Synthesis of spirocyclic diarylfluorenes by one-pot twofold SNAr reactions of diaryl sulfones with diarylmethanes

Article abstract of DOI:10.1021/acs.orglett.5b03384

Treatment of dibenzothiophene dioxides with cyclic diarylmethanes in the presence of KN(SiMe₃)₂ results in the formation of fluorene-based spirocyclic tetraarylmethanes in a single operation. The transformation would proceed via an intermolecular S_NAr reaction of the dioxides with cyclic diarylmethylpotassium followed by intramolecular S_NAr cyclization. This straightforward strategy provides a wide range of spirocyclic diarylfluorenes including unusual ones that are otherwise difficult to synthesize.

Full text of DOI:10.1021/acs.orglett.5b03384

Letter
pubs.acs.org/OrgLett
Synthesis of Spirocyclic Diarylfluorenes by One-Pot Twofold SNAr
Reactions of Diaryl Sulfones with Diarylmethanes
M. Bhanuchandra, Hideki Yorimitsu,* and Atsuhiro Osuka
Department of Chemistry, Graduate School of Science, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan
S
* Supporting Information
ABSTRACT: Treatment of dibenzothiophene dioxides with
cyclic diarylmethanes in the presence of KN(SiMe3)2 results in
the formation of fluorene-based spirocyclic tetraarylmethanes in a
single operation. The transformation would proceed via an
intermolecular SNAr reaction of the dioxides with cyclic
diarylmethylpotassium followed by intramolecular SNAr cyclization. This straightforward strategy provides a wide range of
spirocyclic diarylfluorenes including unusual ones that are otherwise difficult to synthesize.
pirocyclic bifluorene cores have attracted recent attention in
the fields of molecular tectonics,1 chiral molecular
Scheme 1. Approaches to Spirocyclic 9,9-Diarylfluorenes
S
recognition,2 and molecular electronics3 including low-molec-
ular-weight light emitting materials4 and macromolecular ones.5
The bifluorene system is comprised of two conformationally
fixed and well-conjugated biphenyl units, which leads to
interesting photophysical and electrochemical properties such
as high fluorescence quantum yields as well as chemical
robustness favorable for long-lived organic materials. Addition-
ally, its perpendicular framework results in its high solubility,
which is advantageous for easy material processing.
Taking advantage of these fascinating properties of the
bifluorene core, some research groups are interested in other
spirocyclic 9,9-diarylfluorenes such as fluorene−xanthene,6
−thioxanthene,7 and −dihydroanthracene6b,8 hybrids. Due to
the different ring sizes and/or the presence of a heteroatom,
these molecules show intriguing properties that are better than or
different from those of bifluorene-based analogues. Among them,
fluorene−xanthene hybrids have been well investigated not only
because they show fascinating properties but also because they
are readily available through Friedel−Crafts cyclization reactions
under harsh acidic conditions,6,9 especially through Huang’s one-
pot condensation method (Scheme 1).6o,10 Much less attention
has been paid to spirocyclic fluorene−thioxanthene and
−dihydroanthracene analogs, mainly owing to the lack of
efficient synthetic routes.6b,7,8,9b It is thus of great importance
to develop convenient and efficient synthetic routes to these
coveted spirocyclic 9,9-diarylfluorenes.11 From a wider view-
point, it is also interesting to invent a new approach to
tetraarylmethanes, a simple yet synthetically challenging entity,
without resorting to Friedel−Crafts reactions.12
advantageous since one naturally does not suffer from residual
transition metal impurities that could adversely affect the device
performances of the final products.
We thus envisioned that this SNAr-based strategy is applicable
to the efficient synthesis of spirocyclic 9,9-diarylfluorenes in a
single operation, which is disclosed herein. To the best of our
knowledge, this is the first base-mediated strategy for the
synthesis of bispirocyclic tetraarylmethanes.
Recently, our group has been interested in developing
“aromatic metamorphosis”, which represents a transformation
of one aromatic system to another one through partial
disassembly of the starting aromatic ring.13−16 Among our
work, the aromatic metamorphosis of dibenzothiophenes to
carbazoles13b consists of two steps: (1) oxidation into the
corresponding sulfones and (2) transition-metal-free twofold
SNAr reactions with aniline.17−19 This carbazole synthesis is
Despite significant structural differences between aniline and
xanthene (2a), dibenzo[b,d]thiophene 5,5-dioxide (1a) reacted
Received: November 25, 2015
© XXXX American Chemical Society
A
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with 2a by means of KN(SiMe3)2 similarly to afford the
corresponding spiro[fluorene-9,9′-xanthene] (3a) in 93% yield
(eq 1). NaN(SiMe3)2 was inferior to KN(SiMe3)2, yielding 3a in
Next, we investigated the scope of dibenzothiophene dioxides
(Figure 2). Because our strategy does not rely on a Friedel−
only 66% yield. Notably, the reaction did not proceed with
LiN(SiMe3)2. Other potassium bases such as K2CO3, K3PO4, and
KOAc were totally ineffective, whereas KOt-Bu promoted the
reaction, albeit sluggishly.
With the optimized conditions in hand, the scope of cyclic
diarylmethanes was examined in the reaction of 1a (Figure 1).
Figure 2. Scope of dibenzothiophene dioxides.
Crafts cyclization that would lack regioselectivity,6 the synthesis
of 4d−g naturally proceeded with retention of the original
substitution patterns of sulfones 1d−g. Similar to the synthesis of
3h, partial hydrodechlorination of 1i and 4i unfortunately
occurred. Electron-donating methoxy groups did not show any
detrimental effects to afford 4j almost quantitatively.
Our synthetic strategy is totally different from conventional
Grignard/Friedel−Crafts-based approaches, and one can
synthesize spirocyclic diarylfluorenes that had been previously
difficult to access. Benzothiophene-fused xanthene 2m smoothly
underwent reaction with 1a to furnish 3m in excellent yield
(Scheme 2). Furthermore, oxidation of 3m to sulfone 5 followed
by another SNAr reaction with 2a delivered the novel
bispirocyclic derivative 6 in 86% yield.
Taking advantage of the unique reactivity of benzothiophene
dioxide (7) as a dienophile in the Diels−Alder reaction with
isobenzofuran 8, we were able to prepare π-conjugated
benzonaphthothiophene dioxide 1k easily (Scheme 3).22 Despite
the steric hindrance around the sulfur atom of 1k, sterically
congested spirocyclic compound 4k was isolated in high yield.
Figure 1. Scope with respect to cyclic diarylmethanes. a 3.5 equiv
KN(SiMe3)2. b 2.0 equiv KN(SiMe3)2. 3b was concommtantly formed in
27% yield. c At 120 °C. d Under microwave at 150 °C for 12 h.
Thioxanthene (2b) also reacted to afford spiro[fluorene-9,9′-
thioxanthene] (3b) in 95% yield.20 Commercially available 2-
substituted thioxanthenes 2c−f participated in the reaction
regardless of the electronic nature of the substituent.21 With the
aid of one extra equivalent of KN(SiMe3)2, unprotected 2-
hydroxythioxanthene (2g) was converted smoothly. The chloro
group in 2-chlorothioxanthene (2h) partly survived to obtain 3h
in 65% yield, along with dechlorinated 3b as a byproduct in 27%
yield. 9,9-Dimethyl-9,10-dihydroanthracene (2i) bearing no
heteroatom also reacted with 1a to furnish 3i in 64% yield.
Unfortunately, acyclic diphenylmethane (2j) as well as fluorene
(2k) reacted sluggishly, affording the corresponding products 3j
and 3k in moderate yields even with higher temperatures.
Interestingly, 9,10-dihydroanthracene (2l) reacted with 4 equiv
of 1a via a quadruple C−C bond-forming event to furnish 3l in
94% yield (eq 2).
B
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spirocycle 3a in 39% yield (Scheme 5). Subsequently, 11 was
exposed to KN(SiMe3)2 at a higher temperature (80 °C) to yield
3a in 98% yield.23
Scheme 2. Synthesis of Bispirocyclic Compound 6
Scheme 5. Elucidation of Mechanism
In conclusion, we have invented a one-pot, practical strategy to
prepare spirocyclic tetraarylmethanes, especially spirocyclic 9,9-
diarylfluorenes. The twofold SNAr reactions of dibenzothiophene
dioxides with cyclic diarylmethanes proceed with the aid of
KN(SiMe3)2, hence offering an approach complementary to the
conventional Friedel−Crafts cyclization under acidic conditions.
Our synthetic approach is unique to allow the synthesis of
unusual spirocyclic compounds that are otherwise difficult to
achieve. Current efforts are directed toward exploring new π-
conjugated materials through our twofold SNAr strategy.
Scheme 3. Diels−Alder-Based π-Extension Followed by
Twofold SNAr Reaction
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.5b03384.
Experimental details, characterization data, NMR spectra
of the products (PDF)
Crystallographic data for 3f (CIF)
Crystallographic data for 10b (CIF)
To showcase the high synthetic performance of our SNAr-
based spirocycle formation, we envisioned preparing linearly
bispirocyclic compounds such as 10 through repetitive spirocycle
formation (Scheme 4). Oxidation of 3b that was obtained by our
AUTHOR INFORMATION
Corresponding Author
■
Scheme 4. Synthesis of Linearly Bispirocyclic Hexaarylenes
*E-mail: yori@kuchem.kyoto-u.ac.jp.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by Grants-in-Aid from MEXT (No.
25107002 “Science of Atomic Layers”) and from JSPS (Nos.
25220802 (Scientific Research (S)), 24685007 (Young Scientists
(A)), 26620081 (Exploratory Research)) as well as by ACT-C,
JST. H.Y. acknowledges the Japan Association for Chemical
Innovation for financial support.
REFERENCES
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