Pinpoint-fluorinated phenacenes: New synthesis and solubility enhancement strategies

Article abstract of DOI:10.1021/ol503759d

The Pd(II)-catalyzed cyclizations of 2,2-difluorovinylated biaryls, following a Friedel-Crafts-type mechanism, provide a new route to pinpoint-fluorinated phenacenes. The single fluorine substituent stabilized the synthesized fluoropicenes (fluoro[5]phenacenes) toward aerial oxidation and contributed to their solubility in organic solvents. For example, 6- and 13-fluoropicenes were 25- and 15-fold more soluble in THF than nonfluorinated picene. X-ray crystal structure analysis revealed that the fluorine substituent did not alter molecular planarity.

Full text of DOI:10.1021/ol503759d

Letter
pubs.acs.org/OrgLett
Pinpoint-Fluorinated Phenacenes: New Synthesis and Solubility
Enhancement Strategies
Kohei Fuchibe, Toshiyuki Morikawa, Kento Shigeno, Takeshi Fujita, and Junji Ichikawa*
Division of Chemistry, Faculty of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki 305−8571, Japan
*
S Supporting Information
ABSTRACT: The Pd(II)-catalyzed cyclizations of 2,2-difluor-
ovinylated biaryls, following a Friedel−Crafts-type mechanism,
provide a new route to pinpoint-fluorinated phenacenes. The
single fluorine substituent stabilized the synthesized fluoropi-
cenes (fluoro[5]phenacenes) toward aerial oxidation and
contributed to their solubility in organic solvents. For example,
6- and 13-fluoropicenes were 25- and 15-fold more soluble in
THF than nonfluorinated picene. X-ray crystal structure
analysis revealed that the fluorine substituent did not alter
molecular planarity.
henacenes are a subclass of polycyclic aromatic hydro-
carbons (PAHs) that are comprised of ortho-fused
planarity in the π-system, which is necessary for inducing self-
2b
P
assembly of the molecules. In addition, other effects of the
fluorine substituent such as the restriction of intermolecular C−
H/π interactions could also lead to solubility enhancement.
In spite of these promising features, few studies have focused
on the properties and structures of pinpoint-fluorinated
phenacenes, presumably because of their difficult synthesis.
Various methods have been developed to produce non-
fluorinated phenacenes. The photochemical oxidative cycliza-
benzene rings in a zigzag configuration. Linear acenes are
1
,2
established organic semiconducting materials but display air
3
and light sensitivity. Conversely, phenacenes, which exhibit
higher stability than acenes because of their low-lying highest
occupied molecular orbitals, have emerged as important
4
alternatives for these materials.
Our continuing studies on reactions involving fluorinated
5
,6
7
1
2
alkenes
and allenes have prompted our interest in
tion of stilbene derivatives, originally established by Mallory,
13
regioselectively monofluorinated phenacenes. These pinpoint-
fluorinated phenacenes are expected to serve as organic
has given the longest [11]phenacenes. Diels−Alder reac-
14
15
tions, Friedel−Crafts-type ring closures, Ru- or Pt-catalyzed
8
semiconducting materials. Among important factors in
16
cyclizations of alkynyl biaryls, and other metal-catalyzed
developing organic semiconducting materials (e.g., carrier
mobility, on/off ratio, etc.), we have focused our attention
particularly on solubility of pinpoint-fluorinated phenacenes.
The introduction of a single fluorine substituent into
17
reactions have also been addressed. However, the fluorination
of the aromatic nuclei is the only way to synthesize pinpoint-
18
fluorinated phenacenes. This approach has resulted in a
regioselective monofluorination of higher-order phenacene
skeletons, albeit with difficulty. Herein we present strategies
to synthesize pinpoint-fluorinated phenacenes and to solve the
solubility problem of the parent phenacenes.
8
,9
phenacene skeletons presents several advantages as follows:
(
i) The fluorine substituent displays high electronegativity,
which decreases the electron density of phenacene π
systems and, therefore, enhances their resistance to aerial
oxidation.
The electrophilic activation of 1,1-difluoro-1-alkenes was
5
achieved by a cationic palladium(II) catalyst. When treated
(
ii) The fluorine substituent has lone pairs in its 2p orbitals.
These lone pairs present repulsive interaction with
adjacent π-electrons in the 2p orbitals, perturbing the
electron density of the π-systems.
with a catalytic amount of [Pd(MeCN) ](BF ) in the presence
4
4 2
19
of BF ·OEt in 1,1,1,3,3,3-hexafluoropropan-2-ol (HFIP), 1,1-
3
2
difluoroalkenes bearing a 2-arylethyl group provided α-
tetralones upon spontaneous hydrolysis of the formed cyclic
5a
(
iii) The fluorine substituent displays a low steric impact.
Therefore, its introduction into phenacene molecules
should not change their shape.
vinyl fluorides. In this reaction, π-coordination with Pd(II)
promoted a selective Friedel−Crafts-type cyclization at the
vinylic CF carbon. Using this concept, we devised a new
2
strategy for the synthesis of pinpoint-fluorinated phenacenes.
Specifically, the 2-arylethyl moiety was replaced with a biaryl
moiety in the substrates to provide target fluorophenacenes,
The effects of the electronegativity (i) and fluorine lone pairs
ii) induce polarization in molecules. We hypothesized that
the polarization might contribute to enhanced solubility of
10
(
pinpoint-fluorinated phenacenes in polar organic solvents,
which is required in wet processes of device fabrication.
The small steric effect (iii) would result in the preservation of
11
Received: December 31, 2014
©
XXXX American Chemical Society
A
DOI: 10.1021/ol503759d
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
which should be hydrolytically stable because of their
aromaticity.
Table 2. Synthesis of Pinpoint-Fluorinated Chrysenes and
Picenes
a
2,2-Difluorovinylated biaryls such as 1a were readily prepared
(
i) by a Wittig-type difluoromethylenation of biaryl carbalde-
20
hydes (C1 introduction) or (ii) by a palladium-catalyzed
difluorovinylation of biaryl triflates or halides (C2 introduc-
2
1
tion) (see the Supporting Information). Screening of catalysts
for the cyclization of 1a revealed that in situ generated
palladium(II) catalysts exhibit higher catalytic activities (Table
1
). In the presence of [Pd(MeCN) ](BF ) , substrate 1a gave
4 4 2
Table 1. Catalyst Effects on Chrysene Synthesis
a
a
entry
Pd(II) catalyst
2a (%) recovery of 1a (%)
1
2
3
4
5
6
7
[Pd(MeCN) ](BF )
4 2
15
24
81
68
7
63
54
2
4
Pd(MeCN) Cl , AgOTf (1:2)
2
2
PdCl , AgOTf (1:2)
2
PdCl , AgOTf (1:1)
22
93
24
33
2
PdCl2
PdCl , AgBF (1:2)
78
63
a
2
4
Conditions: 1, 15 mol % PdCl , 30 mol % AgOTf, BF ·OEt (1.0
2
3
2
PdCl , AgSbF (1:2)
b
c
2
6
equiv), HFIP, 60 °C. Isolated yield unless otherwise noted. Single
a
19
d
19
F NMR yield based on PhCF as an internal standard.
regioisomer. F NMR yield based on PhCF as an internal standard.
3
e
3
The decrease in the isolated yield results from the difficult
f
chromatographic separation. 25 mol % PdCl , 50 mol % AgOTf,
BF ·OEt (1.0 equiv). Regioisomeric ratio (2d:3) determined by
NMR spectroscopy.
2
the corresponding 5-fluorochrysene ([4]phenacene, 2a) as a
single regioisomer only in 15% yield (60 °C, 2 h, entry 1).
The regiochemistry in the cyclization of 1a stems from the high
reactivity of the naphthalene α carbon toward electrophilic
aromatic substitution. The cationic palladium(II) species
generated from Pd(MeCN) Cl and silver trifluoromethanesul-
g
19
F
5a
3
2
2
2
fonate (AgOTf) afforded 2a in 24% yield (entry 2). Acetonitrile
ligand removal from the starting catalyst dramatically improved
the yield of 2a to 81% (entry 3). The use of two equiv of
2
2,23
AgOTf to PdCl gave the best yield of 2a (entries 3−5).
2
Silver tetrafluoroborate (AgBF ) and hexafluoroantimonate
4
(
6
AgSbF ) provided somewhat less promising results (entries
and 7).
6
The ligand-free palladium(II) catalyst achieved a regiose-
lective synthesis of pinpoint-fluorinated [4]- and [5]-
phenacenes (Table 2). Single isomers of 5- (2a) and 6-
fluorochrysenes (2b) were obtained in 81% and 77% yields,
respectively, starting from the corresponding difluorovinylated
biaryls (entries 1 and 2). Synthesis of 5- (2c), 6- (2d), and 13-
fluoropicenes (2e) was also accomplished under similar
conditions in 47%, 70%, and 85% yields, respectively (entries
3
−5). These fluorinated chrysenes and picenes were stable
enough to conduct chromatographic purifications without
requiring any specific precaution.
A mechanism for the phenacene synthesis is proposed in
Figure 1. Substrates 1 coordinate to the cationic palladium(II)
center to form π complexes A, in which the electron density of
the difluoroalkene moiety is lowered, leading to its electrophilic
activation. This promotes a Friedel−Crafts-type ring closure,
providing cyclic alkylpalladium intermediates B that in turn
Figure 1. Proposed catalytic cycle for fluorophenacene synthesis.
undergo a BF -assisted β-fluorine elimination to yield product
were analyzed by density functional theory (DFT) calculations.
As expected, the introduction of a single fluorine substituent
into the picene skeleton greatly perturbed the electron density
of the π-systems and effectively induced polarization (Figure 2).
The parent picene exhibited symmetrical and even contours in
3
−
2
. The formation of BF4 regenerates the active cationic
palladium(II) species in this process.
The charge distributions of the parent nonfluorinated and
the synthesized 5- (2c), 6- (2d), and 13-fluoropicenes (2e)
B
DOI: 10.1021/ol503759d
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Organic Letters
Letter
Figure 3. X-ray crystal structures of (a) 6-fluoropicene (2d) and (b)
Figure 2. DFT electrostatic potential maps and dipole moments of
pinpoint-fluorinated phenacenes (B3LYP/6-31G* level). White arrows
represent dipole moments for each picene.
1
3-fluoropicene (2e).
The X-ray crystal structure analysis showed that two
the electrostatic potential map (a). In contrast, the potential
maps of the pinpoint-fluorinated picenes were not symmetrical
molecules of pinpoint-fluorinated picenes were coupled
through C−H···F interactions in solid state (2.54 and 2.60 Å
for H···F distances in 2d and 2e, respectively, and 2.67 Å for the
sum of the van der Waals radii of hydrogen and fluorine, Figure
(
b−d). Fluorinated picenes thus showed a higher degree of
desymmetrization in their charge distribution and their dipole
moments rose from 0.06 D (parent picene) to 1.14−1.37 D.
These pinpoint-fluorinated PAHs displayed high solubility in
organic solvents (Table 3). A saturated THF solution presented
26
3) and that the dimer formed face-to-face stacks (see the
Supporting Information). The fluorine substituent thus
decreased C−H/π interactions, which usually organize aromatic
molecules in solid state, and might contribute to the solubility
enhancement.
Table 3. Solubility of Pinpoint-Fluorinated Picenes in THF
27
It should be mentioned that some groups have recently
reported that packing structures of acene derivatives were
controlled by introducing a polyfluorinated aromatic moiety to
their frameworks through the use of C−H···F and Ar−FAr
interactions. However, the compound was hardly soluble in
organic solvents because of the packing structure stabilized by
concentration (mol L−1)
1
a
entry
phenacene
picene
mol L−
wt %
3
1
2
3
4
6.4 × 10− (1)
0.20
0.48
5.3
2
5-fluoropicene (2c)
6-fluoropicene (2d)
13-fluoropicene (2e)
1.4 × 10− (2)
−1
1.6 × 10 (25)
9.3 × 10− (15)
2
27c
3.1
the tight interactions. Again, introduction of a single fluorine
a
substituent to PAHs is a suitable strategy for development of
solution-processable organic semiconducting materials.
In summary, the Pd(II)-catalyzed cyclization of 2,2-
difluorovinylated biaryls was studied to synthesize pinpoint-
fluorinated phenacenes, which are stable toward aerial oxidation
and soluble in THF. Our synthetic and solubility enhancement
strategies by introducing one fluorine substituent provide
promising materials that are suitable for printable organic
electronics.
Solubility relative to that of the parent picene is shown in
parentheses.
a low concentration of parent picene (6.4 × 10−3 mol L ) at
room temperature, based on its molar extinction coefficient ε
determined by UV−vis spectroscopy. On the other hand, the
measured solubilities amounted to 1.4 × 10 , 1.6 × 10 , and
−1
−2
−1
.3 × 10−2 mol L for 2c, 2d, and 2e, respectively. These
−1
9
values correspond to 2-, 25-, and 15-fold that of the parent
picene.
ASSOCIATED CONTENT
Supporting Information
Organic substituents such as tert-butyl or phenyl groups are
often employed to increase the solubility of polycyclic aromatic
compounds. However, these bulky substituents affect molecular
planarity. For example, the previously reported soluble
■
*
S
Experimental procedures, spectra of new compounds, a
25
summary of the patent document, photophysical properties
obtained by UV−vis spectroscopy, and packing structures of 2d
and 2e. This material is available free of charge via the Internet
at http://pubs.acs.org.
[
7]phenacene derivative bearing tert-butyl groups was enor-
24
mously twisted. In contrast, X-ray crystal structure analysis
revealed that the π-systems of the synthesized 2d and 2e
remained planar (Figure 3). Therefore, the fluorine substituent
introduced into the phenacene skeleton did not change the
shape of these molecules because of its small steric require-
ment. Planarity of aromatic compounds is an important factor
that induces self-assembly of their benzene rings into π-stacked
arrays, which leads to high performance as organic semi-
conducting materials. Thus, introduction of a single fluorine
substituent is a suitable strategy for enhancing phenacene
solubility without affecting their performance as materials for
electronic devices. Fluoropicene 2e actually exhibits p-type
AUTHOR INFORMATION
Corresponding Author
■
*
E-mail: junji@chem.tsukuba.ac.jp.
Notes
2
b,c
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This research is supported by JSPS KAKENHI [Grant Nos.
25288016 (J.I.) and 25620158 (K.F.)] and Asahi Glass
■
25
semiconducting behavior (see the Supporting Information).
C
DOI: 10.1021/ol503759d
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Foundation (J.I.). Central Glass Co., Ltd., is acknowledged for
the generous gifts of (CF ) CHOH (HFIP). We thank Dr.
Kyoko Yamamoto, Mr. Shinya Tahara, and Dr. Yoshitomi
Morizawa (Asahi Glass Co., Ltd.) for the analysis of the
semiconducting behavior of 2e and Professor Hiroyuki Isobe
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