Heterogeneous catalytic method for the conversion of aldoximes into nitriles using molecular sieve modified with Copper(II)

Article abstract of DOI:10.1080/00397911.2012.667861

A simple heterogeneous metal-catalyzed method was developed for the transformation of aldoximes into nitriles. Molecular sieve (4 A) modified with copper(II) proved to be an efficient catalyst for the conversion. Supplemental materials are available for this article. Go to the publisher's online edition of Synthetic Communications to view the free supplemental file.

Full text of DOI:10.1080/00397911.2012.667861

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Heterogeneous Catalytic Method for the
Conversion of Aldoximes into Nitriles
Using Molecular Sieve Modified with
Copper(II)
a
a
Árpád Kiss & Zoltán Hell
a
Department of Organic Chemistry and Technology, Budapest
University of Technology and Economics, Budapest, Hungary
Accepted author version posted online: 27 Dec 2012.Version of
record first published: 04 Apr 2013.
To cite this article: Árpád Kiss & Zoltán Hell (2013): Heterogeneous Catalytic Method for the
Conversion of Aldoximes into Nitriles Using Molecular Sieve Modified with Copper(II), Synthetic
Communications: An International Journal for Rapid Communication of Synthetic Organic Chemistry,
43:13, 1778-1786
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1
Synthetic Communications , 43: 1778–1786, 2013
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911.2012.667861
HETEROGENEOUS CATALYTIC METHOD FOR THE
CONVERSION OF ALDOXIMES INTO NITRILES USING
MOLECULAR SIEVE MODIFIED WITH COPPER(II)
´
Arp a´ d Kiss and Zolt a´ n Hell
Department of Organic Chemistry and Technology, Budapest University of
Technology and Economics, Budapest, Hungary
GRAPHICAL ABSTRACT
Abstract A simple heterogeneous metal-catalyzed method was developed for the transform-
˚
ation of aldoximes into nitriles. Molecular sieve (4 A) modified with copper(II) proved to
be an efficient catalyst for the conversion.
Supplemental materials are available for this article. Go to the publisher’s online edition
1
of Synthetic Communications to view the free supplemental file.
Keywords Aldoximes; copper; heterogeneous catalysis; molecular sieve; nitriles
INTRODUCTION
Different nitriles are important intermediates for the fine chemical industry.
There are several methods for their preparation, although these methods generally
involve dangerous reagents, mostly cyanides. The preparation of different aldehydes
is simpler; their conversion into aldoximes is a well-known reaction, and thus the
synthesis of nitriles from aldoximes is an alternative and very useful route, especially
in the case of aromatic nitriles. The aldoxime–nitrile transformation was first
[1]
investigated by Gabriel and Meyer. There are a number of different reagents that
[2]
can be used for this conversion. Some interesting examples of this conversion
[5]
[
3]
[4]
include PPh =CCl ,
3
triflic anhydride,
trifluoroacetic anhydride=pyridine,
4
[6]
[7]
KCN=phase-transfer catalyst, Me N=SO complex, phthalic anhydride under
microwave irradiation, different inorganic materials such as solid metaboric acid,
and anhydrous FeSO4.
3
2
[
8]
[9]
[
10]
Received January 31, 2012.
Address correspondence to Zolt a´ n Hell, Department of Organic Chemistry and Technology,
Budapest University of Technology and Economics, H-1521, Budapest, Hungary. E-mail: zhell@
mail.bme.hu
1
778

II
Cu -4A-CATALYZED CONVERSION OF OXIMES TO NITRILES
1779
In recent decades metal-catalyzed methods have been reported in the literature.
[
11]
Thus Attanasi et al. used Cu(OAc) catalyst.
A series of newer transition-metal-
catalyzed methods using the following reagents have been published: [RuCl (p-
2
2
[
12]
[13]
[14]
[15]
[16]
cymene)]2,
Pd(OAc) in the presence of PPh , ZnCl , and In(NO ) . The big disadvantage
(HO)ReO3,
PtCl (EtCN) ,
Ga(OTf)3,
W-Sn hydroxide,
4
2
[
17]
[18]
2
3
2
3 3
of these homogeneous methods is the tedious separation of the reagents. The metal
can contaminate the product, which is unacceptable for the fine chemical industry.
Our research group has investigated different heterogeneous catalytic methods
in organic syntheses for a number of years. In continuation of our recent studies our
aim was the elaboration of an environmentally benign method for the transform-
ation of aldoximes into nitriles, which could be applicable even on the industrial
scale. Earlier we developed a method using a slightly acidic zeolite under microwave
[
19]
conditions,
in which the desired products were obtained with moderate yields.
RESULTS AND DISCUSSION
Based on the results obtained during the investigation of the A -coupling
3
[20]
and taking into account the published metal-catalyzed methods for the aldoxime–
nitrile transformation, we examined the applicability of different metals supported
˚
on 4 A molecular sieve (MS-4A, 4A) for the preparation of nitriles. Conversion
of benzaldoxime into benzonitrile in boiling acetonitrile was chosen as the model
reaction (Scheme 1). The results are summarized in Table 1.
II
As shown, the best results were obtained using Cu and Pd (entries 1 and 2).
II
I
III
II
II
II
Ag , Fe , Co , and Ni were inefficient (entries 4–7). The results of Cu and Pd
II
II
were similarly good; however, Cu gave a little better yield and no benzamide
II
by-product was obtained under the same reaction conditions. If Pd was reduced
0
to Pd (entry 3), the activity of the catalyst decreased significantly.
Magnesium–aluminium 3:1 hydrotalcite and magnesium–lanthanum 3:1 mixed
oxide were widely used earlier in the research group. We tested these materials also
as support of copper(II). In both cases the yield was less and we detected high copper
leaching in the model reaction. Based on these results, further examinations were
II
made using copper(II) on molecular sieve (Cu -4A).
There are several proposals for the mechanism of different metal-catalyzed
aldoxime–nitrile transformations. In recent years evidence was reported for different
[14,17,18,21,22]
transition metals.
The authors proposed a water transfer from the aldox-
ime to the solvent—mostly acetonitrile—CꢀN bond, which produces equimolar
amount of acetamide by-product. The key step is complex formation among the
metal, the aldoxime, and the CꢀN group. Gas chromatography–mass spectrometry
Scheme 1. Model reaction of the oxime–nitrile conversion.

´
A. KISS AND Z. HELL
1780
a
Table 1. Selection of the catalyst
b
c
Conversion (%)
c
Benzonitrile (%)
c
Benzamide (%)
Entry
Catalyst
II
Cu -4A
1
2
3
4
5
6
7
100
98
26
17
12
10
10
99
96
26
17
12
10
10
0
2
0
0
0
0
0
II
Pd -4A
0
Pd -4A
d
I
Ag -4A
III
Fe -4A
II
Co -4A
II
Ni -4A
a
ꢁ
mmol benzaldoxime, 0.1 g catalyst, 2 mL acetonitrile, 8 h, 80 C.
All the catalysts were prepared by treatment of MS-4A with the corresponding
1
b
metal salts; metal content: ca. 0.85 mmol metal=g support.
c
Determined by GC-MS.
d
0
Catalyst Pd -4A was obtained by reduction of Pd -4A with hydrazine.
II
II
GC-MS) examination of our crude product using Cu -4A as catalyst showed a large
(
amount of acetamide, which can verify this concerted mechanism also in the case of
copper (Scheme 2).
When using toluene instead of acetonitrile, the yield of benzonitrile was only
II
1
which can confirm that acetamide could not form in a consecutive way.
6%. The reaction of water, Cu -4A catalyst, and acetonitrile gave no product,
II
Using Cu -4A as catalyst, the optimal reaction conditions of the benzaldoxime–
benzonitrile model reaction (Scheme 1, Table 2) have been determined. The results
showed that the amount of the acetonitrile solvent has a determinant role in the
formation of the benzamide by-product (compare entries 1 and 3). The product
benzonitrile can compete with the solvent acetonitrile, which could generate the
benzamide by-product (see Scheme 2). An increase in the amount of copper can
Scheme 2. Proposed mechanism of the reaction.

II
Cu -4A-CATALYZED CONVERSION OF OXIMES TO NITRILES
1781
a
Table 2. Determination of the optimal reaction conditions
b
c
c
Cu amount
(mol%)
Acetonitrile
(mL)
Reaction
time (h)
Benzonitril
(%)
Benzamide
(%)
Entry
1
2
3
4
5
1.7
42.5
1.7
2
2
3
1
74
70
91
99
99
26
30
8
10
10
10
16
3
42.5
8.5
0
8
0
a
II
mmol benzaldoxime, Cu -4A catalyst (0,1 g, 2.5 g, 0.5 g), acetonitrile, 80 C.
ꢁ
5
Total amount of copper in mol%.
Determined by GC-MS.
b
c
II
Table 3. Cu -4A catalyzed dehydration of aldoximes
a
b
Conversion (%)
b,c
Nitrile (%)
b
Amide (%)
Entry
Aldoxime
1
2
100
100
99
0
1
99 (91)
3
100
96 (90)
4
4
5
100
100
96 (87)
96 (88)
4
4
6
100
86 (78)
14
7
8
9
100
100
87
90 (82)
83 (76)
25
10
17
62
d
15
10
6
0
1
1
2
100
100
96 (87)
4
0
1
100
a
b
c
II
mmol aldoxime, 0.5 g Cu -4A, 10 mL acetonitrile, 80 C, 8 h.
ꢁ
5
Determined by GC-MS.
Isolated yield in parentheses; the purity was checked by GC-MS.
9% benzisoxazole was also formed.
d

´
A. KISS AND Z. HELL
1782
Scheme 3. Conversion of salicylaldoxime.
significantly decrease the reaction time (compare entries 3 and 4). Because 8.5%
copper gave good results (entry 5), this was used in our further studies.
The optimized reaction conditions were used to synthesize a series of nitrile
derivatives (Table 3).
The starting aldoximes were generally obtained as a mixture of syn and anti
isomers, although in most cases the syn form was the major isomer. There was no
difference between the reactivity of the two isomers; benzaldoxime (syn=anti ratio
9
5:5) and butyraldoxime (syn=anti ratio 56:44) gave the same yield under optimal
reaction conditions.
Most of the aldoximes gave perfect yields, but in some cases the amide
by-products were also detectable (entries 6–9). Pyridine-2-benzaldoxime showed
no complete conversion (entry 9) and gave less yield near the formation of a high
amount of amide.
As shown in Table 3, 2-hydroxy-benzaldoxime (salicylaldoxime) gave a very
poor yield (entry 10) and near the nitrile we also detected the formation of benzisox-
azole (Scheme 3).
The formation of benzisoxazole by dehydration of salicylaldoxime has already
been reported, Yan et al. obtained this compound with a good yield in a Ga(OTf) -
3
[15]
catalyzed reaction. It is well known that in salicylaldoxime there is a strong intra-
molecular hydrogen bond between the aldoxime nitrogen and the o-hydroxyl group
(Scheme 3). This hydrogen bond can hinder the complex formation of the oxime
group with copper (see Scheme 2), which can explain the weak reactivity of salicylal-
doxime. We tried to break this hydrogen bond by adding different bases (K CO ,
2
3
t
KHCO , KO Bu, Na CO , triethylamine) to the reaction mixture. The best result
was obtained using triethylamine; the conversion after 16 h was complete and the
3
2
3
II
Table 4. Cu -4A-catalyzed conversion of salicylaldoximes in the presence of triethylamine
a
b
Conversion (%)
b,c
Nitrile (%)
b
Amide (%)
Entry
1
Aldoxime
100
100
100
100
93 (85)
7
0
1
1
2
3
4
a
100 (98)
99 (91)
99 (90)
II
5
mmol substituted salicylaldoxime, 5 mmol triethylamine, 0.5 g Cu -4A, 10 mL acetoni-
ꢁ
trile, 80 C, 16 h.
b
Determined by GC-MS.
Isolated yield in parentheses; the purity was checked by GC-MS.
c

II
Cu -4A-CATALYZED CONVERSION OF OXIMES TO NITRILES
1783
Figure 1. Recycling of the catalyst.
yield of the salicylonitrile increased to 93%. This modified method was used for the
synthesis of the compounds presented in Table 4.
The conversion of substituted salicyloximes in the presence of triethylamine
was excellent; neither amide nor benzisoxazole were formed in appreciable amounts.
The catalyst could be reused easily without any purification after drying at
ꢁ
1
conditions are summarized in Fig. 1.
50 C for 1 h. The results obtained in the model reaction under optimal reaction
After 10 cycles the yield did not decrease significantly. After the 10th cycle, the
catalyst was examined by inductively coupled plasma–mass spectrometry (ICP-MS).
The results showed a decrease of the copper content of 10.85% (instead of 5.09%, the
catalyst after 10 cycles contained 4.54% copper). X-ray fluorescence (XRF) analysis
showed that no appreciable amount of copper contaminated the product.
In summary, a simple and efficient heterogeneous catalytic method has been
described for the conversion of aldoximes into nitriles using copper(II) on molecular
sieve catalyst in acetonitrile solvent. The catalyst is easily recyclable and could be used
at least 10 times without significant loss of activity. The experimental and workup
procedure is simple. Different nitrile derivatives were obtained with good or excellent
yields. This method had to be modified for salicylaldoximes where the use of triethy-
lamine as an additive resulted in the formation of substituted salicylonitriles with
quantitative yields.
EXPERIMENTAL
1
13
H and C NMR spectra were recorded on a Bruker Avanche-300 instrument
using tetramethylsilane (TMS) as an internal standard in CDCl or dimethylsulfoxide
3
(DMSO-d ). GC-MS measurements were made using a Shimadzu GC-2010, GC-MS
6
QP2010S instrument [column: HP-5MS, 30 m ꢂ 0.25 mm ꢂ 0.25 mm, temperature pro-
ꢁ
ꢁ
ꢁ
gram: 40 C (10 min) ! 10 C=min ! 300 C (26 min)]. Dodecane was used as internal
standard. The Cu content of the samples was determined with ICP-MS using an
Agilent 7500 CE ICP-MS instrument after microwave digestion of the sample in
aqua regia. XRF examinations were performed with a Canberra isotope excitation
1
25
instrument with Si(Li) detector (exciting source: I).
All the products prepared are known compounds and were characterized by
comparing the H and C NMR, MS, elemental analysis, and melting-point data
with those reported in the literature.
1
13

´
A. KISS AND Z. HELL
1784
General Procedure for Catalyst Preparation
˚
Molecular sieve (4 A) used was the commercial product of Merck. The support
was treated with the corresponding metal salts (CuCl ꢃ 2H O, Na PdCl , AgNO ,
2
2
2
4
3
FeCl ꢃ 6H O, NiCl ꢃ 6H O, or CoCl ꢃ 6H O) as follows: 1 mmol of salt was dis-
3
2
2
2
2
2
solved in 100 mL of deionized water and stirred with 1 g MS-4A at room temperature
for 6 h. The resulting solid was filtered, washed with deionized water and acetone,
ꢁ
and dried in an oven at 150 C for 1 h.
General Procedure for Synthesis of Nitriles
A typical reaction was carried out in a 20-mL flask. The mixture of aldoxime
II
5 mmol), appropriate amount of Cu -4A, and acetonitrile (10 mL) was stirred at
(
ꢁ
8
0 C for 8 h. The solid was filtered and washed with acetonitrile, and the filtrate
was evaporated. The residue was subjected to GC-MS analysis or was diluted with
ether, washed with water, and dried over anhydrous sodium sulfate, and then the
solvent was evaporated. When required, the crude product was purified by column
chromatography on silica gel using hexane–acetone 2:1 eluent. The filtered catalyst
ꢁ
can be recycled after drying at 150 C for 1 h.
[
23]
Benzonitrile
1
Yellowish liquid. H NMR (300 MHz, CDCl ) d (ppm): 7.45–7.50 (m, 2H,
3
1
3
ArH), 7.59–7.67 (m, 3H, ArH). C NMR (75 MHz, CDCl ) d (ppm): 112.9, 119.2,
29.5, 132.5, 133.2. GC: t : 14.91 min. MS m=z (%): 103 (M , 100), 76 (35), 75
R
3
þ
1
(10), 52 (10), 50 (18), 39 (8). Anal. calcd. for C H N: C, 81.55; H, 4.85; N, 13.59%.
Found: C, 81.59; H, 4.86; N, 13.56%.
7 5
General Procedure for Synthesis of Salicylonitriles
A typical reaction was carried out in a 20-mL flask. The mixture of substituted
II
salicylaldoxime (5 mmol), triethylamine (5 mmol), Cu -4A (0.5 g), and acetonitrile
ꢁ
(
10 mL) was stirred at 80 C for 16 h. The solid was filtered and washed with acetoni-
trile, and the filtrate was evaporated. The residue was subjected to GC-MS analysis or
was diluted with ether, washed with diluted hydrochloric acid and water, and dried
over anhydrous sodium sulfate. Then the solvent was evaporated. When required,
the crude product was purified by column chromatography on silica gel using
hexane–acetone 2:1 eluent.
[
24]
Salicylonitrile
ꢁ
ꢁ
1
Colorless solid: mp 93–94 C (lit. 91–92 C). H NMR (300 MHz, CDCl ) d
3
1
3
ppm): 6.98–7.03 (m, 2H, ArH), 7.47–7.53 (m, 2H, ArH). C NMR (75 MHz,
(
CDCl ) d (ppm): 99.2, 116.5, 116.7, 120.9, 132.9, 134.9, 158.9. GC t : 18.48 min.
3
R
þ
MS m=z (%): 119 (M , 60), 91 (100), 64 (40), 63 (30), 39 (20), 38 (15). Anal. calcd.
for C H NO: C, 70.59; H, 4.20; N, 11.76%. Found: C, 70.59; H, 4.19; N, 11.74%.
7
5
Complete experimental details are available online in the Supplemental
Material.

II
Cu -4A-CATALYZED CONVERSION OF OXIMES TO NITRILES
1785
ACKNOWLEDGMENTS
The authors are grateful to Chinoin Pharmaceutical and Chemical Works Ltd.,
´
member of the Sanofi Group, for the technical support and A. K. for financial support.
The work was supported within the framework of the Hungarian-French intergovern-
mental scientific and technological cooperation, T e´ T project no. F-3=2009.
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