Oxime Ether Synthesis through O-H Functionalization of Oximes with Diazo Esters under Blue LED Irradiation

Article abstract of DOI:10.1021/acs.orglett.1c02555

A green and sustainable oxime ether formation method via the visible-light-promoted O-H functionalization of oximes with diazo esters is described. The reaction occurs under very mild conditions (catalyst- and additive-free) with a high yield and a high functional group tolerance. When the reaction was performed with a cyclic ether as the solvent (e.g., THF, 1,4-dioxane, tetrahydropyran, ect.), an interesting photochemical three-component reaction product was obtained in good yields.

Full text of DOI:10.1021/acs.orglett.1c02555

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Letter
Oxime Ether Synthesis through O−H Functionalization of Oximes
with Diazo Esters under Blue LED Irradiation
Qian Li, Bao-Gui Cai, Lei Li, and Jun Xuan*
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ABSTRACT: A green and sustainable oxime ether formation method via the visible-light-promoted O−H functionalization of
oximes with diazo esters is described. The reaction occurs under very mild conditions (catalyst- and additive-free) with a high yield
and a high functional group tolerance. When the reaction was performed with a cyclic ether as the solvent (e.g., THF, 1,4-dioxane,
tetrahydropyran, ect.), an interesting photochemical three-component reaction product was obtained in good yields.
xime ether is one of the most important organic
Fenpyroximate is a broad spectrum acaricide.2f,g Driven by
1
Ocompounds. Molecules containing oxime ether moieties their rich biological activities, the development of efficient and
can not only be frequently found in various medicines, showing
excellent drug activity,2 but can also be applied as the synthetic
building blocks in synthetic organic chemistry.3 Some
compounds, such as the selected examples are shown in
Scheme 1, are used to treat infections caused by bacteria,
including the antifungal drug oxiconazole and the antibacterial
drug roxithromycin, which contain the oxime ether scaffol-
d.2a−c Fluvoxamine is used to treat obsessive-compulsive
disorder and exhibits antidepressant activity.2d,e In addition,
practical methods for the synthesis oxime ether is of
widespread interest in synthetic organic chemistry.
In the past several years, a great deal of effort has been
reported for oxime ether synthesis in both industry and
academia. Traditionally, oxime ethers can be accessed through
the condensation of carbonyl compounds with hydroxyl-
amines, which usually requires additional catalysts or additives
to accelerate the dehydration process (Scheme 2a).4 The
cross-coupling of oximes with arylboronic acids or organic
halides is another elegant route to form oxime ether (Scheme
2b).5,6 However, those methods generally require the use of
transition-metal catalysts in high reaction temperatures. In
addition, the allylic substitution of oximes in the presence of
transition-metal catalysts to obtain oxime ether has also
attracted much attention (Scheme 2c).7 Compared with those
well-developed methods, the continuous development of green
and efficient oxime ether synthesis methods without the
addition of catalysts and additives is still appealing.
Scheme 1. Bioactive Molecule Structures Containing Oxime
Ether Motifs
Chemical synthesis using visible light as a green energy
source has attracted much attention in the past decades.8 In
this regard, the photopromoted functionalization of α-diazo
esters though the generation of a free carbene as the key
intermediate was first discovered by Davies et al. in 20189 and
further developed by many other groups.10,11 As one of most
Received: August 1, 2021
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© XXXX American Chemical Society
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component coupling product could be obtained when THF
was used as the reaction solvent (Table 1, entry 6).14 Finally,
the control experiment revealed that light irradiation was
crucial for the formation of the desired oxime ether (Table 1,
entry 7).
Scheme 2. Methods for the Synthesis of Oxime Ethers
After the optimal reaction conditions were established, the
substrate scope and limitations of the method were
subsequently explored. As shown in Table 2, α-diazo esters
Table 2. O−H Functionalization of Oximes with α-Diazo
a b
,
Esters in DCE
representative reaction activities, insertions of the formed
carbene species into the O−H bond of carboxylic acids,
phenols, and alcohols have been reported recently.12
Motivated by those elegant findings, we question whether
the photogenerated carbene species from α-diazo esters might
also be trapped by oximes, thus providing an alternative
method to access oxime ether derivatives (Scheme 2d).13
Compared with reported oxime ether formation processes, this
designed photochemical reaction occurred under green and
sustainable reaction conditions without the requirement of any
additional catalysts or additives. Herein, we would like to
describe the preliminary results of this study.
Initially, methyl 2-diazo-2-phenylacetate 1 and (E)-3,4-
dimethoxybenzaldehyde oxime 2 were selected as model
substrates to optimize the reaction conditions (Table 1). It
a
Table 1. Condition Optimization
b
entry
solvent
yield (%)
1
2
3
4
5
6
DCM
CH3CN
DCE
DMSO
DMF
THF
61
53
70
trace
trace
trace
N.R.
a
Reaction performed with an α-diazo ester (0.6 mmol) and oxime
(0.3 mmol) in dry DCE (3.0 mL) at rt under irradiation with 24 W
blue LEDs for 8 h. Isolated yield. Performmed at a 1.0 mmol scale.
b
c
c
7
DCE
a
Reaction was performed with 1 (0.6 mmol) and 2 (0.3 mmol) in dry
solvent (3.0 mL) at rt under irradiation with 24 W blue LEDs for 8 h.
with different electron-neutral, electron-donating (−Me), or
electron-withdrawing (−Cl) substituents at the para-position
on the phenyl ring were well-tolerated, affording oxime ethers
3, 5, and 6, respectively, in good yields (58−71%). Apart from
phenyl diazoacetate, substrates bearing a 2-naphthyl or
benzo[d][1,3]dioxole moiety were also amenable substrates,
and the corresponding oxime ethers 7 and 8 could be isolated
in 58% and 81% yields, respectively. Next, we turned our
attention to the substituent modification of the ester group in
aryldiazoacetates. Replacing a methyl group in the aryldiazoa-
cetates with a cyclic alkyl substituent or alkyl substituents
containing sensitive functional groups, such as unsaturated
double and triple bonds, successfully produced the correspond-
ing oxime ethers (9−11) in moderate to good yields (54−
70%). Note that the structure of 9 was unambiguously
b
c
Isolated yield. In dark. N.R. , no reaction.
was found that the desired oxime ether 3 could be obtained in
a 61% isolated yield after irradiating the reaction mixture in
DCM for 8 h (Table 1, entry 1). Encouraged by this
preliminary result, the influence of other reaction media was
systematically investigated to further improve the reaction
yield. Apart from DCM, CH3CN and DCE were also suitable
for the formation of the desired oxime ether 3, and DCE was
determined as the best reaction medium (Table 1, entries 2
and 3, respectively). In contrast, only a trace amount of oxime
ether 3 was detected when the reaction was performed in
strong polar solvents, such as DMF and DMSO (Table 1,
entries 4 and 5, respectively). To our delight, a three-
B
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confirmed by X-ray diffraction analysis. To further demonstrate
the synthetic value this visible-light-promoted oxime ether
formation process, we introduced some bioactive complex
molecules, such as L-(−)-borneol, cholesterol, and L-menthol,
into the structure of aryldiazoacetates. To our delight, the
corresponding bioactive complex-molecule-modified oxime
ethers could be obtained in good yields (12−14, 50−71%).
In addition, replacing diazo esters with aryl/aryl diazoalkane
also proved successful, affording oxime ether 15 in a 52%
isolated yield.
apart from phenyl oximes, providing the corresponding
products (34 and 35) in good yields. Note that aliphatic
aldehyde was also a suitable oxime precursor and formed
product 36 in a 63% yield. Moreover, when an oxime derived
from trifluoroacetophenone was involved, the target trifluor-
omethyl-containing oxime ether 37 was obtained in a 72%
yield.
To further show the synthetic potential of this method, we
examined the compatibilities of different cyclic ethers (Table
4). As the structural analogues of THF, 2,3-dihydrofuran and
Next, we evaluated the scope of the oxime components.
Different aryl-substituted aldoximes and acetophenone-derived
oximes reacted well under the standard reaction conditions to
give the corresponding oxime ethers in moderate to good
yields (16−19). In addition, this O−H insertion reaction can
be further extended to modify N-hydroxyphthalimide, giving
oxime ether 20 in a 53% yield.
Table 4. O−H Functionalization of Oximes with α-Diazo
a b
,
Esters in Other Cyclic Ethers
As mentioned above, a three-component coupling oxime
ether product 4 was obtained in a 78% isolated yield by
performing the reaction in THF. Intrigued by this observation,
we next studied the scope of different α-diazo esters for this
visible-light-promoted three-component oxime ether formation
reaction (Table 3). Various halogen-substituted aryldiazoace-
Table 3. O−H Functionalization of Oximes with α-Diazo
a b
,
Esters in THF
a
Reaction performed with an α-diazo ester (0.2 mmol) and oxime
(0.1 mmol) in cyclic ethers (1.0 mL) at rt under irradiation with 24 W
b
blue LEDs for 8 h. Isolated yield.
2,5-dihydrofuran reacted smoothly to give oxime ethers 38 and
39, respectively, in moderate yields. To our delight, the
method could be applied to the synthesis of the cyclohexane-
tethered oxime ether 40 in a 42% yield by using 7-
oxabicyclo[2.2.1]heptane as a carbene trapping reagent.
Apart from five-membered cyclic ethers, the replacement of
tetrahydrofuran with tetrahydropyran and 1,4-dioxane success-
fully afforded oxime ethers 41 and 42 in 64% and 65% yields,
respectively. We also tested the efficiencies of other cyclic
heterocycles, such as ethylene oxide, propylene oxide,
tetrahydrothiophene, and N-methylpiperidine. Unfortunately,
none of those reactions could give the desired three-
component coupling products under the optimal conditions.
FFor more details about unsuccessful substrates, see the
Supporting Information.
a
Reaction performed with an α-diazo ester (0.6 mmol) and oxime
(0.3 mmol) in dry THF (3.0 mL) at rt under irradiation with 24 W
blue LEDs for 8 h. Isolated yield.
b
tates were well-tolerated, leading to oxime ethers (21−24) in
average to good yields (72−85%). When using cyclopentyl and
adamantane formic acid-derived aryldiazoacetates, the desired
oxime ethers (25 and 26, respectively) can be obtained with
high efficiencies. The successful introduction of a citronellol
fragment into the final oxime ether 27 further disclosed the
advantage of this strategy. Then, we turned our attention to
examining the scope of the oxime components. Oximes with
various electron-rich (−Me,) or electron-deficient (−Cl, −Br,
and −CN) groups at meta- or para- positions of the phenyl
ring could be utilized as suitable substrates in the reaction,
affording products 28−33 in moderate to good yields (56−
85%). To our delight, (E)-nicotinaldehyde oxime and (E)-
thiophene-2-carbaldehyde oxime are also suitable substrates
We conducted a gram-scale synthesis using the reaction of 2-
diazo-2-phenylacetate and benzaldehyde oxime in THF as an
example under continuous flow reaction conditions (Section 6
in the Supporting Information). After 8 h of irradiation in a
continuous flow, oxime ether 28 still could be isolated in an
83% yield.
Based on the literature reports,10−12 a plausible reaction
mechanism was proposed to explain the oxime ether formation
process (Section 7 in the Supporting Information). Under
irradiation with a blue LED, the photolysis of α-diazo esters
C
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gives the carbene intermediate A. The insertion of A into the
O−H bond of an oxime afforded the corresponding oxime
ether when the reaction was performed in DCE (path A).12 In
THF, trapping the carbene species A with the solvent delivered
the ylide intermediate B.10,11f Then, the protonation of B,
followed by the ring opening of C, furnished the final three-
component coupling product (path B).
In summary, we have developed a visible-light-promoted O−
H functionalization of oximes with diazo esters for the
synthesis of oxime ether derivatives. In contrast to conven-
tional methods, the reaction described herein proceeded under
extremely mild conditions without any additional additives and
catalysts. An interesting three-component coupling product
was observed when the reaction was performed in THF. The
broad substrate scope, excellent functional group tolerance,
and large-scale preparation in continuous flow conditions
further documented the synthetic potential of this method-
ology.
https://pubs.acs.org/10.1021/acs.orglett.1c02555
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We are grateful to the National Natural Science Foundation of
China (nos. 21971001 and 21702001) for financial support.
■
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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.1c02555.
1
Experimental procedures, characterization data, and H
and 13C NMR spectra (PDF)
Accession Codes
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AUTHOR INFORMATION
Corresponding Author
■
Jun Xuan − Anhui Province Key Laboratory of Chemistry for
Inorganic/Organic Hybrid Functionalized Materials, College
of Chemistry & Chemical Engineering, Anhui University,
Hefei, Anhui 230601, People’s Republic of China; Key
Laboratory of Structure and Functional Regulation of Hybrid
Materials, Ministry of Education, Anhui University, Hefei
230601, People’s Republic of China; Key Laboratory of
Precise Synthesis of Functional Molecules of Zhejiang
Province, School of Science, Westlake University, Hangzhou
310024 Zhejiang Province, China; orcid.org/0000-0003-
0578-9330; Email: xuanjun@ahu.edu.cn
Authors
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Bao-Gui Cai − Anhui Province Key Laboratory of Chemistry
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College of Chemistry & Chemical Engineering, Anhui
University, Hefei, Anhui 230601, People’s Republic of China
Lei Li − Anhui Province Key Laboratory of Chemistry for
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E
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