Furan-Containing Oligoaryl Cyclophanene

Article abstract of DOI:10.1021/ol0356688

(Equation presented) Furan-containing oligoaryl cyclophanene 1 and the corresponding cyclophane 2 were synthesized from propargylic dithioacetal 3. The electrochemical and photophysical properties and the fluxional behavior of these molecules have been ex

Full text of DOI:10.1021/ol0356688

ORGANIC
LETTERS
2003
Vol. 5, No. 23
4381-4384
Furan-Containing Oligoaryl
Cyclophanene
Jui-Chang Tseng,†,‡ Shou-Ling Huang,† Cheng-Lan Lin,‡ Hsin-Chieh Lin,†,‡
,†,‡
Bih-Yaw Jin,† Chun-Yan Chen,† Jen-Kan Yu,† Pi-Tai Chou,† and Tien-Yau Luh*
Department of Chemistry and Institute of Polymer Science and Engineering,
National Taiwan UniVersity, Taipei 106, Taiwan, and Institute of Chemistry,
Academia Sinica, Nangang, Taipei 115, Taiwan
tyluh@chem.sinica.edu.tw
Received September 1, 2003
ABSTRACT
Furan-containing oligoaryl cyclophanene 1 and the corresponding cyclophane 2 were synthesized from propargylic dithioacetal 3. The
electrochemical and photophysical properties and the fluxional behavior of these molecules have been examined. The emission of 1 appeared
at 499 nm whereas that of 2 appeared at 389 nm.
Cyclophanes have provided useful models for the investiga-
tion of through-space interactions between chromophores.1-5
Several factors are known to dictate the nature of such
interactions. Recently, Bazan and co-workers have shown
that the conjugation length and the orientation of the
chromophores in paracyclophanes may play an important role
in this respect.1 Five-membered heteroaromatic ring(s) in
cyclophanes are well documented.2 These compounds may
demonstrate a unique structural feature because the stereo-
chemistry of these molecules is very different from those
constituted of only para-substituted benzene derivatives.2-4
It is noteworthy that oligoaryl cyclophanes whose backbone
contains five-membered heteroaromatic ring(s) have been
sporadically studied.2 Incorporation of double bond(s) in the
tethering aliphatic chain(s) such as cyclophanenes may alter
the geometry of the molecules.5 An extreme case can be seen
in planar and annulene-like porphycenes and related ana-
logues, although they can be considered structurally as
cyclophanes.4 Other than porphycene and related compounds,
study on the influence of the double bond in nonplanar
cyclophanenes on the photophysical properties of these
molecules has been rare.
† National Taiwan University.
‡ Academia Sinica.
(1) (a) Bazan, G. C.; Oldham, W. J., Jr.; Lachicotte, R. J.; Tretiak, S.;
Chernyak, V.; Mukamel, S. J. Am. Chem. Soc. 1998, 120, 9188 and
references therein. (b) Batholomew, G. P.; Bazan, G. C. J. Am. Chem. Soc.
2002, 124, 5183 and references therein. (c) Bartholomew G. P.; Ledoux I.;
Mukamel S.; Bazan G. C.; Zyss, J. J. Am. Chem. Soc. 2002, 124, 13480.
(2) (a) Kaikawa, T.; Takimiya, K.; Aso, Y.; Otsubo, T. Org. Lett. 2000,
2, 4197. (b) Vo¨gtle, F.; Staab, A. Chem. Ber. 1968, 101, 2709. (c) Hart,
H.; Rajakumar, P. Tetrahedron 1995, 51, 1313. (d) Vo¨gtle, F. Cyclophane
Chemistry; Wiley: New York, 1993.
(3) For examples, see: (a) Pahor, N. B.; Calligaris, M.; Randaccio, L.
J. Chem. Soc., Perkin Trans. 2 1978, 42. (b) Haley, J. F.; Rosenfeld, S.
M.; Keehn, P. M. J. Org. Chem. 1977, 42, 1379. (c) Cooke, M. P. J. Org.
Chem. 1981, 46, 1747.
(4) Vogel, E.; Jux, N.; Do¨rr, J.; Pelster, T.; Berg, T.; Bo¨hm, H.-S.;
Behrens, F.; Lex, J.; Bremm, D.; Hohlneicher, G. Angew. Chem., Int. Ed.
2000, 39, 1101 and references therein.
(5) (a) Cram, D. J.; Cram, J. M. Acc. Chem. Res. 1971, 4, 204. (b)
Mitchell, R. H.; Boekelheide, V. J. Am. Chem. Soc. 1974, 96, 1547 and
references therein. (c) Sto¨bbe, M.; Reiser, O.; Na¨der, R.; de Meijere, A.
Chem. Ber. 1987, 120, 1667.
We recently reported a new annulation procedure for the
synthesis of trisubstituted furans from the corresponding
propargylic dithioacetals.6 The strategy has been employed
for the iterative construction of a variety of soluble furan-
and pyrrole-containing oligoaryls leading to molecular wires.
We now wish to report the synthesis and unusual properties
10.1021/ol0356688 CCC: $25.00 © 2003 American Chemical Society
Published on Web 10/18/2003

Scheme 1a
a Key: (a) (1) BuLi, THF, -78 °C, 50 min, (2) 4,4′-diformylbibenzyl (5), THF, -78 °C, 1 h; rt, 30 min, (3) CF3CO2H, rt, 12 h, 65%;
(b) i-Bu2AlH, THF, 0 °C, 3 h, 94%; (c) PBr3, C6H6, rt, 6 h, 97%; (d) PhLi, THF, -78 °C, 2 h, 76%; (e) Na2S‚9H2O, EtOH, reflux, 12 h,
61%; (f) (1) i-Pr2NLi, THF, 0 °C, 1 h, (2) MeI, rt, 1 h, 94%; (g) (1) Me3O‚BF4, CH2Cl2, 0 °C, 2 h; rt, 12 h, (2) t-BuOK, THF, rt, 12 h, 51%;
(h) MnO2, CH2Cl2, rt, 6 h, 95%; (i) 1,2-ethanedithiol, BF3‚OEt2, CH2Cl2, rt, 12 h, 92%; (j) W(CO)6, PhCl, reflux, 48 h, 25%.
of the first furan-containing teraryl cyclophanene 1 and its
saturated analogue 2.
Treatment of allenyllithium 4, prepared from propargylic
dithioacetal 3 and BuLi, with dialdehyde 5 followed by
trifluoroacetic acid afforded 6 in 65% yield. Reduction of 6
with i-Bu2AlH gave 7 in 94% yield, which was further
converted into bromide 8 (PBr3) in 97% yield. Intramolecular
cyclization of 8 with Na2S‚9H2O was achieved in refluxing
EtOH under dilute conditions to give 9 in 61% yield.
Exposure of 9 to i-Pr2NLi at 0 °C followed by trapping the
intermediate with MeI afforded 10 in 94% yield.7 The
formation of the double bond was obtained by treating 10
1
Figure 1. Variable-temperature H NMR spectra of (a) 1 and (b) 2 in CD2Cl2.
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Org. Lett., Vol. 5, No. 23, 2003

with Meerwein reagent followed by t-BuOK, to give 1 in
51% yield (Scheme 1). Alternatively, 1 was obtained in 25%
yield from the W(CO)6-mediated intramolecular desulfur-
dimerization of bisdithioacetal 11 (Scheme 1).8 Compound
11 was synthesized in two steps in 87% yield from 7 by
MnO2 oxidation (giving 12) followed by 1,2-ethanedithiol
in the presence of BF3‚Et2O. A dilute THF solution of 8
(0.001 M) was allowed to react with PhLi at -78 °C to afford
2 in 76% yield (Scheme 1).
0.665 V were observed for 1, one at lower potential than
that of 2 whereas the other at higher potential. The presence
of the double bond might contribute to the interaction of the
two oligoaryl moieties in 1. It is noteworthy that the reference
compound teraryl 13 exhibited lower oxidation potential
(0.593 V) than that of 2. Presumably, the coplanarity of the
teraryl moiety in 13 may account for this observation.
The photophysical properties of 1 and 2 are compared with
those of 13 in Table 1. The absorption profiles for 1, 2, and
1
Dynamic H NMR spectra for 1 and 2 were examined,
coalescence of the signals attributed to the protons on the
benzene rings being observed (Figure 1). The chemical shifts
were assigned based on COSY and NOESY experiments at
ambient temperature and at -95 °C. It is noteworthy that
certain signals of fluxional protons in 1 and 2 were not
completely resolved even at -95 °C, and the signal due to
the absorption of the furan protons in 1 and 2 remained a
sharp singlet at -95 °C. For 1, the signal at δ 6.69 coalesced
at ca. -60 °C and appeared as two broad peaks at ca. δ 6.0
and 7.2. The fluxional barrier for 1 was thus estimated to be
9 kcal/mol.9 In a similar manner, the signal at δ 6.68 for 2
coalesced at ca -50 °C and appeared as two broad peaks at
ca. δ 6.0 and 7.2. The barrier was calculated to be 9.7 kcal/
mol.9
Table 1. Photophysical Properties of 1, 2, and 13
compd
λmax
10-4
ꢀ
λem
φ
1
2
13
321
314
324
5.3
5.5
6.2
372, 390, 416, 499
370, 389, 410
362, 379, 398
0.28
0.59
0.83
13 are similar (Figure 3). There was an additional weak
absorption in the region of 350 to ca. 430 nm for cyclo-
phanene 1. As shown in Figure 3, the emission spectrum
Cyclophane 2 exhibited reversible two-electron redox
process electrochemically and the oxidation potential for 2
appeared at 0.614 V with reference to the ferrocene/
ferrocenium ion (Figure 2).10 The two oligoaryl moieties in
Figure 3. Absorption and emission spectra of 1 (solid line), 2 (dash
line), and 13 (dotted line) in EtOAc.
Figure 2. Cyclic voltammograms of 1 (solid line), 2 (dash line),
and 13 (dotted line) in CH2Cl2 with 0.1 M Bu4NPF6 as supporting
electrolyte. A glassy carbon electrode, a Pt wire, and a Ag/Ag+
(0.1 M AgNO3) electrode were used as the working, counter, and
reference electrodes, respectively. The scan rate was 20 mV/s.
for 2 in EtOAc appeared at 389 nm with vibronic fine
structure. Similarly, compound 13 exhibited fluorescence at
379 nm with vibronic splittings. It is striking to note that
other than the weak emission in the same region, compound
1 showed a strong broad luminescence at 499 nm. In
addition, the quantum yields for 1 and 2 were somewhat
2 were oxidized independently and appeared at the same
potential. Two sequential oxidation potentials at 0.540 and
(7) Mitchell, R. H.; Otsubo, T.; Boekelheide, V. Tetrahedron Lett. 1975,
219.
(8) Yip, Y. C.; Wang, X.-j.; Ng, D. K. P.; Mak, T. C. W.; Chiang, P.;
Luh, T.-Y. J. Org. Chem. 1990, 55, 1881.
(6) (a) Lee, C.-F.; Yang, L.-M.; Hwu, T.-Y.; Feng, A.-S.; Tseng, J.-C.;
Luh, T.-Y. J. Am. Chem. Soc. 2000, 122, 4992. (b) Lee, C.-F.; Liu, C.-Y.;
Song, H.-C.; Luo, S.-J.; Tseng, J.-C.; Tso, H.-H.; Luh, T.-Y. Chem.
Commun. 2002, 2824. (c) Liu, C.-Y.; Luh, T.-Y. Org. Lett. 2002, 4, 4305.
(9) Gunther, H. NMR Spectroscopy, 2nd ed.; Wiley: Chichester, 1994.
Org. Lett., Vol. 5, No. 23, 2003
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lower than that of the model compound 13. Presumably, the
small blue shift of the absorption peak and the small red
shift of the emission peak of 2 relative to 13 can be
rationalized in terms of exciton theory.11 In other words, the
cyclophane structure may cause intramoelcular interaction
between the chromphores and the presence of a double bond
in 1 may further influence the photophysical properties.
In summary, we have depicted the first synthesis of furan-
containing oligoaryl cyclophanene 1 and cyclophane 2. The
double bond in 1 plays a critical role in dictating the
photophysical properties of 1 which is very much different
from those of the saturated counterpart 2.
Acknowledgment. We thank the National Science Coun-
cil and the Ministry of Education of the Republic of China
for support.
(10) Electron-transfer number, n, of 1 and 2 was determined by linear
potential sweep chronoamperometry (Bard, A. J.; Faulkner, L. R.; Elec-
trochemical Methods, 2nd ed.; Wiley: New York, 2001; pp 228-231). With
the peak potential, Ep, and the half-peak potential, Ep/2, determined by using
a low scan rate (3 mV/s), the electron-transfer number was evaluated. For
2, Ep was 0.510 V (vs Fc/Fc+) and Ep/2 was 0.483 V, and the calculated
electron-transfer number was 2.06. For 1, because there were two electron
transfer steps in series, the current responses of each oxidation step were
overlapped. Only the first electron-transfer number was estimated. The Ep
and Ep/2 were estimated to be 0.505 and 0.445 V (vs Fc/Fc+), and the
calculated electron-transfer number of 1 was 0.94.
Supporting Information Available: Experimental details
of the synthesis of 1 and 2, and COSY and NOESY of 1
and 2. This material is available free of charge via the Internet
at http://pubs.acs.org.
(11) Cornil, J.; Beljonne, D.; Calbert, J. P.; Bredas, J. L. AdV. Mater.
2001, 13, 1053.
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