Silver-Catalyzed [3+1+1] Annulation of Nitrones with Isocyanoacetates as an Approach to 1,4,5-Trisubstituted Imidazoles

Article abstract of DOI:10.1002/ejoc.202001536

A silver-catalyzed protocol for [3+1+1] annulation of nitrones and isocyanoacetates is reported. The reaction is proposed to proceed through sequential [3+2] cycloaddition, ring-opening, cyclization-oxidation, and dealkylation, allowing access to a broad scope of valuable polysubstituted imidazoles in high yields.

Full text of DOI:10.1002/ejoc.202001536

European Journal of Organic Chemistry
Accepted Article
Accepted Article
Title: Silver-Catalyzed [3 + 1 + 1] Annulation of Nitrones with
Isocyanoacetates as an Approach to 1,4,5-Trisubstituted
Imidazoles
Authors: Lanlan Lv, Yan Chen, Andrey Shatskiy, Jian-Quan Liu, Xiaoyi
Liu, Markus Kärkäs, and Xiang-Shan Wang
This manuscript has been accepted after peer review and appears as an
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of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
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to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
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content of this Accepted Article.
To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.202001536
Link to VoR: https://doi.org/10.1002/ejoc.202001536
01/2020

10.1002/ejoc.202001536
European Journal of Organic Chemistry
COMMUNICATION
Silver-Catalyzed [3 + 1 + 1] Annulation of Nitrones with
Isocyanoacetates as an Approach to 1,4,5-Trisubstituted
Imidazoles
Lanlan Lv,[a] Yan Chen,[a] Andrey Shatskiy,[b] Jian-Quan Liu,*[a,b] Xiaoyi Liu,[a] Markus D. Kärkäs,*[b] and
Xiang-Shan Wang*[a]
[a]
[b]
School of Chemistry and Materials Science, Jiangsu Key Laboratory of Green Synthesis for Functional Materials, Jiangsu Normal University, Xuzhou,
Jiangsu 221116, China
E-mail: liujq316@jsnu.edu.cn; xswang1974@yahoo.com
Department of Chemistry, KTH Royal Institute of Technology, SE-100 44 Stockholm, Sweden
E-mail: karkas@kth.se
Supporting information for this article is given via a link at the end of the document.
Abstract: A silver-catalyzed protocol for [3 + 1 + 1] annulation of
nitrones and isocyanoacetates is reported. The reaction is proposed
to procced through sequential [3 + 2] cycloaddition, ring-opening,
cyclization-oxidation, and dealkylation, allowing access to a broad
scope of valuable polysubstituted imidazoles in high yields.
Polysubstituted imidazoles constitute a ubiquitous class of five-
membered nitrogen-containing heterocyclic compounds featured
in various bioactive natural products and pharmaceuticals,[1,2]
including antitumor,[3] antiviral,[4] antifungal[5] and antibacterial[6]
agents (Figure 1). Over the years, efficient and versatile
transition metal-catalyzed methods for synthesis of such
heterocycles have been developed.[2,7] Among these, the
cycloaddition between two nitrogen-containing unsaturated
substrate molecules has been established as one of the most
common synthetic strategies. Catalytic protocols based on this
strategy generally exhibit high efficiency and functional group
tolerance; however, they largely rely on elaborate and not
directly accessible substrates[8] and catalyst precursors,[9] and
require precise control of the reaction conditions.[10] As a result,
the methodology developments for accessing polysubstituted
imidazoles from readily available starting materials under mild
reaction conditions remains an important objective in
heterocyclic chemistry.
Figure 1. Relevance of imidazoles and [3 + 1 + 1] annulations of
nitrones with isocyanoacetates.
Nitrones and isocyanides are common commercially available
starting materials that have been widely applied for construction
of nitrogen-containing motifs.[11,12,13] However, the cross-reaction
between nitrones and isocyanides has been utilized only in a
handful of synthetic protocols.[14] A prominent example of such a
reaction manifold was recently demonstrated by Xu and co-
workers.[14e] They disclosed a copper-catalyzed [3 + 1 + 1]
2-imidazolinones.[14g] Surprisingly, attempts of expanding this
protocol to alkyl-substituted nitrones such as 1a resulted in the
formation of the polysubstituted imidazole product 3a in 22%
yield (Table 1, entry 1). To further investigate this divergent
reactivity, we selected N-benzylideneaniline oxide (1a) and ethyl
isocyanoacetate (2a) as the model substrates for optimization of
the reaction conditions. Various silver catalysts including AgF,
Ag2O, Ag2CO3 and AgOTf were evaluated for the reaction
between 1a and 2a in DMF at 80 °C (Table 1, entries 2–5).
Among these, AgOTf displayed the best reactivity (Table 1, entry
4) while the other catalyst precursors consistently provided the
desired product 3a in diminished yields. Thereafter, a range of
non-polar and polar solvents including 1,4-dioxane, MeCN,
EtOH and toluene were evaluated (Table 1, entries 6–9).
Conducting the reaction in toluene resulted in further increase of
the yield of product 3a to 56% (Table 1, entry 9). Based on the
literature precedents,[16] we attempted improving the efficiency of
cycloaddition reaction between
isocyanoacetate molecules, allowing
a
nitrone and two
construction of
polysubstituted pyrroles. (Figure 1). As a continuation of our
efforts in the development of copper- and silver-catalyzed
organic transformations,[15] herein we describe an alternative
silver-catalyzed [3 + 1 + 1] cycloaddition reaction of nitrones and
isocyanoacetates,
which
allows
access
to
diverse
polysubstituted imidazoles in high yields under mild reaction
conditions (Figure 1).
Recently, we described a cooperative silver- and base-
catalyzed diastereoselective [3 + 3] cycloaddition of diaryl-
substituted nitrones with methylene isocyanides to furnish 4-aryl-
1
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10.1002/ejoc.202001536
European Journal of Organic Chemistry
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the reaction by the use of some additives, such as Y(OTf)3,
La(OTf)3, Yb(OTf)3 and KOTf (Table 1, entries 10–15). For all of
the additives, a significant increase in the yield of 3a was
observed, with Yb(OTf)3 providing the desired product in 75%
yield (Table 1, entry 12). Finally, the addition of KOTf greatly
improved the yield of 3a up to 89% (Table 1, entries 14 and 15).
isocyanide (TosMIC) failed to provide the desired product 3r.
The structure of product 3k was unequivocally confirmed
through single crystal X-ray analysis (CCDC 2031851, see
Scheme 1).
Table 1. Optimization of reaction conditions.[a]
Entry
[M]
Additive
Solvent
Yield[b]
1
Ag2CO3
AgF
-
DMF
22
29
39
37
45
43
37
18
56
64
69
75
77
85
89
2
-
DMF
3
Ag2O
-
DMF
4
AgOAc
AgOTf
AgOTf
AgOTf
AgOTf
AgOTf
AgOTf
AgOTf
AgOTf
AgOTf
AgOTf
AgOTf
-
DMF
5
--
DMF
6
-
1,4-dioxane
MeCN
EtOH
7
-
8
-
9
-
toluene
toluene
toluene
toluene
toluene
toluene
toluene
10
11
12
13[c]
14[d]
15[e]
Y(OTf)3
La(OTf)3
Yb(OTf)3
Yb(OTf)3
KOTf
KOTf
[a] Reaction conditions: 1a (0.5 mmol), 2a (1.2 mmol), catalyst (0.1
equiv), additive (0.1 equiv), solvent (2 mL), 80 oC, N2 atmosphere, 2 h. [b]
Isolated yields. [c] Yb(OTf)3 (0.2 equiv) was used. [d] KOTf (1.0 equiv)
was used. [e] KOTf (2.0 equiv) was used.
With the optimized reaction conditions in hand, the scope of
the developed protocol was investigated (Scheme 1). A diverse
range of functionalized nitrones smoothly reacted with ethyl
isocyanoacetate (2a), providing the corresponding products 3 in
high yields. Nitrone substrates featuring ortho-, meta-, and para-
substituted aryl motifs bearing electron-withdrawing or electron-
donating substituents were all compatible with the developed
protocol, providing the expected products 3b–3k in 78–91%
yields. A number of common functional groups including F, Cl,
Br, Me, OMe and NMe2 were well tolerated by the protocol. Di-
and tri-substituted substrates were also effective, providing the
corresponding products 3l–3n in 74–83% yields. Also, the fused
aromatic nitrone 1o was well-tolerated, generating the expected
product 3o in 85% yield. Furthermore, other isocyanides
including methyl isocyanoacetate (2b) and tert-butyl
isocyanoacetate (2c) were evaluated for the cycloaddition
reaction with 1a and delivered the expected products 3p and 3q
in 83% and 79% yields, respectively. Unfortunately, the
Scheme 1. Substrate scope for synthesis of 1,4,5-trisubstituted
imidazoles.
A series of mechanistic experiments were designed in order to
gain insight into the possible reaction mechanism (Scheme 2).
Addition of the radical inhibitor 2,2,6,6-tetramethylpiperidine-1-
oxyl (TEMPO) did not have a significant effect on the reaction
between 1a and 2a, suggesting that the reaction does not
proceed through
a
free-radical paathway.[14e] An alkene
byproduct derived from the nitrone substrate is expected to form
during the course of the reaction (vide infra). For nitrone 1a,
formation of cyclohexene is therefore expected; however,
isolation of this volatile byproduct proved problematic. Therefore,
we devised an alternative nitrone substrate (1p), which smoothly
reacted with 2a, producing the expected product 3a in 71% yield
and the styrenyl byproduct 4 in 79% yields. Meanwhile,
nitronesubstrates 1q and 1r, which lack the ability to form the
alkene byproduct, did not produce the cycloaddition adduct 3a.
alternative
isocyanide
substrate
p-toluenesulfonylmethyl
2
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10.1002/ejoc.202001536
European Journal of Organic Chemistry
COMMUNICATION
Na+] is 297.0846, which is consistent with the detected value
297.0860 through HRMS measurements (for details see the
Supporting Information), thereby supporting the proposed
mechanism. Conducting the reaction between α-imino ester F
and 2a under the standard conditions afforded the
corresponding imidazoles 3a in 89% yield (see Scheme 2), thus
implying the reaction through an α-imino ester intermediate
process.
Based on the mechanistic probes and previously reported
precedents,[13,17,18] a plausible reaction mechanism for the
synthesis of 1-arylated imidazoles 3 was proposed (Scheme 3).
Here, isocyanoacetate 2 coordinates to Ag and undergoes
deprotonation by nitrone 1 to generate tautomer 1’ and silver
adduct B.[19] Then, a formal [3 + 2] cycloaddition of 1’ with B
involving C–N bond formation and 5-exo-dig cyclization
furnishes intermediate D.[16] Deargentation from intermediate D,
generates the key cationic α-imino ester E, which upon
elimination gives one molecule of the alkene byproduct and α-
imino ester intermediate F. Imino ester F reacts with adduct B
through a second formal [3 + 2] cycloaddition process. Finally, a
protodeargentation and oxidation-aromatization process delivers
the desired product 3.[18] The presence of KOTf not only
accelerates the generation of F, but also facilitates regeneration
of the active silver catalyst.
In summary, we have developed a novel silver-catalyzed
protocol for [3 + 1 + 1] annulation between isocyanoacetates
and nitrones, providing
a convenient approach for the
construction of 1,4,5-trisubstituted imidazoles in good yields. A
sequential cascade mechanistic pathways involving a formal [3 +
2] annulation, ring-opening, dealkylation, and cyclization-
oxidation process is proposed. Considering the relevance of
polysubstituted imidazole as potentially useful building blocks,
this practical and novel methodology is expected to find useful
applications in the future.
Scheme 2. Mechanistic investigations.
Experimental Section
Collectively, these results support the involvement of an
alkene elimination process as one of the key steps in the
developed transformation. To further verify the rationality of the
imine process, a competition experiment between 1a (0.5 mmol),
2a (0.6 mmol) and 2b (0.6 mmol) was conducted under standard
conditions (see Scheme 2). The exact mass of 3a’ or 3p’ [M +
Typical synthetic procedure (with 3a as an example): To a 10
mL Schlenk tube, a mixture of 1a (102 mg, 0.5 mmol), AgOTf
(13 mg, 0.05 mmol) and KOTf (188 mg, 1.0 mmol) in toluene
(2.0 mL) was added. Then, 2a (131 µL, 1.2 mmol) was added.
The mixture was stirred at 80 °C under a nitrogen atmosphere
Scheme 3. Proposed mechanism.
3
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chromatography (silica gel; petroleum ether/ethyl acetate 4:1)
1
afforded 3a in 89% yield (128 mg) as a light yellow oil. HNMR
(400 MHz, CDCl3):  7.65 (s, 1H), 7.51–7.49 (m, 3H), 7.36–7.33
(m, 2H), 4.42 (q, J = 7.2 Hz, 2H), 4.24 (q, J = 7.2 Hz, 2H), 1.41 (t,
J = 7.2 Hz, 3H), 1.16 (t, J = 7.2 Hz, 3H); 13C{1H} NMR (101 MHz,
CDCl3):  162.0, 159.9, 138.5, 135.4, 135.3, 129.51, 129.45,
128.0, 125.3, 62.0, 61.3, 14.2, 13.7; HRMS (ESI-TOF) m/z
calculated for C15H17N2O4 [M + H]+: 289.1183, found 289.1199.
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Acknowledgements
This work was supported by the NSFC of China (No. 21702078),
NSF of Jiangsu Province (No. BK20170231), the Postgraduate
Research & Practice Innovation Program of Jiangsu Province
(KYCX20_2253). The Project Funded by the Priority Academic
Program Development of Jiangsu Higher Education Institutions
and KTH Royal Institute of Technology. The Wenner-Gren
Foundations and the Olle Engkvist Foundation are kindly
acknowledged for postdoctoral fellowships to J.-Q.L. and A.S.,
respectively.
Keywords: Annulations • Cascade reactions • Imidazoles •
Isocyanides • Silver
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4
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10.1002/ejoc.202001536
European Journal of Organic Chemistry
COMMUNICATION
Synthetic Methods
A silver-catalyzed protocol for [3 + 1 + 1] annulation of nitrones and isocyanoacetates is reported. The reaction is proposed to procced
through sequential [3 + 2] cycloaddition, ring-opening, cyclization-oxidation, and dealkylation, allowing access to a broad scope of valuable
polysubstituted imidazoles in high yields.
Lanlan Lv, Yan Chen, Andrey Shatskiy, Jian-Quan Liu,* Xiaoyi Liu, Markus D. Kärkäs,* Xiang-Shan Wang*
Author Biographies
Lanlan Lv received her undergraduate degree from Huaiyin Normal University in 2011. In the same year she began
her graduate studies under the direction of Professor Cangsheng Cao at Jiangsu Normal University. After receiving
a Master’s in 2014, she joined Jiangsu Normal University as an Assistant Experimentalist. In late 2016, she joined
Professor Xiangshan Wang’s group at Jiangsu Normal University as an Experimentalist. Her research interests
include studying organic reactions in green media.
Yan Chen received her undergraduate degree from Huaiyin Normal University in 2018. In the same year she began
her graduate studies under the direction of Professor Xiangshan Wang at Jiangsu Normal University. She will
complete her Master’s program in June 2021. Her research interests are focusing on copper/silver-catalyzed
organic reactions.
Andrey Shatskiy graduated as an engineer in polymer processing technology from the D. Mendeleev University of
Chemical Technology of Russia in 2011. In 2014, he completed a Master’s program in Organic Chemistry at
Stockholm University and subsequently joined the group of Professor Björn Åkermark at Stockholm University as a
PhD student. His graduate studies were focused on the development and mechanistic studies of ruthenium-based
water oxidation catalysts, in particular by electrochemical techniques. He received his PhD degree in 2018, and
then joined the Kärkäs group as a postdoctoral researcher at KTH Royal Institute of Technology, Stockholm.
Currently, his research interests include photoredox catalysis, solar fuels and organic electrosynthesis.
Jian-Quan Liu was born in Jiangsu (P. R. of China) in 1990. He received his undergraduate degree from Huaiyin
Normal University in 2011. In the same year he began his graduate studies under the direction of Professor Xihe Bi
at Northeast Normal University. The thesis concerned the study of silver-catalyzed reactions involving isocyanides.
After receiving his PhD degree in 2016, he worked as an Associate Professor at the Department of Organic
Chemistry at Jiangsu Normal University. Currently, he is a postdoctoral researcher in Professor Markus D. Kärkäs’
group at KTH Royal Institute of Technology. His research interests include silver-catalyzed organic reactions and
photoredox catalysis.
Xiaoyi Liu is now a sophomore at Jiangsu Normal University. Because of her interest in organic chemistry, she
joined Dr. Liu’s group in Organic Chemistry at Jiangsu Normal University. Currently, she is committed to exploring
new reactions involving silver-catalyzed [3 + 2] dipolar cycloadditions.
5
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Markus D. Kärkäs received his undergraduate degree from Stockholm University in 2008. In the same year he
began his graduate studies under the direction of Professor Björn Åkermark at Stockholm University. The thesis
concerned the development and mechanistic insight of artificial water oxidation catalysts. After receiving his PhD
degree in 2013, he joined Professor Corey Stephenson’s research group at the University of Michigan as a
postdoctoral fellow supported by the Swedish Research Council (Vetenskapsrådet). His postdoctoral work focused
on the development of photochemical methods for valorization of lignin. In late 2016, he returned to the Department
of Organic Chemistry at Stockholm University. In August 2018, he joined the Department of Chemistry at KTH
Royal Institute of Technology as an Assistant Professor. His research interests include photoredox catalysis, organic electrosynthesis,
and transition metal catalysis.
Xiang-Shan Wang received his undergraduate degree from Jiangsu Normal University in 1996. In 1999, he
completed a Master’s program in Organic Chemistry at Soochow University. Subsequently, he joined the
Department of Chemistry at Jiangsu Normal University as a Lecturer. In 2004, he began his PhD studies in organic
chemistry at China University of Mining and Technology. In late 2007, he returned to the Department of Organic
Chemistry at Jiangsu Normal University. Currently, he is a Professor at Jiangsu Normal University. His research
interests include copper/silver-catalyzed organic reactions and Lewis acid-catalyzed multi-component reactions.
6
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