Aqueous and biphasic nitrile hydration catalyzed by a recyclable Ru(ii) complex under atmospheric conditions

Article abstract of DOI:10.1039/c1gc15950j

[RuCl₂(PTA)₄] (PTA = 1,3,5-triaza-7- phosphaadamantane) was found to be a highly active catalyst for aqueous phase nitrile hydration at 100 °C in air. Near quantitative conversion of aromatic, alkyl, and vinyl nitriles to their corresponding amides was observed. The reaction tolerated ether, hydroxyl, nitro, bromo, formyl, pyridyl, benzyl, alkyl, and olefinic functional groups. Some amides were isolated by simple decantation from the aqueous phase catalyst. Catalyst loading down to 0.001 mol% was examined with turnover numbers as high as 22000 observed. The catalyst was stable for weeks in solution and could be reused more than five times without significant loss of activity.

Full text of DOI:10.1039/c1gc15950j

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COMMUNICATION
Aqueous and biphasic nitrile hydration catalyzed by a recyclable Ru(II)
complex under atmospheric conditions†
Wei-Chih Lee and Brian J. Frost*
Received 3rd August 2011, Accepted 3rd October 2011
DOI: 10.1039/c1gc15950j
2
0
[
RuCl
2
(PTA)
4
] (PTA = 1,3,5-triaza-7-phosphaadamantane)
applied in transfer hydrogenation of carbonyl groups, iso-
21,22
23–26
was found to be a highly active catalyst for aqueous
phase nitrile hydration at 100 C in air. Near quantitative
merization of alkenes,
and hydration of alkynes
and
◦
9–12,16–18
nitriles.
Recently, Cadierno and co-workers reported a
conversion of aromatic, alkyl, and vinyl nitriles to their
corresponding amides was observed. The reaction tolerated
ether, hydroxyl, nitro, bromo, formyl, pyridyl, benzyl, alkyl,
and olefinic functional groups. Some amides were isolated
by simple decantation from the aqueous phase catalyst.
Catalyst loading down to 0.001 mol% was examined with
turnover numbers as high as 22 000 observed. The catalyst
was stable for weeks in solution and could be reused more
than five times without significant loss of activity.
series of Ru(II) and Ru(IV) complexes with nitrogen-containing
9–12,27
phosphines for nitrile hydration in water.
The superior
6
activity of [RuCl
-phosphaadamantane, over [RuCl
2
(h -benzene)(PTA)], where PTA = 1,3,5-triaza-
6
7
2
(h -benzene)(PPh )], was
3
attributed to the cooperative effect of the PTA amine function-
ality activating water via hydrogen bonding. Cadierno’s work
prompted us to investigate the catalytic activity of [RuCl
for nitrile hydration compared with the inactive [RuCl
2
(PTA)
(PPh
4
]
]
2
3
)
3
28
analogue. Darensbourg, Jo o´ , and co-workers first synthesized
the air-stable and water-soluble [RuCl (PTA) ] and utilized it
as a catalyst for the hydrogenation of aldehydes, olefins, and
2
4
28
28
Amides are versatile and important synthetic intermediates
used in the production of a variety of plastics, detergents,
29
CO
2
in aqueous or biphasic media. Herein we describe the
1,2
hydration of various nitriles to their corresponding amides in
aqueous and biphasic media by the air-stable, water-soluble,
lubricants, and polymers. Traditional methods of hydrating
nitriles to amides involve the use of strong bases or acids
under harsh conditions, often inducing undesired hydrolysis to
efficient, and recyclable catalyst: [RuCl
2
(PTA)
4
].
1–5
The catalytic activity of 5 mol% [RuCl
hydration was evaluated in aqueous solution at 100 C with
2
(PTA)
4
] toward nitrile
carboxylic acids. During neutralization salt formation can
◦
1–3
lead to contamination, separation, and pollution problems.
Moreover, poor tolerance for sensitive functional groups is
1
mmol nitrile in a culture tube under air (Scheme 1). Under
the conditions described here RuCl (5 mol%) provided a 54%
conversion of benzonitrile to benzamide in 24 h. Benzonitrile
hydration by 2 mol% RuCl was previously reported to yield
28% benzamide after 3 h at 130 C. No hydration was
1–3
3
often observed under such conditions. A variety of transition
metal complexes have been developed as efficient catalysts for
nitrile hydration in organic media often with air-sensitive or
3
◦
15
1–7
non-recyclable catalysts. Water, with its unique properties,
has seen increased use as a solvent in stoichiometric and
6
observed in the absence of a catalyst, or with PTA, [RuCl
toluene)] , or [RuCl (PPh ] as catalysts. Benzonitrile hydration
by [RuCl
2
(h -
8
9–11
2
2
)
3 3
catalytic reactions. Recently several aqueous homogeneous
◦
12–15
2
(PTA)
4
] did not occur at 50 C and provided only 23%
and heterogeneous
nitrile hydration reactions have been
◦
conversion after 24 h at 75 C. The hydration of benzonitrile
reported under inert atmosphere.
catalyzed by [RuCl
2
◦
(PTA)
4
] showed a >99% conversion to
One common mechanistic pathway proposed for the hydra-
tion of nitriles involves nitrile coordination to a metal followed
benzamide at 100 C after 7 h, in contrast to the inactive
30
1
[RuCl
of the nitrogen-containing PTA versus PPh
nitrile hydration catalyzed by 5 mol% [RuCl
2
(PPh ) ], potentially demonstrating a cooperative effect
3
3
by nucleophilic attack by water on the nitrile carbon. Recent
3
1
3
.
For comparison,
(h -arene)(PTA)]
studies have shown that nucleophilic attack can be promoted
6
2
by activating water via hydrogen bonding to a functionalized
6
9–12,16–18
19
(h -arene = benzene, p-cymene, 1,3,5-trimethylbenzene, and
hexamethylbenzene), showed >98% conversions in 4–9 h for
ligand.
The concept of bifunctional catalysis has been
◦
9
aqueous benzonitrile hydration under N at 100 C.
2
Department of Chemistry, MS 216, University of Nevada, Reno, Nevada,
8
9557, USA. E-mail: frost@unr.edu
†
Electronic supplementary information (ESI) available: Experimen-
tal details including nitrile hydration and catalyst recycling proce-
31
1
dures, P{ H} NMR spectrum of the reaction mixture, and GC
1
13
1
chromatogram, H and C{ H} NMR spectra of isolated amides. See
DOI: 10.1039/c1gc15950j
2 4
Scheme 1 [RuCl (PTA) ]-catalyzed nitrile hydration.
6
2 | Green Chem., 2012, 14, 62–66
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Table 1 Substrate scope for nitrile hydration catalyzed by
a
[
RuCl
2
(PTA)
4
]
b
Entry
1
Substrate
Conversion (%)
Isolated yield (%)
c
99
91
d
2
3
83(99)
67
c
96(99)
84
Fig. 1 Hydration of p-anisonitrile to p-anisamide. (Left) p-Anisamide
crystals appear after cooling the reaction. (Right) Isolated p-anisamide
crystals by filtration.
d
4
5
6
7
8
9
97(99)
90(99)
89(99)
99
77
lower conversion relative to m- and p-tolunitriles (Table 1,
entries 2–4), which is attributed to steric hindrance of the o-
tolunitriles. This indicates nitrile activation by coordinating to
rutheniummaybe necessary. Hydration of4-cyanobenzaldehyde
led to 4-formylbenzamide in a 99% conversion in 7 h with an
intact formyl moiety (entry 9). The coordinating ability of the
pyridyl functionality reduced catalytic activity as hydration of 2-
cyanopyridine to picolinamide resulted in only 81% conversion
after 24 h (entry 10).
d
78
c
90
c
86
d
99
73
[
RuCl
2
(PTA) ] was also effective as a hydration catalyst for
4
c
the less reactive aliphatic nitriles (Table 1, entries 11–13). 4-
Methylbenzyl cyanide was transformed with 99% conversion in
99
87
7
h (entry 11) into the amide. Hydration of the sterically bulky
pivalonitrile to pivalamide proceeded with a 99% conversion in
4 h although a modest conversion of 67% was observed after 7
h (entry 13). The resistance of tertiary nitriles toward hydrolysis
c
1
1
0
1
43(81)
72
2
d
99
81
36
has been noted. The industrially important acrylonitrile was
almost quantitatively converted into acrylamide in 7 h without
observation of polymerization or hydrolysis byproducts (Table
1, entry 14). For all the nitrile hydrations studied, the corre-
sponding amides were the only product observed (no carboxylic
acids were detected by GC-MS). Thus, the catalytic conditions
described here are compatible with ether (entry 5), hydroxyl
c
1
1
2
3
72(99)
67(99)
88
c
85
c
1
4
99
87
(
entry 6), nitro (entry 7), bromo (entry 8), formyl (entry 9),
a
Conditions: nitrile (1 mmol), [RuCl
2
(PTA)
4 2
] (5 mol%), H O (3 mL),
c
◦
b
pyridyl (entry 10), benzyl (entry 11), alkyl (entries 12–13), and
olefinic (entry 14) functional groups, which establishes a wide
synthetic scope.
1
00 C, in air. GC yields after 7 h (24 h yield in parentheses). Isolated
d
by column chromatography. Isolated by decantation.
Little or no induction period was observed in a kinetics plot
The conversion of various nitriles to the corresponding amides
was explored with results summarized in Table 1. All nitriles
were efficiently converted to amides with 67–99% conversion in
of the hydration of benzonitrile by [RuCl
2
(PTA) ] in water (Fig.
4
2). Free PTA was observed in solution while monitoring the
reaction by GC-MS. This suggests that formation of the catalyt-
ically active species may involve PTA dissociation, presumably
followed by nitrile coordination. Dissociation of PTA from
7
h and >99% conversions by 24 h, with the exception of 2-
cyanopyridine (81% after 24h). After completion, the reactions
◦
were cooled to 0 C and, in most cases, the product amides
[RuCl
2
(PTA) ] to allow coordination of unsaturated aldehydes
4
28,29
crystallized out as white needles and were easily isolated in 67–
has been discussed for the hydrogenation of aldehydes.
32
31
1
8
1% yield by decantation (Fig. 1). The identity of the isolated
P{ H} NMR spectroscopy was obtained on the reaction mix-
32
amides was confirmed by GC-MS and NMR spectroscopy.
Substituted benzonitriles bearing electron-withdrawing
groups (Table 1, entries 7–9) exhibited slightly more effi-
cient conversions to amides than those with electron-donating
groups (entries 2–6). Presumably, the presence of the electron-
withdrawing group makes the nitrile carbon more susceptible
to nucleophilic attack by an activated water molecule. This
ture after 2 h in attempt to identify active species in water. The
NMR spectrum was consistent with a series of substitutional
isomers [Ru(PTA)
Jo o´ and coworkers from reaction of [Ru(H
and cis/trans isomerization of [RuCl (PTA)
The durability of [RuCl (PTA) ] was evaluated by reducing the
2
+
x
(H
2
O)
y
(PhCN)6-x-y
]
similar to the results of
2
+
37
2
O)
6
]
with PTA
38
2
4
] by our group.
2
4
catalyst loading (Table 2). We examined catalyst loading down to
0.001 mol% for the hydration of benzonitrile. Turnover numbers
(TONs) up to 22 000 (entry 9, Table 2) were obtained, which are
1
2,33
is in agreement with other catalyst systems
although there
15,34
have been exceptions reported.
As previously reported
18,33–35
1
for ortho-substituted benzonitriles,
o-tolunitrile exhibited
among the highest reported to date. Preparative scale reactions
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Green Chem., 2012, 14, 62–66 | 63

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Table 3 Effect of dilution on the hydration of benzonitrile at 1 mol%
a
catalyst loading
b
Conversion (%)
Entry
H
2
O (mL)
[Nitrile] (M)
2 h
7 h
24 h
1
2
3
1
3
5
1.00
0.33
0.20
19
41
43
62
92
96
99
99
99
a
◦
2 4
Conditions: nitrile (1 mmol), [RuCl (PTA) ] (1 mol%), 100 C, in air.
Conversions are determined by GC.
b
2 4
Fig. 2 Kinetics for the hydration of benzonitrile using [RuCl (PTA) ].
Conversions were determined by GC-MS with each data point a
2 4
Table 4 Raecycling experiments with [RuCl (PTA) ] under aqueous
conditions
separate reaction under identical conditions: benzonitrile (1 mmol),
◦
[
RuCl
2
(PTA)
4
] (5 mol%), H
2
O (3 mL), 100 C, in air.
b
Recycling experiment
Substrate
1
2
3
4
5
6
7
Table 2 Effect of reduced catalyst loading on the hydration of benzoni-
a
trile catalyzed by [RuCl
2
(PTA)
4
]
99
99
99
99
99
99
99
c
(
18)
(53) (67) (55) (61) (52) (66)
Catalyst
Entry (mol%)
b
c
d
-1
d
Time (h)
Conv. (%)
TON
TOF (h )
99
99
99
96
95
85
38
(
69)
(80) (92) (85) (89) (87)
1
2
3
4
5
6
7
8
9
5
1
1
7
7
24
70
99
87
99
99
98(93)
66
(83)
(12)
(22)
19.8
87
99
2.8
12.4
4.1
a
2 4 2
Conditions: nitrile (1 mmol), [RuCl (PTA) ] (5 mol%), H O (3 mL),
b
◦
0.1
0.1
0.01
0.01
0.001
0.001
990
930
14.1
100 C, 7 h, in air. Conversions are determined by GC (isolated yields
by decantation are given in parentheses). A 93% isolated yield can be
obtained by extraction with CH
e
f
g
g
c
31
30
d
504
528
864
2328
6600
8300
12 000
22 000
13.1
15.7
13.9
9.5
2
Cl
2
(3 mL ¥ 5). See ref. 32.
e
e
h
g
g
h
g
g
g
h
g
which addition of 5 mol% chloride ion (0.02 M) reduces nitrile
hydration conversion from 87 to 2.7%.
a
◦
c
42
Conditions: nitrile (1 mmol, 0.33 M), H
2
O (3 mL), 100 C, in
b
air. Determined by GC (isolated yields in parentheses). TON
Further evidence for the robust nature of the catalyst was
d
-1
(
mol product)/(mol catalyst). TOF (mol product)/(mol catalyst) h .
f
e
obtained upon storing an aqueous solution of [RuCl
under air for four weeks at ~0 C. Hydration of benzonitrile
performed using the stored aqueous solution of [RuCl
(
2
(PTA) ]
4
2
Nitrile (20 mmol, 6.66 M in water), H O (3 mL). Isolated yield by
◦
recrystallization from water (2.12 g, 88%) and purification of the filtrate
by chromatography providing an additional 0.13 g (5%) benzamide.
2
(PTA) ]
4
g
h
Based on isolated yield. Isolated yield by column chromatography.
1 mol% loading) resulted in a 83% conversion after 7 h
compared to 87% by freshly-prepared catalyst in water; both
reactions reached >99% conversion by 24 h.
were also examined for the hydration of benzonitrile (Table 2,
entries 5, 7, 8). For example, 2.02 g of benzonitrile was hydrated
at 0.1 mol% catalyst loading to give a 2.25 g (93%) isolated
The ability to store aqueous solutions of [RuCl
2
(PTA) ]
4
prompted us to explore reusability of the catalyst through
recycling experiments with benzonitrile and 4-methylbenzyl
cyanide, Table 4. After 7 h the reaction was cooled overnight and
the aqueous supernatant containing the catalyst was carefully
transferred to another reaction tube followed by addition of
fresh nitrile. The precipitated amide product was collected and
washed with cold water. Further purification was unnecessary
-
1
benzamide with a TON of 930 and a TOF of 30 h (Table 2,
entry 5). The TOF increases with a decrease in catalyst loading
or increase in [nitrile]. For example in Table 2 entries 4 and 5 the
benzonitrile concentration is increased from 0.33 M to 6.66 M
at 0.1 mol% catalyst loading and the TOF increases from 14.1 to
-
1
1
13
1
3
0 h . Faster reactions with higher concentration of the nitrile
as determined by GC-MS, H and C{ H} NMR spectroscopy.
The isolated yields in Table 4 were obtained by decantation;
maximized isolated yields (93%) can be obtained by extraction
with an organic solvent (Table 4, run 1, see footnote c). Recycling
was carried out at least seven times without significant loss of the
catalytic activity or selectivity in the case of benzonitrile. With
4-methylbenzyl cyanide a decrease in activity was observed after
the fifth recycling experiment. We attribute this to incomplete
catalyst recovery during transfer of the aqueous supernatant
15,16,39
have been observed in other ruthenium catalytic systems.
It should be noted that in the preparative scale reactions (2
mL benzonitrile, 3 mL water) two phases were observed during
hydration followed by formation of a homogeneous solution,
indicating hydration was complete. The rate of hydration was
also found to increase with an increase in the volume of water
(
i.e. dilution) possibly indicative of substrate/product inhibition
at higher concentration, Table 3. Better catalytic performance
32,43
(
higher TOF) at lower concentration of the catalyst has also
indicated by a faint yellow color observed in the product.
16,40
been reported for other ruthenium complexes.
Addition of 1
The isolated yields in the aqueous phase recycling experiments
with benzonitrile were modest as decantation left some benza-
mide dispersed in water. Aqueous/organic biphasic hydration
of benzonitrile was thus studied, Table 5. Due to the mixing
mmol (100 mol%, 0.33 M) NaCl to the hydration of benzonitrile
resulted in a slight decrease in conversion from 99 to 93%
41
after 7 h. This is in contrast to a Pd nanoparticle system in
6
4 | Green Chem., 2012, 14, 62–66
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Table 5 Raecycling experiments with [RuCl
2
(PTA)
4
] under biphasic
Support from NSF is also acknowledged for the NMR facilities
CHE-0521191).
conditions
(
b
Recycling experiment
Notes and references
Substrate
1
2
3
4
5
6
7
1
2
3
T. J. Ahmed, S. M. M. Knapp and D. R. Tyler, Coord. Chem. Rev.,
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9
9
99
(93)
99
(89)
98
(91)
98
(88)
89
(89)
88
(86)
2
(
69)
1
a
Conditions: nitrile (1 mmol), [RuCl
tert-amyl alcohol (1.5 mL), 100 C, 24 h, in air. Conversions are
determined by GC (isolated yields given in parentheses).
2
(PTA)
4
] (5 mol%), H
2
O (1.5 mL),
◦
b
1
4
5
6
efficiency of two phases aqueous biphasic hydration of benzoni-
trile to benzamide reached 84% conversion after 7 h compared to
9% conversion in water; therefore, 24 h was used in the biphasic
7
8
S.-I. Murahashi and H. Takaya, Acc. Chem. Res., 2000, 33, 225–233.
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9
1
09, 643–710; (b) N. Pinault and D. W. Bruce, Coord. Chem. Rev.,
reactions. After 24 h the organic phase (tert-amyl alcohol) was
decanted off and benzamide isolated. Fresh benzonitrile and
tert-amyl alcohol were added to the aqueous phase for the
next reaction. The catalyst could be recycled five times without
significant loss of catalytic activity. A slight decrease in catalytic
activity was observed after the fifth cycle attributed to catalyst
leaching into the organic layer (slight yellow color observed over
time in the organic layer). Ruthenium leaching into the organic
layer was indeed observed by ICP-AES with the [Ru] increasing
with each cycle: recycling experiment 1 (2.9 ppm Ru), 4 (24.5
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1
1
1
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32
ppm Ru), and 7 (77.2 ppm Ru).
1
1
Conclusions
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and K. Takai, Organometallics, 2005, 24, 6287–6290.
ˇ
In conclusion, we have described an efficient and recyclable
catalytic system to convert nitriles to amides in aqueous environ-
ments with tolerance of air and a variety of functional groups.
Advantages of the catalytic system discussed here include easy
catalyst preparation, simple reaction setup, and the use of green
solvent (water). The catalyst is robust and highly recyclable
under atmospheric conditions (no inert atmosphere required).
Isolation of many amides by decantation from water largely
decreases or circumvents the use of organic solvents, even in
the workup steps. The gram-scale amide synthesis by hydration
17 T. Smejkal and B. Breit, Organometallics, 2007, 26, 2461–2464.
1
8 M. Muranaka, I. Hyodo, W. Okumura and T. Oshiki, Catal. Today,
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011, 164, 552–555.
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6
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2
2
0 T. Ikariya and A. J. Blacker, Acc. Chem. Res., 2007, 40, 1300–1308.
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2
(PTA) ] in water is simple, practical and
4
1
2233.
environmentally friendly.
24 D. B. Grotjahn, C. D. Incarvito and A. L. Rheingold, Angew. Chem.,
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2
2
2
2
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General procedure for the catalytic nitrile hydration
´
7 R. Garc ´ı a-Alvarez, J. Francos, P. Crochet and V. Cadierno, Tetrahe-
dron Lett., 2011, 52, 4218–4220.
Under air, 1 mmol nitrile, 3 mL water, and 5 mol% [RuCl
2
(PTA) ]
4
8 (a) D. J. Darensbourg, F. Jo o´ , M. Kannisto, A. Katho, J. H.
Reibenspies and D. J. Daigle, Inorg. Chem., 1994, 33, 200–208; (b) D.
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Organometallics, 1992, 11, 1990–1993.
(
40 mg) were added to a Telfon-sealed screw-cap culture tube and
◦
stirred at 100 C for 7 h. The GC yields were obtained by taking
a small aliquot (~50 mL) from the hot solution and extracting
2
9 (a) G. Laurenczy, F. Jo o´ and L. N a´ dasdi, Inorg. Chem., 2000, 39,
with CH
2
2
Cl (2 mL ¥ 3) and analysing by GC-MS. Isolated
5
083–5088; (b) F. Jo o´ , G. Laurenczy, P. Kar a´ dy, J. Elek, L. N a´ dasdi
yields were obtained by either decanting the aqueous layer from
the product crystals or by evaporation of the solvent followed by
column chromatography over silica gel (eluent: ethyl acetate).
and R. Roulet, Appl. Organomet. Chem., 2000, 14, 857–859; (c) F.
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32
3
3
0 It is unlikely that the activity of [RuCl
2
(PTA)
(PTA) ] are slightly acidic (pH
5). Our efforts exploring the effect of pH on ruthenium catalyzed
nitrile hydration will be the subject of a future manuscript.
1 While inactive for nitrile hydration [RuCl (PPh ] is active for
4
] is due to increased
-
[
OH ] as aqueous solutions of [RuCl
2
4
~
Acknowledgements
2
3 3
)
Support of this work by the National Science Foundation
CAREER program (CHE-0645365) is gratefully acknowledged.
isomerization reactions in aqueous solution: (a) C.-J. Li, D. Wang and
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Green Chem., 2012, 14, 62–66 | 65

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2 See ESI†for details.
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3
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2 4
consistent with facile chloride dissociation from [RuCl (PTA) ] in
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2
3
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43 PTA and O PTA were observed by ESI-MS and P{ H} NMR
spectroscopy of the reaction mixtures which may also be responsible
for the loss of activity during recycling.
7 J. Kov a´ cs, F. Jo o´ , A. C. B e´ nyei and G. Laurenczy, Dalton Trans.,
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6
6 | Green Chem., 2012, 14, 62–66
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