Copper-catalyzed (4+1) and (3+2) cyclizations of iodonium ylides with alkynes

Article abstract of DOI:10.1039/d0cc04373g

The copper(ii)-catalyzed (4+1) cyclizations and copper(i)-catalyzed (3+2) cycloadditions of iodonium ylides and alkynes were successfully developed by employing efficient and safe iodonium ylides instead of traditional diazo compounds. Highly functionaliz

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DOI: 10.1039/D0CC04373G
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Copper-Catalyzed [4+1] and [3+2] Cyclizations of Iodonium Ylides
with Alkynes
Received 00th January 20xx,
Accepted 00th January 20xx
Hao Liang,a Xiaobo He,a Yaqi Zhang,a Bin Chen,a Jiasheng Ouyang,a Yongsu Li,a Bendu Pan,a
Chitreddy V Subba Reddy,a Wesley Ting Kwok Chan,b and Liqin Qiu *a
DOI: 10.1039/x0xx00000x
The copper(II)-catalyzed [4+1] cyclizations and copper(I)-catalyzed cyclizations of N-(ortho-chloromethyl)aryl amides and
[3+2] cycloadditions of iodonium ylides and alkynes were iodonium ylides in the presence of K2CO3 at room temperature
successfully developed by employing efficient and safe iodonium 9. Recently, Wang and co-workers reported that iodonium
ylides instead of traditional diazo compounds. Highly ylides and tertiary arylamines could induce a single electron-
functionalized
dimethyl
(E)-3-benzylideneindoline-2,2- transfer (SET) process, leading to the formation of 3,3’-
7c
dicarboxylates and methyl 5-(2-hydroxyphenyl)-2-methoxy-4- disubstituted indoline compounds
. For the oxygen
phenylfuran-3-carboxylates, were conveniently prepared in heterocycles, similar reports indicated that under catalysis like
moderate to excellent yields. The possible reaction mechanisums Rh(Ⅱ), Cu(Ⅱ), or photochemical conditions, annular iodonium
were also discussed.
ylides with stronger nucleophilic ability could react with those
10
active unsaturated bonds, such as terminal acetylenes
,
ketenes 11, nitriles 12, isocyanates 13, isothiocyanates 14, thiones
Nitrogen- or oxygen-containing heterocycles are common
motifs in natural products and bioactive molecules.
Particularly, electron-withdrawing group at C2 position of
indolines or aryl group at C2 position of furans can give
16
15, dienes and carbodiimides 17. Nevertheless, strategies for
the synthesis of C2-aryl furan compounds are still limited,
whether through traditional coupling 18 or the Rh(Ⅲ) catalyzed
1
C-H activation 19
.
invaluable effects in heterocyclic compounds . Pursuit of new
and effective protocols for the synthesis of these tolerant
derivatives using unprotected polar groups remains an
important goal in modern organic synthesis.
Inspired by the previous reports, herein, we report the
copper-catalyzed cyclization reactions of iodonium ylides with
ortho-alkynylaniline or ortho-alkynylphenol derivatives. In
these processes indoline products were obtained through
[4+1] cyclizations in good to excellent yields, while the furans
were subjected to [3+2] cycloadditions and the products were
acquired in moderate yields.
Iodine(III) compounds had broad applications in organic
2
synthesis, such as in oxidation reaction , asymmetric catalytic
3
synthesis
, etc. Among general classes of iodine(III)
compounds, iodonium ylides, as green reagents, efficient
carbene precursors and safe substitutes for diazo compounds,
had attracted progressive interests of scientists due to their
To optimize the synthesis of indoline derivatives, initially,
we studied the reaction of 2-(phenylethynyl) aniline 1a with
dimethyl 2-diazomalonate 2a or iodonium ylide 2b to
investigate the possibility of [4+1] cyclization and the
reactivity. It is worth noting that substrate 2b was more
suitable than 2a in the tested reactions, and a higher yield was
accordingly provided for the desired product 3ab (see
Electronic Supporting Information (ESI) Scheme 5). This
exhibits the effectiveness of iodonium ylide in this kind of
reaction. Therefore, iodonium ylides were selected as the
preferred substrates for the following studies. Further
performing the reaction in DCE at 80 ℃ with Cu(OAc)2 or
Cu(OTf)2 as the catalyst, the former provided better yield along
with excellent E/Z selectivity (Table 1, entries 1, 2). When the
catalyst was switched from Cu(II) to Cu(Ⅰ), interestingly, the
objective reaction did not occur (Table 1, entries 3-7, in ESI).
Solvent was further screened. Among all the solvents, DCE was
still the best choice (Table 1, entry 1). Other solvents like
4
safety, low toxicity and easy preparation . After generating
carbene or carbenoid species, aryl iodonium ylides can
participate in a range of useful transformations such as serving
5
as nucleophiles or electrophiles , X-H (X = C, N, etc.) insertion
6, as reactants in radical reactions 7 and inter or intramolecular
cyclizations
just
like
symmetric
or
asymmetric
cyclopropanation 8. However, in these clyclization reactions,
direct construction of indoline or furan skeletons are still less
presented. In 2013, Liang and co-workers found [4+1]
a. School of Chemistry, The Key Laboratory of Low-Carbon Chemistry & Energy
Conservation of Guangdong Province, Sun Yat-Sen University, Guangzhou
510275, People’s Republic of China. Email: qiuliqin@mail.sysu.edu.cn
b. Department of Applied Biology and Chemical Technology, The Hong Kong
Polytechnic University, Hong Kong, People’s Republic of China
Electronic
Supplementary
Information
(ESI)
available.
See
DOI: 10.1039/x0xx00000x
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toluene and CH3CN were not suitable for the reaction (Table 1,
entries 3 and 4). Although the reaction had similar activity and
selectivity in CHCl3, chloroform was not selected as a preferred
solvent based on the principle of green chemistry (Table 1,
entry 5). Detailed optimizations of the reaction conditions
were given in Table 1 of ESI.
Table 2. Synthesis of dimethyl (E)-3-benzylideneindoline-2,2-dicarboxylate derivatives
View Article Online
a,b,c
DOI: 10.1039/D0CC04373G
R3
R3
IPh
COR4
Cu(OAc)2 10 mol%
COR4
COR4
R2
+
R2
R4OC
DCE, 80
℃
NHR1
NR1
1
2
3
Table 1. Optimization of Reaction Conditions a
Cl
N
R
Ph
Ph
3ab, R = H
3bb, R = F
3cb, R = Cl
3db, R = Br
3eb, R = MeO 64% E/Z > 20/1
3fb, R = Me 68% E/Z > 20/1
3gb, R = CF3 86% E/Z > 20/1
86% E/Z > 20/1
85% E/Z > 20/1
76% E/Z > 20/1
79% E/Z > 20/1
IPh
COOMe
M 10 mol%
COOMe
COOMe
+
COOMe
COOMe
COOMe
COOMe
MeOOC
℃
COOMe
COOMe
Solvent, 80
N
NH2
1a
N
H
N
H
N
H
H
3pb, 62% E/Z > 20/1
3ob, 89% E/Z = 12/1
EtOOC
2b
3ab
R
3hb, R = Cl
3ib, R = Br
3jb, R = CN 82%
3kb, R = Me 70%
80%
76%
E/Z = 10/1
E/Z = 11/1
E/Z > 20/1
E/Z = 12/1
E/Z = 19/1
Cl
COOMe
COOMe
Entry
M
Solvent
Yield (%) b
E/Z c
COOMe
COOMe
N
H
N
H
3lb, R = Ph
65%
1
2
3
4
5
Cu(OAc)2
Cu(OTf)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
DCEd
DCE
86
53
54
68
83
＞20/1
＞20/1
15/1
16/1
＞20/1
3qb, 86% E/Z > 20/1
O
Br
toluene
CH3CN
CHCl3
Cl
COOMe
COOMe
COOMe
COOMe
COOMe
COOMe
N
N
N
N
H
H
H
3nb, 57% E/Z > 20/1
3rb, 56% E/Z = 12/1
3mb, 67% E/Z > 20/1
a
Reaction conditions: 1a (0.2 mmol), 2b (0.26 mmol), M (10 mol%) and Solvent
(2.0 mL) under Ar at 80 ℃. b Isolated yields. c Determined by 1H NMR. d DCE = 1,2-
30o
C
C
C
C
C
C
72% E/Z = 10/1
85% E/Z = 10/1
86% E/Z = 9/1
90% E/Z = 9/1
92% E/Z = 8/1
92% E/Z = 8/1
40o
50o
60o
70o
80o
dichloroethane.
COOEt
COOMe
COOMe
COOCH2CF3
COOCH2CF3
COOEt
N
N
N
With the optimized reaction conditions in hand, the scope of
[4+1] cyclizations for indolines was explored (Table 2). Substrates 1
bearing a para-substituted phenyl group reacted with 2 smoothly,
providing corresponding products 3bb-3gb in 64-86% yields with
E/Z ＞ 20:1. Despite the existence of larger steric hindrance for
ortho- or meta-substitution, products 3hb-3ob could still be
obtained in 57-89% yields. In general, it seems that electron-
withdrawing substituents on the benzene ring have a more positive
effect on the yields than the electron-donating substituents.
Furthermore, the absolute configuration of product 3ma was
determined to be E type by single crystal X-ray diffraction analysis.
Besides the phenyl groups, isoquinoline, ester and pyridine
derivatives also afforded products 3pb-3rb in 56-86% yields.
Additionally, different protecting groups (PG) of aniline were also
examined. When the PG were having electron-withdrawing groups
like Ts or Boc, the reactions didn’t occur. However, methyl-
protected substrate could react with 2b even under lower
temperature, providing product 3sb in good to excellent yields.
Based on this interest, different iodonium ylides were tried, and
target products including spiro product 3se were acquired.
Cyclopropyl and thienyl also delivered 3tb and 3ub in good yields
along with lower E/Z selectivities. But it is noteworthy that no
desired products were obtained without the methyl PG. What’s
more, six-membered ring product 3vb could also be afforded with
an excellent E/Z ratio despite in a lower yield.
3sc e, 82% E/Z = 10/1
3sb
3sd e, 76% E/Z = 10/1
S
Ph
O
N
N
COOMe
COOMe
COOMe
COOMe
O
COOMe
COOMe
N
N
N
N
H
O
3tb e, 90% E/Z = 5/1
3ub e, 71% E/Z = 3/1
3vb, 26% E/Z > 20/1
3se e, 85% E/Z = 10/1
a
Reaction conditions: 1 (0.2 mmol), 2 (0.26 mmol) and DCE (2.0 mL) under Ar at
80 ℃. b Isolated yields. c Determined by 1H NMR. d DCE = 1,2-dichloroethane. e 40
℃.
Table 3. Optimization of Reaction Conditions a
COOMe
Ph
IPh
COOMe
O
M 10 mol%
O
+
COOMe
COOMe
MeOOC
℃
Solvent, 80
OH
4a
OH
O
2b
5ab'
5ab
Yield (%) b
5ab’
Entry
M
Solvent
5ab
-
1
2
Cu(OAc)2
CuI
DCE
DCE
DCE
DCE
Toluene
THF
-
-
-
-
-
-
-
-
-
18
35
32
32
21
23
49
53
70
3
CuCl
CuBr
CuCl
CuCl
CuCl
CuCl
CuCl
CuI
4
5
6
7
DMF
8
Acetone
CH3CN
CH3CN
9
10
a
Reaction conditions: 4a (0.1 mmol), 2b (0.13 mmol), M (10 mol%) and Solvent
(1.0 mL) under Ar at 80 ℃. b Isolated yields. c DCE = 1,2-dichloroethane.
Following the above similar synthetic route, we carried out the
reaction of ortho-alkynylphenol 4a with 2b to synthesize targeted
2 | J. Name., 2012, 00, 1-3
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Ph
Ph
compound 5ab’. However, no product formation was observed.
(Table 3, entry 1). With CuI, CuCl and CuBr instead, interestingly, a
certain amount of 5ab was given and CuCl was found to be the best
catalyst. But the expected compound 5ab’ was not formed in all the
experiments (Table 3, entries 2-4). We assume that the insertion
reaction was weakened due to the electronic effect of o-alkynyl.
This demonstrates a type of new reaction occurred. After screening
the solvents, CH3CN was found to be better than DCE, toluene, THF,
DMF and acetone (Table 3, entries 5-9). Cross experiments showed
that CuI was superior to CuCl when the solvent was CH3CN (Table 3,
entries 9, 10). Detailed optimization of the reaction conditions was
given in Table 2 of ESI.
View Article Online
DOI: 10.1039/D0CC04373G
COOMe
COOMe
COOMe
COOMe
N
H
N
H
IPh
disfavor
3ab
MeOOC
COOMe
PhI
Ph
2b
Cu(OAc)2
CuOAc
O
Cu(OAc)2
HOAc
MeOOC
COOMe
OMe
COOMe
N
H
Ⅰ
Ph
Ⅳ
Conia-ene
Ph
Ph
HOAc
NH2
HO
Cu(OAc)2
Cu(OAc)2
1a
NH COOMe
N
O
H
Me
COOMe
H
O
O
Ⅲ
Ⅱ
Table 4. Synthesis of 5-(2-hydroxyphenyl)-2-methoxy-4-phenylfuran-3-carboxylate
derivatives a
Scheme 1. Proposed mechanism for the [4+1] cyclization
COOMe
R1
R1
R2
IPh
OMe
CuI 10 mol%
Different from the above-mentioned [4+1] cyclization, in the
[3+2] cycloaddition, O-H insertion with copper carbene
intermediate formed in situ from iodonium ylides didn’t occur.
According to the literatures 19, 21, cyclopropenyl ester might been
formed before the formation of furan by cycloisomerization under
copper catalysis. Additional HPLC-MS analysis further confirmed the
existence of another intermediate with the same molecular weight
as the product (Scheme 10 in ESI). Therefore, it is proposed that the
copper carbene intermediate generated by Cu(Ⅰ) and iodonium
ylide (2b) firstly reacts with 4a to yield the cyclopropenyl ester Ⅳ.
O
R2
+
MeOOC
COOMe
℃
CH3CN, 80
OH
OH
4
2b
5
Cl
Br
COOMe
COOMe
COOMe
O
COOMe
O
COOMe
O
O
O
O
O
O
OH
5bb 48%
OH
5cb 64%
OH
5ab 70%
OH
5db 42%
F
Ⅰ
Herein, the hydroxyl group can bind the Cu to control the
R
COOMe
R = CF3 5gb 68%
R = CN
5hb 75%
R = NO2 5ib 63%
COOMe
COOMe
O
regioselectivity. Subsequently,
a metal-carbene species Ⅴ is
O
O
formed by a ring-opening isomerization. Then attack of the carbonyl
oxygen atom to the copper carbene atom leads to a cyclic
O
O
O
5jb 46%
5kb 50%
R = Ac
R = Ph
OH
5fb 47%
OH
5eb 58%
OH
intermediate Ⅵ. Aromatization of
Ⅵ at last gives the
corresponding furan product 5ab and the catalyst is also
regenerated (Scheme 2).
COOMe
O
O
5ab
4a
2b
[Cu]
O
OH
5lb 62%
COOMe
COOMe
MeOOC
Ph
O
Ph
[Cu]
O
a
[Cu]
OH
HO
Reaction conditions: 4 (0.1 mmol), 2b (0.13 mmol), CuI (10 mol%) and CH3CN
(1.0 mL) under Ar at 80 ℃.
MeOOC
Ph
O
Ⅵ
Ⅳ
[Cu]
To further explore the mechanism, some control experiments
and HPLC-MS tests were performed (see ESI). The experiments in
Scheme 6 of ESI showed that a possible intermediate 3aa could be
converted into target product 3ab under the standard conditions.
Two peaks with the same molecule weight were also observed in
the HPLC-MS spectra (Scheme 9 in ESI). Based on the above
investigations, a plausible mechanism of [4+1] cyclizations for
indolines is proposed. As shown in Scheme 1, a copper(Ⅱ) carbene
intermediate Ⅰ, which is formed in situ from the iodonium ylide in
the presence of the copper, reacts with 1a to afford a copper
OH
Ⅴ
Scheme 2. Proposed mechanism for the [3+2] cycloaddition
As reported in the literature 22, the N-H indoline skeleton with
an acyl group at C2 position was convertible to the dearomatized
indole skeleton through rearrangement of the acyl group under acid
condition. Therefore, 3ab was further treated with CF3COOH at
room temperature, and product 6 with a quaternary carbon was
obtained in good yield (Scheme 3).
zwitterionic intermediate Ⅱ 20c. Then 1,2-H shift gives an insertion
20a,b
product Ⅲ
. Subsequent 5-exo-dig carbocyclization onto the
internal alkyne by a copper-catalyzed Conia-ene reaction followed
by decopperization and protonation produces the target indoline.
Due to the steric hindrance effect, 3ab of E-configuration is yielded
finally as the favourable product.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
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Ph
108, 5299 − 5358; (d) A. Yoshimura, V. V. Zhdankin. Chem.
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Ph
Rev. 2016, 116, 3328 − 3435.
DOI: 10.1039/D0CC04373G
CF3COOH 3.0 equiv.
DCE, rt. 12 h
5
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COOMe
COOMe
COOMe
COOMe
N
N
H
3ab 0.10 mmol
6 84%
Scheme 3. Transformation of 3ab
In conclusion, we have successfully developed the
copper(II)-catalyzed [4+1] cyclizations of ortho-alkynylanilines
with iodonium ylides and the copper(I)-catalyzed [3+2]
cycloadditions of ortho-alkynylphenols with iodonium ylides
respectively employing the iodonium ylides as efficient
carbene precursors and safe alternatives to diazo compounds.
The former reactions provide a robust synthetic route for the
new skeleton of five-membered indolines with excellent
stereoselectivity, while the latter ones afford the C2-aryl furan
derivatives as the products. Through these green chemical
methods, two classes of highly functionalized compounds were
therefore prepared directly and conveniently in moderate to
excellent yields. The most interesting finding is that the
difference between ortho amino and hydroxyl groups of
substrate leads to completely different reaction types and
different copper catalyst requirements. These results will help
to expand the applications of iodonium ylides as green
reagents in developing nitrogen- or oxygen-containing
heterocycles and other related reactions.
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This work was supported by the Guangzhou Science and
Technology Program (201604016037, 201510010080),
Guangdong Province Zhu Jiang Talents Plan (2016ZT06C090)
and Guangzhou City Talents Plan (CYLJTD-201609).
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Conflicts of interest
There are no conflicts to declare.
Notes and references
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R2
DOI: 10.1039/D0CC04373G
COR3
R3
R2
X
R1
R2
XH
Cu(OAc)2
DCE
CuI
COR3
COR3
+
O
R1
CH3CN
R1
IPh
COR3
XH
R3OC
X = OH
Yield up to 75%
X = NH, NMe, etc
Yield up to 92%
E/Z up to ＞20/1
The copper(II)-catalyzed [4+1] cyclizations and copper(I)-catalyzed [3+2]
cycloadditions of iodonium ylides and alkynes have been successfully developed by
employing
iodonium
ylides.
Highly
functionalized
dimethyl
(E)-3-
benzylideneindoline-2,2-dicarboxylates and methyl 5-(2-hydroxyphenyl)-2-methoxy-
4-phenylfuran-3-carboxylates, were directly and conveniently prepared in good to
excellent yields