Cobalt/nitrophenolate-catalyzed selective conversion of aldoximes into nitriles or amides

Article abstract of DOI:10.1016/j.catcom.2014.11.025

A novel cobalt/nitrophenolate complex has been synthesized, characterized and studied for their catalytic activities. Conversion of aldoximes to nitriles can be performed via in situ conditions from cobalt(II) acetate and 2,4-dinitrophenol. The rearrangement of aldoximes to amides via cobalt(II) acetate and 2-nitro-1-naphthol has also been demonstrated. A complete reversal of transformation was accomplished by modifying the cobalt salt and careful choice of both the nitrophenol ligand and reaction conditions.

Full text of DOI:10.1016/j.catcom.2014.11.025

Catalysis Communications 60 (2015) 120–123
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Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short communication
Cobalt/nitrophenolate-catalyzed selective conversion of aldoximes into
nitriles or amides
Wonseok Jang a,b, Se Eun Kim a,b, Cheol Mo Yang a,b, Sungwoo Yoon a, Myunghwan Park c, Junseong Lee d,
Youngjo Kim a,b, Min Kim a,b,
⁎
a
Department of Chemistry, Chungbuk National University, Cheongju, Chungbuk 361-763, Republic of Korea
Research Team for Syntheses and Physical Properties of Various Molecules (BK21Plus), Chungbuk National University, Republic of Korea
Department of Chemistry Education, Chungbuk National University, Republic of Korea
Department of Chemistry, Chonnam National University, Gwangju 500-757, Republic of Korea
b
c
d
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 10 October 2014
Received in revised form 19 November 2014
Accepted 21 November 2014
Available online 22 November 2014
A novel cobalt/nitrophenolate complex has been synthesized, characterized and studied for their catalytic activ-
ities. Conversion of aldoximes to nitriles can be performed via in situ conditions from cobalt(II) acetate and 2,4-
dinitrophenol. The rearrangement of aldoximes to amides via cobalt(II) acetate and 2-nitro-1-naphthol has also
been demonstrated. A complete reversal of transformation was accomplished by modifying the cobalt salt and
careful choice of both the nitrophenol ligand and reaction conditions.
© 2014 Elsevier B.V. All rights reserved.
Keywords:
Cobalt
Amides
Nitriles
Homogeneous catalysis
Selective conversions
1. Introduction
complex formation utilizing phosphine ligands and not for catalysis or
other applications [8,9].
Nitro compounds are important organic molecules in both academia
and industrial settings although they are often highly explosive [1–3].
The nitration of organic molecules is a very straightforward and efficient
method to introduce ‘the nitrogen atom’ into a molecular scaffold [1].
Nitro groups have been widely used in transition metal-catalyzed or-
ganic reactions due to its significant electron-withdrawing property.
Meanwhile, transition metal–nitro complexes have been less widely
studied despite the nitro group's potential to coordinate directly to a
metal center through one of the three heteroatoms. Coordination of
the lanthanide metal by the nitro group is routinely used as a method
of stabilization of explosive aromatic nitro compounds such as picric
acid (trinitrophenol) series [3,4]. Since cobalt(II) salts have been widely
studied in various organometallic complexes with tetradentate ligands
such as Salen type molecules [5–7], we wondered whether an ortho-
nitrophenol ligand could form metal–ligand complexes in a similar
fashion. To the best of our knowledge, only two series of nitrophenol-
coordinating cobalt complexes have been reported which studied
With the cobalt/nitrophenolate system, we attempted the conversion
of aldoximes. Aldoximes are easily synthesized from aldehyde through
stoichiometric reaction with hydroxylamine, and are key intermediates
of various organic transformations. The aldoximes could be converted
to primary amides in strong acidic conditions (i.e., Beckmann rearrange-
ment), hydrolyzed to the corresponding aldehydes, changed to the
amines through Hoffman reaction in the presence of alkali hypochlorites,
or dehydrated to the corresponding nitrile compounds [2]. Recently, var-
ious late transition metal-catalyzed conversions of aldoximes have been
reported, especially to formation of nitriles and amides [10–18]. The re-
action pathway was dependent on both the transition metal and reaction
conditions. For example, ruthenium catalyzed the dehydration of
aldoximes to nitriles in acetonitrile with molecular sieve [10], whereas
rhodium catalyzed the rearrangement of aldoximes to amides in toluene
[13]. For the latter, high temperature (150 °C) or the addition of p-
toluenesulfonic acid was essential to perform the reaction with good
yields [15]. For cobalt, several complexes have been studied for the hy-
dration of nitriles [19–21], but the dehydration of aldoximes [11,12]
and direct conversion of aldoximes into amides are limited. Described
herein is our study on the synthesis and characterization of cobalt/
nitrophenolate complex, and their catalytic application to selective con-
version of aldoximes into nitriles, through dehydration, and amides,
through a rearrangement using cobalt(II) catalysts.
⁎
Corresponding author at: Department of Chemistry, Chungbuk National University,
Cheongju, Chungbuk 361-763, Republic of Korea.
E-mail address: minkim@chungbuk.ac.kr (M. Kim).
http://dx.doi.org/10.1016/j.catcom.2014.11.025
1566-7367/© 2014 Elsevier B.V. All rights reserved.

W. Jang et al. / Catalysis Communications 60 (2015) 120–123
121
Table 1
H2O (0.49 + 0.01 mL, 2 v/v% of water in acetonitrile) were added to
an oven-dried vial. The mixture was vigorously stirred at 80 °C for
24 h and then diluted with ethyl acetate. Solvent was removed in
vacuo, and the desired product was purified and isolated by silica gel
column chromatography (ethyl acetate/n-hexane).
Screen of the dehydration reaction conditions.a
2.3. Typical procedures for the synthesis of amides from aldoximes
Aldoxime (0.25 mmol), cobalt acetate(II) tetrahydrate (6.2 mg,
0.025 mmol), 2-nitro-1-naphthol (9.5 mg, 0.05 mmol) and acetoni-
trile/H2O (0.01 + 0.24 mL, 4 v/v% of acetonitrile in water) were added
to an oven-dried vial. The mixture was vigorously stirred at 80 °C for
24 h and then diluted with ethyl acetate. Solvent was removed in
vacuo, and the desired product was purified and isolated by silica gel
column chromatography (ethyl acetate/n-hexane).
Entry
Catalyst system (mol%)
Yieldb (%)
1
2
3
4
5
6
7
8
9
–
b1
b1
31
3a (20)
Co(OAc)2 · 4H2O (10)
Co(OAc)2 · 4H2O (10) + 3a (20)
Co(OAc)2 · 4H2O (10) + 3b (20)
Co(OAc)2 · 4H2O (10) + 3c (20)
Co(OAc)2 · 4H2O (10) + 3d (20)
Co(OAc)2 · 4H2O (10) + 3e (20)
Co(OAc)2 · 4H2O (10) + CH3COOH (20)
90
45c
75
45c
45c
39
3. Result and discussion
Conversion of 4-bromobenzaldoxime (1a) to 4-bromobenzonitrile
(2a) was explored as an initial test reaction (Table 1). Whereas no con-
version of the benzaldoxime was observed in the absence of cobalt (en-
tries 1 and 2), cobalt(II) acetate did displayed some catalytic activity on
the test reaction (entry 3). One of the other side products from this re-
action was identified as 4-bromobenzamide (6% yield). Acetonitrile
was the solvent of choice with the addition of a small amount of
water (2 v/v%) to improve solubility of the cobalt salt. This water and
acetonitrile mixture produced higher product yields than either pure
acetonitrile or water produced on their own. Interestingly, anions
other than acetate were less effective. It was observed that 2,4-dinitro-
phenol (3a) was the highest yielding reactant (entry 4) among the var-
ious ortho-nitrophenol ligands screened (entries 4–8). This could be
attributed to the inherent acidity of ortho-nitrophenol ligands
by the electron withdrawing nature of the substituents. Among the
ligands screened, 2,4-dinitrophenol (pKa = 4.11) was the most
acidic thus corroborating this hypothesis. A more acidic ligand, 2-
nitrobenzoic acid (pKa = 2.16) showed similar efficiency in the dehy-
dration with an 85% yield. However, acidity of the ligand is not the
only contributing factor in this improved reactivity. Acetic acid showed
lower conversion than 2,4-dinitrophenol (3a) although they have simi-
lar pKas (entry 9).
To identify the active catalytic species, cobalt/nitrophenolate com-
plex (Co(C6H3N2O5)2(OH2)2, 4) was synthesized. A brown powder
was obtained from the reaction of cobalt(II) acetate with 2,4-dinitro-
phenol (3a), and was verified via X-ray crystallographic analysis
(Scheme 1 and see Supporting information for detail, CCDC deposit
1009805). Since metal–nitro coordination complexes are rare, a
detailed structure analysis was performed. The cobalt metal center
has six coordinating oxygen atoms leading to an octahedral coordi-
nation geometry; two oxygen atoms from phenolate, two from
a
Reaction condition: A mixture of 4-bromobenzaldoxime (0.25 mmol), cobalt catalyst
(10 mol%) and ligand (if necessary) in acetonitrile (0.49 mL) and water (0.01 mL, 2 v/v%
of water in acetonitrile) was stirred for 24 h at 80 °C.
b
Yield of isolated product, and is reported as an average from at least two independent
measurements.
c
NMR yield, and is reported as an average from at least two independent measurements.
2. Experimental
Concentration of solution was carried out by using a rotary evapora-
tor with a water aspirator, and generally followed by removal of residual
solvents on a vacuum line held at 0.1–1 Torr. Unless otherwise stated, all
commercial reagents and solvents were used without additional purifi-
cation. All chemicals were purchased from Sigma-Aldrich, TCI, and Alfa
Aesar chemical companies.
2.1. Preparation of cobalt/nitrophenolate complex (4)
Cobalt acetate (1 mmol, 177 mg), 2,4-dinitrophenol (2 mmol,
368 mg) and anhydrous methanol (10 mL) were placed in a 50 mL
round-bottom flask with a reflux condenser. The mixture was stirred
at reflux for 1 day. After cooling, the precipitate was filtered, thoroughly
washed with methanol (10 mL, 3 times). The solid was dried under vac-
uum to give a brown powder (259 mg, 56%). The single crystal of com-
plex 4 was obtained from recrystallization in water.
2.2. Typical procedures for the synthesis of nitriles from aldoximes
Aldoxime (0.25 mmol), cobalt acetate(II) tetrahydrate (6.2 mg,
0.025 mmol), 2,4-dinitrophenol (9.2 mg, 0.05 mmol) and acetonitrile/
Scheme 1. Synthesis of cobalt/nitrophenolate complex (4) and its catalytic activity.

122
W. Jang et al. / Catalysis Communications 60 (2015) 120–123
Co(OAc)2 4H2O (10 mol%)
2,4-Dinitrophenol (20 mol%)
H
N
OH
N
MeCN:H2O (49:1), 80 oC, 24 h
R
R
2
1
Cl
CN
O2N
CN
Br
CN
CN
2b, 70%
2e, 82%
2c, 72%
2a, 90%
2d, 65%
CN
Me
MeO
CN
Br
2f, 60%
2i, 75%
CN
CN
CN
Br
2g, 80%
MeO
2h, 71%
Scheme 2. Cobalt/nitrophenol-catalyzed dehydration of aldoximes into nitriles.a,b aReaction condition: A mixture of benzaldoxime (0.25 mmol), cobalt acetate tetrahydrate (10 mol%),
.
2,4-dinitrophenol (20 mol%) in acetonitrile (0.49 mL) and water (0.01 mL, 2 v/v% of water in acetonitrile) was stirred for 24 h at 80 °C. bYield of isolated product, and is reported as an
average from at least three independent measurements.
the nitro group, and two from water. The two coordinating ligands
are anti-coplanar with two axial water molecules. The isolated
cobalt/nitrophenolate complex was directly used in the model re-
action, and displayed conversion of 4-bromobenzaldoxime to 4-
bromobenzonitrile in good yield (80%, Scheme 1). It should be
mentioned that the synthesis of the cobalt complex 4 is very simple
and easily scalable to gram quantities.
Under the optimized conditions, the substrate scope of an aldoxime
conversion to a nitrile was subsequently investigated (Scheme 2). Both
electron-withdrawing and electron-donating substituted aldoximes
showed moderate to high yields in this condition. It is noteworthy
that the position of the functional group did not diminish the efficiency
of the reaction (2a, 2f, and 2g). Not only did the substituted phenyl rings
undergo this transformation but the conjugated cinnamaldehyde oxime
also underwent the dehydration reaction in moderate yield (2i). While
the benzaldoximes proved to be facile, reactions of heteroaromatic and
aliphatic aldoxime showed diminished turnover.
displayed good yields for conversion of aldoximes to amides (5a
and 5f), the ortho-substituted 2-bromobenzaldoxime showed poor
yield in the rearrangement condition (5g). Again, the conjugated
cinnamaldehyde oxime converted to an amide in moderate yield (5i),
and heteroaromatic aldoximes underwent the rearrangement reaction
in moderate to good yield (5j–5l). Not surprisingly, the acetophenone
oxime and bezaldehyde O-methyloxime had no reactivity in the cobalt/
nitrophenolate catalyzed rearrangement even with elevated
O
Co(OAc)2 4H2O (10 mol%)
2-Nitro-1-naphthol (20 mol%)
H
OH
NH2
N
MeCN:H2O (1:24), 80 oC, 24 h
R
R
5
1
O
O
O
NH2
NH2
NH2
O2N
Br
Utilizing cobalt/nitrophenolate complexes, conversion of aldoximes
to amides via a rearrangement was also explored. In 2012, Lu et al. dem-
onstrated the hydration of nitriles catalyzed by a simple cobalt salt [19].
Extrapolating from this study, it was reasonable to conclude that the co-
balt/nitrophenolate complex could also be applied to the hydration of ni-
triles. After screening various reaction conditions, it was found out that
the solvent ratio predominantly determined the mode of the reaction
whether through a dehydration or rearrangement. As seen previously,
pure solvent systems demonstrated poor solubility of either the catalyst
or the substrate, however, a 4% acetonitrile in water was determined to
be best for amide formation in comparison to the 98% that favored
dehydration. Strikingly, the employment of 2-nitro-1-naphthol (3f)
displayed a better yield than 2,4-dinitrophenol (3a) for synthesis of 4-
bromobenzamide (5a) from 4-bromobenzaldoxime (1a). The utility of
these reactions is demonstrated by using the same starting materials
but manipulation of the solvent system and coordinating ligand, a variety
of aldoxime derivatives were successively converted to amides in moder-
ate to high yields (Scheme 3). Regardless of the electronic nature of
substituted benzaldoximes, moderate to good yields were obtained for
all (5a–5h). In contrast to our observations in the dehydration mecha-
nism, the position of the functional group did have an effect on the reac-
tion. Although 4-bromobenzaldoxime and 3-bromobenzaldoxime
Br
Cl
5a, 80%
O
5b, 68%
5c, 67%
O
O
NH2
NH2
NH2
Me
MeO
5d, 65%
5e, 70%
5f, 80%
Br
O
O
O
MeO
NH2
NH2
NH2
5g, 18%
O
5h, 80%
O
5i, 55%
O
S
O
NH2
NH2
NH2
N
5j, 68%
5k, 81%
5l, 88%
Scheme 3. Cobalt/nitrophenol-catalyzed rearrangement of aldoximes into amides.a,b
.
aReaction condition: A mixture of 4-bromobenzaldoxime (0.25 mmol), cobalt acetate
tetrahydrate catalyst (10 mol%), and 2-nitro-1-naphthol (20 mol%) in acetonitrile
(0.01 mL) and water (0.24 mL, 4 v/v% of acetonitrile in water) was stirred for 24 h at 80
°C. bYield of isolated product, and is reported as an average from at least two independent
measurements.

W. Jang et al. / Catalysis Communications 60 (2015) 120–123
123
O
Co(OAc)2 4H2O (10 mol%)
N
2-Nitro-1-naphthol (20 mol%)
NH2
MeCN:H2O (1:24), 80 oC, 24 h
Br
Br
5a (0%)
O
2a
OH
H
N
Co(OAc)2 4H2O (10 mol%)
N
2-Nitro-1-naphthol (20 mol%)
NH2
+
MeCN:H2O (1:24), 80 oC, 24 h
Br
Br
Br
1a
2a (5%)
5a (80%)
Scheme 4. Cobalt/nitrophenol-catalyzed hydration of nitrile and nitrile-additive studies.
temperature, which means the present cobalt system could rearrange
only the hydroxide and proton groups.
Acknowledgment
Generally a two-step dehydration–hydration pathway had been
established for the conversion of aldoximes to amides, which is a similar
mechanism to the Beckmann rearrangement [22,23]. From two differ-
ent optimization studies, it is first presumed that 2,4-dinitrophenol li-
gand is active only for the dehydration of aldoximes to nitriles while
2-nitro-1-naphthol is presumed to accelerate the first dehydration
followed by hydration of the nitrile intermediates. Although
transition-metal catalyzed hydration of nitriles to amides has been re-
ported in several system including cobalt [19–21], no conversion was
observed with the nitrile and cobalt/nitrophenol system (Scheme 4).
Thus, we can conclude that the cobalt/2-nitro-1-naphthol catalyzed
conversion of aldoximes to amides undergoes a one-step rearrange-
ment without the release of water [24]. More recently, Chang et al. re-
vealed a nitrile additive effect on rhodium-catalyzed transformation of
aldoximes to amides and suggested a nitrile-assisted mechanism for
conversion of oximes to amides [15]. In the systems discussed herein,
there were no apparent nitrile additive effects on the cobalt-catalyzed
conversion of aldoximes to amides (Scheme 4). It is suggestive of the ni-
trile molecule being too inert to undergo hydration in the cobalt/nitro-
phenol system. While the rhodium system has intermolecular
transfers between the aldoximes and nitriles, the cobalt system only
has intramolecular rearrangement pathways available for the oximes
(Scheme 4). Since the nitrile molecule is inert in our conditions, we
could develop selective conversion of aldoximes to nitriles and amides.
If the cobalt/nitrophenol catalyst can interact with the nitrile, the ni-
triles in Scheme 2 should be able to be converted to amides directly.
This research was supported by the Basic Science Research Program
through the National Research Foundation of Korea (NRF) funded by the
Ministry of Science, ICT & Future Planning (2013R1A1A1061399) and
the research grant of the Chungbuk National University in 2012.
Appendix A. Supplementary data
Supplementary data to this article can be found online at http://dx.
doi.org/10.1016/j.catcom.2014.11.025.
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4. Conclusion
In conclusion, we have synthesized and characterized a novel cobalt/
nitrophenolate compound that is catalytically active. Cobalt acetate
could bind with two ortho-nitrophenol ligands where the cobalt metal
center is coordinated by 6 oxygens atoms; 2 from phenolate, 2 from
the nitro group, and 2 from water. This unique cobalt/nitrophenolate
complex shows the catalytic conversion of aldoximes to nitriles. Based
on the choice of ligand, 2,4-dinitrophenol or 2-nitro-1-naphthol, we
can selectively synthesize nitriles or amides starting from aldoxime
using these simple cobalt-catalyzed conditions.
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