Nickel-catalyzed C-H/C-O coupling of azoles with phenol derivatives

Article abstract of DOI:10.1021/ja210249h

The first nickel-catalyzed C-H bond arylation of azoles with phenol derivatives is described. The new Ni(cod)₂/dcype catalytic system is active for the coupling of various phenol derivatives such as esters, carbamates, carbonates, sulfamates, triflates, tosylates, and mesylates. With this C-H/C-O biaryl coupling, we synthesized a series of privileged 2-arylazoles, including biologically active alkaloids. Moreover, we demonstrated the utility of the present reaction for functionalizing estrone and quinine.

Full text of DOI:10.1021/ja210249h

Communication
pubs.acs.org/JACS
Nickel-Catalyzed C−H/C−O Coupling of Azoles with Phenol
Derivatives
Kei Muto, Junichiro Yamaguchi, and Kenichiro Itami*
Department of Chemistry, Graduate School of Science, Nagoya University, Nagoya 464-8602, Japan
S
* Supporting Information
cross-coupling (C−M/C−X coupling) by more inexpensive,
readily available, and less toxic participants.
ABSTRACT: The first nickel-catalyzed C−H bond
arylation of azoles with phenol derivatives is described.
The new Ni(cod)2/dcype catalytic system is active for
the coupling of various phenol derivatives such as esters,
carbamates, carbonates, sulfamates, triflates, tosylates,
and mesylates. With this C−H/C−O biaryl coupling, we
synthesized a series of privileged 2-arylazoles, including
biologically active alkaloids. Moreover, we demonstrated
the utility of the present reaction for functionalizing
estrone and quinine.
For example, significant progress has been made in the
development of aromatic C−H bond arylation with aryl halides
(C−H/C−X coupling).2,3 Oxidative C−H bond arylation
reactions using arylboron reagents (C−H/C−B coupling)4 or
arenes (C−H/C−H coupling)5 are also known. Though these
direct arylation reactions are typically catalyzed by Pd, Rh, Ir,
and Ru complexes,2 catalytic systems using inexpensive metals
such as Cu,6 Ni,7 Fe,8 and Co9 are also emerging, indicating an-
other clear direction for the advancement of biaryl coupling.
The search for new aryl electrophiles to replace aryl halides
is also being actively investigated. For example, new Ni-based
catalytic systems have recently been developed for cross-
coupling of aryl C−O bonds of some phenol derivatives (e.g.,
aryl ethers and esters) with arylmetal species (C−M/C−O
coupling).10−13 The advantage of using phenol derivatives as
aryl electrophiles is apparent, as they are often ready available
and inexpensive. Herein we report on our recent findings that
Ni(cod)2/dcype [cod = 1,5-cyclooctadiene, dcype =1,2-bis-
(dicyclohexylphosphino)ethane] serves an efficient catalyst for
the direct arylation of azoles with various phenol derivatives
(C−H/C−O coupling). This represents important progress in
the advancement of biaryl coupling chemistry (Scheme 1).
Motivated by the fact that arylated azoles are privileged
structures present in many biologically active natural products,
pharmaceuticals, and optoelectronic materials,7d we selected
azoles as heteroarene substrates for C−H/C−O biaryl
coupling.14 After careful and thorough investigation, we dis-
covered the new Ni(cod)2/dcype catalyst. For example, the
reaction of benzoxazole (1A, 1.0 equiv) and naphthalen-2-yl
pivalate (2a, 1.5 equiv) in the presence of Ni(cod)2 (10 mol %),
dcype (20 mol %), and Cs2CO3 (1.5 equiv) in 1,4-dioxane at
120 °C (representing our standard conditions) afforded the
corresponding coupling product (3Aa) in 95% isolated yield
(Scheme 2).
he development of efficient methods for constructing
T
privileged biaryl structures is a topic of utmost
importance in various fields of chemistry. Although the Pd-
catalyzed cross-coupling reactions of metalated arene/
heteroarene and halogenated arene/heteroarene species are
undoubtedly among the most reliable methods for making
biaryls and heterobiaryls,1 significant academic efforts are
underway to make metal-catalyzed biaryl couplings “greener”
chemical processes (Scheme 1). Among many aspects of
Scheme 1. Recent Directions in Catalyzed Biaryl Coupling
Listed in Scheme 3 are some examples of variations from
the standard conditions. Among various reaction parameters,
a dramatic ligand effect of dcype is notable. For example,
we found that tricyclohexylphosphine (PCy3), the standard
ligand for activating aryl C−O bonds for C−M/C−O cou-
pling,11,12 was totally ineffective for the present C−H/C−O
coupling. Similarly 1,3-bis(2,6-diisopropylphenyl)imidazolium
chloride (IPr·HCl) was ineffective. Structurally similar ethyl-
and phenyl-substituted ligands (depe and dppe) as well as
biaryl coupling, efforts are being directed toward replacing
conventional aryl nucleophiles (Mg, Zn, B, etc.), electrophiles
(aryl halides), and metal catalysts (Pd) used in conventional
Received: October 31, 2011
Published: December 8, 2011
© 2011 American Chemical Society
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Communication
Scheme 2. Discovery of the Ni(cod)2/dcype Catalyst for
C−H/C−O Biaryl Coupling
Scheme 4. C−H/C−O Biaryl Coupling Catalyzed by the
Ni/dcype System
Scheme 3. Influence of Parameters in the Coupling of 1A
and 2a
2,2′-bipyridyl (bipy), our standard ligand for Ni-catalyzed
C−H/C−X coupling,7a,d were also not effective. As shown in
Scheme 3, usable solvents and bases are not limited to 1,4-
dioxane and Cs2CO3, respectively. Gratifyingly, we found that
the present Ni(cod)2/dcype catalyst is active for the coupling of
other phenol derivatives such as carbamates, carbonates, sulfa-
mates, triflates, tosylates, and mesylates (80−99% yield). We
also confirmed that the reaction also takes place with catalyst
loadings as low as 2 mol % (54% yield with triflate). Although
the mechanism of the C−H/C−O biaryl coupling remains
unknown,15 the observed striking ligand effect is notable. In
particular, PCy311,12,16 and N-heterocyclic carbenes,17 which are
known to be able to activate aryl C−O bonds, do not promote
the present C−H/C−O biaryl coupling.
6-quinolinyl, 5-quinolinyl, and 9-phenanthrenyl pivalates, giving
the corresponding coupling products in moderate to excellent
yields. For installing substituted phenyl groups onto azoles,
we found that the use of the corresponding triflates is more
effective. Under these conditions, both electron-rich and
electron-deficient phenol derivatives were successfully coupled
with 1A. Aryl−methoxy11,12a and aryl−cyano18 bonds, which
could be activated with Ni catalysts, were well-tolerated under
the present conditions. In addition to benzoxazoles, oxazoles,
thiazoles, and benzothiazoles were found to react smoothly
with phenol derivatives to furnish the corresponding coupling
On the basis of these findings, we then examined the direct
arylation of various 1,3-azoles with aryl pivalates and triflates as
representative C−O coupling partners (Scheme 4). 1A was
successfully coupled with 1-naphthyl, 2-naphthyl, 3-pyridinyl,
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Journal of the American Chemical Society
Communication
products.19 During these investigations, we successfully achieved
concise syntheses of the oxazole alkaloids texamine (3Dc)20 and
unguenenazole (3Ec).21
We believe that the C−H/C−O biaryl coupling should find
use in the rapid identification of new biologically active com-
pounds by a late-stage functionalization of naturally occurring
privileged structures.22 Thus, to demonstrate the feasibility of
this approach, we investigated the Ni-catalyzed C−H/C−O
coupling for functionalization of estrone and quinine (Scheme 5).
The coupling of estrone triflate (4) with 5-phenyloxazole
ACKNOWLEDGMENTS
■
This work was supported by the Funding Program for Next
Generation World-Leading Researchers from JSPS (220GR049
to K.I.) and the Grant-in-Aid for Scientific Research on
Innovative Areas “Molecular Activation Directed toward
Straightforward Synthesis” (23105517 to J.Y.) from MEXT.
We thank Dr. Jean Bouffard for fruitful discussions and critical
comments.
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proceeded smoothly under our standard conditions to afford
heteroarylated estrone 5 in 52% yield. The C−H/C−O coupling
of quinine triflate (6)23 with benzoxazole also occurred, giving the
quinine−benzoxazole hybrid molecule 7, albeit with somewhat
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successful application to the functionalization of the quinine
structure, which is known to be sensitive under acidic, basic, and
redox conditions, speaks well for the potential of the present nickel
catalysis for further development and applications.
In summary, we have discovered the first nickel-catalyzed
C−H/C−O coupling reaction of azoles with phenol derivatives.
Under Ni(cod)2/dcype catalysis, various pivalates, triflates,
tosylates, mesylates, carbamates, carbonates, and sulfamates can
be used as phenol derivatives. The present findings not only
push the limit of biaryl coupling into the C−H/C−O manifold
but also should open an avenue for a range of new catalytic
C−O bond transformations of phenol derivatives with
Ni(cod)2/dcype. Works along this line, mechanistic investiga-
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second-generation catalyst with much broader scope are now
the focus of our ongoing efforts.
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ASSOCIATED CONTENT
■
S
* Supporting Information
Detailed experimental procedures and spectral data for all
compounds, including scanned images of H and 13C NMR
1
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spectra. This material is available free of charge via the Internet
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AUTHOR INFORMATION
■
Corresponding Author
itami.kenichiro@a.mbox.nagoya-u.ac.jp
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catalyst.
́
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