Copper-Mediated Regioselective C-H Sulfenylation and Selenation of Phenols with Phenanthroline Bidentate Auxiliary

Article abstract of DOI:10.1021/acs.orglett.0c02012

A copper-mediated, phenanthroline-directed highly ortho-selective C-H sulfenylation of phenols with diaryl disulfides proceeds to form the corresponding unsymmetrical diaryl sulfides in good yield. The key to success is the introduction of a phenanthroline directing group of the bidentate-chelating nature, which is easily attachable, detachable, and even recyclable. Moreover, the same strategy is applicable to the C-H selenation, giving the diaryl selenides with high efficiency and regioselectivity.

Full text of DOI:10.1021/acs.orglett.0c02012

pubs.acs.org/OrgLett
Letter
Copper-Mediated Regioselective C−H Sulfenylation and Selenation
of Phenols with Phenanthroline Bidentate Auxiliary
Rikuo Kajiwara, Kazutaka Takamatsu, Koji Hirano,* and Masahiro Miura*
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ABSTRACT: A copper-mediated, phenanthroline-directed highly ortho-selective C−H sulfenylation of phenols with diaryl
disulfides proceeds to form the corresponding unsymmetrical diaryl sulfides in good yield. The key to success is the introduction of a
phenanthroline directing group of the bidentate-chelating nature, which is easily attachable, detachable, and even recyclable.
Moreover, the same strategy is applicable to the C−H selenation, giving the diaryl selenides with high efficiency and regioselectivity.
iaryl sulfide is an important class of compounds in
Dorganic chemistry because it is frequently occurring in
biologically active compounds and natural products.1 In
particular, the phenol-moiety-containing diaryl sulfides are
pivotal structural motifs in the treatment of cancer, HIV, and
heart disease.2 Accordingly, considerable attention has been
focusing on their concise and selective synthesis. The most
convergent approach is the metal-mediated aryl−S cross-
coupling reactions of aryl halides with SH thiols or their
derivatives.3 However, the preparation of the starting
halogenated aromatic compounds is often tedious and
problematic. The classical aromatic electrophilic sulfenyla-
tion-type reaction of phenols is more straightforward and
attractive from the viewpoint of atom efficiency, but the
regioselectivity is controlled by the innate electronic nature of
the phenol ring; a mixture of ortho- and para-regioisomers is
generally obtained.4 Thus the development of new protocols
for the regioselective direct sulfenylation of phenol derivatives
is strongly desired.
On the contrary, the metal-promoted C−H activation
chemistry has greatly progressed in recent decades and now
provides potentially more effective synthetic methodologies
than the conventional cross-coupling reactions relying on the
organic halides.5 In this context, the aromatic C−H
sulfenylation reactions have also been developed by using
Pd,6 Rh,7 and Cu8 catalysts. However, the viable substrates are
limited to electron-rich (hetero)arenes, phenylpyridines, and
benzamides; the regioselective C−H sulfenylation of phenols
still remains largely elusive. Herein we report a Cu-mediated
highly ortho-selective C−H sulfenylation of phenols with
diaryl disulfides by using a phenanthroline-type bidentate
auxiliary.9 The chelating nature of phenanthroline uniquely
promotes the reaction to form the targeted diaryl sulfides with
the phenol moiety. Additionally, the phenanthroline auxiliary is
readily accessible and easily attachable, detachable, and
recyclable. Moreover, the Cu/phenanthroline system is also
applied to the C−H selenation reaction with diaryl
diselenides.10 We note that during the course of this research
project, the related Co-mediated, carbonyl-directed C−H
sulfenylation of phenols with thiols was reported, but the
reaction includes a radical species, and in some cases, the
regioselectivity is thus complementary to the present work
(vide infra).11
On the basis of the previous success of the Cu-catalyzed C−
H amination with the phenanthroline-type auxiliary,9a our
studies commenced with the phenol derivative 1a and diphenyl
disulfide (2a) as model substrates. In an early experiment, the
treatment of 1a with 2a (4.0 equiv) in the presence of
Cu(OPiv)2 (30 mol %) in heated DMF (90 °C) formed a 1:1.7
mixture of mono- (3aa) and disulfenylated (4aa) products in
64% combined yield (Scheme 1a). Notably, only the ortho-C−
H bonds were selectively sulfenylated, and any para-C−H
sulfenylated products were not detected at all. Additionally, the
structure of 4aa was unambiguously confirmed by X-ray
analysis (CCDC 1989583). Under these conditions, other
directing groups were also tested: The parent phenol (1a-OH)
and its carbamate (1a-CONMe2) resulted in decomposition
and no reaction, respectively. The pyridyl- (1a-Py) and
pyrimidyl- (1a-Pym) substituted substrates, which are effective
for some noble transition-metal-catalyzed C−H activations,12
Received: June 17, 2020
https://dx.doi.org/10.1021/acs.orglett.0c02012
© XXXX American Chemical Society
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Scheme 1. Representative Optimization Studies for Cu-
Mediated Regioselective C−H Sulfenylation of Phenols 1
with Diphenyl Disulfide (2a)
Scheme 2. Cu-Mediated Regioselective C−H Sulfenylation
a
of Phenols 1 with Disulfides 2
also underwent no conversion. On the contrary, the bipyridyl-
type system 1a-bpy showed comparable reactivity (3/4 1:1.2,
68% combined yield). As a different type of bidentate auxiliary,
we also tried the reaction of a pyridylsulfoximine derivative
(1a-MPyS).13 However, no sulfenylated products were
observed. Apparently, the bidentate chelating nature of the
auxiliary as well as its sp2-hybridization-based high planarity
played pivotal roles in this reaction. Given the more ready
availability of the phenanthroline auxiliary (easily prepared on
a decagram scale from commercial phenanthroline),9 further
optimizations were performed with 1a. Extensive investigations
of Cu salts, solvents, and the reaction temperature finally
revealed that the reaction proceeded more efficiently and
selectively at 70 °C with a Cu(OAc)2 or CuTC (TC = 2-
thiophenecarboxylate) promotor to deliver the corresponding
a
Reaction conditions: Cu(OAc)2 or CuTC (0.25 mmol), 1 (0.25
mmol), 2 (1.0 mmol), DMF (1.5 mL), 70 °C, 40−48 h, N2. Isolated
b
c
d
yields are shown. On a 0.10 mmol scale. With 2.0 equiv of 2. On a
e
f
1.0 mmol scale. 1H NMR yield. With Cu(OPiv)2 and DMF (3.0
mL).
1
disulfenylated product 4aa in >80% H NMR yield (68 and
moderate yields of disulfenylated products, the monosulfeny-
lated products were not detected at all. The ortho-substituted
phenols 1j and 1k also underwent the reaction smoothly and
regioselectively, even in the presence of potentially reactive
para-C−H, and the monosulfenylated products 3ja and 3ka
were obtained as the sole regioisomers. In the case of 1l with
the meta substituent, the reaction proceeded preferably at the
more sterically accessible position (3la and 4la), which is
complementary to the Co-mediated, radical-promoted C−H
sulfenylation.11,15 A similar trend was also observed in the 2-
naphthol derivative (3ma and 4ma). The reaction could also
be performed on a preparative scale (3ka), thus indicating the
good reproducibility of this process.16
66% yields after purification, respectively; Scheme 1b).14
With the conditions in Scheme 1b, we next examined the
scope of the reaction (Scheme 2): The more abundant
Cu(OAc)2 was generally used, but in some cases, CuTC
showed a better performance. In addition to the simple 1a,
several electron-rich phenols 1b−d containing t-Bu, Ph, and
MeO groups at the para position were readily coupled to 2a to
form the corresponding C−H sulfenylated products 4ba−da in
good yields. On the contrary, the electron-deficient CF3-
substituted substrate showed moderate reactivity (4ea).
Particularly notable is the halogen compatibility: The
sulfenylation occurred at the ortho-C−H bonds over the C−
halogen bonds to deliver the unsymmetrical diaryl sulfides
4fa−ia, the remaining halogens of which can be useful
synthetic handles for further manipulations by the transition-
metal-catalyzed cross-coupling technology. Even in the cases of
Sterically and electronically diverse diaryl disulfides 2 were
amenable to the reaction. Except for the strongly electron-
donating MeO-substituted disulfide (3kd), functionalized SAr
moieties were successfully introduced to the phenol ring in
B
https://dx.doi.org/10.1021/acs.orglett.0c02012
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good to high yields (3kb,kc,ke−kh). Again, the high halogen
compatibility was observed (3kf and 3kg). Moreover, the
conceivably more challenging heteroaryl disulfides could also
be employed, and the thiophene-, pyridine-, and benzothiazole-
containing diaryl sulfides 3ki−kk were formed in acceptable
yields. On the contrary, the reaction with dialkyl disulfide
provided a reduced yield (3kl).
The phenanthroline-based auxiliary was also applicable to
the Cu-mediated regioselective C−H selenation of phenols
with diselenides (Scheme 3): Under slightly modified
form the corresponding disulfenylated free phenol 4aa-OH in
94% yield. Upon treatment with TFA, the directing group was
concurrently recovered as the 2-phenanthrolinone 8 in 91%
yield, which can be recycled for the 2-chlorophenanthroline.9
The obtained 4aa-OH further underwent the O-methylation
and Pd-catalyzed double intramolecular C−H/C−H cou-
pling18 to afford the bent-type benzobisbenzothiophene 9.
This pentacyclic system is frequently found in organic
semiconductors.19 Interestingly, the Br-containing substrate
4ag, which was prepared from 1a and 2g in 42% yield, was
directly transformed to the phenoxathiine 10 via successive
deprotection and intramolecular C−O coupling under CuI/
KO-t-Bu-mediated conditions. The subsequent Pd-catalyzed
intramolecular C−H/C−Br coupling reaction6b,c successfully
formed the benzothienophenoxathiine (11), the structure of
which was determined by X-ray crystallographic analysis
(CCDC 2010305). On the contrary, the removal of the
directing group from the C−H selenated product 6ka was also
possible (6ka-OH).
Scheme 3. Cu-Mediated Regioselective C−H Selenation of
a
Phenols 1 with Diphenyl Diselenide (5a)
In conclusion, we have developed a Cu-mediated
regioselective C−H sulfenylation and selenation of phenols
with diaryl disulfides and diselenides, respectively. Different
from the conventional aromatic electrophilic substation
reaction, the reaction exclusively occurs at the ortho position.
The key to success is the introduction of the bidentately
coordinating phenanthroline auxiliary, which is readily
prepared and easily attachable, detachable, and recyclable.
The obtained phenol-containing unsymmetrical diaryl sulfides
and selenides are of potent interest in medicinal and
pharmaceutical chemistry as well as a useful platform for the
preparation of highly condensed heterocyclic molecules. The
further development of related C−H activation Cu catalysis
and the application of the phenanthroline auxiliary to other
challenging C−H activation reactions are currently underway
in our laboratory.
a
Reaction conditions: Cu(OPiv)2 (0.10 mmol), 1 (0.10 mmol), 5a
(0.40 mmol for 1a, 0.20 mmol for 1j and 1k), DMF (1.0 mL), 70 °C,
24 h, N2. Isolated yields are shown. 12 h.
b
conditions with Cu(OPiv)2, diphenyl diselenide, (5a) reacted
with some phenol derivatives 1 to afford the corresponding
unsymmetrical diaryl selenides 7aa, 6ja, and 6ka in good
yields. Such diaryl selenides are also important core structures
in bioactive molecules and organic photosensitizers.17 Again,
the exclusive ortho-selectivity was observed in all cases.
Finally, we attempted to derivatize the C−H sulfenylated
products (Scheme 4). The phenanthroline directing group was
readily removed from 4aa by KO-t-Bu-mediated alcoholysis to
ASSOCIATED CONTENT
■
Scheme 4. Derivatizations of Products
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c02012.
1H, 13C{1H}, and 19F{1H} NMR spectra, ORTEP
drawing, and detailed optimization studies (PDF)
Accession Codes
CCDC 1989583 and 2010305 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.
AUTHOR INFORMATION
■
Corresponding Authors
Koji Hirano − Department of Applied Chemistry, Graduate
School of Engineering, Osaka University, Suita, Osaka 565-
0871, Japan; orcid.org/0000-0001-9752-1985;
Email: k_hirano@chem.eng.osaka-u.ac.jp
Masahiro Miura − Department of Applied Chemistry, Graduate
School of Engineering, Osaka University, Suita, Osaka 565-
0871, Japan; orcid.org/0000-0001-8288-6439;
Email: miura@chem.eng.osaka-u.ac.jp
C
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Authors
pubs.acs.org/OrgLett
Letter
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ChemSusChem 2019, 12, 4601.
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Rikuo Kajiwara − Department of Applied Chemistry, Graduate
School of Engineering, Osaka University, Suita, Osaka 565-
0871, Japan
Kazutaka Takamatsu − Department of Applied Chemistry,
Graduate School of Engineering, Osaka University, Suita,
Osaka 565-0871, Japan
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c02012
Notes
́
́
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
(7) Yang, Y.; Hou, W.; Qin, L.; Du, J.; Feng, H.; Zhou, B.; Li, Y.
Chem. - Eur. J. 2014, 20, 416.
This work was supported by JSPS KAKENHI grant nos. JP
17J00349 (Grant-in-Aid for JSPS Research Fellow) to K.T., JP
15H05485 (Grant-in-Aid for Young Scientists (A)) and JP
18K19078 (Grant-in-Aid for Challenging Research (Explor-
atory)) to K.H., and JP 17H06092 (Grant-in-Aid for Specially
Promoted Research) to M.M. K.H. also acknowledges The
Kyoto Technoscience Center for financial support. We thank
Dr. Yuji Nishii (Osaka University) for his assistance with the
X-ray analysis.
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