Synthesis and redox behavior of novel 9,10-diphenylphenanthrenes

Article abstract of DOI:10.1002/hlca.201000238

The synthesis and electrochemical investigations of 9,10- diphenylphenanthrene 2a and its derivatives 2b-2e are reported. The cyclic voltammetry of derivatives 2a-2c and 2e in different solvent/Bu ₄NPF₆ electrolyte systems reveals th

Full text of DOI:10.1002/hlca.201000238

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Helvetica Chimica Acta – Vol. 93 (2010)
Synthesis and Redox Behavior of Novel 9,10-Diphenylphenanthrenes
by Alina Schreivogela), Monika Siegerb), Angelika Baroa), and Sabine Laschat*a)
a) Institut fꢀr Organische Chemie, Universitꢁt Stuttgart, Pfaffenwaldring 55, D-70569 Stuttgart
(phone: þ 49-711-68564565; fax: þ 49-711-68564285; e-mail: sabine.laschat@oc.uni-stuttgart.de)
b) Institut fꢀr Anorganische Chemie, Universitꢁt Stuttgart, Pfaffenwaldring 55, D-70569 Stuttgart
Dedicated to Professor Bernd Giese on the occasion of his 70th birthday
The synthesis and electrochemical investigations of 9,10-diphenylphenanthrene 2a and its
derivatives 2b – 2e are reported. The cyclic voltammetry of derivatives 2a – 2c and 2e in different
solvent/Bu4NPF6 electrolyte systems reveals that the redox properties are dependent on solvent,
temperature, and sweep rate. The oxidation of 9,10-diphenylphenanthrene 2a occurred as an irreversible
process, while two fully reversible oxidation waves were observed for dimethoxy derivative 2c. The room-
temperature oxidation of brominated compound 2b is reversible, whereas AcO-substituted phenan-
threne 2e displayed a reversible oxidation peak only at low temperature. Furthermore, the electronic
nature of the substituent affects the oxidation potentials. In the CH2Cl2-based electrolyte system, the first
oxidation potentials increase in the order 2c < 2e < 2b.
1. Introduction. – Both low- and high-molecular-weight organic compounds which
are suitable for electronic and optoelectronic devices have been intensively inves-
tigated over the last decade1). Phenanthrene derivatives are highly attractive building
blocks for such purposes, because, due to its high resonance energy and high energy
gap, phenanthrene is one of the most stable fused aromatics [2]. In particular,
phenanthrenes are less prone to photochemical degradation in the presence of air as
compared to other aromatics such as pentacenes, which is important for organic field-
effect transistors (OFETs) [3]. As recently shown by Geng and co-workers, and Mꢀllen
and co-workers, phenanthrene oligomers are useful stable p-type semiconductors for
OFETs [4] and 9,10-diarylphenanthrenes can be converted into photoluminescent
blue-emitting polyphenanthrenes [5]2). Despite these promising features, systematic
studies of the redox behavior of diarylphenanthrenes are still lacking, whereas related
systems such as dibenzochrysenes and stilbenoprismands have been investigated by
electrochemical methods [7]. Octamethoxydibenzochrysene, for example, which was
accessible by an oxidative cyclodehydrogenation of the corresponding tetraphenyl-
ethene precursor underwent reversible electrochemical oxidation to a stable cation-
radical salt [7a]. A similar behavior was described for electron-rich stilbenoprismands
[7b]. Previously we reported on tetraphenylethene-derived columnar liquid crystals
bearing gallic acid derivatives and their oxidative photocyclization to the correspond-
ing 9,10-diphenylphenanthrenes [8]. Although the two compound classes differ only by
1
)
)
For reviews, see [1].
For other work on phenanthrene polymers, see [6].
2
ꢂ 2010 Verlag Helvetica Chimica Acta AG, Zꢀrich

Helvetica Chimica Acta – Vol. 93 (2010)
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an additional CꢀC bond, they behaved very differently with regard to their liquid-
crystalline properties3). Moreover, these tetraphenylethenes showed a complex redox
behavior. To gain some insight into the electrochemistry, simple tetraphenylethenes 1
were prepared and studied by cyclic voltammetry and IR spectroelectrochemistry [10],
which reveal not only solvent effects on the comproportionation constants and the two-
electron nature of the composite wave, depending on the substituent, but also two
differently charged and thus structurally different aryl groups at each of the central C-
atoms were identified.
From the observed differences in mesomorphic properties of tetraphenylethenes
and their 9,10-diphenylphenanthrene derivatives, we anticipated that the latter should
also differ in their redox behavior. Therefore, the 9,10-diphenylphenanthrenes 2 with
different substituents (Scheme 1) were prepared and investigated by cyclic voltamme-
try. The results of this study are reported below.
Scheme 1
2. Results and Discussion. – 2.1. Synthesis of Phenanthrenes 2. 9,10-Diphenylphe-
nanthrene (2a) was available in 75% yield by cyclization of 1,1,2,2-tetraphenylethane-
1,2-diol (¼ benzopinacol) with triflic acid (¼trifluoromethanesulfonic acid) in benzene
at room temperature according to the method of Olah et al. [11]. The synthesis of the
differently substituted 9,10-diphenylphenanthrenes 2b – 2e is depicted in Scheme 2.
According to a procedure of Gatzke et al. [12], tetrakis(bromophenyl)ethene 1b [10]
was treated with chromyl chloride (CrO2Cl2) at 08 in CCl4 for 24 h to give the
corresponding phenanthrene 2b in 20% yield4). It should be noted that, under these
conditions, tetraphenylethene 1a reacted to 2a (10%), together with the C¼C cleavage
product benzophenone. This cleavage is not completely unexpected and was already
observed by Lansbury and co-workers [13]. All tetraphenylethenes with MeO, AcO,
BnO, PhO, NHCOCF3 substituents, however, yielded only the respective cleavage
products.
3
)
)
For reviews on columnar liquid crystals, see [9].
4
Low yield due to repeated flash chromatography to separate the poorly soluble product 2b from the
product mixture of 1b, 2b, and 4,4’-dibromobenzophenone, whose ratio strongly depended on the
reaction time.

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Scheme 2. Preparation of Substituted Phenanthrene Derivatives 2b – 2e
Irradiation of tetrakis(methoxyphenyl)ethene 1c [8][10] for 18 h with a 700-W Hg
lamp in toluene at room temperature in the presence of I2 according to a method by
Mallory and Mallory [14] afforded 3,6-dimethoxy-9,10-bis(4-methoxyphenyl)phenan-
threne (2c) in 40% yield. Subsequent demethylation with BBr3 in CH2Cl2 at ꢀ 788 [15]
and workup with MeOH yielded 9,10-bis(4-hydroxyphenyl)phenanthrene-3,6-diol
(2d), which was acetylated with Ac2O in pyridine at room temperature for 18 h to
give the tetraacetate 2e in 70% yield.
2.2. Electrochemistry. The redox properties of 9,10-diphenylphenanthrene deriv-
atives 2a – 2c and 2e were studied by cyclic voltammetry and are summarized in the
Table. As first oxidation and reduction potentials differ by more than 3 V, in all cases,
monitoring of anodic and cathodic processes required different solvents. The oxidations
of 9,10-diphenylphenanthrene (2a) in the CH2Cl2/0.2m Bu4NPF6 electrolyte system at
different temperatures are shown in Fig. 1,a.
At room temperature, an irreversible oxidation wave in the range of 0.6 – 1.6 V was
.
observed, which corresponds to the oxidation of 2a to 2a þ 5). The composite cation is
mainly reduced on the return scan, but an additional electrochemically active species at
0.95 V was observed (Fig. 1,a, thin line). Upon decreasing the temperature to 193 K,
.
this cathodic peak disappeared, and only the oxidation of 2a to 2a þ with an anodic
oxidation at 1.40 V and a return wave at 1.18 V was detected. At both room
5
)
For radical cation generation by using oxidants, see [16].

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Table. Electrochemical Data of Diphenylphenanthrenes 2a – 2c and 2e at 293 K at v ¼ 0.1 V sꢀ1
Employing Bu4NPF6 (0.2m) as Supporting Electrolyte
0=þ
1=2
þ=2þ
1=2
0=ꢀ
1=2
ꢀ=2ꢀ
1=2
Entry
Solvent
E
[V]
E
[V]
DE1/2 [V]
E
[V]
E
[V]
Kcomp
1
2
3
4
5
6
7
8
2a
2a
2b
2c
MeCN
DMF
1.19a)b)
–
1.33
0.68
0.74
–
–
–
–
–
–
0.32
0.19
0.12
–
ꢀ 2.74
ꢀ 2.92
–
–
–
ꢀ 2.89
ꢀ 3.24c)
CH2Cl2
CH2Cl2
MeCN
CH2Cl2
CH2Cl2
CH2Cl2
–
–
–
–
–
–
–
–
–
–
–
–
0.997
0.93
0.53
–
260000
1700
112
2c
1cd)
2e
0.41
1.19e)
0.93e)
–
935
1ed)
1.05c)
0.12
a) At sweep rate 1.2 V sꢀ1 b) Quasi-reversible. c) At 233 K. d) Data from [10]. e) At 193 K.
.
Fig. 1. Cyclic voltammograms of 9,10-diphenylphenanthrene 2a in a) CH2Cl2/0.2m Bu4NPF6 at v ¼
100 mV sꢀ1; b) in MeCN/0.2m Bu4NPF6 at 293 K
temperature and 193 K, the electron transport is inhibited, as suggested by a DE value
of 226 mV, indicating an irreversible oxidation.

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In contrast, in MeCN/0.2m Bu4NPF6 at room temperature, the oxidation of 2a
occurred as a major irreversible process with an anodic potential at 1.23 V vs. ferrocene
(Fig. 1,b, bold line). The voltammetric responses strongly depended on the sweep rate.
At higher sweep rate, the major wave becomes quasi-reversible with a half-wave
potential, E1/2 of 1.19 V, and a smaller follow-up cathodic peak at 0.88 Vappeared. This
second process was clearly observed even at a scan rate of 1200 mV sꢀ1 (Fig. 1,b, thin
line). The DE value of 120 mV in MeCN indicated a less inhibited electron transport
than in CH2Cl2.
The cyclic voltammogram of 3,6-dibromo-9,10-bis(4-bromophenyl)phenanthrene
(2b) in CH2Cl2/0.2m Bu4NPF6 at room temperature indicated a reversible oxidation to
.
the radical cation 2b þ (Fig. 2).
Fig. 2. Cyclic voltammogram of 3,6-dibromo-9,10-bis(4-bromophenyl)phenanthrene (2b) in CH2Cl2/
0.2m Bu4NPF6 at 293 K at v ¼ 100 mV sꢀ1
The voltammetric responses of 2b were found to be influenced by sweep rate and
temperature. As the sweep rate was increased (> 500 mV sꢀ1), the oxidation wave
broadens significantly, probably caused by low electron-transfer kinetics. At low
temperature (193 K), the oxidation became even more kinetically hindered, leading to
irreversible oxidation waves, and chemical follow-up reactions could be monitored.
When ferrocene was added to the electrolyte solution to determine the E1/2 value of 2b,
an overall change of the CV curve was observed, resulting only in a single very broad
irreversible wave, instead of the expected oxidation wave and a reversible wave of the
ferrocene/ferrocenium ion pair both at room temperature and 193 K. The replacement
of ferrocene by decamethylferrocene or cobaltocene gave similar results. Obviously,
the metallocene complex was not as inert as anticipated but interacts with the 3,6-
dibromo-9,10-bis(4-bromophenyl)phenanthrene intermediate. However, thianthrene
as the internal reference was inert, and the E1/2 value of 2b could be determined as
1.33 V.
3,6-Dimethoxy-9,10-bis(4-methoxyphenyl)phenanthrene (2c) was oxidized in the
CH2Cl2/0.2m Bu4NPF6 electrolyte system at room temperature in two well-behaved
reversible waves at E1/2 ¼ 0.68 V and E1/2 ¼ 0.997 V vs. ferrocene as internal standard
(Fig. 3,a), which did not change at 193 K. A comproportionation constant Kcomp
¼

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2.6 · 105 was calculated6). The EPR spectrum (Fig. 3,b) clearly demonstrated the
.
formation of a radical cation 2c þ during the oxidation. The signal is isotropic with a g-
factor of 2.003 and 15-G signal width, being typical for an organic radical. The
hyperfine signal splitting was not simulated, thus coupling constants could not be
determined.
Fig. 3. a) Cyclic voltammogram of 3,6-dimethoxy-9,10-bis(4-methoxyphenyl)phenanthrene (2c) in
CH2Cl2/0.2m Bu4NPF6 at 293 K at v ¼ 100 mV sꢀ1; b) room temperature EPR spectrum for the radical
.
cation 2c þ, taken from in situ electrolytic oxidation of 2c in CH2Cl2/0.2m Bu4NPF6
6
)
DE1/2 Values provide a quantitative measure of the comproportionation constant, which relates the
thermodynamic stabilities of the monocations to those of the neutrals and the fully oxidized forms
according to the equation:
Kcomp ¼ [Aþ]2/[A][A2þ] ¼ exp{(nF/RT) · DE1/2
}
F denotes the Faraday constant, R the gas constant, and T the temperature.

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In the more polar MeCN/0.2m Bu4NPF6 system, a temperature dependence was
observed. At room temperature, the oxidation waves broadened due to slower
electron-transfer kinetics, and only the first wave was fully reversible. Upon cooling to
253 K, however, two completely reversible oxidations at E0=þ ¼ 0.74 V and Eþ=2þ
¼
1=2
1=2
0.93 V were detected, and Kcomp was calculated as 1.7 · 103. Consequently, in the
CH2Cl2-based electrolyte system, the monocation 2cþ is thermodynamically more
stable than in MeCN.
The room-temperature oxidation of the tetraacetate 2e in CH2Cl2/0.2m Bu4NPF6
displayed several irreversible peaks together with two corresponding irreversible
reduction waves (Fig. 4,a). Upon cooling to 193 K, the cathodic reductions disap-
peared, and the first oxidation wave A became fully reversible with a peak current ratio
ipa/ipc ꢁ 1 and DE ¼ 90 mV (Fig. 4,b). The E1/2 value of this couple was determined as
Fig. 4. Cyclic voltammogram of 4-{3,6-bis(acetyloxy)-10-[4’-(acetyloxy)phenyl]phenanthren-9-yl}phenyl
acetate (2e) in CH2Cl2/0.2m Bu4NPF6 at v ¼ 100 mV sꢀ1 at 293 K (a) and 193 K (b)

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1.19 V. A second irreversible wave appeared at a peak potential of 1.47 V, where a
second and third oxidation (B þ C) obviously coincide as indicated by the peak height.
The reductions of the phenanthrene derivatives 2 were outside the potential
window of the CH2Cl2/Bu4NPF6 electrolyte but were monitored in DMF and THF
owing to their cathodic discharge limits.
The room-temperature reduction of 9,10-diphenylphenanthrene (2a) in DMF/0.2m
Bu4NPF6 is represented by a reversible two-electron wave at E1/2 ¼ ꢀ2.89 V vs.
ferroceneþ/0 and by a second irreversible one-electron wave at ꢀ 3.08 V vs. ferrocene.
Both reductions were accompanied by two anodic follow-up waves at ꢀ 1.51 and 0.39 V
(Fig. 5, thin line). If the scanning is terminated prior to the start of the second reduction
event, the anodic wave at 0.39 V is not visible, thus being clearly attributed to the
second reduction. Upon cooling to 233 K, the second reduction process is observed as a
partially reversible couple and is cathodically shifted to E1/2 ¼ ꢀ3.24 V (Fig. 5, bold
line). The second reduction product was completely oxidized on the return scan, and
the anodic follow-up peak at 0.39 V, which is coupled to the second reduction peak,
could not be detected anymore. The distance between the two reduction peaks
increased from 191 mV at room temperature to 406 mV at 233 K, indicating an
inhibited electron transfer, which is caused by rearrangement in the structure during
the absorption of electrons. This observation may also be taken as evidence for the
enhanced stability of the anion 2aꢀ at lower temperatures.
Fig. 5. Cyclic voltammogram of 9,10-diphenylphenanthrene (2a) in DMF/0.2m Bu4NPF6 at v ¼
100 mV sꢀ1
The reduction of 2a in MeCN at room temperature vs. ferrocene as internal
standard gave qualitatively similar results with the first reversible wave at E1/2
¼
ꢀ2.74 Valong with its anodic follow-up wave at ꢀ 1.1 Vand the second quasireversible
reduction wave at E1/2 ¼ ꢀ2.92 V. At 253 K, the behavior of 2a resembled that at room
temperature (data not shown).
The CV of the reduction of Br-substituted diphenylphenanthrene 2b at room
temperature in DMF/0.2m Bu4NPF6 displayed two irreversible reduction peaks close to

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the solvent-discharge limit. Again, the addition of an equimolar amount of ferrocene
caused a complete change of the CV curve, which was found to depend on the sweep
direction. The two reduction peaks at ꢀ 2.82 and ꢀ 3.10 V, respectively, and the broad
oxidation wave of ferrocene were observed upon cathodic-to-anodic scan direction.
Upon reversal of the initial sweep direction, however, only the broad ferrocene wave
was visible (Fig. 6), being typical for electrolyte solutions of metal complexes with high
molecular mass such as polyvinyl ferrocene [17] or poly-Fe-complexes with tetra-
(ethinylphenyl)ethene subunits, as was recently recorded by Akita and co-workers [18].
A chemical reaction with the cyclopentadienyl ring seems to be rather unlikely because,
upon new reversal of the sweep direction, both irreversible reduction waves
reappeared.
Fig. 6. Cyclic voltammogram of 3,6-dibromo-9,10-bis(4-bromophenyl)phenanthrene (2b) with an
equimolar amount of ferrocene in DMF/0.2m Bu4NPF6 at 293 K, v ¼ 100 mV sꢀ1
In contrast, in the DMF/0.2m Bu4NPF6 electrolyte system any redox processes of
bis(methoxyphenyl)phenanthrene 2c were not detected. Acetoxy derivative 2e
displayed irreversible reductions in MeCN/0.2m Bu4NPF6 at ꢀ 2.99 V and in DMF/
0.2m Bu4NPF6 at ꢀ 2.87 V.
The data listed in the Table reveal the influence of the substituent on oxidation
potentials. In the system CH2Cl2/Bu4NPF6, compound 2e with electron-withdrawing
AcO group showed an oxidation potential at 1.19 V (Entry 7), while the Br substituent
has an even larger effect (Entry 3). The electron-donating MeO groups, however,
shifted the oxidation potential to a value by 0.65 V lower as compared to the Br
substituent (Entry 4). These data reflect an oxidation tendency of the studied
phenanthrene derivatives in the order 2c > 2e > 2b.
Whereas tetraphenylethenes 1 generally displayed two consecutive one-electron
oxidations, which are combined with a structure change [10], the appearance of a one-

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electron oxidation wave is typical of phenanthrene derivatives 2 in our study. The
cyclization of tetraphenylethenes 1 to phenanthrenes 2 caused an extension and
flattening of the aromatic system, and the second oxidation is inhibited with the MeO
compound 2c as exception. The increased electron density in the aromatic system by
the MeO groups apparently enabled a second oxidation. The direct comparison of
electrochemical data of tetraphenylethene 1c listed in the Table with its phenanthrene
counterpart 2c shows that, in the CH2Cl2/0.2 m Bu4NPF6 electrolyte system, the E1/2
values of 2c are shifted to values by 0.27 and 0.46 V higher as compared to 1c (Entries 4
and 6). The values of DE1/2 [10]6) vary between 120 mV for 1c and 320 mV for 2c, and
thus reflect the relative stabilities of the mono-oxidized forms (2c > 1c). As DE1/2
values provide a measure of the comproportionation constant Kcomp, the values 1c vs. 2c
clearly differ (112 and 260,000, Entries 4 and 6). Consequently, 2c requires more energy
to be oxidized than the corresponding tetraphenylethene 1c.
A similar trend was observed for the low-temperature oxidation of derivatives 1e
and 2e bearing AcO groups in the CH2Cl2-based electrolyte system, where the higher
E1/2 value of 2e indicates a higher relative stability as compared to 1e (Entries 7 and 8).
In contrast to tetraphenylethene derivative 1e, however, the second oxidation wave of
phenanthrene 2e is irreversible.
The need of different solvents in electrochemical experiments does not allow a
direct comparison of the data determined for tetraphenylethenes 1a and 1b, and
phenanthrenes 2a and 2b. The electrochemistry of both 9,10-diphenylphenanthrene
(2a) and tetraphenylethene (1a) was influenced by the solvent, leading, in the case of
2a, to an irreversible oxidation in the CH2Cl2-based electrolyte system and a partly
reversible oxidation wave in MeCN. In contrast, 1a showed a reversible first oxidation
in CH2Cl2 but irreversible processes in MeCN [19]. The Br derivatives 1b and 2b differ
in their behavior towards ferrocene. Under the experimental conditions, analyte 2b
interacted with ferrocene resulting in a complete change of the CV curves. This
behavior was not observed for 1b.
3. Conclusions. – Some novel 9,10-diphenylphenanthrene derivatives 2 were pre-
pared, and their electrochemical behavior has been investigated by cyclic voltammetry.
The first oxidation potentials of phenanthrenes 2 in the range of 0.68 – 1.33 Vare higher
than those of the corresponding tetraphenylethenes 1 (0.41 – 1.08 V). This result
indicates not only a more difficult oxidation of the conjugated diphenylphenanthrenes
but also an increased relative stability of the mono-oxidized forms as compared to the
electron-rich tetraphenylethene analogues, which have a decreased conjugation.
The generous financial support by the Deutsche Forschungsgemeinschaft, the Fonds der Chemischen
Industrie, and the Ministerium fꢀr Wissenschaft, Forschung und Kunst des Landes Baden-Wꢀrttemberg is
gratefully acknowledged. We would like to thank Lisa Steudle for the EPR spectroscopy.
Experimental Part
General. Solvents CH2Cl2 (Fluka) and MeCN (Roth) for electrochemical work were distilled over
CaH2 prior to use. DMF (Fluka) was stored over molecular sieves (4 ꢃ) and purged with Ar prior to use.
Flash chromatography (CC): silica gel 60 (mesh 40 – 63 mm; Fluka) with hexanes (b.p. 30 – 758). IR
Spectra: Bruker Vector 22 FT-IR spectrometer with ATR technique; in cmꢀ1. 1H- and 13C-NMR spectra:
Bruker ARX 500 instrument; at 500 and 125 MHz, resp., d in ppm, J in Hz, Me4Si as internal standard; 13
C

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Helvetica Chimica Acta – Vol. 93 (2010)
multiplicities assigned by DEPTexperiments. MS and ESI-MS: Varian MAT 711, Finnigan MAT 95, and
Bruker micrOTOF_Q; in m/z (rel. %). EPR Spectra in the X band: Bruker System ESP 300; a two-
electrode capillary served to generate intermediates for X band EPR studies [20].
Electrochemical Experiments. All electrochemical experiments were performed in a home-built
cylindrical vacuum-tight one-compartment cell. Typical three-electrodes arrangement was used. A spiral-
shaped Pt wire and a Ag wire as the counter and reference electrodes, resp., were sealed directly into
opposite sides of the glass wall, while the respective working electrode (Pt, 1.1 mm, polished with 0.25-mm
diamond paste (Buehler– Wirtz) before each experiment) was introduced via a Teflon screw cap with a
suitable fitting. The cell was attached to a conventional Schlenk line via two side arms equipped with
Teflon screw valves and allowed experiments to be performed under Ar with ca. 2.5 ml of analyte soln. In
all experiments, Bu4NPF6 (0.2m in the respective solvent) was used as the supporting electrolyte. In all
experiments, ferrocene or thianthrene were used as the internal standard, whereas the E1/2 value of
thianthrene was also calibrated vs. ferrocene. For each compound, cyclic voltammograms were run at
sweep rates from 0.05 to 1.0 V sꢀ1 at r.t. and low temp. (193 K in CH2Cl2, 233 K in MeCN or DMF). Scans
recorded at high sweep rates suffer, however, from distortions owing to Ohmic drop and somewhat slow
electron-transfer kinetics, especially at low temp.
9,10-Diphenylphenanthrene (2a). Trifluoromethanesulfonic acid (¼triflic acid; 1.72 ml, 8.60 g,
57.0 mmol) was added to a soln. of 1,1,2,2-tetraphenylethane-1,2-diol (¼ benzopinacol; 1a; 0.73 g,
2.00 mmol) in benzene (4 ml) at 108, and the mixture was stirred at r.t. for 20 h. The mixture was poured
onto ice, and the aq. layer was extracted with CH2Cl2 (3 ꢂ 10 ml). The combined org. layers were washed
with H2O (3 ꢂ 20 ml) and dried (MgSO4). After removal of the solvent, the solid residue was
recrystallized from CH2Cl2 to give 2a (490 mg, 1.50 mmol, 75%). Colorless crystals. 1H-NMR (500 MHz,
CDCl3): 7.13 – 7.18 (m, 4 arom. H, Ph); 7.19 – 7.23 (m, 2 arom. H, Ph); 7.24 – 7.27 (m, 4 arom. H, Ph); 7.49
(ddd, J ¼ 8.5, 6.8, 1.2, 2 arom. H); 7.56 (ddd, J ¼ 8.3, 1.4, 0.5, 2 arom. H); 7.67 (ddd, J ¼ 8.3, 6.8, 1.4, 2
arom. H); 8.82 (ddd, J ¼ 8.5, 1.2, 0.5, 2 arom. H). 13C-NMR (125 MHz, CDCl3): 122.5, 126.4, 126.6, 127.8
(C(1), C(2), C(3), C(4), C(5), C(6), C(7), C(8)); 126.5 (C(4’)); 127.6, 131.0 (C(2’), C(3’), C(5’), C(6’));
129.9, 131.9, 137.2, 139.6 (C(1a), C(4a), C(5a), C(8a), C(9), C(10), C(1’)). EI-MS: 330 (100, Mþ), 313
(10), 289 (5), 252 (20), 156 (10).
3,6-Dibromo-9,10-bis(4-bromophenyl)phenanthrene (2b). At 08, a soln. of CrO2Cl2 (0.60 ml, 0.11 g,
0.70 mmol) in CCl4 (4 ml) was added dropwise to a soln. of tetrakis(4-bromophenyl)ethene (1b; 0.20 g,
0.31 mmol) in CCl4 (20 ml). The mixture was stirred at 08 for 1 h and at r.t. for 24 h. The precipitate was
filtered off, and the filtrate was washed with a Na2S2O5 soln. (3 ꢂ 20 ml), dried (MgSO4), and
concentrated i.v. The solid residue was purified twice by CC (SiO2; hexanes/CH2Cl2 20 :1) to give 2b
(38.0 mg, 0.06 mmol, 20%). Yellow crystals. ATR-FT-IR: 3301, 3078, 3026w, 2923m, 2854m, 1839m,
1738m, 1592vs, 1504vs, 1390vs, 1212m, 1070vs, 1010vs, 919, 794vs. 1H-NMR (500 MHz, CDCl3): 6.97 (d,
J ¼ 8.6, 2 HꢀC(2’), 2 HꢀC(6’)); 7.35 (d, J ¼ 8.6, 2 HꢀC(3’), 2 HꢀC(5’)); 7.42 (d, J ¼ 8.4, HꢀC(1),
HꢀC(8)); 7.60 (dd, J ¼ 8.4, 1.9, HꢀC(2), HꢀC(7)); 8.82 (d, J ¼ 1.9, HꢀC(4), HꢀC(5)). 13C-NMR
(125 MHz, CDCl3): 121.4, 121.7 (C(3), C(6), C(4’)); 125.5 (C(1), C(8)); 129.3 (C(4), C(5)); 130.4, 130.5
(C(1a), C(4a), C(5a), C(8a)), 130.7 (C(2), C(7)); 131.3, 132.3 (C(2’), C(3’), C(5’), C(6’)); 136.1 (C(1’));
137.3 (C(9), C(10)). EI-MS: 645 (100, Mþ), 565 (5), 485 (10), 406 (10), 326 (30), 202 (10), 163 (75), 80
(10). HR-EI-MS: 641.7830 (C26H14Brþ4 ; calc. 641.7829).
3,6-Dimethoxy-9,10-bis(4-methoxyphenyl)phenanthrene (2c). A soln. of tetrakis(4-methoxyphenyl)-
ethene (1c; 0.52 g, 1.20 mmol) and I2 (0.30 g, 1.20 mmol) in freshly distilled toluene (750 ml) was
irradiated for 18 h with vigorous stirring in a reactor equipped with 700-W Hg lamp at r.t. The solvent was
removed i.v., and the residue was washed with a dil. Na2S2O3 soln. and dried (MgSO4). After removal of
the solvent, the residue was purified by CC (SiO2; hexanes/AcOEt 10 :1) and recrystallized from AcOEt
1
to give 2c (0.12 g, 0.30 mmol, 40%). Colorless crystals. H-NMR (500 MHz, CDCl3): 3.79 (s, 2 MeO);
4.03 (s, 2 MeO); 6.78 (d, J ¼ 8.7, 2 HꢀC(3’), 2 HꢀC(5’)); 7.04 (d, J ¼ 8.7, 2 HꢀC(2’), 2 HꢀC(6’)); 7.12
(dd, J ¼ 9.1, 2.6, HꢀC(2), HꢀC(7)); 7.50 (d, J ¼ 9.1, HꢀC(1), HꢀC(8)); 8.06 (d, J ¼ 2.6, HꢀC(4),
HꢀC(5)). 13C-NMR (125 MHz, CDCl3): 55.1, 55.6 (MeO); 104.4 (C(4), C(5)); 113.0 (C(3’), C(5’));
116.1 (C(2), C(7)); 127.4 (C(1a), C(8a)); 129.5 (C(1’)); 130.7 (C(1), C(8) C(9), C(10)); 132.2 (C(2’),
C(6’)); 132.3 (C(4a), C(5a)); 134.7 (C(3), C(6)); 157.9 (C(4’)). ESI-MS: 473.18 ([M þ Na]þ).

Helvetica Chimica Acta – Vol. 93 (2010)
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9,10-Bis(4-hydroxyphenyl)phenanthrene-3,6-diol (2d). In a Schlenk flask under inert gas atmos-
phere, a soln. of BBr3 (1m in CH2Cl2, 0.60 ml) was added slowly to a soln. of 2c (0.06 g, 0.14 mmol) in abs.
CH2Cl2 (10 ml) at ꢀ 608, and the mixture was stirred at ꢀ 608 for 1 h and at r.t. for 20 h. After cooling to
08, H2O was added dropwise, and the precipitate was filtered off and recrystallized from EtOH/H2O 1:1
1
to give 2d (55.0 mg, 0.14 mmol, 99%). Fine light-red crystals. H-NMR (500 MHz, (D6)acetone): 5.50
(br. s, 4 OH); 6.61 (d, J ¼ 8.2, 2 HꢀC(3’), 2 HꢀC(5’)); 6.83 (d, J ¼ 8.2, 2 HꢀC(2’), 2 HꢀC(6’)); 6.94
(dd, J ¼ 8.8, 2.5, HꢀC(2), HꢀC(7)); 7.26 (d, J ¼ 8.8, HꢀC(1), HꢀC(8)); 7.93 (d, J ¼ 2.5, HꢀC(4),
HꢀC(5)). 13C-NMR (125 MHz, (D6)acetone): 107.1 (C(2), C(7)); 115.3 (C(3’), C(5’)); 117.7 (C(4),
C(5)); 127.7, 130.1, 131.8 (C(1a), C(4a), C(5a), C(8a), C(1’)); 132.3 (C(1), C(8)); 133.0 (C(2’), C(6’));
135.2 (C(9), C(10)); 156.46, 156.51 (C(3), C(6), C(4’)).
[3,6-Bis(acetyloxy)phenanthrene-9,10-diyl]dibenzene-4,1-diyl Diacetate (2e). Ac2O (0.50 ml,
180 mg, 1.73 mmol) was added to a soln. of 2d (55.0 mg, 0.14 mmol) in pyridine (10 ml), and the
mixture was stirred at r.t. for 15 h. The solvent was removed i.v., the residue was dissolved in CH2Cl2
(10 ml), successsively washed with dil. HCl (3 ꢂ 10 ml) and a sat. soln. of NaHCO3 (3 ꢂ 10 ml), and dried
(MgSO4). The solvent was removed i.v., and the residue was purified by CC (SiO2; hexanes/AcOEt 3 :2)
to give 2e (55.0 mg, 0.10 mmol, 70%). Colorless crystals. ATR-FT-IR: 3063, 3031w, 2937s, 2323w, 1753vs,
1609m, 1526, 1499, 1428, 1366vs, 1180vs, 1010s, 908s. 1H-NMR (500 MHz, CDCl3): 2.29 (s, 2 MeꢀC(4’));
2.40 (s, MeꢀC(3), MeꢀC(6)); 7.00 (d, J ¼ 8.8, 2 HꢀC(3’), 2 HꢀC(5’)); 7.12 (d, J ¼ 8.8, 2 HꢀC(2’),
2 HꢀC(6’)); 7.25 (dd, J ¼ 8.9, 2.6, HꢀC(2), HꢀC(7)); 7.61 (d, J ¼ 8.9, HꢀC(1), HꢀC(8)); 8.36 (d, J ¼
2.6, HꢀC(4), HꢀC(5)). 13C-NMR (125 MHz, CDCl3): 21.2, 21.3 (Me); 114.9 (C(4), C(5)); 120.9 (C(3’),
C(5’)); 121.6 (C(2), C(7)); 129.5 (C(1), C(8)); 129.9 (C(1a), C(8a)); 130.7 (C(4a), C(5a)); 131.9 (C(2’),
C(6’)); 136.1, 136.5 (C(1’), C(10), C(9)); 149.3, 149.4 (C(3), C(6), C(4’)); 169.3, 169.7 (C¼O). EI-MS:
562 (100, Mþ), 520 (50), 478 (60), 436 (40), 394 (50), 365 (5), 321 (5), 254 (1), 197 (1), 127 (1), 99 (1), 43
(10). HR-ESI-MS: 585.1518 ([M þ Na]þ, C34H26NaO8þ ; calc. 585.1525).
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Received June 21, 2010