Copper-Catalyzed Three-Component Domino Cyclization for the Synthesis of 4-Aryl-5-(arythio)-2-(trifluoromethyl)oxazoles

Article abstract of DOI:10.1021/acs.orglett.9b02934

A copper-catalyzed oxidative cyclization of oxime, arylthiol, and trifluoroacetic anhydride for the construction of trisubstituted oxazoles has been developed. This transformation combines N-O bond cleavage, C-H functionalization, and intramolecular annulation, providing a practical protocol for the introduction of a trifluoromethyl (-CF₃) group at oxazole rings.

Full text of DOI:10.1021/acs.orglett.9b02934

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Copper-Catalyzed Three-Component Domino Cyclization for the
Synthesis of 4‑Aryl-5-(arythio)-2-(trifluoromethyl)oxazoles
Fuhong Xiao,*,† Shanshan Yuan,† Huawen Huang,*,† Feng Zhang,‡ and Guo-Jun Deng†
†Key Laboratory of Environmentally Friendly Chemistry and Application of Ministry of Education, College of Chemistry, Key
Laboratory for Green Organic Synthesis and Application of Hunan Province Xiangtan University, Xiangtan 411105, China
‡College of Science, Hunan Agricultural University, Changsha 410128, China
S
* Supporting Information
ABSTRACT: A copper-catalyzed oxidative cyclization of
oxime, arylthiol, and trifluoroacetic anhydride for the
construction of trisubstituted oxazoles has been developed.
This transformation combines N−O bond cleavage, C−H
functionalization, and intramolecular annulation, providing a
practical protocol for the introduction of a trifluoromethyl
(−CF3) group at oxazole rings.
he oxazole skeleton is an important N-heterocyclic
structural unit, existing in naturally occurring compounds
T
and drugs.1 Therefore, a series of efficient and simple methods
for synthesizing polysubstituted oxazoles have been devel-
oped.2 Despite these applications, few methods for the
introduction of a trifluoromethyl (−CF3) group onto oxazole
rings,3 which may change the metabolic stability, lipophilicity,
and biological properties of oxazole compounds,4 have been
reported. In this regard, the development of efficient methods
for preparing trifluoromethyl-decorated oxazoles has long been
a hot and challenging topic in molecular synthesis.
Oximes play an important role in the area of synthetic
organic chemistry because of the prevalent molecular skeleton
for various organic transformations.5 In particular, O-acyl
oximes have been among the most extensively studied in an
attempt to provide viable precursors of active imino radicals
due to their stability and ease of preparation.6 Over the past
decade, major improvements in the development of transition
metal-catalyzed transformations of methylketoxime esters have
been realized, especially in the synthesis of nitrogen hetero-
cycles such as thiazoles,7 pyridines,8 pyrazoles,9 pyrroles,10
imidazo[1,2-a]pyridines,11 etc. (Figure 1, a−e). Very recently,
we have developed novel approaches for synthesis of
benzo[4,5]thieno[3,2-d]thiazoles via C−H bis-heteroannula-
tion of acetophenone oximes with elemental sulfur.12 To the
best of our knowledge, there are few methods for constructing
the biologically important trifluoromethyl nitrogen-containing
heterocycles using arylmethylketoximes as a versatile N−C−C
building block.13 Herein, we disclose the copper-catalyzed
three-component oxidative annulation reactions of ketoximes,
trifluoroacetic anhydride, and arylthiols, which provide efficient
access to 4-aryl-5-(arythio)-2-(trifluoromethyl)oxazoles (Fig-
ure 1, f). Hence, salient features of our finding include (a) easy
operational handling, (b) readily available starting materials,
Figure 1. Copper-catalyzed N-heteroarenes from methylketone
oximes.
(c) access to highly functionalized trifluoromethyl oxazoles,
and (d) broad functional group tolerance.
We commenced our studies by exploring the three-
component tandem cyclization of acetophenone oxime 1a, 4-
methylbenzenethiol 2a, and trifluoroacetic anhydride 3a
(Table 1). To our delight, the domino cyclization proceeded
smoothly to afford product 4a in 65% yield using Cu(OAc)2 as
the catalyst, N-iodosuccinimide (NIS) as the additive, and
K2S2O8 as the oxidant in DCE (Table 1, entry 1). Encouraged
Received: August 18, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b02934
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Organic Letters
Letter
a
a
Table 1. Optimization of the Reaction Conditions
Scheme 1. Scope of Ketoximes
b
entry
catalyst
additive
oxidant
K2S2O8
solvent
yield (%)
1
2
3
4
5
6
7
8
Cu(OAc)2
CuSO4
Cu(OTf)2
Cu2O
CuI
CuCl2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
−
NIS
NIS
NIS
NIS
NIS
NIS
I2
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
PhCH3
CH3CN
TTCE
DMF
DMSO
DCE
DCE
DCE
65
58
76
57
49
56
52
67
62
60
40
42
50
37
70
63
64
0
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
Na2S2O8
(NH4)2S2O8
oxone
ICl
9
NaIO4
IBX
NIS
NIS
NIS
NIS
NIS
NIS
NIS
NIS
NIS
NIS
−
10
11
12
13
14
15
16
17
18
19
20
21
22
S
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
−
0
20
22
5
Cu(OTf)2
Cu(OTf)2
NIS
a
Conditions: 1 (0.2 mmol), 2a (0.3 mmol), 3a (0.8 mmol),
a
Conditions: 1a (0.2 mmol), 2a (0.3 mmol), 3a (0.8 mmol), catalyst
Cu(OTf)2 (5 mol %), NIS (0.12 mmol), K2S2O8 (0.3 mmol), DCE
(5 mol %), additive (0.12 mmol), oxidant (0.3 mmol), solvent (1
mL), 80 °C, 20 h, air. Isolated yield.
b
(1 mL), 80 °C, 20 h, air. Isolated yield based on 1. On a 6 mmol
b
c
scale. 2a (2.4 mmol), 3a (3.2 mmol).
by this result, we screened other copper catalysts, including
CuSO4, Cu(OTf)2, Cu2O, CuI, and CuCl2 (entries 2−6,
respectively). Among them, Cu(OTf)2 was the most effective,
resulting in the formation of 4a in 76% yield (entry 3).
Replacing N-iodosuccinimide with other iodine-containing
additives such as I2, ICl, NaIO4, and IBX led to lower yields
(entries 7−10, respectively). A suitable oxidant is crucial to this
reaction. The use of Na2S2O8, (NH4)2S2O8, oxone, and sulfur
significantly decreased the reaction yields (entries 11−14,
respectively). Solvents also played an important role. Moderate
yields were obtained when reactions were carried out in
toluene, acetonitrile, and 1,1,2,2-tetrachloroethane (TTCE)
(entries 15−17, respectively). DMF and DMSO as the reaction
media completely quenched the reaction (entries 18 and 19,
respectively). The control experiments in the absence of a
copper catalyst or NIS decreased the product yields to ∼20%
(entry 20 or 21, respectively). An only 5% yield of 4a was
achieved without an external oxidant (entry 22).
With the effective oxidative cyclization procedure estab-
lished, the substrate scope of the copper-catalyzed three-
component oxidative annulation was probed. First, various aryl
methyl ketoximes were reacted with 4-methylbenzenethiol
(2a) and trifluoroacetic anhydride (3a) (Scheme 1). The
isolated yield of model reaction product 4a is 76%. Moreover,
when the reaction was conducted on a 6.0 mmol scale, the
yield of 4a slightly decreased to 70%. Acetophenone oximes
with methyl, isobutyl, and phenyl electron-donating groups
were quite compatible, giving the desired products 4b−4d in
65%, 38%, and 61% yields, respectively. The structure of 4d
was confirmed by X-ray crystallography. Good to high yields
were obtained with ketoximes with the fluoro, chloro, bromo,
and trifluoromethyl substituents at the para or meta positions
on aromatic rings (4e−4g and 4m−4o). The steric effect of
the substituents is obvious in this oxidative cyclization (82%
for p-Br vs 49% for o-Br). Other substrates with electron-
withdrawing groups such as nitro and cyano performed well in
the reaction, affording products 4h and 4i, respectively, in high
yields. Notably, ester or sulfone groups on the benzene ring
were also feasible for this transformation (4j or 4k,
respectively). Aryl methyl ketoximes with two functional
groups reacted well and gave the trisubstituted oxazole in
good yields (4q and 4r). Furthermore, 1-(naphthalen-2-
yl)ethanone oxime and 1-(phenanthren-3-yl)ethanone oxime
afforded products 4s and 4t in 60% and 47% yields,
respectively. The heteroaromatic substrate bearing an indole
moiety underwent cyclization with 2a and 3a to afford the
corresponding product 4v, albeit in 28% yield. Active amino or
hydroxyl group substituents on acetophenone oxime were
acylated by trifluoroacetic anhydride, and the desired products
4w−4y were obtained in 55%, 63%, and 68% yields,
respectively. However, the active hydroxyl group was not
acylated by trifluoroacetic anhydride when using 1-(4-hydroxy-
3-nitrophenyl)ethan-1-one oxime as a substrate, giving an
excellent yield (4u). Notably, 4,4′-diacetylbiphenyl-derived
oximes could give 4z in a 40% yield.
To further investigate the scope and limitation of this three-
component system, various thiophenols were examined under
B
DOI: 10.1021/acs.orglett.9b02934
Org. Lett. XXXX, XXX, XXX−XXX

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Letter
the optimized reaction conditions (Scheme 2). Good yields
Scheme 3. Control Experiments
were obtained with electron-donating groups (−tBu and
a
Scheme 2. Scope of Thiophenols and Anhydrides
On the basis of the control experiments and previous
reports,13,14 a possible mechanism was proposed in Scheme 4.
a
Conditions: 1a (0.2 mmol), 2 (0.3 mmol), 3a (0.8 mmol),
Scheme 4. Tentative Reaction Mechanism
Cu(OTf)2 (5 mol %), NIS (0.12 mmol), K2S2O8 (0.3 mmol), DCE
(1 mL), 80 °C, 20 h, air. Isolated yield based on 1a. Diphenyl
disulfide.
b
−OCH3) at the para position of the benzene ring (4ab and
4ac). Thiophenol derivatives containing halogen substituents,
including fluoro, chloro, and bromo, were also available for
cyclization, giving the desired products in moderate yields,
with no cleavage of the aryl−halide bond observed (4ad−4af).
The steric effects of the substituents are apparent. For example,
when the methyl was located in the para, ortho, and meta
positions on the benzene ring, the corresponding products 4a,
4ag, and 4ak were obtained in 76%, 48%, and 66% yields,
respectively. Moderate yields were observed when 2-ethyl, 2-F,
and 2-Br thiophenols were used as substrates (4ah−4aj,
respectively). To be specific, upon treatment with 2,3-
dichlorobenzenethiol as the coupling reagent, the yield of the
desired product 4al sharply decreased to 22%. Naphthalene-1-
thiol was compatible with the reaction but gave 4am in a lower
yield. It is noteworthy that diphenyl disulfide was also a
suitable substrate in this reaction (4an). In current systems,
acetic anhydride afforded 4ao in 60% yield. Among other
polyfluorinated substituted anhydrides, pentafluoropropionic
acid anhydride and perfluorobutyric anhydride could also react
to afford products (4ap and 4aq, respectively) in modest
yields. Unfortunately, no corresponding products were
obtained when alkylthiols were used as substrates.
To understand the mechanistic information, some control
experiments were performed (Scheme 3). When we directly
used O-trifluoroacetyl oximes 1a′ as the substrate, the desired
product 4a was formed in 70% yield (Scheme 3, a). During the
optimization of the reaction, an only 5% yield of 4a was
achieved in the absence of K2S2O8 (Table 1, entry 22), but
thioenamine 4a′ was observed in 62% yield when acetophe-
none oxime (1a) reacted with thiophenol (2a) and trifluoro-
acetic anhydride (3a) (Scheme 3, b). Finally, oxidative
annulations of 4a′ occurred to produce the corresponding
product 4a in 83% yield with the assistance of only K2S2O8
(Scheme 3, c).
First, p-toluenethiol 2a was oxidized by NIS to form sulfur
radical intermediate 2a′. On the other hand, oxime
trifluoroacetate 1a′ is generated in situ from the acylation of
ketoxime. Then, imino radical A is formed from 1a′ by single-
electron transfer (SET) with Cu(I), which is formed in situ by
disproportionation from Cu(II). Intermediate A becomes
intermediate B by sequential acylation and isomerization,
which coordinates with Cu(I) and 2a′ to generate alkyl-
Cu(III) intermediate C. Then, reductive elimination of C
followed by isomerization occurs to form enamine inter-
mediate product 4a′. The SET oxidation of 4a′ generates
intermediate E, which further delivers intermediate F via a
radical isomerization process. Cyclo radical intermediate G is
generated by intramolecular cyclization of F, which may also
be promoted by a copper catalyst. The single-electron
oxidation of intermediate G followed by deprotonation gives
final product 4a.
In summary, we have developed a copper-catalyzed oxidative
system that enabled the highly selective formation of 4-aryl-5-
(arythio)-2-trifluoromethylated oxazoles. With O-trifluoroace-
C
DOI: 10.1021/acs.orglett.9b02934
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b02934.
1
Experimental procedures, characterization data, and H,
13C, and 19F NMR spectra for all new products (PDF)
Accession Codes
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CCDC 1954698 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-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: fhxiao@xtu.edu.cn.
*E-mail: hwhuang@xtu.edu.cn.
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ORCID
Fuhong Xiao: 0000-0003-0862-0557
Huawen Huang: 0000-0001-7079-1299
Guo-Jun Deng: 0000-0003-2759-0314
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
Support by the National Natural Science Foundation of China
(21602187 and 21502160) and the Collaborative Innovation
Center of New Chemical Technologies for Environmental
Benignity and Efficient Resource Utilization is gratefully
acknowledged.
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