2,6-Bis(phenylethynyl)biphenyls and their cyclization to pyrenes

Article abstract of DOI:10.1055/s-0031-1290754

We present a new protocol for pyrene synthesis via transition-metal cross-couplings. The initially prepared 2,6-bis(phenylethynyl)biphenyls were transformed to pyrenes with uncommon 4,10-disubstitution through an electrophilic cyclization. The precursors were synthesized by Suzuki-Miyaura cross-coupling, which provides 2,6-dibromobiphenyls; these were subsequently coupled with phenylethynyl derivatives via Kumada cross-coupling. Georg Thieme Verlag Stuttgart · New York.

Full text of DOI:10.1055/s-0031-1290754

PAPER
▌1405
p2a,p6er-Bis(phenylethynyl)biphenyls and Their Cyclization to Pyrenes
Synthesis and Cyclization of 2,6-Bis(phenylethynyl)biphenyls
Mareike M. Machuy,a Christian Würtele,b Peter R. Schreiner*a
a
Institut für Organische Chemie, Justus-Liebig University, Heinrich-Buff-Ring 58, 35392 Gießen, Germany
Fax +49(641)34309; E-mail: prs@org.chemie.uni-giessen.de
b
Institut für Anorganische und Analytische Chemie, Justus-Liebig University, Heinrich-Buff-Ring 58, 35392 Gießen, Germany
Received: 12.02.2012; Accepted after revision: 17.02.2012
2) and employed density functional theory (DFT) compu-
tations to gain insight into the geometric constraints of the
cyclization precursors.
Abstract: We present a new protocol for pyrene synthesis via tran-
sition-metal cross-couplings. The initially prepared 2,6-bis(phenyl-
ethynyl)biphenyls were transformed to pyrenes with uncommon
4,10-disubstitution through an electrophilic cyclization. The precur-
sors were synthesized by Suzuki–Miyaura cross-coupling, which
provides 2,6-dibromobiphenyls; these were subsequently coupled
with phenylethynyl derivatives via Kumada cross-coupling.
R2
R2
R1
R1
R1
R1
Key words: alkynes, biaryls, pyrenes, cyclizations, transition
PtCl2 or AuCl
metals
R4
R4
R4
R4
R3
R3
Pyrenes are associated with attractive electronic and opto-
electronic properties for use as building blocks for organic
electronics, and they are already in use in organic light-
emitting diodes (OLEDs), organic field-effect transistors
Scheme 1 Davis’ pyrene synthesis: R1 = I, H; R2–R4 = various sub-
stituents (OBu, NBoc, OMe, etc.), generally leading to a uniform sub-
stitution pattern
(OFETs) and organic photovoltaic devices (OPVs).1–4
A
special property of pyrenes is their blue emission fluores-
cence when substituents are placed so that undesirable
face-to-face π–π stacking interactions are avoided.2 Tradi-
tional synthetic pathways to substituted pyrenes typically
involve electrophilic halogenation of the parent pyrene
and its derivatives leading to substitution in the 1-, 1,3-,
1,6-, 1,8-, 2,7-, 1,3,6,8- and 4,5,9,10-positions, but mostly
not with different substituents.4,5 Alternatively, 2,6-dieth-
ynylbiphenyls and analogous phenanthrene derivatives
have been proposed as precursors for electrophilic cycli-
zations to form pyrenes with diverse and complementary
substitution patterns (Scheme 1). Davis’ approach3 and
our approach6 utilizing electrophile-induced ring
closures6–8 enables the introduction of substituents in po-
sitions 4 and 10 without substitution in positions 5 and 9.
Furthermore, the use of electrophilic cyclizations is a via-
ble path to achieve uncommonly functionalized pyrene
derivatives without employing toxic preformed pyrene.
In addition to the use of our ethynyl precurors for pyrene
cyclizations, these materials are also valuable building
blocks for a unique family of compounds for various ap-
plications, such as liquid crystal engineering and drug de-
sign.10 The 4′-substituted 2,6-dibromobiphenyls 3 are
known compounds but have not yet been fully character-
ized in the literature.11 2,6-Dibromobiphenyls 3a–e were
synthesized in moderate to good yields (Scheme 2) by
Suzuki–Miyaura cross-coupling, starting from the appro-
priate 2,6-dibromoiodobenzene 1 and boronic acid 2,
utilizing Organ’s PEPPSI-iPr catalyst.12
I
Br
Br
R2
R1
1
K2CO3 (3 equiv)
PEPPSI-iPr (5 mol%)
Here, we present an extension of this strategy to prepare
functionalized pyrenes from 2,6-bis(phenylethynyl)bi-
phenyls, anticipating that the latter can also be conve-
niently employed for cyclization reactions.8,9 We found
that various cyclization attempts utilizing the established
procedures of Fürstner and Mamane were to no avail.7 We
also tackled the question of what factors prevent such sys-
tems from giving pyrenes. In order to elucidate this unex-
pected behavior, we compared our systems with those of
the pioneering work of Larock and co-workers (see Figure
+
1,4-dioxane
Br
Br
B(OH)2
R1
3
R2
2
Scheme 2 R group definitions: 1a R1 = H, 1b R1 = F; 2a R2 = H, 2b
R2 = F, 2c R2 = OMe, 2d R2 = CF3; 3a R1 = R2 = H, 62%; 3b R1 = H,
R2 = F, 45%; 3c R1 = H, R2 = OMe, 49%; 3d R1 = H, R2 = CF3, 18%;
3e R1 = F, R2 = H, 58%.
SYNTHESIS 2012, 44, 1405–1409
Advanced online publication: 10.04.2012
DOI: 10.1055/s-0031-1290754; Art ID: SS-2012-Z0142-OP
© Georg Thieme Verlag Stuttgart · New York
Purification by flash chromatography (3a–c and 3e) gave
colorless crystals. Compound 3d was purified by prepara-
0
0
3
9
-
7
8
8
1
1
4
3
7
-
2
1
0
X

1406
M. M. Machuy et al.
PAPER
tive GC and was also obtained as colorless crystals. All tography was mostly adequate for purification; compound
the biphenyls were characterized spectroscopically and by 4a required purification by preparative HPLC.
X-ray crystal structure analyses. Structure 3a is shown in
Figure 1 with its elementary cell; the X-ray crystal struc-
tures of compounds 3c and 3e were measured (see Sup-
porting Information). These structures were also used for
DFT computations at the M06-2X/6-31G(d,p) level indi-
cate that 4a is twisted, with the phenyl substituent in the
1-position being almost exactly perpendicular (89°) to the
plane of the other aryl moieties. The twisted structure lies
structural validation of the optimized computed geome-
14.7 kcal·mol–1 below a hypothetical planar arrangement,
tries at the M06-2X/6-31G(d,p)13 level of theory, for
which is not a stationary point on the potential energy
which we found that the experimental bond lengths and
hypersurface. In contrast, the corresponding energy dif-
ference of the rotational profile of Larock’s 2-phenyl-
angles were well reproduced.
ethynylbiphenyl system is only about 7.2 kcal·mol–1.15
Between the carbon atoms in 4a that eventually form the
new carbon–carbon bond we observed a distance of 3.85
Å, while an angle of 79° was determined between these
two carbon atoms and the ethynyl-substituted carbon
atom of the biphenyl; Larock’s precursor has a critical an-
gle of 73° and a distance of 3.40 Å (Figure 2). Hence, the
steric arrangement of structure 4a is rather unfavorable for
the desired 1,6-cyclization.
Figure 1 X-ray structure of 2,6-dibromobiphenyl (3a) with selected
bond lengths compared with the computed M06-2X/6-31G(d,p)
values
Coupling of 2,6-dibromobiphenyls 3 with phenylacety-
lenes via Sonogashira–Hagihara cross-coupling14 gave, if
at all, the twofold coupling product in low yields; often,
the singly phenylethynylated biphenyl was formed in-
stead. However, Kumada coupling of the 2,6-dibromobi-
Figure 2 Optimized geometries of Larock’s structure (left) vs 4a:
phenyls with phenylethynylmagnesium bromides in
tetrahydrofuran readily gave the 2,6-bis(phenylethy-
nyl)biphenyls 4a–d (Scheme 3). The catalyst
(Ph3P)2PdCl2 was used for compounds 4a, 4b and 4d; for
4c, Pd2(dba)3 proved to be more suitable. Flash chroma-
different spatial adjustment of the unsubstituted phenyl ring in the bi-
phenyl [M06-2X/6-31G(d,p) level]
Owing to the larger distances and angles between the cru-
cial C atoms, we expected our system to require higher re-
action temperatures; however, the cyclization of 4a and
4b in refluxing toluene with platinum(II) chloride resulted
in the expected pyrenes 5a and 5b (Scheme 4).16 After pu-
rification by preparative HPLC and subsequent high-
vacuum sublimation, 5a was characterized by 1H and 13C
NMR spectroscopy with the aid of HSQC and DEPT-135
experiments.
R2
R2
R3
R3
ArMgBr
Pd cat.
Br
Br
THF
R1
The optimized geometry of pyrene 5a also shows twisted
(almost perpendicular) phenyl substituents, which is sup-
ported by the spectroscopic finding that 5a displays pro-
nounced blue fluorescence and does not therefore display
undesirable π–π stacking.
4
3a,b
Scheme 3 ArMgBr prepared from phenylacetylene (6a) or 4-fluoro-
phenylacetylene (6b) via H–MgBr exchange with EtMgBr;
catalyst = (Ph3P)2PdCl2 or Pd2(dba)3; 3a R1 = R2 = H, 3b R1 = H,
R2 = F; 4a R2 = R3 = H, 53%; 4b R2 = R3 = F; 4c R2 = H, R3 = F,
15%; 4d R2 = F, R3 = H, 40%.
The main quality for the potentional use of these pyrene
derivatives in optical devices is their strong fluorescence.
R2
R2
R3
R3
R3
R3
PtCl2
toluene, reflux
5
4a,b
Scheme 4 1,6-Cyclization towards functionalized pyrenes 5a (R2 = R3 = H) and 5b (R2 = R3 = F)
Synthesis 2012, 44, 1405–1409
© Georg Thieme Verlag Stuttgart · New York

PAPER
Synthesis and Cyclization of 2,6-Bis(phenylethynyl)biphenyls
1407
was refluxed for 36 h. Sat. aq NaHCO3 (80 mL) was added and the
crude product was extracted with CH2Cl2 (100 mL). The extract was
dried over MgSO4 and purified by column chromatography
[Rf = 0.32 (pentane–CH2Cl2, 99:1)]. Evaporation of the eluent pro-
vided colorless crystals; yield: 0.97 g (3.1 mmol, 62%); mp 71 °C.
1H NMR (400 MHz, CDCl3): δ = 7.05 (t, J = 8.0 Hz, 1 H), 7.20 (d,
J = 7.2 Hz, 2 H), 7.4–7.5 (m, 3 H), 7.62 (d, J = 8.0 Hz, 2 H).
13C NMR (100 MHz, CDCl3): δ = 124.54 (2 CBr), 128.06 (ArCH),
128.19 (2 ArCH), 129.14 (2 ArCH), 129.81 (2 ArCH), 131.78
(ArCH), 141.14 (ArCq), 143.02 (ArCq).
We recorded a blue shift (–20 nm) of the absorption bands
compared to the parent pyrene (338 nm).17 The emission
spectrum of 5a (as an example) shows a maximum at
about 380 nm and nicely mirrors the UV–vis absorptions
(Figure 3). The Stokes shift was determined to be around
50 nm. We identified the long-wavelength absorption at
310 nm of pyrene 5a as a π → π* transition, which mainly
involves the HOMO–LUMO transition (B → A) of the S0
ground state to the open-shell singlet state S1.
HRMS: m/z calcd for C12H8Br2: 309.899; found: 309.897.
2,6-Dibromo-4′-fluorobiphenyl (3b)
Into a 100-mL nitrogen flask were placed 2,6-dibromoiodobenzene
(1a; 1.8 g, 5 mmol), PEPPSI-IPr (0.2 g, 0.25 mmol, 5 mol%),
K2CO3 (2.48 g, 18 mmol) and dioxane (25 mL) under argon and an-
hydrous conditions. The mixture was stirred at r.t. for 1 h, then 4-
fluorophenylboronic acid (2b; 0.7 g, 5 mmol) was added and the so-
lution was refluxed for 36 h. Sat. aq NaHCO3 (80 mL) was added
and the crude product was extracted with CH2Cl2 (100 mL). The ex-
tract was dried over MgSO4 and purified by column chromatogra-
phy [Rf = 0.26 (pentane–CH2Cl2, 99:1)]. This afforded colorless
crystals; yield: 0.74 g (2.24 mmol, 45%); mp 56 °C.
1H NMR (400 MHz, CDCl3): δ = 7.04–7.18 (m, 5 H), 7.61 (d,
J = 8.9 Hz, 2 H).
13C NMR (100 MHz, CDCl3): δ = 115.3 (d, 2JC-F = 22 Hz, 2 ArCH),
124.7 (2 CBr), 130.04 (2 ArCH), 131.04 (d, JC-F = 8.2 Hz, 2
Figure 3 UV–vis absorption (left) and emission (right) spectra of 5a
(λexc = 310 nm) in CH2Cl2, and optimized geometry [M06-2X/6-
31G(d,p) level]
3
ArCH), 131.87 (ArCH), 137.01 (ArCq), 142.02 (ArCq), 162.77 (d,
1JC-F = 243 Hz, ArCF).
HRMS: m/z calcd for C12H7Br2F: 327.889; found: 327.890.
In conclusion, we have synthesized the 2,6-dibromobi-
phenyls 3a–e from their respective boronic acid precur-
sors 2a–d using Suzuki–Miyaura cross-couplings as the
decisive step. The 2,6-dibromobiphenyls can be readily
coupled with alkynyl derivatives. The resulting 2,6-
bis(phenylethynyl)biphenyls were electrophilically cy-
clized to pyrene derivatives with a substitution pattern that
2,6-Dibromo-4′-methoxybiphenyl (3c)
A 100-mL nitrogen flask was charged with 2,6-dibromoiodoben-
zene (1a; 1.8 g, 5 mmol), PEPPSI-IPr (0.17 g, 0.25 mmol, 5 mol%),
K2CO3 (2.48 g, 18 mmol) and dioxane (25 mL) under argon and an-
hydrous conditions. The mixture was stirred at r.t. for 1 h, then 4-
methoxyphenylboronic acid (2c; 0.76 g, 5 mmol) was added and the
solution was refluxed for 36 h. Sat. aq NaHCO3 (80 mL) was added
is not readily available utilizing common approaches to and the crude product was extracted with CH2Cl2 (100 mL). The ex-
tract was dried over MgSO4 and purified by column chromatogra-
phy [Rf = 0.1 (hexane–CH2Cl2, 95:5)]. This afforded colorless
pyrenes.
crystals; yield: 0.83 g (2.43 mmol, 49%); mp 60 °C.
1H NMR spectra were recorded at 400 MHz or 600 MHz, and 13
C
1H NMR (400 MHz, CDCl3): δ = 3.85 (s, 3 H), 6.97 (d, J = 8.7 Hz,
2 H), 7.03 (t, J = 8.05 Hz, 1 H), 7.13 (d, J = 8.4 Hz, 2 H), 7.60 (d,
J = 7.9 Hz, 2 H).
13C NMR (100 MHz, CDCl3): δ = 55.2 (OCH3), 113.5 (2 ArCH),
125.2 (2 CBr), 129.7 (ArCq), 130.4 (2 ArCH), 131.8 (2 ArCH),
133.7 (ArCH), 142.7 (ArCq), 159.2 (CO).
NMR spectra at 100 MHz or 151 MHz, on a Bruker AV 400 or AV
600 spectrometer, respectively. Chemical shifts are reported in ppm
(δ scale) using TMS as internal standard or the solvent signal as sec-
ondary standard. Structural assignments were made using 2D NMR
spectra (HSQC) and DEPT-135 NMR spectra. UV–vis spectra were
measured with a HP 8423 spectrometer (mercury lamp). Emission
spectra were measured with an Ocean Optics SD 200 spectrometer
(lamp: Beckmann Xe 1045, monochromatic illuminator: Beckmann
GM 1139). Preparative GC was carried out using an OV-17 column
and a temperature program 100 °C to 250 °C, with 4 °C/min ramp-
ing. Preparative HPLC was carried out using a Knauer HPLC Pump
64 system (RP-18 column). Melting points are not corrected and
were measured with a Büchi Dr. Tottoli Typ S apparatus or with a
Krüss KSP I N melting point meter. HRMS were recorded using a
Finnigan MAT 95 instrument under electron impact (EI) conditions.
Solvents were dried according to literature procedures. Compounds
1a, 1b, 2a–d and 6b were prepared according to literature proce-
dures.18
HRMS: m/z calcd for C13H10Br2O: 339.909; found: 339.911.
2,6-Dibromo-4′-(trifluoromethyl)biphenyl (3d)
Into a 100-mL nitrogen flask were placed 2,6-dibromoiodobenzene
(1a; 0.362 g, 1 mmol), PEPPSI-IPr (0.1 g, 0.15 mmol, 15 mol%),
K2CO3 (0.42 g, 3 mmol) and dioxane (10 mL) under argon and an-
hydrous conditions. The mixture was stirred at r.t. for 1 h, then 4-
(trifluoromethyl)phenylboronic acid (2d; 0.19 g, 1 mmol) was add-
ed and the solution was refluxed for 36 h. Sat. aq NaHCO3 (20 mL)
was added and the crude product was extracted with CH2Cl2 (60
mL). The extract was dried over MgSO4 and subsequently purified
by preparative GC. This afforded colorless crystals; yield: 0.07 g
(0.18 mmol, 18%); mp 46 °C.
2,6-Dibromobiphenyl (3a)
1H NMR (400 MHz, CDCl3): δ = 7.09 (t, J = 8.1 Hz, 1 H), 7.33 (d,
J = 8 Hz, 2 H), 7.63 (d, J = 8.1 Hz, 2 H), 7.71 (d, J = 8 Hz, 2 H).
A 100-mL nitrogen flask was charged with 2,6-dibromoiodoben-
zene (1a; 1.8 g, 5 mmol), PEPPSI-IPr (0.2 g, 0.25 mmol, 5 mol%),
K2CO3 (2.48 g, 18 mmol) and dioxane (25 mL) under argon and an-
hydrous conditions. The mixture was stirred at r.t. for 1 h, then
phenylboronic acid (2a; 0.6 g, 5 mmol) was added and the solution
13C NMR (100 MHz, CDCl3): δ = 122.75 (1JC-F = 273 Hz, CF3),
124.0 (2 CBr), 125.3 (3JC-F = 4 Hz, 2 ArCH), 129.8 (2 ArCH), 130.1
© Georg Thieme Verlag Stuttgart · New York
Synthesis 2012, 44, 1405–1409

1408
M. M. Machuy et al.
PAPER
(2JC-F = 32 Hz, ArCq), 131.1 (2 ArCH), 132.9 (ArCH), 141.7 (Ar-
Cq), 144.5 (ArCq).
2,6-Bis(4-fluorophenylethynyl)biphenyl (4c)
To a stirred mixture of 3a (0.312 g, 1 mmol) and Pd2(dba)3 (0.137
g, 0.15 mmol) in THF (10 mL) under argon atmosphere and anhy-
drous conditions was added 4-fluorophenylethynylmagnesium bro-
mide [prepared beforehand from ethynylmagnesium bromide (1
equiv) and 4-fluorophenylacetylene (6b, 6 mmol) by transmetala-
tion at –40 °C]. The solution was refluxed for 48 h. Then, NH4Cl
(50 mL) was added to the dark brown solution and the mixture was
extracted with CH2Cl2 (150 mL). The crude product was purified by
flash column chromatography with silica gel [Rf = 0.36 (cyclohex-
ane–CH2Cl2, 95:5)] to provide a pale yellow, viscous oil; yield:
0.057 g (0.15 mmol, 15%).
HRMS: m/z calcd for C13H7Br2F3: 377.886; found: 377.885.
2,6-Dibromo-4-fluorobiphenyl (3e)
Into a 100-mL nitrogen flask were placed 2,6-dibromo-4-fluo-
roiodobenzene (1b; 1.9 g, 5 mmol), PEPPSI-IPr (0.17 g, 0.25 mmol,
5 mol%), K2CO3 (2.1 g, 15 mmol) and dioxane (25 mL) under argon
and anhydrous conditions. The mixture was stirred at r.t. for 1 h,
then phenylboronic acid (2a; 0.6 g, 5 mmol) was added and the so-
lution was refluxed for 36 h. Sat. aq NaHCO3 (80 mL) was added
and the crude product was extracted with CH2Cl2 (100 mL). The ex-
tract was dried over MgSO4 and purified by column chromatogra-
phy [Rf = 0.35 (hexane–CH2Cl2, 9:1)]. This afforded colorless
crystals; yield: 0.96 g (2.9 mmol, 58%); mp 110 °C.
1H NMR (400 MHz, CDCl3): δ = 6.93 (t, J = 8.8 Hz, 4 H), 7.12–
7.16 (m, 4 H), 7.30 (t, J = 8.8 Hz, 1 H), 7.4–7.5 (m, 3 H), 7.56–7.58
(m, 4 H).
13C NMR (100 MHz, CDCl3): δ = 88.5 (2 CCAr), 91.9 (2 CCAr),
115.5 (2JC-F = 21 Hz, 4 ArCH), 119.2 (4JC-F = 4 Hz, 2 ArCq), 123.1
(2 ArCq), 127.1 (ArCH), 127.4 (ArCH), 127.7 (2 ArCH), 130.3 (2
ArCH), 132.0 (2 ArCH), 133.3 (3JC-F = 8.5 Hz, 4 ArCH), 139.1
(ArCq), 146.4 (ArCq), 163.7 (1JC-F = 250 Hz, 2 ArCF).
1H NMR (400 MHz, CDCl3): δ = 7.18 (d, J = 7.3 Hz, 2 H), 7.4–7.5
(m, 5 H).
13C NMR (100 MHz, CDCl3): δ = 119.4 (2JC-F = 23 Hz, 2 ArCH),
124.4 (2 CBr), 128.3 (3 ArCH), 129.4 (2 ArCH), 139.4 (4JC-F = 4
Hz, ArCq), 140.3 (ArCq), 161.1 (1JC-F = 254 Hz, ArCF).
HRMS: m/z calcd for C28H16F2: 390.122; found: 390.121.
HRMS: m/z calcd for C12H7Br2F: 327.889; found: 327.890.
4′-Fluoro-2,6-bis(phenylethynyl)biphenyl (4d)
2,6-Bis(phenylethynyl)biphenyl (4a)
To a stirred mixture of 3b (0.330 g, 1 mmol) and (Ph3P)2PdCl2
(0.105 g, 0.15 mmol) in THF (10 mL) under argon atmosphere and
anhydrous conditions was added phenylethynylmagnesium bro-
mide [prepared beforehand from ethynylmagnesium bromide (1
equiv) and phenylacetylene (6a, 6 mmol) by transmetalation at
–40 °C]. The solution was refluxed for 48 h. Then, NH4Cl (50 mL)
was added to the dark brown solution and the mixture was extracted
with CH2Cl2 (150 mL). The crude product was purified by flash
chromatography [Rf = 0.17 (hexane–CH2Cl2, 95:5)] to provide a
pale yellow, viscous oil; yield: 0.148 g (0.40 mmol, 40%).
To a stirred mixture of 3a (0.312 g, 1 mmol) and (Ph3P)2PdCl2
(0.105 g, 0.15 mmol) in THF (15 mL) under argon atmosphere and
anhydrous conditions was added phenylethynylmagnesium bro-
mide [prepared beforehand from ethynylmagnesium bromide (1
equiv) and phenylacetylene (6a, 6 mmol) by transmetalation at
–40 °C]. The solution was refluxed for 48 h. Then, NH4Cl (50 mL)
was added to the dark brown solution and the mixture was extracted
with CH2Cl2 (150 mL). The crude product was purified by prepara-
tive HPLC (MeCN–H2O, 80:20; 3 mL/min; 220 nm). This afforded
a pale yellow, viscous oil; yield: 0.19 g (0.53 mmol, 53%).
1H NMR (400 MHz, CDCl3): δ = 7.1–7.3 (m, 5 H, ArH), 7.5–7.6
(m, 12 H, ArH).
1H NMR (400 MHz, CDCl3): δ = 7.1–7.2 (m, 10 H), 7.24 (t, J = 8
Hz, 1 H), 7.34–7.43 (m, 3 H), 7.52–7.55 (m, 4 H).
13C NMR (100 MHz, CDCl3): δ = 88.6 (2 CCAr), 93.1 (2 CCAr),
114.5 (2JC-F = 21 Hz, 2 ArCH), 123.0 (2 ArCq), 123.4 (2 ArCq),
127.2 (2 ArCH), 128.3 (3JC-F = 6 Hz, 3 ArCH), 129.4 (4 ArCH),
131.4 (4 ArCH), 132.1 (2 ArCH), 135.0 (ArCq), 145.1 (ArCq), 163.7
(1JC-F = 243 Hz, ArCF).
13C NMR (100 MHz, CDCl3): δ = 88.9 (2 CCAr), 92.9 (2 CCAr),
123.2 (2 ArCq), 123.3 (2 ArCq), 127.1 (ArCH), 127.4 (ArCH), 127.7
(2 ArCH), 128.2 (6 ArCH), 130.4 (2 ArCH), 131.4 (4 ArCH), 132.1
(2 ArCH), 139.1 (ArCq), 146.4 (ArCq).
HRMS: m/z calcd for C28H18: 354.140; found: 354.139.
HRMS: m/z calcd for C28H17F: 372.131; found: 372.130.
4′-Fluoro-2,6-bis(4-fluorophenylethynyl)biphenyl (4b)
To a stirred mixture of 3b (0.330 g, 1 mmol) and (Ph3P)2PdCl2
(0.105 g, 0.15 mmol) in THF (15 mL) under argon atmosphere and
anhydrous conditions was added 4-fluorophenylethynylmagnesium
bromide [prepared beforehand from ethynylmagnesium bromide (1
equiv) and 4-fluorophenylacetylene (6b, 6 mmol) by transmetala-
tion at –40 °C]. The solution was refluxed for 48 h. Then, NH4Cl
(50 mL) was added to the dark brown solution and the mixture was
extracted with CH2Cl2 (150 mL). The crude product was purified by
flash column chromatography with silica gel [Rf = 0.18 (hexane–
CH2Cl2, 95:5)] to provide a pale yellow solid; conversion almost
complete (probable spectroscopically invisible impurity); mp
82 °C.
4,10-Diphenylpyrene (5a)
Biphenyl 4a (118 mg, 0.6 mmol) and PtCl2 (40 mg, 0.6 mmol) were
placed in a nitrogen flask, and degassed and purged with argon. An-
hyd toluene (3 mL) was added and the mixture was heated at reflux
for 20 h. The reaction mixture was filtered through Celite® and the
dark brown solid was purified by preparative HPLC (MeCN–H2O,
80:20; 3 mL/min; 220 nm) and sublimation [UHV (10–6 mbar),
heating with a heat gun]. This afforded a pale yellow solid; yield: 28
mg (0.08 mmol, 13%).
1H NMR (600 MHz, CDCl3): δ = 7.47–7.58 (m, 8 H, 8 ArH), 7.65–
7.7 (m, 4 H, 4 pyrene-H), 8.03–8.05 (m, 2 H, 2 pyrene-CH), 8.18–
8.24 (m, 4 H, 2 pyrene-H, 2 ArH).
1H NMR (400 MHz, CDCl3): δ = 6.96 (t, J = 8.6 Hz, 4 H), 7.1–7.2
(m, 6 H), 7.34 (t, J = 7.5 Hz, 1 H), 7.5 (m, 4 H).
13C NMR (151 MHz, CDCl3): δ = 124.1 (ArCH), 125.0 (ArCH),
127.5 (ArCH), 128.7 (ArCH), 130.1 (ArCH), 130.6 (Cq), 131.8
(Cq), 139.7 (Cq), 140.9 (Cq).
13C NMR (100 MHz, CDCl3): δ = 88.2 (2 CCAr), 92.1 (2 CCAr),
114.3 (2JC-F = 21 Hz, 2 ArCH), 115.6 (2JC-F = 22 Hz, 4 ArCH),
119.05 (4JC-F = 3 Hz, 2 ArCH], 123.3 (2 ArCq), 127.4 (ArCH),
131.3 (ArCq), 132.1 (2 ArCH), 133.2 (3JC-F = 8 Hz, 2 ArCH), 135.0
(2 ArCH), 145.0 (ArCq), 161.3 (1JC-F = 245 Hz, 2 ArCF), 163.8
(1JC-F = 248 Hz, ArCF).
HRMS: m/z calcd for C28H18: 354.141; found: 354.142.
2-Fluoro-4,10-bis(4-fluorophenyl)pyrene (5b)
Fluorinated biphenyl 4b (122 mg, 0.3 mmol) and PtCl2 (8 mg, 0.03
mmol) were placed in a nitrogen flask, and degassed and purged
with argon. Anhyd toluene (1.5 mL) was added and the mixture was
heated at reflux for 20 h. The reaction mixture was filtered through
Celite® and the dark brown solid was purified by preparative HPLC
(MeCN–H2O, 80:20; 3 mL/min; 220 nm) and then sublimation
HRMS: m/z calcd for C28H15F3: 408.113; found: 408.112.
Synthesis 2012, 44, 1405–1409
© Georg Thieme Verlag Stuttgart · New York

PAPER
Synthesis and Cyclization of 2,6-Bis(phenylethynyl)biphenyls
1409
[UHV (10–6 mbar) heating with a heat gun]. The NMR spectra were
not analyzed because of unidentified side products.
2011, 133, 10716. (f) Lee, Y. O.; Pradhan, T.; No, K.; Kim,
J. S. Tetrahedron 2012, 68, 1704.
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(b) Goldfinger, M. B.; Crawford, K. B.; Swager, T. M.
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HRMS: m/z calcd for C28H15F3: 408.112; found: 408.112 (the frag-
mentation pattern of the mass spectrum unambiguously supports the
assignment of 5b).
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Acknowledgment
We thank Volker Lutz for discussions and help with the initial pre-
parative work. We are grateful to Anika Bernhard for HPLC puri-
fications. Dr. Heike Hausmann is gratefully acknowledged for help
with the NMR analysis. M.M.M. also thanks Dr. Dennis Gerbig for
technical help with the computations.
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© Georg Thieme Verlag Stuttgart · New York
Synthesis 2012, 44, 1405–1409