Photoinduced Rapid Multicomponent Cascade Reaction of Aryldiazonium Salts with Unactivated Alkenes and TMSN3

Article abstract of DOI:10.1021/acs.orglett.0c04148

The first example of a photoinduced rapid multicomponent cascade reaction of aryldiazonium salts with unactivated alkenes and trimethylsilyl azide (TMSN3) under oxidant-free conditions is described. This approach provides the synthetic route for a wide range of unsymmetric azo compounds. The compounds are obtained in good to excellent yields under mild reaction conditions. This transformation is applicable to various aryldiazonium salts and alkenes, providing an alternative route for the synthesis of unsymmetric azo compounds. The control experiment demonstrates that the reaction mechanism follows a radical pathway.

Full text of DOI:10.1021/acs.orglett.0c04148

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Letter
Photoinduced Rapid Multicomponent Cascade Reaction of
Aryldiazonium Salts with Unactivated Alkenes and TMSN3
Jiabin Shen, Jun Xu, Lei He, Yani Ouyang, Lin Huang, Wanmei Li,* Qing Zhu,* and Pengfei Zhang*
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ABSTRACT: The first example of a photoinduced rapid multi-
component cascade reaction of aryldiazonium salts with
unactivated alkenes and trimethylsilyl azide (TMSN3) under
oxidant-free conditions is described. This approach provides the
synthetic route for a wide range of unsymmetric azo compounds.
The compounds are obtained in good to excellent yields under
mild reaction conditions. This transformation is applicable to
various aryldiazonium salts and alkenes, providing an alternative route for the synthesis of unsymmetric azo compounds. The control
experiment demonstrates that the reaction mechanism follows a radical pathway.
romatic azo compounds have been used for the
preparation of dyes, pigments,1 therapeutic agents,2
Scheme 1. Radical Reaction Involving Diazonium Salts
A
food additives, and indicators (Figure 1).3 The compounds
Figure 1. Representative azo compounds.
can be obtained through the azo coupling reaction,4 Mills
reaction,5 and Wallach reaction.6 However, these synthetic
methods for forming the azo group suffer from various
disadvantages. The reactions need to be carried out at high
temperatures (>100 °C) in the presence of special substrates
and additives.7 Furthermore, most of these approaches are only
suitable for the synthesis of diaryl azo compounds. Therefore,
it is necessary to develop a green and efficient method to
synthesize aryl−alkyl azo compounds.
workers demonstrated a visible-light-induced formation of C−
N bonds following an intermolecular amino-arylation process
involving alkenes.8b In 2017, Jiang’s group established the
unprecedented [2 + 2] cycloaddition/S-centered radical-
induced 1,4-addition cascade reactions involving benzene-
linked allene−ynes. The reactions were conducted under mild
and redox neutral conditions.8c In the same year, Wu et al.
reported an efficient route to synthesize sulfonated tetrahy-
dropyridine derivatives via a radical pathway in the presence of
1,6-enynes, sulfur dioxide, and aryl diazonium tetrafluorobor-
ates.8d Reports on radical reactions where diazonium salts are
We note that diazonium salts containing the NN group
can be used as a precursor to prepare azo compounds. Aryl
diazonium salts are widely available and can be easily produced
from anilines that are present in abundance. In recent years,
numerous attempts have been made to synthesize azo
compounds via multicomponent reaction processes using
diazonium salts as aryl radical precursors (Scheme 1a).8 Studer
and co-workers developed a new method for radical oxy-
arylation of alkenes using the readily available aryl diazonium
Received: December 15, 2020
Published: February 4, 2021
salts as aryl radical precursors.8a In 2014, Konig and co-
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© 2021 American Chemical Society
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used as the radical acceptors are scarce (Scheme 1b).9 In 2017,
Hao’s group developed an efficient method for the synthesis of
thermally stable and pharmaceutically important unsymmetric
azo compounds.9a Subsequently, Clayden and co-workers
reported an elegant strategy for the preparation of alkyl−aryl
azo compounds via a radical addition reaction involving alkyl
radicals and diazonium ions.9b Despite these considerable
advances, such transformations are still restricted to two-
component reactions. The reactions employing aryl diazonium
salts as intermolecular radical acceptors in multicomponent
reactions have been rarely realized.10
Organoazides have been extensively used for structuring
complex pharmaceutical molecules and preparing diversified
chemical libraries.11 The photocatalyzed three-component
cascade reaction of alkenes has recently attracted attention
and is an elegant example of a method that can be used for the
synthesis of organoazides.12 In 2017, Lu and co-workers
reported the photocatalyzed metal-free aerobic hydroxyazida-
tion reaction of alkenes in the presence of trimethylsilyl azide
(TMSN3).12a They were the first to report this reaction. They
also reported a photocatalyzed three-component carboazida-
tion reaction involving alkenes, TMSN3, and acrylonitrile.12b
Yang et al. developed a method for the synthesis of α-
azidoketones. The photocatalyzed oxyazidation of alkenes in
the presence of TMSN3 afforded the desired compounds.12c
Recently, our group developed a radical three-component
cascade reaction involving quinoxalinones, TMSN3, and
alkenes.13 In addition, we later disclosed a photocatalytic
decarboxylative coupling strategy for the mild and efficient
synthesis of 3-alkylated quinoxalinones.14 In a continuation of
our efforts, we herein demonstrate a rapid, photocatalyzed
three-component cascade reaction of diazonium salts in the
presence of unactivated alkenes and TMSN3. This strategy
provides an alternative route for the synthesis of unsymmetric
azo compounds for the efficient construction of two C−N
bonds (Scheme 1c).
a,b
Table 1. Optimization of Reaction Conditions
b
entry
photocatalyst
solvent
time
yield (%)
1
2
3
4
5
6
7
8
PC-1
PC-2
PC-3
PC-4
MeCN
MeCN
MeCN
MeCN
MeCN
DCE
DCM
THF
toluene
DMSO
DMF
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
1 h
1 h
1 h
1 h
1 h
1 h
1 h
1 h
1 h
34
12
21
0
0
PC-1
PC-1
PC-1
PC-1
PC-1
PC-1
PC-1
PC-1
PC-1
PC-1
PC-1
PC-1
PC-1,dark
PC-1
PC-1
74
87
17
25
32
18
81
86
63
89
88
68
0
9
10
11
12
13
14
15
16
17
18
19
20
1 h
1 h
2 h
15 min
10 min
15 min
15 min
2h
15 min
15 min
15 min
c
d
e
d
d
d
,
,
f
f
95
93
,
g
a
Reaction conditions: 1a (0.2 mmol), 2a (1.3 equiv), 3 (1.3 equiv),
photocatalyst (4 mol %), solvent (1.0 mL), room temperature,
b
irradiation with 6 W blue LEDs under air. Isolated yields.
c
d
e
Photocatalyst (5 mol %). Photocatalyst (2 mol %). Photocatalyst
f
g
(1 mol %). 10 W blue LEDs were used as the light source. N2
atmosphere.
reaction (Table 1, entries 15−17). However, the product was
not formed in the absence of the photocatalyst or light (Table
1, entries 5 and 18). When the light intensity was increased
from 6 to 10 W, the yield of the desired product increased to
95% (Table 1, entry 19). When the model reaction was carried
out under a nitrogen atmosphere, 93% yield of the desired
product was detected (Table 1, entry 20).
With the optimized reaction conditions in hand, the
substrate scope of the cascade reaction was explored by
reacting various alkenes (2) with 4-methoxybenzenediazonium
tetrafluoroborate (1a) and TMSN3 (3) (Scheme 2). Non-
functionalized linear and cyclic olefins afforded the desired
products in excellent yields (4a−e). However, linear alkenes
bearing the sensitive hydroxyl groups and multiple substituted
aromatic alkenes yielded the corresponding products (4f−j) in
relatively low yields (76−94%). It should be mentioned that
the olefins bearing various functional groups such as ester,
ether, sulfonyl, and halo substituents were well tolerated under
the optimized reaction conditions (4k−r), affording the
corresponding products in 75−93% yields.
Initially, 4-methoxybenzenediazonium tetrafluoroborate
(1a), 1-amylene (2a), and TMSN3 (3) were chosen as the
starting substrates to optimize the reaction conditions (Table
1). When 4 mol % of Acr+MesClO4 (a visible-light
−
photocatalyst) was used for the reactions, the target product
(4a) was obtained in 34% yield. The reaction was carried out
in MeCN (1.0 mL) at room temperature. The desired product
formed within 1 h (Table 1, entry 1). Other photocatalysts,
such as eosin Y, Acid Red 94, and Ir(ppy)2(dtb-bpy)PF6,
afforded the products in relatively low yields under the same
reaction conditions (Table 1, entries 2−5). The reaction
solvent plays a very important role in this reaction. Various
organic solvents, such as dichloroethane (DCE), dichloro-
methane (DCM), tetrahydrofuran (THF), toluene, dimethyl
sulfoxide (DMSO), and N,N-dimethylformamide (DMF),
were screened. It was found that the optimal product yield
was obtained when DCM was used as the reaction solvent
(Table 1, entries 1 and 6−11). The reaction time did not
significantly effect on this transformation (Table 1, entries 12−
14). To our delight, the reaction proceeded to completion
within 15 min (Table 1, entries 13 and 14). This makes this
method of synthesizing azo compounds highly efficient and
economical. The amount of the catalyst did not influence the
product yield (4a). Even when the catalyst loading was
reduced to 2 mol %, the reaction afforded 4a in 88% yield.
When the catalyst loading was 1 mol %, the product yield
could not be improved by increasing the duration of the
Following this, the substrate scope of aryl diazonium salt (1)
in the presence of 1-amylene (2a) and TMSN3 (3) was
evaluated (Scheme 3). It was observed that aryldiazonium salts
bearing electron-withdrawing or electron-donating groups, at
the para-, meta-, or ortho-positions afforded the corresponding
products (4s−4ac) in good yields. Moreover, the reactions
with 1-naphthyldiazonium salt proceeded smoothly to afford
the target product (4ad) in 71% yield.
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a,b
Scheme 2. Substrate Scope of Alkenes
Scheme 4. Gram-Scale Synthesis and Further Chemistry
Finally, natural products (−)-ß-citronellol and nootkatone
were structurally modified to yield the products 4ah and 4ai in
good yields (Scheme 5).
a
Reaction conditions: 1a (0.2 mmol), 2 (1.3 equiv), 3 (1.3 equiv),
PC-1 (2.0 mol %), DCM (1.0 mL), 10 W blue LEDs, 15 min, under
air. Isolated yields. Reaction was performed on a 1 mmol scale.
b
c
Scheme 5. Modification of Natural Products
a,b
Scheme 3. Substrate Scope of Aryl Diazonium
We conducted a preliminary mechanistic investigation to
gain insight into the reaction mechanism.16 The reaction did
not proceed when two equivalents of 2,2,6,6-tetramethylpiper-
idine-1-oxyl (TEMPO) were added as the radical inhibitor.
The corresponding adducts formed were detected by the high-
resolution mass spectrometry technique, revealing that a free-
radical process can be potentially operative. The radical clock
reaction of diethyl 2,2-diallylmalonate 2aj was performed to
afford the radical ring-closure product 4aj in 62% yield,
suggesting that this protocol advances via a radical pathway
(Scheme 6). This was further proved by the fluorescence
quenching study, as shown in Figure 2. The fluorescence of the
photocatalyst could be quenched in the presence of TMSN3.
This observation was consistent with our hypothesis.17
a
Reaction conditions: 1 (0.2 mmol), 2a (1.3 equiv), 3 (1.3 equiv),
PC-1 (2.0 mol %), DCM (1.0 mL), 10 W blue LEDs, 15 min, under
air. Isolated yields.
b
The practical synthetic application of the procedure was also
investigated. First, a gram-scale synthesis was performed. The
product (4d) was obtained in 87% yield (Scheme 4a).
Following this, a potentially bioactive molecule (4ae) was
easily synthesized in 84% yield via a dehalogenation pathway.
The molecule 4q was reacted with 4-hydroxycoumarin15 to
afford the said bioactive molecule (Scheme 4b). Additionally,
triazole 4af could be obtained through “click” reaction with
phenylacetylene (Scheme 4c). 1,2-diaminocyclohexane 4ag
was also obtained through hydrogenation (Scheme 4d).
Based on the above results and previous reports,16,18 we
proposed the possible reaction mechanism (Scheme 7). We
proposed that, initially, the photocatalyst PC-1 was photo-
promoted to the excited state producing the *PC-1 species
(Ered* = 2.06 V vs SCE).19 Following this, a single electron
transfer (SET) process occurred between TMSN3 (3) and
*PC-1 to produce the azido radical and PC-1‑ radical anion.
The oxidation potential of the free azide anion was found to be
+1.32 V (NHE).20 Subsequently, the azido radical attacked
olefin 2a rapidly to generate an alkyl radical (A), which was
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Scheme 6. Control Experiments
AUTHOR INFORMATION
■
Corresponding Authors
Wanmei Li − College of Material, Chemistry and Chemical
Engineering, Hangzhou Normal University, Hangzhou
311121, China; Email: liwanmei@hznu.edu.cn
Qing Zhu − Collaborative Innovation Center of Yangtze River
Delta Region Green Pharmaceuticals, Zhejiang University of
Technology, Hangzhou 311121, China; orcid.org/0000-
0002-0761-9434; Email: zhuq@zjut.edu.cn
Pengfei Zhang − College of Material, Chemistry and Chemical
Engineering, Hangzhou Normal University, Hangzhou
311121, China; orcid.org/0000-0001-9859-0237;
Email: pfzhang@hznu.edu.cn
Authors
Jiabin Shen − College of Material, Chemistry and Chemical
Engineering, Hangzhou Normal University, Hangzhou
311121, China; Collaborative Innovation Center of Yangtze
River Delta Region Green Pharmaceuticals, Zhejiang
University of Technology, Hangzhou 311121, China
Jun Xu − College of Material, Chemistry and Chemical
Engineering, Hangzhou Normal University, Hangzhou
311121, China
Lei He − College of Material, Chemistry and Chemical
Engineering, Hangzhou Normal University, Hangzhou
311121, China
Yani Ouyang − College of Material, Chemistry and Chemical
Engineering, Hangzhou Normal University, Hangzhou
311121, China
−
Figure 2. Quenching of Acr+MesClO4 fluorescence emission in the
presence of TMSN3.
Scheme 7. Plausible Mechanism
Lin Huang − College of Material, Chemistry and Chemical
Engineering, Hangzhou Normal University, Hangzhou
311121, China
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c04148
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the National Natural Science Foundation of China
(No. 21871071) for financial support.
trapped by the aryldiazonium salt 1a to produce the radical
cation (B).21 Finally, another SET process between the radical
cation (B) and the PC-1‑ radical anion afforded the target
product 4a with the simultaneous regeneration of the
photocatalyst PC-1.
In conclusion, we have reported a photocatalyzed, rapid
three-component cascade reaction involving aryl diazonium,
unactivated alkenes, and TMSN3. The substrates bearing
various functional groups were well tolerated under the mild
catalytic conditions, affording the corresponding products in
good to excellent yields. This reaction provides a simple and
efficient method for the synthesis of various useful azo
compounds. It was hypothesized that this three-component
transformation proceeded via a radical mechanism.
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ASSOCIATED CONTENT
* Supporting Information
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The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c04148.
Experimental procedures and characterization data for
all the compounds (PDF)
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Letter
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