Cu-catalyzed cyanomethylation of imines and α,β-alkenes with acetonitrile and its derivatives

Article abstract of DOI:10.1039/d0ra10693c

We describe copper-catalyzed cyanomethylation of imines and α,β-alkenes with a methylnitrile source and provide an efficient route to synthesize arylacrylonitriles and β,γ-unsaturated nitriles. This method tolerates aliphatic and aromatic alkenes substituted with a variety of functional groups such as F, Cl, Br, Me, OMe,tert-Bu, NO₂, NH₂and CO₂H with good to excellent yields (69-98%). These systems consist of inexpensive, simple copper catalyst and acetonitrile with its derivatives (α-bromo/α-iodo-acetonitrile) and are highly applicable in the industrial production of acrylonitriles.

Full text of DOI:10.1039/d0ra10693c

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Cu-catalyzed cyanomethylation of imines and a,b-
alkenes with acetonitrile and its derivatives†
Cite this: RSC Adv., 2021, 11, 5427
Muhammad Siddique Ahmad *a and Atique Ahmadb
We describe copper-catalyzed cyanomethylation of imines and a,b-alkenes with a methylnitrile source and
provide an efficient route to synthesize arylacrylonitriles and b,g-unsaturated nitriles. This method tolerates
aliphatic and aromatic alkenes substituted with a variety of functional groups such as F, Cl, Br, Me, OMe,
Received 21st December 2020
Accepted 19th January 2021
2 2 2
tert-Bu, NO , NH and CO H with good to excellent yields (69–98%). These systems consist of
DOI: 10.1039/d0ra10693c
inexpensive, simple copper catalyst and acetonitrile with its derivatives (a-bromo/a-iodo-acetonitrile)
rsc.li/rsc-advances
and are highly applicable in the industrial production of acrylonitriles.
approaches for the synthesis of b,g-unsaturated nitriles have
been progressed in recent decades. However, the cyanation of
Introduction and importance
4
Acrylonitrile and cyanomethyl are versatile functional units allyl substrates containing leaving groups such as carbonate, or
found in many dyes, herbicides, agrochemicals, pharmaceuti- ester alcohol, halide, acetate, phosphate, are frequently used in
1
4a–f
cals, and natural products. For example, b,g-unsaturated the transformation into b,g-unsaturated nitriles.
Our many
nitriles are found in natural products such as alkanenitriles, b- efforts have been paid attention in developing non-toxic and
2
amino nitriles, nitrilosids (Scheme 1). The biologically active slow-releasing cyano-methyl reagents like alkyl nitriles, espe-
ruxolitinib, alkylnitrile and acrylonitrile containing entacapone cially acetonitrile. However, due to its high pK
a
value [pK
a
2
are also shown in Scheme 1. These b,g-unsaturated nitriles and (CH
3
CN) ¼ 31.3 in DMSO], relatively difficult to be used as
alkenyl nitriles are also key structural units as antifungal agents a nucleophile. The catalytic C–H bond activation of acetonitrile
3
2b
and vitamin D receptor. Besides, the cyano group serves as by transition metals has rarely been explored in last decades.
A
a valuable intermediate for transformation into aldehydes, few strategies has been reported for cyano-methylation by using
2
amines, amides, tetrazoles, and carboxyl derivatives. A lot of acetonitrile for various substrates such as phenazines, 2,2,6,6
tetramethylpipyridine, C -quaternary indolin-3-ones, cyclo-
2
alkene, simple arenes, aryl-ketone, diarylethenes, azoles, alde-
hydesaliphatic amides, allylic alcohols, diazonium salts,
arylacrylamides, alkenes, 1,3-dicarbonyls, benzaldehyde and
5
coumarins substrates.
Consequently, the reactivity of imines have been rarely
explored for chiral cyanomethyl product by transfer of hydrogen
6
atom. However, synthesis of phenylacrylonitriles from imines
7
not yet explored so for (Scheme 2). A number of pharmaceutical
reagents contain a,b-unsaturated cyanide moiety such as enta-
capone and rilpvirine, which can be used as anti-Parkinson's
2
and anti-HIV agents.
Scheme
1 The natural with biologically active alkylnitriles and
acrylonitriles.
a
Institute of Chemical Sciences, Bahauddin Zakariya Unversity, Multan, 66000,
Pakistan. E-mail: doctormsahmad@gmail.com
b
Department of Physical Sciences, Air University, Islamabad Campus, Pakistan
Scheme 2 Our Cu-catalyzed cyanomethylation of aromatic imines
and styrenes.
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/d0ra10693c
1
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2021 The Author(s). Published by the Royal Society of Chemistry
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catalysts (CuI, CuBr, CuCl) were screened (Table 1, entries 11–
Results and discussion
17).
For this copper-catalyzed cyanomethylation of aromatic imines
To our delight, these Cu(II) catalysts have good reactivity,
with green MeCN solvent, we used (E)-N-benzylidene-4- which gave the corresponding alkenyl cyanated product (2a) in
methylbenzenesulfonamide (1a) as a model substrate (Table 31% to 79% yield (Table 1, entries 11–14). Moreover, copper(I)
1
). The compound 1a was treated with Cu(OAc)
2
(20 mol%) and halides (I, Br and Cl) catalytic system such as CuI, CuBr and
ꢀ
atmosphere at 135 C, which gave CuCl elaborated cyanomethyl products in 63%, 58% and 46%
2
HOAc (1.0 eq.) under N
cyano-methylated product (2a) in 10% yield (Table 1, entry 1). (Table 1, entries 15–17) respectively. Gratifyingly, all these
Moreover, the amination occurs and
4-methyl- copper catalysts have worse reactivity than the commercially
benzenesulfonamide obtained as directing auxiliary (3a) with abundant Cu(OAc) which produced good yield of phenyl-
low yield (11%) and decomposition of remaining substrate into acrylonitrile (2a) product. In this context, a various quantities of
complex mixture (Table 1, entry 1). For further week acid Cu(OAc) such as 5 mol%, 10 mol%, 15 mol%, 25 mol% and
screening such as HCO
H and alcohols (t-BuOH, i-PrOH) were 30 mol% were examined to nd best quantity of Cu(OAc) as
2
2
2
2
elaborated low to mild yields (10–39%) (Table 1, entries 2–4). catalyst (Table 1, entries 18–22). Notably, the yields of desired
However, strong acid (HCl) unable to produce desired product product (2a) dramatically varied when using 5 mol%, 10 mol%,
(
Table 1, entry 5). To further explore the reaction parameters, 15 mol%, 25 mol% and 30 mol% of Cu(OAc)
2
(Table 1, entries
a variety of boronsted bases such as KOBu , NaOBu , LiOBu and 18–22). For example, 62% to 98% yields were obtained by using
Cs CO were screened. However, these bases are not suitable for wide range of quantities for the Cu(OAc) , instead of 20 mol%
reaction and gave the product (2a) in lower to medium yields (Table 1, entries 18–22). Aer optimization, we elaborate the
25–51%) (Table 1, entries 6–9). Importantly, Cu(OAc)
evalu- scope of substrate by varying the substituent on the N-
t
t
t
2
3
2
(
2
ated 98% yield of phenylacrylonitrile (2a) with more than 99% benzylidene-4-methylbenzenesulfonamide (Scheme 3). A variety
of directing auxiliary (4-methylbenzenesulfonamide) in the of N-benzylidene-4-methylbenzenesulfonamide (1a–1d) with
absence of additive and base or acid (Table 1, entry 10). For electron-donating group such as Me, OMe, tert-butyl at para
further Cu catalyst optimization, a wide variety of Cu(II) catalysts position of benzene ring afforded the corresponding desired
such as Cu(OTf)2, Cu(ClO ) , Cu(C H O ) , CuCl , and Cu(I) products (2a–2d) in 75% to 98% yields (Scheme 3).
4 2
2
5
2 2
2
a,b
3
Table 1 Optimization of conditions for imine using CH CN.
b
b
Entry
[Acid/base]
HOAc
[Cu(X)
n
]
[Cu quant.]
Yield 2a (%)
Yield 3a (%)
1
2
3
4
5
6
7
8
9
Cu(OAc)
Cu(OAc)
Cu(OAc)
Cu(OAc)
Cu(OAc)
Cu(OAc)
Cu(OAc)
Cu(OAc)
Cu(OAc)
2
20 mol%
20 mol%
20 mol%
20 mol%
20 mol%
20 mol%
20 mol%
20 mol%
20 mol%
20 mol%
20 mol%
20 mol%
20 mol%
20 mol%
20 mol%
20 mol%
20 mol%
5 mol%
10
10
46
39
0
11
36
51
55
n.d
60
53
40
49
>99
47
71
79
81
88
80
59
74
83
91
99
99
HCO
2
H
2
2
2
2
2
2
2
2
t-BuOH
i-PrOH
HCl
t
KO Bu
51
41
25
28
98
31
59
68
79
63
58
46
62
75
88
97
98
t
NaO Bu
t
LiO Bu
2 3
Cs CO
1
1
1
1
1
1
1
1
1
1
2
2
2
0
1
2
3
4
5
6
7
8
9
0
1
2
None
None
None
None
None
None
None
None
None
None
None
None
None
Cu(OAc)
Cu(OTf)
Cu(ClO
Cu(C
2
2
4 2
)
2
H
5
O
2
)
2
CuCl
CuI
2
CuBr
CuCl
Cu(OAc)
Cu(OAc)
Cu(OAc)
Cu(OAc)
Cu(OAc)
2
2
2
2
2
10 mol%
15 mol%
25 mol%
30 mol%
a
ꢀ
b
Conditions: 1a (0.2 mmol), Cu(X)
n
(Cu-catalysts), acid/base (1.0 eq.), N
2
, 135 C, CH
3
CN (1.2 mL), 24 h. Isolated yield. n.d; not determined.
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a,b a
Scheme 3 Substrate scope for imines using CH
3
CN.
Conditions: 1
ꢀ
(
0.2 mmol), Cu(OAc)
2
(20 mol%), N
2
, 135 C, CH
3
CN (1.2 mL), 24 h.
b
Isolated yield.
a,b
Scheme 4 Substrate scope for styrenes using CH
3
CN derivatives.
a
Conditions: 4 (0.2 mmol), CuCl (20 mol%), K
2
CO (1.0 eq.), bro-
CN (1.2 mL), 24 h. Isolated
3
ꢀ
b
moacetonitrile (2.0 eq.), N
2
, 135 C, CH
3
In this context, electron-rich methoxy substituent at ortho
position of benzene ring such as (E)-N-(2-methoxybenzylidene)-
-methylbenzenesulfonamide (1e) efficiently evaluated the cor-
c
yield. iodoacetonitrile (2.0 eq.).
4
responding product (2e) in 88% yields (Scheme 3). Gratifyingly,
scope of substrate extended to electron sensitive electron
functional groups such as nitro (1f), uoro (1g) and free amino
non-fused ring (ethene-1,1-diyldibenzene) was installed with
electron donating substituent methyl (4j), and electron with-
drawing substituents (bromo and chloro) for b,g-unsaturated
products (5j, 5k, 5l) in good to excellent yields (81–88%) with
(1h) groups were attached to the benzene of N-benzylidene-4-
methylbenzenesulfonamide, which worked well and formed
aryl-alkenyl cyanated products (1f–1h) in 69%, 71% and 87%
yields (Scheme 3) respectively. Delightfully, we used the 4-
methyl-N-(naphthalen-1-ylmethylene)benzenesulfonamide (1i)
(
1 : 0.17 and 1 : 0.1) E/Z respectively. Additionally, both non-
fused rings of ethene-1,1-diyldibenzene installed with electron
donating substituent methyl (4m), methoxy (4n), and electron
withdrawing substituents bromo (4o) and chloro (4p) have low
impact on the reaction efficiency, resulting b,g-unsaturated
products (5m to 5p) with good to excellent yields (78–88%).
Moreover, the product 3-(10,11-dihydro-5H-dibenzo[a,d][7]
annulen-5-ylidene)propanenitrile (5q), mainly found in the
biological active compounds, could synthesize in our reaction
system by allowing cyanomethyl functionalization through
a cross-coupling of 5-methylene-10,11-dihydro-5H-dibenzo[a,d]
and
2-napthalene
4-methyl-N-(naphthalen-2-ylmethylene)
benzenesulfonamide (1j) for the Cu-catalyzed cyanomethyla-
tion and obtained excellent yields (87% for 1i and 90% for 1j)
(Scheme 3). Additionally, our optimization shows that the
reaction system was signicantly improved with CuCl as
ꢀ
a catalyst with K CO as base at 120 C for styrene derivatives as
2
3
a
substrate with a-haloacetonitriles (a-bromo/a-iodo-
acetonitrile) as cyanomethyl source (Scheme 4). In order to
elaborate the scope of substrate, a variety of styrenes were
examined to get the variety of b,g-unsaturated nitriles products
[
(
7]annulene (4q) and a-bromo-acetonitrile with 94% yield
Scheme 4).
On the basis of reported mechanistic studies we examined
8,9
(Scheme 4). The electron donating substituted styrenes such as
this reaction and propose a possible pathway of reaction as
described in Scheme 5. Accordingly, the C(sp )–H activation of
acetonitrile was possibly promoted by copper species (Scheme
5
p-1-methyl-4-vinylbenzene (4a), 1-methoxy-4-vinylbenzene (4b),
3
1
-(tert-butyl)-4-vinylbenzene (4c), 1-methoxy-2-vinylbenzene (4d)
0
and 4-vinyl-1,1 -biphenyl (4e) were allowed 73% to 90% yields of
b,g-unsaturated cyanated products (5a–5e) (Scheme 4). Grati-
fyingly, prop-1-en-2-ylbenzene (4f) underwent into desired 4-
phenylpent-3-enenitrile (5f) products with 69% yields and (1 : 1)
E/Z (Scheme 4). Moreover, by using a-iodo-acetonitrile as cross
coupling partner of the (1-bromovinyl)benzene (4g) to form 4-
bromo-4-phenylbut-3-enenitrile (5g) in 70% yield (Scheme 4).
To our surprise, 2-vinylbenzoic acid (4h) gave the corresponding
product (E)-2-(3-cyanoprop-1-en-1-yl)benzoic acid (5h) product
with high yield (71%) (Scheme 4). Remarkably, the reaction with
10
a). Firstly, cyano of acetonitrile could coordinates with Cu
species and speculate that the acetonitrile deprotonated via
capture of proton by imine substrate (1) to generate nucleophile
of acetonitrile (Scheme 5a). Further, it can coordinates with
proposed 6a and produces 6b possible species (Scheme 5a).
Moreover, 3a (methylbenzenesulfonamide) and 2a (phenyl-
acrylonitrile) could be formed by dehydrogenation and recov-
ered proton transferred to imine (Scheme 5a).
Similarly, bromo-acetonitrile activated by copper metal into
radical species 8a (Scheme 5b). Consequently, substrate (4) was
converted into 7a with Cu(I) species through single electron
transfer (SET), was observed by adding 2 equivalent of TEMPO,
1,1-diphenylethylene (4i) worked well and afforded the target
product (5i) with 94% yield (Scheme 4). Delightfully, when one
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5
Conclusion
We report copper catalyzed cyanomethylation of imines and
a,b-alkenes with acetonitrile (MeCN) and its derivatives for the
synthesis of arylacrylonitriles and b,g-unsaturated nitriles.
Moreover, considering the importance of acrylonitrile and b,g-
unsaturated nitriles, this protocol has potential in the industrial
production. This method could tolerates broad scope of
substrate with substitution of variety of functional groups led
good to excellent yields (69–98%). These aromatic acrylonitriles
and b,g-unsaturated nitriles has application in organic reac-
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2021 The Author(s). Published by the Royal Society of Chemistry
RSC Adv., 2021, 11, 5427–5431 | 5431