Palladium-Assisted "Aromatic Metamorphosis" of Dibenzothiophenes into Triphenylenes

Article abstract of DOI:10.1002/anie.201501992

Abstract Two new palladium-catalyzed reactions of aromatic sulfur compounds enabled the conversion of dibenzothiophenes into triphenylenes in four steps. This transformation of one aromatic framework into another consists of 1) 4-chlorobutylation of the dibenzothiophene to form the corresponding sulfonium salt, 2) palladium-catalyzed arylative ring opening of the sulfonium salt with a sodium tetraarylborate, 3) an intramolecular S_N2 reaction to form a teraryl sulfonium salt, and 4) palladium-catalyzed intramolecular C-S/C-H coupling through electrophilic palladation. Symmetrical as well as unsymmetrical triphenylenes of interest were synthesized in a tailor-made fashion in satisfactory overall yields. A change of heart: The invention of two palladium-catalyzed arylation reactions of organosulfur compounds enabled the transformation of dibenzothiophenes into triphenylenes and thus a fundamental change in the core aromatic structure (see scheme). Both symmetrical and unsymmetrical triphenylenes were synthesized in a tailor-made fashion in satisfactory overall yield.

Full text of DOI:10.1002/anie.201501992

Angewandte
Chemie
International Edition: DOI: 10.1002/anie.201501992
German Edition: DOI: 10.1002/ange.201501992
Aromatic Compounds
Palladium-Assisted “Aromatic Metamorphosis” of Dibenzothiophenes
into Triphenylenes**
Dhananjayan Vasu, Hideki Yorimitsu,* and Atsuhiro Osuka
Abstract: Two new palladium-catalyzed reactions of aromatic
sulfur compounds enabled the conversion of dibenzothio-
phenes into triphenylenes in four steps. This transformation of
one aromatic framework into another consists of 1) 4-chloro-
butylation of the dibenzothiophene to form the corresponding
sulfonium salt, 2) palladium-catalyzed arylative ring opening
of the sulfonium salt with a sodium tetraarylborate, 3) an
intramolecular SN2 reaction to form a teraryl sulfonium salt,
the transformation of dibenzothiophenes into triphenylenes
with the aid of palladium catalysis. Triphenylenes have been
attracting increasing attention owing to their broad applica-
tion in liquid crystals, OLEDs, and photoconductive and
optical data-storage devices.[7] However, the tailor-made
synthesis of multisubstituted triphenylenes remains a chal-
lenge because of their symmetrical and planar structures.[8,9]
Our strategy began with the activation of dibenzothio-
phenes 1 as sulfonium salts 2 with diminishment of the Lewis
basicity of the sulfur atom (Scheme 1). The activated
À
À
and 4) palladium-catalyzed intramolecular C S/C H cou-
pling through electrophilic palladation. Symmetrical as well
as unsymmetrical triphenylenes of interest were synthesized in
a tailor-made fashion in satisfactory overall yields.
T
he fundamental nature of aromatic compounds is governed
by the specific characteristics of the aromatic rings they
contain. Aromatic rings are generally stable and thus
unbreakable except under harsh or special conditions. The
conversion of an aromatic system into a different aromatic
system is challenging.[1] Developments in transition-metal
catalysis in the last decade now enable some transformations
of this type. Specifically, triazoles and related high-energy
triazaarenes undergo catalytic denitrogenative transannula-
tion with unsaturated compounds, such as alkynes and nitriles,
to afford other aromatic systems.[2] There still remains ample
room to develop novel strategies for the conversion of
aromatic systems into other aromatic systems with the aid
of catalysts.
Scheme 1. Strategy for the aromatic metamorphosis of dibenzothio-
phenes into triphenylenes (M=metal).
sulfonium salt 2 had been expected to undergo smooth
cross-coupling with an aryl nucleophile.[10–12] The product of
ring opening, a 4-chlorobutyl teraryl sulfide 3, would be
activated again through the formation of the corresponding
Sulfur-containing heteroaromatic skeletons, such as thio-
phene, are found widely in bioactive compounds and organic
electronics as well as abundantly in unrefined oils.[3] The
catalytic cleavage of thiophenes is usually difficult because
sulfur species strongly poison transition-metal catalysts.
Although desulfurization–hydrogenation is an important
heterogeneous catalytic process in oil refining,[4] there are
no reports on the catalytic conversion of thiophene units into
other aromatic molecules.[5,6] Herein we report a strategy for
À
cyclic sulfonium salt 4. Intramolecular C H arylation of 4
should finalize our aromatic metamorphosis, although there
[13]
À
have been no reports of catalytic C H arylation with an
aromatic sulfur compound as an electrophilic partner.
The first step occurred readily through a AgBF4-mediated
SN2 reaction[14] of dibenzothiophenes with a 1-halo-4-chloro-
butane [Eq. (1); see also the Supporting Information].
[*] Dr. D. Vasu, Prof. Dr. H. Yorimitsu, Prof. Dr. A. Osuka
Department of Chemistry, Graduate School of Science
Kyoto University
Sakyo-ku, Kyoto 606-8502 (Japan)
E-mail: yori@kuchem.kyoto-u.ac.jp
Prof. Dr. H. Yorimitsu
ACT-C, JST
Sakyo-ku, Kyoto 606-8502 (Japan)
We next examined the arylative ring opening of 2.
However, 2 suffered from rapid dealkylative decomposition
owing to the high leaving-group ability of the dibenzothio-
phene unit. Typical Suzuki–Miyaura or Migita–Kosugi–Stille
conditions were ineffective and provided 1a as the unwanted
main product (see the Supporting Information for details).
The former reaction requires basic conditions that are not
[**] This research was supported by Grants-in-Aid from MEXT (No.
25107002 “Science of Atomic Layers”) and from the JSPS (Nos.
24685007 (Young Scientists (A)) and 26620081 (Exploratory
Research)). D.V. acknowledges a JSPS Postdoctoral Fellowship.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201501992.
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À
compatible with 2, and the latter led to the direct dealkylation
of 2 with a phenyltin reagent. These unsuccessful results
suggested that desired aryl metal species should show
exquisite reactivity in the smooth transfer of their aryl
group to palladium without the aid of an additional base
and should remain unreactive toward the sulfonium salt 2.
With these guidelines in mind, we eventually devised
a combination of a sodium tetraarylborate[15] and a [Pd-
(PtBu3)2] catalyst[16] for the desired transformation under mild
additive-free conditions (Table 1). A variety of tetraaryl-
borates participated in the arylative ring-opening reaction.
All reactions proceeded to completion within 45 min at 358C,
which highlights the high efficiency of the ring opening.
ring opening at the less hindered C S bond to form 3o
exclusively. This kinetic control also operated in the reactions
of benzonaphthothiophenes 2g–i, albeit with modest regio-
selectivity.[17]
The treatment of 3 with AgSbF6 led to highly efficient SN2
cyclization to give the corresponding teraryl sulfonium salts 4
[Eq. (2); see also the Supporting Information].
With compounds 4 in hand, we screened reaction con-
À
ditions extensively for the final intramolecular C H arylation
Table 1: Arylative ring opening of 2a with NaBAr4.
(see the Supporting Information for details). Fortunately,
a Pd/SPhos catalyst[18] proved to be uniquely effective for the
generation of triphenylenes 5 (Table 2, Scheme 3; see also the
Supporting Information). This transformation is a rare exam-
À
ple of C H arylation involving organosulfur compounds as
R
3
Yield [%]
R
3
Yield [%]
electrophilic arylating agents.[19] The reactions generally
furnished the desired triphenylenes 5 in high to excellent
yields. The four-step overall yields are satisfactory in most
cases. The electronic nature of a substituent at the 4-position
had little effect on the reaction efficiency (Table 2). The
compatibility of a formyl group with the reaction conditions
enabled the formation of 5j. The cyclization reactions of 3-
substituted substrates 4b, 4d, 4h, and 4i posed a regioselec-
tivity issue (Scheme 3; see also the Supporting Information).
The regioselectivity was broadly in favor of the formation of
the more congested triphenylenes. Notably, the cyclization of
3,4-methylenedioxy-substituted 4h yielded 5h with exclusive
regioselectivity. The regioselectivity of the formation of
naphthyl-derived 5i/5i’ was as high as 4.8:1, whereas that
observed for the reactions of 4b and 4d (R = 3-Me and 3-
OMe, respectively, in the precursor structure 3 in Table 1) was
almost negligible. Steric repulsion around the sulfur atom in
4l, 4n, and 4o had no influence on the efficiency of the
cyclization. In contrast, 5m was obtained in only 22% yield
along with the elimination product 3m’, probably because of
the outer-sphere steric repulsion in the binaphthyl backbone.
The reaction of an isomeric mixture of 4p and 4p’ led to the
convergent formation of 5i. Similarly, a mixture of 4q and 4q’
H
3a
3b
3c
3d
3e
81
79
80
89
83
4-F
4-Cl
3,4-OCH2O
(2-naphthyl)[a]
4-CHO[b]
3 f
3g
3h
3i
88
77
84
73
88
3-Me
4-Me
3-MeO
4-MeO
3j
[a] NaB(2-C10H7)4 was used. [b] NaB[C6H4-4-CH(OMe)2]4 was used.
Compound 3j was obtained after hydrolysis with I2 in aqueous acetone.
The results of the arylative ring opening of substituted
dibenzothiophene sulfonium salts are shown in Scheme 2. The
ring-opening reaction was powerful enough to yield the highly
crowded products 3l and 3n as well as the quinquephenyl
derivative 3k. Unsymmetrical 2 f underwent regioselective
Table 2: Palladium-catalyzed cyclization of 4.
R
5
Yield from 4 [%]
(yield from 1 [%])
R
5
Yield from 4 [%]
(yield from 1 [%])
H
Me
5a 92 (70)
5c 89 (66)
F
Cl
5 f 98 (70)
5g 84 (54)
MeO[a] 5e 98 (73)
CHO 5j
86 (52)
[a] The reaction was carried out with 6 mol% of [Pd2dba3] and 12 mol%
of SPhos. dba=dibenzylideneacetone, DME=dimethoxyethane,
SPhos=2-dicyclohexylphosphanyl-2’,6’-dimethoxybiphenyl.
Scheme 2. Products of the arylative ring opening of 2b–i (the struc-
tures of which are shown in the Supporting Information).
2
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Scheme 5. Synthesis of multisubstituted triphenylenes through Tf2O-
Scheme 3. Triphenylene products 5h, 5i, and 5k–o. Reaction condi-
tions are described in Table 2. For the reaction of 4h, 10 mol% of
[Pd2dba3] and 20 mol% of SPhos were used.
mediated sulfonium formation: a) Tf2O (2 equiv), (CH2Cl)2, 258C, 15–
30 min; b) see reaction conditions in Table 1, Equation (2), and
Table 2. Tf=trifluoromethanesulfonyl.
converged into the sole product 5i’. Given that a mixture of
5i/5i’ was obtained from 4i, well-designed synthetic pathways
would offer tailor-made syntheses of appropriately substi-
tuted triphenylenes.
were converted into triphenylenes 10 in good overall yields in
five steps from 8. This strategy will be useful for synthesizing
multisubstituted triphenylenes in a precisely controlled
manner.
In conclusion, we have developed an “aromatic meta-
morphosis” of dibenzothiophenes into triphenylenes by
inventing two new palladium-catalyzed arylation reactions.
Owing to the invaluable roles played by aromatic core
structures, aromatic metamorphosis could be a game-chang-
ing strategy in organic chemistry. Investigations to develop
new forms of aromatic metamorphosis are under way in our
laboratory.
À
To clarify the reaction mechanism for the C H arylation
process, ortho-monodeuterated [D1]4a was prepared and
subjected to the cyclization conditions (Scheme 4). We found
À
À
Keywords: C H arylation · C S cleavage · cross-coupling ·
palladium · sulfur
Scheme 4. Deuterium-labeling experiment.
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Angewandte
Chemie
1808; d) B. Martꢃn-Matute, C. Mateo, D. J. Cꢁrdenas, A. M.
Echavarren, Chem. Eur. J. 2001, 7, 2341; e) B. S. Lane, M. A.
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1995, chap. 3.4.
[23] For the TfOH-mediated cyclization of 2-arylphenyl sulfoxides,
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[21] M. Gꢄmez-Gallego, M. A. Sierra, Chem. Rev. 2011, 111, 4857.
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Received: March 3, 2015
Published online: && &&, &&&&
Angew. Chem. Int. Ed. 2015, 54, 1 – 6
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

.
Angewandte
Communications
Communications
Aromatic Compounds
D. Vasu, H. Yorimitsu,*
A. Osuka
&&&& — &&&&
Palladium-Assisted “Aromatic
Metamorphosis” of Dibenzothiophenes
into Triphenylenes
A change of heart: The invention of two
palladium-catalyzed arylation reactions of
organosulfur compounds enabled the
transformation of dibenzothiophenes
into triphenylenes and thus a fundamen-
tal change in the core aromatic structure
(see scheme). Both symmetrical and
unsymmetrical triphenylenes were syn-
thesized in a tailor-made fashion in
satisfactory overall yield.
6
www.angewandte.org
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2015, 54, 1 – 6
These are not the final page numbers!