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Synlett 2006(18): 2981-2984  
DOI: 10.1055/s-2006-947329
LETTER
© Georg Thieme Verlag Stuttgart · New York

1,2,3,4-Tetraethynylbenzene as a Template for Cobalt-Catalyzed Alkyne Cocyclizations: Synthesis of 2,3,8,9-Tetrakis(trimethylsilyl) Angular [3]Phenylene and Bent [5]Phenylene (Benzo[1′′,2′′:3,4;3′′,4′′:3′,4′]dicyclo­buta[1,2-b:1′,2′-b′]bisbiphenylene)

Debra L. Mohlera, Sriram Kumaraswamya, Amnon Stangerb, K. Peter C. Vollhardt*a
a Department of Chemistry, University of California at Berkeley, Berkeley, California 94720-1460, USA
Fax: +1(510)6435208; e-Mail: kpcv@berkeley.edu;
b Department of Chemistry and The Lise Meitner-Minerva Center for Computational Quantum Chemistry, Technion-Israel Institute of Technology, Haifa 32000, Israel
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Publication History

Received 6 April 2006
Publication Date:
04 August 2006 (online)

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Abstract

An approach to the synthesis of bent phenylenes is described, which features 1,2,3,4-tetraethynylbenzene as starting material. Application of this synthon in CpCo-catalyzed alkyne cocyclizations allows the total synthesis of bent [5]phenylene, the seventh isomer of the family of twelve [5]phenylenes to be isolated.

Key words

alkynes - coupling - cyclizations - palladium - transition metals

    References and Notes

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8

1,2,3,4-Tetrakis(trimethylsilylethynyl)benzene (2). Trimethylsilylethyne (900 mg, 9.16 mmol), 1,2,3,4-tetrabromobenzene (317 mg, 0.805 mmol), Pd[P(C6H5)3]2Cl2 (69 mg, 0.098 mmol), CuI (7 mg, 0.04 mmol), and Et3N (12 mL) were combined in a glass bomb with a teflon stopcock. The reaction mixture was placed in a 110 °C sand bath for 36 h, after which it was cooled and the solvent removed under reduced pressure. Chromatography of the residue on silica gel with 5% CH2Cl2-hexane followed by recrystallization from EtOH gave 2 (252 mg, 68%). Tetrakis(triisopropylsilylethynyl) Angular [3]Phenyl-ene 7. A solution of tetrakis(trimethylsilylethynyl)benzene (2; 221 mg, 0.477 mmol), KF·2H2O (1.95 g, 22.6 mmol), and 18-crown-6 (50 mg, 0.19 mmol) in DME (20 mL) was allowed to stir for 15 min at r.t. The green mixture was decanted from the solids into a 100-mL pear-shaped flask and diluted with additional DME (30 mL). After flushing with argon, CpCo(CO)2 (120 µL, 0.942 mmol) was added via syringe. This solution was injected over an 8 h period to bis(triisopropylsilyl)hexatriyne (6, 853 mg, 2.21 mmol) in boiling toluene (50 mL). During the addition and for an additional 10 h, the reaction mixture was kept at reflux and irradiated with a slide projector lamp. The solution was cooled to r.t., the toluene removed by rotary evaporation, and the residue subjected to chromatography on silica gel, starting with hexane and gradually changing to 4% CH2Cl2-hexane, eluting initially some 6. Further elution of an orange band, removal of the solvents, and recrystallization from CH2Cl2-EtOH (2:1) gave 7 (165 mg, 37%).
Tetrakis(trimethylsilyl) Bent [5]Phenylene 8. To a solution of 7 (116 mg, 0.122 mmol) in THF (15 mL) was added Bu4N+F- (0.52 mL of a 1.0 M solution in THF, 0.52 mmol). The mixture was allowed to stir for 30 min, then treated with activated charcoal and filtered through silica gel, washing the plug with additional THF (10 mL). The combined solutions were diluted with more THF (25 mL), and the mixture was placed in a 100-mL pear-shaped flask and flushed with argon. To this was added CpCo(CO)2 (110 µL, 0.863 mmol) and the solution injected over an 8 h period into a 250 mL round-bottom flask containing boiling bis(trimethylsilyl)acetylene (50 mL) under argon. During the addition and for an additional 12 h, heat and irradiation with a slide projector lamp were maintained. After cooling to r.t., the THF was removed by rotary evaporation, and the excess bis(trimethylsilyl)acetylene was recovered by vacuum transfer. The remaining solid was subjected to chromatography on silica with 5% CH2Cl2-hexane, followed by recrystallization from THF (by evaporation at 2 °C) to give 8 (52 mg, 65%).
Bent [5]Phenylene 1.
In a 25-mL round-bottom flask, 8 (24.2 mg. 0.0366 mmol), CF3COOH (100 µL, 0.88 mmol), and CHCl3 (5 mL) were combined and allowed to stir for 16 h, generating an initial crop of 1 (3.5 mg) as a red precipitate that was filtered off. The filtrate was diluted with hexane (30 mL) and Et2O (10 mL), washed with 10% aq NaOH (30 mL) and H2O (50 mL), and dried over MgSO4 to give an additional crop of 1 (total of 5.9 mg, 43%).
All new compounds gave satisfactory analytical and spectral data, with the exception of unstable 3. For selected data, compound 2: white flakes, mp 128 °C; UV/Vis (hexane): λmax (log ε) = 227 (3.83), 258 (sh, 4.71), 265 (4.75), 273 (4.86), 287 (4.69), 296 (4.47), 307 (4.74) nm. IR (CHCl3): 3004, 2957, 2891, 2141, 1461, 1387, 1251, 1013, 945, 873, 833, 695, 641 cm-1. MS (70 eV): m/z (rel. intensity) = 462(5) [M+], 428(6), 355(9), 327(9), 239(20), 131(20), 73(100). 1H NMR (300 MHz, CDCl3): δ = 7.29 (s, 2 H), 0.27 (s, 18 H), 0.24 (s, 18 H). 13C{H} NMR (75 MHz, CDCl3): δ = 131.3, 128.4, 125.9, 103.6, 102.6, 101.1, 100.6, 0.09, -0.03.
Compound 4: bright yellow crystals, mp 190 °C. UV/Vis (hexane): λmax (log ε) = 228 (4.61), 245 (4.52), 277 (4.33), 287 (4.67), 292 (4.65), 301 (4.88), 310 (4.54), 316 (4.54), 326 (4.90), 366 (3.84), 388 (3.82), 410 (3.73) nm. IR (CHCl3): 2949, 1402, 1253, 1054, 851, 823, 684, 636 cm-1. MS (70eV): m/z (rel intensity) = 514(100) [M+], 499(7), 483(9), 442(6), 411(7), 74(8), 73(97). 1H NMR (300 MHz, CDCl3): δ = 7.25 (s, 2 H), 7.18 (s, 2 H), 6.16 (s, 2 H), 0.37 (s, 18 H), 0.35 (s, 18 H). 13C{H} NMR (75 MHz, CDCl3): δ = 150.2, 149.8, 148.6, 148.5, 147.9, 137.3, 125.1, 124.0, 114.6, 2.64, 2.58.
Compound 7: orange crystals, mp 233 °C. UV/Vis (hexane): λmax (log ε) = 234 (4.41), 260 (sh, 4.64), 279 (4.78), 288 (4.80), 314 (4.73), 329 (4.94), 361 (4.91), 412 (3.91), 430 (3.76), 440 (3.75) nm. IR (CHCl3): 2930, 2872, 2149, 1470, 1418, 1321, 1020, 993, 891, 685, 650 cm-1. MS (70 eV): m/z (rel. intensity) = 948(16) [M+], 947(16), 401(16), 400(35), 399(38), 323(18), 322(45), 321(100). 1H NMR (300 MHz, CDCl3): δ = 7.00 (s, 2 H), 6.92 (s, 2 H), 6.22 (s, 2 H), 1.12 (br s, 42 H), 1.11 (br s, 42 H). 13C{H} NMR (75 MHz, CDCl3): δ = 148.9, 148.5, 147.3, 135.7, 126.4,125.7, 123.3, 122.3, 115.8, 106.1, 106.0, 96.9, 96.6, 18.81,18.79,11.4, 11.3.
Compound 8: square red plates, mp >250 °C. UV/Vis (hexane): λmax (log ε) = 229 (4.48), 283 (4.83), 302 (4.85), 362 (4.34), 379 (4.48), 403 (4.70), 430 (5.11), 495 (4.07), 529 (3.72) nm. IR (CHCl3): 2942, 2864, 1339, 1251, 1084, 996, 836 cm-1. MS (70 eV): m/z (rel. intensity) = 662(61) [M+], 590(12), 460(10), 400(36), 399(47), 323(20), 322(42), 321(100), 262(22), 261(20). 1H NMR (400 MHz, CDCl3): δ = 6.84 (s, 4 H), 6.47 (d, J = 1.5 Hz, 2 H), 6.44 (d, J = 1.5 Hz, 2 H), 5.89 (s, 2 H), 0.314 (s, 18 H), 0.311 (s, 18 H). 13C{H} NMR (125 MHz, CDCl3): δ = 153.7, 153.2, 150.7, 149.44, 149.36 (2 C), 148.4, 148.3, 146.5, 131.0, 121.7, 121.6, 111.3, 110.7, 110.4, 2.10 (2 C).
Compound 1: red powder, mp >250 °C. UV/Vis (CH2Cl2): λmax (log ε) = 280 (4.78), 293 (4.80), 354 (4.26). 373 (4.34), 398 (sh, 4.64), 424 (5.09), 488 (4.04), 523 (3.86) nm. IR (KBr): 3040, 2935, 1245, 807, 722 cm-1. MS (70 eV): m/z (rel. intensity) = 374(100) [M+], 373(13), 372(34), 371(11), 370(20), 187(19), 186(15), 185(13). 1H NMR (400 MHz, CDCl3): δ = 6.68 (m, 4 H), 6.50 (m, 4 H), 6.43 (s, 2 H), 6.40 (s, 2 H), 5.88 (s, 2 H).