Tf2NH-catalyzed formal [3 + 2] cycloaddition of oxadiazolones with ynamides: a simple access to aminoimidazoles

Article abstract of DOI:10.1039/c7ob00701a

Oxadiazolones are first employed as the three-atom coupling partners in the Tf₂NH-catalyzed cycloaddition with ynamides. This formal [3 + 2] cycloaddition allows a rapid synthesis of aminoimidazoles with a broad substrate scope. The approach also features a metal-free catalytic cycloaddition process, which may find applications in the synthesis of bioactive molecules. Besides, the resulting N-methyl products can further be readily converted to free N-H aminoimidazoles.

Full text of DOI:10.1039/c7ob00701a

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Takeharu Haino et al.
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Tf2NH-Catalyzed formal [3+2] cycloaddition of oxadiazolones with
ynamides: a simple access to aminoimidazoles†
Yingying Zhao,§[a,b] Yancheng Hu,§[a] Xincheng Li,*[a] Boshun Wan*[a]
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Oxadiazolones are first employed as the three-atom coupling
partners in the Tf2NH-catalyzed cycloaddition with ynamides. This
a) Gold- versus acid-catalyzed cycloaddition of ynamides with nitriles:
R2
EWG
R1
R2
R1
N
N
EWG
formal [3+2] cycloaddition allows
a
rapid synthesis of
[2+2+2]
R1
[2+2+2]
N
R1
N
aminoimidazoles with broad substrate scope. The approach also
features a metal-free catalytic cycloaddition process, which may
find applications in the synthesis of bioactive molecules. Besides,
the resulting N-methyl products can further be readily converted
to free N-H aminoimidazoles.
EWG
R2
gold/acid
catalyst
gold/acid
catalyst
R2
+
N
N
N
MeCN
EWG
R1 = Ph
TfOH
[4+2]
Gold catalysis: Liu et al
Gold catalysis: Liu et al
Acid catalysis: Maulide
et al; Tang et al
Acid catalysis: Wang and
Chang et al; Zhang and
Sun et al; Maulide et al
EWG
N
R2
Combinations of various unsaturated building units in
cycloadditions have created remarkable opportunities to construct
heterocycles of high complexity. In this regard, the development of
new cycloaddition partner has great significance. 1,2,4-Oxadiazol-
5(4H)-one (abbreviated as oxadiazolone) has emerged as a versatile
precursor for the synthesis of useful amidine motifs.1 Seeking its
new synthetic utility, oxadiazolone could possibly serve as a three-
atom building unit in the cycloaddition process. To the best of our
N
Maulide et al
b) Gold- versus acid-catalyzed cycloaddition of ynamides with dioxazoles:
R1
R2
N
Liu et al:
EWG
R2
IPrAuNTf2 (5 mol%)
O
O
R1
N
O
EWG
+
Our group:
N
N
Tf2NH (5 mol%)
R
R
knowledge, the employment of oxadiazolone as
cycloaddition partner has never been reported before.
a potential
EWG
N
c) This work
R2
EWG
R2
N
O
Tf2NH (15 mol%)
O
The cycloaddition of ynamides with various unsaturated
precursors, owing to its rapid assembly of structurally complex
heterocycles in a convergent and step-economical fashion, has
attracted considerable attention over the past decade. Most of the
N
R1
N
+
R
Toluene, 90 o
C
R1
N
R
N
R'
R'
Oxadiazolone as a three-atom unit
Broad substrate scope
Simple access to aminoimidazoles
4
‒
reactions rely on transition-metal catalysis,2 gold catalysis in
particular3,4. For example, in 2014, Liu and co-workers developed an
elegant gold-catalyzed [2+2+2] cycloaddition of ynamides with
nitriles for the synthesis of pyrimidines (Scheme 1a).4a Notably, the
chemoselectivity can be switched by employing terminal ynamides
as the substrates, enabling an easy access to pyridine core (Scheme
1a).4b Based on these seminal works, Zhang and Sun et al,5 Wang
and Chang et al,6 and Maulide et al7 independently found that the
similar cycloadditions toward pyridines also proceeded smoothly in
the presence of acid catalysts (Scheme 1a). In the process, ynamide
was first activated by acid catalysts to form keteniminium ion
High eff iciency, up to 94% yield
Scheme
precursors.
1. Cycloadditions of ynamides with unsaturated
intermediate, which then underwent the reactions with nitrile.
Following the same strategy, pyrimidines and isoquinolines can also
be constructed by varying the reaction conditions and the
substituents (Scheme 1a).8 Very recently, gold- and Tf2NH-catalyzed
formal [3+2] cycloadditions of ynamides with dioxazoles were
developed by Liu et al3l and our group9, respectively (Scheme 1b).
Prompted by these works and our long-standing interests in
exploring new cycloaddition patterns,10 we envisaged that strong
Brønsted acids could also promote the cycloaddition of
oxadiazolones with ynamides. Just as anticipated, in the presence of
Tf2NH catalyst, the [3+2] cycloaddition of oxadiazolones with
ynamides proceeded efficiently, enabling a facile synthesis of
a Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan
Road, Dalian 116023, China
E-mail: xcli@dicp.ac.cn; bswan@dicp.ac.cn;
Fax: +86-411-84379223; Tel: +86-411-84379260
b University of Chinese Academy of Sciences, Beijing 10049, China
§ Y.Z. and Y.H. contributed equally to this work.
†Electronic Supplementary Information (ESI) available: Experimental details,
characterization data, and copies of NMR spectra. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
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aminoimidazoles (Scheme 1c). It is noted that aminoimidazoles are inferior result (Table 1, entry 2). Gratifyingly, 77% yield of 3aa was
core structures of various bioactive molecules and medically achieved in the presence of 15 mol% of Tf2NH (Table 1, entry 3).
relevant compounds.11 Their conventional synthesis often involves However, a lower catalyst loading (10 mol%) provided 3aa in a
the palladium-catalyzed C‒N cross-coupling of haloimidazoles and decreased yield (Table 1, entry 4). After stirring for 6 h, the
amines.12 From an economical and environmental point of view, our substrates were not completely consumed (Table 1, entry 5). The
metal-free catalytic cycloaddition is much more preferred, yield of 3aa increased to 79% when varying the solvent to toluene
especially in the medicinal chemistry since the removal of the (Table 1, entry 6). A screen of the temperature indicates that the
resulting metal residues can be expensive.13 We herein present the best result was obtained when the cycloaddition was conducted at
results of our investigations.
90 oC (Table 1, entries 7‒9). It is noteworthy that the attempt of
using oxadiazolone 2a’’ as the potential partner failed (Table 1,
entry 10). To facilitate the purification step,15 the amount of
ynamide 1a was increased to 1.2 equivalent, by which 3aa was
isolated in 81% yield (Table 1, entry 11).
EWG
R2
N
EWG
R2
N
O
N
Tf2NH (15 mol%)
R1
N
+
R1
Toluene, 90 o
C
R
R
O
N
R'
2
N
Table 1. Optimization of the reaction conditionsa
R'
1
3
Ts
Ts
N
Bn
N
Bn
Z = 2-Me, 3ba, 70% Z = 4-Me, 3ea, 77%
Z = 2-F, 3ca, 88% Z = 4-F, 3fa, 80%
N
N
Ph
Ph
N
Ph
N
Z = 3-F,
Z = 4-Cl,
3da
, 77%
3ga
, 87%
Me
Me
Z
3aa, 81%
Ts
Ts
Ts
Ts
Bn
N
Bn
nBu
N
N
Bn
Oxa-
diazolone
Yield of
Bn
N
Entry
Acid
z
Solvent
T (oC)
N
3aa (%)b
N
N
S
from
Ph
Ph
TIPS (H)
Ph
N
N
N
1
2
Tf2NH 0.20
TfOH 0.20
Tf2NH 0.15
Tf2NH 0.10
Tf2NH 0.15
Tf2NH 0.15
Tf2NH 0.15
Tf2NH 0.15
Tf2NH 0.15
Tf2NH 0.15
Tf2NH 0.15
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a’’
2a
DCE
DCE
80
80
80
80
80
80
60
90
100
90
90
71
61
TIPS
1i
Me
Me
Me
3ja, 42%a
3ia, 0%a
3ha, 90%
3
DCE
77
Ts
SO2Ph
p-Ns
SO2(4-FC6H4)
N
Bn
N
N
N
4
5c
DCE
67
Bn
Ph
Bn
Bn
N
N
N
N
DCE
66
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
N
N
N
N
6
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
79
Me
Me
Me
Me
3ka
, 49%a
3la, 77%
3ma, 82%
3na, 72%
7
61
O
8
86
O
Ms
SO2(2-Naphthyl)
Ts
N
N
N
Bn
Ph
9
81
Bn
N
N
N
N
N
N
10
11d
0
Ph
Ph
Ph
Ph
Ph
Ph
Ph
N
N
87 (81)e
N
Me
Me
Me
Me
aReaction conditions: Acid catalyst was added to a mixture of 1a
(0.1 mmol), 2a or 2a’’ (0.11 mmol), and solvent (1.0 mL), then
3oa, 86%
3pa, 74%
3qa, 73%
3ra, 0%
b
R = 2-BrC6H4, 3ab, 82%
R = 3-BrC6H4, 3ac, 82%
R = 4-BrC6H4, 3ad, 83%
R = 4-ClC6H4, 3ae, 94%
R = 4-OMeC6H4, 3af, 74%
R = 2-thienyl, 3ag, 69%
R = 1-naphthyl, 3ah, 84%
Ts
stirred at indicated temperature for 12 h. Yields were detected by
Ts
HPLC using naphthalene as the internal standard. c6 h. d1a (0.12
mmol) and 2a (0.10 mmol) were employed. eIsolated yield.
N
Bn
Ts
N
Bn
N
N
N
Bn
Ph
N
Ph
Ph
N
N
Ph
R
At the outset, free N-H oxadiazolone 2a’ and ynamide 1a were
selected as the model substrates to test our hypothesis. When a
catalytic amount of Tf2NH (20 mol%) was introduced to the mixture
N
Bn
Me
Me
3aj, 79%
3ai, 77%
of 1a and 2a’, the expected cycloaddition product 3a was not Scheme 2. Substrate scope. Reaction conditions: Tf2NH (15
mol%) was added to a mixture of 1 (0.24 mmol), 2 (0.20
mmol), and toluene (2.0 mL), then stirred at 90 oC for 12 h.
Isolated yields were given. aTf2NH (1.0 equiv.), 30 min.
observed, while imidazole 3aa’ was obtained as the main product
(Eqn 1).14 It is because that the in situ formed N-H imidazole 3a can
further undergo the addition with ynamide 1a in the presence of
acid catalyst. To solve this problem, N-Me oxadiazolone 2a was
chosen as the cycloaddition partner in the process. Indeed, when
the reaction of 1a and 2a was carried out under the same
conditions, the desired aminoimidazole 3aa was afforded in 71%
yield (Table 1, entry 1). The utilization of TfOH as catalyst gave an
Subsequently, we explored the scope of ynamides with the
optimized conditions and the results are shown in Scheme 2. A
variety of aromatic groups in R1 were all tolerated in the process.
The electronic nature and position of the substituents had no
significant impact on the reactivity. For example, 2-Me and 2-F
2 | J. Name., 2012, 00, 1-3
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substituted ynamides could be readily converted to the free N-H imidazole 5aa with a high efficiency, when the reaction
corresponding imidazoles 3ba and 3ca in 70% and 88% yield, was conducted at room temperature for 1 h. This reaction provided
respectively. The meta- and para-substituents (F, Me and Cl) on the a mild alternative for the removal of methyl protecting group.
phenyl ring also gave satisfactory results (3da‒3ga). Remarkably, Besides, subjecting 3aa to a mixture of Pd/C, HCOONH4 and MeOH
the cycloaddition of 2-thienyl derived ynamide 1h with 2a furnished resulted in the formation of debenzylated product 6aa in 94% yield.
the imidazole 3ha in an excellent yield. However, no desired
product was obtained when using TIPS-derived ynamide 1i as the
substrate even in the presence of a stoichiometric amount of acid.
In the cases of other alkyl groups such as n-butyl and cyclohexyl in
R1, 1.0 equivalent of Tf2NH was required to achieve moderate
yields (3ja, 3ka) and the reactions can be finished within 30 min.
The scope of the sulfonyl group on the nitrogen atom was also
investigated. When Ph, p-FPh, and p-NO2Ph were employed in the
sulfonyl group, the reactions proceeded smoothly, providing the
corresponding aminoimidazoles 3la‒3na in good yields. Besides,
sterically hindered 2-naphthylsulfonyl substituted ynamide 1o
underwent the process with a high efficiency. The methylsulfonyl
group on the nitrogen atom was tolerated as well, producing 3pa in
74% yield. Gratifyingly, submitting N-Ph ynamide 1q to the standard
conditions delivered the desired imidazole 3qa in 73% yield.
Scheme 3. Further transformations of the product.
Disappointedly, the more electron-rich ynamide 1r bearing an
oxazolidine group cannot participate in the reaction because of its
relatively low reactivity.
According to those works on the acid-catalyzed
cycloaddition of ynamides with nitriles,5‒8
plausible
mechanism was proposed in Scheme 4. Initially, ynamide 1a is
protonated by Tf2NH to generate keteniminium ion A 16
Then,
a nucleophilic attack of oxadiazolone 2a on the intermediate
forms the adduct , which undergoes ring fragmentation to
eliminate one molecule of CO2, resulting in the formation of
benzylic carbocation subsequent intramolecular
cyclization of furnishes the iminium ion intermediate
Finally, deprotonation of affords the imidazole 3aa and
a
Furthermore, oxadiazolones with different substituents were
surveyed. 2-Br, 3-Br, and 4-Br on the phenyl ring in R were all
compatible with the process, leading to the formation of imidazoles
3ab‒3ad in good yields. The electron-withdrawing substituents
such as 4-Br and 4-Cl gave good results (3ad, 3ae), while the
electron-donating group 4-OMe afforded the product 3af in a
moderate yield (74%). 2-Thienyl was also tolerated in the reaction
albeit with a slightly decreased yield (3ag). Notably, 84% yield of
3ah was achieved when sterically hindered 1-naphthyl was
employed. The benzyl-substituted oxadiazolone 2i could also
undergo the cycloaddition with 1a, generating 3ai in 77% yield.
When the methyl group on the nitrogen atom was changed to
.
A
B
C
.
A
C
D.
D
regenerates the acid catalyst. It is noted that the substrate
scope supports this cationic-type mechanism. The annulations
of alkyl-derived ynamides with oxadiazolone 2a afforded the
products in low yields, while the aryl-substituted substrates
gave good results. Because the benzylic cation in intermediate
benzyl, the cycloaddition proceeded efficiently as well (3aj).
Ts
C
(aryl groups) is more stable than alkyl carbocation (alkyl
N
Bn
N
O
Ts
Bn
groups).
Tf2NH (15 mol%)
N
(2)
Ph
N
+
Ph
O
Toluene, 90 o
C
N
Ph
Ph
N
Me
2a, 3 mmol
Me
1a, 3.3 mmol
3aa, 1.187 g
80% yield
To illustrate the practicability of this approach, a gram-scale
experiment was performed (Eqn 2). Ynamide 1a (3.3 mmol) and
oxadiazolone 2a (3.0 mmol) were dissolved in 30 mL toluene. Then
15 mol% of Tf2NH was introduced and the mixture was stirred at 90
oC for 12 h. The reaction was quenched by Et3N solution (10 vol.% in
pentane) and extracted with ethyl acetate. Removal of the solvent
and purification by column chromatography afforded imidazole 3aa
in 80% yield (1.187 g).
Given the prevalence of aminoimidazoles in the core of
bioactive molecules, the synthetic transformations of the product
were further studied (Scheme 3). The tosyl group on the nitrogen
atom of 3aa could be easily removed by
a treatment with
Na/naphthalene at -50 oC. Surprisingly, it is found that the methyl
Scheme 4. Proposed mechanism.
substituent on the core ring was removed as well, producing the
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8
9
(a) L.-G. Xie, S. Niyomchon, A. J. Mota, L. González and N.
Maulide, Nat. Commun., 2016, , 10914; (b) P. Chen, C. X.
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80055.
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82, doi: 10.1021/acs.joc.7b00076.
In conclusion, we have developed a formal [3+2] approach
to access aminoimidazoles through the Tf2NH-catalyzed
cycloaddition of oxadiazolones with ynamides, in which
oxadiazolones are first employed as the three-atom building
units. The approach also features a metal-free catalytic
cycloaddition process, broad substrate scope, and high
efficiency. Besides, the N-methyl products can be readily
converted to free N-H aminoimidazoles. We believe that this
metal-free catalytic cycloaddition will have potential
applications in the medicinal chemistry.
7
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Financial support from the National Natural Science
Foundation of China (no. 21572225) is gratefully
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14 The structure of 3aa’ was unambiguously confirmed by X-ray
crystallography. The ellipsoid contour percent probability
level is 50%. Detailed information can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif (CCDC 1534957).
15 When 1.1 equivalent of oxadiazolone 2a was used, it is
difficult to isolate the mixture of 3aa and the remaining 2a
during the column chromatography.
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