESTEK RTX-1 (15 m x 0.32 mm)). In Tables 1 and 2, the yields of
benzamide were calculated from response factors relative to an
internal naphthalene standard. In Table 3, amides were isolated
by a silica-gel short column using ethyl acetate as an eluent. The
amides were identified spectroscopically by comparison to authen-
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