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Synlett 2007(16): 2559-2563  
DOI: 10.1055/s-2007-986641
LETTER
© Georg Thieme Verlag Stuttgart · New York

Synthesis and Photoluminescence Studies of Siloles with Arylene Ethynylene Strands

Changhua Ding, Govindarajulu Babu, Akihiro Orita*, Takayoshi Hirate, Junzo Otera*
Department of Applied Chemistry, Okayama University of Science, Ridai-cho, Okayama 7000005, Japan
Fax: +81(86)256 4292; e-Mail: orita@high.ous.ac.jp; e-Mail: otera@high.ous.ac.jp;
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Publication History

Received 22 July 2007
Publication Date:
12 September 2007 (online)

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Abstract

Siloles attached to phenylene ethynylene strands were prepared successfully by Sonogashira coupling of 2,5-dibromo­silole with ethyne-terminated phenylene ethynylenes. Elongation of the arylene ethynylene chain involving up to at least three aromatic rings leads to high quantum yields in solution.

Key words

alkynes - sulfones - aldehydes - eliminations - siloles

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UV-Vis and photofluorescence were recorded with JASCO V-560 and JASCO FP-6500 instruments at r.t., respectively. Absolute quantum yields of photofluorescence were recorded by an integration sphere system (Hamamatsu photonics C9920-02).

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Representative Experimental Procedure:
(i) Preparation of 3e: To a THF solution (40 mL) of phenyl 4-(trimethylsilylethynyl)phenylmethyl sulfone (1.57 g, 4.8 mmol) was added LiHMDS (4.8 mL, 1.0 M THF solution, 4.8 mmol) at -78 °C, and the mixture was stirred for 0.5 h. To this solution was added a THF solution (5 mL) of 2,5-diethyl-4-(4-methoxyphenylethynyl)benzaldehyde (1.16 g, 4.0 mmol), and the mixture was stirred for 1 h. After ClP(O)(OEt)2 (0.694 mL, 4.8 mmol) had been added, the mixture was stirred at r.t. for 2 h. After LiHMDS (20.0 mL, 1.0 M THF solution, 20.0 mmol) had been added at -78 °C, the reaction mixture was stirred at -78 °C for 1 h and then at 30 °C for 17 h. After usual workup with EtOAc and aq NH4Cl, the organic layer was dried over MgSO4 and filtered. The solvents were evaporated, and the residue was chromatographed (CH2Cl2-hexane, 3:7) to give 3e (1.73 g, 94%) as a colorless solid. 1H NMR (500 MHz, CDCl3): δ = 0.26 (s, 9 H), 1.29 (t, J = 7.6 Hz, 6 H), 2.81-2.86 (m, 4 H), 3.82 (s, 3 H), 6.87 (d, J = 8.8 Hz, 2 H), 7.36 (s, 2 H), 7.45 (s, 4 H), 7.46 (s, 1 H), 7.48 (s, 1 H). 13C NMR (125 MHz, CDCl3): δ = -0.12, 14.6, 14.7, 27.1, 27.2, 55.1, 86.9, 90.3, 93.5, 94.3, 96.2, 104.7, 114.0, 115.5, 121.7, 122.8, 123.1, 123.6, 131.2, 131.3, 131.5, 131.8, 132.9, 143.1, 143.3, 159.6. Other TMS-protected phenylene ethynylenes 3c and 3d were prepared similarly from the corresponding sulfones and aldehydes by one-shot double elimination process.6l,m
(ii) Preparation of 4e: A 100-mL flask was charged with 3e (1.73 g, 3.76 mmol), K2CO3 (2.6 g, 18.8 mmol), THF (20 mL) and MeOH (20 mL). The mixture was stirred at r.t. for 2 h. After usual workup with EtOAc and aq NH4Cl, the organic layer was dried over MgSO4 and filtered. The solvents were evaporated, and the residue was chromatographed (CH2Cl2-hexane, 3:7) to give 4e (1.35g, 93%) as a colorless solid. 1H NMR (500 MHz, CDCl3): δ = 1.27-1.31 (m, 6 H), 2.80-2.86 (m, 4 H), 3.16 (s, 1 H), 3.77 (s, 3 H), 6.84 (d, J = 8.6 Hz, 2 H), 7.36 (s, 2 H), 7.45 (s, 5 H), 7.47 (s, 1 H). 13C NMR (125 MHz, CDCl3): δ = 14.6, 14.7, 27.1, 30.2, 55.2, 78.9, 83.3, 86.8, 90.3, 93.3, 94.4, 114.0, 115.4, 121.6, 121.7, 123.1, 123.9, 131.3, 131.4, 131.5, 132.0, 132.9, 143.2, 143.4, 159.6.
(iii) Preparation of 10a: A solution of lithium naph-thalenide was prepared by stirring a mixture of naphthalene (1.17 g, 9.16 mmol) and lithium (63.6 mg, 9.16 mmol) in THF (8 mL) at r.t. for 3 h. To the above lithium naph-thalenide solution, diethylbis(phenylethynyl)silane (0.66 g, 2.29 mmol) in THF (8 mL) was added over 15 min, and the mixture was stirred at r.t. for 15 min. ZnCl2 (1.56 g, 11.4 mmol) in THF (12 mL) was added at -20 °C, and the mixture was stirred for 20 min. After N-bromosuccinimide (1.01 g, 5.72 mmol) had been added at -78 °C, the flask was shielded from light by covering with foil and stirred at -78 °C for 1 h. After usual workup with Et2O and aq NH4Cl, the organic layer was washed with half-saturated Na2S2O3 solution, dried over MgSO4 and filtered. The solvents were evapor-ated, and the crude dibromosilole 7 was used for the next step without further purification. To a solution of ZnCl2 (0.625 g, 4.58 mmol) in THF (8 mL) and Et3N (1.85 g, 18.3 mmol) were added dibromosilole 7 in THF (4 mL) and 4e (1.779 g, 4.58 mmol) in THF (4 mL). After Pd(Ph3P)4 (0.264 g, 0.22 mmol) had been added, the mixture was stirred at 65 °C for 13 h. After evaporation of the solvents, the residue was chromatographed (EtOAc-hexane, 1:9) to give 10a (2.82 g, 58%) as a yellow solid. 1H NMR (500 MHz, CDCl3)): δ = 1.13-1.31 (m, 22 H), 2.84 (m, 8 H), 3.84 (s, 6 H), 6.90 (d, J = 8.5 Hz, 4 H), 7.19-7.24 (m, 14 H), 7.36 (s, 4 H), 7.42 (d, J = 8.0 Hz, 4 H), 7.48 (d, J = 9.0 Hz, 4 H). 13C NMR (125 MHz, CDCl3): δ = 2.6, 7.1, 14.6, 14.7, 27.1, 55.2, 86.8, 90.2, 92.7, 93.8, 94.2, 99.6, 114.0, 115.4, 121.8, 122.2, 122.7, 122.9, 124.2, 127.2, 127.5, 129.3, 131.2, 131.4 (2), 132.9, 137.5, 143.1, 143.3, 159.6, 162.7. ESI-MS: m/z [M + H] calcd for C78H67O2Si: 1063.49; found: 1063.1.