Nucleophilic substitution in 16π-antiaromatic system: Synthesis of heteroatom-substituted dibenzopentalenes

Article abstract of DOI:10.1246/cl.140583

An antiaromatic compound, 5-bromo-10-iododibenzopentalene, underwent nucleophilic substitution of heteroatoms such as R₂NLi, ROLi, and RSLi. This subst itution proceeded preferentially at the bromopentalene moiety than at the iodo-counterpart.

Full text of DOI:10.1246/cl.140583

CL-140583
Received: June 11, 2014 | Accepted: June 26, 2014 | Web Released: July 4, 2014
Nucleophilic Substitution in 16π-Antiaromatic System:
Synthesis of Heteroatom-substituted Dibenzopentalenes
1
1
2
1
1
Feng Xu, Lifen Peng, Kan Wakamatsu, Akihiro Orita,* and Junzo Otera*
1
Department of Applied Chemistry, Okayama University of Science, Kita-ku, Okayama 700-0005
2
Department of Chemistry, Okayama University of Science, Kita-ku, Okayama 700-0005
(
E-mail: orita@dac.ous.ac.jp)
An antiaromatic compound, 5-bromo-10-iododibenzopenta-
lene, underwent nucleophilic substitution of heteroatoms such as
to aryl- and/or arylethynyl-substituted dibenzopentalenes
through Suzuki­Miyaura and Sonogashira couplings. A series
of oligo(dibenzopentalene)s were also synthesized successfully
by repeating the Suzuki­Miyaura coupling of 1a and 1b with
borodibenzopentalene, which was prepared from 1b. Currently,
in the hope of preparing new types of dibenzopentalenes
with heteroatom substituent(s), we have devoted our efforts to
develop nucleophilic substitution of halo-dibenzopentalenes
R NLi, ROLi, and RSLi. This substitution proceeded preferen-
2
tially at the bromopentalene moiety than at the iodo-counterpart.
Combining this substitution and Sonogashira coupling enabled
syntheses of π-systems expanded by a dibenzopentalene array
and heteroatom(s).
(
Scheme 2).
We expected that if generation of aromatic indenyl anion
Expanded π-systems have been extensively investigated for
applications in a variety of optoelectronic devices such as
intermediate B could facilitate the addition of a nucleophile to
halo-substituted dibenzopentalene A, the desired nucleophilic
substitution would proceed smoothly to provide C through
1
organic light-emitting diodes (OLEDs), organic field-effect
2
3
4
transistors (OFETs), and organic photovoltaic cells (OPVs).
¹
7
In addition to a number of polycyclic aromatic compounds,
elimination of the halide anion X (Scheme 2). When diiodo-
substituted dibenzopentalene 1a was treated with 1.1 equiv of
PhMeNLi, the expected substitution occurred, but an inseparable
mixture of monoadduct 3a (R = 0.30, CH Cl /hexane 1:6) and
which have been explored for their optoelectronic properties
so far, recently, great attention has been paid to the synthesis
of antiaromatic compounds and their application to organic
f
2
2
5
5d,5e
5f,5g,5o
materials. For instance, Tilley
and Kawase
independ-
bisadduct 4 (0.26) was obtained (Scheme 3). In sharp contrast to
this, treatment of bromo- and iodo-substituted dibenzopentalene
1b with PhMeNLi gave 3a in 89% yield as the sole product
(Scheme 3). 1b could also be used for the synthesis of various 5-
iodo-10-heteroatom-substituted dibenzopentalenes; the reaction
ently prepared 16π-electron-system dibenzopentalenes by tran-
sition-metal-catalyzed coupling reactions, and Kawase evaluated
their FET properties. Saito obtained bis(trialkylsilyl)-substituted
dibenzopentalene by the reduction of silylethynylbenzene with
5
h
lithium. Although this procedure afforded the desired penta-
lene scaffold in a rather low yield, Saito succeeded in the
X
X
Nu
Nu
5
i
isolation of the di- and monoanions of the dibenzopentalene.
Nu-
X-
-
We have reported a new methodology for the synthesis of
dihalo-substituted dibenzopentalenes 1a and 1b by invoking
-
A
C
B
electrophilic addition of I2 and IBr to ethyne moieties in
6
5
,6,11,12-tetradehydrodibenzo[a,e]cyclooctene (2) accompa-
Scheme 2. Expected nucleophilic substitution of halogen-
substituted dibenzopentalene.
5q
nied by transannulation (Scheme 1). The halo-substituted
dibenzopentalenes 1a and 1b thus obtained could be transformed
Ph
Ph
Me
Me
N
N
PhMeNLi
1.1 equiv)
(
Y
1
a
+
THF,
°C, 4 h
0
I
N
Me
4 2%
Ph
2
1
2
) Sonogashira
coupling
) Suzuki-Miyaura
coupling
3a 70%
R
0.30 (CH2Cl2/hexane 1:6)
R
0.26 (CH2Cl2/hexane 1:6)
I2 or IBr
f
f
I
Z
R1
R2
N
Y
1
2
R R NLi
1.1 equiv)
(
S
I
1
b
C10H7SLi
(0.83 equiv)
Y
X
THF,
-78 °C, 20 min
1
1
b
b
X= I (1a), Br (1b)
I
THF,
-100 °C, 1 h
1
) Sonogashira
coupling
R , R2 = Ph, Me (3a) 89%
1
5
43%
2
) borylation
Ph, Et (3b) 82%
n
4
-Cl-C6H4-, Me (3c) 80%
OC8H17
C8H17OLi
(10 equiv)
Y
B(OR)2
THF,
reflux, 5 h
oligo(dibenzopentalene)
I
6
85%
Scheme 1. Syntheses of dibenzopentalenes by using 1 as
starting compound.
Scheme 3. Nucleophilic substitution of 1.
1548 | Chem. Lett. 2014, 43, 1548–1550 | doi:10.1246/cl.140583
© 2014 The Chemical Society of Japan

R1
R1
OC8H17
R2
R2
N
N
I
3
4
R R NLi
3.0 equiv)
(
Ph
Me
N
THF,
reflux, 1.5 h
7
CN
[Pd(PPh3)4], CuI
3
R3
N
R4
[Pd(PPh3)4], CuI
3a
6
1
2
3
4
13 90%
(
(
R , R ) = (Ph, Me), (R , R ) = (Ph, Et) (7a) 45% from 3a
R , R ) = (Ph, Et), (R , R ) = (p-MeO-C6H4-, Me) (7b) 76% from 3b
1
2
3
4
1
2
3
4
(
R , R ) = (p-Cl-C6H4-, Me), (R , R ) = (O(C2H4)2-) (7c) 81% from 3c
12 91%
Ph
NC
Me
Ph
OC8H17
N
S
Me
2
N
Na2S-9H2O
CuI, K2CO3
p-MeC6H4SLi
3.0 equiv)
(
3
a
Scheme 5. Sonogashira coupling of iododibenzopentalenes 3a
and 6.
3a
DMF,
THF,
°C, 1.0 h
110 °C, 25 min
0
8
92%
S
9
75%
PhMeNLi
(3.0 equiv,
Me
Me
Br
-78 °C, 0.5 h)
or
C8H17OLi
10 equiv,
R
ODMO
[Pd(PPh3)4], CuI
(
OC8H17
S
S
reflux, 5 h)
p-MeC6H4SLi
3.0 equiv)
p-MeC6H4SLi
(3.0 equiv)
1b
THF
(
5
6
DMOO: 3,7-dimethyloctyloxy
THF,
THF,
rt, 3.0 h
rt, 1.0 h
S
1 89%
1
14 76%
1
0 67%
DMOO
DMOO
R = PhMeN- (15) 95%
C8H17O- (16) 72%
Me
Me
Scheme 6. Sonogashira coupling of 1b followed by nucleo-
philic substitution.
Scheme 4. Nucleophilic substitution of 3, 5, and 6.
of 1b with nucleophiles such as lithium alkyl(aryl)amides,
naphthyl sulfide, and octyloxide in THF gave Ar(R)N-, C10H7S-,
and C8H17O-substituted iododibenzopentalenes 3b and 3c, 5,
and 6, respectively. In these reactions, the bromine-substituted
moiety of 1b underwent substitution preferentially compared
with the iodo-counterpart.8
The iodine of iododibenzopentalene 3, 5, and 6 could be
transformed to other functional groups by further nucleophilic
substitution. Treatment of 3 with 3.0 equiv of lithium amide
gave unsymmetrically diamino-substituted dibenzopentalenes
In this substitution, the bromopentalene moiety exhibited
higher reactivity than the iodo-counterpart, and the bromo-
substitution proceeded preferentially. Combining this protocol
and Sonogashira coupling afforded the pentalene derivatives
substituted by heteroatom and arylethynyl groups. Further
synthesis of substituted pentalenes and their application to
organic materials such as OFETs are under investigation.
This work was supported by a Grant-in-Aid for Scientific
Research on Innovative Areas “Organic Synthesis based on
Reaction Integration. Development of New Methods and
Creation of New Substances” (No. 2105), matching fund
subsidy for private universities from the Ministry of Education,
Culture, Sports, Science and Technology, Japan, the Japan
Society for the Promotion of Science (JSPS) through its
“Funding Program for World-Leading Innovative R&D on
Science and Technology (FIRST Program),” and Okayama
Prefecture Industrial Promotion Foundation.
7a­7c in good yields (Scheme 4). Lithium 4-methylphenyl
sulfide and disodium sulfide served well as the nucleophile in
the substitution of 3a to provide amino- and thio-substituted
pentalene 8 and bis(dibenzopentalene) sulfide 9, respectively.
When 5 and 6 were reacted with 3.0 equiv of lithium 4-
methylphenyl sulfide, 10 and 11 were obtained, respectively.
Although substitution proceeded both at the iodine and
naphthylthio moieties in 5, substitution occurred only at the
iodine moiety in 6, leaving the octyloxy group untouched.
The iododibenzopentalenes could undergo a C­C bond-
forming reaction by Sonogashira coupling to furnish phenyl-
ethynyl-substituted derivatives (Scheme 5). When 3a and 6 were
subjected to Sonogashira coupling, the desired ethynyl-substi-
tuted dibenzopentalenes 12 and 13 were produced, respectively.
When 1b was subjected to Sonogashira coupling, the
iodine-substituted moiety underwent substitution preferentially,
and bromodibenzopentalene 14 was obtained in 76% yield
Supporting Information is available electronically on J-STAGE.
References and Notes
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Chem. Lett. 2014, 43, 1548–1550 | doi:10.1246/cl.140583
© 2014 The Chemical Society of Japan | 1549

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© 2014 The Chemical Society of Japan