Nickel Catalysis Enables Hetero [2+2+1] Cycloaddition between Yne-Isothiocyanates and Isonitriles with Low Catalyst Loading

Article abstract of DOI:10.1002/adsc.201601271

Nickel(II) can be used to catalyze the hetero [2+2+1] cycloaddition of 2-alkynylaryl isothiocyanates and isonitriles in 2-methyltetrahydrofuran (2-MeTHF) to give a wide array of thieno[2,3-b]indoles in excellent yields. The reaction is featured by employi

Full text of DOI:10.1002/adsc.201601271

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DOI: 10.1002/adsc.201601271
Nickel Catalysis Enables Hetero [2+2+1] Cycloaddition between
Yne-Isothiocyanates and Isonitriles with Low Catalyst Loading
a
a
a
a,
a,
Rui-Juan Liu, Peng-Fei Wang, Wen-Kui Yuan, Li-Rong Wen, * and Ming Li *
a
State Key Laboratory Base of Eco-Chemical Engineering, College of Chemistry and Molecular Engineering, Qingdao
University of Science and Technology, Qingdao 266042, Peopleꢀs Republic of China
E-mail: wenlirong@qust.edu.cn or liming928@qust.edu.cn
Received: November 15, 2016; Revised: January 24, 2017; Published online: && &&, 0000
Supporting information for this article is available under http://dx.doi.org/10.1002/adsc.201601271.
Abstract: Nickel(II) can be used to catalyze the sence of any additives (additional reducing agents
hetero [2+2+1] cycloaddition of 2-alkynylaryl iso- and external ligands). This is the first successful ex-
thiocyanates and isonitriles in 2-methyltetrahydrofur- ample to apply nickel(II) directly in hetero [2+2+1]
an (2-MeTHF) to give a wide array of thieno[2,3- cycloadditions.
b]indoles in excellent yields. The reaction is featured
by employing as little as 0.3 mol% nickel(II) acetyla- Keywords: 2-alkynylaryl isothiocyanates; hetero
cetonate [Ni(acac) ] under air conditions in the ab- [2+2+1] cycloaddition; isonitriles; nickel catalyst
2
[
8]
Introduction
and sulfur heteroatoms. For example, Saito and co-
[8e]
workers
reported an Mo(CO) -catalyzed [2+2+1]
6
Thienodolin, a new plant growth-regulating substance cycloaddition of yne-isothiocyanates for the synthesis
containing a thieno[2,3-b]indole skeleton, was isolated of thieno[2,3-b]indol-2-ones, in which only 4 examples
from the fermentation broth of a streptomycete strain were disclosed and used 2 equiv. of Mo(CO)6
[
8f]
identified as Streptomyces albogriseolus MJ286-76F6, (Scheme 1a). Cai and co-workers developed a cop-
which has been reported as the basis for both growth per(I)-catalyzed tandem reaction of yne-isothiocya-
[
1]
promoting and inhibiting activities in rice seedlings.
Some thieno[2,3-b]indole derivatives are known as zo[d]imidazo[5,1-b][1,3]thiazines (Scheme 1b).
electron transfer agents which can reduce the residual Owing to their low cost and unique properties,
potential and improve sensitivity of electrophoto- nickel-catalyzed carbon-carbon and carbon-heteroa-
nates with ethyl 2-isocyanoacetate to obtain 5H-ben-
[
2]
graphic photoreceptors.
thieno[2,3-b]indole motif still remains an important tools in organic syntheses. The most common Ni(0)
task. source for catalysis is Ni(cod) , however, it is expen-
So construction of the tom bond-forming reactions have become powerful
[
9]
2
The hetero [2+2+1] cycloaddition process has also sive and must be used under rigorously air-free condi-
become one of the most powerful methodologies for tions. The inexpensive nickel catalysts such as Ni(II)
[
3]
[4]
the construction of bicyclic frameworks. Mukai
compounds have been preferable to Ni(cod) because
2
[5]
and Saito
reported a Co (CO) -catalyzed and they are readily available, cheaper, and convenient to
2 8
[
10]
[11]
a Rh(I)-catalyzed hetero [2+2+1] cycloaddition of manipulate. Isonitriles have long proved to be ir-
yne-carbodiimides leading to pyrrolo[2,3-b]indol-2- replaceable building blocks in modern organic
[
6]
ones, respectively. Tu and co-workers
achieved chemistry. Based on our ongoing research interest
a Co(acac) -catalyzed double insertion of two identi- and preceding work of others on the utility of isoni-
2
[
12]
cal isonitriles into 2-ethynylanilines to obtain func- triles as synthons, herein, we report a Ni(II)-cata-
tionalized pyrrolo[2,3-b]indoles. Very recently, Zhang lyzed hetero [2+2+1] cycloaddition of yne-isothiocya-
[7]
and co-workers reported a synthesis of pyrrolo[2,3- nates with isonitriles to form thieno[2,3-b]indoles
b]indoles through self-relay Rh(I)-catalyzed cycliza- without any additive or ligand (Scheme 1c).
tion of alkyne-azides with two isonitriles via tandem
nitrene transformation.
Alkynyl isothiocyanates have been used to synthe-
size various fused ring compounds containing nitrogen
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[a]
Table 1. Optimization of the reaction conditions.
En- Ni
try (mol%)
Solvent
T
Time Yield
[b]
o
[ C] [h]
[%]
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
none
none
Ni(acac) (10)
Ni(ClO ) (10)
Ni(OAc) ·4H O (10) toluene
toluene
toluene
toluene
toluene
r.t. 48
NR
NR
80
80
80
80
80
80
80
80
80
80
80
80
48
5 min 95
2
24
1.5
2
3 min 95
4
trace
85
48
4
2
2
2
NiCl ·6H O (10)
toluene
toluene
toluene
2
2
Ni(cod) (10)
Ni(acac) (1.0)
Ni(acac) (1.0)
Ni(acac) (1.0)
Ni(acac) (1.0)
Ni(acac) (1.0)
Ni(acac) (1.0)
Ni(acac) (1.0)
Ni(acac) (0.5)
Ni(acac) (0.3)
Ni(acac) (0.1)
Ni(acac) (0.3)
2
96
71
75
82
80
2
H O
5
2
2
0
1
2
3
4
5
6
7
8
EtOH
1.5
1.5
0.5
2
CH CN
2
3
DCE
THF
2
5 min 93
5 min 93[
2
2-MeTHF 80
2-MeTHF 80
2-MeTHF 80
2-MeTHF 80
2-MeTHF 50
2
c]
d]
e]
2.5
5
93
2
[
94[
80
82
2
17
8
2
2
[
a]
Reaction conditions: 1a (0.3 mmol), 2a (0.36 mmol), sol-
vent (1 mL).
b]
Scheme 1. The reactions of yne-isothiocyanates.
[
[
Isolated yield.
c]
d]
e]
The reaction was performed on a 1 mmol scale in 2-
MeTHF (3 mL).
The reaction was performed on a 2 mmol scale in 2-
MeTHF (6 mL).
The reaction was performed on a 4 mmol scale in 2-
MeTHF (12 mL).
Results and Discussion
[
[
We commenced with an optimization of the reaction
conditions by conducting the model reaction of 2-al-
kynylphenyl isothiocyanate 1a with cyclohexyl isoni-
trile 2a, and the results are summarized in Table 1.
Initially, without any catalyst, the model reaction
did not occur in toluene at room temperature or at
8
08C after 48 h (Table 1, entries 1 and 2). Gratifying- these two solvents and the free coordination sites
[
13]
ly, when 10 mol% Ni(acac) was used in the reaction, could be occupied by the solvents. Inspired by this
2
the reaction was accomplished within 5 min, and the result, the lower amount of Ni(acac) was investigated
2
[
14]
yield of 3a reached up to 95% (entry 3). Then other in 2-MeTHF,
it was found that the reaction time
inexpensive inorganic nickel salts such as Ni(ClO ) , was prolonged but the yield of 3a was not reduced
4
2
Ni(OAc) , and NiCl were also examined. Unfortu- when the amount of Ni(acac) was decreased (en-
2
2
2
nately, they did not work as well as Ni(acac) (en- tries 15 and 16). However, when 0.1 mol% Ni(acac)2
2
tries 4–6). Next, the same amount of Ni(cod) was was used, the yield declined to 80% (entry 17). Final-
2
tested, and the yield of 3a reached 95% within just ly, the reaction temperature was tested in 2-MeTHF,
3
min (entry 7). Considering the air-instability and but use of a lower temperature prolonged the reac-
uneasy handling of expensive Ni(cod) , Ni(acac) was tion time and reduced the yield of 3a (entry 18).
2
2
selected as the catalyst to screen other reaction condi- Taking temperature, reaction time and amount of
tions. When the amount of Ni(acac) was reduced to nickel catalyst into consideration, 0.3 mol% of
2
1
mol%, a 96% yield was afforded even though the Ni(acac) as the catalyst in 2-MeTHF at 808C for 5 h
2
reaction time was prolonged to 4 h (entry 8). Subse- (entry 16) were selected as the best reaction condi-
quently, other solvents such as H O, EtOH, CH CN, tions.
2
3
DCE, THF, and 2-MeTHF were screened (entries 9–
Successively, the reactions of various 2-alkynylaryl
4). More gratifyingly, the yield of 3a still reached isothiocyanates 1 with isonitriles 2 were performed
3% within 5 min in THF and 2-MeTHF, which might under the optimal conditions for exploring the sub-
1
9
be due to the fact that Ni(acac) could dissolve well in strate scope and limitations (Table 2).
2
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[a]
Table 2. Scope of the substrates.
[
a]
Reaction conditions: 1 (1 mmol), 2 (1.2 mmol), 2-MeTHF (3 mL). Isolated yields.
As shown in Table 2, for substrates 1, a wide variety Furthermore, aromatic isonitriles such as 1,3-dime-
of 2-alkynylphenyl isothiocyanates (1a–1q) were per- thylphenyl (3u) and 1-naphthyl (3v) afforded relative-
fectly tolerated, giving the desired products (3a–3q) ly low 66% and 60% yields, respectively. Notably,
in excellent yields of 81–95% except for 1i and 1l. benzyl isonitrile gave a isomerized product (3t) in
Due to the special property of the trifluorome- a yield of 96%. Unfortunately, however, some active
[
8f]
thylgroup, 1i only gave a trace of product 3i that isonitriles such as ethyl isocyanoacetate and p-tol-
cannot be identified by spectroscopy. Additionally, 1l uenesulfonyl isonitrile gave complex mixtures.
containing an alkyne with a trimethylsilyl group gave
In order to demonstrate the synthetic utility of this
a relatively low 59% yield (3l), which might be due to method, the model reaction was performed on
steric hindrance. For substrates 2, various aliphatic a 10 mmol scale under the standard conditions, and
isonitriles such as cyclohexyl, tert-butyl, n-butyl and 2- the desired product 3a was provided in 93% yield
dimethoxyphenylethyl (3w) provided excellent yields, after 7 h (Scheme 2).
but indolylethyl isonitrile gave only a 65% yield (3x).
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Scheme 2. Gram-scale experiment.
To better understand the present catalytic reaction,
three control experiments were investigated
(
Scheme 3). For ensuring the catalyst to fully exert its
catalytic activity, 1 mol% Ni(acac) was used in 2-
2
MeTHF at 808C. When 1,2-diphenylethyne 4 and
phenyl isothiocyanate 5 were separately employed to
react with 2a, both reactions did not occur even
though the reaction time was prolonged to 5 h
Scheme 4. Comparative experiments with Ni(0) and Ni(II)
catalysis.
(
Scheme 3a and b). Next, a three-component reaction
of 4, 5 with 2a was also performed under the same
conditions, similarly, no reaction took place catalytic activity for this reaction at room temperature
Scheme 3c). These results suggest that the present (Scheme 4, bottom). Apparently, the key active spe-
(
transformation results from a synergistic effect of Ni cies of the catalyst in this reaction is Ni(0). This ob-
coordination with the alkyne bond and the isothiocya- served result was accordance with the findings of
nate group followed by an isonitrile insertion.
Ranu, who reported that the acetylacetone moiety in
Notably, both Ni(acac) and Ni(cod) could catalyze Ni(acac) catalyst is responsible for the reduction of
2
2
2
[
15]
this reaction efficiently under the tested conditions. Ni(II) to Ni(0).
To elucidate key mechanistic features of this Ni-cata- In order to verify the reducing effect of the acetyla-
lyzed hetero [2+2+1] cycloaddition, two comparative cetone moiety, another two control experiments with
experiments using the model reaction were conducted the model reaction using NiCl (0.1 equiv.) as Ni(II)
2
in 2-MeTHF with 0.3 mol% and 0.1 mol% Ni(cod) as source were conducted in THF at 808C for 2 h
2
the catalyst, respectively (the reaction was performed (Scheme 5). It was found that free acetylacetone
on the vacuum line) (Scheme 4, top). The results (0.2 equiv.) did not play the role of a reducing agent
showed that the two reaction times were 20 min and [Scheme 5, Eq. (1)], only giving a 45% yield, the
8
h, respectively, which were all significantly faster same as NiCl2 alone; while the addition of zinc
than those with the counterpart Ni(acac) . Additional- powder (0.2 equiv.) increased the yield of 3a to 83%
2
ly, the other two comparative experiments were also [Scheme 5, Eq. (2)]. These results indicated that the
investigated on the model reaction at room tempera- reduction of Ni(II) to Ni(0) might be the result of
ture with 10 mol% Ni(cod) and Ni(acac) in toluene a synergistic action of several substances in the reac-
2
2
for 10 h, respectively, it was found that Ni(II) had no tion system in a specific chemical environment.
Scheme 3. Control experiments.
Scheme 5. Control experiments.
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On the other hand, in order to check for in situ a convenient, efficient, and less expensive alternative
generated Ni(0), two XPS experiments was attempt- for the synthesis of thieno[2,3-b]indoles on a large
ed, (i) the model reaction system with 0.5 equiv. of scale.
Ni(acac)2 as catalyst at 808C for 1 min; (ii) only
adding cyclohexyl isonitrile (2a) without substrate 1a
at 808C for 30 min to verify if isonitriles can reduce Experimental Section
Ni(II) to Ni(0). Unfortunately, in the two cases, we
did not trap an Ni(0) signal but only observed an General Procedure for the Synthesis of Thieno[2,3-
[15b]
Ni(II) peak.
This might be due to the fact that the b]indoles 3
trace of in situ generated Ni(0) from Ni(II) is too re-
active to immediately take part in the catalytic reac-
tion or be oxidized by air during XPS and TEM tests.
Although it is not yet clear what plays the reduc-
The mixture of 2-alkynylaryl isothiocyanate 1 (1.0 mmol),
isonitrile 2 (1.2 mmol), Ni(acac) (0.3 mol%), and 2-MeTHF
2
(
3 mL) was stirred at 808C in a 25-mL round-bottom flask
for the indicated time until complete consumption of start-
tion role in the catalytic reaction, based on the above ing materials as monitored by TLC. After the reaction was
experimental results, a plausible reaction scenario to finished, the mixture was purified by silica gel column chro-
account for the formation of
3
is proposed matography (petroleum ether/ethyl acetate=20:1) to afford
the desired product 3 as a red solid or an oil.
(
Scheme 6). Firstly, the existing donor–acceptor inter-
CCDC 1038976 (3a) and CCDC 1514722 (3t) contain the
supplementary crystallographic data for this paper.
These data can be obtained free of charge from
The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
actions between the alkyne and the C=S bond of the
-alkynylaryl isothiocyanate 1 with empty d orbitals
of Ni(0) exist in the p-complex A are predicted to
2
[16]
play a key role. The subsequent oxidative coupling
of the alkyne with N=C=S bonds and the coordination
of isonitriles 2 deliver a fused bicyclic nickel(II) com-
plex B. Next, a migratory insertion of isonitriles 2 to
the NiÀS bond provide the intermediate C. Finally,
Acknowledgements
a reductive elimination of nickel from C gives the de-
sired products 3.
This work was financially supported by the National Natural
Science Foundation of China (21572110 and 21372137) and
In conclusion, we have demonstrated a low-cost
and air-stable nickel(II)-catalyzed hetero [2+2+1] cy- the Natural Science Foundation of Shandong Province
cloaddition of yne-isothiocyanates and isonitriles for (ZR2014M006).
the synthesis of thieno[2,3-b]indoles in excellent
yields. The protocol can be performed by employing
as little as 0.3 mol% Ni(acac) catalyst in 2-MeTHF.
2
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2
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These are not the final page numbers!

FULL PAPERS
Nickel Catalysis Enables Hetero [2+2+1] Cycloaddition
between Yne-Isothiocyanates and Isonitriles with Low
Catalyst Loading
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Adv. Synth. Catal. 2017, 359, 1 – 7
Rui-Juan Liu, Peng-Fei Wang, Wen-Kui Yuan,
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ꢁ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÞÞ
These are not the final page numbers!