Phenyl groups versus tert-butyl groups as solubilizing substituents for some [5]phenacenes and [7]phenacenes

Article abstract of DOI:10.1021/jo3020819

In recent years, we have used the photocyclizations of diarylethylenes to synthesize a number of [n]phenacenes in the hope that they might be useful as the bridging groups for electron transfer processes in donor-bridge-acceptor molecules. Because [n]phenacenes with n > 5 are very insoluble, their synthesis and characterization has required the attachment of solubilizing substituents such as tert-butyl. The studies of Pascal and co-workers of some large polynuclear aromatic compounds having multiple phenyl substituents prompted us to explore the use of phenyls as alternative solubilizing groups for [n]phenacenes. Although phenyl groups turned out to provide significantly less solubilization than tert-butyl groups in these compounds, we found some interesting structural comparisons of the phenyl-substituted and tert-butyl-substituted [n]phenacenes.

Full text of DOI:10.1021/jo3020819

Article
pubs.acs.org/joc
Phenyl Groups versus tert-Butyl Groups as Solubilizing Substituents
for Some [5]Phenacenes and [7]Phenacenes
Frank B. Mallory,*,† Clelia W. Mallory,†,‡ Colleen K. Regan,† Rebecca J. Aspden,† Annie Butler Ricks,†
Joy M. Racowski,† Abigail I. Nash,† Ahmara V. Gibbons,† Patrick J. Carroll,‡ and Joseph M. Bohen†
†Department of Chemistry, Bryn Mawr College, Bryn Mawr, Pennsylvania 19010-2899, United States
‡Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6323, United States
S
* Supporting Information
ABSTRACT: In recent years, we have used the photocyclizations of
diarylethylenes to synthesize a number of [n]phenacenes in the hope that
they might be useful as the bridging groups for electron transfer processes in
donor−bridge−acceptor molecules. Because [n]phenacenes with n > 5 are
very insoluble, their synthesis and characterization has required the attach-
ment of solubilizing substituents such as tert-butyl. The studies of Pascal and
co-workers of some large polynuclear aromatic compounds having multiple
phenyl substituents prompted us to explore the use of phenyls as alternative
solubilizing groups for [n]phenacenes. Although phenyl groups turned out to
provide significantly less solubilization than tert-butyl groups in these compounds, we found some interesting structural
comparisons of the phenyl-substituted and tert-butyl-substituted [n]phenacenes.
INTRODUCTION
■
We have been investigating the family of large polycyclic
aromatic compounds, for which we proposed the name
[n]phenacenes1−3 because they have n fused benzene rings
in an extended zigzag phenanthrene-like structural motif. At the
photocyclization5 for the sequential elongations of their aromatic
outset of our work, the largest known member of this family
was the unsubstituted compound with n = 6, and our objective
backbones. As shown in Scheme 1, the syntheses of 1 and 2 each
was to synthesize a series of [n]phenacenes with larger values of n.
Much of our effort has been spent on finding suitable solubilizing
proceeded through the common intermediate 1-bromo-8-methyl-
3,5-diphenylphenanthrene 3. This compound was synthesized
starting from the diazotization of 3-bromo-4-methylaniline 4
with nitrous acid followed by the addition of aqueous fluoro-
groups that will allow the synthesis and characterization of these
compounds. In our earliest work on [7]phenacene systems,1,2
n-pentyl substituents were employed for this purpose, but after
further exploration we switched to tert-butyl substituents because
they lack reactive benzylic hydrogens and therefore allow a wider
scope of synthetic pathways to be employed in the multistep
boric acid to give the tetrafluoroborate salt, and the subsequent
treatment of the dry salt with potassium acetate in a benzene/
acetonitrile solution under Gomberg−Bachmann6,7 conditions
to give biphenyl 5. Treatment of 5 with N-bromosuccinimide
construction of these compounds. Although we were successful in
gave benzylic bromide 6, which was combined with triphenyl-
phosphine to give phosphonium salt 7. The treatment of 5 with
n-BuLi followed by dimethylformamide gave aldehyde 8, which
was used in a Wittig reaction with phosphonium salt 7 to pre-
pare a mixture of the E and Z isomers (as represented in Scheme
1 by a wavy bond) of stilbene 9. This mixture of isomers was
synthesizing several tert-butyl-substituted [7]phenacene and
[11]phenacene systems,1−3 we felt that the solubilizing capability
of tert-butyl groups might be insufficient for our plans to syn-
thesize a [15]phenacene. Inspired by Pascal’s work4 on the syn-
theses of large polynuclear aromatic molecules having multiple
phenyl substituents, we explored whether phenyl groups might be
irradiated with visible light in a dilute cyclohexane solution con-
more effective than tert-butyl groups in solubilizing [n]phenacenes.
taining a trace of iodine as a catalyst to give exclusively the pure E
isomer for purification and characterization. Subsequent ultra-
violet irradiation of the E isomer in hexanes with iodine as the
oxidant caused a reversible E-to-Z isomerization accompanied by
the oxidative photocyclization of the Z isomer to produce the
RESULTS AND DISCUSSION
■
Syntheses. We report here the syntheses of two previously
unknown compounds, the tetraphenyl-substituted [7]phenacene 1
(11-bromo-4-methyl-1,13,15,17-tetraphenyl[7]phenacene) and the
triphenyl-substituted [5]phenacene 2 (9-bromo-4-methyl-1,11,13-
triphenyl[5]phenacene).
Special Issue: Howard Zimmerman Memorial Issue
All of our syntheses of [n]phenacenes over the years have
employed the stilbene-to-phenanthrene type of oxidative
Received: September 21, 2012
Published: December 3, 2012
© 2012 American Chemical Society
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a
round-bottom flasks containing small stir bars. The weight of
each flask and its contents was recorded. Using a Pasteur pipet,
benzene was added slowly with stirring to each flask at room
temperature until only a small residue of solid remained, after
which the four samples were allowed to stir for several hours.
Additional benzene was then added dropwise very slowly to
each of the stirred flasks until it appeared that the dissolution of
the solids was complete. Each flask was then weighed, and the
weight of benzene used to dissolve each sample was calculated
by subtracting the initial weight of the flask, stir bar, and sample
from the final weight that included the added benzene. The
results of these measurements are given in Table 1. The phenyl
Scheme 1. Syntheses of [n]Phenacenes
Table 1. Solubility Measurements in Benzene Solution
ab
,
sample amt
benzene amt
solubility
entry cpd
mg
mmol
mg
mL
mol/L
ratio 15/2
1a
1b
2a
2b
15
2
4.0
4.0
4.0
4.0
0.0074
0.0067
0.0074
0.0067
93.5
822.9
92.6
0.107
0.939
0.106
0.954
0.069
9.7
9.7
10
0.0071
0.070
15
2
836.6
0.0070
10
a
b
Benzene density = 0.8765 g/mL. Qualitatively, the solubility of 15 is
also significantly larger than that of 2 in both hexanes and diethyl
ether.
a
Reagents and conditions: (a) HONO, aq HBF4; (b) KOAc, benzene,
CH3CN; (c) NBS, CCl4; (d) PPh3; (e) n-BuLi, DMF; (f) 50% aq
NaOH, CH2Cl2; (g) hν, I2, hexanes; (h) n-BuLi, i-Pr2NH; (i) hν, I2,
toluene; (j) hν, I2, benzene.
substituents in 2 proved to be about 10 times less effective
than the tert-butyl substituents in 15 at solubilizing these two
[5]phenacene systems in benzene. As discussed below, we believe
this results from the crystal packing being more compact for 2
than for 15. Qualitatively consistent with this, the melting point of
2 (290−291.5 °C) is higher than that of 15 (246.5−248.0 °C).
The intramolecularly adjacent pairs of phenyl substituents in
2 and tert-butyl substituents in 15 each undergo significant
steric crowding against one another. As a consequence, these two
compounds both exist as racemic mixtures as indicated below.
substituted phenanthrene 3. Treatment of 3 with N-bromo-
succinimide gave the benzylic bromide 10, and the subsequent
reaction of 10 with triphenylphosphine gave the phosphonium
salt 11. The Wittig reaction of 11 with aldehyde 8 gave a
mixture of the E and Z isomers of diarylethylene 12, from
which the pure E isomer was obtained by the method described
above for stilbene 9. The ultraviolet photocyclization of the E
isomer of 12 in benzene with iodine as the oxidant, following
the method described above for the analogous preparation of 3,
produced the triphenyl-substituted [5]phenacene 2. Treatment
of the common intermediate 3 with n-BuLi followed by di-
methylformamide gave aldehyde 13, and a Wittig reaction of 13
with the phosphonium salt 11 gave diarylethylene 14. The
ultraviolet photocyclization of 14 in toluene with iodine as the
oxidant produced the tetraphenyl-substituted [7]phenacene 1.
Solubility Studies. In order to assess the effectiveness of
phenyl substituents relative to tert-butyl substituents for the
solubilization of phenacenes, the room-temperature solubility in
benzene solution of triphenyl[5]phenacene 2 was compared
directly with that of tri-tert-butyl[5]phenacene 15.3
Our X-ray crystallographic determinations of the molecular
structure and the crystal packing for the racemic mixture 2a and
2b are shown in Figure 1. The two phenyl groups that are attached
in the two adjacent bay regions of the nearly planar [5]phenacene
backbone each experience intramolecular steric crowding with the
bay hydrogen on the other side of the same bay. As a consequence,
these two phenyl groups are each twisted by about 60° around the
single CC bonds that connect them to the [5]phenacene backbone,
and they also are sterically forced by this crowding to bend in
opposite directions above and below the approximate plane of the
backbone. In contrast, the third phenyl group in the crystal of 2,
Two 4-mg samples of 2 and two 4-mg samples of 15
were each placed individually in one of four separate 10-mL
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Figure 1. ORTEP representations of the X-ray crystallographic results
for one molecule and also for a six-molecule cluster of the racemic
phenyl-substituted [5]phenacene 2.
which is attached to a nonbay carbon, is twisted out of the ap-
proximate plane of the backbone by only about 3°. This is rem-
iniscent of the structure of biphenyl,8 which in the gas phase
has its two phenyl groups twisted by 44.4° around the bond
linking them together, whereas in the crystal they are nearly
coplanar because of the energetically favorable crystal packing
between adjacent molecules.
Figure 2. RHF/6-31G calculation9 of the structure of a molecule of 16
and a visual representation of the magnitudes of the calculated twist
angles between the phenyl substituents labeled A, B, C, and D and the
[7]phenacene backbone.
For example, in the six-molecule cluster shown in Figure 1,
the horizontal in-plane phenyl substituent on the molecule of 2
that is shown on the left half of the second layer from the top of
the cluster experiences three intermolecular attractions: (a) a
CH/π interaction between one of its ring hydrogens and the π
system of the twisted phenyl ring that is bent slightly down
from the phenacene molecule in the first layer of the cluster;
(b) a CH/π interaction between its π system and an ortho-
hydrogen from the twisted phenyl ring that is bent up from the
molecule in the first layer; and (c) the π/π stacking of the
horizontal in-plane extended π system of the phenyl group of
interest in the second layer with the nearly planar phenacene
backbone of the enantiomeric isomer of the molecule on the
left half of the third layer of the cluster. Another example of a
CH/π interaction involves the downward-pointing phenyl on
the molecule of 2 on the right half of the second layer from the
top and the phenacene backbone of the molecule of 2 in the
fourth layer of the cluster. We suggest that this close packing
accounts for the order-of-magnitude lower solubility reported
in Table 1 for compound 2 relative to compound 15.
Since tert-butyl groups are sterically more demanding than
phenyl groups, they are forced to bend further above and
further below the phenacene backbone in 17 than the analogous
out-of-plane bending of the phenyl groups in 2 as shown in Figure
1. The phenacene backbone in 17 is somewhat more twisted than
the phenacene backbone in 2. The crystal packing pattern for the
four-molecule cluster of 17 shown in Figure 3 is dramatically
Figure 2 shows the results of an RHF/6-31G calculation of
an isolated single molecule of the analogous [7]phenacene system
16 using Gaussian 03, Revision B.05.9 The nonbay phenyl sub-
stituent labeled A is calculated to be twisted by 43°, which suggests
that a similar twist angle is likely for the nonbay phenyl group in
the isolated single molecule of the related [5]phenacene 2. This
supports our conclusion that the twist angle of only 3° shown in
Figure 1 for the analogous phenyl substituent in the crystal of 2
results from crystal packing effects.
We have been hampered in developing an explanation for
the 10-fold greater solubility of the tert-butyl-substituted [5]-
phenacene 15 as compared to that of the phenyl-substituted
[5]phenacene 2 because we have been unable to grow an X-ray
quality crystal of 15. However, we previously determined the
X-ray crystal structure3 of the related tert-butyl-substituted
[7]phenacene 17, which we hoped would provide some insight
into what would have been found from the analogous tert-butyl-
substituted [5]phenacene 15.
Figure 3. ORTEP representations of the previously reported3 X-ray
crystallographic results for the tert-butyl-substituted [7]phenacene 17.
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246/248 (M+). Anal. Calcd for C13H11Br: C, 63.18; H, 4.49. Found: C,
63.26; H, 4.54.
different from the crystal packing pattern shown for the six-
molecule cluster of 2 shown in Figure 1. The packing index for
a crystal is defined as the percentage of the volume of the unit
cell that is occupied by the atoms of the molecule. This is calcu-
lated by summing the volumes of all the individual atoms in the
molecule using standard values of their van der Waals radii. We
calculated these percentages for 2 and 17 using the PLATON
program10 and found that the packing index for the tert-butyl-
substituted [7]phenacene 17 is only 64.8% whereas the packing
index for the phenyl-substituted [5]phenacene 2 is 70.1%. On
the assumption that the crystal packing index for the tert-butyl-
substituted [5]phenacene 15 might be numerically similar to
that for the tert-butyl-substituted [7]phenacene 17, we suggest
that this calculation may offer some support for our hypothesis
that crystal packing is an important factor in accounting for the ex-
perimental finding given in Table 1 that the tert-butyl-substituted
[5]phenacene 15 is 10-fold more soluble in benzene than the
phenyl-substituted [5]phenacene 2.
(2-Bromo-4-phenylbenzyl)triphenylphosphonium bromide
(7). A solution of 15.1 g (61 mmol) of 3-bromo-4-methylbiphenyl
5, 10.9 g (61 mmol) of N-bromosuccinimide, and a small amount of
benzoyl peroxide in 130 mL of CCl4 was heated at reflux for 24 h. The
reaction mixture was allowed to cool, and the succinimide was
removed by vacuum filtration. The filtrate was rotary evaporated to
give the benzyl bromide 6 as a yellow solid. Without further purifica-
tion, 6 was dissolved in 91 mL of reagent grade acetone, and 16.1 g
(62 mmol) of triphenylphosphine was added. The mixture was allowed
to stir overnight at room temperature, after which the solid was
collected by vacuum filtration, washed with toluene, and dried under a
vacuum to give 26.2 g (71%) of phosphonium salt 7 as a white solid:
1H NMR (CDCl3, 300 MHz) δ 7.83−7.72 (m, 9H), 7.68−7.61 (m, 8H),
7.50−7.34 (m, 6H), 5.75 (d, J = 14.2 Hz, 2H); 13C NMR (CDCl3, 100.6
MHz) δ 30.6, 31.0, 117.0, 117.8, 126.2, 126.9, 127.6, 128.3, 129.0, 130.2,
131.1, 133.4, 134.3, 135.2, 138.4, 143.3. HRMS (ES+) Calcd for
C31H25BrP (M+): 507.0877. Found: 507.0876.
2-Methyl-5-phenylbenzaldehyde (8). Biphenyl 5 (13.8 g, 56
mmol) was dissolved in 300 mL of anhydrous ether under N2, and the
solution was cooled in an ice bath while 55 mL (110 mmol) of 2 M
n-BuLi in pentane was added dropwise, and the solution was then
stirred in the ice bath for an additional 1 h. After 19 mL (245 mmol)
of dimethylformamide was added dropwise, the mixture was allowed to
warm to room temperature, after which it was quenched with 300 mL of
5% H3PO4. The aqueous layer was extracted twice with ether, and the
ether extracts were washed 3 times with water and finally dried over
Na2SO4. Removal of the solvent by rotary evaporation gave an orange
oil that was vacuum distilled (bp 113−125 °C, 0.04 mmHg) to give a
pale yellow oil that solidified on standing to give 10.0 g (92%) of 8 with
mp 41.0−41.5 °C (lit.12 mp 35.5−37.0 °C): 1H NMR (CDCl3, 300 MHz)
δ 10.34 (s, 1H), 8.03 (d, J = 2.1 Hz, 1H), 7.71 (dd, J = 7.9, 2.1 Hz, 1H),
7.63−7.59 (m, 2H), 7.49−7.43 (m, 2H), 7.37 (tt, J = 7.3, 1.3 Hz, 1H),
7.34 (br d, J = 7.7 Hz, 1H), 2.71 (s, 3H); GC−MS m/z 196 (M+). Anal.
Calcd for C14H12O: C, 85.68; H, 6.16. Found: C, 85.70; H, 6.23.
(E)-2-Bromo-2′-methyl-4,5′-diphenylstilbene (9). Phosphonium
salt 7 (26.2 g, 44 mmol) was added to a solution of 9.8 g (50 mmol) of
2-methyl-5-phenylbenzaldehyde 8 in 60 mL of CH2Cl2. The resulting
mixture was cooled in an ice bath, and 16 mL of 50% aqueous NaOH
solution was added dropwise with vigorous stirring that was continued
at room temperature for 6 h, after which the reaction mixture was
poured into 100 mL of water. The aqueous layer was extracted 3 times
with CH2Cl2, and the combined organic extracts were washed 3 times
with water and then dried over Na2SO4. The solvent was removed by
rotary evaporation, the orange residue was dissolved in hexanes, and
the solution was filtered though a plug of silica to remove triphenyl-
phosphine oxide. Rotary evaporation of the filtrate gave a mixture of E
and Z isomers of alkene 9 as a yellow oil that was redissolved in 100 mL
of cyclohexane, and a few crystals of iodine were added. The purple
solution was stirred and irradiated with visible light from a 100-W
tungsten bulb for 2 days to achieve Z to E isomerization. The solution
was then washed with aqueous NaHSO3 to remove the iodine, and the
organic layer was dried over Na2SO4. The solvent was rotary evaporated
to give a pale yellow solid that was recrystallized from 1:1 hexanes/
toluene to give 14.7 g (79%) of 9 as a white crystalline solid with mp
115−117 °C: 1H NMR (CDCl3, 300 MHz) δ 7.85 (d, J = 1.9 Hz, 1H),
7.85 (d, J = 1.9 Hz, 1H), 7.76 (d, J = 8.2 Hz, 1H), 7.64 (br d, J = 7.1 Hz,
2H), 7.59 (br d, J = 7.2 Hz, 2H), 7.56 (dd, J = 8.1, 1.8 Hz, 1H), 7.463
(t, J = 7.5 Hz, 2H), 7.456 (t, J = 7.3 Hz, 2H), 7.450 (A of AB q, J =
16.0 Hz, 1H), 7.447 (dd, J = 7.8, 2.0 Hz, 1H), 7.37 (br t, J = 7.2 Hz,
1H), 7.36 (br t, J = 7.3 Hz, 1H), 7.34 (B of AB q, J = 15.9 Hz, 1H), 7.27
(br d, J = 7.9 Hz, 1H), 2.49 (s, 3H); 13C NMR (CDCl3, 100.6 MHz) δ
141.6, 140.8, 139.2, 139.0, 136.2, 136.0, 134.9, 131.2, 130.7, 129.1, 128.7,
128.53, 128.49, 127.6, 126,9, 126.8, 126.7, 126.5, 126.0, 124.4, 124.3,
19.4; GC−MS m/z 424/426 (M+). Anal. Calcd for C27H21Br: C, 76.24;
H, 4.98. Found: C, 76.37; H, 5.16.
CONCLUSIONS
■
We have found that phenyl substituents are significantly less
effective than tert-butyl substituents as solubilizing groups for
[n]phenacenes such as 1 and 2. We attribute this to the especi-
ally tight crystal packing that is illustrated in Figure 1 for the
phenyl-substituted [5]phenacene 2 as contrasted with the crystal
packing illustrated in Figure 3 for the tert-butyl-substituted
[7]phenacene 17. Phenyl substituents on [n]phenacenes can
also be experimentally disadvantageous because the large number
1
of phenyl hydrogens can sometimes obscure some of the H
NMR signals that are important for the verification of the
molecular structures of those [n]phenacenes and their synthetic
precursors. In our current work on the syntheses of large
[n]phenacenes, we have solved the solubility problems by using
modified tert-butyl groups in which one of the three methyl
groups has been replaced by an n-octyl group.
EXPERIMENTAL SECTION
■
Photocyclizations were carried out in stirred solvents in Pyrex vessels
surrounded by 16 300-nm lamps. H NMR and 13C NMR spectra
1
were measured in CDCl3 solution at ambient temperature using TMS
as an internal standard. Melting points were determined with an ap-
propriate melting point apparatus and are uncorrected. Mass spectra
were obtained using either GC−MS or HRMS instrumentation.
3-Bromo-4-methylbiphenyl (5). Concentrated HCl (50 mL) was
added to a mixture of 30 g (0.16 mol) of 3-bromo-4-methylaniline 4
and 50 mL of water, and the mixture was cooled to −10 °C using an
i-PrOH/dry ice bath. A solution of 15 g (0.22 mol) of NaNO2 in 50 mL
of water was added dropwise. After the addition was complete, 58.9 g
(0.32 mol) of 48% HBF4 was added slowly. The white precipitate that
formed was collected by vacuum filtration, rinsed sparingly with cold
water, cold methanol and cold diethyl ether, and then was dried
overnight under a vacuum. A mixture of the dry fluoroborate salt, 1.44 L
of benzene, 160 mL of acetonitrile, and 31.4 g (0.32 mol) of KOAc
was then allowed to stir at room temperature for 6 h. The dark red
mixture was filtered to remove insoluble impurities, and the solvent
removed from the filtrate by rotary evaporation. The remaining oil was
dissolved in 5% benzene in hexanes and poured though a plug of 1:1
silica/alumina. The solvent was removed, the oil redissolved in hexanes,
and the solution poured though a short column of alumina. The oil re-
maining after rotary evaporation was vacuum distilled (bp 103−106 °C,
0.02 mmHg) to give 24.8 g (63%) of 3-bromo-4-methylbiphenyl 5 as a
yellow oil (lit11 mp 9 °C): 1H NMR (CDCl3, 300 MHz) δ 7.76 (d, J =
1.8 Hz, 1H), 7.54−7.51 (m, 2H), 7.43−7.38 (m, 3H), 7.32 (tt, J =
7.2, 1.4 Hz, 1H), 7.26 (d, J = 7.9 Hz, 1H), 2.42 (s, 3H); GC−MS m/z
1-Bromo-8-methyl-3,5-diphenylphenanthrene (3). A solution
of 2.63 g (6.18 mmol) of stilbene 9 and 0.80 g (3.2 mmol) of iodine in
1 L of silica-filtered hexanes was irradiated with 300-nm light for 42 h.
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When the reaction was complete as judged by GC−MS analysis, the
hexanes were removed by rotary evaporation, and the resulting brown
solid was dissolved in toluene. The red-purple toluene solution was
washed with aqueous NaHSO3 to remove the iodine. The toluene was
removed by rotary evaporation to give a yellow solid that was recrys-
tallized from 1:1 hexanes/toluene to give 2.38 g (91%) of beige crystals of
phenanthrene 3 with mp 173.5−175.5 °C: 1H NMR (CDCl3, 300 MHz)
δ 8.29 (br d, J = 9.4 Hz, 1H), 8.23 (dd, J = 1.5, 0.8 Hz, 1H), 8.07 (d, J =
9.4 Hz, 1H), 8.01 (d, J = 1.7 Hz, 1H), 7.56−7.49 (m, 3H), 7.51 (d, J =
7.4 Hz, 1H), 7.41 (br d, J = 7.4 Hz, 1H), 7.47−7.41 (m, 2H), 7.32−7.27
(m, 3H), 7.06−7.03 (m, 2H), 2.81 (s, 3H); 13C NMR (CDCl3, 100.6
MHz) δ 145.8, 139.2, 138.7, 136.9, 134.3, 132.19, 132.17, 130.7, 130.3,
129.3, 128.9, 128.7, 128.4, 127.7, 127.3, 127.2, 126.9, 126.5, 125.2, 124.4,
123.2, 20.0; GC−MS m/z 422/424 (M+). Anal. Calcd for C27H19Br: C,
76.60; H, 4.52. Found: C, 76.42; H, 4.40.
was recrystallized from 95% EtOH to give 2 as a white powder, mp
290−291.5 °C: 1H NMR (CDCl3, 300 MHz) δ 8.90 (d, J = 9.7 Hz, 1H),
8.83 (br d, J = 9.4 Hz, 1H), 8.69 (br d, J = 9.3 Hz, 1H), 8.29 (d, J =
9.4 Hz, 1H), 8.14 (v br s, 1H), 8.02 (d, J = 1.7 Hz, 1H), 7.96 (br s,
1H), 7.53−7.41 (m, 8H), 7.36−7.26 (m, 5H), 7.03−7.00 (m, 2H),
6.91−6.88 (m, 2H), 2.87 (s, 3H); UV peaks (cyclohexane) 279, 308,
340, and 355 nm (similar to the peaks of the tert-butyl analogue 153 at
271, 305, 342, and 357 nm). Anal. Calcd for C41H27Br: C, 82.13; H,
4.54. Found: C, 82.37; H, 4.45.
8-Methyl-3,5-diphenylphenanthrene-1-carboxaldehyde (13).
Phenanthrene 3 (2.2 g, 5.2 mmol) was dissolved in 190 mL of an-
hydrous diethyl ether. The solution was cooled to 0 °C under N2, and
1.6 M n-BuLi in hexanes (8.3 mL, 13.3 mmol) was added dropwise.
When the addition was complete, the reaction mixture was allowed to
stir for 1 h, after which 2.0 mL (26 mmol) of dimethylformamide was
added dropwise, the ice bath was removed, and the solution was allowed
to stir for 2 h. The reaction was quenched with 5% H3PO4, and the
layers were separated. The aqueous layer was extracted 3 times with
toluene, and the combined organic layer was washed with aqueous
NaHCO3 and then with brine. After drying over Na2SO4, the solvent
was rotary evaporated to give a solid that was recrystallized from toluene
to give 0.66 g (34%) of 13 as a light yellow crystalline solid with a broad
melting range (mp 117−145 °C): 1H NMR (CDCl3, 300 MHz) δ 10.57
(s, 1H), 9.19−9.16 (d, J = 9.5 Hz, 1H), 8.58 (d, J = 1.8 Hz, 1H), 8.21−
8.18 (m, 2H), 7.56−7.43 (m, 7H), 7.33−7.32 (m, 1H), 7.31−7.30 (m,
2H), 7.10−7.09 (m, 2H), 2.83 (br s, 3H); 13C NMR (CDCl3, 100.6
MHz) δ 193.4, 145.9, 139.2, 138.7, 135.7, 134.5, 133.3, 133.2, 131.6,
131.0, 130.7, 129.6, 129.1, ca.128.71, 128.7, 127.9, 127.7, 127.0, 126.9,
126.1, 122.2, 20.1; GC−MS m/z 372 (M+). Anal. Calcd for C28H20O:
C, 90.29; H, 5.41. Found: C, 90.55; H, 5.54.
(8-Bromo-4,6-diphenyl-1-phenanthrylmethyl)triphenylphos-
phonium bromide (11). Phenanthrene 3 (0.90 g, 2.1 mmol) was
dissolved in 15 mL of CCl4, to which 0.37 g (2.1 mmol) of N-bromo-
succinimde and a small amount of benzoyl peroxide were added. After
the solution was heated at reflux for 24 h, the succinimide was re-
moved by vacuum filtration, and the solvent was evaporated to give the
bromomethyl compound 10 as a yellow solid. Without further purifica-
tion, 10 and 0.63 g (2.4 mmol) of triphenylphosphine were dissolved
in 10 mL of xylenes, and the solution was heated at gentle reflux over-
night. After allowing the reaction mixture to cool to room temperature,
vacuum filtration gave a pale yellow solid that was washed with cold
xylenes and dried overnight under a vacuum to give 1.31 g, (82%) of the
1
phosphonium salt 11: H NMR (CDCl3, 300 MHz) δ 8.12 (br s, 1H),
7.98 (d, J = 1.6 Hz, 1H), 7.83−7.71 (m, 12H), 7.60−7.46 (m, 10H),
7.35−7.26 (m, 5H), 7.03−7.00 (m, 2H), 6.14 (d, J = 14.0 Hz, 2H); 13C
NMR (CDCl3, 100.6 MHz) δ 144.9, 141.1, 138.9, 137.1, 134.9, 134.8,
134.6, 134.4, 132.8, 131.4, 131.0, 130.60, 130.56, 130.1, 130.0, 129.8,
129.5, 129.1, 128.7, 128.6, 127.6, 127.1, 127.0, 126.9, 125.5, 123.6, 123.5,
123.4, 123.1, 118.1, 117.3, 28.3, 27.8. HRMS (ES+) Calcd for
C45H33BrP (M+): 683.1503. Found: 683.1503.
(E)-1-(8-Bromo-4,6-diphenyl-1-phenanthryl)-2-(8′-methyl-
3′,5′-diphenyl-1′-phenanthryl)ethene (14). Phosphonium salt 11
(1.03 g, 1.3 mmol) was combined with 30 mL of THF under N2.
A solution of 1.2 mL (1.9 mmol) of 1.6 M n-BuLi in hexanes, 0.4 mL
(2.4 mmol) of diisopropylamine, and 7 mL of THF was added slowly
though an addition funnel. The reaction solution turned red indicating
the formation of the ylid. A solution of 0.50 g (1.3 mmol) of aldehyde
13 dissolved in 6 mL of THF was added with a syringe to the reaction
flask, which was then heated slowly and allowed to stir overnight at
gentle reflux. The reaction was quenched by the addition of saturated
aqueous NH4Cl solution, and the aqueous layer was extracted twice
with toluene. The combined organic layer was then washed with brine
and finally dried over Na2SO4. The brown oil that remained after
rotary evaporation was dissolved in hexanes and stirred overnight with
an equal volume of 70% EtOH to remove triphenylphosphine oxide. A
solid had formed, and the mixture was sonicated for 2 h before collecting
0.85 g (83%) of a yellow solid that was recrystallized from toluene to give
(E)-1-(8-Bromo-4,6-diphenyl-1-phenanthryl)-2-(2′-methyl-
5′-phenyl)ethene (12). Phosphonium salt 11 (1.74 g, 2.3 mmol)
was placed in 26 mL of THF under N2, and to this white suspension
was added a solution prepared from 6.5 mL of THF, 0.44 mL (2.5 mmol)
of diisopropylamine, and 1.2 mL (2.4 mmol) of 2 M n-BuLi in pentane.
This produced a bright red reaction mixture, indicating the formation of
the ylid, to which was added a solution of 0.44 g (2.2 mmol) of 2-methyl-
5-phenylbenzaldehyde 8 in 6.5 mL of THF. The resulting mixture was
heated at reflux overnight, after which it was quenched with saturated
aqueous NH4Cl and then extracted three times with toluene. After the
combined toluene extracts were washed with 70% EtOH to remove
triphenylphosphine oxide, they were dried over Na2SO4. Evaporation of
the solvent gave a mixture of the E and Z isomers of product 12. This
mixture was dissolved in hexanes, several crystals of iodine were added,
and the purple solution was irradiated overnight with visible light from a
100-W tungsten bulb to achieve the Z to E isomerization. After the purple
solution was washed with aqueous NaHSO3 solution to remove the
iodine, it was dried over Na2SO4. The solvent was removed by rotary
evaporation to give a brown oil. A small amount of acetonitrile was added
to the oil, and the mixture was sonicated for several hours to give 0.94 g
(70%) of a yellow solid that was recrystallized from 1:1 hexanes/
toluene to give 12 with mp 193−194 °C: 1H NMR (CDCl3, 300 MHz)
δ 8.28 (br s, 2H), 8.24 (br d, J = 1.6 Hz, 1H), 8.02 (d, J = 1.6 Hz, 1H),
7.96 (br d, J = 1.7 Hz, 1H), 7.90 (br d, J = 15.7 Hz, 1H), 7.89 (d, J =
2.3 Hz, 1H), 7.71−7.68 (m, 2H), 7.56−7.27 (m, 15H), 7.07−7.04
(m, 2H), 2.53 (s, 3H). Anal. Calcd for C41H29Br: C, 81.86; H, 4.86.
Found: C, 81.75; H, 4.64.
1
14 as a yellow powder, mp 237−248 °C (dec): H NMR (CDCl3,
300 MHz) δ 8.34 (d, J = 9.4 Hz, 1H), 8.29 (d, J = 9.4 Hz, 1H), 8.29 (br s,
1H), 8.27 (br s, 1H), 8.09−8.01 (m, 6H), 7.62−7.42 (m, 14H), 7.33−
7.28 (m, 6H), 7.21−7.18 (m, 2H), 7.08−7.05 (m, 2H), 2.82 (s, 3H).
Anal. Calcd for C55H37Br: C, 84.93; H, 4.80. Found: C, 84.76; H, 5.02.
11-Bromo-4-methyl-1,13,15,17-tetraphenyl[7]phenacene
(1). A solution of 0.40 g (0.52 mmol) of alkene 14 and 0.24 g (0.95 mmol)
of iodine in 300 mL of toluene was irradiated with 300-nm light for
6 h. The solid that formed during the irradiation was collected by
vacuum filtration and washed with cold hexanes. Recrystallization of
the solid from benzene gave 0.31 g (79%) of cream-colored phenacene
1
1, mp >350 °C: H NMR (CDCl3, 300 MHz) δ 9.10−8.89 (m, 4H),
8.51−8.49 (d, J = 8.86 Hz, 1H), 8.33−8.30 (d, J = 8.82 Hz, 1H), 8.13
(s, 1H), 8.02−7.99 (d, J = 6.7 Hz, 2H), 7.88 (s, 1H), 7.55−7.19 (m,
16H), 7.01 (m, 2H), 6.88−6.85 (d, J = 6.75 Hz, 4H), 2.36 (s, 3H).
Anal. Calcd for C55H35Br: C, 85.15; H, 4.55. Found: C, 84.80; H, 4.80.
9-Bromo-1,11,13-triphenyl-4-methyl[5]phenacene (2). A solu-
tion of 100 mg (0.17 mmol) of alkene 12 and 110 mg (0.43 mmol) of
iodine in 140 mL of benzene was irradiated with 300-nm light for 24 h,
after which the reaction was judged complete by NMR analysis. The
solvent was rotary evaporated, and the residue redissolved in toluene.
Extraction with aqueous NaHSO3 removed residual iodine, and the
color of the solution faded to pale yellow. The toluene was removed by
rotary evaporation, and the resulting 0.10 g (100%) of brown powder
ASSOCIATED CONTENT
■
S
* Supporting Information
1
Copies of H NMR spectra, 13C NMR spectra, GC−MS data,
HRMS data, and UV spectral data for key reaction products, X-ray
crystallographic results (CIF) for compound 2, and computational
2044
dx.doi.org/10.1021/jo3020819 | J. Org. Chem. 2013, 78, 2040−2045

The Journal of Organic Chemistry
Article
data for the structure of compound 16. This material is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: fmallory@brynmawr.edu.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Acknowledgement is made to the donors of the Petroleum
Research Fund, administered by the American Chemical Society,
for partial support of this research through Grant 41685-AC1,
and also to Bristol-Myers Squibb for providing summer research
support for R.J.A.
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