Cu-Catalyzed direct cyanation of terminal alkynes with AMBN or AIBN as the cyanation reagent

Article abstract of DOI:10.1039/c5cc04987c

A Cu-catalyzed direct cyanation of terminal alkynes was reported with broad substrate generality in moderate to high yield, and AMBN (azobisisoamylonitrile)/AIBN (azobisisobutyronitrile) were used as less toxic and effective cyanating sources in open air. Interestingly, addition products were selectively achieved as the major product under the same conditions in argon.

Full text of DOI:10.1039/c5cc04987c

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DOI: 10.1039/C5CC04987C
Journal Name
COMMUNICATION
Cu‐Catalyzed direct cyanation of terminal alkynes with AMBN or
AIBN as cyanation reagents
Guangwei Rong,a Jincheng Mao,*a,b Yang Zheng,a Ruwei Yaoa and Xinfang Xu*a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
A Cu‐catalyzed direct cyanation of terminal alkynes was reported
Terminal alkynes are very useful compounds, which are abundant
with broad subsrate generality in moderated to high yield, and in natural products, and they can be easily manipulated to form
AMBN (Azobisisoamylonitrile)/AIBN (Azobisisobutyronitrile) are other compounds via additions with aryl halides,6a amines,6b
used as less toxic and effective cyanating source in open air. arylboronic acids,6c etc.6d‐6g Besides, the terminal position tend to
Interestingly, addition products were selectively achieved as be served as ideal reaction sites for further introducing the
major product in same condition under argon.
functional group by replacement of the C(sp)‐H bond. Relevant
researches on the functionalization of terminal alkyne were well
explored, including methylation,7a trifluoromethylation,7b‐7c
halogenation,7d cyanation, etc.7h‐7n For the cyanation reactions,
metal cyanides proved to be straightforward and effective reagents,
despite the metal waste and toxicity (Scheme 1, eq 1).7h‐7j Very
recently, Okamoto et al. have successfully achieved the cyanation
of internal alkynes by using cyanogen iodide as cyanation reagent.7n
Other non‐metal cyanide regents such as 1‐cyanoimidazole were
reported for the same transformations, while harsh conditions were
required (eq 2).7k‐7m What's more, these kind of cyanation
surrogates are not readily available and toxic. Herein, we disclosed
a copper‐catalyzed cyanation of terminal alkynes under mild
conditions with broad substrates generality, more importantly,
AMBN or AIBN are used as a less toxic cyanation reagent under air
atmosphere (eq 3). And the addition products were observed as
major product when the reaction carried out in argon (eq 4).
The nitrile group plays an important role in organic chemistry,
which not only served as a key unit in plentiful pharmaceuticals and
bioactive compounds1, but also as a versatile group that can be
easily transformed into various derivatives, such as nitrogen‐
containing heterocycles, amides, amines, etc.2 In this context,
cyanation reaction has been an important research orientation in
organic chemistry, including traditional cyanation reactions such as
Sandmeyer reaction, which provide effective methods to prepare
aromatic nitriles.3 And various cyanation reagents have been well
explored by many researchers in recent decades.3‐5 Convenient
methods for the synthesis of aromatic nitriles are the transition
metal‐catalyzed cyanation of aryl halides like Rosenmund‐von
Braun reaction.4a However, prefunctionalization is required for the
substrate and stoichimetric toxic metal cyanation regents are
adopted, including MCN(M= Cu, K, Na).4 Although there are few of
new organic cyanation reagents were developed, it costs money or
takes steps to prepare them.5 Therefore, a range of green cyanation
regents were discovered. Cheng and co‐workers reported a copper‐
catalyzed cyanation of arylboronic acids, in which safe reagent DDQ
was employed as the cyanide source.5a In 2012, Wang’s group
disclosed using benzyl cyanide as an operational benign cyanide
surrogate in copper‐catalyzed cyanation of 2‐phenylpyridines.5b
Acetonitrile was firstly reported as a cyanide source for palladium‐
catalyzed cyanation of aryl halide by Cheng and co‐workers.5c
Interestingly, in 2010 Chen’s group illustrated that DMF can be
employed as the source of “C” atom and aqueous ammonia as the
source of “N” atom in their cyanation reaction.5d Very recently, Han
and co‐workers reported a copper‐mediated cyanation of 2‐
phenylpyridines using AIBN as cyanating reagent, which provided a
novel protocol for C‐H cyanation via a free radical “CN” process.5e
Inspired by these results, herein, we present a copper‐catalyzed
cyanation of terminal alkyne with AIBN as cyanation reagent.
Scheme 1. Approaches to the cyanation of terminal alkynes.
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Scheme 2. Substrate scope of arylacetylenes 1 in Cu‐catalzyed
We initiated our study with 4‐bromophenylacetylene 1a as a
model substrate. To our delight, desired product 3a was obtained in
26% yield when the reaction was treated with AIBN (0.6 mmol) and
Cu(OAc)2 (20 mol%) in acetonitrile at 80 oC for 12 hours in air (Table
1, entry 1). No cyanation product was detected at the absence of
the copper catalyst. Then a range of copper catalysts were tested,
and the yield was increased to 48% by switching Cu(OAc)2 with
Cu(acac)2 (entry 9). Other catalysts such as CuCl, CuBr2 and CuI
showed inferior results for this transformation (entries 3‐8). Cupric
nitrate was proved to be the best and afforded 2a with 70% isolated
yield (entry 10). The use of other solvents such as DMSO, DCE and
1,4‐dioxane resulted in much lower yields (entries 11‐14). In order
to improve the reaction yield, AMBN (Azobisisoamylonitrile) was
tried as cyanation reagent, which is a derivative of AIBN. It’s
pleased to find that slightly higher yield 73% was obtained under
the same reaction condition (entry 15). Further investigation
showed this reaction cannot proceed under argon atmosphere
(entry 16), indicating that oxygen is indispensable for this process.
However, the yield was not further promoted under oxygen
atmosphere (entries 17). It’s noteworthy that no self‐coupling
product of acetylene was generated in this reaction.
View Article Online
cyanation a
DOI: 10.1039/C5CC04987C
a
Reaction condition: aryacetylene (0.3 mmol), AMBN (0.6 mmol),
.
o
b
Cu(NO3)2 H2O (20 mol%), CH3CN (4 mL), 80 C, 12h, air. Rection
carried out in 4.0 mmol scale. c AIBN (0.6 mmol) instead of AMBN. d
AMBN (0.9 mmol).
By comparing the results, it’s not difficult to find that substrates
bearing electron‐withdrawing groups on the benzene rings tend to
accomplish the reaction in much higher yields than those with
electron‐donating analogues. Moderate yield was obtained when
phenylacetylene was applied to react with AMBN. Methyl and ethyl
substituted substrates afforded the corresponding products (2b, 2c)
in slightly lower yields. Due to the strong electron‐withdrawing
property of trifluoromethyl group, 2d was afforded in much higher
yield compared with 2b. Notably, Halogen substituted
phenylacetylenes also gave the corresponding products in good
yields (2a, 2e, 2f), and the halogen group survived well after
reaction which were applicable for further functionalization. It’s
noteworthy when the reaction was conducted on gram scale by
using pheylacetylene (1g) as substrate and 2g was obtained in the
yield of 52%. For some substrates (2h, 2i), AIBN shows better
efficiency than AMBN. Nitro and cyano groups were well tolerated
and the reactions were furnished in good to excellent yields. For 2‐
naphthyl and 2‐thienyl substrates, the cyanation reactions can
proceed smoothly and satisfied yields were obtained (2m, 2n).
Furthermore, 1,3‐diethynylbenzene was also suitable for this
reaction, affording 2o in 62% yield. Interestingly, no single
cyanation products were detected in the reaction, which shows the
high efficiency of this cyanation protocol. However, 3‐
ethynylpyridine is not suitable for this reaction, mainly because of
the coordination effect between copper catalyst and substrate.
Likewise, amino substituted phenylacetylene can not successfully
cyanated under standard reaction conditions, unless the amino
group was protected as an amide (2l).
Table 1. Reaction condition optimizationa
entry
1
2
3
4
5
6
7
8
catalyst
Cu(OAc)2.H2O
solvent
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
DCE
Dioxane
Ethanol
DMSO
CH3CN
CH3CN
CH3CN
yield (%)b
26
‐
Cu
nd
27
19
30
38
28
29
48
CuCl
.
CuCl2 2H2O
CuBr
CuBr2
CuI
9
Cu(acac)2
.
10
11
12
13
14
15c
16d
17e
Cu(NO3)2 3H2O
70
.
Cu(NO3)2 3H2O
trace
trace
18
12
73
.
Cu(NO3)2 3H2O
.
Cu(NO3)2 3H2O
.
Cu(NO3)2 3H2O
.
Cu(NO3)2 3H2O
.
Cu(NO3)2 3H2O
nd
59
.
Cu(NO3)2 3H2O
a Reaction condition: 4‐Bromophenylacetylene 1a (0.3 mmol),
o
AIBN (0.6 mmol), catalyst (20 mol%), solvent (4.0 mL), 80 C,
b
c
12 h in open air. Isolated yield; nd: not detected. AMBN
d
(0.6 mmol) instead of AIBN. Under Ar atmosphere. e Under
oxygen atmosphere.
After the exploration of the reaction scope with aryl alkynes, we
next turned our attention to the aliphatic alkynes (Scheme 3). To
our delight, 56% yield of 2p was isolated when 1‐dodecyne was
performed under standard condition with AMBN. Encouraged by
this result, we next tried various alkylacetylenes. Obviously, the
substrate with cyan group gave high yield (2q, 92%). Silylacetylene
With optimized reaction condition in hand, we proceeded to
investigate the scope generality of this cyanation reactions. A
number of arylacetylenes were first examined under the optimized
reaction conditions, and the results were summarized in Scheme 2.
As expected, a variety of functional groups, including
trifluoromethyl, methoxy, halogen and nitro, were well tolerated.
2 | J. Name., 2012, 00, 1‐3
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a
Scheme 4. Reaction scope of preparationViewtAhrteicle Ovninlinyel
Scheme 3. Cyanation with various alkylacetylene
isobutyronitrilesa
DOI: 10.1039/C5CC04987C
NC
Cu(NO3)2.3H2O
N
Alkyl
Alkyl
CN
CN
N
80oC
CN
2
1
CN
n-C10H21
CN
NC
TIPS
O
CN
( )3
O
Ph
2s, 78%
2q, 92%
2r, 42%
2p, 56%
NC
Bn
CN
CN
( )3
Ph
O
CN
2u, 72%
HO
CN
2t, 50%
2v, 70%
2w, 45%
CN
O
O
O
a
Reaction condition: Arylacetylene (0.6 mmol), AIBN (0.15 mmol),
CN
O
.
Cu(OAc)2 H2O (20 mol%), pyridine (4 mL), 80 oC, 12h, Ar; yields were
N
O
b
O
O
CN
based on limiting reagent of AIBN or AMBN. AMBN (0.15 mmol)
O
CN
2x, 58%
CN
instead of AMBN.
2y, 68%
O
2z, 86%
a
Reaction condition: Aliphatic alkyne 1 (0.3 mmol), AMBN (0.6
mmol), Cu(NO3)2 3H2O (20 mol%), CH3CN (4.0 mL), 80 oC, 12 h, air.
.
initiated via the generating of an isobutyronitrile radical from AIBN
by heating,6 which then reacted with arylacetylene to generate 4 in
argon atmosphere. The radical process was confirmed by control
reaction in the precence of TEMPO, in which the reaction was
inhibited obviously (Scheme S1, eq 3).9 Intermediate 4 then reacted
with CuII species to yield vinyl copper(III) complexes 5. Finally,
intermediate 5 went through a protonation process by trapping a
proton from arylacetylene, or partly from H2O in the system, to
produce desired product 3 and regenerated Cu(II) catalyst.10 And
this proton trasfer procee was confirmed by observation a certain
amount of coupling pyprodcut,8 which was not detected under the
same condition in the absence of AIBN, and the deuterium labeling
experiment.9
When the reaction was conducted in air, the initially generated
isobutyronitrile radical was then oxidized to radical 6 by O2. After
releasing one equivalent of acetone, radical 6 converted to the
cyano radical;5f and the (phenylethynyl)copper could be a very
possible intermediate in this reaction to trap the cyano radical to
give 7 through single‐electron transfer,5g since moderate yield was
obtained for the same product when (phenylethynyl)copper was
used as the srarting materal instead of alkyene (Scheme S1, eq 5).9
Finally, reductive elimination of intermediate 7 delivered the
product 2 and Cu(I).11 The Cu(I) was next oxidized to Cu(II) by O2
and went back to the reaction cycle.
was also compatible by our method, giving 2r in the yield of 42%.
Good yields were obtained when substrates 1s and 1v were tested,
which provides an efficient strategy to introduce cyan group to
specific molecules containing ether or ester groups. It’s encouraging
to see that 1w was also suitable for this reaction which contains an
free hydroxyl group, although moderate yield was obtained. It’s
quite excited to find out that diynes also react smoothly under this
condition (1t, 1y). Even unsymmetrical diyne (1x) can be tolerated
by the reaction and afford the desired product in good yield.
Moreover, alkyne 1z which contains an amide group was suitable
for this reaction, giving 2z in 86% isolated yield.
During the reaction optimization, 28% of compound 3h was
obtained when the reaction was conducted under argon
atmosphere with cupric acetate as catalyst. Literature investigation
found out that such vinyl isobutyronitrile compounds are except as
the byproducts in some reactions.8a‐8b Besides, Onaka’s group
developed a method to access this vinyl isobutyronitrile from
unsaturated alcohols with TMSCN.8c However, this strategy suffers
the drawback of using expensive catalyst and combined with
limited substrates generality. Therefore, preparing vinyl
isobutyronitriles from commercial available starting materials with
exclusive E isomers seems to be very attractive. After optimization
(Table S1),9 we found that excess amount of alkynes should be used
due to the self‐coupling byproducts were inevitably generated
under this condition. Generally, moderate yields were obtained
when arylacetylenes were employed to react with AIBN in pyridine
with cupric nitrate as catalyst at 80 oC under argon atmosphere for
12 hours (Scheme 4). Obviously this reaction was little influenced by
the electron nature of the substrates, as both 4‐fluoro and 4‐
methoxyl phenylacetylenes provided the products in moderate
yields. However, aliphatic acetylene was not tolerated for this
reaction, and no conversion of the substrate was detected.
In summary, we have developed an efficient copper catalyzed
cyanation of terminal alkynes with AMBN/AIBN as effective
According to the reported literatures and our observations
(Scheme S1),9 a plausible mechanism was proposed to rationalize
the formation of both 2 and 3 (Scheme 6). Firstly, the process was
Scheme 6. Plausible reaction pathways.
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Wang, Angew. Chem. Int. Ed., 2014, 53, 2186; ( ) J. Li and L.
o
cyanating source. Various alkynes were tolerated with this protocol,
including not only aryl, alkyl or silyl substituted alkynes, but also
their ether, ester and amide derivatives. Interestingly, vinyl
isobutyronitrile products were selectively achieved from the same
starting materials by simply altering the reaction condition under
inert gas atmosphere. Comparing with previous reports on the
synthesis of cyanoacetylenes, our method has the remarkable
advantages in less toxic cyanating reagent and wide substrate scope.
But drawbacks are still exist such as excess amout of alkynes were
used in the systhesis of vinyl isobutyronitrile compounds. Further
studies on the cyanating reactions with AIBN/AMBN as cyanating
reagent are ongoing in our group.
View Article Online
DOI: 10.1039/C5CC04p9)87YC.
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(a
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,
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9
7
(a
b
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Acknowledgements
8
f
We are grateful to the grants from the Research Grant under
the SA/China Agreement on Cooperation in Science and
Technology; the Scientific Research Foundation for the
Returned Overseas Chinese Scholars, State Education Ministry;
the grants from the Priority Academic Program Development
of Jiangsu Higher Education Institutions; the startup funding
from Soochow University and the Key Laboratory of Organic
Synthesis of Jiangsu Province.
(
g
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2
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