Oxoammonium Salt-Mediated Vicinal Oxyazidation of Alkenes with NaN3: Access to β-Aminooxy Azides

Article abstract of DOI:10.1002/adsc.202100783

An approach to the vicinal oxyazidation of alkenes has been achieved under mild and transition metal-free conditions. This method utilizes NaN₃ as the azidation agent and 2,2,6,6-tetramethylpiperidine-1-oxoammonium tetrafluoroborate (TEMPO

Full text of DOI:10.1002/adsc.202100783

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doi.org/10.1002/adsc.202100783
Oxoammonium Salt-Mediated Vicinal Oxyazidation of Alkenes
with NaN3: Access to β-Aminooxy Azides
Fei Chen,a,* Yu-Ting Tang,a Xin-Ru Li,a Yan-Yan Duan,a Chao-Xing Chen,a and
Yang Zhenga,*
a
Henan Provincial Engineering and Technology Research Center for Precise Synthesis of Fluorine-Containing Drugs, College of
Chemistry and Chemical Engineering, Anyang Normal University, Anyang, 455000, People’s Republic of China
E-mail: chenf2019@aynu.edu.cn; zhengy2019@aynu.edu.cn
Manuscript received: June 23, 2021; Revised manuscript received: August 30, 2021;
Version of record online: ■■■, ■■■■
Supporting information for this article is available on the WWW under https://doi.org/10.1002/adsc.202100783
et al. demonstrated a novel TEMPONa mediated
Abstract: An approach to the vicinal oxyazidation
of alkenes has been achieved under mild and
transition metal-free conditions. This method uti-
azidooxygenation of alkenes with Zhdankin reagent.
Lu’s,[5d] Wei’s[5f] and Kashyap’s[5g,h] groups independ-
ently disclosed visible-light-promoted oxyazidation
lizes NaN3 as the azidation agent and 2,2,6,6-
of olefins. Lin[5e] and co-workers developed a
tetramethylpiperidine-À1-oxoammonium tetrafluoro-
borate (TEMPO+BF4 ) as the single-electron oxi-
convenient electrochemical azidooxygenation of al-
kenes (Scheme 1b). Despite these elegant works
dant as well as the oxygen source. By using this
toward this end, however, the progress is far from
protocol, various β-aminooxy azides were synthe-
sized and several complex bioactive molecules were
meeting the need of synthesis, and the exploration of
an efficient and low-cost approach to this structur-
functionalized.
ally important β-oxy azides is still in demand.
As a continuation of our research on the
difunctionation of alkenes,[6] herein we wish to
present a new and efficient method for the oxy-
azidation of various alkenes by using commercially
Keywords: vicinal oxyazidation; alkenes; TEMPO;
β-aminooxy azides; single electron transfer
Organic azides as a versatile reagent have been
widely applicated in synthetic chemistry, biochemis-
try and materials science.[1] In particular, β-oxy
azides not only exists in biologically active mole-
cules but also can be transformed into other
important compounds that contain two vicinal nitro-
gen and oxygen atoms.[2] For example, zidovudine is
Figure 1. Bioactive molecules containing β-oxy azide/1,2-
a prodrug used in the treatment of HIV/AIDS.[2a] amino alcohol/α-amino ketone moiety.
Azidocillin is an anti-inflammatory drug.[2b] Salbuta-
mol is an important bronchodilator.[2c] Bupropion is
an antidepressant (Figure 1).[2d]
available NaN3 as theÀ azidation reagent and easily
Thus, the efficient synthesis of β-oxy azides has prepared TEMPO+BF4 as the oxidant as well as the
attracted much attention from organic and pharma- oxygen source (Scheme 1c).[7] This strategy not only
ceutical chemists.[3–5] In the past few years, a series provides a practical way for the convenient synthesis
of metal-catalyzed and metal-free methods on oxy- of biologically important β-aminooxy azides but also
azidation of alkenes have been developed.[4,5] For can be applied to late-stage modification of bioactive
instance, Mn-catalyzed hydroxyazidation,[4e] Cu-cat- molecule derivatives.
alyzed alkoxyazidation[4f]/oxo-azidation[4g] and Ag-
We commenced our study by subjecting styrene
catalyzed azidotrifluoromethoxylation[4h] of olefins (1a) (1.0 equiv.) to NaN3 (1.0 equiv.) and TEMPO+
À
were individually reported by several different BF4 (1.0 equiv.) in CH3CN under an argon atmos-
research groups (Scheme 1a). Moreover, Studer[5a] phere at room temperature for 24 h. To our delight,
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entry 3). Amazingly, 2a was still obtained in 85%
yield under air atmosphere (Table 1, entry 4). Be-
sides CH3CN, several other solvents, such as THF,
PhCl, DMSO, DMF, and DCM were also inves-
tigated, but no better yield was obtained (Table 1,
entries 5–9). In addition, other azidation agent and
other oxidant were also examined, but the outcomes
received under these conditions were not satisfying
(Table 1, entries 10 and 11).
With the optimized conditions in hand (Table 1,
entry 2), the scope of styrenes was surveyed (Ta-
ble 2). First, ortho/meta/para-substituted styrenes
with a variety of electronic properties were well
compatible in the reaction, delivering the desired
products 2a–q in good to excellent yields. Polysub-
stituted styrene also participated well in the proce-
dure, affording the expected product 2r in 65%
yield. ReactioÀn of 2-vinylnaphthalene with NaN3 and
TEMPO+BF4 yielded the product 2s in 84% yield.
The heterocyclic substrates such as 2-vinylpyridine,
4-vinylpyridine, 4-methyl-5-vinylthiazole, and 1-
vinyl-1H-imidazole were also good candidates for
Scheme 1. Representative oxyazidation of alkenes and our
À
work using TEMPO+BF4 .
the desired product 1-(2-azido-1-phenylethoxy)- this process, providing the corresponding oxyazida-
2,2,6,6-tetramethylpiperidine (2a) was generated in tion products 2t–w in 24% to 79% yields. The
79% yield (Table 1, entry 1). When the usage structure of 2w was confirmed by a single-crystal X-
À
amounts of NaN3 and TEMPO+BF4 were both ray diffraction study.[8] Significantly, this strategy
increased to 1.1 equivalent, the yield of 2a was was also suitable to 1,2-disubstituted substrates, as
improved to 89% (Table 1, entry 2). However, fur- exhibited in the cases of 2x–aa.
ther increasinÀg the application amounts of NaN3 and
Having successfully achieved the oxyazidation of
TEMPO+BF4 did not give a better result (Table 1, styrenes, we next shifted our attention to explore the
applicability of this new approach to aliphatic
alkenes (Table 3). Gratifyingly, both simple and
various functionalized terminal alkenes participated
Table 1. Optimization of the reaction conditions.[a]
smoothly in the reaction, delivering the correspond-
ing oxyazidation products 2ab–ag in 23–38% yields
with starting material being recovered in some cases
(66% of 1ad, 64% of 1ae and 32% of 1af).
Entry NaN3
TEMPO+BF4 Solvent Yield (%)[b]
Unfortunately, pent-4-en-1-ol was not applicable for
this method and only unidentifiable complex mix-
tures were observed. Significantly, internal alkenes
such as cyclopentene, cyclohexene, and (Z)-cyclo-
octene were very suitable for this strategy, providing
the desired products 2ai–ak in good to excellent
yields. When buta-1,3-dien-1-ylbenzene was used as
the substrate, the reaction did not yield the expected
oxyazidation product; instead, compound 2al’ was
acquired in a yield of 23%.[9] In addition, phenyl-
acetylene was also investigated, but only unidentifi-
able complex mixtures were obtained.
To demonstrate the synthetic potentiality of this
strategy, this reaction was applied to the late-stage
modification of complex molecules derived from
natural products and drugs (Figure 2). For example,
when estrone and deoxycholic acid derivatives were
used as substrates, the desired oxyazidation product
2an and 2ao were gained in good yields. Moreover,
styrenes derived from drugs such as aspirin, ibupro-
1
1.0 equiv. 1.0 equiv. CH3CN 79
2
1.1 equiv. 1.1 equiv.
CH3CN 89
CH3CN 89
CH3CN 85
3
1.5 equiv. 1.5 equiv.
1.1 equiv. 1.1 equiv.
1.1 equiv. 1.1 equiv.
1.1 equiv. 1.1 equiv.
1.1 equiv. 1.1 equiv.
1.1 equiv. 1.1 equiv.
1.1 equiv. 1.1 equiv.
1.1 equiv. 1.1 equiv.
1.1 equiv. 2.2 equiv.
4[c]
5
THF
PhCl
DMSO
DMF
DCM
49
63
0
28
47
6
7
8
9
10[d]
11[e]
CH3CN 15
CH3CN
0
[a] Unless otherwise specified, all reactions were carried out by
stirring a mixture of 1a (0.3 mmol, 1.0 equiv.), NaN3 and
TEMPO+BF4 in 2 mL of solvent under argon atmosphere
À
(1 atm) at room temperature for 24 h.
[b] Isolated yield.
[c] Under air atmosphere (1 atm).
[d] TMSN3 was used instead of NaN3.
[e] TEMPO was used instead of TEMPO+BF4 .
À
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Table 2. Oxyazidation of styrenes.[a,b]
Table 3. Oxyazidation of aliphatic alkenes.[a,b]
[a] Reaction conditions:
1
(0.3Àmmol, 1.0 equiv.), NaN3
(1.8 equiv.) and TEMPO+BF4 (1.8 equiv.) in CH3CN
(2 mL) at room temperature under argon atmosphere (1 atm)
for 24 h.
[b] Isolated yields.
[c] The yield of recovered substrate is listed in brackets.
[d] The ratio of diastereomers was determined by 1H NMR
spectroscopy.
[a] Reaction conditions:
1
(0.3Àmmol, 1.0 equiv.), NaN3
(1.1 equiv.) and TEMPO+BF4 (1.1 equiv.) in CH3CN
(2 mL) at room temperature under argon atmosphere (1 atm)
for 24 h.
[b] Isolated yields.
À
À
À
[c] NaN3 (1.7 equiv.) and TEMPO+BF4 (1.7 equiv.) were used.
[d] NaN3 (1.3 equiv.) and TEMPO+BF4 (1.3 equiv.) were used.
[e] NaN3 (2.1 equiv.) and TEMPO+BF4 (2.1 equiv.) were used.
[f] The ratio of diastereomers was determined by 1H NMR
spectroscopy.
fen, probenecid, and oxaprozin were also applicable
for this protocol, affording the expected products
2ap–as in moderate to excellent yields.
To further examine the practicability of this
protocol in the organic synthesis chemistry, a gram-
scale reaction of styrene 1a was performed under the
standard conditions. Cheerfully, the reaction still
Figure 2. Oxyazidation of natural product derivatives and drug
1
derivatives.[a] The ratio of diastereomers was determined by H
NMR spectroscopy.
worked smoothly and the product 2a was obtained are functional synthetic building blocks in organic
in 89% yield (Scheme 2). Moreover, β-oxy azides synthesis. Follow-up conversions of β-oxy azides
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Scheme 2. Gram-scale reaction and follow-up transformations
of 2a. For the reaction conditions in details, see Supporting
Information.
Scheme 4. Proposed mechanism.
can supply multifarious important N/O-containing
compounds that are potentially bioactive molecules
or significant organic synthetic precursors. Trans- ated vicinal oxyazidation process. As shown in
formations of product 2a via click reaction, Stau- Scheme 4, NaN3 is firÀstly oxidized to the azide
dinger reaction, reduction, and oxidation to afford radical by TEMPO+BF4 via single elecÀtron transfer
corresponding triazole, amine, alcohol, and α-azido- (SET). At the same time, TEMPO+BF4 is reduced
ketone have previously been described in the to TEMPO. Then, azide radical adds to alkene 1 to
literature.[5a] Notably, the α-azidoketone skeleton is give the carbon-centered radical A. Finally, A is
very useful in the synthesis of pyrroles,[10] trapped immediately by TEMPO, yielding the oxy-
imidazoles,[11] isoquinoline,[12] and other azacyclic azidation product 2.
compounds.[13] Furthermore, as illustrated in
In summary, a protocol for the vicinal oxy-
Scheme 2, some important synthetic intermediates azidation of alkenes has been developed. This
such as α-diazoketone 3, 4-phenyl-1H-1,2,3-triazole method employs NaN3 as theÀ azide source and
4, and N-(2-oxo-2-phenylethyl)benzamide 5 can be readily accessible TEMPO+BF4 as the oxidant as
obtained separately as well through corresponding well as the oxygen donor. Consequently, this proto-
subsequent transformations of 2a.
col has the advantages of transition metal free, broad
To gain insights into the reaction mechanism, the substrate applicability, mild conditions, simple oper-
control experiments were conducted as shown in ation, and late-stage functionalization of bioactive
Scheme 3. When dimethyl 2,2-diallylmalonate was molecules. By using this new method, various
subjected to the standard conditions, the cyclic alkenes can be very easily converted into β-amino-
product 7 was obtained in 9% yield, with 75% of 6 oxy azides. We hope that the reaction depicted
being recovered. When (2-vinylcyclopropyl)benzene herein will find applications in the synthesis of
was applied to the same conditions, the ring opening important bioactive compounds which contain vici-
product 9 was gained in 53% yield along with nal nitrogen and oxygen atoms. Further studies of
recovered substrate 8 in 18% yield. These results the oxoammonium salt-mediated reactions are in
demonstrated clearly that the oxyazidation reaction progress in our laboratory.
is triggered by the azide radical.
Based on the above results and previous
Experimental Section
literatures,[5e,14] a free radical mechanism is sug-
gested to rationalize the oxoammonium salt-medi-
General experimental procedure for synthesis of products 2
Substrates 1 À (0.30 mmol, 1.0 equiv.), NaN3 (1.1–2.2 equiv.),
TEMPO+BF4 (1.1–2.2 equiv.) and MeCN (2 mL) were added
to a 25 mL round bottom flask. The mixture was stired at room
temperature under argon atmosphere (1 atm) for 24 h. Upon
completion of the reaction, the solvent was then removed under
vacuo. The residue was purified with chromatography column
on silica gel (gradient eluent of EtOAc/petroleum ether: 1/100
to 1/1) to give the corresponding product 2.
Scheme 3. Mechanistic investigations.
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Hossain, A. Vidyasagar, C. Eichinger, C. Lankes, J.
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We gratefully acknowledge the National Natural Science
Foundation of China (22001012), the Foundation of Henan
Educational Committee (21A150008 and 20B150003), the
Foundation of Henan Science and Technology Department
(212102311061), the Foundation of Anyang Science and
Technology Bureau (2021C01GX016), and the Foundation of
Anyang Normal University (AYNUKPY-2019-08 and AYNUK-
PY-2019-10) for financial support. We thank Dr. Jun-Jie Wang
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Oxoammonium Salt-Mediated Vicinal Oxyazidation of
Alkenes with NaN3: Access to β-Aminooxy Azides
Adv. Synth. Catal. 2021, 363, 1–7
F. Chen*, Y.-T. Tang, X.-R. Li, Y.-Y. Duan, C.-X. Chen, Y.
Zheng*