Synthesis and photophysical properties of expanded dehydrobenzoannulenoannulene trefoils

Article abstract of DOI:10.1021/ol101542e

A sequential Sonogashira cross-coupling/Pd-mediated oxidative homocoupling strategy affords two-dimensional dehydrobenzoannulene trefoils containing different sizes of the central annulenic ring system. Use of these conditions instead of Cu-mediated homocoupling conditions yields a structural isomer possessing a triphenylene ([6]annulene) core. Noticeable differences in the absorption and emission spectra are observed depending upon the core unit.

Full text of DOI:10.1021/ol101542e

ORGANIC
LETTERS
2010
Vol. 12, No. 17
3824-3827
Synthesis and Photophysical
Properties of Expanded
Dehydrobenzoannulenoannulene Trefoils
Takashi Takeda, Aaron G. Fix, and Michael M. Haley*
Department of Chemistry and Materials Science Institute, UniVersity of Oregon,
Eugene, Oregon 97403-1253
haley@uoregon.edu
Received July 5, 2010
ABSTRACT
A sequential Sonogashira cross-coupling/Pd-mediated oxidative homocoupling strategy affords two-dimensional dehydrobenzoannulene trefoils
containing different sizes of the central annulenic ring system. Use of these conditions instead of Cu-mediated homocoupling conditions
yields a structural isomer possessing a triphenylene ([6]annulene) core. Noticeable differences in the absorption and emission spectra are
observed depending upon the core unit.
Highly conjugated carbon-rich compounds1 continue to
receive considerable attention not only because of the
fundamental science aspects2 (e.g., aromaticity/antiaroma-
ticity) of these aesthetically appealing compounds but also
due to the interesting chemical and physical properties that
these π-electron-rich species possess.3 Our studies have
focused on the synthesis and optoelectronic attributes of a
particular subset of carbon-rich compounds named dehy-
drobenzoannulenes (DBAs).4 We and others have shown that
DBAs exhibit a myriad of technologically relevant properties
including nonlinear optical activity,5 two-photon absorption,6
and liquid crystalline behavior,7 among many others.8 More
recent work has turned to the investigation of increasingly
larger and synthetically more complex systems (e.g., trefoils)
(4) (a) Marsden, J. A.; Palmer, G. J.; Haley, M. M. Eur. J. Org. Chem.
2003, 2355–2369. (b) Hisaki, I.; Sonoda, M.; Tobe, Y. Eur. J. Org. Chem.
2006, 833–847. (c) Haley, M. M. Pure Appl. Chem. 2008, 80, 519–532.
(5) Sarkar, A.; Pak, J. J.; Rayfield, G. W.; Haley, M. M. J. Mater. Chem.
2001, 11, 2943–2945.
(1) (a) Topics in Current Chemistry (Carbon Rich Compounds I); de
Meijere, A., Ed.; Springer-Verlag: Berlin, 1998; Vol. 196. (b) Topics in
Current Chemistry (Carbon Rich Compounds II); de Meijere, A., Ed.;
Springer-Verlag: Berlin, 1999; Vol. 201. (c) Acetylene Chemistry: Chemistry,
Biology, and Material Science; Diederich, F., Stang, P. J., Tykwinski, R. R.,
Eds.; Wiley-VCH: Weinheim, 2005. (d) Carbon-Rich Compounds: From
Molecules to Materials; Haley, M. M., Tykwinski, R. R., Eds.; Wiley-VCH:
Weinheim, 2006.
(6) (a) Bhaskar, A.; Guda, R.; Haley, M. M.; Goodson, T., III. J. Am.
Chem. Soc. 2006, 128, 13972–13973. (b) Slepkov, A. D.; Hegmann, F. A.;
Tykwinski, R. R.; Kamada, K.; Ohta, K.; Marsden, J. A.; Spitler, E. L.;
Miller, J. J.; Haley, M. M. Opt. Lett. 2006, 31, 3315–3317. (c) Kamada,
K.; Antonov, L.; Yamada, S.; Ohta, K.; Yoshimura, T.; Tahara, K.; Inaba,
A.; Sonoda, M.; Tobe, Y. ChemPhysChem 2007, 8, 2671–2677. (d) Guzman,
(2) Hopf, H. Classics in Hydrocarbon Chemistry; Wiley-VCH:
Weinheim, 2000; pp 197-249.
(3) (a) Functional Organic Materials; Mu¨ller, T. J. J., Bunz, U. H. F.,
Eds.; Wiley-VCH: Weinheim, 2007. (b) Organic Light Emitting DeVices:
Synthesis, Properties and Applications; Müllen, K., Scherf, U., Eds.; Wiley-
VCH: Weinheim, 2006.
¨
A. R.; Harpham, M. R.; Su¨zer, O.; Haley, M. M.; Goodson III, T. J. Am.
Chem. Soc. 2010, 132, 7840–7841.
(7) (a) Zhang, D.; Tessier, C. A.; Youngs, W. J. Chem. Mater. 1999,
11, 3050–3057. (b) Seo, S. H.; Jones, T. V.; Seyler, H.; Peters, J. O.; Kim,
T. H.; Chang, J. Y.; Tew, G. N. J. Am. Chem. Soc. 2006, 128, 9264–9265.
10.1021/ol101542e  2010 American Chemical Society
Published on Web 08/03/2010

Figure 1. Target dehydrobenzoannulene trefoils 1-4.
containing multiple fused DBAs.9 In a vast majority of these
molecules, the annulene rings have been the same size,
typically 12-membered9b,d-f or 18-membered rings.9a,c We
were curious to learn what effect, if any, that variation of
the ring size of the central arene in a series of expanded
molecular trefoils might have. To that end, we report herein
the synthesis and optoelectronic properties of trefoil-shaped
DBAs 1-3 possessing [6]-, [12]-, and [18]annulenes,
respectively, as the central fused ring (Figure 1). In addition,
we also present trefoil 4, a structural isomer of 1, prepared
from a common intermediate via an alternative homocoupling
procedure.
Scheme 1. Synthesis of Triphenylene Core Trefoils 1 and 4
Given prior problems of expanded DBAs with poor
molecule solubility,4a,9 we designed our syntheses such that
1-4 incorporated 12 decyl units. Starting from 5,9e Sono-
gashira cross-coupling of (triisopropylsilyl)acetylene (TIPSA)
afforded differentially protected diyne 6 (Scheme 1). Pro-
tiodesilylation and a second Sonogashira reaction with
hexabromotriphenylene10 gave dodecayne 8 in a reasonable
ca. 50% yield for the two steps. Molecular modeling
suggested that once desilylated, the terminal alkynes were
sufficiently close to one another that either trefoil 1,
(8) Recent examples, inter alia: (a) Tahara, K.; Johnson, C. A., II; Fujita,
T.; Sonoda, M.; De Schryver, F. C.; De Feyter, S.; Haley, M. M.; Tobe, Y.
Langmuir 2007, 23, 10190–10197. (b) Hisaki, I.; Sakamoto, Y.; Shigemitsu,
H.; Tohnai, N.; Miyata, M.; Seki, S.; Saeki, A.; Tagawa, S. Chem.sEur. J.
2008, 14, 4178–4187. (c) Lei, S.; Tahara, K.; De Schryver, F. C.; Van der
Auweraer, M.; Tobe, Y.; De Feyter, S. Angew. Chem., Int. Ed. 2008, 47,
2964–2968. (d) Tahara, K.; Fujita, T.; Sonoda, M.; Shiro, M.; Tobe, Y.
J. Am. Chem. Soc. 2008, 130, 14339–14345. (e) Hisaki, I.; Shigemitsu, H.;
Sakamoto, Y.; Hasegawa, Y.; Okajima, Y.; Nakano, K.; Tohnai, N.; Miyata,
M. Angew. Chem., Int. Ed. 2009, 48, 5465–5469. (f) Tahara, K.; Okuhata,
S.; Adisoejoso, J.; Lei, S.; Fujita, T.; De Feyter, S.; Tobe, Y. J. Am. Chem.
Soc. 2009, 131, 17583–17590.
possessing three 14-membered DBA rings, or 4, containing
three 18-membered rings, could form during the homocou-
pling reaction. We recently reported reaction conditions that
made it possible to discriminate between annulene ring size,
depending upon ring strain in the organometallic intermediate
prior to reductive elimination.11 Gratifyingly, desilylation of
8 with TBAF and subsequent treatment with catalytic
PdCl2(dppe) and stoichiometric I2 as oxidant furnished a
single product. Conversely, use of excess Cu(OAc)2 in the
(9) (a) Wan, W. B.; Brand, S. C.; Pak, J. J.; Haley, M. M. Chem.sEur.
J. 2000, 6, 2044–2052. (b) Bunz, U. H. F. J. Organomet. Chem. 2003,
683, 269–287. (c) Marsden, J. A.; Haley, M. M. J. Org. Chem. 2005, 70,
10213–10226. (d) Yoshimura, T.; Inaba, A.; Sonoda, M.; Tahara, K.; Tobe,
Y.; Williams, R. V. Org. Lett. 2006, 8, 2933–2936. (e) Johnson, C. A., II;
Lu, Y.; Haley, M. M. Org. Lett. 2007, 9, 3725–3728. (f) Tahara, K.;
Yoshimura, T.; Ohno, M.; Sonoda, M.; Tobe, Y. Chem. Lett. 2007, 36,
838–839.
(11) (a) Marsden, J. A.; Miller, J. J.; Haley, M. M. Angew. Chem., Int.
Ed. 2004, 43, 1694–1697. (b) Marsden, J. A.; Miller, J. J.; Shirtcliff, L. D.;
Haley, M. M. J. Am. Chem. Soc. 2005, 127, 2464–2476.
(10) Breslow, R.; Jaun, B.; Kluttz, R. Q.; Xia, C.-Z. Tetrahedron 1982,
38, 863–867.
Org. Lett., Vol. 12, No. 17, 2010
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homocoupling step afforded a sole yet different compound.
We have provisionally assigned these as trefoils 1 and 4,
respectively, based on the upfield NMR chemical shift of
the triphenylene protons of 4 (δ 9.20 for 1 vs 8.29 for 4,
starting from 8.72 for 8) which lie in the shielding cone of
the aromatic 18-membered π-system,12 as well as based on
the aforementioned “organometallic intermediate” argument.
Simply put, the cis-oriented Pd(dppe) intermediate that could
possibly lead to the 18-membered ring is too highly strained
to form, whereas the analogous “trans-like” dimeric Cu
species leading to 4 is relatively strain-free. The reverse
argument applies for 1: the cis-oriented Pd(dppe) intermedi-
ate that leads to the 14-membered ring is relatively strain-
free, whereas a “trans-like” dimeric Cu-species leading to 1
is highly strained.11
cyclotrimerization using the conditions of Iyoda14 fur-
nished precursor 12 in 39% yield. Removal of the six TIPS
groups by TBAF followed by Pd-mediated homocoupling
under pseudohigh dilution conditions gave 2 in a respect-
able 69% yield.
Starting again with triazene 9, sequential Sonogashira
reactions with 2 equiv of 7 and then excess TIPSA provided
13 (Scheme 3). Triazene conversion as before and then cross-
Scheme 3. Synthesis of [18]DBA Core Trefoil 3
Preparation of trefoil 2 began with known triazene 9
(Scheme 2).9c Sequential Sonogashira reactions with 2
Scheme 2. Synthesis of [12]DBA Core Trefoil 2
coupling with (trimethylsilyl)butadiyne gave heptyne 14.
Selective protiodesilylation with K2CO3 in MeOH/THF
followed by Sonogashira reaction with another 1 equiv of 9
afforded 15. While triazene iodide conversion with HI/I2
occurred in similar fashion, subsequent cross-coupling with
excess 7 proceeded poorly such that the overall yield of the
equiv of 7 and then excess TMSA provided triazene 10,
which was followed by triazene conversion to iodoarene
11 using HI/I2 at rt.13 Protiodesilylation and subsequent
(12) A model system containing a single 18-membered subunit of 4
exhibited an analogous upfield shift of the intra-annular protons: Jones, C. S.
Postdoctoral Report, University of Oregon, 2005.
(14) Iyoda, M.; Sirinintasak, S.; Nishiyama, Y.; Vorasingha, A.; Sultana,
F.; Nakao, K.; Kuwatani, Y.; Matsuyama, H.; Yoshida, M.; Miyake, Y.
Synthesis 2004, 1527–1531.
(13) Barbero, M.; Degani, I.; Diulgheroff, N.; Dughera, S.; Fochi, R.
Synthesis 2001, 2180–2190.
3826
Org. Lett., Vol. 12, No. 17, 2010

two steps furnished precursor 16 in no more than 12%.
Nonetheless, a final desilylation with TBAF and subsequent
Pd-mediated homocoupling gave 3.
the same low energy band at 436 nm. Closer inspection
of 2 (as well as precursor 12) reveals the characteristic
vibronic spectral pattern of the antiaromatic [12]DBA
core,9e,16 with discernible shoulders at 450 and 470 nm, a
spectral feature lacking in 3.
The most notable differences arising from variation of core
size are depicted in the emission spectra in Figure 2 (bottom)
and Table 1. Trefoils 1, 3, and 4 possess maxima roughly
As desired, all four trefoils possessed sufficient solubility
such that they could be easily purified by column chroma-
tography and their complete spectral data secured. Other than
the aforementioned triphenylene proton shifts, NMR analysis
of 1-4 did not provide sufficient detail about core size
differences. The periphery arene protons of the trefoils did
exhibit lower field shifts compared with the corresponding
precursors (∆δ 0.05-0.15 ppm), typical of the weak aromatic
character of the fused 14 and/or 18 π-electron systems.15
Similar to our previous results of purely hydrocarbon
expanded DBAs, no evidence for self-association in solution
was observed.
Table 1. Quantum Yields of Trefoils 1-4 and Their Precursors
8, 12, and 16
compd
quantum yielda (%)
stokes shift (nm/cm-1
)
1
2
3
4
8
12
16
15
2.7
16
43/2430
66/3020
22/1100
40/2200
48/3010
93/4510
21/1130
More instructive are the electronic absorption and emission
spectra of trefoils 1-4. As shown in Figure 2 (top), 2-4
16
9.1
9.9
25
a Measured in CH2Cl2 and determined by comparison 9,10-diphenyl-
anthracene as reference.
20 nm red-shifted compared to their precusor molecules,
emitting in the general range of 400-550 nm. In contrast,
trefoil 2 and precursor 12 emit at lower energy (475-625
nm), with the spectra exhibiting the vibrational splitting often
observed for [12]DBA derivatives.9d,e While the additional
alkyne units in 3 afford a slightly longer linear conjugation
pathway compared to 2, the ca. 90 nm lower energy λem
maximum of 2 suggests that the longest conjugation length
does not dictate the fluoresence properties of these trefoil-
shaped hydrocarbons; instead, it is the intrinsic spectral
features of the particular core unit that predominate.
In summary, we have synthesized a series of expanded
annulene derivatives that possess different sizes of the central
ring system. Optoelectronic measurements clearly show that
the absorption and emission properties of the trefoil-shaped
molecules can vary significantly depending on use of an
aromatic (1, 3, 4) or antiaromatic (2) annulene core. Further
studies on additional expanded annulene derivatives will be
reported in due course.
Acknowledgment. We thank the National Science Foun-
dation (CHE-0718242 and 1013032) for financial support.
T.T. gratefully acknowledges the JSPS for a Fellowship for
Young Scientists (18-4425). We also thank the NSF for
support in the form of instrumentation grants (CHE-0639170
and CHE-0923589).
Figure 2. Electronic absorption (top) and fluorescence (bottom)
spectra of trefoils 1-4 and their precursors 8, 12, and 16.
Supporting Information Available: Experimental details
and spectroscopic data for all new compounds. This
material is available free of charge via the Internet at
http://pubs.acs.org.
red-shift ca. 30 nm to lower energy compared to their
acyclic precursors, whereas the shift of 1 is more modest
(ca. 10 nm). Interestingly, trefoils 2 and 3 appear to have
OL101542E
(15) Bell, M. L.; Chiechi, R. C.; Johnson, C. A.; Kimball, D. B.; Matzger,
A. J.; Wan, W. B.; Weakley, T. J. R.; Haley, M. M. Tetrahedron 2001, 57,
3507–3520.
(16) (a) Staab, H. A.; Graf, F. Tetrahedron Lett. 1966, 751–757. (b)
Wirz, J. In Excited States in Organic Chemistry and Biology; Pullman, B.,
Goldblum, N., Eds.; D. Reidal Publishing Co.: Dordrecht, 1977; p 283.
Org. Lett., Vol. 12, No. 17, 2010
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