Three-step synthesis of end-substituted pentacenes

Article abstract of DOI:10.1021/jo7017284

(Chemical Equation Presented) A concise, three-step synthesis of 1,4,8,11-substituted 2,3,9,-10-tetrakis(methoxycarbonyl)pentacenes from commercially available 1,2,4,5-tetrakis(bromomethyl)benzene was established. Efficient alkynylation, followed by formation of four fused rings via a zirconacyclopentadiene intermediate, and then oxidation with DDQ gave pentacenes 1a-c. The process was compatible with methyl, phenyl, and trimethylsilyl substituents, which have good solubility in various organic solvents.

Full text of DOI:10.1021/jo7017284

electronics and would allow the evaluation of theoretical studies
of their properties.11-14 Our previous investigation of a â-cy-
clodextrin anthracene rotaxane supported this concept as the
insulated acene moiety was significantly more resistant to
fluorescence quenching and photobleaching than the uninsulated
analog.10 However, to extend this approach to longer acenes,
we were presented with the challenge of synthesizing a
pentacene with sterically bulky end groups.
Three-Step Synthesis of End-Substituted
Pentacenes
Matthew T. Stone* and Harry L. Anderson
Department of Chemistry, Chemistry Research
Laboratory, UniVersity of Oxford, 12 Mansfield Road,
Oxford OX1 3TA, U.K.
While a number of pentacene derivatives have been isolated,
relatively few synthetic approaches have been established to
access such molecules. The most commonly employed strategies
proceed via the reduction of a pentacenequinone precursor which
can be used to synthesize a variety of 6,13-substituted penta-
cenes.15,16 However, using this methodology to install substit-
uents at other positions can require a number of additional steps
in order to access the substituted pentacenequinone precur-
sor.8,17,18 An alternative homologation approach was reported
by Takahashi and co-workers that allowed the synthesis of
highly substituted acenes; however, this methodology required
eight steps to synthesize pentacenes.19,20 Building on Takahashi’s
homologation approach, here we report a symmetric route to
synthesize end-substituted pentacenes in only three steps. We
demonstrated its application to the synthesis of end-function-
alized pentacenes 1a-c.
matthew.stone@chem.ox.ac.uk
ReceiVed August 6, 2007
A concise, three-step synthesis of 1,4,8,11-substituted 2,3,9,-
10-tetrakis(methoxycarbonyl)pentacenes from commercially
available 1,2,4,5-tetrakis(bromomethyl)benzene was estab-
lished. Efficient alkynylation, followed by formation of four
fused rings via a zirconacyclopentadiene intermediate, and
then oxidation with DDQ gave pentacenes 1a-c. The process
was compatible with methyl, phenyl, and trimethylsilyl
substituents, which have good solubility in various organic
solvents.
Pentacenes represent a promising class of molecules for
application as organic semiconductors and have been shown to
have excellent mobilities in thin film transistors.1-5 However,
unsubstituted pentacene is difficult to work with due to its lack
of stability and solubility. To address these problems and control
organization in the solid state, a number of pentacene derivatives
have been synthesized and characterized.6-9 Toward the same
end, we have begun to explore a supramolecular strategy to
control the properties of acenes by threading them inside
macrocycles to form rotaxanes.10 The advantage of this approach
is that it should permit the isolation of longer and less stable
acene systems that have potential applications in organic
Synthesis of end-substituted pentacenes 1a-c proceeded from
commercially available 1,2,4,5-tetrakis(bromomethyl)benzene
2 by reaction with various alkynes to give tetraalkynes 3a-c
(Scheme 1). All attempts at reacting 2 with alkynyllithium
compounds gave an unidentified mixture of products, which
might be attributed to the lithiation of the relatively acidic
benzylic hydrogens generated after displacement of the bromide.
Successful synthesis of tetraalkynes 3a-c was achieved by
reaction with the appropriate alkynylmagnesium bromides in
the presence of copper(I) bromide as described by Gopalsam-
uthiram and Wulff for coupling of trimethylsilylacetylene.21
* To whom correspondence should be addressed. Tel: +44 01865 275983.
Fax: +44 01865 285002.
(1) Bendikov, M.; Wudl, F.; Perepichka, D. F. Chem. ReV. 2004, 104,
4891-4945.
(11) Kivelson, S.; Chapman, O. L. Phys. ReV. B. 1983, 28, 7236-7242.
(12) Lowe, J. P.; Kafafi, S. A. J. Phys. Chem. 1986, 90, 6602-6610.
(13) Houk, K. N.; Lee, P. S.; Nendel, M. J. Org. Chem. 2001, 66, 5517-
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1736.
(16) Vets, N.; Smet, M.; Dehaen, W. Synlett 2005, 217-222.
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A. C.; Malliaras, G. G. Org. Lett. 2005, 7, 3163-3166.
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71, 2155-2158.
(19) Takahashi, T.; Kitamura, M.; Shen, B.; Nakajima, K. J. Am. Chem.
Soc. 2000, 122, 12876-12877.
(2) Anthony, J. E. Chem. ReV. 2006, 106, 5028-5048.
(3) Payne, M. M.; Parkin, S. R.; Anthony, J. E.; C., K. C.; Jackson, T.
N. J. Am. Chem. Soc. 2005, 127, 4986-4987.
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(5) Sheraw, C. D.; Jackson, T. N.; Eaton, D. L.; Anthony, J. E. AdV.
Mater. 2003, 15, 2009-2011.
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Steigerwald, M. L.; Nuckolls, C. J. Am. Chem. Soc. 2006, 128, 1340-
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Lett. 2004, 6, 1609-1612.
(20) Takahashi, T.; Li, S.; Huang, W. Y.; Kong, F. Z.; Nakajima, K.;
Shen, B. J.; Ohe, T.; Kanno, K. J. Org. Chem. 2006, 71, 7967-7977.
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(10) Stone, M. T.; Anderson, H. L. Chem. Commun. 2007, 2387-2389.
10.1021/jo7017284 CCC: $37.00 © 2007 American Chemical Society
Published on Web 11/14/2007
9776
J. Org. Chem. 2007, 72, 9776-9778

SCHEME 1. Synthesis of Pentacenes 1a-ca
a Modified conditions were used for oxidation to give 1c.
SCHEME 2. Modified Conditions for Oxidation To Give 1c23
These conditions were also found to be compatible with methyl-
and phenyl-substituted acetylenes. Full conversion was crucial
to isolation of the pure compounds since incompletely coupled
byproducts were impractical to separate.
solvents. Consistent with previously reported pentacene deriva-
tives, 1a-c were photosensitive, and thus, reactions were
performed under nitrogen and shielded from light.4,24-26 The
compounds were purified and characterized open to the air in
the dark and showed no evidence of degradation under these
conditions.
Tetraalkynes 3a-c were then converted to 5,7,12,14-tetrahy-
dropentacenes 4a-c providing four of the fused rings of the
pentacene skeleton. First, tetraalkynes 3a-c were treated with
dibutylzirconocene (Negishi’s reagent), generated from zir-
conocene dichloride and n-butyllithium, to afford two zircona-
cyclopentadienes that formed the second and fourth ring of the
pentacene skeleton. The first and fifth rings of the pentacene
skeleton were formed by reacting the zirconacyclopentadiene
compound in situ with copper(I) chloride and dimethyl-
acetylenedicarboxylate acid (DMAD).19,20 The yield of the
reaction was highly dependent on the quality of zirconocene
dichloride; exposure to moisture in the air was found to
deactivate the reagent.
Tetrahydropentacenes 4a-c were then transformed into
pentacenes 1a-c by reacting with 2,3-dicyano-5,6-dichloropa-
rabenzoquinone (DDQ) at 100 °C in toluene. DDQ (2 equiv)
was used for tetramethyl-substituted pentacene 1a and tetraphe-
nyl-substituted pentacene 1b, and any incompletely oxidized
byproducts were separated by precipitation with methanol.
However, trimethylsilyl-substituted pentacene 1c was relatively
soluble even in methanol and was difficult to separate from
incompletely oxidized byproducts by chromatography. This
problem was addressed by reacting 4c with an excess of DDQ
(5 equiv) to eliminate incompletely oxidized byproducts.
However, under these conditions, pentacene 1c then underwent
cycloaddition with the excess DDQ to give Diels-Alder adduct
5 (Scheme 2). To recover the pentacene, Diels-Alder adduct 5
was treated with 5 equiv of 1,3-cyclohexadiene at 100 °C in
toluene to drive the retro-Diels-Alder reaction.22 The evolution
of pentacene 1c after addition of the 1,3-cyclohexadiene was
apparent as the solution developed a deep indigo color.
Pentacenes 1a-c are deep blue powders when precipitated
from solution and have good solubility in a variety of organic
The photophysical properties of compounds 1a-c are con-
sistent with pentacene and it derivatives.4,6,9,27,28 However, the
substituents cause some differences between the colors of the
three pentacene derivatives. This was noticeable to the eye, as
in solution pentacenes 1b and 1c were both indigo whereas
tetramethyl derivative 1a was red-violet. The absorbance spectra
are consistent with these observations, as pentacenes 1b and
1c have nearly identical absorbance maxima at 600 and 602
nm, respectively, while the absorbance maximum of 1a is
hypsochromically shifted 20 nm to 580 nm (Figure 1, Table
1). The same hypsochromic shift was observed in the fluores-
cence spectra where methyl-substituted pentacene 1a was also
about 20 nm less than 1b and 30 nm less than 1c. Since all
three substituents are only weakly electron-donating/withdraw-
ing groups, the shift in the spectra might be attributed to their
steric influence on the carboxylate groups. The larger phenyl
and trimethylsilyl substituents act to force the carboxylates
further out of conjugation than the smaller methyl groups.
(22) A thermally driven retro-Diels-Alder reaction to produce substituted
pentacenes from thin films of the corresponding p-chloranil pentacence
adduct is reported in the patent: Yoshitoku, N.; Kazuto, O. (Asahi Kasei
Co.). Jpn. Kokai Tokkyo Koho 2005232136, 2005.
(23) Regioselectivity of Diels-Alder adduct 5 was assumed on the basis
of ref 24.
(24) Zhou, X.; Kitamura, M.; Shen, B. J.; Nakajima, K.; Takahashi, T.
Chem. Lett. 2004, 33, 410-411.
(25) Chan, S. H.; Lee, H. K.; Wang, Y. M.; Fu, N. Y.; Chen, X. M.;
Caid, Z. W.; Wong, H. N. C. Chem. Commun. 2005, 66-68.
(26) Reichwagen, J.; Hopf, H.; Desvergne, J. P.; Del Guerzo, A.; Bouas-
Laurent, H. Synthesis 2005, 3505-3507.
(27) Herwig, P. T.; Mullen, K. AdV. Mater. 1999, 11, 480-+.
(28) Weidkamp, K. P.; Afzali, A.; Tromp, R. M.; Hamers, R. J. J. Am.
Chem. Soc. 2004, 126, 12740-12741.
J. Org. Chem, Vol. 72, No. 25, 2007 9777

filtered, and concentrated in vacuo. The crude product was purified
by silica gel column chromatography (ethyl acetate/dichlo-
romethane, 3/7) to give 433 mg (0.833 mmol, 58%) of pure 3b as
1
a white solid: mp 162-163 °C; H NMR (500 MHz, CDCl3) δ
7.74 (s, 2H), 7.40 (dd, J ) 8.0, 1.5, 8H), 7.24 (s, 12H), 3.92 (s,
8H); 13C NMR (125 MHz, CDCl3) δ 133.5, 131.7, 129.5, 128.2,
127.8, 123.5, 86.7, 83.2, 23.3; HRMS (EI) m/z [M]+ calcd for
C42H30 534.2347, found 534.2327. Anal. Calcd for C42H30: C,
94.34; H, 5.66. Found: C, 94.33; H, 5.68.
Preparation of 1,4,8,11-Tetraphenyl-2,3,9,10-tetrakis(methox-
ycarbonyl)-5,7,12,14-tetrahydropentacene (4b). To a 25 mL
round-bottom flask outfitted with a side arm adapter was added
zirconocene dichloride (377 mg, 0.44 mmol) and then tetrahydro-
furan (9 mL). The flask was evacuated and filled with nitrogen
(3×) and then cooled to -76 °C (dry ice/2-propanol). Then a
solution of n-butyllithium (1.7 mL, 1.6 M) in hexane was added,
and the reaction mixture was allowed to stir for 1 h. A solution of
3b (292 mg, 0.55 mmol) in tetrahydrofuran (12 mL) was added
via cannula, and the solution was allowed to warm to room
temperature and react for 3 h. Then dimethyl acetylenedicarboxylate
(0.55 mL, 4.4 mmol) was added followed quickly by copper(I)
chloride (340 mg, 3.4 mmol). The reaction mixture was then heated
to 40 °C for 18 h and then quenched with acetic acid (1.5 mL).
The precipitated product was collected by centrifugation and washed
with ether (3×) to remove the more soluble byproduct formed by
trimerization of dimethyl acetylenedicarboxylate. The solid was
dissolved in chloroform and passed through a plug of silica gel
eluting with ethyl acetate/chloroform (2/3) to give 279 mg (0.341
mmol, 62%) of pure 4b as a white solid: mp 309 °C dec; 1H NMR
(400 MHz, CDCl3) δ 7.41 (m, 12H), 7.23 (d, J ) 7.9, 8H), 6.84
(s, 2H), 3.63 (s, 8H), 3.43 (s, 12H); 13C NMR (100 MHz, CDCl3)
δ 168.5, 138.3, 138.2, 138.1, 134.1, 130.4, 129.2, 128.2, 127.6,
125.8, 52.0, 34.0; MS (ESI) m/z [M + Na]+ calcd for C54H42NaO8
841.2772, found 841.2776. Anal. Calcd C54H42O8: C, 79.20; H,
5.17. Found: C, 79.14; H, 5.19.
FIGURE 1. UV (a) and fluorescence emission (b) spectra of
pentacenes 1a (bold red), 1b (blue), and 1c (dotted black) in dichlo-
romethane at 120 µM and 6 µM, respectively. Fluorescence spectra
were excited at 500 nm.
TABLE 1. Spectroscopic Data for Compounds 1a-c
compd
R
π
maxabs (nm)
π
maxem (nm)
1a
1b
1c
Me
Ph
TMS
580
600
602
594
615
625
In conclusion, a concise three-step synthesis of end-substituted
pentacenes has been established. Efficient conditions were
determined to give complete alkynylation of 1,2,4,5-tetrakis-
(bromomethyl)benzene. Subsequently, a powerfully simplifying
transform utilizing Negishi’s reagent was used to form the four
remaining fused rings of the pentacene skeleton. Oxidation
afforded pentacenes with good solubility in various organic
solvents. The scheme is compatible with methyl, phenyl, and
trimethsilyl groups, although further optimization is necessary
to obtain the larger quantities required for rotaxane synthesis.
Our next goal will be to identify clipping reactions that will
allow the synthesis of rotaxanes from these end-substituted
pentacenes. We are particularly interested in aromatic macro-
cycles that would facilitate charge transfer in the solid state.
Preparation of 1,4,8,11-Tetraphenyl-2,3,9,10-tetrakis(methox-
ycarbonyl)pentacene (1b). To a 50 mL round-bottom flask
outfitted with a sidearm adapter were added 4b (100 mg, 0.12
mmol) and 2,3-dichloro-5,6-dicyano-p-benzoquinone (55 mg, 0.24
mmol). The flask was evacuated and backfilled with nitrogen (3×),
and then toluene (25 mL) was added. The solution was then
degassed (3×), and the flask was heated to 100 °C for 18 h. After
the solution was cooled to room temperature, the solvent was
removed in vacuo. The residue was dissolved in a minimum amount
of dichloromethane and then precipitated with methanol. The
product was recovered by centrifugation and washed with methanol
(3×). The crude product was purified by silica gel column
chromatography (dichloromethane) to give 32 mg (0.039 mmol,
1
32%) of pure 1b as a dark blue solid: mp 330 °C dec; H NMR
(400 MHz, CDCl3) δ 8.62 (s, 2H), 8.34 (s, 4H), 7.55-7.50 (m,
12H), 7.45-7.43 (m, 8H), 3.52 (s, 12H); 13C NMR (100 MHz,
CDCl3) δ 186.6, 140.0, 137.5, 130.7, 130.3, 129.9, 128.5, 128.2,
128.14, 128.08, 127.6, 52.1; MS (ESI) m/z [M + Na]+ calcd for
C54H38NaO8 837.2464, found 837.2459.
Experimental Section
Preparation of 1,2,4,5-Tetrakis(3-phenyl-2-propynyl)benzene
(3b). To a 100 mL two-neck round-bottom flask outfitted with a
reflux condenser were added tetrahydrofuran (5 mL) and pheny-
lacetylene (2.0 mL, 1.86 g, 18 mmol) under nitrogen. The flask
was subsequently cooled to 0 °C, and a solution of ethylmagnesium
bromide (17 mL, 1.0 M) in tetrahydrofuran was added via syringe.
The solution was allowed to react for 30 min and then allowed to
warm to room temperature for 30 min. Copper(I) bromide (65 mg,
0.45 mmol) was then added to the resulting slurry and allowed to
react for 30 min. Then 1,2,4,5-tetrakis(bromomethyl)benzene 2 (0.64
g, 1.4 mmol) was added, and the reaction mixture was refluxed for
72 h. After the mixture was cooled to room temperature, the reaction
was quenched with saturated aqueous ammonium chloride (100 mL)
and then extracted with ether (3 × 75 mL). The combined organic
layers were washed with brine (100 mL), dried with sodium sulfate,
Acknowledgment. We thank the National Science Founda-
tion for support under Grant No. CHE-0501170 through the
MPS Distinguished International Postdoctoral Research Fel-
lowship. We also thank the EPSRC for support.
Supporting Information Available: Experimental procedures
for 1a,c, 3a,c, and 4a,c and spectroscopic data (1H NMR and 13C
NMR) for all products. This material is available free of charge
via the Internet at http://pubs.acs.org.
JO7017284
9778 J. Org. Chem., Vol. 72, No. 25, 2007