Copper-catalyzed decarboxylative cross-coupling of cinnamic acids and ACCN via single electron transfer

Article abstract of DOI:10.1039/c5ob02677f

A novel and convenient method for the synthesis of β,γ-unsaturated nitriles using ACCN (1,1′-azobis(cyclohexane-1-carbonitrile)) as a cyano source was described. This reaction was amenable to a broad range of substrates and provided the desired β,γ-unsatu

Full text of DOI:10.1039/c5ob02677f

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Copper-catalyzed decarboxylative cross-coupling of cinnamic
acids and ACCN via single electron transfer
Received 00th January 20xx,
Accepted 00th January 20xx
a,b
a
a
a
Bao Gao, Yinjun Xie, Lei Yang, and Hanmin Huang*
DOI: 10.1039/x0xx00000x
www.rsc.org/
8
,9
A novel and convenient method for the synthesis of β,γ- radical. Our recent work has demonstrated that such kind of
unsaturated nitriles using ACCN (1,1’-azobis(cyclohexane-1- radical derived from AIBN (azodiisobutyronitrile) could be
carbonitrile)) as a cyano source was described. This reaction was activated by coordination to a copper catalyst with its nitrile
amenable to a broad range of substrates and provided the desired functional group. The coordinated radical species possessing a
β,γ-unsaturated nitriles containing an all-carbon quaternary lower-energy SOMO could react with an enolate generated
carbon center in moderate yields with high selectivity via single from a cinnamic acid via single electron transfer, and a
electron transfer and decarboxylation.
cyclization reaction has been established by incorporation of
1
0
two isobutyronitrile radicals. Inspired by this result, we
1
Nitriles are very useful intermediates in synthetic organic envisioned that a decarboxylative coupling reaction of
1
1
chemistry, which can be easily converted to amines, amides
2
cinnamic acids with one molecular of radical species could
3
and carboxylic acids. As a result, the incorporation of the proceed via single electron transfer under suitable reaction
cyano group into a simple starting material has attracted conditions. Herein, we reported a convenient copper-catalyzed
4
considerable attention. The β,γ-unsaturated nitriles served as method for the synthesis of β,γ-unsaturated nitriles with
a special class of nitriles in synthetic chemistry, which were cinnamic acids and ACCN (1,1’-azobis(cyclohexane-1-
5
viewed as important building blocks. Until now, several carbonitrile)) as substrates via single electron transfer and
methods have been developed for the preparation of this kind decarboxylation.
6
of compounds. Generally, the synthesis of β,γ-unsaturated
Previous work:
nitriles was achieved from the cyanation process of different
NaCN or KCN
starting materials such as allylic alcohols, halides, acetates,
R
X
R
CN
carbonates, ethers, phosphates, and carbonyl compounds
7
Scheme 1). However, most of these methods required toxic
X=Cl, Br, OH, OAc etc
(
reagents or expensive catalysts. Therefore, simple and safe
methods for the synthesis of β,γ-unsaturated nitrile
compounds are highly desirable and remain a significant
challenge.
This work:
O
CN
N
+
N
R
OH
R
CN
NC
Scheme 1 Synthetic methods of β,γ-unsaturated nitriles.
1,1'-Azobisalkylcarbonitriles were probably one of the most
popular commercially available radical initiators in polymer
chemistry and radical-mediated organic synthesis. The typical
donor-acceptor carbon-centered radicals could be generated
on heating. Although these nitriles containing radical species
could be utilized as a kind of useful cyano source in synthetic
organic chemistry, they usually did not get involved as
reactants in either radical or polar reactions in part due to the
inherently low nucleophilicity and electrophilicity of the
10
Initially, on the basis of our previous work, we examined
the feasibility of the cross-coupling reaction between cinnamic
acid 1a and ACCN
and 33% yield of the desired product 3a was obtained when
the reaction was conducted with 10 mol% of CuCl as catalyst
and xylene as solvent under nitrogen atmosphere at 100 C for
0 h (Table 1, entry 1). Encouraged by this preliminary result, a
series of copper catalysts were investigated and the results
showed that the reaction performed well with CuBr as
catalyst (36% yield) (Table 1, entry 4), which was better than
other copper catalysts. Thus, CuBr was determined to be the
most appropriate catalyst for this transformation.
2. To our delight, the reaction could occur,
2
o
1
2
a
State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute
of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China;
E-mail: hmhuang@licp.cas.cn
University of Chinese Academy of Sciences, Beijing 100049, P. R. China
2
b
†
Electronic supplementary information (ESI) available. DOI: 10.1039/0000000x/
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Subsequently, a variety of silver salts serving as oxidants, showed higher levels of reactivity towards this reaction. In
which were effective for many decarboxylation coupling addition, no obvious steric hindrance effect was observed. For
reactions, were screened, and the results were shown in example, the reactions of the cinnamic acids bearing chlorine-
Table 1. Among the silver salts examined, the AgOAc substituent at its o, m, p-position gave the corresponding β,γ-
demonstrated the best oxidative efficiency for giving the unsaturated nitriles in similar yields (Table 2, 3f
desired product 3a in 38% yield in the presence of CuBr substrate 1j with strong electron-withdrawing group had lower
Table 1, entry 5). Next, the amount of the AgOAc was reactivity, giving corresponding product 3j in 15% yield.
-3h). The
2
(
surveyed, and the results demonstrated that the best result Interestingly, naphthyl- and heteroaryl-substituted acrylic
was afforded when AgOAc (1.0 equiv) was utilized (Table 1, acids were also compatible with this reaction under the
entry 9). When the loading of CuBr
to 20 mol%, the product 3a was obtained in 44% yield. yields, Table 2, 3k
Further experiment conducted for searching proper Once we had identified that the cinnamic acids were
2
increased from 10 mol% optimized conditions to give the desired products (26%-61%
-
3n).
reaction time found that 55% yield was furnished when adequate substrates for this reaction system, we aimed to
reaction time was prolonged to 24 hours (Table 1, entry 13). expand the substrate scope to more complex α,β-unsaturated
A control experiment was also conducted in the absence of acids. As such, substrates containing two carbonyl groups were
CuBr
2
, and showed that no desired product was obtained.
synthesized and investigated under the optimized reaction
conditions (Table 2, 3o 3r). All substrates examined here
a
-
Table 1 Optimization of reaction conditions
bearing both electron-withdrawing and -donating groups in
a
Table 2 Substrate scope of the reaction
NC
O
CuBr2, AgOAc
DABCO
xylene, 100 oC, 24 h
R
N
N
OH+
b
R
Entry
[Cu]
10 mol%
Oxidant
t (h) Yield (%)
CN
NC
1
2
3
4
5
6
7
8
9
CuCl
2
Ag
2
Ag
2
Ag
2
Ag
2
CO
3
3
3
3
, 1.0 equiv
10
10
10
10
10
10
10
10
10
10
10
12
24
24
24
24
33
11
1
2
3
CuCl, 10 mol%
CuBr, 10 mol%
CO
CO
CO
, 1.0 equiv
, 1.0 equiv
, 1.0 equiv
MeO
34
NC
a, 52%
NC
3b, 59%
NC
3
3c, 56%
CuBr
CuBr
CuBr
CuBr
CuBr
CuBr
CuBr
CuBr
CuBr
CuBr
2
, 10 mol%
2
, 10 mol%
2
, 10 mol%
2
, 10 mol%
2
, 10 mol%
2
, 10 mol%
2
, 10 mol%
2
, 20 mol%
2
, 20 mol%
2
, 20 mol%
/
36
MeO
AgOAc, 2.0 equiv
AgOTf, 2.0 equiv
38
Cl
Br
MeO
MeO
MeO
26
NC
MeO
NC
3d, 64%
NC
3e, 57%
AgClO
4
, 2.0 equiv
trace
31
3f, 45%
AgOAc, 0.5 equiv
AgOAc, 1.0 equiv
AgOAc, 3.0 equiv
AgOAc, 1.0 equiv
AgOAc, 1.0 equiv
AgOAc, 1.0 equiv
AgOAc, 1.0 equiv
/
40
Cl
Cl
NC
NC
NC
3
g, 42%
3h, 46%
3i, 43%
1
1
1
1
1
1
0
1
2
3
4
5
38
44
N
NC
45
O2N
NC
NC
3j, 15%
3k, 53%
3l, 26%
b
55(52)
O
CN
nr
N
O
NC
NC
CuBr
CuBr
2
, 20 mol%
, 20 mol%
9
3m, 61%
3n, 31%
3o, 45%
c
1
6
2
AgOAc, 1.0 equiv
23
O
O
O
CN
CN
CN
a
Reaction conditions: cinnamic acid 1a (0.5 mmol), ACCN 2 (0.5 mmol),
o
DABCO (0.75 mmol), [Cu], xylene (2 mL), 100 C, yields determined by GC
using n-hexadecane as an internal standard. Isolated yield. In the absence of
Cl
b
c
3
p, 48%
3q, 44%
3r, 36%
DABCO
a
Reaction conditions: cinnamic acid 1a (0.5 mmol), ACCN 2 (0.5 mmol),
DABCO (0.75 mmol), CuBr (20 mol%), AgOAc (0.5 mmol), xylene (2 mL),
With the optimized reaction conditions in hand, we
proceeded to evaluate the scope of this decarboxylative cross-
coupling reaction with a variety of substituted cinnamic acids
shown in Table 2. The results revealed that the optimized
procedure could be applied successfully to a range of different
cinnamic acids. The substrates bearing electron-donating and -
withdrawing groups were converted to the corresponding β,γ-
unsaturated nitriles in the range of 15%-64% isolated yields.
Cinnamic acids bearing electron-donating groups in particular
2
o
1
00 C, 24 h. Isolated yields.
the aromatic rings could afford the desired products in
moderate yields with high selectivity (only the E-isomer was
observed, determined by NMR). No obvious steric hindrance
effect was observed in these reactions. However, only 9%
desired product 4a could be obtained when AIBN was used as
a cyano source.
2
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Scheme 2 The reaction of cinnamic acid 1a and AIBN
On the basis of the above results and previous report,
plausible reaction mechanism is depicted in Scheme 3. Initially,
cinnamoyloxy)copper was formed in the presence of DABCO
and at the same time radical would be generated by heating
of ACCN. Addition of the radical at the α-position of the
double bond of would give the benzylic radical , which then
1
0
a
1
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Cu(II)X2
1a + DABCO
AgOAc
Cu(I)X
Ph
2
CO Cu(II)X
A
NC
Ph
CN
CO2Cu(II)X
CN
+
Ph
B
CO2
ACCN
3
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C
Scheme 3 Plausible reaction mechanism.
(
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In summary, we have developed a new protocol for the
synthesis of β,γ-unsaturated nitriles via copper-catalyzed
decarboxylative cross-coupling of cinnamic acids with ACCN.
The reaction was amenable to a broad range of substrates and
provided an easy access to β,γ-unsaturated nitriles, which had
an all-carbon quaternary carbon-center that was difficult to be
obtained from other methods.
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This research was supported by the Chinese Academy of
Sciences, the National Natural Science Foundation of China
(21222203, 21372231 and 21133011).
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