Diels-alder reaction of 1,3-diarylbenzo[ c ]furans with thiophene S,S -dioxide/indenone derivatives: A facile preparation of substituted dibenzothiophene S,S-dioxides and fluorenones

Article abstract of DOI:10.1021/ol501175q

One pot syntheses of substituted dibenzothiophene S,S-dioxides and fluorenones were successfully achieved by Diels-Alder reaction of benzo[c]furans with thiophene S,S-dioxides and indenones, respectively. Photophysical properties of representative seven- and nine-membered dibenzothiophene S,S-dioxide acenes were also reported.

Full text of DOI:10.1021/ol501175q

Letter
pubs.acs.org/OrgLett
Diels−Alder Reaction of 1,3-Diarylbenzo[c]furans with Thiophene
S,S-Dioxide/Indenone Derivatives: A Facile Preparation of
Substituted Dibenzothiophene S,S-Dioxides and Fluorenones
Meganathan Nandakumar, Jayachandran Karunakaran, and Arasambattu K. Mohanakrishnan*
Department of Organic Chemistry, School of Chemical Sciences, University of Madras, Guindy Campus, Chennai 600 025, Tamil
Nadu, India
S
* Supporting Information
ABSTRACT: One pot syntheses of substituted dibenzothio-
phene S,S-dioxides and fluorenones were successfully achieved
by Diels−Alder reaction of benzo[c]furans with thiophene S,S-
dioxides and indenones, respectively. Photophysical properties
of representative seven- and nine-membered dibenzothio-
phene S,S-dioxide acenes were also reported.
ibenzothiophene1 and fluorenone-based2 π-conjugated
materials have attracted renowned interest due to their
fluorenone derivatives. Recently, Iniesta and co-workers16
achieved the preparation of dibenzothiophene S,S-dioxides
through Diels−Alder reaction of benzo[b]thiophene S,S-
dioxide and tetraphenylthiophene S-oxide. They have also
carried out the Diels−Alder reaction of benzo[b]thiophene S,S-
dioxide with dienes such as substituted butadiene, furan,
cyclopentadiene, and tetracyclone to afford the respective
adducts in moderate yields. As a representative case, the Diels−
Alder reaction of 1,3- diphenylbenzo[c]furan 1a with benzo-
[b]thiophene S,S-dioxide 217 in toluene at reflux for 12 h
afforded corresponding adduct 3a in 84% yield.
Subsequent aromatization of the adduct 3 using p-
toluenesulfonic acid (PTSA) led to the formation of the
expected benzo[d]naphtho[b]thiophene S,S-dioxide 4a in 83%
yield. The Diels−Alder reaction of benzo[c]furan 1a with 2
followed by aromatization without the isolation of intermediate
adduct slightly improved the yield of 4a (Scheme 1 and the
Supporting Information). The structure of thiophene S,S-oxide
4a was confirmed by a single-crystal X-ray analysis.18
D
promising applications in the area of organic electronics.
Barbarella et al. first demonstrated that an introduction of one
or more thiophene S,S-dioxide unit into the conjugated
oligothiophene backbone provides a considerable reduction of
HOMO−LUMO band gap value as well as an enhancement in
the electron affinity compared to the parent oligothiophenes.3
The oligothiophene S,S-dioxide also displayed a better solid-
state photoluminescence efficiency.4 The greater fluorescence
efficiency and electron-transport properties of the dibenzothio-
phene S,S-dioxide-based materials5 prompted chemists to
undertake a comprehensive study on these compounds.6−8
Perepichka et al. extensively utilized the dibenzothiophene S,S-
dioxide unit as material for dual-fluorescence,7a strong
solvatochromism,7b and high luminescence efficiency7c by
incorporating into the main chain of oligofluorene. Recently,
Yang and co-workers successfully synthesized alcohol-soluble
amino-functionalized polyfluorene-tethered dibenzothiophene
S,S-dioxides and explored their applications in polymer light
emitting diodes (PLEDs) and polymer solar cells (PSCs).8
Oligo- and polyfluorenes have been explored as OLEDs with
high quantum efficiencies.9 To overcome the stability issue of
fluorenes,10 9,9-dialkylfluorenes are being explored.11 Recently,
Goel and co-workers reported the donor−acceptor-type
fluorene- and fluorenone-based small molecules as stable blue
light emitters.12 The Diels−Alder reaction is one of the
ubiquitous reactions in organic synthesis and used for
construction of polycyclic aromatics.13 Over the years, the
benzo[c]furan analogues were being exploited as a dienes in the
Diels−Alder reactions to synthesize complex π-conjugated
acenes.14
Scheme 1. Synthesis of Benzo[b]naphtho[d]thiophene S,S-
Dioxide 4a
Our aim is to explore the synthetic utility of 1,3-disubstituted
benzo[c]furans15 as a diene in the Diels−Alder reaction to
prepare π-conjugated dibenzothiophene S,S-dioxide and
Received: April 23, 2014
© XXXX American Chemical Society
A
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Letter
Table 1. Synthesis of Dibenzothiophene S,S-Dioxides
Table 2. Synthesis of Substituted Fluorenones
a
The Diels−Alder reaction was carried out in 1,2-DCE at reflux until
disappearance of furan in TLC (10 h), and for aromatization it was
refluxed until adduct disappearance (9 h) was noted. Reaction was
b
a
Diels−Alder reaction was allowed to reflux in toluene until the
carried out in toluene for 14 h, and aromatization was allowed for 12 h.
Isolated yield by column chromatography.
c
disappearance of furan (∼12 h), and for aromatization (PTSA) it was
refluxed until disappearance of the adduct (∼10 h) was noted.
b
Isolated yield by column chromatography.
Scheme 2. Diels−Alder Reaction of Indenone 11 with Furan
1a
Figure 1. Single-crystal X-ray structures of 6a and 16e.
Inspired by this result, the one-pot syntheses of dibenzo-
thiophene S,S-dioxides 4b−d, 6a−c, 8, and 10 have been
successfully achieved in good to excellent yields (Table 1). The
Diels−Alder reaction of benzo[c]furans 1b−d with benzo[b]-
thiophene S,S-dioxide 2 followed by aromatization afforded
benzo[d]naphtho[b]thiophene S,S-dioxides 4b−d in good
yields (entry 1). Under identical conditions, naphtho[b]-
thiophene S,S-dioxide 5 reacts with benzo[c]furans 1c−e to
furnish dinaphtho[b:d]thiophene S,S-dioxides 6a−c in 76−84%
yields (entry 2). Interestingly, the bent-naphtho[b]thiophene
S,S-dioxide 7 also underwent smooth Diels−Alder reaction
followed by aromatization to give dinaphtho[1,2-b:2′,3′-d]thi-
ophene S,S-dioxide 8 in 81% yield (entry 3). The electron-rich
bis(hexyloxy)dinaphtho[b:d]thiophene S,S-dioxide 9 success-
fully participated in the one-pot synthesis to give tetrasubsti-
tuted five-membered acene 10 in 75% yield (entry 4).
aromatization furnished diphenylbenzo[b]fluorenone 12a in
88% yield (Scheme 2).
Next, the indenone 11, naphtho[b]indenone 13, as well as
the diaryl-substituted naphtho[b]indenone 15a,b also partici-
pated in the Diels−Alder reaction followed by aromatization to
afford the corresponding benzo/naphthofluorenones 12b−d,
14a−d, and 16a−e in excellent yields (Table 2). It is worth
mentioning that the indenones 11 and 13 require lower
temperature than that of diaryl substituted indenone 15a,b. The
structures of thiophene S,S-dioxide 6a and fluorenone 16e were
confirmed by single-crystal X-ray analyses (Figure 1).18
After demonstrating the preparation of a variety of benzo/
naphtho dibenzothiophene S,S-dioxides and fluorenones by the
Diels−Alder reaction of 1,3-diarylbenzo[c]furans, we next
initiated the reduction1e of dibenzothiophene S,S-dioxides. As
a representative case, diphenyldibenzothiophene S,S-dioxide 4a
upon reduction with lithium aluminum hydride (LAH) in
diethyl ether at 0 °C to rt for 2 h afforded diphenyldibenzo-
thiophene derivative 17a in 74% yield (Scheme 3). Similarly,
We next investigated the Diels−Alder reaction of benzo[c]-
furans with a range of indenones. It should be noted that the
indenone 11 and benzo[b]thiophene S,S-dioxide 2 have similar
motifs, differing only at the C-1 position. To our delight,
indenone 1119 upon Diels−Alder reaction with furan 1a in 1,2-
dichloroethane (DCE) at reflux for 10 h followed by
B
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Organic Letters
Letter
Scheme 3. Preparation of Dibenzothiophene Derivatives
17a,b
Figure 2. Absorption and emission spectra of thiophene S,S-dioxides
20a, 20b, 23b, and 23c.
Scheme 4. Synthesis of Seven-Membered Thiophene S,S-
Dioxide Acenes 20a−e
Table 3. Photophysical Studies of Thiophene S,S-Dioxides
Stokes
a
a
,
b
c
absorption
emission
shift
quantum
a
entry product λmax(abs) (nm) λmax(em) (nm)
(cm−1
)
yield Φ
1
2
3
4
5
6
20a
20b
20c
20d
23b
23c
275
265
274
266
382
392
478
488
479
486
467
466
15443
17244
15619
17017
4764
0.210
0.019
0.180
0.018
0.054
0.045
4051
a
b
Recorded in CH3CN at 25 °C. Excited at the longest wavelength of
c
the absorption maxima. Stokes shift = λmax(abs) − λmax(em) (cm−1).
dibenzothiophene S,S-dioxide 4b also converted into the
corresponding dibenzothiophene 17b in good yield.
To explore the scope further, synthesis of higher acenes
containing thiophene S,S-dioxide units has been initiated. The
bis(hexyloxy)benzodithiophene and N-dodecyldithieno carba-
zole derivatives have been synthesized and incorporated in
polymers due to their potential optical properties.20 Hence, the
required bis(hexyloxy)benzodithiophene S,S-dioxide 19a/19b
was prepared by the routinely used HCO2H/H2O2 proce-
dure.17 Gratifyingly, the one-pot synthesis of hexaaryl-substitu-
ted thiophene S,S-dioxide fused seven member acenes 20a−e
have been successfully achieved in good yields (Scheme 4).
Next, we began to construct more complex and challenging
higher acenes containing carbazole-fused thiophene S,S-
dioxides. The Diels−Alder reaction of diphenylbenzo[c]furan
1a with thiophene S,S-dioxide 22 followed by adduct bridge
cleavage furnished dithienocarbazole-fused nine-membered
acene 23a in 75% yield. Finally, the anisyl/thienyl-tethered
dithienocarbazole-fused nine-membered acenes 23b/23c have
also been achieved using a one-pot Diels−Alder synthetic
strategy (Scheme 5).
Figure 3. Luminescence color upon illumination at 365 nm.
Table 4. Fluorescence Lifetimes of 20a, 20b, 23b, and 23c*
compd τ1 (ps) τ2 (ns) τ3 (ns) A1 (%) A2 (%) A3 (%)
χ2
20a
20b
23b
23c
1.20
0.90
3.39
1.86
100
22.74
1.148
0.943
1.081
1.226
43.30
0.25
1.50
0.627
16.13
61.13
10.84
25.97
89.16
74.03
*
Measured using the TCSPC technique and excited at 370 nm and
monitored at their respective emission maximum.
The optical properties of the novel thiophene S,S-dioxides
20a, 20b, 23b, and 23c were investigated by UV−vis and
fluorescence spectroscopy (Figure 2, Table 3). The absorption
spectra of thiophene S,S-dioxides 23b and 23c shifted to lower
energies compared to the compounds 20a and 20b. Figure 3
shows the emission color of 20a, 20b, 23b, and 23c in
acetonitrile upon illumination at 365 nm. The thiophene S,S-
dioxides 20a and 20b exhibited broad emission band and larger
Stokes shift compared to the thiophene S,S-dioxides 23b and
23c (Table 3). The fluorescence quantum yield was measured
using quinine sulfate as a standard (0.1 N H2SO4) Φf = 0.54 at
an excitation wavelength of 366 nm. It is worth mentioning that
among thiophene S,S-dioxide-based acenes, compounds 20a
and 20c displayed much enhanced fluorescence quantum yields
(Table 3).
Scheme 5. Synthesis of Nine-Membered Thiophene S,S-
Dioxide Acenes 23a−c
Next, the time-correlated single photon counting (TCSPC)
measurement of thiophene S,S-dioxides 20a, 20b, 23b, and 23c
were carried out in acetonitrile at room temperature (Table 4
and the Supporting Information). It should be noted that the
fluorescence lifetime decay of compounds 20a and 20b
C
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Letter
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1
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ASSOCIATED CONTENT
* Supporting Information
Experimental procedure, copies of NMR spectra, and X-ray
crystallographic data of 4a, 6a, and 16e (CIF). This material is
available free of charge via the Internet at http://pubs.acs.org.
■
S
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̈
AUTHOR INFORMATION
Corresponding Author
■
*mohanakrishnan@unom.ac.in; mohan_67@hotmail.com.
(12) Goel, A.; Chaurasia, S.; Dixit, M.; Kumar, V.; Prakash, S.; Jena,
B.; Verma, J. K.; Jain, M.; Anand, R. S.; Manoharan, S. S. Org. Lett.
2009, 11, 1289.
Notes
The authors declare no competing financial interest.
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Pergamon Press: New York, 1990.
ACKNOWLEDGMENTS
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Financial assistance from DST, New Delhi, is acknowledged.
M.N. and J.K. thank CSIR, New Delhi, for fellowships. We
thank DST-FIST for the high-resolution NMR facility. We also
thank Prof. P. Ramamurthy, Director, National Centre for
Ultrafast Processes, University of Madras, for providing
photoluminescence and lifetime studies. We thank SAIF, IIT
Madras, for crystallography and VT NMR studies.
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DEDICATION
■
This paper is dedicated to Prof. Ganesh Pandey on the occasion
of his 60th birthday.
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