Cyclopropenone-catalyzed direct conversion of aldoximes and primary amides into nitriles

Article abstract of DOI:10.1002/ejoc.201300059

Efficient conversion of aldoximes and primary amides into nitriles by employing cyclopropenone as an organocatalyst is reported. The reaction proceeds smoothly under mild conditions with 5 mol-% catalyst loading to afford nitriles in excellent yields (78-94 %) in a single operation. This method is equally applicable to both aldoximes and primary amides bearing aromatic, heterocyclic, and aliphatic moieties. The convenient and catalytic procedure widens the scope of the utilization of cyclopropenones in organic synthesis. Cyclopropenone- catalyzed conversion of aldoximes and primary amides into nitriles in a one-pot procedure is described. The reaction proceeds smoothly under mild conditions with low catalyst loading. The convenient and catalytic procedure widens the scope of the utilization of cyclopropenones in organic synthesis.

Full text of DOI:10.1002/ejoc.201300059

SHORT COMMUNICATION
DOI: 10.1002/ejoc.201300059
Cyclopropenone-Catalyzed Direct Conversion of Aldoximes and Primary
Amides into Nitriles
Ankita Rai[a] and Lal Dhar S. Yadav*[a]
Keywords: Organocatalysis / Aldoximes / Amides / Cyanides / Small-ring systems
Efficient conversion of aldoximes and primary amides into
nitriles by employing cyclopropenone as an organocatalyst is
reported. The reaction proceeds smoothly under mild condi-
tions with 5 mol-% catalyst loading to afford nitriles in excel-
lent yields (78–94%) in a single operation. This method is
equally applicable to both aldoximes and primary amides
bearing aromatic, heterocyclic, and aliphatic moieties. The
convenient and catalytic procedure widens the scope of the
utilization of cyclopropenones in organic synthesis.
However, only a few reports are available on the dehy-
dration of aldoximes and primary amides with the same
reagent system.[19] Some of these methods have only limited
value because of low yields, expensive or hard-to-obtain
reagents, the use of strong acids or bases (toxic and hazard-
ous chemicals), drastic reaction conditions, involvement of
inconveniently low temperatures (–78 °C), long reaction
times, and tedious workup. More importantly, the methods
are not equally applicable to alkyl, aryl, and heterocyclic
compounds and involve the use of certain reagents that
might cause severe storage and handling problems as well
as significant waste generation. Hence, the search for conve-
nient, catalytic, less-toxic, more-reliable, reproducible meth-
ods that can be applied to both aldoximes and primary
amides is still desirable.
In view of the above discussion along with the impor-
tance of organocatalysis, we thought that an organocata-
lytic dehydration of aldoximes and amides would offer a
method of choice for the synthesis of nitriles (Scheme 1).
Cyclopropenones[20,21] were first synthesized independently
in the laboratories of Breslow[21a–21c] and Vol’pin[21d,21e] in
1957. Since then, numerous reports have appeared on the
physical properties and reactions of these interesting com-
pounds.[20]
Introduction
Nitriles are synthetic intermediates that are useful for the
production of pharmaceuticals, polyamides, pigments, dyes,
agrochemicals, and many other fine chemicals.[1] They are
endowed with rich chemistry and serve as precursors in sev-
eral group transformations to other functionalities.[2] In ad-
dition, the cyano group plays a significant role in biology
by hydrogen bonding to certain biological receptors.[3] It is
a prominent functional motif found in numerous natural
products and bioactive molecules.[3a,3b]
The synthesis of nitriles by dehydration of aldoximes and
primary amides is well established. However, most reagents
employed for these transformations suffer from practical
disadvantages such as inconvenient reaction conditions or
workup methods, problematic byproducts, narrow substrate
scope, toxicity, and poor reactivity. A plethora of reagents
is available for the transformation of aldoximes into nitriles,
and some selected methodologies have been performed in
the presence of acidic dehydrating agents;[4] the combina-
tion of acyl, thionyl, silyl, or sulfonyl chloride and a base;[5]
phosphorus-based reagents;[6] basic reagents;[7] polychloro-
heteroaromatic compounds;[8] certain main group/transi-
tion metals and their complexes,[9] Lewis acids,[10] ion ex-
changers,[11] modified or unmodified montmorillonite
clays,[12] enzymatic systems,[13] or heating without[14] or with
[17]
a catalyst.[15] Very recently, Pd[16] and [RuCl2(p-cymene)]2
catalysts have been employed for the conversion of oximes
into nitriles.
Notably, an opulence of reagents is available to effect
only the conversion of primary amides into nitriles.[18]
[a] Green Synthesis Lab, Department of Chemistry, University of
Allahabad,
Scheme 1. Synthesis of nitriles from aldoximes and primary amides.
Allahabad 211002, India
Fax: +91-5322460533
E-mail: ldsyadav@hotmail.com
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201300059.
Importantly, diarylcyclopropenones have recently gained
prominence as a result of the work of Lambert and co-
workers.[22a–22d] Owing to aromatic stabilization, cycloprop-
Eur. J. Org. Chem. 2013, 1889–1893
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A. Rai, L. D. S. Yadav
SHORT COMMUNICATION
enones are highly polarized (Figure 1), and thereby their Table 1. Conversion of benzaldoxime into benzonitrile 4a.[a]
physical and chemical properties are remarkably different
from those of other α,β-unsaturated ketones. The above fea-
tures, convenient synthesis, and tunable electronic and steric
properties are well suited for the organocatalytic utilization
of cyclopropenones. Recently, we reported a cyclopropen-
ium ion catalyzed Beckmann rearrangement, which pres-
ents a unique application of the cyclopropenium ion as an
Entry
Solvent
Base
Time [h][b]
Yield [%][c]
organocatalyst.[23] Following this publication, Vanos and
Lambert reported a cyclopropenium-activated Beckmann
rearrangement and provided an ingenious discussion on ca-
talysis versus self-propagation in previously reported
organocatalytic Beckmann rearrangements.[24]
1
2
3
4
5
6
7
8
CH3CN
DCM
THF
pyridine
pyridine
pyridine
pyridine
Et3N
Et3N
DBU
DBU
DBU
–
11
9
8
9
5
10
4
6
11
11
11
86
89
90
77
91
88
94
89
0[d]
0[e]
0[f]
Et2O
DCM
CH3CN
DCM
CH3CN
DCM
DCM
DCM
9
10
11
DBU
[a] The reaction was performed with benzaldoxime (1a, 1 equiv.),
3a (5 mol-%), (COCl)2 (1 equiv.), and base (3 equiv.) in refluxing
solvent, unless otherwise indicated. [b] Time taken to complete the
reaction. [c] Yield of isolated pure product 4a. [d] Reaction was per-
formed in the absence of (COCl)2. [e] Reaction was performed in
the absence of base. [f] Reaction was performed in the absence of
3a.
Figure 1. Aromatic stabilization of cyclopropenones.
Result and Discussions
Intrigued by our study,[23] we hypothesized that a cyclo-
propenium ion catalyzed reaction in the presence of a base
could result in the dehydrative formation of nitriles from
aldoximes and primary amides.
Next, we examined the conversion to optimize the cata-
Interestingly, the reaction did take place, but it required lytic activity of various cyclopropenium ions and the quan-
a stoichiometric amount of chlorocyclopropenium ion 5Ј tity of (COCl)2, which acts as an activating agent for the
and afforded cyclopropenone as a byproduct (Scheme 2), catalyst. Thus, after screening a range of cyclopropenium
which is in accordance with earlier reports.[22a–22c,22e] In- ions bearing different Ar groups, it was found that variation
spired by the work of Vanos and Lambert[22d] and on the of the size of the aryl substituents showed a little change in
basis of the fact that (COCl)2 readily converts 2,3-disubsti- the yield obtained (Table 2).
tuted cyclopropenones[21e] into the corresponding 3,3-
Curiously, however, it was also found that a change in
dichlorocyclopropenones,[22] which are precursors of chloro- the electronic properties of the aryl substituents affected the
cyclopropenium chloride, we turned our attention to the results considerably. Thus, the 3-NO2 electron-withdrawing
catalytic utilization of the cyclopropenone byproduct by group was unsatisfactory and decreased the yield consider-
employing (COCl)2 as an activator.[25a,25b] To our delight, ably (Table 2, entry 9). Again, the influence of the molar
this strategy worked well in the presence of a base. There concentrations of the catalyst and activating agent were
are several advantages associated with the use of cyclopro- studied. The complete results are compiled in Table 2, and
penone over other reagents/catalysts: it can be readily syn- the best results were obtained with 5 mol-% of (4-methoxy-
thesized in the laboratory by using economic substrates, it phenyl)cyclopropenone and 1 equiv. of (COCl)2 (Table 2,
can be recycled, it does not require low temperatures, and entry 2). Thus, 3a in DCM in the presence of DBU and the
it can dehydrate both aldoximes and primary amides to the (COCl)2 activating agent was found to be the method of
corresponding nitriles.
choice for the transformation of aldoximes into nitriles
In our initial investigation, benzaldoxime (1a) was se- (Table 3). The generality of the method was further demon-
lected as a model substrate to identify and optimize critical strated by transforming primary amides into the corre-
reaction parameters for its cyclopropenone-catalyzed dehy- sponding nitriles (Table 4). For this purpose, benzamide
dration to benzonitrile (Table 1). From the results compiled was used as a model substrate and was converted into
in Table 1, it is evident that the best results in terms of yield benzonitrile under the optimized conditions for obtaining
and reaction time with catalyst 3a were obtained when nitriles from aldoximes. Oxalyl chloride was added to a
dichloromethane (DCM) was used as the solvent and 1,8- stirred mixture of benzamide and 3,3-dichloro-1,2-bis(4-
diazabicyclo[5.4.0]undec-7-ene (DBU) was used as the base methoxyphenyl)cyclopropenone (3a) in DCM and DBU at
(Table 1, entry 7). Interestingly, it was found that the refluxing temperature to afford benzonitrile in 92% yield
reaction did not proceed in the absence of 3a, DBU, or (Table 4, entry 1). It was noted that the reaction did not
(COCl)2 (Table 1, entries 9–11). Similar results were ob- proceed in the absence of 3a, DBU, or (COCl)2 (Table 4,
tained with primary amides (Table 4, entries 2–4).
entries 2–4).
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Eur. J. Org. Chem. 2013, 1889–1893

Conversion of Aldoximes and Primary Amides into Nitriles
Table 2. Optimization of the catalyst and activator for the conver-
Table 4. Catalytic conversion of various primary amides into ni-
sion of benzaldoxime into benzonitrile 4a.[a]
triles 4.[a]
Entry Precursor 2, R
Nitrile 4
Time
[h][b]
Yield
[%][c,d]
1
2
3
4
5
6
7
8
C6H5
C6H5
C6H5
C6H5
4a
4a
4a
4a
4b
4c
4d
4e
4f
4g
4h
4k
4l
5.5
11
11
11
4.5
4.5
6
5
4.5
5
4.5
6
5
4.5
5
92
0[e]
0[f]
0[g]
94
91
87
91
89
78
83
89
89
84
88
Entry Ar
Cat. 3 Activator (COCl)2
Yield
[%][b]
[mol-%]
[equiv.]
4-MeOC6H4
4-ClC6H4
4-O2NC6H4
4-MeC6H4
3-pyridyl
3-methyl-2-thienyl
benzyl
3,4-(MeO)2C6H3
2-H2NC6H4
4-MeOCOC6H4
EtOOC–CH=CH–CH=CH
1
2
3
4
5
6
7
8
9
4-MeOC6H4
3
5
10
5
5
5
5
5
5
1
1
1
1.5
0.5
1
1
1
1
76
94
94
94
44
80
84
89
71
4-MeOC6H4
4-MeOC6H4
4-MeOC6H4
4-MeOC6H4
C6H5
4-tBuC6H4
2,4-(MeO)2C6H3
3-NO2C6H4
9
10
11
12
13
14
15
4m
4n
[a] The reaction was performed with benzaldoxime (1a, 1 equiv.)
and DBU (3 equiv.) in refluxing DCM for 4 h. [b] Yield of isolated
pure product 4a.
[a] Reaction was performed according to the general procedure (see
the Experimental Section), unless otherwise indicated. [b] Time re-
quired for completion of the reaction. [c] Yield of isolated and puri-
fied product. [d] All of the products are known compounds[17,18,26]
(see the Supporting Information). [e] Reaction was performed in
the absence of (COCl)2. [f] Reaction was performed in the absence
of base. [g] Reaction was performed in the absence of 3a.
Table 3. Catalytic conversion of various aldoximes into nitriles 4.[a]
chains are tolerated in this protocol to afford smoothly the
corresponding nitriles in excellent yield with high purity.
Notably, the presence or absence of an electron-with-
drawing or electron-donating group does not significantly
affect the yield.
A mechanistic rationale of the present dehydration reac-
tion is depicted in (Scheme 2). Cyclopropenone catalyst 3
undergoes rapid reaction with (COCl)2 to generate 5, which
ionizes to form cyclopropenium chloride salt 5Ј. The salt
then reacts with aldoxime 1 or primary amide 2 to produce
protonated O-cyclopropenyl derivatives 6 and 8, respec-
tively, which afford nitriles 4 through intermediates 7 and
9, respectively, with regeneration of catalyst 3 to complete
the catalytic cycle. It should be noted that proton abstrac-
tion is required not only at intermediates 6 and 8 but also
at 7 and 9.
Entry Precursor 1, R
Nitrile 4
Time
[h][b]
Yield
[%][c,d]
1
2
3
C6H5
4-MeOC6H4
4-ClC6H4
4a
4b
4c
4d
4e
4f
4g
4h
4i
4.5
4
4
4.5
4.5
4
4.5
5.5
5.5
4
90
94
91
92
86
88
85
87
84
87
86
89
4
5
4-O2NC6H4
4-MeC6H4
6
7
8
3-pyridyl
3-methyl-2-thienyl
benzyl
9
C6H5CH=CH2
3-MeOC6H4
4-MeOCOC6H4
EtOOC-CH=CH-CH=CH
10
11
12
4j
4m
4n
4
4
[a] See the Experimental Section for the general procedure. [b] Time
required for completion of the reaction. [c] Yield of isolated and
purified product. [d] All of the products are known com-
pounds[17,18,26] (see the Supporting Information).
The protocol requires mild heating probably because of
the high energy required to break the strong N–O and C–
O bonds of intermediates 7 and 9 during the elimination
As the final stage in the development of this method, we process. Initial studies revealed that in the absence of either
assessed the generality, utility, and scope of the transforma- the cyclopropenone catalyst or DBU the reaction does not
tion under the optimized conditions. Thus, the dehydration proceed in the presence of (COCl)2; this is because an active
of a variety of aldoximes and primary amides was under- carbocation is required for nucleophilic attack of the lone
taken and excellent results were obtained (Tables 3 and 4). pairs of electrons of the oxygen atom of corresponding ald-
Various functional groups including methoxy, aromatic, oximes 1 and primary amides 2 and extraction of a proton
heteroaromatic, halogens, nitro, ester, amino, and aliphatic of corresponding intermediates 6/8 and 7/9 is necessary.
Eur. J. Org. Chem. 2013, 1889–1893
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1891

A. Rai, L. D. S. Yadav
SHORT COMMUNICATION
Scheme 2. A plausible mechanism for the synthesis of nitriles 4.
were recorded with a Bruker AVII 400 spectrometer in CDCl3 by
using TMS as an internal reference. 13C NMR spectra were re-
corded on the same instrument at 100 MHz in CDCl3 and TMS
was used as an internal reference. Mass (EI) spectra were recorded
with a JEOL d-300 mass spectrometer. Elemental analyses were car-
ried out in a Coleman automatic carbon, hydrogen, and nitrogen
analyzer. The structure of products 4 were confirmed by their ele-
mental and spectral analyses and by comparison of their melting
This is in accordance with earlier reports in which
(COCl)2 was used as an activating agent with other rea-
gents, and it alone could not bring about the conversion of
aldoximes and primary amides into nitriles.[25] It has been
reported that many of the reagents used are not selective
and that they may react with the primary amides and ald-
oximes to give side products other than nitriles or give
products other than their dehydration products.[25] None-
theless, herein is reported a reagent catalyst system that ex-
1
points, TLC data, and IR, H NMR and 13C NMR spectroscopic
data with authentic samples obtained commercially or prepared by
clusively gives nitriles from both aldoximes and primary literature methods[17,18,26] (see the Supporting Information).
amides.
General Procedure for the Synthesis Nitriles through the Dehydration
of Aldoximes: To a stirred mixture of aldoxime 1 (2 mmol) and 3,3-
dichloro-1,2-bis(4-methoxyphenyl)cyclopropenone (3a; 26 mg,
5 mol-%) in DCM (10 mL) in a flame-dried, round-bottomed flask
Conclusions
was added a solution of (COCl)2 (251 mg, 2 mmol) over 1 h by
In summary, we have utilized the cyclopropenium ion ac-
using a syringe pump followed by the slow addition of DBU
tivation strategy for the synthesis of nitriles through the cy-
(0.9 mL, 6 mmol) in DCM (2 mL), and the reaction mixture was
clopropenone-catalyzed dehydration of aldoximes and pri-
heated at reflux for the specified time (see Table 3). After comple-
mary amides. The compatibility of this method with base
tion of the reaction, as indicated by TLC, the reaction mixture was
offers rapid access to nitriles. The reaction can be con-
ducted in a simple way under mild conditions and offers
significant advantages over existing methods in terms of
high yields, easily handled reagent system, and wide appli-
cability to heterocyclic, aromatic, as well as aliphatic sub-
strates. The present study widens the scope of the utilization
of cyclopropenones in organic synthesis.
diluted with DCM and washed with water (3ϫ 5 mL). The organic
layer was dried with Na2SO4 and evaporated under reduced pres-
sure to give the crude product, which was purified by flash
chromatography (hexane/ethyl acetate, 9:1) to afford analytically
pure 4.
General Procedure for the Synthesis Nitriles through the Dehydration
of Primary Amides: Products 4 were synthesized from primary
amides 2 (see Table 4) by following the above procedure.
Supporting Information (see footnote on the first page of this arti-
cle): Experimental details and characterization data for all com-
pounds.
Experimental Section
General Methods: Reagents were obtained from commercial suppli-
ers and used without further purification unless otherwise speci-
fied. All reactions were performed by using oven-dried glassware
under a nitrogen atmosphere. Organic solutions were concentrated
by using a Buchi rotary evaporator. Flash column chromatography
was carried out over silica gel (Merck 300–400 mesh), and TLC
was performed with silica gel GF254 (Merck) plates. Melting points
were determined by open glass capillary method. 1H NMR spectra
Acknowledgments
The authors sincerely thank SAIF, Punjab University, Chandigarh,
for providing microanalyses and spectra. A. R. is grateful to the
Council of Scientific and Industrial Research (CSIR), New Delhi,
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Conversion of Aldoximes and Primary Amides into Nitriles
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Received: January 13, 2013
Published Online: February 18, 2013
Eur. J. Org. Chem. 2013, 1889–1893
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