Aromatic Metamorphosis of Dibenzofurans into Triphenylenes Starting with Nickel-Catalyzed Ring-Opening C-O Arylation

Article abstract of DOI:10.1021/acs.orglett.6b03861

A new class of aromatic metamorphosis has been developed in which dibenzofurans were converted into triphenylenes. This transformation is composed of three successive operations: (1) nickel-catalyzed ring-opening C-O bond arylation with arylmagnesium bromides, (2) trifluoromethanesulfonylation (triflation) of the resulting hydroxy moiety with Tf₂O, and (3) palladium-catalyzed or photoinduced ring closure. In the last ring-closing step, the photoinduced process has proven to be more productive than the palladium-catalyzed one. By employing π-extended dinaphthofuran as the substrate, dorsally benzo-fused [5]helicene was obtained in a satisfactory yield.

Full text of DOI:10.1021/acs.orglett.6b03861

Letter
pubs.acs.org/OrgLett
Aromatic Metamorphosis of Dibenzofurans into Triphenylenes
Starting with Nickel-Catalyzed Ring-Opening C−O Arylation
Yuto Kurata, Shinya Otsuka, Norihito Fukui, Keisuke Nogi, Hideki Yorimitsu,* and Atsuhiro Osuka
Department of Chemistry, Graduate School of Science, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan
S
* Supporting Information
ABSTRACT: A new class of aromatic metamorphosis has been developed in
which dibenzofurans were converted into triphenylenes. This transformation is
composed of three successive operations: (1) nickel-catalyzed ring-opening C−
O bond arylation with arylmagnesium bromides, (2) trifluoromethanesulfony-
lation (triflation) of the resulting hydroxy moiety with Tf2O, and (3)
palladium-catalyzed or photoinduced ring closure. In the last ring-closing step,
the photoinduced process has proven to be more productive than the palladium-catalyzed one. By employing π-extended
dinaphthofuran as the substrate, dorsally benzo-fused [5]helicene was obtained in a satisfactory yield.
riphenylenes have gained increasing attention because of
their unique physical and optical properties, which can be
utilized as liquid crystals as well as materials in organic light-
emitting diodes (OLEDs).1 Nevertheless, owing to the high
symmetry and planarity of the triphenylene core, efficient and
selective synthesis of triphenylenes remains difficult and can be
further improved.2
Scheme 1. Strategy for Transformation of Dibenzofurans
into Triphenylenes
T
Recently, we have become interested in developing novel
methodologies, “aromatic metamorphosis”, in which an
aromatic compound is converted into another cyclic compound
through aromatic ring cleavage and reconstruction processes.3
Along this line, we achieved transformations of dibenzothio-
phenes into triphenylenes in four steps assisted by palladium
catalysts.3a However, this four-step protocol has several
drawbacks: (1) organosulfur substrates have to be activated
as sulfonium salts by alkylation of the sulfur atom before both
ring-breaking and ring-closing steps, (2) stoichiometric
amounts of silver salts are necessary for the alkylation reactions,
and (3) resulting sulfonium species are often unstable. To
address these problems and expand the utility of aromatic
metamorphosis, here we report conversions of dibenzofurans
into triphenylenes in three steps powered by nickel-catalyzed
ring-opening C−O bond arylation and palladium-catalyzed or
photoinduced ring closure.
Our strategy is depicted in Scheme 1. The first step is ring-
opening arylation of the C−O bond with an arylmetal reagent
under nickel catalysis (step a). After the ring-opening arylation,
the resulting aryloxy moiety would be readily converted into
triflate with trifluoromethanesulfonic anhydride (Tf2O) (step
b). Since the C−O bond of aryl triflate is highly reactive, we
expected that the subsequent ring closure would proceed easily
(step c).
alkylation of the challenging endocyclic C−O bonds of
heteroaromatic furan analogues was also accomplished under
nickel5a,n,6 or rhodium catalysis.7 However, dibenzofurans have
rarely undergone C−O bond cleavage. Although Chatani and
Tobisu recently succeeded in nickel-catalyzed ring-opening
alkylation of dibenzofuran with an alkylmagnesium reagent,5n
there has been no example of arylative ring-opening of
dibenzofurans.
We first screened a series of nickel catalysts in the presence
of p-tolylmagnesium bromide to execute ring-opening arylation
of dibenzofuran (1a) as a model (Table 1). Phosphine ligands
as well as a widely used carbene ligand IPr did not show
catalytic activity (entries 1−3). Similar reaction conditions for
the arylative C−O bond cleavage of benzofuran reported by
Martin was not applicable to the conversion of dibenzofuran
(entry 4).6c Fortunately, 1,3-dialkylimidazol-2-ylidene ligands,
especially I(1-Ad) and ICy, afforded the arylated product 2a in
First, we needed to develop ring-opening arylation of
dibenzofurans with arylmagnesium reagents. Although an
ethereal C−O bond is much stronger than C−X (X = I, Br,
Cl) bonds, a variety of catalytic C−O bond cleavages are
rapidly emerging with the aid of electron-rich transition metal
catalysts such as nickel complexes.4,5 Ring-opening arylation or
Received: December 27, 2016
© XXXX American Chemical Society
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DOI: 10.1021/acs.orglett.6b03861
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Organic Letters
Letter
Table 1. Optimization of Catalyst System
Scheme 2. Scope of Ring-Opening Arylation
entry
Ni cat.
ligand
SPhos
NMR yield (%)
1
2
3
NiCl2(PCy3)2
Ni(cod)2
8
6
NiCl2(IPr) (PPh3)
Ni(cod)2
0
a
4
SIPr·HCl
trace
3
5
6
7
8
Ni(cod)2
ItBu·HBF4
I(1-Ad)·HBF4
ICy·HBF4
ICy·HCl
Ni(cod)2
36
77
80
91
Ni(cod)2
Ni(cod)2
b
9
Ni(cod)2
I(2-Ad)·HCl
a
b
1.0 equiv of LiCl was added. Performed for 2 h.
a
Triflation was performed with 2 equiv of Tf2NPh and 2 equiv of NEt3
moderate to good yields (entries 6−8). Eventually, I(2-Ad)5d,8
was found to be optimal for efficient ring opening to afford
hydroxyteraryl 2a in 91% NMR yield (entry 9).
b
at −78 °C to room temperature for 24 h. Nickel-catalyzed reaction
was performed for 5 h. 3.0 mmol scale.
c
After aqueous workup of the arylation, treatment of the
crude reaction mixture with Tf2O furnished the corresponding
teraryl triflate 3a in 88% isolated yield over two steps (Scheme
2). We then conducted the ring-opening C−O bond arylation
and subsequent triflation with an array of arylmagnesium
bromides. Although electron-rich 4-(dimethylamino)-
phenylmagnesium bromide underwent the C−O bond
arylation to give the product 3c in good yield, electron-
deficient 4-fluoro- or 4-(trifluoromethyl)phenylmagnesium
bromide did not afford the corresponding product, and 1a
was recovered. Sterically congested 1-naphthyl- or 9-phenan-
thrylmagnesium bromide provided the coupling products 3e or
3f albeit in moderate yields as well as in a longer reaction time.
As for the scope of dibenzofurans, 2,8-diphenyldibenzofuran
(1g) was also applicable to this transformation. The C−O bond
arylation also proceeded with π-extended dinaphthofurans to
furnish 3h and 3i.
With teraryl triflates 3 in hand, we attempted the last ring-
closing transformation to form triphenylenes (Scheme 3). In
our previous report, ring closure of teraryl sulfonium
compounds was accomplished by means of a palladium/
SPhos catalyst.3a As we expected, under similar reaction
conditions (procedure A), ring closure of 3a also proceeded
to afford 2-methyltriphenylene (4a) in 89% yield. However,
when other teraryl triflates were employed, the yields of
products were moderate or significantly low, and 4i was not
obtained at all. Furthermore, teraryl triflate 3d afforded a
mixture of two regioisomers 4d and 4d′ with moderate
selectivity.
As an alternative method to execute smooth ring closure, we
then focused on photoinduced cyclization with a loss of TfOH.
In the literature,9 under photoirradiation conditions, 2-
halogenated stilbenes or o-terphenyls undergo dehydrohaloge-
native cyclization to afford phenanthrenes or triphenylenes.
Moreover, this method was applicable to the synthesis of
polycyclic aromatic hydrocarbons such as dibenzotetracenes.9b,e
We thus conducted photoinduced ring closure of teraryl
triflates 3. A dilute benzene solution of 3a was irradiated by
means of a 100-W high-pressure mercury lamp at room
temperature for 30 min. To our delight, 4a was successfully
obtained in 90% yield. In contrast to the palladium-catalyzed
system, this method was applicable to a range of teraryl triflates
3 and provided the corresponding triphenylenes 4 in moderate
to good yields. It is noteworthy that photoinduced ring closure
of 3d took place with exclusive regioselectivity to furnish 4d as
a single isomer in 88% yield. It is assumed that the ring closure
proceeds through photoinduced 6π electrocyclization followed
by elimination of TfOH. The stronger double-bond character
of the C1−C2 bond of the naphthalene ring than that of the
C2−C3 bond would lead to the perfect 4d/4d′ regioselectivity.
The ring closure of 3i gave benzo[5]helicene 4i in 44% yield. A
larger scale synthesis (1.0 mmol of 3i in 100 mL of benzene)
improved the yield of 4i up to 59%. This three-step access to
B
DOI: 10.1021/acs.orglett.6b03861
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Organic Letters
Letter
Scheme 3. Formation of Triphenylenes 4
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: yori@kuchem.kyoto-u.ac.jp
ORCID
Shinya Otsuka: 0000-0001-5598-9707
Keisuke Nogi: 0000-0001-8478-1227
Hideki Yorimitsu: 0000-0002-0153-1888
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by JSPS KAKENHI Grant Numbers.
JP23655037, JP24685007, JP25107002, JP16H01019,
JP16H01149, JP16H04109, and JP16H06887 as well as by
ACT-C, JST. H.Y. thanks the Japan Association for Chemical
Innovation, Tokuyama Science Foundation, and The Naito
Foundation for financial support. S.O. and N.F. acknowledge
support through JSPS Predoctoral Fellowships.
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ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.6b03861.
Detailed experimental procedures as well as full
spectroscopic data for all new compounds (PDF)
C
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