Direct synthesis of benzoxazinones via Cp*Co(III)-catalyzed C–H activation and annulation of sulfoxonium ylides with dioxazolones

Article abstract of DOI:10.1016/j.cclet.2020.09.020

A highly novel and direct synthesis of benzoxazinones was developed via Cp*Co(III)-catalyzed C–H activation and [3 + 3] annulation between sulfoxonium ylides and dioxazolones. The reaction is conducted under base-free conditions and tolerates various functional groups. Starting from diverse readily available sulfoxonium ylides and dioxazolones, a variety of benzoxazinones could be synthesized in one step in 32%-75% yields.

Full text of DOI:10.1016/j.cclet.2020.09.020

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CCLET 5849 No. of Pages 4
Chinese Chemical Letters xxx (2019) xxx–xxx
Contents lists available at ScienceDirect
Chinese Chemical Letters
journal homepage: www.elsevier.com/locate/cclet
Communication
Direct synthesis of benzoxazinones via Cp*Co(III)-catalyzed C–H
activation and annulation of sulfoxonium ylides with dioxazolones
Yongqi Yua, Zhen Xiaa, Qianlong Wua, Da Liua, Lin Yua, Yuanjiu Xiaoa, Ze Tana,
,
*
b
Wei Denga, , Gangguo Zhu
*
a
State Key Laboratory of Chemo/Biosensing and Chemometrics, College of Chemistry and Chemical Engineering, Hunan University, Changsha 410082, China
Department of Chemistry, Zhejiang Normal University, Jinhua 321004, China
b
A R T I C L E I N F O
A B S T R A C T
Article history:
Received 19 July 2020
Received in revised form 14 September 2020
Accepted 15 September 2020
Available online xxx
A highly novel and direct synthesis of benzoxazinones was developed via Cp*Co(III)-catalyzed C–H
activation and [3 + 3] annulation between sulfoxonium ylides and dioxazolones. The reaction is
conducted under base-free conditions and tolerates various functional groups. Starting from diverse
readily available sulfoxonium ylides and dioxazolones, a variety of benzoxazinones could be synthesized
in one step in 32%-75% yields.
© 2020 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences.
Published by Elsevier B.V. All rights reserved.
Keywords:
C–H activation
Cobalt catalysis
Annulations
Sulfoxonium ylides
Synthetic methods
Benzoxazinones are an important class of fused heterocycles
that are widely present in diverse natural products and pharma-
ceuticals [1], such as TEI-5624 [2], Cetilistat [3], and AX-9657
(Fig. 1) [4]. In addition, they are also valuable intermediates in
organic synthesis [5]. For example, 2-substituted benzoxazinones
could be converted to bioactive quinazolinone derivatives. As a
result, developing efficient protocols for the synthesis of benzox-
azinones has been an important research topic in organic synthesis
[6–9]. Conventionally, benzoxazinones can be synthesized through
the annulation of N-acyl anthranilic acid or anthranilic acid [7].
Furthermore, benzoxazinones can also be prepared via Pd-
catalyzed carbonylation of ortho-haloanilines [8] and oxidation
of indoles [9]. Despite all these progress, most of these approaches
tend to suffer from either requiring prefunctionalized substrates or
harsh reaction conditions. Recently, transition-metal-catalyzed C–
H activation has become a powerful synthetic tool in contemporary
organic synthesis due to its high atom/step economy [10]. Hence,
developing direct and efficient methods for the construction of
benzoxazinones via C–H activation mode can be of great
significance.
benzoxazinones can be synthesized via Pd(II)-catalyzed C–H
carbonylation of aniline derivatives (Scheme 1a). It should be
noted that these reactions require the use of toxic carbon
monoxide gas. Kim [11c], Wang [11d] and Dong [11e] reported
Rh(III) or Ir(III)-catalyzed C–H amidation of aldehydes or their
equivalents to deliver 2-aminobenzaldehydes, which could further
undergo intramolecular oxidative cyclization to give benzoxazi-
nones (Scheme 1b). In these cases, the additional oxidation step
was essential for the synthesis of benzoxazinones. Very recently,
Zou reported a Rh(III)-catalyzed cascade reaction of benzoic acids
with dioxazolones for the construction of 2,5-substituted benzox-
azinones (Scheme 1c) [11g]. Though highly efficient, due to the cost
issues associated with the use of palladium, iridium and rhodium
metals, it would be highly desirable if similar transformations can
be accomplished by employing cheaper catalysts. Recent studies
have shown that Cp*Co(III) catalysts are not only promising
alternatives to Cp*Rh(III) catalysts, they also show unique catalytic
reactivity in C–H activation [12]. It is worth noting that enormous
advances in this area have been accomplished by the groups of
Cheng [13], Kanai [14], Matsunaga [15], Chang [16], Ackermann
[17], Li [18], Glorius [19], Shi [20] and others [21]. Inspired by these
studies and in continuation of our research on C–H bond
functionalizations [22], we herein present a direct and practical
method for the synthesis of benzoxazinones via Cp*Co(III)-
catalyzed C–H activation and [3 + 3] annulation reaction of
sulfoxonium ylides [23] with dioxazolones (Scheme 1d). It should
be mentioned that during the course of our investigation, Li
Indeed, the preparation of benzoxazinones through transition-
metal-catalyzed C–H activation has been reported (Scheme 1) [11].
Lloyd-Jones, Booker-Milburn [11a] and Yu [11b] reported that
* Corresponding authors.
E-mail addresses: ztanze@gmail.com (Z. Tan), weideng@hnu.edu.cn (W. Deng).
https://doi.org/10.1016/j.cclet.2020.09.020
1001-8417/© 2020 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences. Published by Elsevier B.V. All rights reserved.
Please cite this article as: Y. Yu, Z. Xia, Q. Wu et al., Direct synthesis of benzoxazinones via Cp*Co(III)-catalyzed C–H activation and annulation of
sulfoxonium ylides with dioxazolones, Chin. Chem. Lett., https://doi.org/10.1016/j.cclet.2020.09.020

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Table 1
Optimization of the reaction conditionsa.
Entry
Catalyst
Ag salt
Solvent
Additive
Yield (%)b
1
2
3
4
5
6
7
8
Cp*Co(CO)I2
Cp*Co(CO)I2
Cp*Co(CO)I2
Cp*Co(CO)I2
Cp*Co(CO)I2
Cp*Co(CO)I2
Cp*Co(CO)I2
Cp*Co(CO)I2
Co2(CO)8
AgSbF6
AgSbF6
AgSbF6
AgSbF6
AgSbF6
AgSbF6
AgSbF6
AgSbF6
AgSbF6
AgSbF6
AgOTf
AgOAc
AgSbF6
AgSbF6
AgSbF6
AgSbF6
AgSbF6
None
DCE
DCE
DCE
DCE
None
NaOAc
Zn(OAc)2
KOAc
PivOH
None
None
None
None
None
None
None
None
None
None
None
None
None
69
65
61
63
DCE
58
Fig. 1. Biologically active benzoxazinones.
1,4-dioxane
CH3CN
TFE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
Trace
NDg
NDg
NDg
NDg
43
9
10
11
12
13c
14d
15e
16f
17
18
Co(acac)3
Cp*Co(CO)I2
Cp*Co(CO)I2
Cp*Co(CO)I2
Cp*Co(CO)I2
Cp*Co(CO)I2
Cp*Co(CO)I2
None
NDg
67
58
71
68
NDg
NDg
Cp*Co(CO)I2
a
Unless otherwise noted, the reactions were performed using 1a (0.2 mmol), 2a
(0.3 mmol), Cp*Co(CO)I2 (10 mol%), AgSbF6 (25 mol%) and additive (20 mol%) in a
solvent (2 mL) at 130 ꢀC for 18 h under N2.
b
Isolated yield.
c
At 140 ꢀC.
d
At 120 ꢀC.
Under air atmosphere.
Under O2 atmosphere.
ND: not detected.
e
f
g
lowered to 70 ꢀC, only ortho C–H amidation product of sulfoxonium
ylide was obtained, and no cyclization product 3aa was detected.
When the reaction was carried out under an air atmosphere, the
yield of 3aa could be improved to 71% (entry 15). Control
experiments revealed that both Cp*Co(CO)I2 and AgSbF6 were
necessary for the formation of 3aa (entries 17 and 18). It should be
mentioned that when another amidating reagent 5-phenyl-1,3,2,4-
dioxathiazole 2-oxide was employed in place of 2a, no desired
product was detected (not shown in Table 1). Therefore, we
decided to set reacting 1a with 1.5 equiv of 2a in the presence of
10 mol% of Cp*Co(CO)I2 and 25 mol% of AgSbF6 in DCE under air at
130 ꢀC for 18 h as our optimized reaction conditions. It should be
noted that only 5 mol% of cobalt catalyst is enough to obtain similar
yields when the reaction is run at larger scale, details see
Supporting information.
We next examined the substrate scope with respect to the
sulfoxonium ylides under the optimized conditions, and the results
are summarized in Scheme 2. It was observed that sulfoxonium
ylides substituted with electron-donating or electron-withdraw-
ing groups on the phenyl rings all reacted smoothly with
dioxazolone 2a to provide the corresponding benzoxazinones in
moderate to good yields (3aa-3oa). From the table, we can see that
when ortho-substituted sulfoxonium ylides were used as the
substrates, the corresponding products were obtained with lower
yields (3ba and 3ca), which is perhaps due to steric hindrance. As
for the meta-substituted sulfoxonium ylides, the C–H activation
occurred exclusively on the less hindered site (3da-3ga). It should
be mentioned that 3,4-disubstituted sulfoxonium ylide 1n was
found to be a viable substrate, affording the desired product 3na in
56% yield. Furthermore, sulfoxonium ylide with a naphthalene ring
could also participate in the reaction to give the desired product
3oa in 61% yield. Unfortunately, heterocyclic sulfoxonium ylides
Scheme 1. Comparison of previous works with this work.
reported a similar cobalt(III)-catalyzed C–H functionalization of
sulfoxonium ylides with dioxazolones under different reaction
conditions and only amidation products were obtained
(Scheme 1e) [24].
Our initial plan was to realize Cp*Co(III)-catalyzed C(aryl)–H
amidation of sulfoxonium ylides with dioxazolones as the
amidating reagents. Much to our surprise, when we treated
sulfoxonium ylide 1a (0.2 mmol) with dioxazolone 2a (0.3 mmol)
in the presence of Cp*Co(CO)I2 (0.02 mmol), and AgSbF6
(0.05 mmol) in DCE (2 mL) under N2 at 130 ꢀC for 18 h,
benzoxazinone derivative 3aa was obtained in 69% yield (Table 1,
entry 1). Encouraged by this result, we further studied other
reaction conditions for the transformation. It was found that
introduction of additives such as NaOAc, Zn(OAc)2, KOAc and PivOH
could not improve the yield of 3aa (entries 2–5). Subsequent
screening of solvents suggested that DCE was the most efficient
medium for the transformation, while other solvents such as 1,4-
dioxane, CH3CN, and TFE were totally ineffective (entries 6–8).
Other cobalt catalysts such as Co2(CO)8 and Co(acac)3 were also
examined and found to be ineffective for this reaction (entries 9
and 10). It was observed that the choice of Ag salt had an important
effect on the reaction. When AgOTf was used in place of AgSbF6,
3aa could be isolated in 43% yield (entry 11). In sharp contrast, no
desired product was detected when AgOAc was employed (entry
12). The yield of 3aa was not improved under elevated (67%,140 ꢀC)
or lower (58%, 120 ꢀC) reaction temperatures (entries 13 and 14). It
is worthwhile to note that when the reaction temperature was
Please cite this article as: Y. Yu, Z. Xia, Q. Wu et al., Direct synthesis of benzoxazinones via Cp*Co(III)-catalyzed C–H activation and annulation of
sulfoxonium ylides with dioxazolones, Chin. Chem. Lett., https://doi.org/10.1016/j.cclet.2020.09.020

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Y. Yu, Z. Xia, Q. Wu et al. / Chinese Chemical Letters xxx (2019) xxx–xxx
3
Scheme 2. Substrate scope of sulfoxonium ylides. Reaction conditions:
1
(0.2 mmol), 2a (0.3 mmol), Cp*Co(CO)I2 (10 mol%), AgSbF6 (25 mol%), DCE (2 mL),
130 ꢀC, air, 18 h. Isolated yield.
Scheme 4. Mechanistic studies.
such as α-2-furoyl sulfoxonium ylide were found to be incompati-
ble for the system (Scheme 2, 3pa).
annulation process, NH-benzoyl-substituted sulfoxonium ylide
4aa was prepared [24]. Treatment of the compound 4aa with
10 mol% of Cp*Co(CO)I2 and 25 mol% of AgSbF6 in DCE at 130 ꢀC
afforded product 3aa in 91% yield, indicating that 4aa is an
intermediate in the reaction (Scheme 4a, entry 1). Lowering the
reaction temperature significantly decreased the yield of 3aa
(Scheme 4a, entries 2 and 3), suggesting that high temperature is
necessary for the cyclization process to proceed. It is worthwhile to
note that in the presence of Cp*Co(CO)I2 or AgSbF6, 3aa could be
obtained in excellent yield (Scheme 4a, entries 1, 4 and 5). In sharp
contrast, when both Cp*Co(CO)I2 and AgSbF6 are omitted, 3aa
could only be obtained in 24% yield (Scheme 4a, entry 6). These
results revealed that Cp*Co(CO)I2 and AgSbF6 may play an
important role in the cyclization process. We suspected that
Cp*Co(CO)I2 and AgSbF6 may serve as Lewis acid to activate the
carbonyl of sulfoxonium ylides. To verify this conjecture, we
Subsequently, the scope of dioxazolones was explored as well
(Scheme 3). From the scheme, we can see that aryl-substituted
dioxazolones bearing either electron-donating (methyl and
methoxy) or electron-withdrawing (fluoro, chloro, bromo, and
nitro) groups are all viable in this system, delivering the
corresponding products in moderate to good yields (3abÀ3al).
However, 2-methylphenyl and 2-fluorophenyl-substituted dioxa-
zolones (2b and 2c) were less reactive possibly due to steric effect.
It should be noted that naphthalene ring and heterocycle
substituted dioxazolones also reacted smoothly with 1a to deliver
3am and 3an in yields of 72% and 32%, respectively. We were also
delighted to find that alkyl-substituted dioxazolone 2o and 2p also
proved to be viable substrates, giving 3ao and 3ap in 38% and 53%
yield, respectively. It is worthwhile to point out that compound
3gq, a high-density lipoprotein elevator [25], could also be
synthesized in 63% yield under our reaction conditions.
performed
a control experiment with Lewis acid Zn(OTf)2
To elucidate the reaction mechanism, several mechanistic
experiments were conducted (Scheme 4). At first, to probe the
(Scheme 4b). Gratifyingly, treatment of 4aa with 20 mol% of Zn
(OTf)2 delivered 3aa in 88% yield, showing the importance of Lewis
acid participation. Then, an intermolecular competition reaction
between electronically different sulfoxonium ylides 1i and 1k were
conducted (Scheme 4c). The result suggested that more electron-
rich substrate 1i was favored for the reaction. Furthermore, we
carried out several experiments to study the kinetic isotopic effect
(KIE). Both intermolecular competition reaction (kH/kD = 1.00) and
parallel reaction (kH/kD = 1.38) gave relatively small KIE values,
revealing that C–H activation may not be involved in the turnover-
limiting step (Scheme 4d). Combined with Li's work [24], we
suspected that the cyclization of intermediate 4 may be the rate-
limiting step of the whole reaction process.
On the basis of mechanistic experiments and previous
literatures [22f,26], a plausible mechanism for the reaction is
proposed in Scheme 5. First, the active cationic Co(III) complex A is
generated upon treatment of Cp*Co(CO)I2 with AgSbF6. Then, A
coordinates to the carbonyl oxygen of 1a and activates the ortho C–
H bond to give a five-membered cobaltacyclic intermediate B.
Subsequent coordination of the nitrogen of dioxazolone 2a to the
Co center delivers the intermediate C, which subsequently is
converted to intermediate D via extrusion of a molecule of CO2 and
migratory insertion process. Next, protonolysis of D regenerates
the Co(III) catalyst for a new catalytic cycle and releases the key
intermediate 4aa. Finally, intramolecular nucleophilic attack of the
Scheme 3. Substrate scope of dioxazolones. Reaction conditions: 1 (0.2 mmol), 2
(0.3 mmol), Cp*Co(CO)I2 (10 mol%), AgSbF6 (25 mol%), DCE (2 mL), 130 ꢀC, air, 18 h.
Isolated yield.
Please cite this article as: Y. Yu, Z. Xia, Q. Wu et al., Direct synthesis of benzoxazinones via Cp*Co(III)-catalyzed C–H activation and annulation of
sulfoxonium ylides with dioxazolones, Chin. Chem. Lett., https://doi.org/10.1016/j.cclet.2020.09.020

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Scheme 5. Possible mechanism.
amide carbonyl oxygen on the sulfoxonium ylide carbonyl group
with the assistance of Cp*Co(CO)I2 or AgSbF6 gives the product 3aa
via the loss of a molecule of Corey’s ylide (please see the
Supporting information for its trapping experiment) [26d].
In conclusion, we have developed a highly novel and direct
route for the synthesis of benzoxazinones via Cp*Co(III)-catalyzed
C–H activation and [3 + 3] annulation of sulfoxonium ylides with
dioxazolones. This strategy employs cost-effective and air-stable
Cp*Co(CO)I2 as the catalyst and features operational simplicity.
Another remarkable feature is that the reaction is conducted under
base-free conditions. Starting from diversely substituted sulfoxo-
nium ylides and dioxazolones, a variety of benzoxazinones could
be prepared in one step in moderate to good yields. Further
mechanistic investigation and application of this protocol are in
progress, and the results will be forthcoming.
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Declaration of competing interest
The authors report no declarations of interest.
Acknowledgments
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We acknowledge the financial support from the Hunan
Provincial Innovation Foundation for Postgraduate (No.
CX20190267), Natural Science Foundation of Hunan Province
(No. 2020JJ4171).
Appendix A. Supplementary data
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Please cite this article as: Y. Yu, Z. Xia, Q. Wu et al., Direct synthesis of benzoxazinones via Cp*Co(III)-catalyzed C–H activation and annulation of
sulfoxonium ylides with dioxazolones, Chin. Chem. Lett., https://doi.org/10.1016/j.cclet.2020.09.020