Acid-catalyzed synthesis of condensed polycyclic diaryl ethers from arenols

Article abstract of DOI:10.1039/c9cc07172e

Diaryl ethers containing condensed polycyclic aryl groups were synthesized from arenols in the presence of a catalytic amount of p-chlorobenzenesulfonic acid. Symmetrical binaphthyl, biphenanthryl, and bipyrenyl ethers were obtained in high yields. Unsymmetrical derivatives were also synthesized from 9-phenanthrol and arenols, using combinations of reactive and unreactive substrates.

Full text of DOI:10.1039/c9cc07172e

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Acid-catalyzed synthesis of condensed polycyclic
diaryl ethers from arenols†
Cite this: Chem. Commun., 2019,
55, 14078
Saori Tanii, Mieko Arisawa * and Masahiko Yamaguchi
*
Received 13th September 2019,
Accepted 31st October 2019
DOI: 10.1039/c9cc07172e
rsc.li/chemcomm
Diaryl ethers containing condensed polycyclic aryl groups were
Naphthols can readily tautomerize between enol- and
synthesized from arenols in the presence of a catalytic amount of keto-forms in the presence of an acid catalyst.7 Nucleophilic
p-chlorobenzenesulfonic acid. Symmetrical binaphthyl, biphenanthryl, addition at the carbonyl carbon of the keto-form followed by
and bipyrenyl ethers were obtained in high yields. Unsymmetrical dehydration can result in a substitution reaction of hydroxy
derivatives were also synthesized from 9-phenanthrol and arenols, groups, and naphthyl sulfides, amines, and ethers were obtained
using combinations of reactive and unreactive substrates.
by reactions with thiols, amines, and alcohols, respectively. Acids
including p-toluenesulfonic acid,8 CF3SO3H,9 BF3OEt2,10 and
11
Diphenyl ethers are an important class of organic compounds, Bi(OTf)3 were used as catalysts. Application of these methods
which can exhibit various biological activities and material to phenols, however, is restricted to the scattered synthesis
functions,1 and generally have been synthesized by substitution of 1- and 2-naphthyl phenyl ethers.8c–10,11b We herein report an
reactions using halobenzenes and phenols under basic conditions.2 acid-catalyzed method to substitute the hydroxy groups by
In contrast to diphenyl ethers with a monocyclic benzene moiety, condensed polycyclic arenols (Scheme 1). The method provides
the synthesis of diaryl ethers with condensed polycyclic aryl various condensed polycyclic and polysubstituted diaryl ethers
groups is quite limited. A few syntheses of binaphthyl ethers with symmetrical and unsymmetrical structures, which were
have been reported, using naphthyl halides and naphthoxides not known previously.
via employing Ullmann-type reactions with copper or palladium
When 2-naphthol in o-dichlorobenzene was heated at reflux
catalysis3 and nucleophilic aromatic substitution reactions.4,5 for 10 h in the presence of p-chlorobenzenesulfonic acid
Such methods, however, have not been applied to the synthesis (10 mol%), bis(2-naphthyl)ether 2a was obtained in 85% yield
of higher condensed polycyclic arenols, despite potential appli- (Table 1). The reaction did not occur in the absence of the acid.
cations of such electron-rich diaryl ethers with simple structures The use of other Brønsted acid catalysts, such as triflic acid
in optical and electrical materials. This lack may partly be due to (46%), triflic imide (27%), methanesulfonic acid (80%),
the instability of the arenols, which readily undergo oxidation p-toluenesulfonic acid (70%), and Nafions (22%), generally
and oligomerization under basic conditions.
decreased the yield of 2a.
Alternatively, naphthols were reported to give symmetrical
Substituted 2-naphthols were reacted to give the corres-
binaphthyl ethers when heated at extremely high temperatures.6 ponding bis(2-naphthyl)ethers 2b–2e in high yields (Table 1).
Apparently, this high temperature method is more convenient
than the above-mentioned method because it uses readily avail-
able substrates without stoichiometric amounts of bases. Its
applicability, however, is limited to simple binaphthyl ethers,
and the synthesis of substituted binaphthyl ethers and higher
condensed polycyclic diaryl ethers has not been reported.
Another subject not examined previously is its applicability to
the synthesis of unsymmetrical diaryl ethers.
Department of Organic Chemistry, Graduate School of Pharmaceutical Sciences,
Tohoku University, Aoba, Sendai, 980-8578, Japan.
Scheme 1 Acid-catalyzed synthesis of condensed polycyclic diaryl ethers
E-mail: arisawa@m.tohoku.ac.jp, yama@m.tohoku.ac.jp
from arenols.
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c9cc07172e
14078 | Chem. Commun., 2019, 55, 14078--14080
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Table 1 Synthesis of symmetrical polycyclic diaryl ethers
Table 2 Synthesis of unsymmetrical polycyclic diaryl ethers
a
In parentheses are yields of bis(9-phenanthryl)ether 2k. N.D.: not
b
detected. 4% yield of 2e was obtained.
a
Reacted for 24 h.
A possible mechanism of this reaction to provide unsymme-
trical diaryl ethers is shown below (Fig. 1). Condensed polycyclic
Methyl and halogen groups were not affected. 1-Naphthol gave arenols tautomerize between their keto- and enol-forms, a process
bis(1-naphthyl)ether 2f in 85% yield when the reaction time accelerated by acid catalysis. An arenol with a higher keto-form
was extended to 24 h. 1-Naphthols with polymethyl groups concentration may then be attacked at the carbonyl carbon by
smoothly reacted to provide 2g–2j, and bis(2-phenanthryl) and another arenol with a lower keto-form concentration, and
bis(4-pyrenyl)ethers 2k–2m were also obtained in high yields. dehydration provides the unsymmetrical diaryl ether. The
Several arenols including 6,7-dibromo-1-naphthol and 2-phenanthrol unreactive arenol does not provide the symmetrical diaryl ether.
are less reactive substrates providing lower yields of diaryl ethers.
To confirm the proposed mechanism, the reaction of
2-Pyrenol and monocyclic phenols are unreactive substrates, and 18O-labeled phenol was examined (Scheme 2). When
did not give diaryl ethers under the present conditions. Note that 9-phenanthrol 1k and Ph18OH (1 equilvalent, 18O-enrichment of
the symmetrical diaryl ethers obtained in this study are new 85%) were reacted in refluxing o-dichlorobenzene for 10 h in
compounds except for 2a, 2f, and 2k.12 This catalytic reaction the presence of p-chlorobenzenesulfonic acid (20 mol%),
can be a conventional method for the synthesis of symmetrical 9-phenoxyphenanthrene 4j was obtained in 63% yield along with
condensed polycyclic diaryl ethers.
2k (28%). The 18O-enrichment of 4j was determined by ESI-MS to
The synthesis of unsymmetrical diaryl ethers was examined be 83%. 2k was not labelled with 18O. This result supported the
under the same conditions using the combination of reactive reaction mechanism between the keto-form of 1k and arenol.
9-phenanthrol and less reactive substrates such as monocyclic
phenols, 6,7-dibromo-1-naphthol, 2-phenanthrol, and 3-pyrenol
(Table 2). When 9-phenanthrol and 3-fluorophenol (1 equivalent)
were reacted in refluxing o-dichlorobenzene for 20 h in the
presence of p-chlorobenzenesulfonic acid (20 mol%), 9-(3-fluoro-
phenoxy)phenanthrene 4a was obtained in 68% yield. A small
amount of symmetrical 2k (14%) was formed, although no bis(3-
fluorophenyl)ether was detected, which was consistent with the
low reactivity of 3-fluorophenol towards diaryl ether formation.
Substituted phenols also reacted, and halogen and methoxy
groups in phenols were tolerated. 1-Naphthols, 2-naphthol,
2-phenanthol, 2-triphenylenol, and 3-pyrenol reacted with
9-phenanthrol to provide the corresponding 9-phenanthryl
ethers 4b–4i in high yields. All the unsymmetrical diaryl ethers
Fig. 1 Possible reaction mechanism.
are previously unreported compounds.
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oligomerization byproducts from the arenols, which readily
occur under basic conditions. Condensed polycyclic aryl ethers
are not common, and the present method may find use in the
development of novel optical and electronic materials.15
The authors would like to thank Mr Kohei Fukumoto for his
efforts in this study. This research was supported by the Platform
Project for Supporting Drug Discovery and Life Science Research
from AMED (Grant Number JP18am0101100) and Tohoku Univer-
sity Center for Gender Equality Promotion (TUMUG). S. T. acknowl-
edges Grants-in-Aid for Young Scientists (B) (no. 19K15551) from
the Japan Society for the Promotion of Science.
Scheme 2 Reaction of 9-phenanthrol 1k and 18O-labeled phenol.
Conflicts of interest
There are no conflicts to declare.
Notes and references
Fig. 2 Reactivity of arenols.
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14080 | Chem. Commun., 2019, 55, 14078--14080
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