Silver-Catalyzed Acyl Nitrene Transfer Reactions Involving Dioxazolones: Direct Assembly of N-Acylureas

Article abstract of DOI:10.1002/ejoc.202001460

Dioxazolones and isocyanides are useful synthetic building blocks, and have attracted significant attention from researchers. However, the silver-catalyzed nitrene transfer reaction of dioxazolones has not been investigated to date. Herein, a silver-catalyzed acyl nitrene transfer reaction involving dioxazolones, isocyanides, and water was realized in the presence of Ag₂O to afford a series of N-acylureas in moderate to good yields.

Full text of DOI:10.1002/ejoc.202001460

Communications
doi.org/10.1002/ejoc.202001460
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Silver-Catalyzed Acyl Nitrene Transfer Reactions Involving
Dioxazolones: Direct Assembly of N-Acylureas
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Zheng-Lin Yang,[a] Xin-Liang Xu,[b] Xue-Rong Chen,[a] Zhi-Feng Mao,*[a] and Yi-Feng Zhou*[a]
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Dioxazolones and isocyanides are useful synthetic building
blocks, and have attracted significant attention from research-
ers. However, the silver-catalyzed nitrene transfer reaction of
dioxazolones has not been investigated to date. Herein, a silver-
catalyzed acyl nitrene transfer reaction involving dioxazolones,
isocyanides, and water was realized in the presence of Ag2O to
afford a series of N-acylureas in moderate to good yields.
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The incorporation of an amide moiety into a molecule is of
considerable importance because of the presence of this
substructure in many biologically active molecules and natural
products.[1] In the late 1960s, 1,4,2-dioxazol-5-ones (dioxazo-
lones) 1 were identified by Sauer and Mayer as safe alternatives
to acyl azides for N-acyl nitrene formation.[2] Owing to their low
cost, ready availability, and environmental friendliness, 1,4,2-
dioxazol-5-ones have been extensively studied as amidating
reagents in the past several years. In 2009, Dubé et al. published
a pioneering method to prepare isocyanates by employing
1,4,2-dioxazol-5-ones.[3] Later, Bolm and co-workers reported
the first light-induced ruthenium-catalyzed N-acyl nitrene trans-
fer to sulfides and sulfoxides by the decarboxylation of 1,4,2-
dioxazol-5-ones (eq 1, Scheme 1).[4] Subsequently, dioxazolones
were reported as amidating reagents for Rh-catalyzed CÀ H
amidation by the Chang group.[5] Thereafter, a series of Rh-,[6]
Co-,[7] and Ir- catalyzed[8] CÀ H amidation reactions were
developed using this amidating reagent (eq 2, Scheme 1).
Furthermore, Li, Zhu, Glorius, Ackermann, and other research
groups described the cascade CÀ H amidation and cyclization
Scheme 1. Background for this research.
reactions for the syntheses of various heterocycles.[9] Bruin and
co-workers recently reported the fast N-acyl amidations via Cu-
catalyzed three-component reaction of aryl acetylenes, amines,
and 1,4,2-dioxazol-5-ones (eq 3, Scheme 1).[10] However, to the
best of our knowledge, the silver-catalyzed reaction of 1,4,2-
dioxazol-5-one via acyl nitrene has not been reported in the
literature.
Isocyanides, as versatile building blocks with diverse
reactivities, have been extensively investigated in the transition-
metal-catalyzed transformations to construct numerous valua-
ble nitrogen-containing compounds.[11] Among the studied
transition-metal-catalysts investigated, silver catalyst exhibits a
remarkable capacity for the activation of isocyanides, allowing
their reactions with many different types of electrophilic
reagents, such as carbonyls,[12] imines,[13] alkenes,[14] α-imino
esters,[15] allenoates,[16] fulvenes,[17] and alkynes[18] for construct-
ing diverse heterocyclic compounds.[19] Additionally, silver salts
have been also utilized in the radical chemistry of 2-
isocyanobiphenyls for the construction of 6-substituted
phenanthridines.[11h] Furthermore, the reactions between two
different isocyanides catalyzed by silver salts have been
reported.[20] Despite these elegant strategies, the reactions of
isocyanides with acyl nitrene precursors, i.e., dioxazolones, have
[a] Z.-L. Yang, X.-R. Chen, Dr. Z.-F. Mao, Dr. Y.-F. Zhou
Department of Pharmacy, College of Life Sciences
China Jiliang University
Hangzhou 310018, China
E-mail: zhouyifeng@cjlu.edu.cn
18a0904100@cjlu.edu.cn
[b] X.-L. Xu
Apeloa Pharmaceutical Co., Ltd.
Hengdian 322118, China
E-mail: xinliang.xu@apeloa.com
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/ejoc.202001460
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Table 1. Optimization of reaction conditions.[a]
not been reported. Recently, Wei and co-workers reported the
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syntheses of pyrimidinediones via the Ag2O catalyzed difunc-
tionalization of an isocyano group with cyclic oximes (eq 4,
Scheme 1).[21] Inspired by their work and considering that
dioxazolones and cyclic oximes have structural similarities, it
was speculated that the reactions between dioxazolones and
isocyanides may proceed by silver catalysis. Herein, the one-pot
syntheses of a series of N-acylureas via silver-catalyzed acyl
nitrene transfer reactions involving dioxazolones, isocyanides,
and water are reported for the first time (eq 5, Scheme 1).
Notably, N-acylureas, a class of acyclic compounds, have
been widely applied in agricultural and pharmaceutical
chemistry.[22] Therefore, a number of strategies for the con-
struction of N-acylurea compounds have been developed. A
typical approach relies on the reactions of isocyanates with
amides (Scheme 2, path A).[23] Another strategy involves the N-
acylation of substituted ureas (Scheme 2, path B, and C).[22a,24]
The reactions of carboxylic acids with carbodiimides have also
been frequently used for the syntheses of N-acylureas
(Scheme 2, path D).[25] Recently, Zhu et al. reported a method to
prepare N-acylureas via the CÀ H/NÀ H bond activation between
N-alkoxy aromatic amides and N,N-disubstituted formamides
(Scheme 2, path E).[26] Yadav and co-workers reported the
syntheses of N-acylureas via the reactions of two different
carboxamides (Scheme 2, path F).[27] Some other methods to
generate N-acylureas have also been reported.[28] Herein, a
different pathway toward preparing N-acylurea derivatives
utilizing 3-substituted-1,4,2-dioxazol-5-ones, isocyanides, and
water as the starting materials is reported.
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Entry
Catalyst
[mol %]
Solvent
T
Yield[b]
[%]
°
[ C]
1
2
3
4
5
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7
8
Ag2O
Ag2O
Ag2O
Ag2O
Ag2O
Ag2O
Ag2O
1,4-dioxane
MeCN
THF
toluene
EtOH
80
80
80
80
80
80
80
80
80
80
80
80
80
80
60
100
80
80
57
34
59
21
13
68
73
NR
65
63
52
31
59
16
67
61
73
66
DMF
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
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AgOAc
Ag2CO3
AgCO2CF3
AgNO3
Ag2O
Ag2O
Ag2O
Ag2O
Ag2O
Ag2O
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11
12
13[c]
14[d]
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17[e]
18[f]
[a] Reaction conditions: All the solvents were commercially available and
used as received. 1a (1.1 equiv), 2a (0.5 mmol), catalyst, solvent (2 mL),
temperature, stirred for 6 h in air. [b] Isolated yield. NR=no reaction. [c]
Reaction was performed in DMSO/H2O (10/1, v/v). [d] 0.5 g of 4 Å MS was
added and the reaction was performed under N2 atmosphere. [e] 15 mol%
Ag2O was added. [f] 5 mol% Ag2O was added.
First, 3-phenyl-1,4,2-dioxazol-5-one (1a) and ethyl isocya-
noacetate (2a) were selected as the model substrates to
investigate the reaction conditions. The resulting product N-
acylurea 3aa was isolated in 57% yield (entry 1, Table 1) after
the reaction using 10 mol% Ag2O as a catalyst in 1,4-dioxane for
(73% yield, entry 7, Table 1). When ethanol was used as the
solvent, the yield of the target product decreased significantly
owing to the formation of some unidentified by-products
(entry 5, Table 1). In the absence of Ag2O, no reaction occurred
(entry 8, Table 1). Several commercially available Ag salts were
then tested as catalysts. The reactions proceeded to afford
product 3aa in acceptable yields (AgOAc, 65% yield, entry 9;
Ag2CO3, 63% yield, entry 10; AgCO2CF3 52% yield, entry 11).
When AgNO3 was used, the target product 3aa was obtained in
only 31% yield. To determine the effect of water on the
reaction, its content was varied. When DMSO/H2O (10:1) was
used as the solvent, the yield of the product decreased to some
extent (entry 13, Table 1) along with a side product, sym-
metrical N,N’-diphenyl urea.[2,3,4] When the reaction was per-
formed in strictly dry DMSO under N2, the reaction yield
decreased dramatically, and an unknown side product was
formed (entry 14, Table 1). The above results indicated that
water traces in the reaction mixture were necessary for the
reaction. Further screening of the reaction temperature and
°
six hours at 80 C in air. Considering the presence of two H
atoms and one O atom in the product, it was speculated that
residual H2O in undried solvent could participate in this reaction
to provide the N-acylurea product. Based on this preliminary
result, further optimization of the reaction conditions was
performed. Investigation of various solvents (entries 2–7, Ta-
ble 1) led to the identification of DMSO as an optimal solvent
°
catalyst amount showed that 80 C and 10 mol% catalyst
loading were appropriate. Considering these results, the
optimal conditions included 10 mol% Ag2O in DMSO at 80 C in
air.
°
Scheme 2. The approaches for the synthesis of N-acylureas.
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With the optimum reaction conditions in hand, a wide
substituents (p-Cl, p-Br, p-CF3; 1e–1g) reacted to provide the
corresponding products (3ea–3ga) in good yields (71–73%
yields). Dioxazolones bearing a heteroarene substituent at the
3-position also reacted with 2a to afford the desired products
in moderate yields (furan, 3ha, 53%; thiophene, 3ia, 57%).
Interestingly, when 3-naphthyl-1,4,2-dioxazol-5-one (1j) was
utilized, the reaction occurred smoothly in good yield (82%).
Fortunately, aliphatic 1,4,2-dioxazol-5-ones were also identified
as suitable substrates resulting in the desired products (3ka–
3na) in good yields. Furthermore, styryl-substituted 1,4,2-
dioxazol-5-one 1o furnished the corresponding product 3oa in
69% yield. Interestingly, the drug-like product 3pa could be
also prepared from 1p in good yield.
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range of 3-substituted-1,4,2-dioxazol-5-ones 1 was reacted with
ethyl isocyanoacetate (2a) to examine the generality of the
reaction (Scheme 3). The reactions of 3-substituted-1,4,2-dioxa-
zol-5-ones containing para-methoxyphenyl and meta-meth-
ylphenyl groups proceeded smoothly to afford the desired
products (3ba–3ca) in good yields (63–76% yields), and the
structure of 3ba was confirmed by X-ray crystallography (CCDC
1995067, for details, see Supporting Information). The sterically
hindered ortho-methylphenyl group was also compatible in this
reaction, albeit affording a lower yield (3da, 31% yield). The 2-
substituted-1,4,2-dioxazol-5-ones with electron-withdrawing
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Thereafter, the reactions of 1a with various isocyanides
were also examined (Scheme 4). Isocyanoacetates bearing
different ester groups were all suitable substrates, affording
products 3ab–3ad in moderate to good yields (48–71%).
Surprisingly, when methyl 2-isocyano-3-phenylpropanoate 2d
was used as the substrate, the reaction proceeded to afford the
desired product 3ad in moderate yield (48%). Unfortunately,
when 2-isocyano-1-morpholinoethan-1-one 2e and tosylmeth-
ylisocyanide (TosMIC) 2f were tested in the reactions with 1a,
the desired products were not obtained under any optimized
condition. Similarly, 1-isocyano naphthalene 2g failed to yield
the desired product in the reaction.
To test the practicality of the methodology, a gram-scale
synthesis of N-acylurea was performed under standard con-
ditions. Here, 8.8 mmol of ethyl 2-isocyanoacetate 2a was
reacted with 9.7 mmol of (italic E)-3-styryl-1,4,2-dioxazol-5-one
1o and water to afford 1.82 g of the corresponding N-acylurea
3oa in 75% yield (Scheme 5a). To investigate the mechanism of
Scheme 3. Reaction substrate scope: dioxazolones. [a] Reaction conditions:
The solvent was commercially available and used as received. 1 (1.1 mmol),
Scheme 4. Reaction substrate scope: isocyanides. [a] Reaction conditions:
The solvent was commercially available and used as received. 1a (1.1 mmol),
°
°
2a (1.0 mmol), Ag2O (10 mol%), DMSO (4 mL), 80 C, 6 h in air. [b] Isolated
2 (1.0 mmol), Ag2O (10 mol%), DMSO (4 mL), 80 C, 6 h in air. [b] Isolated
yield of 3ab–3ag.
yield of 3aa–3pa.
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In conclusion, a new protocol for the syntheses of N-
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acylureas through a one-pot acyl nitrene transfer reaction is
described. To the best of our knowledge, the silver-catalyzed
reaction of 1,4,2-dioxazol-5-one via acyl nitrene has not been
reported to date. Further investigations of the reaction
mechanism and the synthetic potential of this reaction are in
progress.
Deposition Number 1995067 (for 3ba)
contains the
supplementary crystallographic data for this paper. These data
are provided free of charge by the joint Cambridge Crystallo-
graphic Data Centre and Fachinformationszentrum Karlsruhe
Access Structures service www.ccdc.cam.ac.uk/structures.
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Acknowledgements
Financial support of this research provided by the Hangzhou
Ledun Technology Co., Ltd is greatly acknowledged.
Scheme 5. (a) Gram-scale synthesis of 3oa and (b) Isotopic labeling experi-
ment.
Conflict of Interest
the reaction, O18-labelled water was added to the reaction
mixture to determine the oxygen atom source of the urea
moiety (Scheme 5b). Mass spectrometry analysis of the target
product revealed that the O18 atom was incorporated into the
product [O18]3aa.
The authors declare no conflict of interest.
Keywords: CÀ N bond formation · Silver · Synthetic methods ·
Transition metals
Based on the results and previously reported studies,[5,29]
a
possible reaction mechanism shown in Scheme 6 is postulated.
Through one of the heteroatoms, 3-substituted-1,4,2-dioxazol-
5-one 1 interacts with the silver(I) salt to generate silver
complex A.[5] The subsequent loss of CO2 provides silver N-acyl
nitrene complex B.[29] Then, the interaction of isocyanide 2 with
N-acyl nitrene complex B allows an N-acyl nitrene transfer
reaction via complex C, generating intermediate D and active
silver(I) catalyst for the next cycle. The intermediate D is then
converted into the desired product 3 by reaction with H2O.
However, the reason for the limitation of the substrate scope to
isocyanoacetates remains unclear.
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Manuscript received: November 7, 2020
Revised manuscript received: December 3, 2020
Accepted manuscript online: December 4, 2020
Eur. J. Org. Chem. 2021, 648–652
www.eurjoc.org
652
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