Ligand-free nickel-catalyzed conversion of aldoximes into nitriles

Article abstract of DOI:10.1055/s-0030-1260119

Catalytic dehydration of aldoximes can be performed efficiently with NiCl₂ in acetonitrile under neutral and mild conditions. Under these conditions, various functionalized aldoximes produce the corresponding nitriles in good to excellent yields. Georg Thieme Verlag Stuttgart. New York.

Full text of DOI:10.1055/s-0030-1260119

PAPER
2639
Ligand-Free Nickel-Catalyzed Conversion of Aldoximes into Nitriles
a
a
b
a
C
Y
onversionof
A
ld
e
oximes into
n
Nitriles -Ting Li, Bei-Sih Liao, Hsin-Pei Chen,* Shiuh-Tzung Liu*
a
Department of Chemistry, National Taiwan University, Taipei 106, Taiwan
Fax +886(2)23636359; E-mail: stliu@ntu.edu.tw
Department of Mechanical Engineering, Tungnan University, Shenkeng Dist., New Taipei City 222, Taiwan
b
Received 25 April 2011; revised 29 May 2011
complexes with pp ligands were selected as catalysts for
the dehydration of aldoximes. In the presence of 5 mol%
metal complex and molecular sieves, with acetonitrile as
the solvent, benzonitrile was obtained from benzaldoxime
under heating for seven hours; the results are summarized
in Table 1.
Abstract: Catalytic dehydration of aldoximes can be performed ef-
ficiently with NiCl in acetonitrile under neutral and mild condi-
2
tions. Under these conditions, various functionalized aldoximes
produce the corresponding nitriles in good to excellent yields.
Key words: aldehydes, nickel, nitrile, catalysis, elimination
2+
Nitriles, which serve as precursors in several functional
group transformations, are an important class of organic
compound. The cyano group is also a known functional
motif for hydrogen bonding to certain biological
R
R
–
N
N
N
N
N
N
N
N
N
N
(Cl )2
Ni
(H2O)3
R
R
1
receptors as well as a donating ligand in coordination
R = H, pp
R = Me, dmpp
1
2
chemistry. For these reasons, remarkable efforts have
been made to develop efficient methods for the prepara-
tion of many nitrile compounds. Among the numerous
methodologies available, the dehydration of aldoxime or
primary amide functionalities appears to be attractive be-
Figure 1 Ligands and nickel complexes
Table 1 Dehydration of Benzaldoximea
cause of the ready availability of the starting materials. A Entry
number of reagents have proved to be effective for this
Metal
Ligand
Yield of
benzonitrile (%)
b
3
transformation. However, the approach suffers from a
number of disadvantages such as the use of stoichiometric
1
FeCl2
pp
pp
–
Trace
38
amounts of reagents, and limitations arising from the sen-
sitivity of some functional groups to the reaction condi-
tions. Thus, the development of metal-catalyzed
dehydration of aldoxime has received much attention. A
number of catalytic systems are available, such as
2
3
4
5
6
7
8
9
0
CoCl2
[Ni(pp)(H O) ]Cl
2
100
Trace
100
71
2
3
No metal ions
NiCl ·2H
–
4
5
2
2
O
pp
dmpp
pp
pp
pp
pp
Pd(OAc) /Ph P,
[(CH CH CN) PtCl ],
[RuCl (p-
2
3
3
2
2
4
2
6
7
8
9
cymene)] , Ga(OTf) , Cu(CH COO) , O Re(OH), iron
2
3
3
2
3
NiCl ·2H O
2
2
1
0
11
porphyrin, tungsten–tin mixed hydroxide, CuCl under
2
1
2
13
NiBr2
70
ultrasound conditions, and Raney nickel. Despite the
success of these reactions, there is still a strong need for
an efficient method for the conversion of aldoximes into
nitriles through the use of a simple catalytic system. We
report here a catalytic method for this transformation that
involves thermal heating of aldoximes in acetonitrile in
the presence of catalytic amounts of NiCl ·2H O.
Ni(NO ) ·6H O
56
3
2
2
Ni(CH COO) ·6H O
58
3
2
2
1
FeCl2
Trace
a
Reaction conditions: benzaldoxime (0.5 mmol), catalyst (0.025
2
2
mmol), MeCN (1 mL), 4 Å MS (0.1 g), reflux, 7 h.
GC yield.
Initially, the dehydration of benzaldoxime catalyzed by
various transition-metal complexes to yield benzonitrile
was examined to screen the best catalytic system. Transi-
tion-metal complexes associated with polydentate ligands
are generally active in catalysis, and we have recently
been working with N-donor ligands derived from pyra-
b
As shown in Table 1, entries 1–3, use of the nickel com-
plex was clearly the best catalyst for this conversion; the
corresponding iron complex demonstrated no activity at
all. It should be noted that performing the reaction by gen-
erating the metal complex in situ gave similar results to
those obtained with 1. However, it appeared that the yield
decreased with the use of dmpp (entry 6), presumably due
to the steric hindrance of this ligand. This catalytic con-
version was also influenced by the anion of the nickel
1
4
zole and pyridine such as pp (Figure 1). Thus, metal
SYNTHESIS 2011, No. 16, pp 2639–2643
x
x.
x
x
.
2
0
1
1
Advanced online publication: 14.07.2011
DOI: 10.1055/s-0030-1260119; Art ID: H44111SS
Georg Thieme Verlag Stuttgart · New York
©

2
640
Y.-T. Li et al.
PAPER
salts; chloride was established as the best choice (entries in the aromatic ring caused a decrease in the yield of the
7
–9).
desired product (entry 7) accompanied by the formation of
p-nitrobenzamide as a side product. The present method is
also applicable to the preparation of aliphatic nitriles
Further efforts were then focused on optimizing the reac-
tion conditions; the results are summarized in Table 2. To
our surprise, it was found that the activity of nickel dichlo-
ride itself was as good as that of complex 1 in the catalytic
conversion of benzaldoxime into benzonitrile (Table 2,
entry 1). Apparently, the metal ion plays the key role in
the catalysis, not the associated ligand, thus, a simple cat-
alyst for the dehydration of oximes is available. Other
modified reaction conditions were also examined. When
the reaction was carried out in the presence of air, the
yield dropped due to decomposition of the metal com-
plexes, as evidenced by the formation of black precip-
(
entries 11–14). Thus, butyaldoxime was quantitatively
converted into butanenitrile, as was cyclohexanecarbal-
doxime. Neither an amino- nor an alkoxy substituent in
the aliphatic substrate affected the efficiency. For exam-
ple, 3-methoxybutylaldoxime and 2-dimethylaminopropi-
onaldoxime were both smoothly converted into 3-
methoxybutanenitrile and 2-dimethylaminopropaneni-
trile, respectively (Table 3, entries 12 and 13).
Table 3 Conversion of Aldoximes into Nitrilesa
itates (entry 2). The reaction rate was reduced at lower Entry Substrate
temperatures, and low conversion was observed at 50 °C
Product
Yield (%)
1
2
3
4
PhCH(=NOH)
PhCN
100
46
(
<35%). Among several solvents tested, acetonitrile was
the medium of choice (entries 6 and 7). Running the reac-
tion under basic conditions did not provide better results
o-MeC H CH(=NOH)
o-MeC H CN
6
4
6
4
p-MeOC H CH(=NOH)
p-MeOC H CN
82
6
4
6
4
(
entries 4 and 5).
p-BrC H CH(=NOH)
p-BrC H CN
78
6
4
6
4
Table 2 Conversion of Benzaldoxime into Benzonitrilea
EtO
EtO
Entry
Conditions and additives
Yield (%)b
5
6
57
16
CH(=NOH)
CN
NiCl + pp NiCl2
EtO
EtO
2
HO
HO
1
2
3
4
5
6
7
8
–
76
82
HO
CH(=NOH)
CH(=NOH)
HO
CN
under air
52
57
50 °C
34
30
7
8
9
54b
O2N
2
O N
CN
Cs CO (0.025 mmol)
26
38
2
3
–
–
CH(=NOH)
DBU (0.025 mmol)
EtOH as solvent
27
27
N
NRc
NRc
21
NRc
NRc
Trace
65
100
CH=CHCH(=NOH)
CH(=NOH)
CH=CH-CN
CN
toluene as solvent
microwave (150 W), 10 min
without molecular sieves
1
1
0
1
80
9
68
a
Reaction conditions: benzaldoxime (0.5 mmol), catalyst (0.025
mmol), MeCN (1 mL), 4 Å MS (150 mg), reflux, 3 h.
CH (CH ) CH(=NOH)
CH (CH ) CN
100
100
3
2 2
3
2 2
b
GC yield.
NR: no reaction.
OMe NOH
H
OMe
c
12
CN
Et
N
NOH
Et
N
The generality of this method was demonstrated by the 13
conversion of various kinds of aldoximes into the corre-
sponding nitriles, as illustrated in Table 3. The reactions
79
CN
Et
H
Et
NOH
1
4
100
CN
were carried out at reflux temperature for seven hours
with the use of 5 mol% nickel chloride in the presence of
a
4
Å molecular sieves. Substituent variation on the aromat-
Reaction conditions: aldoxime (0.5 mmol), catalyst (0.025 mmol),
ic ring did influence the efficiency (entries 1–6), and a MeCN (1 mL), 4 Å MS (150 mg), reflux, 3 h.
b
p-Nitrobenzamide (24%) was also obtained.
moderate yield was obtained from the reaction of a steri-
cally congested oxime (entry 2). Conversion of 3,4-dihy-
droxybezaldoxime and 2-pyridylaldoxime into the
When aromatic bis-aldoxime compounds were treated un-
corresponding nitriles were quite poor because of chela-
tion of these substrates with metal ions. Cinnamaldoxime
was smoothly converted into the corresponding nitrile
der the same conditions, a mixture of bis-nitrile and
amide–nitrile products were obtained (Equation 1). Pre-
sumably, the cyano group formed in the first stage acts an
electron-withdrawing group, which affects the dehydra-
(
entry 9). However, a strong electron-withdrawing group
Synthesis 2011, No. 16, 2639–2643 © Thieme Stuttgart · New York

PAPER
Conversion of Aldoximes into Nitriles
2641
tion on the second group. This electronic effect is consis- ment competes with the dehydration process to yield the
tent with the observed dehydration of p-nitro- amide product. However, nickel(II) ions acting as a Lewis
benzaldoxime (Table 3, entry 7).
acid to assist the dehydration is an alternative pathway.
NOH
N
N
G
H
+
NiCl2⋅2H2O
H
NH2
N
N
H
G: electron-withdrawing group
NOH
m-
O
45%
42%
Ni(0)Ln
H2O
55%
58%
OH
p-
G
Equation 1
O
NH2
Ln–1
H
G
As shown in Figure 2, complete conversion of benzald-
oxime into benzonitrile was accomplished within seven
hours; however, the catalytic reaction did not proceed in
Ni
N
HO
N
G
the first 20 minutes, i.e., there was an induction period for Scheme 1 Pathway for catalysis
the catalysis. This indicates that the nickel dichloride
complex is not the active catalyst. It has been proposed
In summary, the efficient dehydration of a range of al-
doximes into nitriles by using a nickel(II) dichloride/ace-
tonitrile system under mild conditions was developed.
The advantages of this method are the general applicabil-
ity to aldoximes, the good to excellent yields obtained, the
inexpensive catalyst, and the mild reaction conditions. A
detailed study on the mechanistic pathway is currently un-
der investigation.
that the Pd(II)-catalyzed dehydration of aldoxime pro-
ceeds through an oxidative addition of N–O bond to the
4
Pd(0). In addition, treatment of aldoximes with Raney
1
3
nickel led to dehydration to afford nitriles. We suspected
that reduction of nickel took place under the reaction con-
ditions, and that the lower oxidation state nickel species
were responsible for the catalysis. This explains why su-
perior results were obtained when the reaction was carried
out under a nitrogen atmosphere. In a separate experi-
ment, it was found that the dehydration of benzaldoxime
NMR spectra were recorded in CDCl with a Bruker AVANCE 400
3
spectrometer. Chemical shifts are given in parts per million (ppm)
proceeded in the presence of [Ni(COD) ] as the catalyst
2
1
relative to TMS for H NMR. Infrared spectra were measured with
under similar conditions. However, this Ni(0) complex is
quite air-sensitive and thus not easy to handle.
a Nicolet Magna-IR 550 spectrometer (Series-II) as KBr pellets, un-
less otherwise noted. Chemicals and solvents were of analytical
grade and used as received unless otherwise stated.
Nickel Complex 1
A mixture of NiCl (61.4 mg, 0.47 mmol) and pp (100 mg, 0.47
2
mol) in THF (10 mL) was heated at reflux overnight. The reaction
mixture was centrifuged and the residue was washed with THF
(
2 × 2 mL) and Et O (2 mL). The obtained light-blue solid was re-
2
crystallized from MeOH to give 1.
Yield: 70 mg (69%); blue-green crystals.
–
1
IR: 1619, 1589 cm .
Anal. Calcd. for C H Cl NiN O : C, 33.46; H, 3.83; N, 17.74.
1
1
15
2
5
3
Found: C, 33.72; H, 3.89; N, 17.52.
General Procedure
A mixture of aldoxime (0.5 mmol), Ni complex (0.025 mmol) and
4
Å molecular sieves (150 mg) in MeCN (1 mL) was placed in a re-
action tube under a nitrogen atmosphere. The mixture was stirred
and heated to 80 °C. After the completion of the reaction, brine (3
mL) and CH Cl (5 mL) were added, and the organic layer was sep-
Figure 2 Reaction profile for the dehydration of benzaldoxime.
Reagents and conditions: benzaldoxime (1 mmol), NiCl ·2H O
2
2
arated, dried over MgSO and concentrated. Products obtained in
4
2
2
(
0.05 mmol), 4 Å MS, MeCN (1 mL), 80 °C.
this work are all known compounds and were characterized by stan-
dard spectroscopic methods, particularly NMR (CDCl ) and IR; the
3
data were consistent with those reported.
The catalytic pathway is depicted in Scheme 1. Accord-
ingly, formation of the nitrile involves the oxidative addi- o-Methylbenzonitrile15
–
1
tion of oxime followed by b-elimination, which yields the IR (KBr): 2226 (CN) cm .
final product by de-coordination of nitrile from the metal
center. When the aromatic aldoxime contains a strong J = 7.2 Hz, 1 H), 7.57 (d, J = 7.6 Hz, 1 H).
1
H NMR (400 MHz): d = 2.54 (s, 3 H), 7.23–7.30 (m, 2 H), 7.46 (t,
electron-withdrawing substituent, Beckmann rearrange-
13
C NMR (100 MHz): d = 113.2, 118.4, 126.5, 130.2, 133.0, 142.2.
Synthesis 2011, No. 16, 2639–2643 © Thieme Stuttgart · New York

2
642
Y.-T. Li et al.
PAPER
MS (EI): m/z = 117 [M]+.
1
H NMR (400 MHz): d = 5.86 (d, J = 16.8 Hz, 1 H), 7.35–7.43 (m,
6
13
H).
p-Methoxybenzonitrile1
6
C NMR (100 MHz): d = 96.3, 118.2, 127.2, 130.0, 131.1, 133.5,
50.4.
–
1
IR (KBr): 2217 (CN) cm .
1
1
H NMR (400 MHz): d = 3.85 (s, 3 H, OCH ), 6.93 (d, J = 8.0 Hz,
MS (EI): m/z = 129 [M]+.
3
2
H), 7.56 (d, J = 8.0 Hz, 2 H).
1
3
n-Butyronitrile23
IR (KBr): 2260 (CN) cm .
C NMR (100 MHz): d = 56.8, 104.3, 115.8, 119.4, 134.1, 163.0.
–
1
MS (EI): m/z = 113 [M]+.
1
H NMR (400 MHz): d = 1.06 (t, J = 7.4 Hz, 3 H), 1.68 (m, 2 H),
p-Bromobenzonitrile17
IR (KBr): 2224 (CN) cm .
2.30 (t, J = 6.8 Hz, 2 H).
–
1
13
C NMR (100 MHz): d = 13.1, 19.0, 19.3, 119.9.
1
H NMR (400 MHz): d = 7.58 (d, J = 8.2 Hz, 2 H), 7.63 (d,
MS (EI): m/z = 69 [M]+.
J = 8.2 Hz, 2 H).
1
3
24
C NMR (100 MHz): d = 111.5, 118.2, 128.1, 132.5, 134.0.
3-Methoxybutanenitrile
–
1
+
+
IR (KBr): 2251 (CN) cm .
MS (EI): m/z = 181 [M] , 183 [M + 2] .
1
H NMR (400 MHz): d = 1.28 (d, J = 6.0 Hz, 3 H), 2.49 (d, J =
5.6 Hz, 2 H), 3.47 (s, 3 H), 3.58–3.62 (m, 1 H).
p-Nitrobenzonitrile16
Solid; mp 149–151 °C.
13
C NMR (100 MHz, CDCl ): d = 19.5, 24.9, 56.7, 72.5, 117.
3
–
1
IR (KBr): 2232 (CN) cm .
MS (EI): m/z = 99 [M]+.
1
H NMR (400 MHz): d = 7.86 (d, J = 8.0 Hz, 2 H), 8.34 (d, J =
2
5
8
.0 Hz, 2 H).
N,N-Diethylaminoacetonitrile
IR (KBr): 2174 (CN) cm .
–
1
1
3
C NMR (100 MHz): d = 50.2, 134.1, 125.0, 118.4, 116.5.
1
MS (EI): m/z = 148 [M]+.
H NMR (400 MHz): d = 1.08 (t, J = 7.0 Hz, 6 H), 2.56 (d, J =
.8 Hz, 4 H), 3.58 (s, 2 H).
6
p-Nitrobenzamide18
IR (KBr): 1685 (CO) cm .
13
C NMR (100 MHz): d = 13.6, 40.8, 48.1, 114.5.
–
1
MS (EI): m/z = 112 [M]+.
1
H NMR (400 MHz): d = 7.96 (d, J = 8.4 Hz, 2 H), 8.29 (d, J =
2
6
8
.4 Hz, 2 H).
Cyclohexanecarbonitrile
IR (KBr): 2228 (CN) cm .
–
1
1
3
C NMR (100 MHz): d = 124.6, 130.0, 141.4, 149.2, 168.2.
1
MS (EI): m/z = 166 [M]+.
H NMR (400 MHz): d = 1.38–1.58 (m, 4 H), 1.66–1.76 (m, 4 H),
1
.81–1.86 (m, 2 H), 2.59–2.61 (m, 1 H).
1
9
13
4
-(Diethoxymethyl)benzonitrile
C NMR (100 MHz): d = 24.2, 25.3, 28.1, 29.6, 122.7.
–
1
IR (KBr): 2229 (CN) cm .
1
MS (EI): m/z = 109 [M]+.
H NMR (400 MHz): d = 1.24 (t, J = 7.0 Hz, 6 H), 3.50–3.63 (m,
H), 5.52 (s, 1 H), 7.59 (d, J = 8.0 Hz, 2 H), 7.65 (d, J = 6.8 Hz,
4
2
1,4-Dicyanobenzene27
IR (KBr): 2212 (CN) cm .
–
1
H).
1
3
1
C NMR (100 MHz): d = 15.6, 61.3, 100.1, 109.1, 118.4, 127.1,
H NMR (400 MHz, CDCl ): d = 7.79 (s, 4 H).
3
1
31.6, 143.7.
13
C NMR (100 MHz, CDCl ): d = 116.9, 117.0, 132.8.
3
MS (EI): m/z = 205 [M]+.
MS (EI): m/z = 128 [M]+.
2
0
3
,4-Dihydroxylbenzonitrile
28
p-Cyanobenzamide
–
1
IR (KBr): 2217 (CN) cm .
1
1
H NMR (400 MHz, DMSO-d ): d = 7.65 (br, 1 H), 7.92 (d, J =
7.6 Hz, 2 H), 8.00 (d, J = 7.6 Hz, 2 H), 8.02 (br, 1 H).
6
H NMR (400 MHz): d = 90 (d, J = 8.8 Hz, 1 H), 7.10 (d, J = 6.8 Hz,
2
H), 7.13 (s, 1 H).
13
C NMR (100 MHz): d = 114.6, 118.9, 129.1, 133.8, 139.5, 169.0.
IR (KBr): 2207 (CN), 1698 (CO) cm–1.
MS (EI): m/z = 146 [M]+.
1
3
C NMR (100 MHz): d = 103.0, 115.5, 118.2, 119.3, 125.8, 144.3,
1
48.8.
MS (ESI): m/z = 136 [M + 1]+.
1
,3-Dicyanobenzene29
1
-Naphthonitrile21
–1
IR (KBr): 2233 (CN) cm .
–
1
IR (KBr): 2222 (CN) cm .
1
H NMR (400 MHz): d = 7.64 (t, J = 8.0 Hz, 1 H), 7.88 (d, J =
1
H NMR (400 MHz): d = 7.46–7.50 (m, 1 H), 7.58 (t, J = 7.4 Hz,
8
.0 Hz, 2 H), 7.94 (s, 1 H).
1
8
H), 7.66 (t, J = 7.6 Hz, 1 H), 7.84–7.89 (m, 2 H), 8.03 (d, J =
.4 Hz, 1 H), 8.20 (d, J = 8.0 Hz, 1 H).
1
3
C NMR (100 MHz): d = 114.2, 116.9, 130.5, 136.1, 136.8.
MS (EI): m/z = 128 [M]+.
1
3
C NMR (100 MHz): d = 110.0, 117.5, 124.8, 125.1, 127.7, 128.5,
28.6, 132.2, 132.5, 132.8, 133.2.
1
3
0
m-Cyanobenzamide
MS (EI): m/z = 153 [M]+.
1
H NMR (400 MHz): d = 7.63 (br, 1 H), 7.68 (t, J = 7.6 Hz, 1 H),
7
.99 (d, J = 7.6 Hz, 1 H), 8.15–8.17 (m, 2 H), 8.26 (s, 1 H).
trans-Cinnamonitrile22
IR (KBr): 2217 (CN) cm .
–
1
Synthesis 2011, No. 16, 2639–2643 © Thieme Stuttgart · New York

PAPER
Conversion of Aldoximes into Nitriles
2643
1
3
C NMR (100 MHz): d = 111.8, 128.1, 129.4, 131.0, 132.4, 134.5,
(11) Yamaguchi, K.; Fujiwara, H.; Ogasawara, Y.; Kotani, M.;
Mizuno, N. Angew. Chem. Int. Ed. 2007, 46, 3922.
1
35.6, 166.5.
MS (EI): m/z = 146 [M]+.
(12) Jiang, N.; Ragauskas, A. J. Tetrahedron Lett. 2010, 51,
4479.
(
13) Zuidema, D. R.; Dennison, A. L.; Park, E. Y.; Mebane, R. C.
Synth. Commun. 2008, 38, 3810.
Acknowledgment
(
(
14) Morgan, G. T.; Burstall, F. H. J. Chem. Soc. 1932, 20.
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Lett. 2009, 11, 2461.
This work was financially supported by the National Science
Council, Taiwan (NSC97-2113-M002-013).
(
16) Reddy, K. R.; Maheswari, C. U.; Venkateshwar, M.;
Prashanthi, S.; Kantam, M. L. Tetrahedron Lett. 2009, 50,
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Synthesis 2011, No. 16, 2639–2643 © Thieme Stuttgart · New York