Metal-Free Synthesis of N-(Carboselenoate) Benzimidazolones by Cascade Cyclization of ortho-Diisocyanoarenes and Selenosulfonates

Article abstract of DOI:10.1021/acs.orglett.9b01886

A facile synthesis of N-(carboselenoate) benzimidazolones through metal-free reactions of ortho-diisocyanoarenes with selenosulfonates is reported here. The desired products are obtained in moderate to good yields with good functional group compatibility. The ortho-diisocyanoarenes are applied to the construction of 2-benzimidazolone derivatives for the first time.

Full text of DOI:10.1021/acs.orglett.9b01886

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Metal-Free Synthesis of N‑(Carboselenoate) Benzimidazolones by
Cascade Cyclization of ortho-Diisocyanoarenes and
Selenosulfonates
Yi Fang,† Can Liu,† Weidong Rao,‡ Shun-Yi Wang,*,† and Shun-Jun Ji*,†
†Key Laboratory of Organic Synthesis of Jiangsu Province, College of Chemistry, Chemical Engineering and Materials Science, and
Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University, Suzhou 215123, China
‡Jiangsu Key Laboratory of Biomass-based Green Fuels and Chemicals, College of Chemical Engineering, Nanjing Forestry
University, Nanjing 210037, China
S
* Supporting Information
ABSTRACT: A facile synthesis of N-(carboselenoate) benzi-
midazolones through metal-free reactions of ortho-diisocya-
noarenes with selenosulfonates is reported here. The desired
products are obtained in moderate to good yields with good
functional group compatibility. The ortho-diisocyanoarenes are
applied to the construction of 2-benzimidazolone derivatives
for the first time.
Scheme 1. Radical Reactions of Isocyanides with
Selenosulfonates
enzimidazolones are important heterocyclic compounds
1
Bwith diverse biological activities. Besides, they are useful
building blocks in pharmaceuticals and pigments.2 For these
reasons, the synthetic methods for benzimidazolone derivatives
have drawn considerable attention. Conventionally, the
reactions of phosgene with o-phenylenediamines were
employed in the synthesis of 2-benzimidazolones.3 However,
it was user- and environment-unfriendly, because of the high
toxicity and corrosiveness of phosgene. In recent years, several
phosgene-free approaches have been reported. N,N′-Carbon-
yldiimidazole,4 urea,5 dimethyl carbonate,6 carbon monoxide,7
and carbon dioxide8 were used as new carbonyl sources in the
reactions with o-phenylenediamines. Most of these synthetic
methods required harsh reaction conditions such as metal
catalysts, high temperature, and high gas pressure (e.g., CO
and CO2), which led to poor chemoselectivity and low product
yields. Therefore, developing facile and environment-friendly
methods for the construction of benzimidazolones is still
highly desirable.
As important C1 synthon, isocyanides have witnessed great
developments in the synthesis of nitrogen-containing com-
pounds. In past few years, o-diisocyanoarenes have been
successfully employed in the synthesis of quinoxaline
derivatives by a radical cascade cyclization strategy. In 2016,
Studer’s group11 and Yu’s group12 reported the synthesis of
(perfluoro)alkyl quinoxalines starting from o-diisocyanoarenes
and (perfluoro)alkyl iodides, respectively. Similarly, a wide
range of 2-phosphoryl-substituted quinoxalines were success-
fully synthesized by Zhao and co-workers.13 Based on the
above reports, we tried to construct 2-selanylquinoxaline
derivatives by reacting o-diisocyanoarenes with selenosulfo-
nates.
Recently, our group reported the preparation of secondary
selenocarbamates through radical-chain reactions of isocya-
nides, selenosulfonates and water under metal-free conditions
(Scheme 1A).9 Inspired by this work and in continuation of
our work on organoselenium chemistry,10 we herein report a
new method for the construction of N-(carboselenoate)
benzimidazolones via reactions of o-diisocyanoarenes with
selenosulfonates (Scheme 1B). This strategy is easy to handle
with bench-stable materials. The corresponding products were
obtained in good yields with excellent chemical selectivity and
functional group tolerance. To the best of our knowledge, this
is the first report on the construction of 2-benzimidazolone
derivatives from o-diisocyanoarenes under metal-free con-
ditions.
Subsequently, we initiated the studies by the reaction of 1,2-
diisocyano-4,5-dimethylbenzene (1a) with Se-(2-phenoxyeth-
Received: June 2, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b01886
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a b
,
yl) benzenesulfonoselenoate (2a) in the presence of water at
30 °C. To our surprise, we did not detect the expected product
2-selanylquinoxaline. However, we could isolate the 2-
benzimidazolone derivative 3a as a major product. First, we
investigated a series of reaction solvents. The reaction did not
work well when employing dimethylformamide (DMF) and
dimethylsulfoxide (DMSO) as solvents (see Table 1, entries 1
Scheme 2. Substrate Scope of Isocyanides 1
a
Table 1. Optimization of the Reaction of 1a with 2a
b
c
entry
solvent
temperature, T (°C)
yield (%)
1
2
3
4
5
6
7
8
9
DMF
DMSO
THF
MeCN
HFIP
H2O
t-BuOH
t-BuOH
t-BuOH
t-BuOH
EtOH
30
30
30
30
30
30
30
50
80
trace
trace
57
63
60
51
74
68
52
a
Reaction conditions: 1 (0.10 mmol), 2a (0.20 mmol), t-BuOH (2
mL), 30 °C, 12 h, then 50 °C for x h (the value of x is indicated in
b
parentheses). Isolated yields. c1 (1.0 mmol), 2a (2.0 mmol), t-BuOH
(20 mL), 30 °C, 12 h, then 80 °C for 4 h.
d
10
30 → 50
30 → 50
30 → 50
81
38
81
d
11
,
ployed in the reaction, the products 3e and 3e′ could be
obtained in 70% total yield with a 1:0.7 ratio. When using o-
diisocyanide bearing electron-withdrawing groups as a
substrate, we could isolate 3f and 3f′ in 37% and 43% yields,
respectively.
Next, we investigated the substrate scope of selenosulfonates
2 (Scheme 3). First, we investigated a series of benzyl
benzeneselenosulfonates bearing different functional groups.
We could isolate the desired products in yields of 68%−71%
(3g−3i). The target product 3j bearing carboxylic ester
structure could be obtained in 67% yield. Branched
selenosulfonate worked smoothly to give the corresponding
de
12
t-BuOH
a
Reaction conditions: 1a (0.10 mmol), 2a (0.20 mmol), H2O (0.20
b
mmol), solvent (2 mL), 12 h. DMF = N,N′-dimethylformamide.
DMSO = dimethyl sulfoxide. THF = tetrahydrofuran. HFIP =
c
d
1,1,1,3,3,3-hexafluoro-2-propanol. Isolated yields. 30 °C for 12 h,
then 50 °C for 4 h. H2O was not added.
e
and 2). When acetonitrile (MeCN), tetrahydrofuran (THF),
1,1,1,3,3,3-hexafluoro-2-propanol (HFIP), and water (H2O)
were used as solvents, we could obtain 3a in moderate yields
(Table 1, entries 3−6). The reaction went smoothly in t-
BuOH with an isolated yield of 74% (Table 1, entry 7). The
reaction temperature then was investigated. When the
reactions were conducted at 50 and 80 °C, the yields
decreased to 68% and 52%, respectively (Table 1, entries 8
and 9). Although a diminished yield was obtained, we found
that the reaction system was cleaner when conducting the
reaction at 50 °C. Therefore, we tried to heat the reaction at 50
°C after reacting at 30 °C in order to improve the yield. To our
delight, after the combination of 12-h reaction at 30 °C and 4-
h reaction at 50 °C, we could obtain 3a in 81% yield (Table 1,
entry 10). When ethanol (EtOH) was used instead of tert-
butanol (t-BuOH) as a solvent, the yield decreased
significantly to 38% (Table 1, entry 11). When performing
the reaction without water, the yield remained unchanged
(Table 1, entry 12).
a b
,
Scheme 3. Substrate Scope of Selenosulfonates 2
With the optimized conditions in hand, we investigated the
substrate scope of various isocyanides 1 (Scheme 2).
Delightedly, we could obtain 3a in 81% yield. Subsequently,
a series of 4,5-disubstituted o-diisocyanoarenes were inves-
tigated. Methoxyl- and bromide-substituted isocyanides could
react smoothly under standard conditions. The target products
3b and 3c were obtained in 63% and 70% yields, respectively.
When 2,3-diisocyanonaphthalene was used for the reaction, the
desired product 3d could be obtained in 91% yield. In
addition, we explored the monosubstituted o-diisocyanoarenes
substrates. When 1,2-diisocyano-3-methylbenzene was em-
a
Reaction conditions: 1a (0.10 mmol), 2 (0.20 mmol), t-BuOH (2
mL), 30 °C, 12 h, then 50 °C for x h (the value of x is indicated in
b
parentheses). Isolated yields.
B
DOI: 10.1021/acs.orglett.9b01886
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
product 3k in 73% yield. In addition, cycloalkane-substituted
selenosulfonates were well-tolerated in the reactions and
provided the desired products in yields of 64%−75% (3l−3n).
The selenoesters have been proven to be reactive
intermediates in the ligation reactions, because of the weakness
of the C−Se bond.14 For instance, by aminolysis with
benzylamine, 3a could be converted to urea derivative 4 in
the presence of N,N′-diisopropylethylamine (DIPEA) (see
Scheme 4).
reaction with isocyanide 1a. We could isolate the product 3g in
72% yield; however, 3a was not detected. It indicated that
diselenide was not involved in the selenylative process (see eq
5 in Scheme 5).
To gain a deep insight into the radical formation, the
electron paramagnetic spectroscopy (EPR) experiments were
performed in the presence of 5,5-dimethyl-1-pyrrolidine-N-
oxide (DMPO) (see Scheme 6). When isocyanide 1a or
a
Scheme 6. EPR Experiments
Scheme 4. Further Transformation of 3a
In order to gain insight into the reaction mechanism, several
controlled experiments were conducted (see Scheme 5). When
a
Legend: the yellow trace denotes data for a solution of 1a and
Scheme 5. Investigation of the Reaction Mechanism
DMPO in t-BuOH at 50 °C for 10 min; the red trace denotes data for
a solution of 2a and DMPO in t-BuOH at 50 °C for 10 min; and the
blue trace represents data for a solution of 1a, 2a, and DMPO in t-
BuOH at 50 °C for 10 min.
selenosulfonate 2a was mixed with DMPO in t-BuOH, we
could not observe the radical signal (Scheme 6, yellow and red
traces). However, when 1a, 2a, and DMPO was mixed in t-
BuOH, a strong radical signal was detected (Scheme 6, blue
trace). The results suggested the formation of radical
intermediate should involve intermolecular interaction be-
tween 1a and 2a. The Se−S bond of 2a was relatively weak, the
Se atom was electropositive and the S atom was electro-
negative. As we known, the isocyanide is both electrophilic and
nucleophilic, it might interact with the Se and S atoms to
weaken the Se−S bond. As a result, the homolysis of
selenosulfonate 2a was easier to occur.
Based on the above results and a previous report,9 we depict
a plausible mechanism involving the radical process (see
Scheme 7). By interaction with 1a, the homolysis of
selenosulfonate 2a will occur in the mixture to give
heavy-oxygen water (H218O) was added under the standard
reaction condition, the 18O-labeled product [18O]-3a could not
be detected by HRMS in the reaction mixture (see eq 1 in
Scheme 5). When the reaction was conducted under N2
protection with degassed t-BuOH, we could isolate 3a in
38% yield (see eq 2 in Scheme 5). The above results indicated
that the carbonyl oxygen of 3a did not originate from the water
in the reaction solvent and oxygen in the air. Besides 3a, we
could isolate t-butyl benzenesulfinate (5) and 1,2-bis(2-
phenoxyethyl)diselane (6) in yields of 34% and 79%,
respectively (see eq 3 in Scheme 5). The results indicated
the carbonyl oxygen of 3a should originate from the SO
group of selenosulfonate 2a in the reaction and the in situ
formation of diselenide was also reasonable. Subsequently, we
conducted the radical-inhibition investigations. Diminished
yields of 3a were observed when 2,2,6,6-tetramethylpiper-
idinyloxy (TEMPO) or 2,6-di-tert-butyl-4-methylphenol
(BHT) was added to the reaction system, which indicated
that a radical path might be involved in the reaction (see eq 4
in Scheme 5). Finally, the selenylative step was investigated via
a competition experiment. Se-benzyl benzenesulfonoselenoate
2b and 1,2-bis(2-phenoxyethyl)diselane 6 were mixed in the
Scheme 7. Plausible Mechanism
C
DOI: 10.1021/acs.orglett.9b01886
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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benzenesulfonyl radical A and selenium radical B. A then reacts
with o-diisocyanide 1a to deliver C. Intermediate C
subsequently reacts with B to give intermediate D through a
synergism process. The second addition of selenosulfonate 2a
to the other isocyanide affords intermediate E. The alcoholysis
of intermediate E gives F and 5. An intramolecular cyclization
process subsequently will occur and the final product 3a is
obtained, along with intermediate G. The diselenide 6 can be
obtained by homocoupling of B or G.
In summary, we have developed a facile method for the
synthesis of N-(carboselenoate) benzimidazolones via the
reaction of o-diisocyanoarenes with selenosulfonates under
metal-free conditions. The reaction is easy to handle and
proceeds smoothly with moderate to good yields.
ASSOCIATED CONTENT
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AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: shunyi@suda.edu.cn (S.-Y Wang).
*E-mail: shunjun@suda.edu.cn (S.-J. Ji).
ORCID
Weidong Rao: 0000-0002-6088-7278
Shun-Yi Wang: 0000-0002-8985-8753
Shun-Jun Ji: 0000-0002-4299-3528
Notes
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We gratefully acknowledge the National Natural Science
Foundation of China (Nos. 21772137, 21542015, and
21672157), PAPD, the Project of Scientific and Technologic
Infrastructure of Suzhou (No. SZS201708), the Major Basic
Research Project of the Natural Science Foundation of the
Jiangsu Higher Education Institutions (No. 16KJA150002),
Postgraduate Research & Practice Innovation Program of
Jiangsu Province (No. KYCX17_1980), Soochow University,
and State and Local Joint Engineering Laboratory for Novel
Functional Polymeric Materials for financial support. We thank
Bei-Bei Liu in this group for reproducing the results of 3d, 3f,
3g, and 3m.
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