Visible-light-enabled oxyazidation of alkenes leading to α-azidoketones in air

Article abstract of DOI:10.1039/c8gc01245h

A new and facile visible-light-enabled method for the synthesis of α-azidoketones has been developed via oxyazidation of alkenes with TMSN₃ in air at room temperature. A series of α-azidoketones could be easily and efficiently obtained in moder

Full text of DOI:10.1039/c8gc01245h

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Visible-light-enabled oxyazidation of alkenes
leading to α-azidoketones in air†
Cite this: DOI: 10.1039/c8gc01245h
a,c
Wei Wei,
*
a,b,c Huanhuan Cui,c Huilan Yue*b and Daoshan Yang
*
Received 20th April 2018,
Accepted 12th June 2018
DOI: 10.1039/c8gc01245h
rsc.li/greenchem
A new and facile visible-light-enabled method for the synthesis of sulfate (BTPBPD) or in situ-generated modified IBX12 have also
α-azidoketones has been developed via oxyazidation of alkenes been developed. Very recently, Szpilman also reported the
with TMSN3 in air at room temperature. A series of α-azidoketones α-azidation of the preformed enolonium species leading to
could be easily and efficiently obtained in moderate to excellent α-azidoketones in the presence of Koser’s hypervalent iodine
yields via cascade C–N and CvO bond formation by simply using reagents at low temperature.13 Nevertheless, most of these
low-toxic and inexpensive Rose Bengal (1 mol%) and PhSeSePh methods still involve the use of a large excess of expensive and
(5 mol%) as co-catalysts.
hazardous oxidants, toxic metal reagents, or multi-step synthetic
sequences, which would lead to increased cost and environ-
mental concerns and limit their practical utility. Therefore, the
development of a facile, efficient, economic and eco-friendly
method to construct α-azidoketones is still highly desirable.
From green and sustainable points of view, the direct utiliz-
ation of O2 as an oxidant and oxygen source is one of the most
ideal methodologies for the construction of oxygen-containing
compounds. In particular, air demonstrates more advantages
over pure dioxygen in terms of safety, accessibility, and low
cost. It is still an appealing but challenging task to develop
mild and selective aerobic oxidation reaction systems to access
important and structurally diverse oxygenated compounds.14
In recent years, visible-light photocatalysis has offered a tech-
nically attractive and energy-saving platform for promoting
various aerobic chemical transformations under mild con-
ditions.15 On the other hand, organoselenium compounds as
an attractive alternative to transition-metal-catalysts are
increasingly utilized in organic synthesis owing to their easy
availability, cheap and less toxic characteristics.16 Although
some achievements have been made in this field, the study of
organoselenium catalysis is still in its infancy and new reaction
systems remain to be further explored.
Organic azides have attracted a great deal of interest owing to
their wide applications in synthetic chemistry1 and materials
science.2 In particular, α-azidoketones are very important
building blocks in the synthesis of diverse triazoles,3 aziri-
dines,4 and other heterocyclic compounds,5 which exhibit a
variety of biological activities such as anti-allergic, anti-microbial,
and anti-HIV properties.6 Furthermore, α-azidoketones can
also serve as useful intermediates for the synthesis of vicinal
amino alcohols, which are key parts of some important com-
pounds such as sphingosines, daunosamine and hydroxy
amino acids.7 In light of their widespread utilities, consider-
able efforts have been devoted towards α-azidoketones.
Traditionally, α-azidoketones are prepared by direct nucleo-
philic substitution of α-halo ketones8 and α-tosyloxy ketones
with azide ions.9 Alternative methods such as the direct azida-
tion of arylketones with NaN3 in the presence of a hypervalent
iodine reagent,10 the azidation of silyl enol ethers using a ben-
ziodoxole-derived azide,11 and the conversion of olefins to
α-azidoketones in the presence of a stoichiometric amount of
chromium trioxide (CrO3), cerium(IV) ammonium nitrate
(CAN), cis-1,4-bis(triphenylphosphonium)-2-buteneperoxodi-
aState Key Laboratory Base of Eco-Chemical Engineering, College of Chemistry and
Molecular Engineering, Qingdao University of Science and Technology, Qingdao,
266042, P. R. China. E-mail: weiweiqfnu@163.com, yangdaoshan@tsinghua.org.cn
bKey Laboratory of Tibetan Medicine Research, Northwest Institute of Plateau
Biology, Chinese Academy of Sciences and Qinghai Provincial Key Laboratory of
Tibetan Medicine Research, Qinghai 810008, China. E-mail: hlyue@nwipb.cas.cn
cSchool of Chemistry and Chemical Engineering, Qufu Normal University,
Qufu 273165, Shandong, China
ð1Þ
In continuation of our interest in the development of new
aerobic oxidation reactions,17 here, we wish to report a mild
†Electronic supplementary information (ESI) available: Experimental details.
See DOI: 10.1039/c8gc01245h
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Table 1 Optimization of the reaction conditionsa
and facile visible-light-induced method for the construction of
α-azidoketones via Rose Bengal (1 mol%) and PhSeSePh
(5 mol%) co-catalyzed oxyazidation of alkenes with TMSN3 and
dioxygen (air) (eqn (1)). The present protocol offers an efficient
and green route to a series of α-azidoketones with good func-
tional-group tolerance, in which the C–N and CvO bonds are
formed in a single operation under mild and metal-free
conditions. To the best of our knowledge, this is the first
example of a metal-free photocatalytic approach to access
α-azidoketones.18
Initially, styrene 1a and TMSN3 were selected as the model
substrates to optimize conditions such as photocatalysts, addi-
tives, solvents, and amounts of reactants under irradiation
with 3 W blue LED lamps. Fortunately, when the model reac-
tion was carried out in the presence of Eosin Y (2 mol%) and
PhSeSePh (5 mol%) in MeCN in air, the desired α-azidoketone
3a was obtained in 66% yield (Table 1, entry 1). The replace-
ment of Eosin Y with other commercially available photocata-
lysts found that Rose Bengal was the best one to give the
product 3a in 74% yield (Table 1, entry 6), while others such as
Na2-Eosin Y, Eosin B, Rhodamine B, and Acridine Red were
deleterious to the product yield (Table 1, entries 2–5). The
screening of the loading of the photocatalyst found that
1 mol% Rose Bengal gave the best yield (92%) of the desired
product 3a (Table 1, entry 8). No transformation was observed
in the absence of the photocatalyst (Table 1, entry 9). Solvent
was also crucial for this oxyazidation reaction. A notable
decrease of yield was observed when other solvents such as
THF, 1,4-dioxane, DME, DCE, toluene, DMF, EtOH, or acetone
were employed as the reaction media to replace MeCN
(Table 1, entries 10–17). Increasing or decreasing the loading
of PhSeSePh failed to improve the reaction efficiency (Table 1,
entries 18 and 19). Only 5% product was obtained in the
absence of PhSeSePh (Table 1, entry 20). The replacement of
PhSeSePh with PhSSPh did not improve the reaction efficiency
(Table 1, entry 21). In addition, when the reaction was con-
ducted under irradiation with 3 W white or green LED lamps,
the desired product was only obtained in 71% and 52% yields,
respectively (Table 1, entries 22 and 23). In contrast, the oxyazi-
dation of alkenes did not occur in the dark or replacing
TMSN3 with NaN3 (Table 1, entries 24 and 25).
Photocatalyst
(mol%)
Additive
(mol%)
Yieldb
(%)
Entry
Solvent
1
2
3
4
5
6
7
8
Eosin Y (2)
Na2-Eosin Y (2)
Eosin B (2)
Rhodamine B (2)
Acridine Red (2)
Rose Bengal (2)
Rose Bengal (5)
Rose Bengal (1)
—
Rose Bengal (1)
Rose Bengal (1)
Rose Bengal (1)
Rose Bengal (1)
Rose Bengal (1)
Rose Bengal (1)
Rose Bengal (1)
Rose Bengal (1)
Rose Bengal (1)
Rose Bengal (1)
Rose Bengal (1)
Rose Bengal (1)
Rose Bengal (1)
Rose Bengal (1)
Rose Bengal (1)
Rose Bengal (1)
PhSeSePh(5)
PhSeSePh(5)
PhSeSePh(5)
PhSeSePh(5)
PhSeSePh(5)
PhSeSePh(5)
PhSeSePh(5)
PhSeSePh(5)
PhSeSePh(5)
PhSeSePh(5)
PhSeSePh(5)
PhSeSePh(5)
PhSeSePh(5)
PhSeSePh(5)
PhSeSePh(5)
PhSeSePh(5)
PhSeSePh(5)
PhSeSePh(1)
PhSeSePh(10)
—
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
THF
1,4-Dioxane
DME
DCE
Toluene
DMF
EtOH
Acetone
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
66
25
Trace
33
20
74
48
92
0
Trace
10
30
27
12
23
15
77
76
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
68
5
PhSSPh(5)
Trace
71c
52d
0e
PhSeSePh(5)
PhSeSePh(5)
PhSeSePh(5)
PhSeSePh(5)
0 f
a Reaction conditions: 1a (0.2 mmol), 2 (0.4 mmol), photocatalyst
(1–5 mol%), additive (1–10 mol%), solvent (analytical reagent) 1 mL,
3 W blue LED lamps, rt, air (O2), 2 h. b Isolated yields based on 1a.
With optimized visible-light photoredox conditions in
hand, the substrate scope for oxyazidation of alkenes with
TMSN3 was surveyed. As shown in Table 2, aromatic alkenes c 3 W white LED lamps. d 3 W green LED lamps. e In the dark. f NaN3
was used instead of TMSN3.
containing electron-rich or electron-poor groups on the aryl
rings were suitable for this process to provide the corres-
ponding products in moderate to good yields. The electronic
nature of the substrates has great influence on the reaction
efficiency, and bearing electron-donating group substituted nitro groups survived well in this reaction leading to the
aromatic alkenes (3a–3h) showed better activity in comparison corresponding products 3g–3q, which can be applied in
with those containing strong electron-withdrawing groups (3o– further modifications. In addition, 2-vinylnaphthalene could
3q). The reaction efficiency was not obviously affected by the smoothly convert to the expected product 3r in 82% yield.
steric effect, and substrates bearing a methyl or chloro group Notably, internal aromatic alkenes such as β-methyl styrene,
in the para-, meta-, or ortho-position of the aryl ring were all 1,2-dihydronaphthalene and 1,2-diphenylethene could also
suitable for this reaction (3b–3d, 3k–3m). Also, functional work smoothly under the current conditions, affording the
groups such as chloromethyl, acetoxy, halogen, cyano and desired products 3s–3u in good yields. Nevertheless, when ali-
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Table 2 Results for visible-light-induced oxyazidation of alkenes
leading to α-azidoketonesa,b
Fig. 1 Synthetic application of α-azidoketones.
Subsequently, we turned our attention to explore the poss-
ible reaction mechanism. As presented in eqn (2), the model
reaction was completely inhibited when 2,2,6,6-tetramethyl-1-
piperidinyloxy (TEMPO) was added under standard conditions,
suggesting that the present transformation might involve a
radical process. Only a trace amount of the desired product
was detected when the model reaction was carried out under a
nitrogen atmosphere (eqn (3)). Furthermore, when the reaction
of styrene 1a and TMSN3 was carried out in the presence of
H2O18 in dry CH3CN in air, the corresponding product 3a was
obtained in 81% yield and none of O18-3a was detected by
LC-MS (eqn (4)). These results suggested that dioxygen should
be the key oxidant and oxygen source in the present reaction
system. Moreover, in addition to the desired product 3a,
β-hydroperoxy azide 5a was isolated in 74% yield when the
model reaction was performed in the absence of PhSeSePh
(eqn (5)). Furthermore, when the preformed β-hydroperoxy
azide 5a was treated with PhSeSePh (5 mol%) in CH3CN at
room temperature, the desired product 3a was isolated in 94%
yield (eqn (6)). These results suggested that β-hydroperoxy
azide 5a should be the key intermediate in this reaction and
PhSeSePh could promote the elimination of water from 5a.
a Reaction conditions: 1 (0.2 mmol), 2 (0.4 mmol), Rose Bengal
(1 mol%), PhSeSePh(5 mol%), CH3CN (analytical reagent, 1 mL), 3 W
blue LED lamps, rt, air (O2), 2 h. b Isolated yields based on 1.
ð2Þ
ð3Þ
phatic alkenes such as hex-1-ene and cyclopentene were
used as the substrates, none of the desired products were
detected.
It is noteworthy that α-azidoketones as useful building
blocks could be transformed into a series of valuable com-
pounds.19 For example, as shown in Fig. 1, β-azido alcohol 4a
and 2-nitroacetophenone 4b can be easily obtained from
product 3a through the simple reduction and oxidation reac-
tion, respectively.19a,b 3-Acylisoquinoline 4c was obtained from
the reaction of phenacyl azide 3a with o-phthalaldehyde and
triphenylphosphine.19c Five membered rings such as imid-
azole 4d and pyrazole 4e were efficiently synthesized by the
condensation of phenacyl azide 3a.19d,e Undoubtedly, a click
reaction was employed to yield β-carbonyl triazole 4f.19f
ð4Þ
ð5Þ
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ð6Þ
Next, on/off visible-light irradiation experiments were per-
formed to prove the effect of photo-irradiation, and the result
demonstrated that continuous irradiation of visible-light is
necessary for this oxyazidation reaction (Fig. 2). Finally, fluo-
rescence quenching (Stern–Volmer) experiments were also
carried out to certify an energy transfer process between Rose
Bengal and TMSN3 under visible-light irradiation. As shown in
Fig. 3 and 4, the emission intensity of excited Rose Bengal
gradually decreased with the increasing of the concentration
of TMSN3. In contrast, such an effect was not found when
Rose Bengal was separately mixed with styrene 1a and
PhSeSePh (see the ESI†). These results indicated that the
single-electron transfer (SET) process actually occurred
between Rose Bengal and TMSN3 under visible-light
irradiation.
Fig. 4 Stern–Volmer plots.
Based the above results and previous reports,15–17,20–22
a
possible reaction pathway is demonstrated in Scheme 1.
Firstly, the excited RB* was generated from Rose Bengal under
visible-light irradiation. Subsequently, a single electron trans-
fer (SET) process occurred between TMSN3 2 and RB* to give
Scheme 1 Possible reaction pathway.
azido radical 6 and RB•− radical anions. Furthermore, the
•−
ground state Rose Bengal and O2 would be formed through
the oxidation of RB•− by molecular oxygen (air). Then, the
addition of azido radical 6 to alkene 1 produced alkyl radical 7.
•−
Next, the interaction of radical 7 with O2 and H2O afforded
hydroperoxide intermediate 5. The oxidation of (PhSe)2 by hydro-
peroxide 5 yielded the corresponding reactive organoselenium
species 8 and PhSeOH.22 Finally, the interaction of PhSeOH with
reactive intermediate 8 produced the desired product 3 and
regenerated (PhSe)2 along with the elimination of water.
Fig. 2 On/off experiments.
In conclusion, we have presented a facile visible-light-
enabled method for the construction of α-azidoketones via
metal-free Rose Bengal and (PhSe)2 co-catalyzed oxyazidation
of alkenes. Considering the desirable features, such as the
cheap and non-toxic catalyst, clean oxidant, eco-friendly
energy source, mild reaction conditions and simplicity of oper-
ation, this reaction offers an efficient and green route to
produce a series of α-azidoketones. Further studies on the
detailed reaction mechanism and the synthetic application are
currently ongoing in our lab.
Conflicts of interest
Fig. 3 Quenching of Rose Bengal fluorescence emission in the pres-
ence of TMSN3.
There are no conflicts to declare.
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Acknowledgements
This work was supported by the Natural Science Foundation of
Shandong Province (ZR2018MB009 and ZR2016JL012), the
International Cooperation Project of Qinghai Province
(2018-HZ-806; 2017-HZ-806), and the National Natural Science
Foundation of China (No. 21302109 and 21302110).
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