Syntheses of tetraquinolinyl substituted pyrene, diphenylacetylene, and trans-stilbene ligands

Article abstract of DOI:10.1016/j.tetlet.2011.12.056

We report the syntheses and characterizations of quinolinyl functionalized di-t-butyl-pyrene, diphenylacetylene, and trans-stilbene ligands.

Full text of DOI:10.1016/j.tetlet.2011.12.056

Tetrahedron Letters 53 (2012) 980–982
Contents lists available at SciVerse ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Syntheses of tetraquinolinyl substituted pyrene, diphenylacetylene,
and trans-stilbene ligands
⇑
Runyu Tan, Datong Song
Davenport Chemical Research Laboratory, Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario, Canada M5S 3H6
a r t i c l e i n f o
a b s t r a c t
Article history:
We report the syntheses and characterizations of quinolinyl functionalized di-t-butyl-pyrene, diphenyl-
acetylene, and trans-stilbene ligands.
Received 16 October 2011
Accepted 13 December 2011
Available online 22 December 2011
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Multidentate ligand
Suzuki coupling reaction
Quinoline functionalization
Multidentate ligands have been widely used in coordination
chemistry to support metal centers, often taking advantage of che-
late effect.1–4 Recently, research in multimetallic systems has gained
increasing interest, largely inspired by the remarkable synergistic
effects between metal centers in catalysis and small-molecule
activation exhibited by metalloenzymes.5–10 The key to achieve
cooperation between metal centers is the design and synthesis of
multinucleating ligands which could accommodate multiple metal
centers in a proper spatial arrangement.11
Our group has reported a series of tetraquinolinyl benzene (ttqb)
ligands and discovered that the dinuclear platinum complexes of
these ligands could undergo the tandem C–Cl activation and reduc-
tion of the central benzene ring of ttqb ligands (Scheme 1).12 If the
resulting dinuclear Pt(IV) species can be reversed back to the start-
ing Pt(II) species and give up the two chloride ligands to a trapping
reagent, we can close the cycle of dechlorination of chlorinated
organics. We envision that the reverse reaction can be triggered
in two ways: (1) chloride removal followed by reduction of Pt(IV)
to Pt(III), which will then oxidatively re-aromatize the central ring;
(2) photolytic re-aromatization of the central ring, which reduces
Pt(IV) to Pt(III), followed by the elimination of chlorine to a trapping
reagent. To examine the generality of the tandem C–Cl activation
and reduction of the ligand as well as the possibility of triggering
the reverse reaction photolytically, a series of new ligands have
been designed, where the central benzene ring of ttqb is replaced
by extended
and trans-stilbene15 moieties for better photon absorptions. If sim-
ilar tandem C–Cl activation and reduction of the ligand systems
can be observed in these new ligand systems, the resulting reduced
systems might be able to restore their original form through
p
systems such as pyrene,13 diphenylacetylene,14
p
p
Scheme 1.
⇑
Corresponding author. Tel.: +1 416 978 7014; fax: +1 416 978 7013.
E-mail address: dsong@chem.utoronto.ca (D. Song).
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.12.056

R. Tan, D. Song / Tetrahedron Letters 53 (2012) 980–982
981
materials. For example, the syntheses of 4,5,9,10-tetrabromo-2,
7-di-tert-butylpyrene compound (1)17 and many substituted quin-
olinylboronic acids12 have been documented. The new compound
3,4,30,40-tetrabromodiphenylacetylene (2) was synthesized in good
yield via a Sonogashira coupling reaction18 between 3,4-dibromo-
iodobenzene and trimethylsilylacetylene, catalyzed by Pd(PPh3)2
Cl2/CuI (Scheme 2). Next, the McMurry coupling reaction19 of
two molecules of 3,4-dibromophenylaldehyde, which was synthe-
sized in two steps following a literature procedure,20 produced
3,4,30,40-tetrabromo-trans-stilbene in a high yield (Scheme 3). A
minor product, presumably the cis-isomer, was also observed.
Due to the extreme difficulty in resolving the two species, they
can be subjected to the next step as a mixture without purification
and the final ligand is readily separable using silica-gel column
chromatography.
Scheme 2. Synthesis of 2.
The condition of the Suzuki coupling reactions between the tetr-
abromides and 8-quinolinylboronic acid was adopted from the syn-
theses of ttqb ligands.12 After the initial screening, we noticed that
the best result was obtained using DMF/H2O as solvents, K3PO4 as
the base, and Pd(PPh3)4 as the catalyst. The most widely used condi-
tion,16 where toluene/EtOH/H2O, Na2CO3, Pd(PP3)4 were employed
as solvents, base, and catalyst, respectively, also gave the desired
products, albeit with considerably lower yields. Other solvent
systems or bases gave either no or only a trace amount of desired
products. It is also worth pointing out that the freshly made catalyst
gave much better yields than the purchased one.
With the optimized condition in hand, we synthesized tetr-
aquinolinyl substituted di-t-butyl-pyrene, diphenylacetylene, and
trans-stilbene compounds (Scheme 4). Generally, the yields of
these products are higher than those of ttqb, presumably due to
the reduced steric congestion between the quinoline moieties.
Scheme 3. Synthesis of 3.
photolysis, enabling the reverse reaction. In this manner, a stepwise
catalytic dechlorination might be achieved. Additionally, these
ligands could also act as versatile multidentate ligands for the con-
struction of other bimetallic complexes. In this Letter, we report the
syntheses and characterizations of tetraquinolinyl functionalized
di-t-butyl-pyrene, diphenylacetylene, and trans-stilbene ligands.
We chose Suzuki coupling reactions16 between quinolinylbo-
ronic acids and the corresponding tetrabromides to construct the
final ligand frameworks because of the availability of the starting
Scheme 4. Syntheses of 1a–3b.

982
R. Tan, D. Song / Tetrahedron Letters 53 (2012) 980–982
the reaction mixture. Bulkier phosphine ligands might be required
in the reaction in order to obtain the desired product.
In summary, we have prepared a series of tetraquinolinyl
substituted compounds which could potentially serve as binucleat-
ing ligands. With the possibility of introducing the substitutes on
the quinolinyl arms, the solubility as well as the steric and elec-
tronic properties of these ligands could be tuned accordingly.
Complexation of these ligands is currently under investigation in
our laboratories.
Acknowledgments
This research is supported by grants to D.S. from the Natural
Science and Engineering Research Council (NSERC) of Canada, the
Canadian Foundation for Innovation, the Ontario Research Fund,
and the ERA program of Ontario. R.T. is grateful for a postgraduate
scholarship from the OGS program of Ontario.
Figure 1. POV-Ray plots of molecular structures of 1a (left), 2a (middle), and 3a
(right). All the hydrogen atoms (except for the olefinic ones in 3a) are omitted for
clarity.
Previously, we observed a hindered rotation behavior in the ttqb
compounds, which caused line-broadening in the 1H NMR spectra
at room temperature.12 Interestingly, no hindered rotation was
observed in 1a although one might expect a relatively large rota-
tion energy barrier because of the presence of tert-butyl groups.
Only one set of sharp quinolinyl and tert-butyl resonances was ob-
served, suggesting fast rotations in solution. In contrast, the 1H
NMR spectra of 2a and 3a showed several broad peaks at ambient
temperature in the aromatic region, while heating the sample to
50 °C sharpened the resonance peaks, suggesting a similar hin-
dered rotation as displayed by ttqb. The prominent molecular ion
peaks of these three compounds in ESI-MS spectra provided strong
evidence for the formation of the desired tetraquinolinyl substi-
tuted compounds. To further prove the identity of these highly
complex species, the solid state structures of 1a–3a were unambig-
uously revealed by X-ray crystallography and shown in Figure 1.
All of them are the desired tetrasubstituted compounds. Each mol-
ecule of compound 1a has a crystallographically imposed twofold
symmetry. The quinolinyl groups are nearly perpendicular to the
pyrene ring with a dihedral angle of ꢀ72° and the two nitrogen
atoms from the adjacent quinolinyl groups are situated on the
opposite sides of the pyrene ring to minimize steric repulsion. Each
molecule of compounds 2a and 3a has a crystallographically
imposed inversion center. The two adjacent quinolinyl groups in
Supplementary data
Supplementary data (crystallographic data (excluding structure
factors) for the structures in this paper have been deposited with
the Cambridge Crystallographic Data Centre as supplementary
publication (CCDC 849009-849011). Copies of the data can be
obtained, free of charge, on application to CCDC, 12 Union Road,
Cambridge CB2 1EZ, UK, (fax: +44-(0)1223-336033 or e-mail:
deposit@ccdc.cam.ac.Uk)) associated with this article can be found,
in the online version, at doi:10.1016/j.tetlet.2011.12.056.
References and notes
1. Aguila, D.; Escribano, E.; Speed, S.; Talancon, D.; Yerman, L.; Alvarez, S. Dalton
Trans. 2009, 6610–6625.
2. Kuzu, I.; Krummenacher, I.; Meyer, J.; Armbruster, F.; Breher, F. Dalton Trans.
2008, 5836–5865.
3. Hierso, J.; Smaliy, R.; Amardeil, R.; Meunier, P. Chem. Soc. Rev. 2007, 36, 1754–
1769.
4. Vila, J. M.; Pereira, M. T.; Suarez, A.; Fernandez, J. J.; Ortigueira, J. M.; Fernandez,
A.; Torres, M. L.; Rodriguez, C. Trends Organomet. Chem. 1999, 3, 71–98.
5. Chandrasekhar, V.; Murugesapandian, B. Acc. Chem. Res. 2009, 42, 1047–1062.
6. Lang, H.; Packheiser, R.; Walfort, B. Organometallics 2006, 25, 1836–1850.
7. Maggini, S. Coord. Chem. Rev. 2009, 253, 1793–1832.
8. Rosenthal, J.; Nocera, D. G. Acc. Chem. Res. 2007, 40, 543–553.
9. Kim, E.; Chufan, E. E.; Kamaraj, K.; Karlin, K. D. Chem. Rev. (Washington, DC, US)
2004, 104, 1077–1133.
10. Belle, C.; Pierre, J. Eur. J. Inorg. Chem. 2003, 4137–4146.
11. Gavrilova, A. L.; Bosnich, B. Chem. Rev. (Washington, DC, US) 2004, 104, 349–
383.
12. Tan, R.; Jia, P.; Rao, Y.; Jia, W.; Hadzovic, A.; Yu, Q.; Li, X.; Song, D.
Organometallics 2008, 27, 6614–6622.
2a are on opposite sides of the central
p system, while those in
3a they are on the same side. In all the three compounds, the
potential coordination environment provided by two adjacent
quinolinyl groups closely resembles that of ttqb ligands, albeit
the central linkers have much larger
p systems.
13. Liu, X.; Zhao, Y.; Ren, A.; Feng, J. J. Mol. Model. 2011, 17, 1413–1425.
14. Khoo, I. C.; Webster, S.; Kubo, S.; Youngblood, W. J.; Liou, J. D.; Mallouk, T. E.;
Lin, P.; Hagan, D. J.; Van Stryland, E. W. J. Mater. Chem. 2009, 19, 7525–7531.
15. Ozawa, R.; Hamaguchi, H. Chem. Lett. 2001, 736–737.
16. Suzuki, A. J. Organomet. Chem. 1999, 576, 147–168.
17. Hu, J.; Era, M.; Elsegood, M. R. J.; Yamato, T. Eur. J. Org. Chem. 2010, 72–79.
18. Mio, M. J.; Kopel, L. C.; Braun, J. B.; Gadzikwa, T. L.; Hull, K. L.; Brisbois, R. G.;
Markworth, C. J.; Grieco, P. A. Org. Lett. 2002, 4, 3199–3202.
19. McMurry, J. E.; Fleming, M. P.; Kees, K. L.; Krepski, L. R. J. Org. Chem. 1978, 43,
3255–3266.
To explore the generality of these ligand frameworks and tune
the solubility of the ligands as well as the potential complexes,
6-trifluoromethoxy-8-quinolinylboronic acid12 was used as the
coupling partner in the Suzuki reaction. As shown in Scheme 4
and the Supplementary data, the syntheses of 2b and 3b are
comparable to those of 2a and 3a. However, the reaction between
1 and the boronic acid did not give the desired tetrasubstituted
product 1b, presumably due to the sterics. Instead, an appreciable
amount of di- and tri-substituted by-products were observed in
20. Swoboda, P.; Saf, R.; Hummel, K.; Hofer, F.; Czaputa, R. Macromolecules 1995,
28, 4255–4259.