Thermal generation of pentacenes from soluble 6,13-dihydro-6,13- ethenopentacene precursors by a Diels-Alder-retro-Diels-Alder sequence with 3,6-disubstituted tetrazines

Article abstract of DOI:10.1021/jo202450u

3,6-Substituted tetrazines 2 (a: R² = 2-pyridyl or b: CO ₂Me) react with 2,3,9,10-(R¹)₄-dihydro-6,13- ethenopentacene 3 in solution at elevated temperature to the corresponding pentacene 1 (a: R¹ = H, b: OBn, c: F).

Full text of DOI:10.1021/jo202450u

Note
pubs.acs.org/joc
Thermal Generation of Pentacenes from Soluble 6,13-Dihydro-6,13-
ethenopentacene Precursors by a Diels−Alder-retro-Diels−Alder
Sequence with 3,6-Disubstituted Tetrazines
Rafael P. Bula,† Iris M. Oppel,‡ and Holger F. Bettinger*,†
†Institut fur Organische Chemie, Universitat Tubingen, Auf der Morgenstelle 18, 72076 Tubingen, Germany
̈
̈
̈
̈
̈
‡Institut fur Anorganische Chemie, RWTH Aachen, Landoltweg 1, 52056 Aachen, Germany
S
* Supporting Information
ABSTRACT: 3,6-Substituted tetrazines 2 (a: R2 = 2-pyridyl or b: CO2Me) react with 2,3,9,10-(R1)4-dihydro-6,13-
ethenopentacene 3 in solution at elevated temperature to the corresponding pentacene 1 (a: R1 = H, b: OBn, c: F).
entacene is a promising molecule for application as an
treatment with 3,6-disubstituted 1,2,4,5-tetrazines at elevated
temperatures.27−30 After a Diels−Alder reaction between the
ethene unit and the tetrazine and loss of N2, a subsequent retro-
Diels−Alder step results in elimination of the etheno unit as
part of pyridazine. We investigate the feasibility of this Diels−
Alder-retro-Diels−Alder sequence to synthesize pentacenes
from 6,13-dihydro-6,13-ethenopentacenes. This approach has
been used previously for the synthesis of naphthalene and
anthracene subunits.27 In view of the quickly increasing Diels−
Alder reactivity in the acene series, it is not clear if a
cycloreversion reaction to a pyridazine can be employed for
generating the pentacene π-system at all. Refluxing 3a at 186
°C with an equimolar amount of 3,6-di(2-pyridyl)tetrazine 2a
in dipentylether for 2 h gave a dark blue solid upon cooling that
was isolated in a yield of 28%. This was identified as pentacene
1a by comparison of its MS and UV−vis spectral properties
with an authentic sample, and by 1H and 13C NMR
spectroscopy at elevated temperature. The low yield of 28%
is due to a side reaction. The retro-Diels−Alder reaction of
intermediate 5 can compete with dehydrogenation by an
unreacted tetrazine molecule (Scheme 1).31 This produces
pyridazine 8 and 1,4-dihydro-3,6-(2-pyridyl)tetrazine 7.32 The
latter, along with unreacted 1a, could by identified by EI-MS
1−5
Porganic semiconductor.
Because of its high degree of
crystallinity in the solid state, pentacene has high charge carrier
mobility in thin films, and therefore it is an ideal p-type
semiconductor in organic thin-film transistors (OTFT).1−5
Thin films of pentacene show mobilities greater than 1.5 cm2/
(V s),6 and even values greater than 5 cm2/(V s)7 have been
reported. The mobility depends critically on the crystal
morphology.8−11 The OTFT devices are generally constructed
by vapor-phase deposition of pentacene under high vacuum.
The low air stability and the low solubility of pentacene in most
solvents is the major problem that limits solution processing in
the fabrication of OTFTs.8−11 Many groups have described
soluble pentacene precursors to solve the problem in
fabrication of devices by spin coating.12−16 Solid precursor
films were created and underwent a chemical reaction by light
or heat to gain a pentacene thin film.17,18 Ono et al. were the
first to use a pentacene precursor containing an α-diketo-
bridge.19 This precursor undergoes photoinduced elimination
of two molecules of carbon monoxide, known as Strating−
Zwanenburg reaction,20 to yield pentacene. The α-diketone is
availble from a 6,13-dihydro-6,13-ethenopentacene19,21 in two
steps by cis-dihydroxylation followed by Swern or Anelli
oxidation. The photobisdecarbonylation was applied several
times for synthesis of substitueted pentacenes,17,18,22−26 but
this procedure may be of limited use for pentacene derivatives
with photolabile or strongly absorbing substituents. Herwig and
1
and H NMR from the reaction solution. To demonstrate the
dehydrogenation of 5 to the side product 8, we refluxed 2 equiv
of tetrazine 2a with 1 equiv of dienophile 3a in 1,4-dioxane.
Under these conditions the reaction produced 8 and 7 in a
yield of 48%.
Side product 8 does not react in a retro-Diels−Alder
sequence, neither in solution nor in the solid phase (heated up
Mullen generated a soluble pentacene precursor by reacting
̈
6,13-dihydro-6,13-ethenopentacene with tetrahalothiophenedi-
oxide in a Diels−Alder cycloaddition at high pressure (6
kbar).21 Removal of the cyclohexadiene bridge at temperatures
below 180 °C allowed generation of pentacene films on glass
after spin coating.21 Another way to remove etheno units is
Received: November 29, 2011
Published: March 23, 2012
© 2012 American Chemical Society
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Note
Scheme 1. Diels−Alder-retro-Diels−Alder Sequence for Formation of Pentacene 1a from 3a and 2a (Route A) and the Proposed
Side Reaction (Route B)
Scheme 2. Synthesis of Pentacenes
to 250 °C), to form the target acene. As it was not possible to
increase the yields by changing the reaction conditions, we
reasoned that a more reactive tetrazine is required to keep its
concentration low and thus suppress the dehydrogenation
pathway. 3,6-Di(carboxylic acid methyl ester)tetrazine 2b is
more reactive in cycloaddition reactions than 2a by 3 orders of
magnitude.33 In the reaction of pentacene precursor 3a with the
more reactive tetrazine 2b the yield increases to 48% at a
decreased reaction time of only 2 min. Such yields are typically
reported for the removal of etheno units using tetrazines in the
literature.27 To investigate the scope of the reaction we studied
the formation of donor substituted 2,3,9,10-tetra(benzyloxy)-
pentacene 1b and acceptor substituted 2,3,9,10-tetrafluoropen-
tacene 1c. We generate the required substituted 6,13-dihydro-
6,13-ethenopentacene 3b and 3c in two steps (14% and 49%
yield over both steps for 3b and 3c, respectively) starting with
the 1,2-bis(benzyloxy)-4,5-dibromobenzene 9b34,35 or 1,2-
dibromo-4,5-difluorobenzene 9c and 2,3,5,6-tetrakis-
(methylene)bicyclo[2.2.2]oct-7-ene 1036 (Scheme 2). In the
synthesis of 10 we obtained crystals structures of the all-trans
ethylester precusor 11 and the alcohol 12 (see Supplementary
Figures S25 and S26) that were of sufficient quality for X-ray
crystallography. The bond distances and bond angles are quite
similar to published molecules.37,38 For example, 12 forms H-
bridges in the crystal with O−O distances (2.711(5)−2.776(5)
Å) similar to Iwasaki’s 2,3-di-tert-butyl-5,6,7,8-endotetrakis-
(hydroxymethyl)bicyclo[2.2.2]oct-2-ene (O···H−O distances
1.96(5)−2.04(4) Å).37
The formation of 1b from 3b proceeds with higher yields
than that of parent pentacene with both tetrazines 2a and 2b.
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Note
were taken from a MBraun MB SPS-800 (solvent purification system).
Methanol and dipenthylether were stored over N2 and molecular
sieves (3 Å). 13C{1H} NMR spectra of 1a and 1b were measured at
120 °C using the UDEFT pulse sequence.39
With the more reactive 2b the pentacene 1b is formed in 62%
yield. The tetrafluoro pentacene 1c, on the other hand, is
formed from 3c with tetrazine 2b with a lower yield of only
13%. Due to the low solubility of 1b and 1c at room
temperature, NMR spectra were obtained in 1,1,2,2-tetra-
2,3,9,10-Tetrabenzyloxy-5,6,7,12,13,14-hexahydro-6,13-
ethenopentacene (13b). To a solution of 4.1 g (9.19 mmol) of 1,2-
dibenzyloxy-4,5-dibromobenzene (9b) and 0.7 g (4.48 mmol) of
2,3,5,6-tetrakis(methylene)bicyclo[2.2.2]oct-7-ene (10) at −30 °C in
80 mL of toluene was added dropwise a solution of 6 mL of n-BuLi
(1.6 M in n-hexane) diluted in 20 mL of n-hexane over 2 h. After the
addition the reaction mixture was stirred 2 h at −30 °C and at room
temperature overnight and then quenched with methanol. After the
solvent was removed under reduced pressure, and the residue was
purified by column chromatography (silica; DCM) to obtain 1.585 g
1
chloroethane (TCE) at 120 °C. Only a H NMR spectrum
could be obtained for 1c; its solubility in TCE was too low to
measure a 13C NMR spectrum even at 120 °C.
In the UV−vis spectrum of 1b and 1c (in CH2Cl2 solution at
room temperature, Figure 1) the p and the α bands show the
1
(48%) of a white solid. Mp 161 − 163 °C; H NMR (400 MHz,
CDCl3) δ 3.45 (s, 8H), 4.22 (m, 2H), 5.10 (s, 8H), 6.66 (s, 4H), 6.81
(m, 2H) 7,36 (m, 20H); 13C{1H} NMR (100 MHz, CDCl3) δ 32.8,
54.2, 71.6, 115.4, 127.1, 127.3, 127.7, 128.4, 137.5, 139.2, 140.2, 147.2.
Mass spectrum (EI) m/z M+ 732.4 (7), 642.4 (15), 552.4 (13), 390.3
(15), 300.2 (7), 91.2 (100). HREI C52H44O4 calcd m/z 732.323960,
measured m/z 732.32235.
2,3,9,10-Tetrabenzyloxy-6,13-dihydo-6,13-ethenopenta-
cene (3b). A 1.5 g (2.05 mmol) portion of 13b and 1.043 g (4.24
mmol) of chloranil in 100 mL of toluene were heated under reflux
overnight. After cooling to room temperature a part of 3b precipitated
and was collected by filtration followed by washing with n-hexane. The
filtrate was washed three times with 20% aqueous NaOH (3 × 100
mL) and two times with water (2 × 100 mL). After drying over
MgSO4 the solvent was removed under reduced pressure, and the
residue was purified by column chromatography (silica; DCM). Total
yield: 0.774 g (50%). Mp 216−219 °C; 1H NMR (400 MHz, CDCl3)
δ 5.17 (m, 2H), 5.22 (s, 8H), 6.98 (m, 2H), 7.08 (s, 4H), 7.45 (m,
24H); 13C{1H} NMR (100 MHz, CDCl3) δ 50.2, 71.1, 110.0, 120.2,
127.2, 127.3, 128.0, 128.7, 137.4, 138.7 141.4, 148.8. Mass spectrum
(EI) m/z M+ 728.5 (40), 638.4 (10), 610 (5), 581.4 (5), 91.2 (100).
HREI C52H40O4 calcd m/z 728.29266, measured m/z 728.29398.
2,3,9,10-Tetra(benzyloxy)pentacene (1b). Method A. A 40
mg (0.055 mmol) portion of 3b in dipentylether (10 mL) was
brought to reflux, and 14.2 mg (0.06 mmol) of 3,6-di(2-
pyridyl)tetrazine (2a) was added. The violet mixture was
heated under reflux for 2 h. After cooling to room temperature,
a red solid formed. The solid was collected by filtration and
washed with dipentylether (10 mL), methanol (20 mL,) and n-
hexane (20 mL). Yield: 13.1 mg (38%)
Figure 1. UV−vis spectrum (in CH2Cl2 solution at room temper-
ature) of 1b (top) and 1c (bottom).
typical vibrational fine structure of pentacenes. The recorded
UV−vis spectrum of 1b is very similar to that reported for
2,3,9,10-tetramethoxypentacene.23 The longest wavelength
absorption of the p band system in 1b undergoes a red-shift
of 5 nm compared to 2,3,9,10-tetramethoxypentacene, and the
longest wavelength absorption of the α band is red-shifted by
only 2 nm. In pentacene 1c the longest wavelength absorption
of the p band system is blue-shifted by 15 nm and the β band
by 4 nm compared to 1a.
Method B. A 50 mg (0.069 mmol) portion of 3b in dipentylether
(10 mL) was brought to reflux, and 13.6 mg (0.069 mmol) of 3,6-
di(carboxylic acid methyl ester)tetrazine (2b) was added. The red
mixture was heated under reflux for 1.5 min. The reaction was stopped
by quickly immersing the reaction flask into an ice bath (0 °C). The
red solid was collected by filtration and washed with dipentylether (10
mL), methanol (20 mL), and n-hexane (20 mL). Yield: 28.9 mg (62%)
1H NMR (250 MHz, d2-TCE, 120 °C) δ 5.26 (s, 8H), 7.21 (s, 4H),
7.27−7.40 (m, 12H), 7.47−7.54 (m, 8H), 8.33 (s, 4H), 8.65 (s, 2H).
13C{1H} NMR (126 MHz, d2-TCE, 120 °C) δ 71.5, 108.2, 123.7,
In summary, our study shows that the aromatic pentacene
core can be obtained from 6,13-dihydro-6,13-ethenopentacene
by a Diels−Alder-retro-Diels−Alder sequence employing
tetrazines in moderate yields. We expect that the tetrazine
route may be useful in case where the photobisdecarbonylation
strategy of Ono et al. is not feasible for pentacene generation.
124.8, 127.5, 128.1, 128.7, 129.4, 129.9, 137.5, 150.6. Mass spectrum
(EI) m/z M+ 91 (100), 207 (18), 432 (4), 522 (8), 613 (18), 702
(19). HREI C50H38O4 calcd m/z 702.277010, measured m/z
702.27647.
2,3,9,10-Tetrafluoro-5,6,7,12,13,14-hexahydro-6,13-etheno-
pentacene (13c). To a solution of 2.7 g (10.08 mmol) of 1,2-dibrom-
4,5-difluorbenzene (9c) and 0.75 g (4.8 mmol) of 2,3,5,6-tetrakis-
(methylene)bicyclo[2.2.2]oct-7-ene (10) at −55 °C in 100 mL of
toluene was added dropwise a solution of 7.5 mL of n-BuLi (1.6 M in
n-hexane) diluted in 20 mL of n-hexane over 2 h. After the addition
the reaction mixture was stirred for 2 h at −55 °C and at room
temperature overnight and then quenched with methanol. After the
solvent was removed under reduced pressure, the residue was purified
by column chromatography (silica; n-hexane/DCM, 5/1) to obtain
EXPERIMENTAL SECTION
■
All commercially available reagents were used as received. Compound
10 was prepared following a literature procedure36 (yield 16% over 5
steps), and compound 9b was prepared by following literature
procedures34,35 (54%, over two steps), and compound 3a was
prepared following literature procedures19,21 (30% over two steps).
Air- and/or water-sensitive reactions were carried under N2 in oven-
dried glassware. Dry solvents (hexane, toluene, and dichlormethane)
1
1.08 g (51%) bright yellow solid. Mp 113.5 − 114.6 °C; H NMR
(400 MHz, CDCl3) δ 3.53 (s, 8H), 4.27 (m, 2H), 6.84−6.90 (m, 6H);
3540
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Note
13C{1H} NMR (100 MHz, CDCl3) δ 32.8, 54.0, 116.8 (t), 130.6,
(400 MHz, CDCl3) δ 7.33 (s, 2H), 7.36−7.40 (m 4H), 7.51 (ddd,
2H), 7.95 (m, 6H), 8.41 (ddd, 2H), 9.05 (ddd, 2H); 13C{1H} NMR
(100 MHz, CDCl3) δ 48.4, 123.5, 124.0, 124.6, 126.1, 127.8, 132.2,
137.2, 139.9, 145.2, 149.1, 153.2, 156.0; mass spectrum (EI) m/z M+
510 (90), 432 (100), 405 (28), 327 (17), 302 (5), 278 (85), 255 (25),
239 (15), 202 (10); HREI: M+• (C36H22N4) calcd 510.18444,
measured 510.18131.
139.5, 140.2, 19F{1H} NMR (400 MHz, CDCl3) δ −141.8. Mass
spectrum (EI) m/z M+ 380.1 (100), 350.1 (22), 337.1.4 (15), 319.1
(5), 251.1 (15), 239.1 (55), 214.1 (70), 203.2 (80), 177.1 (95), 164.0
(45), 151.0 (25), 140.1 (10). HREI C24H16F4 calcd m/z 380.118813,
measured m/z 380.11841.
2,3,9,10-Tetrafluoro-6,13-dihydo-6,13-ethenopentacene
(3c). One gram (2.63 mmol) of 13c, 1.92 g (7.89 mmol) of chloranil,
and 1.45 g of K2CO3 in 150 mL toluene were heated under reflux
overnight. The reaction mixture was washed three times with 20%
aqueous NaOH (3 × 100 mL) and two times with water (2 × 100
mL). After drying over MgSO4 the solvent was removed under
reduced pressure, and the residue was purified by column
chromatography (silica; n-hexane/DCM, 4/1). Total yield: 960 mg
ASSOCIATED CONTENT
■
S
* Supporting Information
1H and 13C NMR and mass spectra of new compounds. This
material is available free of charge via the Internet at http://
pubs.acs.org.
1
(96%). Mp 218.1 − 223.2 °C; H NMR (400 MHz, CDCl3) δ 5.29
AUTHOR INFORMATION
Corresponding Author
*E-mail: Holger.Bettinger@uni-tuebingen.de.
(m, 2H), 7.03 (m, 2H), 7.43 (tAxx′, J = 9 Hz, 4H), 7.63 (s, 4H);
13C{1H} NMR (100 MHz, CDCl3) δ 49.9, 113.4, 120.9, 128.4 (t),
138.3, 142.6, 148.6 - 151.3 (m), 19F{1H} NMR (400 MHz, CDCl3) δ
−138.0. Mass spectrum (EI) m/z M+ 376.2 (90), 375.1 (100), 357.1
(20), 214.1 (8), 187.1 (45), 178 (40), 162.8 (15). HREI C24H12F4
calcd m/z 376.087513, measured m/z 376.08776.
■
Notes
The authors declare no competing financial interest.
Pentacene (1c). Method B. A 100 mg (0.266 mmol) portion
of 3c in dipentylether (10 mL) was brought to reflux, and 52.7
mg (0.266 mmol) of 3,6-di(carboxylic acid methyl ester)-
tetrazine (2b) was added. The red mixture was heated under
reflux for 1.5 min. The reaction was stopped by quickly
immersing the reaction flask into an ice bath (0 °C). The blue
solid was collected by filtration and washed with DCM (20
ACKNOWLEDGMENTS
■
This work was supported by DFG Fonds der Chemischen
Industrie. We thank Prof. Dr. K.-P. Zeller for helpful discussion
concerning the mass spectrum of acene 1b.
REFERENCES
1
■
mL) and n-hexane (20 mL). Yield: 12.1 mg (13%) H NMR
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4H), 8.61 (s, 4H), 8.91 (s, 2H). C{1H} NMR (126 MHz, d2-
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The Journal of Organic Chemistry
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