Construction of Benzofuran-3(2 H)-one Scaffolds with a Quaternary Center via Rh/Co Relay Catalyzed C-H Functionalization/Annulation of N-Aryloxyacetamides and Propiolic Acids

Article abstract of DOI:10.1021/acs.orglett.9b00181

An unprecedented synthesis of valuable benzofuran-3(2H)-one scaffolds with a quaternary center was developed via Rh/Co relay catalyzed C-H functionalization/annulation of N-aryloxyacetamides with propiolic acids in moderate to good yields. The reaction features the simultaneous construction of the benzofuran motif containing a C2 quaternary center and a C3 carbonyl group. The preliminary mechanism study verified that the O atom of C3 carbonyl group originates from molecular oxygen.

Full text of DOI:10.1021/acs.orglett.9b00181

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Construction of Benzofuran-3(2H)‑one Scaffolds with a Quaternary
Center via Rh/Co Relay Catalyzed C−H Functionalization/Annulation
of N‑Aryloxyacetamides and Propiolic Acids
Wen-Kui Yuan, Ming-Hui Zhu, Rui-Sen Geng, Guang-Yi Ren, Lin-Bao Zhang,* Li-Rong Wen,*
and Ming Li*
State Key Laboratory Base of Eco-Chemical Engineering, College of Chemistry and Molecular Engineering, Qingdao University of
Science and Technology, Qingdao 266042, P.R. China
S
* Supporting Information
ABSTRACT: An unprecedented synthesis of valuable
benzofuran-3(2H)-one scaffolds with a quaternary center
was developed via Rh/Co relay catalyzed C−H functionaliza-
tion/annulation of N-aryloxyacetamides with propiolic acids in
moderate to good yields. The reaction features the
simultaneous construction of the benzofuran motif containing
a C2 quaternary center and a C3 carbonyl group. The
preliminary mechanism study verified that the O atom of C3
carbonyl group originates from molecular oxygen.
enzofuran-3(2H)-one scaffolds, which represent one of
the most important heterocycles, widely exist in natural
products and bioactive molecules (Figure 1). For example,
owing to their high activity and excellent tolerance of
substrate/functional group.5 N-Phenoxyacetamides as the
privileged substrates offer opportunities and possibilities to
develop new transformations by utilizing the unique reactivity
of O-NHAc acting as a redox-active directing group (DG).6
Cp*Rh(III)-catalyzed C−H functionalization/[3 + 2] annula-
tion of aromatic substrates bearing DG with internal alkynes to
synthesize benzofuran scaffolds has been well documented.7 Lu
and co-workers reported development of a mild Rh(III)-
catalyzed redox-neutral C−H functionalization of N-phenox-
yacetamides with alkynes for the synthesis of valuable
benzofuran derivatives through C−C/C−O bond formation
(Scheme 1, eq 1).8 Then they reported an efficient Rh(III)-
catalyzed coupling reaction of N-phenoxyacetamides with
propargyl carbonates to yield 3-alkylidene dihydrobenzofuran
derivatives via C−H functionalization/cyclization (Scheme 1,
eq 2).9 Very recently, Zhang and co-workers reported a
Rh(III)-catalyzed C−H activation of N-phenoxyacetamides
with propiolates to afford benzofuran-2(3H)-ones bearing an
exocyclic enamino motif with exclusive Z configuration
selectivity via C−H functionalization/isomerization/lactoniza-
tion (Scheme 1, eq 3).10 Compared with numerous examples
of constructing benzofuran scaffolds, directly constructing
benzofuran-3(2H)-one scaffolds with a quaternary center via
Rh(III)-catalyzed C−H functionalization/[3 + 2] annulation
has so far remained unexplored. In continuation of our
research on the development of N-aryloxyacetamides as a
building block,11 when N-aryloxyacetamides were utilized to
react with propiolic acids under Rh/Co catalysis, the
B
Figure 1. Selected natural products with benzofuran-3(2H)-one
scaffolds.
coptichic aldehyde, isolated from the Coptidis rhizoma
Euodiae, possesses cytotoxicity against human gastric carcino-
ma (NCI-N87) and human colon adenocarcinoma (Caco-2)
cells.1 Armeniaspiroles A−D, isolated from Streptomyces
armeniacus, exhibit good activity against a range of multi-
drug-resistant Gram-positive bacterial pathogens.2 Griseoful-
vin, first isolated from filamentous fungi, is the only drug
available for treatment of tineacapitis caused by dermato-
phytes.3 Therefore, the synthesis of benzofuran-3(2H)-one
derivatives may be of great significance.
Transition-metal-catalyzed direct C−H functionalization has
received considerable interest in recent years as a powerful and
straightforward synthetic approach for complex molecules
without the need for prefunctionalized substrates.4 Among
these transition-metal catalysts, Cp*Rh(III) catalysts have
significantly contributed to the arsenal of heterocyclic synthesis
Received: January 16, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b00181
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Letter
a
Scheme 1. Rh-Catalyzed C−H Functionalization/[3 + 2]
Annulation of N-Aryloxyacetamides
Table 1. Optimization of Reaction Conditions for 3a
b
yield
(%)
entry
1
catalyst (equiv)
oxidant
solvent
MeOH
(Cp*RhCl2)2 (0.05)
Co(OAc)2·
4H2O
83
2
3
4
5
(Cp*RhCl2)2 (0.05)
(Cp*RhCl2)2 (0.05)
(Cp*RhCl2)2 (0.05)
CoCl2·6H2O
K2S2O8
Oxone
MeOH
MeOH
MeOH
MeOH
NR
NR
NR
82
[Cp*Rh(MeCN)3](SbF6)2 Co(OAc)2·
(0.05)
4H2O
Co(OAc)2·
4H2O
Co(OAc)2·
4H2O
Co(OAc)2·
4H2O
Co(OAc)2·
4H2O
Co(OAc)2·
4H2O
Co(OAc)2·
4H2O
Co(OAc)2·
4H2O
6
(CpERhCl2)2 (0.05)
MeOH
MeOH
MeOH
DCE
78
92
7
(Cp*RhCl2)2 (0.025)
(Cp*RhCl2)2 (0.0125)
(Cp*RhCl2)2 (0.025)
(Cp*RhCl2)2 (0.025)
(Cp*RhCl2)2 (0.025)
unexpected product benzofuran-3(2H)-one derivatives were
obtained (Scheme 1, eq 4). Significantly, in this trans-
formation, the O atom in the carbonyl group of benzofuran-
3(2H)-one motif originates from molecular oxygen.
8
82
9
78
We commenced our investigation by examining reactions of
N-phenoxyacetamide (1a) with 3-phenylpropiolic acid (2a)
using Rh(III) catalyst [see the Supporting Information (SI)].
The reaction proceeded under an atmosphere of air, with no
special precautions taken to exclude moisture. When
(Cp*RhCl2)2 (0.05 equiv) was used as the catalyst, NaOPiv·
H2O (1 equiv) as the base, Co(OAc)2·4H2O (1 equiv) as the
oxidant, and MeOH (0.1 M) as the solvent, the desired
product 3a was obtained in 83% yield at room temperature
after 12 h (Table 1, entry 1). No conversion was observed
when Co(OAc)2·4H2O was replaced by CoCl2·6H2O, K2S2O8,
or Oxone (Table 1, entries 2−4). [Cp*Rh(MeCN)3](SbF6)2
was also employed and gave 3a in 82% yield (Table 1, entry 5).
Although (CpERhCl2)2 is deemed to possess higher catalytic
activity than (Cp*RhCl2)2 in the literature,12 it provided only
78% of 3a (Table 1, entry 6). Gratifyingly, decreasing the
loading of (Cp*RhCl2)2 to 0.025 equiv could give 92% yield of
3a (Table 1, entry 7). However, further decreasing the loading
of(Cp*RhCl2)2 to 0.0125 equiv showed a reduction in 82%
yield (Table 1, entry 8). Screening of other solvents such as
DCE, HFIP, and t-AmylOH showed that MeOH was the best
solvent in this transformation (Table 1, entries 9−11). Control
experiments revealed that 3a could not be obtained in the
absence of (Cp*RhCl2)2 and Co(OAc)2·4H2O (Table 1,
entries 12 and 13). Therefore, the optimal reaction conditions
were determined as (Cp*RhCl2)2 (0.025 equiv), NaOPiv·H2O
(1 equiv), and Co(OAc)2·4H2O (1 equiv) in MeOH at room
temperature for 12 h.
After establishing the optimal reaction conditions, we
investigated the substrate scope using various N-phenoxyace-
tamides 1 and propiolic acids 2. As shown in Scheme 2, para-
substituted N-phenoxyacetamides with electron-withdrawing
groups such as halogens and CF3 delivered the desired
benzofuran-3(2H)-one derivatives in moderate to good yields
(3b−e). However, N-(3,5-dichlorophenoxy)acetamide did not
provide the corresponding product 3f, which might be due to
the steric hindrance of Cl. Similar results could be obtained for
N-phenoxyacetamides bearing electron-donating groups (3g−
k). To our delight, when a long-chain N-phenoxyamide such as
n-Bu was applied in the reaction, the corresponding product 3l
could be isolated in yield of 81%. Next, a variety of propiolic
acids 2 were utilized, and the results showed the propiolic acids
10
11
12
13
HFIP
73
t-AmylOH
MeOH
MeOH
87
NR
NP
(Cp*RhCl2)2 (0.05)
a
Reaction conditions: 1a (0.20 mmol), 2a (0.25 mmol), NaOPiv·
H2O (1 equiv), oxidant (1 equiv), solvent (2 mL), rt, under air, 12 h.
b
Isolated yields. NR = no reaction, NP = no product. Cp* =
pentamethylcyclopentadienyl, CpE = 1,3-bis(ethoxycarbonyl)-2,4,5-
trimethylcyclopentadienyl, HFIP = 1,1,1,3,3,3-hexafluoro-2-propanol,
t-AmylOH = 2-methyl-2-butanol.
bearing both electron-withdrawing and electron-donating
substituents at different positions could react smoothly with
1a to afford the corresponding benzofuran-3(2H)-ones in good
to excellent yields (3m−z).
Interestingly, a benzofuran-3-carboxylic acid product 4
(confirmed by X-ray single-crystal diffraction, see the SI) was
obtained in moderate yield when 3-(2-methoxyphenyl)-
propiolic acid (2o) and pent-2-ynoic acid (2p) were applied
in this reaction under the optimized conditions (Scheme 3).
In order to demonstrate the synthetic utility of this method,
the model reaction was performed on a 5 mmol scale under the
standard conditions, and the desired product 3a was provided
in 76% yield (Scheme 4).
To obtain a better understanding of the reaction mechanism,
a series of control experiments were carried out. Two radical-
trapping experiments were conducted with 2,2,6,6-tetrame-
thylpiperidine N-oxide (TEMPO) and 2,6-di-tert-butyl-4-
methylphenol (BHT) as radical scavengers. The results
showed the reactions were inhibited, which supports a radical
pathway (Scheme 5, eq 1). To probe the active intermediate, a
stable five-membered rhodacycle complex A was prepared.6b,m
Significantly, only 1 mol % of A could catalyze effectively the
model reaction to give 3a in 75% yield. The reaction of a
stoichiometric rhodacycle A with 2a facilely conversed into 3a
in 95% yield. These results suggest that rhodacycle A was
involved in the catalytic cycle as the active species (Scheme 5,
eqs 2 and 3). Since this cyclization process includes a C−H
activation step, a kinetic isotopic effect (KIE) of 2.5 was
observed in a competitive experiment. Another, a KIE of 1.6,
B
DOI: 10.1021/acs.orglett.9b00181
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Letter
a,b
Scheme 2. Synthesis of Benzofuran-3(2H)-ones
Scheme 5. Control Experiments
carboxylic acid (4c) was prepared to react with 1a. However,
3t was not detected. Considering that 2 may play an important
role in this conversion reaction, the three-component reaction
of 1a, 2a ,and 4c was conducted. Unfortunately, only 3a was
detected, but 3t still could not be obtained (Scheme 5, eqs 6
and 7) (see the SI). The results suggest the benzofuran-3-
carboxylic acid could not be an intermediate in the reaction.
Isotopic labeling experiments were conducted to further
elucidate the source of the O atom of the benzofuran-3(2H)-
one C3 carbonyl group (Scheme 6). First, two labeled
reagents, H218O and Na18OAc, were applied in the reaction,
and 3a-18O was not detected by HRMS (Scheme 6, eqs 1 and
2). Next, the reaction of 1a with 2a was carried out in degassed
MeOH at rt in the glovebox, only trace 3a was obtained
a
Reaction conditions: 1 (0.20 mmol), 2 (0.25 mmol), (Cp*RhCl2)2
(0.025 equiv), NaOPiv·H2O (1 equiv), Co(OAc)2·4H2O (1 equiv),
b
MeOH (2 mL), rt, 12 h. Isolated yields.
a,b
Scheme 3. Synthesis of Benzofuran-3-carboxylic Acid
Scheme 6. Isotopic Labeling Experiments
a
b
Reaction conditions: 1a (0.20 mmol), 2 (0.25 mmol). Isolated
yield.
Scheme 4. Gram-Scale Experiment
was observed from two side-by-side reactions using 1a and 1a-
d5. These results suggest that C−H bond cleavage occurs in the
rate-determining step (Scheme 5, eqs 4 and 5). Finally, to
demonstrate whether benzofuran-3-carboxylic acids 4 are an
intermediate in this transformation, 2-(p-tolyl)benzofuran-3-
C
DOI: 10.1021/acs.orglett.9b00181
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Letter
(Scheme 6, eq 3) (see the SI). These results suggest that O2 is
the exclusive source of the O atom in the carbonyl group of
benzofuran-3(2H)-one scaffolds.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b00181.
■
S
To probe the role of Co(OAc)2·4H2O in the catalytic cycle,
the solutions of two control reactions were analyzed by LC−
MS. (1) The model reaction was carried out for 1 h under the
standard conditions, and the intermediates E and 3a were
detected (E [M − H]− 296.07; 3a [M + H]+ 268.06. (2) The
model reaction without added Co(OAc)2·4H2O was carried
out for 1 h, and the intermediate E was still detected but 3a
could not be detected. Then Co(OAc)2·4H2O was added into
the reaction system for 20 min, and 3a was obtained in 13%
yield (see the SI). The results suggest Co(OAc)2·4H2O could
play a key role in decarboxylation/cyclization.
Experimental procedures, characterization and spectral
data (PDF)
Accession Codes
Accession Codes CCDC 1831078, 1855373 contain 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 Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: + 44
1223 336033.
On the basis of the above observations, a tentative
mechanism for the reaction of 1a with 2a to afford the
product 3a is depicted in Scheme 7. First, the coordination of
AUTHOR INFORMATION
Corresponding Authors
■
Scheme 7. Possible Mechanism for the Transformation
*E-mail: zhang_linbao@126.com.
*E-mail: wenlirong@qust.edu.cn.
*E-mail: liming928@qust.edu.cn.
ORCID
Li-Rong Wen: 0000-0001-7976-0878
Ming Li: 0000-0003-4906-936X
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was financially supported by the National Natural
Science Foundation of China (21572110, 21801152, and
21372137).
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