Facile synthesis and coupling of functionalized isomeric biquinolines

Tetrahedron Letters 53 (2012) 285–288

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Facile synthesis and coupling of functionalized isomeric biquinolines Manisha Tomar a, Nigel T. Lucas b, Michael G. Gardiner c, Klaus Müllen d, Josemon Jacob a,⇑ a

Centre for Polymer Science and Engineering, Indian Institute of Technology, Hauz Khas, New Delhi 110016, India Department of Chemistry, University of Otago, Dunedin 9054, New Zealand School of Chemistry, University of Tasmania, Private Bag 75, Hobart 7001, Australia d Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany b c

a r t i c l e

i n f o

Article history: Received 7 September 2011 Revised 1 November 2011 Accepted 3 November 2011 Available online 17 November 2011 Keywords: Blue-emitters Fluorene Biquinoline Optical properties

a b s t r a c t A simple route toward functionalized biquinolines, namely 8,50 -dibromo-5,80 -biquinoline 1 and 5,50 -dibromo-8,80 -biquinoline 2, was developed using Skraup syntheses. Both the dibromo compounds undergo facile Suzuki coupling to afford fluorene-coupled products 3 and 4, respectively, an important transformation in designing conjugated materials based on these cores. X-ray structural analyses of 1 and 4 provide insight into the mode of packing within materials containing these units. Ó 2011 Elsevier Ltd. All rights reserved.

Organic electronics based on semiconducting polymers has attracted much interest in recent years.1–3 N-Heterocyclic based polymers having n-type electrically conducting properties are of particular interest. However they are not readily synthesized and examples include the compound classes like oxidazoles,4,5 quinoxalines,6,7 pyridines,8–10 and quinolines.11–13 Quinoline based materials have been widely studied for applications as electron-transport material in light emitting diodes due to their electronic and optical properties.14–19 Polyquinolines possess n-type electrically conducting properties along with good thermal, mechanical, and oxidative properties.20,21 There are many reports on quinoline based polymers but limited work has been done on biquinoline based polymers.4,22– 30 The acid-catalyzed Friedlander condensation reaction is the most commonly used for the synthesis of polyquinolines and results in polymers containing 6,60 -biquinoline units.16,31–36 Incorporation of 5,50 - or 5,80 -linked biquinoline units along a polymer backbone can lead to materials with extended conjugation. Herein we report the synthesis of two isomers of biquinoline with different linkages, namely 8,50 -dibromo-5,80 -biquinoline 1 and 5,50 -dibromo-8,80 biquinoline 2, with the bromo substituents ideally placed for further functionalization. For example, both the dibromides undergo facile Suzuki coupling to generate conjugated systems with extended chromophores. Further to the synthesis, we present X-ray crystallographic studies on two of these materials which give insight into the mode of packing and significant solid state intermolecular contacts

⇑ Corresponding author. Tel.: +91 99 1106 1890; fax: +91 11 2659 1421. E-mail address: [email protected] (J. Jacob). 0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.tetlet.2011.11.008

N

N

Br

Br

Br

Br

N

2

1

N

N

N

3

N

N

4

Chart 1. Structures of biquinoline isomers and model compounds.

in these systems. The structures of the two isomers and their coupled products are shown in Chart 1. Scheme 1 illustrates the synthetic approach to the biquinoline molecules and their coupled products. Both the isomers were synthesized from the corresponding diamino compounds. The precursors 4,40 -dibromo-2,30 -diaminobiphenyl 5 and 4,40 -dibromo-2,20 diaminobiphenyl 6 were synthesized according to the literature.37–39 Compounds 5 and 6 were then ring closed to the corresponding biquinoline molecules 1 and 2 via Skraup synthesis using glycerol in the presence of sulfuric acid and iodine; yields for these reactions were 52% and 35%, respectively. Use of this bromo-containing precursor approach helps to avoid undesirable and nonselective direct bromination on biquinoline. Coupling of 1 and 2 with a 2-boronate ester of 9,9-dimethylfluorene gave 8,50 -di(9,9-dimethylfluoren-2-yl)-5,80 -biquinoline 3 and 5,50 -di(9,9-dimethylfluoren2-yl)-8,80 -biquinoline 4 in 57% and 52% yields, respectively. All the

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M. Tomar et al. / Tetrahedron Letters 53 (2012) 285–288 NH2 Br

i

Br

5

ii

3

NH2

NH2 Br

1

Br

i

2

ii

4

H2N

6 Scheme 1. Synthesis and coupling reactions of dibromo biquinolines. Reagents: (i) glycerol, H2SO4, I2; (ii) 2-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-dimethylfluorene, Pd(PPh3)4, dioxane/2 M Na2CO3.

4 (328 nm) 1.0 3 (337 nm) 4 (424 nm) 0.8 3 (433 nm)

8.0x103

6.0x103 0.6 4.0x10

3

0.4

2.0x103

0.2

0.0

0.0 300

400

500

Normalized Emission (a.u.)

Extinction coefficient (M -1cm-1 )

four molecules were purified by chromatography and then recrystallized from THF/methanol (5:1). Compounds 3 and 4 are soluble in common organic solvents such as THF, CHCl3, CH2Cl2 at concentrations >10 mg/mL. The synthesized molecules were characterized by 1H and 13C NMR, elemental analysis, mass spectroscopy, UV–vis absorption, and emission spectroscopies and in two cases, by Xray crystallography. The optical absorption and photoluminescence (PL) emission of 3 and 4 were measured in dilute THF solution and are shown in Figure 1. Compounds 1 and 2 show nearly identical absorption spectra with absorption maxima at 308 and 307 nm, respectively. On the other hand, when 1 and 2 are coupled with fluorene units at either end, that is 3 and 4, the absorption spectra showed a slight dependence on the linkage between the two quinoline units. The absorption maximum for 3 is seen at 337 nm while for 4 the absorption maximum shifts at 328 nm (extinction coefficients for 3 and 4 are 8.5  104 and 2.3  104 M1 cm1, respectively). The absorption position for 3 at a slightly longer wavelength compared with that of 4 suggests that 3 adopts, on average, a more planar conformation in solution. The photoluminescence behavior in THF solution also shows a small shift in emission maximum between 3 and 4 ( Fig. 1). For the fluorenyl-coupled compound with a 5,80 -biquinoline unit, a bathochromic shift of about 10 nm is observed compared to the compound comprising a 8,80 -biquinoline unit; both emit in the blue region in solution. To gain further insight into the preferred conformations of these biquinoline compounds, crystals for X-ray diffraction studies were successfully grown for 1 and 4. 8,50 -dibromo-5,80 -biquinoline 1 crystallized from THF/methanol in the centrosymmetric C2/c space group ( Fig. 2). The molecule sits on an inversion center at the

600

Wavelength (nm) Figure 1. UV–vis absorption and normalized emission spectra of model compounds 3 and 4 in THF solution.

Figure 2. ORTEP representation of 1 showing the numbering scheme (for one orientation only). Displacement ellipsoids are depicted at 50% probability.

midpoint of the biquinoline bond, despite the fact that the 5,80 -isomer does not possess inversion symmetry. This observation indicates 50:50 orientational disorder about molecular axis, such that the 1 and 4 positions of each quinoline ring are occupied by N and C atoms in equal proportions in the best-fit crystallographic model. The biquinoline core of 1 adopts a transoid non-planar conformation, with a dihedral angle of 107.3° between the two intersecting least-squares quinolyl ring planes. The most notable supramolecular interaction is p-stacking between each quinoline and a neighboring molecule (separation 3.401(6) Å), resulting in columns of biquinolines, albeit with the aforementioned intramolecular twist. The bromines are not involved in any significant supramolecular interactions. Diffusion of methanol into a chloroform solution of the 8,80 biquinoline derivative 4 formed small crystals that diffracted sufficiently in synchrotron-sourced X-rays40 to enable a solid state structure to be determined (Fig. 3). As is the case with 1, solvent is not present in the structure of 4. A dihedral angle of 129.6° between the quinoline rings places them in a non-planar, transoid conformation, while the fluorenyl groups are disposed to the biquinoline core with angles of 40.9° (C5–C12) and 43.5°(C35– C42). Inversion-related pairs of molecules exhibit a face-to-face arrangement; quinolines N1–C10 with a separation of 3.642(3) Å, and quinolines N31–C40 by 4.000(3) Å, both suggesting little to no interaction between these groups (Fig. 4, red). Short N31H55C (2.634(3) Å) contacts within pairs (green) and edge-to-face C– HC(p) interactions (orange) appear to be a major stabilizing element. The N1 atoms in 4 are also involved in intermolecular hydrogen bonds with a neighboring fluorene (N1H16, 2.733(3) Å) that appears to twist the quinoline rings from planarity. No 5,80 -linked biquinolines have been structurally characterized, the structure 1 herein being the first reported. Of the 15 structural reports41 of 8,80 -biquinoline and its derivatives, none is functionalized in the 5/50 -position(s), with 7/70 -substitution the most commonly investigated due to the possibility of atropisomerism.42–45 The parent 8,80 -biquinoline adopts a transoid configuration in the solid state, however is far from planar with the angle between the two halves being 96.8°.46 In conclusion, we have developed a new and efficient synthetic route toward the synthesis of difunctional biquinoline compounds. The regiospecific incorporation of two bromo groups into 8,50 -dibromo-5,80 -biquinoline and 5,50 -dibromo-8,80 -biquinoline provides useful building blocks for new conjugated organic materials. Facile Suzuki coupling of the dibromo compounds to fluorenyl groups provides a demonstration that these cores can be readily partnered with other conjugated moieties to access new classes of conjugated materials containing biquinolines.

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Figure 3. ORTEP representation of 4 showing the numbering scheme. Displacement ellipsoids are depicted at 50% probability.

Figure 4. Packing diagram of 4 showing quinoline face-to-face stacking (red), C–HN (green) and C–HC p (orange).

Acknowledgments The authors acknowledge Department of Science and Technology, India and Max Plank Society, Germany for generous financial support. M.T. acknowledges the research fellowship from Indian Institute of Technology Delhi, India. Data for the structure of 4 were obtained on the MX1 beamline at the Australian Synchrotron, Victoria, Australia. 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 nos. CCDC843080(1), 843081 (4). Copies of the data can be obtained, free of charge, from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Supplementary data (synthesis, NMR data) associated with this article can be found, in the online version, at doi:10.1016/j.tetlet. 2011.11.008. References and notes 1. Guenes, S.; Neugebauer, H.; Sariciftci, N. S. Chem. Rev. 2007, 107, 1324–1338. 2. Klauk, H. Chem. Soc. Rev. 2010, 39, 2643–2666. 3. Grimsdale, A. C.; Leok Chan, K.; Martin, R. E.; Jokisz, P. G.; Holmes, A. B. Chem. Rev. 2009, 109, 897–1091. 4. Zhan, X.; Liu, Y.; Wu, X.; Wang, S.; Zhu, D. Macromolecules 2002, 35, 2529–2537. 5. Concilio, S.; Bugatti, V.; Iannelli, P.; Piotto, S. P. Int. J. Polym. Sci. 2010, 1–6. 6. Kanbara, T.; Yamamoto, T. Chem. Lett. 1993, 419–422. 7. Kanbara, T.; Yamamoto, T. Macromolecules 1993, 26, 3464–3466. 8. Liu, S.-P.; Ng, S.-C.; Chan, H. S. O. Synth. Met. 2005, 149, 1–11.

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