Diels-Alder reaction of tetraarylcyclopentadienones with benzo[: B] thiophene S, S-dioxides: An unprecedented de-oxygenation vs. sulfur dioxide extrusion

Article abstract of DOI:10.1039/d0cc05842d

Diels-Alder reaction of tetraarylcyclopentadienones with benzo[b]thiophene dioxides in xylenes at reflux led to the formation of tetra aryl-substituted dibenzothiophene as well as penta aryl-substituted benzene analogues depending on the influence of aryl substituents present on cyclopentadienones. The intermediate dihydrodibenzothiophene-dioxides underwent aromatization either through de-oxygenation or extrusion of sulfur dioxide to furnish substituted dibenzothiophenes or benzenes. This journal is

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DOI: 10.1039/D0CC05842D
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Diels-Alder Reaction of Tetraarylcyclopentadienones with
Benzo[b]thiophene S, S-dioxides: An Unprecedented De-
Oxygenation Vs Sulfur Dioxide Extrusion
Received 00th January 20xx,
Accepted 00th January 20xx
Palani Manikandan,a Jayachandran Karunakaran,a Elumalai Varathan,b Georg Schreckenbach,b and
Arasambattu K Mohanakrishnan*a
DOI: 10.1039/x0xx00000x
www.rsc.org/
Diels-Alder reaction of tetraarylcyclopentadienones with the formation of bishexaphenylbenzenes,13 cleft-containing
benzo[b]thiophene dioxides in xylenes at reflux led to the polycyclic aromatic hydrocarbon “albatrossenes”,14 polyphenylene
formation of tetra aryl-substituted dibenzothiophene as well as dendrimers,15 arylamino substituted stilbenes,16 graphitic molecules
penta aryl-substituted benzene analogues depending on the with
influence of aryl substituents present on cyclopentadienones. The polyphenylbenzenedendrons,18 polytriphenylene dendrimers,19 non-
intermediate dihydrodibenzothiophene-dioxides underwent planar pentaphenylbenzene functionalized benzo[2,1,3]thiadiazole
aromatization either through de-oxygenation or extrusion of sulfur derivatives,20 hexaphenylbenzenes,21 and triphenylamine derivatives
dioxide to furnish substituted dibenzothiophenes or benzenes.
containing tetraphenylbenzene units as “molecular glasses”.22 The
Polycyclic aromatic compounds (PACs) have been widely explored DA reaction of TACPDs with an in situ generated benzyne has been
partial
zig-zag
periphery,17
solution
processable
in materials science1 as well as medicinal chemistry.2 In particular,
PACs such as acenes and fluoranthenes received prominent
attention due to their optoelectronic properties, as conductors,
ferromagnets and liquid crystalline materials.3,4 Heteroatoms
incorporated in polycyclic hydrocarbons have been extensively
studied due to the large π-conjugation length, high fluorescence
quantum yield and increased charge mobility.5 Over the years,
naphthalene and its higher analogues namely pyrene, perylene,
terrylene and quaterrylene have been explored in dye chemistry,
supramolecular assemblies and also for optoelectronic applications
due to their high fluorescence quantum yields, their outstanding
photo stability coupled with low photobleaching ability.6-10 On the
other hand, thiophene-based π-conjugated materials have been
frequently investigated as components in organic light emitting
diodes (OLEDs), organic solar cells (OSCs), organic field-effect
transistors (OFETs), chemo-sensors, biosensors and electrochromic
devices.11
utilized
for
accessing
derivatives
of
9,14-
diphenylbenzo[b]triphenylenes,23 helical meso benzanthrones,24 and
pyrene-fused twistacenes.25 The DA reaction of TACPDs with benzo
analogs of quinones furnished hairpin furans as well as giant
biaryls.26 Based on the above mentioned reports, it is obvious that
there is plenty of scope for exploring the DA reaction of TACPDs with
relatively less explored double bonded dienophiles.
Ar
Ar/Het-Ar
Ar
S
O
S
O
Ar
[4+2]
O
O
O
Ar
Pd/CuI
Arylation
1
,3
Ref-30
R1
R2
ArI/Het-ArI
Ref-29
[2+2+2]
Rh(I)
R1/R2
MetamorphSNAr
=
Ref-28
S
O
O
X
X
R1/R2 = H, H
Ref-27
osis
S
O
Ar
O
[4+2]
Ar
Ar
X
X = NTs, O
O
Present
work
X = NAr, P=OPh,
Ar
Ar
C
Ar
Ar
Ar
Ar
Ar
Ar
Y
Ar
Y = S, SO2
For more than five decades, [4+2] cycloadditions of
tetraarylcyclopentadienones (TACPDs) have been exploited for
accessing a wide variety of non-planar and structurally complex
polycyclic aromatic hydrocarbons.12 In particular, the inverse-
Scheme 1 Synthetic Transformations of Benzothiophene dioxides.
Recent synthetic applications of benzothiophene dioxides
(Scheme 1) include accessing diverse carbocycles as well as
heterocycles through SNAr reactions by Yorimitsu and co-workers,27
Rh(I)-catalyzed [2+2+2] cycloaddition,28 Pd- as well as CuI-mediated
arylations,29 [4+2] cycloaddition with IBF derivatives,30 and also self-
Diels-Alder dimerization.31 The annulative π-extension (APEX) of
benzo[b]heterocyles32 and benzo[b]thiophene S, S-dioxides,30
inspired us to undertake a detailed study on DA reaction of the latter
with TACPDs. As per the objective, the Diels-Alder reaction of
tetraaryl cyclopentadienones 1a-d with benzo[b]thiophene S, S-
dioxide 2a in xylenes at reflux for 16 h didn’t give the expected
dihydrotetra aryldibenzothiophene S, S-dioxide, instead, the
electron
demand
Diels-Alder
(DA)
reaction
of
tetraarylcyclopentadienones with triple bonded dienophiles led to
aDepartment of Organic Chemistry, School of Chemical Sciences, University of
Madras Guindy Campus, Chennai 600 025, Tamil Nadu, India.
Email: mohanakrishnan@unom.ac.in/mohan_67@hotmail.com
bDepartment of Chemistry, University of Manitoba, Winnipeg, MB R3T 2N2, Canada.
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
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corresponding aromatized 1,2,3,4-tetraaryldibenzothiophenes 3a- the respective tetraphenyldibenzothiophene S, S-dioxides 3a"-c" as
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d33 could be isolated in 64-72% yields (Scheme 2). Most likely, the
unexpected aromatization of dihydro-dibenzothiophenes may
DOI: 10.1039/D0CC05842D
exclusive products in 65-71% yields. Most likely, the aromatization of
the intermediate dihydrodibenzothiophene S, S-dioxides might have
proceed through de-oxygenation to afford thiophenes 3a-d. The
occurred through an involvement of DMF radical.36 The
structures of thiophenes 3a-d have been thoroughly characterized by
tetraphenyldibenzothiophene S, S-dioxides 3a"-c" are also prepared
NMR and HRMS studies.
via Diels-Alder reaction of respective tetraarylcyclopentadienone
1a-c with 3-bromobenzo[b]thiophene S, S-dioxide 2aʹ37 in xylenes at
R1
R1
R2
R1
R2
R2
S
O
O
reflux for 24 h.
2a
O
xylenes, reflux, 16 h
R2
R2
S
S
O
O
Control Experiments
64-72%
R2
Ar
3a-d
1a-c
1a-d
R1
H
R1
Ar
R1
3a R1 = H, R2 = H
3b R1 = OMe, R2 = H
3c R1 = H, R2 = OMe
3d R1 = OMe, R2 = OMe
expected
2a
p-xylene
Br
1a R1 = H, R2 = H
Me
CHO
3a/3d
1a/1d
xylenes
1b R1 = OMe, R2 = H
1c R1 = H, R2 = OMe
1d R1 = OMe, R2 = OMe
reflux
8 h
reflux
S
Ar
reflux, 24 h
(Identified by 1H NMR,
13C NMR and MS)
H
O
51-55%
O
Ar
S
5a Ar = phenyl
5b
O
2a'
O
Ar = p-anisyl
R1
R1
Ar1
CHO
2a
R2
Ph
Ar2
DMF/Cs2CO3
p-xylene
Ph
Ph
S
S
O
mCPBA
1a-c
O
3a
110 oC, 16 h
65-71%
3a
X
Ar2
2a
S
BF3.OEt2
58%
reflux, 24 h
O
O
1e/1f
S
O
xylenes
O
S
Ar1
Me
O
reflux,16 h
Ph
S
3a" Ar1, Ar2 = phenyl
1e R1, R2 = H
1i
3a'
3e (CCDC 1891300)
R2
R1 R1
Ar1= p-anisyl; Ar2 = phenyl
Ar1= phenyl; Ar2 =p-anisyl
1f R1 = Br, R2 = H
1g R1 = OMe, R2 = H
1h R1, R2 = OMe
3b"
3c"
3e R1 = H (67%)
3f R1 = Br (70%)
R1
R2
Me
2a
Me
2a
CHO
4a
4a
X
reflux
p
,
-xylene
CHO
3e
S
,
-xylene
p
reflux
Me
Me
Me
2a
Me
Me
Me
1g
2a
-xylene
reflux
2a
1e
1g/1h
2a
OX
xylenes
N
1i
p
O
,
3e
3a/3d
TEMPO
TEMPO
xylenes
reflux,16 h
,
N
-xylene
p
Me
reflux, 16 h
reflux
Me
Me
S
R2
Me
5a/5b
4c (67%)
(Identified by 1H NMR,
13C NMR and MS)
p-xylene, TEMPO
reflux
R1
4a R1 = OMe, R2 = H (77%)
4b R1, R2 = OMe (76%)
4a (CCDC 1960428)
4c (CCDC 1891974)
Plausible Mechanism
Ph
Ph
Ph
H
Scheme 2 Diels-Alder Reaction of TACPDs 1a-i with 2a
H
Ph
Ph
Ph
Ph
Ph
Ph
1, 3-H~
1,5-H~
Ar
3a
5a
S
-OH
-OH
S
S
OH
Ph
OH
O
III
O
Ph
II
Ph
I
Next, towards generalizing the unexpected deoxygenation
phenomena, the Diels-Alder reactions were explored with
acenaphthene fused CPDs 1e-i.34 The phenyl as well as
4-bromophenyl tethered 8H-cyclopenta[a]acenaphthylen-8-one
1e/1f underwent [4+2] cycloaddition with 2a with concurrent
extrusion of carbon monoxide followed by deoxygenation to afford
respective dibenzothiophenes 3e and 3f in 67% and 70% yields
(Scheme 2). To our delight, the structure of 3e could be confirmed by
a single crystal X-ray analysis. On the other hand, the 8H-
cyclopenta[a]acenaphthylen-8-ones 1g-i containing electron-rich p-
anisyl, veratryl and 2-thienyl units upon Diels-Alder reaction with
benzo[b]thiophene S, S-dioxide 2a in xylenes at reflux underwent
successive extrusions of CO and SO2 to give appropriate triaryl-
substituted fluoranthenes 4a-c in 67-77% yields. The structures of
fluoranthenes 4a and 4c were confirmed by single crystal X-ray
diffraction analyses (Scheme 2).
Ar
H
2a
4a
1g
-SO2
S
S
O
Ar
IV
O
Ar
V
O
HO
Ar = p-anisyl
Scheme 3 Control Experiments and Plausible Mechanism for Deoxygenation
and Sulfur dioxide Extrusion
Further, sulfoxide 3aʹ (prepared via selective oxidation of 3a) upon
refluxing with p-xylene failed to produce the sulfide 3a, and the
starting sulfoxide 3aʹ was recovered unchanged. A careful TLC
analysis unambiguously ruled out the involvement of the sulfoxide
3aʹ (being more polar could be easily identified using TLC) as an
intermediate during deoxygenation as well as Cs2CO3/DMF mediated
dehydrogenation. Next, all our attempts to isolate the intermediate
dihydrodibenzothiophene S, S-dioxide with representative
acenaphthene fused CPDs 1e and 1g by reducing the time periods of
the DA reaction with 2a were found to be unsuccessful. However, as
expected, the cyclopenta[a]acenaphthylen-8-one 1e upon Diels-
Alder reaction with 2a in p-xylene at reflux for 16 h followed by
spectral analyses did confirm the formation of p-tolualdehyde. On
the other hand, the observed failure to produce the p-tolualdehyde
(see SI) upon the Diels-Alder reaction of acenaphthene fused CPD 1g
with 2a in p-xylene at reflux clearly supported the sulfur dioxide
extrusion mechanism (Scheme 3).
To get an insight into the reaction mechanisms, the intermediate
dihydrodibenzothiophene S, S-dioxides 5a and 5b were isolated by
reducing the time periods of the DA reaction (see SI for details).
These intermediates 5a and 5b upon refluxing with p-xylene for 8 h
1
followed by removal of solvent and H, 13C NMR and mass spectral
analyses unambiguously confirmed the formation of p-
tolualdehyde35 along with expected dibenzothiophenes 3a and 3d
(Scheme 3). As a representative case, dibenzothiophene S, S-dioxide
5a upon heating with diphenylmethane/fluorene also produced the
respective benzylic oxidation products, benzophenone as well as
fluorenone (See SI).
To our delight, under basic conditions (Cs2CO3 in DMF at 110 oC
for 16 h), the aromatization pathway of the intermediate
dihydrodibenzothiophene S, S-dioxides could be altered to produce
Next, thermolysis of 5a and 5b in p-xylene in the presence of
TEMPO (2,2,6,6-tetramethylpiperidine 1-oxyl free radical)
completely suppressed the formation of p-tolualdehyde, thus
unequivocally confirming the involvement of a radical mechanism.
Additionally, the formation of 2,2,6,6-tetramethyl-1-((4-
methylbenzyl)oxy) piperidine38 could also be confirmed through 1H,
2 | J. Name., 2012, 00, 1-3
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13C NMR and mass spectral analyses. As expected, among Diels-Alder on the benzo[b]thiophene S, S-dioxides 2b-d. Further,ViienwtAhreticcleaOsenlionef
DOI: 10.1039/D0CC05842D
reaction of acenaphthene fused CPDs 1e and 1g with 2a, only in the 1g, electron withdrawing substituent (Cl/NO2) of benzothiophene S,
case of 1e, the tetramethyl-1-((4-methylbenzyl)oxy)piperidine S-dioxide 2e/2f altered aromatization pattern of the intermediate
formation could be confirmed (Scheme 3).
dihydrodibenzothiophene S, S-dioxide to furnish thiophenes 3q and
Based on control experiments, as a representative case, the 3s. From these examples, it is obvious that an increase in the electron
plausible mechanism for the formation of tetraaryldibenzothiophene density of dihydrodibenzothiophene S, S-dioxide unit by aryl
3a as well as pentaarylbenzene 4a is outlined in Scheme 3. The substituents is favouring SO2 extrusion.
OMe
R1
dihydrodibenzothiophene S, S-dioxide 5a will undergo 1,3-H
hydrogen shift to give an intermediate I, which upon deoxygenation
through homolytic rupture of hydroxyl radical might lead to II. The
intermediate II via a 1,5-H atom abstraction followed by second
homolytic rupture of hydroxyl radical may give 3a. On the other
hand, the cyclopenta[a]acenaphthylen-8-one 1g containing an
electron-rich p-anisyl units underwent a β-elimination followed by
sulfur dioxide extrusion under refluxing conditions of xylenes to
furnish 4a (Scheme 3).
To gain insights into relative stabilities of the radical species
proposed for deoxygenation, DFT calculations (see SI for full details)
were performed on the radical species II and III. It is clearly evident
from the Figure S1 (SI), radical III was found to be more stable than
radical II by ~30 kcal/mol (both gas and solvent phase), which was
attributed to the inability of radical delocalization in II when
compared to III. To further understand the proposed mechanism, the
detailed DFT calculations were performed and the results are given
the in the Figure S2 and S3 (SI).
Finally, the substrate scope for the preparation of
dibenzothiophenes, penta-aryl benzenes and triaryl-substituted
fluoranthenes is presented in Scheme 4. The cyclopentadienone
1a/1b upon Diels-Alder reaction followed by deoxygenation afforded
dibenzothiophenes 3g/3h (with 2b) or 3i/3j (with 2d). Under
identical conditions, the acenaphthene based cyclopentadienone
1e/1f also furnished respective benzothiophene 3k/3l (with 2b) or
3m/3n (with 2c). Additionally, the Diels-Alder reaction of 1e could
also performed with 2d to give 3o in 79% yield. The
cyclopentadienone 1e/1g upon reaction with benzothiophene S, S-
dioxide 2e/2f containing an electron withdrawing substituent
(Cl/NO2) furnished the expected benzothiophenes (3p & 3q, 3r & 3s)
in 44-63% yields. Further, cyclopentadienone 1a/1e/1f upon
reaction with 2g led to the corresponding heterocycles 3t-v in 61-
67% yields. On the other hand, the [4+2] cycloaddition of
cyclopentadienone 1a/1b with 2c gave biphenyl 4d/4e via successive
extrusions of CO and SO2. Finally, the acenaphthene based
cyclopentadienones 1g-i also underwent smooth Diels-Alder
reaction followed by aromatization to furnish respective biaryls 4f-h
(with 2b), 4i-k (with 2c), 4l-n (with 2d) and 4o-q (with 2g) in 61-76%
yields. Except for 4d and 4e, the exclusive formation of the
dibenzothiophenes 3g-r as well as substituted benzenes 4f-q are
controlled only by the electron density of the aryl units present in the
diene portion, specifically at 2- and 5-positions. Surprisingly, the 3,4-
dimethoxy, 3,4-methylenedioxy and 1,4-dimethoxy substituents
present on the benzo[b]thiophene S, S-dioxides 2b-d are not exerting
any influence on the observed aromatization pattern. However, the
exclusive formation of dibenzothiophenes 3g-j and tetra-aryl
benzenes 4d & 4e did confirm the influence of substituents present
PhO2S
MeO
O
O
N
S
S
O
S
S
O
S
O
MeO
O
O
O
2e R1 = Cl
2f R1 = NO2
O
O
O
O
2b
OMe
2c
2g
2d
Synthesis of dibenzothiophenes
2b-g
1a/1b/1e/1f
3g-v
Ar1
OMe
OMe
Ar1
Ar1
Ar1
Ph
Ph
O
Ph
Ph
MeO
OMe
O
S
MeO
S
OMe
S
S
Ar1
Ar1
Ar1
Ar1
3i Ar1 = phenyl (75%)
3k Ar1 = phenyl (74%)
3m Ar1 = phenyl (57%)
3g Ar1 = phenyl (75%)
3j Ar1 = p-anisyl (70%) 3l Ar1 = 4-bromophenyl (70%) 3n Ar1 = 4-bromophenyl (63%)
3h Ar1 = p-anisyl (72%)
R2
Ar1
Ph
Ar1
PhO2
S
SO2Ph
Ph
Ph
R1
N
N
S
S
S
Ar1
Ph
Ar1
3u Ar1 = phenyl (65%)
3v Ar1 = 4-bromophenyl (67%)
3o Ar1 = phenyl; R1-R2 = OCH2O (79%)
3p Ar1 = phenyl; R1 = H, R2 = Cl (63%)
3q Ar1 = p-anisyl; R1 = H, R2 = Cl (59%)
3r Ar1 = phenyl; R1 = H, R2 = NO2 (46%)
3s Ar1 = p-anisyl; R1 = H, R2 = NO2 (44%)
3t (61%)
Synthesis of Pentaarylbenzenes and Triarylfluoranthenes
2b-d, 2g
Ar1
Ar1
1a,1b, 1g-i
4d-q
Ar1
SO2Ph
N
O
R3
R4
Ar1
O
Ar1
OMe
Ph
Ph
R2
Ar1 = p-anisyl (71%) 4o
Ar1 = p-anisyl (72%)
Ar1 = veratroyl (63%)
Ar1 = 2-thienyl (65%)
4l
R1
Ar1 = veratroyl (61%) 4p
Ar1 = 2-thienyl (71%) 4q
1 4m
Ar
MeO
4d
4e
Ar1
4n
Ar1
Ar1 = phenyl (67%)
1
4f
R1, R4 = H; R2, R3 = OMe
(72%)
Ar 1= p-anisyl;
Ar1 = p-anisyl (68%)
4g
4h
4i
R1, R4 = H; R2, R3 = OMe
R1, R4 = H; R2, R3 = OMe
Ar1 = veratroyl;
(71%)
Ar = 2-thienyl;
1
(72%)
(76%)
(71%)
Ar1 = p-anisyl;
Ar 1= veratroyl;
Ar = 2-thienyl;
R1, R4 = OMe; R2, R3 = H
R1, R4 = OMe; R2, R3 = H
4j
4k
R1, R4 = OMe; R2, R3 = H
(73%)
Scheme 4 Scope of Benzo[b]thiophene S,S-dioxides
Gratifyingly, a two-fold Diels-Alder reaction of benzodithiophene
1,1,8,8-tetraoxide 6 with tetraphenylcyclopentadienone 1a in dry
xylenes at reflux followed by column chromatographic purification
afforded octaphenyl thienylheterocycle 7 in 61% yield (Scheme 5). A
similar reaction of 2,5-di-(p-anisyl)-3,4-diphenylcyclopentadienone
1b with benzodithiophene tetraoxide 6 led to the formation of hexa-
aryl pentaphenylene 8 in 56% yield. As expected, acenaphthene
based cyclopentadienone 1e also underwent smooth Diels-Alder
reaction with 6 followed by aromatization to afford thienyl
heterocycle 9 as a green, fluorescent solid. Further, the synthesis of
R1
OMe OMe
S
S
S
O
S
O
O
O
O
O
O
O
6
6
O
xylenes
reflux, 24 h
xylenes
reflux, 24 h
S
S
R1 = H
R1 = OMe
1a R1 = H
8
(56%)
7 (61%)
1b R1 = OMe
R1
OMe
MeO
Ar1
Ar1
6
6
Ph
Ph
xylenes
xylenes
1e
1g/1h
reflux, 24 h
reflux, 24 h
55%
S
S
Ar1
Ar1
Ph
N
9
10a Ar1 = p-anisyl (71%)
Ph
10b Ar1 = veratryl (61%)
S
S
O
O
O
O
11
C6H13
1e
S
xylenes, reflux, 24 h
58 %
S
N
12
C6H13
Scheme 5 Diels-Alder Reaction of Cyclopentadienones with S, S-tetraoxides
1,4-bis(fluoroanthenyl)benzenes 10a and 10b could also be
accomplished. As a representative case, double Diels-Alder reaction
of cyclopentadienone 1e with N-hexylcarbazolyldithieno S, S-
This journal is © The Royal Society of Chemistry 20xx
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(15) (a) U.-W. Wiesler and K. Müllen, Chem. Commun. 1999, 2293-2294. (b)
tetraoxide 11 in xylenes at reflux for 24 h followed by column
chromatographic purification furnished fluoroanthene capped mixed
heteroacene 12 as a green fluorescent solid in 58% yield (Scheme 5).
View Article Online
J. Qu, D. Liu, S. De Feyter, J. Zhang, F. C. De Schryver and K. Müllen, J. Org.
DOI: 10.1039/D0CC05842D
Chem. 2003, 68, 9802-9808.
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The
optical
properties
of
some
representative
dibenzothiophenes 3a, 3e, 3f, 3i, 3a" and 3a', fluoranthenes 4a, 4c,
4f and 10b and hetero acenes 7 and 12 were investigated by UV-vis
and fluorescence spectroscopy (see SI for full details).
In summary, we have accomplished a detailed study on the Diels-
Alder reaction of TACPDs with benzo[b]thiophene S, S-dioxides to
produce dibenzothiophenes, highly aryl-substituted benzenes as well
as fluoranthenes in acceptable yields. A reasonable mechanism has
been proposed for the observed de-oxygenation as well as sulfur
dioxide extrusion. Thermolysis of dihydrodibenzothiophene S, S-
dioxides in the presence of TEMPO confirmed the involvement of a
radical pathway for the deoxygenation reaction.
Financial assistance from UGC, New Delhi in the form of UGC-Mid-
career award grant is acknowledged. J. K thanks CSIR, New Delhi for
SRF fellowship. We thank DST-FIST for NMR & HRMS facilities.
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