Cu-Catalyzed Cross-Dehydrogenative Coupling of Heteroaryl C(sp2)-H and Tertiary C(sp3)-H Bonds for the Construction of All-Carbon Triaryl Quaternary Centers

Article abstract of DOI:10.1021/acs.orglett.9b01755

A Cu-catalyzed protocol for cross-dehydrogenative coupling of benzofuranones with quinolines, indoles, carbazoles, and thiophene, which furnishes highly functionalized 3,3-diaryl benzofuranones bearing a three aryl quaternary carbon center at the C3 posit

Full text of DOI:10.1021/acs.orglett.9b01755

Letter
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/OrgLett
Cu-Catalyzed Cross-Dehydrogenative Coupling of Heteroaryl
2
3
C(sp )−H and Tertiary C(sp )−H Bonds for the Construction of All-
Carbon Triaryl Quaternary Centers
Zhi Tang, Zhili Liu, Zhou Tong, Zhihui Xu, Chak-Tong Au, Renhua Qiu,*,†
†
†
†
†
†,§
,
†,‡
and Nobuaki Kambe*
†State Key Laboratory of Chemo/Biosensing and Chemometrics, College of Chemistry and Chemical Engineering, Hunan
University, Changsha 410082, P.R. China
‡
Department of Applied Chemistry, Graduate School of Engineering, Osaka University, Suita, Osaka 565-0871, Japan
§
College of Chemistry and Chemical Engineering, Hunan Institute of Engineering, Xiangtan 411104, P.R. China
*
S Supporting Information
ABSTRACT: A Cu-catalyzed protocol for cross-dehydrogenative coupling of benzofur-
anones with quinolines, indoles, carbazoles, and thiophene, which furnishes highly
functionalized 3,3-diaryl benzofuranones bearing a three aryl quaternary carbon center at
the C3 position in good yields, has been developed. A radical mechanism is proposed.
arbon−carbon bond forming reactions are among the
Scheme 1. Cross-Dehydrogenative Coupling Reaction for
Constructing C−C Bonds
C
most important processes in organic synthesis because
they are essential to construct carbon skeletons of organic
1
molecules. Transition-metal-catalyzed cross-coupling reactions
between organic (pseudo)halides with organometallic reagents
2
have widely been employed as a reliable tool for this purpose.
However, recently, much attention has been paid to the cross-
dehydrogenative coupling (CDC) reaction as an ideal and
practical and straightforward strategy for carbon−carbon bond
3
formation. The pioneering work of this transformation dates
4
back to the 1980s, and this procedure can now be employed for
constructing various carbon−heteroatom bonds including C−
5
N, C−O, C−P, C−S, or C−Si. To achieve C−C bond
formation efficiently by a CDC procedure, cleavage of two C−H
bonds should take place selectively. To date, many excellent
CDC systems have been developed for constructing secondary
3
and tertiary sp carbon centers via C−C bond formation
3
,4
(
Scheme 1a); however, procedures for constructing all-carbon
quaternary centers by CDC reactions are still limited (Scheme
6
1
b). In the CDC reactions to construct quaternary carbon
example of a CDC reaction for constructing triaryl quaternary
carbon centers via intermolecular arylation with various
heteroarenes.
All-carbon triaryl quaternary centers are commonly present in
natural products, semisynthetic and synthetic pharmaceuticals,
3
centers, activated (sp )C−H compounds to generate stabilized
carbon radicals or anions are often selected as the substrates,
which react with the coupling partners with the aid of catalysts.
Although heteroatom compounds or benzylic hydrocarbons are
often employed as the coupling partners, use of aryl compounds
is only limited to the intramolecular cyclization via homolytic
Received: May 18, 2019
6f−l
substitution on aromatic rings.
Herein, we disclose the first
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b01755
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
and agrochemical ingredients (Figure S1 in the Supporting
Information, SI), such as Hopeachainol A, Oxyphenisatin, and
its analogue, TOP216. In recent years, development of catalytic
Scheme 2. Substrates Scope with Respect to Quinoline
a
Amides
7
methods for the synthesis of triarylmethanes has attracted much
8
9
attention as an attractive strategy in drug design. In previous
1
0
studies of the synthesis of 3,3-diarylbenzofuranone, which is a
key skeleton in natural products such as Hopeachainol A, cross-
coupling or addition reactions were employed. The present
CDC arylation procedure will provide a more straightforward
method for their synthesis. This reaction is catalyzed by copper
in the presence of K S O as an oxidizing reagent under mild
2
2
8
conditions to afford 3-phenylbenzofuran-2(3H)-ones by the
site-selective cross-coupling of benzofuranones with various π-
electron-rich heteroarenes such as quinolines, indoles, carba-
zoles, and thiophenes (Scheme 1c).
As a model reaction, the cross-coupling between 5-methyl-3-
phenylbenzofuran-2(3H)-one (1a) and N-(quinolin-8-yl)-
pivalamide (2a) was examined (Table S1 in SI). After
3
optimization of the reaction conditions, the oxidative C(sp )−
2
H/C(sp )−H coupling took place to give 3a in 75% yield when
the reaction was conducted in acetonitrile using copper(I)
bromide as the catalyst in the presence of KH PO base and
2
4
K S O oxidant at 80 °C under a N atmosphere. The CDC
2
2
8
2
reaction of 1a and 2a can be also regarded as a type of C(5)−H
11
functionalization of an 8-aminoquinoline ring.
With these optimized conditions in hand, we explored the
scope of 3-phenylbenzofuran-2(3H)-ones and various quinoline
amides. As shown in Scheme 2, a number of aliphatic amides
gave the products in moderate to good yields (3a−3j).
Moreover, the system could be applied to a variety of
benzamides, with electron-donating or electron-withdrawing
groups at the para, meta, or ortho position of the aryl ring, leading
smoothly to the desired products (3k−3x). It was found that
vinyl (3y) and heteroaromatic groups such as thienyl and furanyl
were tolerated, giving the corresponding products 3y−3za in
satisfactory yields. The configuration of 3za was confirmed by X-
ray crystallography. This system also tolerates functional groups
on the aryl ring of quinoline, such as methyl and methoxyl
a
Reaction conditions: 1 (0.24 mmol), 2 (0.20 mmol), CH CN (2.0
3
(
3zb−3zc). We extended the substrate scope to 3-arylbenzofur-
mL), 80 °C, N , 12 h, a sealed tube, isolated yield of 3.
2
an-2(3H)-ones bearing methyl, ethyl, isopropyl, methoxyl, and
phenyl substituents and obtained good yields (3ze−3zj),
whereas the nonsubstituted benzofuraone gave a lower yield
phenyl substituents also reacted with indoles to give the
corresponding products in good yields (5q−5v), in particular, in
which the yield of 5s was up to 91%. For gram-scale synthesis of
5s, a yield of 93% (1.17g) could be achieved. Moreover, halide-
substituted substrates reacted smoothly to form the desired
products (5t, 5u, 5y, 5z). Product 5w could not be obtained due
to the steric effect of the methyl group in benzofuranone.
(3zd). The halide-substituted substrates can also react to form
the desired product 3zk and 3zl, which indicates high potential
for further transformation using traditional Suzuki−Miyaura
cross-coupling reactions.
2
We hypothesized that other heteroarenes with a C(sp )−H
bond, such as indoles, carbazoles, and thiophenes, would also
give triarylmethane products using the present system. First, we
confirmed that this catalytic system is suitable for the CDC
reaction of indoles (Scheme 3a). In general, the reaction
proceeded smoothly to afford the desired products in good to
excellent yields. For the reaction with 3-phenylbenzofuranone
12
However, the multisubstituted substrate antioxidant HP-136
gave 5zb in 77% yield.
We then extended the scope to carbazoles (Scheme 3b). A
number of carbazoles gave the products in moderate yields (7a−
7f). The carbazoles bearing ethyl, isopropyl, formyl, and phenyl
substituents reacted and gave good product yields. In general,
benzofuranons with electron-donating and electron-withdraw-
ing groups at the para, meta, or ortho position of the aryl ring
could proceed smoothly to afford the desired products (7g−7s).
In addition, various thiophenes are also acceptable to the CDC
process (Scheme 3c). It was found that different thiophenes and
benzofuranones could undergo CDC smoothly to give the
corresponding products 9a−9k in 30−85% yields. The
configurations of 7a and 9c were confirmed by X-ray
crystallography.
(
1a), a wide range of indoles was examined (5a−5p), and it was
observed that with both electron-withdrawing and electron-
donating groups on the indole ring of 4, the desired products
were obtained in satisfactory yields. This CDC system tolerates a
diversity of valuable functional groups on the aryl ring of the
indoles, such as alkoxy, amino, halo, cyano, ester, and formyl
groups (products 5a−5p), reflecting the high potential for
further synthetic applications. The configuration of 5o was
confirmed by X-ray crystallography. Various 4-substituted
benzofuranone species bearing methyl, ethyl, isopropyl, and
B
DOI: 10.1021/acs.orglett.9b01755
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 3. Substrates Scope with Respect to Indoles (5),
Carbazoles (7), and Thiophenes (9)
reaction of 11 with 2a in the presence of 1b afforded 3a and 3ze
in 43 and 23% yields, respectively (Scheme S1c, c7, in the SI).
These results suggest that the present reaction involves a radical
process and that 11 would be a possible intermediate or a resting
state, as proposed in ref 6e. When 2a was reacted with 1.0 equiv
a
of CuBr or CuBr in the absence of 1, a brominated product 12
2
was formed in high yields in the presence of oxidant with or
without a base (Scheme S1d in the SI). However, 12 did not
afford 3a in the presence of Cu(0) or CuBr under the optimized
conditions, indicating that 12 cannot be an intermediate in the
13
present CDC reaction.
Although the detailed reaction mechanism is still unclear, we
proposed a possible catalytic pathway in Scheme 4 according to
Scheme 4. Plausible Reaction Pathways
the above results and those reported previously in the
14
literature. First, substrate 1a is metalated with CuX and
oxidant to form intermediate RCu(II)X complex A, which can
easily generate a radical B and Cu(I)X. Compound 11, formed
from A or via a radical B, can be a resting state of the present
reaction. The coupling partner 2a also reacts with CuX and
oxidant to produce intermediate R′Cu(II)X complex C via a 1,5-
H shift after deprotonation, which forms brominated 12 as a
dead-end product in the absence of 1a. Then the intermediate C
reacts with radical B to form the possible intermediate
RR′Cu(III)X complex D, which undergoes reductive coupling
to give the target product 3a and regenerates CuX following the
Cu(I)/Cu(II)/Cu(III) radical process.
In summary, the first CDC reaction for the construction of
quaternary carbon centers bearing three aryl groups was
developed using copper catalyst in the presence of K S O as
a
2
2
8
Reaction conditions: 1 (0.24 mmol), 4/6/8 (0.20 mmol), CH CN
3
an oxidizing reagent. This reaction provides a simple and
efficient method for arylation of 3-phenylbenzofuranones with
various heteroarenes including quinolines, indoles, carbazoles,
and thiophenes. The present CDC reaction proceeds site-
selectively with respect to both reactants to give 3,3-diary-
lbenzofuranones, which would be candidates as potential
synthetic pharmaceutical and agrochemical ingredients. Pre-
liminary mechanistic studies indicate that this reaction involves a
Cu-catalyzed radical process.
(
2.0 mL), 80 °C, N , 12 h, a sealed tube, isolated yield of 5/7/9.
2
To gain insights into the reaction mechanism, we conducted
several control experiments (Scheme S1 in the SI). First, a
competition reaction between 2a and dideuterated substrate
2
indicating that C−H bond cleavage of 2 would not be the
rate-determining step (Scheme S1a in the SI). Furthermore, the
addition of 2.0 equiv of a radical scavenger, such as TEMPO, and
butylated hydroxytoluene largely, but not completely, sup-
pressed the reaction (Scheme S1b in the SI). Also, without 2a,
D-2a revealed the kinetic isotope effect, k /k = 1.44,
H D
ASSOCIATED CONTENT
Supporting Information
■
*
S
1
a reacted with itself under similar conditions in THF to form
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.9b01755.
the dimerization product 11 in 41% yield. Without CuBr, base,
or oxidant, no dimerization product 11 was observed (Scheme
S1c, c3−c6, in the SI). When 2a was reacted with 2.0 equiv of 11
under standard conditions, 3a was formed in 45% yield. A similar
Detailed experimental procedures, characterization data,
1
19
13
copies of the H, F, and C NMR spectra (PDF)
C
DOI: 10.1021/acs.orglett.9b01755
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Accession Codes
Letter
R. E. J. Am. Chem. Soc. 1982, 104, 5557. (d) Nicolaou, K. C.; Petasis, N.
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CCDC 1879999−1880003 contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif, or by email-
ing data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road, Cam-
bridge CB2 1EZ, UK; fax: +44 1223 336033.
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(
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AUTHOR INFORMATION
Corresponding Authors
1c. (l) Tsuji, J.; Nagashima, H. Tetrahedron 1984, 40, 2699. (sp )CH−
■
3
(
sp )CH: (m) DeBoef, B.; Pastine, S. J.; Sames, D. J. Am. Chem. Soc.
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1
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*
E-mail: renhuaqiu@hnu.edu.cn.
3
3
*E-mail: kambe@chem.eng.osaka-u.ac.jp.
27, 3672.
(
5) Selected examples of the CDC reaction for C−N bond formation:
a) Louillat, M.-L.; Patureau, F. W. Chem. Soc. Rev. 2014, 43, 901.
b) Shi, Z.; Zhang, C.; Tang, C.; Jiao, N. Chem. Soc. Rev. 2012, 41, 3381.
ORCID
(
(
Renhua Qiu: 0000-0002-8423-9988
Notes
Selected examples of the CDC reaction for C−O bond formation:
(
(
c) Xu, W.; Huang, Z.; Ji, X.; Lumb, J.-P. ACS Catal. 2019, 9, 3800.
d) Guin, S.; Rout, S. K.; Banerjee, A.; Nandi, S.; Patel, B. K. Org. Lett.
The authors declare no competing financial interest.
2
012, 14, 5294. (e) Tang, Z.; Tong, Z.; Xu, Z.; Au, C.-T.; Qiu, R.; Yin,
S.-F. Green Chem. 2019, 21, 2015. Selected examples of the CDC
reaction for C−P bond formation: (f) Basle, O.; Li, C.-J. Chem.
ACKNOWLEDGMENTS
The authors thank the Natural Science Foundation of China
21676076 and 21878071), Hu-Xiang High Talent in Hunan
■
(
́
Commun. 2009, 4124. (g) Wendlandt, A. E.; Suess, A. M.; Stahl, S. S.
Angew. Chem., Int. Ed. 2011, 50, 11062. Selected examples of the CDC
reaction for C−S bond formation: (h) Chen, Q.; Yu, G.; Wang, X.; Ou,
Y.; Huo, Y. Green Chem. 2019, 21, 798. (i) Siddaraju, Y.; Prabhu, K. R. J.
Org. Chem. 2016, 81, 7838. Selected examples of the CDC reaction for
C−Si bond formation: (j) Zhang, J.; Park, S.; Chang, S. J. Am. Chem.
Soc. 2018, 140, 13209.
Province (2018RS3042) and Recruitment Program for Foreign
Experts of China (WQ20164300353) for financial support. R.Q.
thanks Prof. Shuang-Feng Yin (Hunan University) for helpful
discussion. C.-T.A. thanks the HNU for an adjunct professor-
ship.
(
6) Selected examples of constructing all-carbon quaternary centers by
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D
DOI: 10.1021/acs.orglett.9b01755
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
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DOI: 10.1021/acs.orglett.9b01755
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