Time-Economical Radical Cascade Cyclization/Haloazidation of 1,6-Enynes: Construction of Highly Functional Succinimide Derivatives

Article abstract of DOI:10.1021/acs.orglett.0c00682

A "time-economical" radical cascade cyclization/haloazidation of 1,6-enynes provides a direct approach to access highly functional succinimide compounds. Moderate to excellent yields along with excellent Z/E ratio were obtained under the reaction features

Full text of DOI:10.1021/acs.orglett.0c00682

pubs.acs.org/OrgLett
Letter
Time-Economical Radical Cascade Cyclization/Haloazidation of 1,6-
Enynes: Construction of Highly Functional Succinimide Derivatives
Yan Gu, Lei Dai, Kaimin Mao, Jinghang Zhang, Chang Wang, Liming Zhao, and Liangce Rong*
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ABSTRACT: A “time-economical” radical cascade cyclization/
haloazidation of 1,6-enynes provides a direct approach to access
highly functional succinimide compounds. Moderate to excellent
yields along with excellent Z/E ratio were obtained under the
reaction features of broad substrate scope, good functional group
tolerance, and mild reaction conditions.
uccinimide is one of the most important five-membered N-
containing motifs, which widely exists in a large number of
Scheme 1. Radical Haloazidation of Enynes to Construct
Heterocycle Compounds
S
products with biological and pharmaceutical activities.1 Indeed,
some succinimide derivatives can be employed as drug
candidates.2−4 For example, the promotion effect of
succinimide on amyloid fibrillation of hen egg white lysozyme
was revealed by Liu and co-workers,3 which indicated that the
compounds with a succinimide scaffold have the potential role
in treating a series of neurodegenerative diseases. In addition,
Obniska and Kaminski4 reported that the succinimide
derivatives appear to possess anticonvulsant activity. Driven
by their bioactive significance, several efficient strategies for the
synthesis of these motifs have been well-established.5 Despite
the obtained achievements, a practicable approach to access
the highly functional succinimide derivatives is still highly
desirable.
Recently, the transition-metal-catalyzed6 cyclization and
radical cascade cyclization7,8 employing 1,n-enynes as agents
have emerged as versatile methodologies for building five- or
six-membered carbocycles and heterocycle compounds, for
which radical cascade cyclizations of 1,n-enynes especially draw
more attention owing to their ability to construct various
functional cycles by adjusting the versatile radical precursor.7,8
In addition, halogen and azide groups are significantly useful
motifs due to their unique reactivity and generally easy to vary
functionalities.9 Since 2015, Valiulin,10 Chen,11 and Lefor-
estier’s groups12 subsequently disclosed a 1,2-chloroazidation
radical reaction of various olefins, presenting several approach
to access products containing halogen and azide groups. In
2016, Wang et al.8b achieved the metal-free radical cascade
cyclization/iodoazidation of 1,7-enynes to prepare pyridinone
compounds containing iodine and azide group (Scheme 1a). In
2018, Wei’s group8d reported a mild radical haloazidation of
1,6-enynes to construct the functional 2-pyrrolidinones bearing
halogen and azide groups (Scheme 1b). Moreover, chemists
often consider several economic factors (such as atom
economy,13 step economy,14 redox economy,15 and pot
economy16) in the field of organic synthesis, whereas we
propose that time economy is equally important. We herein
report a “time-economical” radial cascade cyclization/haloazi-
dation of new 1,6-enynes to furnish highly functional
succinimide derivatives (Scheme 1c).
Received: February 21, 2020
https://dx.doi.org/10.1021/acs.orglett.0c00682
© XXXX American Chemical Society
Org. Lett. XXXX, XXX, XXX−XXX
A

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Our initial efforts focused on the optimization of reaction
conditions by employing N-phenyl-N-(3-phenylpropioloyl)-
methacrylamide 1a to react with azidotrimethylsilane 2 and N-
bromosuccinimide (NBS) in the reaction media at room
temperature under a N2 atmosphere (Table 1). As shown in
to 90% with 1.2 equiv of PIDA (entry 14). However, more
loading of PIDA was detrimental to the reaction, resulting in
lower yield (entries 15 and 16). In addition, 1.2 equiv of a
bromo source was always proven to be the optimized loading
in this reaction after several investigations of bromo source
loading (entry 14 vs entries 17−19). It was found that further
increasing of the reaction temperature distinctly impaired the
reactivity (entries 20 and 21), and it should be noted that the
yield decreased significantly when the reaction was carried out
in air (entry 22), which meant that neither high temperature
nor air was suitable for this reaction. According to the above
experiments, the optimal reaction system was confirmed as
follows: reaction of 1,6-enyne 1a with 1.2 equiv of
azidotrimethylsilane 2 and 1.2 equiv of NBS was performed
in DCM (2 mL) with 1.2 equiv of PIDA at room temperature
under a N2 atmosphere.
a
Table 1. Reaction Optimization of 1a with 2
b
oxidant
(x equiv)
Br source
yield
(%)
Z/E
entry
solvent
DCM
DCE
CH3CN
EtOH
DMF
THF
(x equiv)
ratio
1
2
3
4
5
6
7
NBS (1.2)
NBS (1.2)
NBS (1.2)
NBS (1.2)
NBS (1.2)
NBS (1.2)
NBS (1.2)
45
40
38
23
93:7
93:7
93:7
93:7
93:7
93:7
93:7
Based on the optimal reaction condition, we evaluated the
scope and limitation of the cascade reaction using various N-
substituted phenyl-N-(3-phenylpropioloyl)methacrylamides 1
along with azidotrimethylsilane 2 and NBS as an active partner.
As indicated in Table 2, good to excellent yields with excellent
trace
trace
trace
1,4-
dioxane
a
8
9
TBHP (1.0) DCM
NBS (1.2)
NBS (1.2)
NBS (1.2)
NBS (1.2)
NBS (1.2)
NBS (1.2)
NBS (1.2)
NBS (1.2)
NBS (1.2)
NBS (1.0)
NBS (1.5)
NBS (2.0)
NBS (1.2)
NBS (1.2)
NBS (1.2)
60
68
NR
NR
NR
85
90
87
83
75
93:7
93:7
ND
ND
ND
93:7
93:7
93:7
93:7
93:7
93:7
93:7
93:7
93:7
93:7
Table 2. Three-Component Bromoazidation of 1,6-Enynes
TBPB (1.0)
BPO (1.0)
H2O2 (1.0)
DCM
DCM
DCM
10
11
12
13
14
15
16
17
18
19
20
21
22
DTBP (1.0) DCM
PIDA (1.0)
PIDA (1.2)
PIDA (1.5)
PIDA (2.0)
PIDA (1.2)
PIDA (1.2)
PIDA (1.2)
PIDA (1.2)
PIDA (1.2)
PIDA (1.2)
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
89
88
c
82
d
75
e
51
a
Reaction conditions: 1a (0.20 mmol), TMSN3 (0.24 mmol), and the
oxidant were combined with NBS in solvent (2.0 mL) for 3−5 min
(monitored reactions by TLC) under a nitrogen atmosphere.
b
c
d
e
Isolated yields. Reaction at 40 °C. Reaction at 50 °C. Under air.
a
Reaction conditions: unless otherwise noted, 1 (0.20 mol), TMSN3
entries 1−7 of Table 1, moderate yields along with excellent Z/
E ratios of the desired cycloaddition/bromoazidation product
3aa were obtained in only 3−5 min using chlorinated solvents
(dichloromethane (DCM), 1,2-dichloroethane (DCE), and
polar solvents (acetonitrile, ethanol)) as reaction media,
whereas poor reactivity was observed using basic solvents
(dimethylformamide (DMF)) and ether solvents (tetrahydro-
furan, 1,4-dioxane). To further improve the reaction efficiency,
other parameters were systematically investigated in DCM.
Due to the notable role in the generation of a radical, several
oxidants were then screened to find their promoting effect in
this cyclization (entries 8−13). For example, corresponding
cascade reactions with an obvious increase in yields of product
3aa smoothly proceeded with the addition of TBHP (60%,
entry 8) or TBPB (68%, entry 9), whereas no reaction
occurred in the presence of BPO, H2O2, or DTBP. To our
delight, with the oxidation of phenyliodine diacetate (PIDA),
the desired cycloadduct 3aa was obtained with significantly
improved yield (85%, entry 13). Obviously, the loading of
oxidant had a certain influence on the reactivity of this tandem
cycloaddition reaction, providing further improvement of yield
(0.24 mmol), NBS (0.24 mmol), and PIDA (0.24 mol) in DCM (2.0
mL) at room temperature for 3−5 min (monitored reactions by
b
c
TLC) under a nitrogen atmosphere. Isolated yields. Ratio of Z/E
isomer is detected by H NMR.
1
configurational selectivity (Z/E ratio) could be obtained in all
cases. Notablely, the Z/E ratio of the reaction had an obvious
steric effect. For example, by adjusting the position of
substituent R from ortho to para, the Z/E ratio of the products
was significantly increased (3ad vs 3ab). The ortho-
substituents appeared to play a crucial role in configurational
selectivity when both ortho- and para-substituents existed in
1,6-enyne (3ak vs 3ab and 3ad), whereas meta-substituents
were proven to be dispensable (3ac vs 3al). In addition, the
reaction was tolerated for para-substituents with various
electronic properties, furnishing a series of cycloaddition/
bromoazidation adducts in 82−92% yield along with excellent
Z/E ratio (3ad−3ah, 3aj). In general, it could be concluded
that the electronic properties of the substituents have little
effect on the reaction, and the yield of the reaction was good
B
https://dx.doi.org/10.1021/acs.orglett.0c00682
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a
whether it was the electron-donating group (Me-, MeO-), the
strong electron-withdrawing group (CF3-, F-), or the weak
electron-withdrawing group (Cl-, Br-) on the benzene ring.
To further emphasize the compatibility of this strategy, we
then investigated the tolerance of other halogen sources by
exchanging NBS with N-chlorosuccinimide (NCS) (Table 3).
Table 4. Cycloaddition/Azidation Reaction of 1,6-Enyne
a
Table 3. Three-Component Chloroazidation of 1,6-Enynes
a
Reaction conditions: unless otherwise noted, 1 (0.20 mmol), 2
TMSN3 (0.24 mmol), and PIDA (0.24 mmol) in DCM (2.0 mL) at
room temperature for 3−5 min (monitored reactions by TLC) under
b
a nitrogen atmosphere. Isolated yields.
To showcase the practicality and scalability of the strategy, a
large-scale reaction and further transformation were carried out
in our research (Scheme 2). The reaction on a gram scale (1.0
Scheme 2. Gram-Scale Experiment and Further
Transformation
a
Reaction conditions: unless otherwise noted, 1 (0.20 mmol),
TMSN3 (0.24 mmol), NCS (0.24 mmol), and PIDA (0.24 mmol)
in DCM (2.0 mL) at room temperature for 3−5 min (monitored
b
reactions by TLC) under a nitrogen atmosphere. Isolated yields.
c
1
Ratio of Z/E isomer is detected by H NMR.
When NCS was used as the halogen source, the chloroazida-
tion reactions occurred smoothly and good to excellent yields
(73−90%) were obtained with the Z isomer as the major
product in almost all cases. Similar to that in the
bromoazidation reaction, ortho-substituents have a significant
effect on configurational selectivity of the desired product with
an impaired Z/E ratio (4ad vs 4ak). The para-substituents
containing both electron-withdrawing and electron-donating R
aryl groups were suitable for the chloroazidation, leading to the
formation of the desired multifunctional succinimide com-
pounds in 79−90% yield with excellent Z/E ratio (4ad−4ah,
4aj). From the X-ray crystallography of product 4aj, the
configuration of the cycloadducts was established. Similar to
synthetic compound 3, the benzene ring of 1,6-enyne bearing
different substituents, such as electron-donating (Me-, MeO-)
and electron-withdrawing groups, had no significant effect on
the yield of the reaction.
Moreover, without the addition of a halogen source, the
cascade azidation/cyclization reaction of 1,6-enyne 1 and
azidotrimethylsilane 2 also occurred smoothly (Table 4),
affording the corresponding non-halogen multifunctional
succinimide product 5 with good yields (80−88%) and
excellent Z/E ratios. The R group containing both electron-
withdrawing and electron-donating groups was compatible for
the cascade reaction and had no obvious electronic effect on
the reactivity.
g of 1a) proceeded successfully, furnishing corresponding
succinimide adduct 3aa in a satisfactory yield (86%). Due to
the easily transformable reactivity of the azide group,17 the
cycloaddition of 3aa with phenylacetylene was performed
under the catalysis of cuprous iodide, furnishing the cyclo-
adduct triazole compound 6 in 78% yield along with a 99:1 Z/
E ratio. In the presence of catalyst Pd(PPh3)4, the coupling
reaction of adduct 3aa with phenylboronic acid occurred
smoothly, providing the coupling product 7 in 85% yield.
We then proceeded with control experiments to obtain the
reaction mechanistic insight into the formation of highly
functional succinimide compounds (Scheme 3). The desired
cyclization of 1a was completely inhibited in the presence of
2.0 equiv of radical scavenger 2,2,6,6-tetramethylpiperidine-1-
oxyl or 2,6-di-tert-butyl-4-methylphenol under the standard
conditions, which disclosed that the cascade reaction under-
went a radical pathway. On the basis of the above experimental
results and previous works,7,8 we described a proposed
mechanism for a radical cascade reaction (for example, 3aa)
in Scheme 3. The reaction was initiated by the ligand exchange
between PIDA and TMSN3, in which intermediate A can be
easily obtained. Driven by the weak I−N bond of intermediate
A, it subsequently underwent thermal homolytic cleavage to
C
https://dx.doi.org/10.1021/acs.orglett.0c00682
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Letter
Authors
Scheme 3. Control Reaction and Proposed Reaction
Mechanism
Yan Gu − School of Chemistry and Materials Science, Jiangsu
Normal University, Xuzhou, Jiangsu 221116, P.R. China
Lei Dai − School of Chemistry and Materials Science, Jiangsu
Normal University, Xuzhou, Jiangsu 221116, P.R. China
Kaimin Mao − School of Chemistry and Materials Science,
Jiangsu Normal University, Xuzhou, Jiangsu 221116, P.R.
China
Jinghang Zhang − School of Chemistry and Materials Science,
Jiangsu Normal University, Xuzhou, Jiangsu 221116, P.R.
China
Chang Wang − School of Chemistry and Materials Science,
Jiangsu Normal University, Xuzhou, Jiangsu 221116, P.R.
China
Liming Zhao − School of Chemistry and Materials Science,
Jiangsu Normal University, Xuzhou, Jiangsu 221116, P.R.
China
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c00682
Notes
The authors declare no competing financial interest.
We note that chlorinated azide (N3Cl) is a dangerous species,
which is generally produced by the inorganic reaction of
chlorine gas with silver azide or the reaction of sodium
hypochlorite with sodium azide in acetic acid. However, no
method has been found for the synthesis of the species in
organic reactions. Although we have carried out these reactions
in our laboratories without any incidents, for safety concerns,
one should pay careful attention not to deviate from the
reaction conditions described in the Supporting Information of
this paper.
generate the azide radical along with the release of radical B.
Then, the azide radical attacked the alkene moiety of 1,6-enyne
1a, giving alkyl radical C. Radical C is transformed into radical
intermediate D by way of 5-exo-dig. Finally, under the
promotion of radical B, the bromine radical was released in
situ from NBS, which combined radical D to deliver the
desired product 3aa.
In conclusion, we developed an efficient “time-economical”
radical cascade cyclization/haloazidation of 1,6-enynes and
TMSN3 in the presence of different halogen sources to access
the highly functional succinimide product containing halogen
and an azide group in moderate to excellent yields with up to
99:1 Z/E ratio. Additional, the significant features of this
process are large-scale synthesis and easy further trans-
formation. Most importantly, the reaction time is very short
(3−5 min), which is in line with the concept of green
chemistry.
ACKNOWLEDGMENTS
■
This work is supported by National Natural Science
Foundation of China (21571087) and the Natural Science
Foundation of the Jiangsu Higher Education Institutions of
China (18KJA150004).
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ASSOCIATED CONTENT
* Supporting Information
■
sı
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c00682.
Experimental procedure, characterization data, NMR
spectra and X-ray crystallographic data (PDF)
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Accession Codes
CCDC 1984640 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
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Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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Corresponding Author
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Liangce Rong − School of Chemistry and Materials Science,
Jiangsu Normal University, Xuzhou, Jiangsu 221116, P.R.
China; orcid.org/0000-0003-0561-1137; Email: lcrong@
jsnu.edu.cn
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