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C. Klein et al. / Tetrahedron Letters 51 (2010) 6161–6165
Table 2
Characteristics of functionalized bipyridine derivatives 22a–f
Entry
Benzaldehyde
Product
R1
R2
R3
Yield (%)
1
2
3
4
5
6
21a
21b
21c
21d
21e
21f
22a
22b
22c
22d
22e
22f
H
H
H
OCH3
H
H
OC6H13
OC9H19
OC6H13
OC9H19
N(CH3)C6H13
OC6H13
85
87
79
78
87
78
OC6H13
H
H
H
–(CH@CH)2–
–(CH@CH)2–
2. Chen, C.-Y.; Wu, S.-J.; Wu, C.-G.; Chen, J.-G.; Ho, K.-C. Adv. Mater. 2007, 19,
3888.
3. Gao, F.; Wang, Y.; Zhang, J.; Shi, D.; Wang, M.; Humphry-Baker, R.; Wang, P.;
Zakeeruddin, S.-M.; Grätzel, M. Chem. Commun. 2008, 2635.
4. Choi, H.; Baik, C.; Kim, S.; Kang, M.-S.; Xu, X.; Kang, H. S.; Kang, S.-O.; Ko, J. J.;
Nazeeruddin, M. K.; Grätzel, M. New J. Chem. 2008, 32, 2233.
5. Bouder, T. L.; Massiot, P.; Le Bozec, H. Tetrahedron Lett. 1998, 39, 6869.
6. Bouder, T. L.; Viau, L.; Guegan, J.-P.; Maury, O.; Le Bozec, H. Eur. J. Org. Chem.
2002, 3024.
7. Maury, O.; Guégan, J.-P.; Renouard, T.; Hilton, A.; Dupau, P.; Sandon, N.; Toupet,
L.; Le Bozec, H. New J. Chem. 2001, 25, 1553.
8. Viau, L.; Maury, O.; Le Bozec, H. Tetrahedron Lett. 2004, 45, 125.
9. Aubert, V.; Guerchais, V.; Ishow, E.; Hoang-Thi, K.; Ledoux, I.; Nakatani, K.; Le
Bozec, H. Angew. Chem., Int. Ed. 2008, 47, 577.
10. Chatterjee, T.; Sarma, M.; Das, S. K. Tetrahedron Lett. 2010, 51, 1985.
11. Gillaizeau-Gauthier, I.; Odobel, F.; Alebbi, M.; Argazzi, R.; Costa, E.; Bignozzi, C.
A.; Qu, P.; Meyer, G. J. Inorg. Chem. 2001, 40, 6073.
chains bearing six carbons seemed to be sufficient to ensure a good
solubility of the resulting ligands. Taking into account this observa-
tion, substituted benzaldehyde adducts 21a–d bearing n-alkyl
chains (n = 6 and 9) located in the para position regarding the
required aldehyde functionality were then synthesized (Scheme 6).
Compounds 21a–d were obtained from the corresponding com-
mercially available hydroxybenzaldehydes 14–16 upon reacting
with 1-haloalkyls RX (R = C6H13 or C9 H19; X = Br or I) and K2CO3
in refluxing CH3CN. Classical work-up, followed by flash column
chromatography purification afforded the desired benzaldehydes
21a–d in good to excellent yields (Table 1).
In order to increase further the light harvesting ability of conju-
gated bipyridyl ligands, the synthesis of benzaldehyde reactants
21e–f was also investigated (Scheme 7).
Compounds 21e–f were then both obtained by a two-step syn-
thesis involving N or O-alkylation of commercially available com-
pounds 17 and 18 with 1-halohexyl to afford 19 and 20 followed
by a Vilsmeier formylation with an overall yield of 40% and 46%,
respectively.
12. Fraser, C. L.; Anastasi, N. R.; Lamba, J. S. J. Org. Chem. 1997, 62, 9314.
13. Savage, S. A.; Smith, A. P.; Fraser, C. L. J. Org. Chem. 1998, 63, 10048.
14. 4,40-Bis(chloromethyl)-2,20-bipyridine. Compound 7 was synthesized according
to published procedure11 with the following modifications. A solution of 612
(2 g, 6.1 mmol), hexachloroethane (5.8 g, 24.3 mmol), and KF (1.42 g,
24.3 mmol) in anhydrous DMF (30 ml) was stirred overnight at room
temperature under nitrogen. EtOAc (200 ml) and water (100 ml) were then
added. The organic layer was separated and washed with water (5 ꢂ 100 ml),
dried over MgSO4 and evaporated to dryness. The resulting solid was dissolved
in the minimum volume of hexane and let to stand in the freezer for few hours.
The resulting white crystalline solid was filtered and washed with small
portions of cold hexane to afford 1.4 g (91%) of 7 as a slightly yellow solid. 1H
NMR was in agreement with those reported in the literature.
Benzaldehyde adducts 21a–f were used for the synthesis of
functionalized disubstituted bipyridines 22a–f following the gen-
eral procedure depicted in Scheme 8.
Bipyridines 22a–f were obtained upon reacting compound 8
with 2.5 equiv of the desired benzaldehyde 21a–f and 4 equiv of
t-BuOK in anhydrous DMF. Evaporation of the solvent followed
by trituration of the resulting solids with methanol and filtration
afforded pure compounds 22a–f in yields up to 78% (Table 2).20
This general and rather easy work-up procedure gives an addi-
tional advantage to the Horner–Emmons–Wadsworth route for the
synthesis of styryl-functionalized bipyridines. As previously
observed for compounds 10 and 13a–b, bipyridyl derivatives
22a–f were exclusively obtained in their (E,E0)-conformations.
In summary, a series of new symmetrically functionalized 4,40-
4,40-Bis(diethylmethylphosphonate)-2,20-bipyridine 8.
A solution of 7 (2.6 g,
10.3 mmol) in triethylphosphite (50 ml) was heated to 160 °C overnight
under nitrogen. Excess P(OEt)3 was evaporated to afford a brown oil. The
crude product was purified by column chromatography (SiO2, EtOAc/MeOH,
80:20) to afford 4 g (85%) of 8 as a white solid. 1H and 31P NMR were in
agreement with those reported in literature.11
15. Chen, C.-Y.; Wu, S.-J.; Wu, C.-G.; Chen, J.-G.; Ho, K.-C. Adv. Funct. Mater. 2007,
17, 29.
16. Qin, H.; Wenger, S.; Xu, M.; Gao, F.; Jing, X.; Wang, P.; Zakeeruddin, S.-M.;
Grätzel, M. J. Am. Chem. Soc. 2008, 130, 9202.
17. Cao, Y.; Bai, Y.; Yu, Q.; Cheng, Y.; Liu, S.; Shi, D.; Gao, F.; Wang, P. J. Phys. Chem. C
2009, 113, 6290.
18. Renouard, T.; Grätzel, M. Tetrahedron 2001, 57, 8145.
19. 4,40-Bis(ethyl 2-thienylvinyl-5-carboxylate)-2,20-bipyridine 10. To a solution of 8
(1 g, mmol) and 924 (2.5 equiv) in anhydrous DMF (50 ml) was added t-BuOK
(2.5 equiv). A copious precipitate formed after few minutes. The resulting
slurry was stirred overnight at room temperature under nitrogen. DMF was
evaporated and MeOH (150 ml) was added. The resulting mixture was let to
stand in the fridge for 1 h before being filtered. The obtained solid was washed
with small portions of cold MeOH (3 ꢂ 10 ml) and dried to afford compound 10
(53%) as a white solid. 1H NMR (CDCl3, 298 K, 200 MHz, d ppm) 1.41 (t, J = 7 Hz,
6H), 4.39 (q, J = 7 Hz, 4H), 7.07 (d, J = 16 Hz, 2H), 7.16 (m, 2H), 7.37 (d, J = 5 Hz,
p
-conjugated-2,20-bipyridine bidentate ligands have been synthe-
sized using the Horner–Emmons–Wadsworth reaction. This easy
and convenient synthetic strategy allowed us to introduce either
electron-withdrawing or electron-donating end-capped groups on
the p-conjugated-bipyridine core. This methodology was relatively
tolerant toward most of the functionalities desired for dye-sensi-
tized solar cell application. 1H NMR analysis revealed that all the
new synthesized ligands were exclusively obtained in their fully E
isomers. This simple synthetic approach is expected to expedite
the development of antenna-type ligands for ruthenium sensitizers.
2H), 7.55 (d, J = 16 Hz, 2H), 7.73 (m, 2H), 8.54 (s, 2H), 8.69 (d, J = 5 Hz, 2H). 13
C
NMR (CDCl3, 298 K, 50 MHz, d ppm) 14.3, 61.3, 118.0, 121.2, 125.6, 127.8,
128.2, 133.3, 133.8, 144.6, 147.7, 149.7, 156.4, 161.9.
20. General procedure for the synthesis of functionalized bipyridines 22a–f. To a
solution of
8 (1 g, 2.2 mmol) and the corresponding benzaldehyde 21a–f
(2.5 equiv) in anhydrous DMF (50 ml) was added solid t-BuOK (4 equiv). A
copious precipitate formed after few minutes. The resulting mixture was
stirred overnight at room temperature under nitrogen. DMF was evaporated
and MeOH (150 ml) was added. The resulting mixture was sonicated 2 min and
let to stand in the fridge for 1 h before being filtered. The obtained solid was
washed with small portions of cold MeOH (3 ꢂ 10 ml) and air-dried to afford
pure compounds 22a–f.
Acknowledgments
This work was supported by the Swiss Science Foundation, Swiss
FederalOffice for Energy andthe EuropeanOffice of U.S. Air Force un-
der contract No. F61775-00-C0003. MKN thanks the World Class
University (WCU) program funded by the Ministry of Education,
Science and Technology (Grant No. R31-2009-000-10035-0).
4,40-Bis(p-hexyloxystyryl)-2,20-bipyridine 22a. This compound was obtained as a
white solid in 85% yield starting from 8 and 21a. 1H NMR (CDCl3, 298 K,
200 MHz, d ppm) 0.93 (t, J = 6.3 Hz, 6H), 1.2–1.5 (m, 12H), 1.85 (m, 4H), 4.00 (t,
J = 6.4 Hz, 4H), 6.93 (d, J = 8.7 Hz, 4H), 7.00 (d, J = 17 Hz, 2H), 7.38 (d, J = 5 Hz,
2H), 7.43 (d, J = 17 Hz, 2H), 7.51 (d, J = 8.7 Hz, 4H), 8.53 (s, 2H), 8.66 (d, J = 5 Hz,
2H). 13C NMR (CDCl3, 298 K, 50 MHz, d ppm) 14.0, 22.6, 25.7, 29.2, 31.6, 68.1,
114.8, 118.0, 120.8, 123.8, 128.4, 128.8, 133.0, 146.1, 149.4, 156.5, 159.7.
4,40-Bis(p-nonyloxystyryl)-2,20-bipyridine 22b. This compound was obtained as a
white solid in 87% yield starting from 8 and 21b. 1H NMR (CDCl3, 298 K,
References and notes
1. Chen, C.-Y.; Wu, S.-J.; Wu, C.-G.; Chen, J.-G.; Ho, K.-C. Angew. Chem., Int. Ed.
2006, 45, 5822.