Photolability of per-arylated butadienes: En Route to dihydronaphthalenes

Article abstract of DOI:10.1021/jo502293q

Arylated butadienes were prepared employing transition-metal coupling techniques and characterized via UV/vis spectroscopy and X-ray crystal structure analysis. Identification of byproducts led to a photochemical route toward novel multiarylated dihydrona

Full text of DOI:10.1021/jo502293q

Note
pubs.acs.org/joc
Photolability of Per-Arylated Butadienes: En Route to
Dihydronaphthalenes
Jan Freudenberg,† Andrea C. Uptmoor,† Frank Rominger,† and Uwe H. F. Bunz*,†,‡
†Organisch-Chemisches Institut, Ruprecht-Karls-Universitat Heidelberg, Im Neuenheimer Feld 270, 69120 Heidelberg, Germany
̈
‡Centre of Advanced Materials, Ruprecht-Karls-Universitat Heidelberg, Im Neuenheimer Feld 225, 69120 Heidelberg, Germany
̈
S
* Supporting Information
ABSTRACT: Arylated butadienes were prepared employing transition-
metal coupling techniques and characterized via UV/vis spectroscopy
and X-ray crystal structure analysis. Identification of byproducts led to a
photochemical route toward novel multiarylated dihydronaphthalenes.
Arylbutadiene−dihydronaphthalene cyclization occurs in solution and in
the solid state. Upon substitution of hexaphenylbutadiene, absorption is
red-shifted and stability under ambient light is even more reduced.
Scheme 1. Synthetic Routes to Hexaphenylbutadiene 1a
erein, the photoreactivity of the light-sensitive arylated
Hbutadienes 1a−3a is reported, leading to pentaarylated
dihydronaphthalenes 1b−3b.
Fluorophores exhibiting aggregation-induced emission (AIE)
are attractive in optoelectronic applications, as chemical noses
or biological probes.1 In the aggregated state, strong
fluorescence is observed due to restriction of intramolecular
rotation, which allows relaxation of the excited state in
solution.2 Well-studied examples of AIE fluorophores are
tetraphenylethenes (TPEs) and their derivatives.3 An un-
disputed premise to apply AIE materials is their photostability.
Zhu and co-workers demonstrated that TPE-based fluoro-
phores undergo photocyclization into dihydrophenanthrenes,
which are readily oxidized to phenanthrenes under aerobic
conditions.4 However, these characteristics should not affect
device performance much, as such cyclization is fully reversible
under anaerobic conditions.
Crystallization of “pure” 1a from dichloromethane/methanol
accidentally revealed the structure of the side product as
dihydronaphthalene isomer 1b (Figure 1).
Little is known about TPEs’ higher homologues, the
hexaaryl-butadienes. Do these materials exhibit AIE phenomena
and are they sufficiently stable for applications? Very recently,
aggregation-induced emission characteristics of hexaphenyl-
butadiene 1a, a “second generation” emitter in luminescent
solar concentrators (LSCs), were reported5 but nothing is
known about its photostability. Our group is interested in
combining these attractive AIE properties with the possibility of
further functionalization compared to TPEs to extend our
portfolio of sensory materials.6
As we attempted to synthesize 1a as a potential transport
material for organic light-emitting diodes (OLEDs), employing
a modified version of Miura’s two-component reaction
(Scheme 1, eq 1),7 we observed a small singlet at approximately
4.5 ppm in the NMR spectrum. This impurity (<1%) formed
independently of the method of preparation, as we tested
Spring’s coupling of vinyl bromide derivatives (Scheme 1, eq
2)8 and developed our own protocol (eq 3; see Experimental
Section for details).
Aryl-substituted butadienes undergo aryl vinyl photocycliza-
tions under oxidative as well as nonoxidative conditions, leading
to substituted phenyl naphthalenes9 or dihydronaphthalenes.10
To test our hypothesis of 1b being a product of a
photochemical 6-π-electron electrocyclization reaction, we
irradiated 1a at different wavelengths (250−419 nm) in
cyclohexane under an inert atmosphere; formation of 1b,
occurring regardless of the wavelength irradiated, was
monitored by GC/MS (Table 1). Stirring 1a at reflux
temperature in toluene or DMSO with or without a catalytic
amount of toluenesulfonic acid did not induce reaction.
To the best of our knowledge, however, photocyclization of
per-substituted 1,4-diphenylbutadienes with conformational
flexibility at the butadiene moiety has never been reported.
To broaden the scope of the photoreaction, we prepared the
arylated butadienes 2a and 4a (Table 1). We could isolate 3a
Received: October 7, 2014
Published: November 4, 2014
© 2014 American Chemical Society
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Cyclization also occurs in the solid state under solvent-free
conditions (50 mg scale, Table 1, entry 2). To further illustrate
this reactivity, samples of 1a and 1b were dropcast on a TLC
plate. After irradiation of 1a with UV light (λ = 365 nm), the
greenish emission of 1a is replaced with the blue solid state
fluorescence of 1b (Figure 2).
Figure 1. Crystal structures of compounds 1a and 1b.
Figure 2. Photographs of compounds 1a (before and after irradiation
with a 3 W UV-LED at 365 nm) and 1b dropcast on silica TLC plates
under excitation with a hand-held UV-lamp at an emission wavelength
of 365 nm. A blank TLC plate is shown as a reference.
Table 1. Photoreactions to Dihydronaphthalenes
Figure 3. Molar absorption coefficient spectra of compounds 1−4 in
dichloromethane.
a
b
4-Tol: 4-tolyl. Pr: n-propyl. A: N2 atmosphere, toluene, 1 h, 350 nm.
B: N2 atmosphere, solvent-free, 34 h, 300 nm. C: N2 atmosphere,
c
d
toluene, 1 h, 300 nm. Isolated yields. Not observed.
UV/vis spectra (dichloromethane, Figure 3) illustrate the
different absorptivities of 1a−4a and 1b−3b. Table 2
summarizes these data.
employing brown-glass and red-light techniques. Miura et al.
did not succeed in the isolation of 3a; however, the authors
obtained 3b as the only thus far known example of this
structural motif.7 Compounds 1a−3a were transformed into
their cycloisomers 1b−3b in excellent yields after irradiation in
a photoreactor at λ = 350 nm (all structures were
unambiguously confirmed by X-ray analysis). However, 4a
did not undergo cyclization to 4b. After 1 h of irradiation at 300
nm and subsequent purification, only geometric (E,E)- and
(E,Z)-isomers (1:2 ratio) could be detected via NMR. This
result indicates that the aryl rings situated on the butadiene
backbone are necessary for the cyclization reaction.
As gleaned from X-ray structure analyses (see Supporting
Information and Figure 1), butadienes 1a−3a are sterically
overcrowded; the aryl rings are twisted out of the plane, and the
butadiene backbone is pushed into a helical s-cis conformation.
As comparable photocyclization reactions occur in fulgamid-
es,10e which are forced into this s-cis geometry by a tether, the
steric pressure in butadienes must enable the force behind the
isomerization reaction.
Substitution of the phenyl rings of 1a (λmax = 303 nm) with
methyl or methoxy groups leads to an increasing red shift of the
spectra thus rendering 2a (λmax = 312 nm) and 3a (λmax = 320
nm) even more photolabile to ambient light, as the absorption
onset is further shifted into the visible regime and ambient light
suffices to induce cyclization for 3a. The same trend is observed
for their isomerized counterparts 1b−3b. As the dihydronaph-
thalenes absorb weakly from 350 to 400 nm, this range is ideal
for the cyclization reaction, thus avoiding photobleaching of the
products. In contrast, the absorption spectrum of 4a differs
from those of 1a−3a. It does only exhibit a weak shoulder at
261 nm. Additionally, the onset of absorption is hypsochromi-
cally shifted by 90 nm and absorptivity is reduced. These results
suggest that, although being bent out of the plane of the
chromophore backbone in the solid state, the aromatic rings in
the per-arylated species freely rotate in solution, allowing
electronic communication with the chromophore backbone
under extension of the delocalized π-system. This free rotation
results in the very weak fluorescence of 1a−3a in solution at
room temperature, whereas rotation is hindered in the solid
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Table 2. Optical Properties of Compounds 1−4 in Dichloromethane
a
b
c
Absorption maximum peak refers to nearest absorption maximum in the absorption spectra to the solar spectrum threshold. At λmax. Shoulder.
Figure 4. Top: Time-dependent monitoring [mm:ss] of the photocyclization reaction (λ = 350 nm) via NMR in benzene-d6 under an inert
atmosphere ([1a] = 22.9 mg/mL). Bottom left: Time-dependent relative concentrations (first 30 min) of 1a and 1b during the photoreaction in
deuterated benzene. Bottom right: linearization and linear fit of [1a] and [1b]. [1a]0 denotes the starting concentration.
state and greenish to yellowish aggregation-induced emission
occurs (vide supra).
To gain further insight into the reaction, we performed a
time-dependent NMR experiment in C6D6 ([1a] = 22.9 mg/
mL) under an argon atmosphere. The sample was illuminated
in a photoreactor at λ = 350 nm for different time intervals, and
the composition was subsequently analyzed via NMR (Figure 4,
top). (1) The photocyclization reaction proceeds smoothly, as
no side products could be detected. (2) Photocyclization is fast;
the reaction was complete in less than 20 min. (3) Under these
reaction conditions, both the formation of 1b and consumption
of 1a follow a first-order kinetic, assuming an excess of photons
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Note
Scheme 2. Photocyclization of 1a
were performed in brown glassware and under red light. Under an
inert gas atmosphere, to 1,2-bis(4-methoxyphenyl)ethyne (200 mg,
893 μmol, 1.0 equiv), (4-methoxyphenyl)boronic acid (191 mg, 1.28
mmol, 1.5 equiv), palladium(II) acetate (4.71 mg, 21.0 μmol, 2.5 mol
%), silver(I) carbonate (231 mg, 839 μmol, 1.0 equiv), and tri-p-tolyl
phosphate (7.73 mg, 20.1 μmol, 2.5 mol %) was added a mixture of 1-
propanol/water (2.5 mL, 9:1). The reaction mixture was stirred at 120
°C for 30 min. The reaction mixture was allowed to cool to room
temperature and was taken up in diethyl ether (50 mL) and water (50
mL), and insoluble materials were removed via filtration. The organic
layer was washed thrice with water (50 mL) and dried over magnesium
sulfate, and the volatiles were removed under reduced pressure. The
crude product was subjected to column chromatography (silica,
petroleum ether/ethyl acetate 5:1, Rf = 0.07) to yield the title
compound as a yellow solid (171 mg, 248 μmol, 59%). Mp: 207−208
°C. IR (cm−1): 3027, 3007, 2993, 2949, 2935, 2835, 1603, 1508, 1463,
is available. The data can be fitted employing Bodenstein’s
quasi-stationary principle (Figure 4, bottom right). This is in
agreement with the mechanism of the reaction, which consists
of a cyclization step and subsequent [1,5] hydrogen shift
(Scheme 2).11 (4) After an irradiation cycle, the formation of a
deeply yellow to orange intermediate was observed, which
disappeared quickly and can be ascribed to 5.10e
In conclusion, per-arylated butadienes are photolabile, both
in solution and in the solid state, rendering 1a−3a less than
ideal for long-term aggregation-induced emission applications,
even under anaerobic conditions. Proceeding from these
butadienes, we described a simple approach toward the
synthesis of penta-arylated dihydronaphthalenes.
EXPERIMENTAL SECTION
1
■
1287, 1240, 1170, 1112, 1028, 826, 805, 773, 556. H NMR (600
All reagents and solvents were obtained from commercial suppliers
and were used without further purification unless specified otherwise.
Preparation of light sensitive materials was carried out employing
brown glass and red-light techniques. Preparation of air and moisture
sensitive materials was carried out in oven-dried flasks under an
atmosphere of nitrogen or argon employing standard Schlenk
techniques. Photocyclization reactions were carried out employing
3500, 3000, and 4190 Å light sources. 1H NMR spectra were recorded
at 298 K on a 300, 400, or 600 MHz spectrometer, and 13C NMR
spectra were recorded on a 75, 100, or 150 MHz spectrometer.
Chemical shifts (δ) are reported in parts per million (ppm) and
referenced internally to the solvent signal. Melting points are reported
uncorrected. Infrared (IR) spectra are reported in wavenumbers
(cm−1) and were recorded neat. Photographs were taken under UV-
light irradiation (λ = 365 nm) using a digital camera (objective: EF-
S60 mm f/2.8 Macro USM). MS spectra were recorded using electron
impact detected by magnetic sector and FT-ICR techniques,
respectively. Absorption spectra were recorded in dichloromethane.
Crystal-structure analysis was accomplished on diffractometers with
Mo Kα radiation, λ = 0.71073 Å. Arylbutadienes 1a, 2a, and 4a were
prepared according to literature procedures.7,8 Consequent exclusion
of UV radiation and blue to yellow light resulted in the isolation of
noncyclized 3a instead of 3b.7
MHz, CDCl3): δ 7.09 (d, J = 8.8 Hz, 4 H), 6.81−6.88 (m, 8 H), 6.59−
6.62 (m, 8 H), 6.49 (d, J = 9.1 Hz, 4 H), 3.73 (s, 6 H), 3.71 (s, 6 H),
3.66 (s, 6 H). 13C NMR (150 MHz, CDCl3): δ 158.2, 158.1, 157.3,
140.6, 138.8, 137.4, 137.1, 134.4, 132.6, 132.6, 131.2, 113.0, 112.8,
112.6, 55.3, 55.2, 55.1. HRMS (EI (+)): m/z [M]•+calcd for C46H42O6
690.2981, found 690.2963. Elemental analysis calcd (%) for C46H42O6:
C, 79.98; H, 6.13. Found: C, 79.90; H, 6.34.
General Procedure for the Photocyclization Reaction (GP).
Under a nitrogen atmosphere, the butadiene (200 μmol) was dissolved
in thoroughly degassed toluene (25 mL) in a quartz flask. The solution
was stirred for 1 h under UV-light irradiation (λ = 350 nm). The
solvent was removed under reduced pressure, and the crude product
was subjected to column chromatography (silica, petroleum ether) to
yield the dihydronaphthalene. Recrystallization from dichlorome-
thane/methanol yielded single crystals suitable for X-ray diffraction
analysis.
1,1,2,3,4-Pentaphenyl-1,2-dihydronaphthalene (1b). GP was
carried out with 1a (102 mg, 200 μmol) in toluene (25 mL). Column
chromatography (silica, petroleum ether, Rf = 0.10) yielded 1b (97.0
mg, 190 μmol, 95%) as a colorless solid. Solid State: Under a nitrogen
atmosphere, powdered 1a (97.9 μmol) was added to a quarz flask
equipped with a stirring bar and stirred without addition of solvent
under UV light irradiation (λ = 300 nm) for 34 h. Purification via
column chromatography on silica yielded 1b as a colorless solid (68.5
μmol, 70%). Recrystallization from dichloromethane/methanol
yielded single crystals with an identical unit cell as that for the
crystals obtained after reaction in solution. Mp: 235−237 °C. IR
(cm−1): 3081, 3055, 3026, 3022, 1597, 1488, 1442, 1190, 1176, 1157,
1071, 1030, 999, 967, 911, 843, 793, 754, 722, 696, 637, 623, 585, 568.
1H NMR (600 MHz, CDCl3): δ 7.28−7.51 (m, 3 H), 6.67−7.26 (m,
22 H), 6.62 (d, J = 7.2 Hz, 2 H), 6.48 (bs, 2 H), 4.35 (s, 1 H). 13C
NMR (150 MHz, CDCl3): δ 145.8, 143.4, 142.6, 139.7, 139.6, 139.4,
139.3, 138.5, 137.0, 131.1, 131.0 (bs), 130.9, 130.0, 129.6 (bs), 128.7,
128.1, 127.9 (bs), 127.6, 127.5 (bs), 127.5, 126.94, 126.87, 126.8,
1,1,2,3,4,4-Hexaphenyl-1,3-butadiene (1a). Under an argon
atmosphere, 1,1′,1″-(2-bromo-1,1,2-ethenetriyl)tribenzene (300 mg,
895 μmol, 2.0 equiv) was dissolved in anhydrous diethyl ether (4 mL)
and cooled to −78 °C. tert-Butyllithium (1.05 mL, 1.79 mmol, 1.7 M
in pentane, 4.0 equiv) was added dropwise, and the mixture was
allowed to stir at −78 °C for 1 h. Copper(II) chloride (120 mg, 895
μmol, 2.0 equiv) was added, and the reaction mixture was very slowly
warmed to 45 °C and was stirred for additional 2 h. The reaction
mixture was filtered through a plug of silica, eluting with a mixture of
petroleum ether/diethyl ether 1:1. After removal of the solvents under
reduced pressure, the crude product was subjected to column
chromatography (silica, petroleum ether/diethyl ether 100:1), to
yield the title compound as a slightly yellow solid (136 mg, 266 μmol,
60%). 1H NMR (300 MHz, CDCl3): δ 7.19 (d, J = 7.4 Hz, 4 H), 7.06
(m, 12 H), 6.91−6.94 (m, 12 H), 6.87−6.89 (m, 2 H). Analytical data
were in accordance with literature.7
1
126.6, 126.5 (bs), 126.3, 126.1, 60.0, 57.6. (Both H and 13C NMR
spectra exhibit broad signals.) HRMS (EI (+)): m/z [M]•+ calcd for
C40H30 510.2348, found 510.2363.
7-Methyl-1,1,2,3,4-pentakis(4-methylphenyl)-1,2-dihydro-
naphthalene (2b). GP1 was carried out with 2a (72.0 mg, 121 μmol)
in toluene (15 mL). Column chromatography (silica, petroleum ether/
1,1′,1′′,1′′′,1′′′′,1′′′′′-Buta-1,3-diene-1,1,2,3,4,4-hexayl-
hexakis(4-methoxybenzene) (3a). The reaction and purification
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Note
ethyl acetate 100:1, Rf = 0.12) yielded 2b (71.0 mg, 119 μmol, 99%) as
a colorless solid. Mp: 303−305 °C. IR (cm−1): 3053, 3023, 2917,
2859, 1510, 1480, 1447, 1183, 1115, 1021, 843, 829, 820, 807, 786,
774, 767, 752, 740, 727, 718, 659, 626, 601, 585, 553, 536, 529, 509,
471. 1H NMR (400 MHz, CDCl3): δ 7.13−7.25 (m, 3 H), 7.01 (d, J =
8.4 Hz, 2 H), 6.88−6.97 (m, 3 H), 6.84 (d, J = 7.9 Hz, 2 H), 6.60−
6.81 (m, 9 H), 6.54 (d, J = 8.1 Hz, 2 H), 6.36 (d, J = 6.8 Hz, 2 H), 4.19
(s, 1 H), 2.32 (s, 3 H), 2.28 (s, 3 H), 2.26 (s, 6 H), 2.24 (s, 3 H), 2.18
(s, 3 H). 13C NMR (100 MHz, CDCl3): δ 143.2, 141.0, 140.1, 140.0,
138.4, 137.1, 136.7, 136.3, 136.2, 136.09, 136.08, 135.8, 135.4, 135.3,
135.2, 131.2, 130.9 (bs), 130.6, 129.5 (bs), 128.63, 128.57, 128.3,
128.1, 127.1 (bs), 127.2, 127.1, 59.4, 57.5, 21.7, 21.4, 21.3, 21.2, 21.09,
(7) Horiguchi, H.; Tsurugi, H.; Satoh, T.; Miura, M. Adv. Synth.
Catal. 2008, 350, 509−514.
(8) Aves, S.; O’Connell, K.; Pike, K.; Spring, D. Synlett 2012, 2012,
298−300.
(9) (a) Fonken, G. J. Chem. Ind. (London) 1962, 1327. (b) Baddar, F.
G.; El-Assal, L. S.; Doss, N. A.; Shehab, A. H. J. Chem. Soc. 1959,
1016−1020. (c) Baddar, F. G.; El-Assal, L. S.; Gindy, M. J. Chem. Soc.
1948, 1270−1272.
(10) (a) Hart, R. J.; Heller, H. G.; Salisbury, K. Chem. Commun.
1968, 1627−1628. (b) Srinivasan, R.; Merritt, V. Y.; Hsu, J. N. C.; op
het Veld, P. H. G.; Laarhoven, W. H. J. Org. Chem. 1978, 43, 980−985.
(c) Heller, H. G.; Strydom, P. J. J. Chem. Soc., Chem. Commun. 1976,
50b−51. (d) Datta, P. K.; Yau, C.; Hooper, T. S.; Yvon, B. L.;
Charlton, J. L. J. Org. Chem. 2001, 66, 8606−8611. (e) Krawczyk, K.
K.; Madej, D.; Maurin, J. K.; Czarnocki, Z. Tetrahedron: Asymmetry
2011, 22, 1103−1107.
(11) Mallory, F. B.; Mallory, C. W. Photocyclizations of Stilbenes and
Related Molecules. In Organic Reactions; Dauben, W. G., Boswell, G.
A., Jr., Danishefsky, S., Gschwend, H. W., Heck, R. F., Hirschmann, R.
F., Paquette, L. A., Posner, G. H., Reich, H. J., Eds.; John Wiley &
Sons, Inc.: New York, 1984; pp 1−456.
1
21.06. (Both H and 13C NMR spectra exhibit broad signals.) HRMS
(EI (+)): m/z [M]•+ calcd for C46H42 594.3287, found 594.3271.
7-Methoxy-1,1,2,3,4-pentakis(4-methoxyphenyl)-1,2-
dihydronaphthalene (3b). GP1 was carried out with 3a (50.0 mg,
72.4 μmol) in toluene (8.4 mL). Column chromatography (silica,
petroleum ether/ethyl acetate) yielded 3b (49.0 mg, 70.9 μmol, 98%)
1
as a colorless solid. H NMR (300 MHz, CDCl3): δ 7.22 (d, J = 8.3
Hz, 2 H), 6.90 (d, J = 2.6 Hz, 1 H), 6.45−6.85 (m, 18 H), 6.37 (d, J =
7.3 Hz, 2 H), 4.14 (s, 1 H), 3.80 (s, 3 H), 3.76 (s, 3 H), 3.74 (s, 3 H),
3.72 (s, 6 H), 3.68 (s, 3 H). Analytical data were in accordance with
literature.7
ASSOCIATED CONTENT
■
S
* Supporting Information
Copies of 1H, 13C NMR spectra of 3a, 1b, and 2b as well as the
images of crystal structures for compounds 1a−3b. This
material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail uwe.bunz@oci.uni-heidelberg.de.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This work was supported by a grant from the “Lan-
■
desgraduiertenforderung (LGFG)”.
̈
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