International Journal of Organic Chemistry, 2011, 1, 162-166 doi:10.4236/ijoc.2011.14024 Published Online December 2011 (http://www.SciRP.org/journal/ijoc) Copyright © 2011 SciRes. IJOC The Ring-Opening Reaction of 7,7’-Dimethyl-2, 5-bis(trimethylsilyl)-dithieno[2,3-b:3’,2’-d]silole in the Presence of NXS (X = Cl, Br, I) Caiyun Zhao1, Li Xu2, Jianwu Shi1, Chunli Li1, Zhihua Wang1,2, Hua Wang1,2* 1Key Lab for Special Functi o nal Mat eri al s of Mini st ry of Educt i on , Henan University, Kaifeng, China 2College of Chemistry and Chemical Engineering, Henan University, Kaifeng, China E-mail: *hwang@henu.edu.cn Received September 23, 2011; revised October 25, 2011; accepted November 6, 2011 Abstract In this paper, the synthetic method for making 7,7’-dimethyl-2,5-bis(trimethylsilyl)-dithieno[2,3-b:3’,2’-d] silole (1) was developed by using 2,2’-dibromo-5,5’-bis-trimethyl-silanyl[3,3’]bithiophenyl as starting mate- rial in one pot reaction. In the presence of NXS (X = Cl, Br, I), a novel ring-opening reation was occurred on the silole ring of 1 in DMF or THF. By using such kind of reaction, two types of ring opened products, (2’-halo-5,5’-bis(trimethylsilanyl)[3,3’ ]bi thiop heny l-2-yl)-dimethylsilanols and 2,2’-dihalo-5,5’-bis(trimethylsi- lanyl)[3,3’]bithiophenyls were obtained efficiently. Keywords: Dithieno[2,3-b:3’,2’-d]silole, Halogenation, Ring-Opening, Silanol 1. Introduction The compound with silole unit as building block has low reduction potential and low-lying lowest unoccupied molecular orbital (LUMO) energy level [1,2] arising fr- om the σ*-π* conjugation between the σ* orbital of the silicon atom and the π*-orbital of the butadiene moiety. The unique electronic structure make silole-based π-con- jugated compounds, such as dithieno[3,2-b:2’,3’-d]silole (DTS-1, Figure 1) based compounds can be used as func- tional materials in organic field effect transistor (OFET) [3], light-emitting diodes (LEDs) [4], hole transport in double-layer EL devices [5-7], photovoltaic devices[8,9] and dye-sensitized solar cells (DSSCs) [10,11]. The typical method for formation of DTS-1 has been reported by many research groups [4,6,8,9,12-17] in re- cent years, in which DTS-1 is formed in two steps. First one is the Br/Li exchange on 3,3’-dibromo-2,2’-bi- H 2 Si SS DTS-1 S S Si H 2 dithieno[2 ,3-b:3', 2'-d] siloledithieno[3 ,2-b:2', 3'-d] silole DTS-2 S S Si TMS TM Me Me 1 Figure 1. Chemical structures of DTS and 1. thiophene and the second one is followed by reaction with dihalosilane to generate DTS-1 in good yield. However, few examples are reported about the formation of the isomer, DTS-2. Iyoda et al. [18] reported the syn- thesis of DTS-2 by the palladium catalyzed cyclization of bis-(3-bromo-thiophen-2-yl)-dimethylsilane in presence of Me3SnSnMe3 (0.9 equiv) with high loading (20%) of pal- ladium catalyst. The bromination of fused aromatic compound is an important step in the synthesis of functional materials. The bromination for derivatives of DTS-1, such as 4,4’- di-n-hexylsilylene-dithieno[3,2-b:2’,3’-d]silole and 2,6- bis-(trimethylsilyl)-4,4’-di-n-hexylsilylene-dithieno-[3, 2-b:2’,3’-d]silole in prescene of NBS [8,9,12,13,19] or bromine [4,20,21] has been reported to generate di- bromo-DTS-1s efficiently. In which, TMS group could be replated by Br group and structure of silole ring in DTS-1 is stable. Different from the case of DTS-1, in our work, we found the ring opening phenomena occurred to DTS-2,7’-dimethyl-2,5-bis-(trimethylsilyl)-dithieno-[2, 3-b:3’,2’-d]silole (1) in the presence of NXS (X = Cl, Br, I) in DMF or THF at ambient temperature. To the best of our knowledge, few of ring opening reactions about si- lole compounds were reported. Braddock-Wilking [22] et al. gave an interesting example of bromination of 1,1- dimethoxy-2,5-bis(trimethylsilyl)-3,4-diphenylsilole tr-
163 C. Y. ZHAO ET AL. eated with Br2, and the ring-opened product, 1-bromo- 4-bromodi(methoxy)silyl-1,4-bis(trimethylsilyl)-2,3-dip- henyl-1,3-butadiene was obtained. Recently, Xi [23] et al. showed an efficient and specific cleavage of the endo- C(sp2)-Si bond of silole rings by using an AcOH/ROH system. Different from the above cases of substituted silole, DTS-2 is a type of fused heterocyclic compound, its chemical stability is more important issue in both the- ory of organic chemistry and synthesis of materials. 2. Results and Discussion 2.1. Ring-Opening Reation of 1 1 could be prepared with 2,2’-dibromo-5,5’-bistrimethyl- silanyl[3,3’]bithiophenyl (3) [24] as starting material in one pot reaction (step a, Scheme 1). Double Br/Liex- change on 3 by n-BuLi at –78˚C for 2 h, followed by reac- tion with dichlorodimethylsilane, 1 was obtained in 81.0% as white solid. With two TMS groups, 3 has a very good solubility and there is no solubility problem in both Br/Li-exchange step and subsequent silole formation step. On the other hand, TMS-protected 3 also prevented pos- sible side reactions occurred on the acidic α positions. Compared with the method reported by Iyoda [18], our method offers a practical route without expensive palla- dium catalyst and poisonous Me3SnSnMe3 to offer 1 in 81% yield. Bromination of DTS-2 is interested to us, because mo- no- and di-bromo-DTS-2 should be same important inter- mediates as dibromo-DTS-1 in synthesis of organic func- tional mateials [3-11]. Different from the case of bromi- nation of DTS-1, however, in bromination of 1 with NBS, instead of formation of bromo-DTS-2 (4) (step b, Scheme 1), a ring opening product, (2’-bromo-5,5’-bistri- methylsilanyl[3,3’]bithiophenyl-2-yl)-dimethylsilanol (6) was obtained in 78.5% yields when 1.05 equiv NBS was employed in DMF (Step c, Scheme 1). None of 4 could be observed. With 1.05 equiv of NCS and NIS instead of NBS, the corresponding ring opening products, 5 and 7 were attain- ed in 39.3% and 71.2% yields, respectively (Step c, Sch- eme 1). In addition, when the solvent was changed to THF, the yield of 5 could be increased to 55.3%, but the yields of 6 (78.3%) and 7 (61.2%) have a little changed. With 2.05 equiv of NXS employed in THF, 2,2’-di- halo-5,5’-bis(trimethylsilanyl)-[3,3’]bithiophenyls (8, 3 and 9) were obtained in 83.4%, 94.7% (isolated yields) and 43.5% (GC-MS analysis), respectively (Step d, Sch- eme 1). In which, iodination of 1 showed low selectivity, the crude product gave weak polarity with one spot (Rf = 0.70) on TLC (eluent: hexane). We failed in separation of 9 by column chromatography on silica gel or recrys- tallization. From the GC-MS analysis for the crude pro- duct, 43.5% of 9 was observed. The main of other possi- ble side-products were ring opened compouds with one TMS group or one to three iodine groups. In order to understand the possible reaction process worked by two steps: monohalogenation and dihalogena- tion, we tried step d in Scheme 1. 1.05 equiv of NXS (X = Cl, Br and I) was used to react with 5, 6, 7 in THF (Step e, Scheme 1), 8, 3 and 9 could be obtained smoothly in 86.2%, 92.6% and 44.7%, respectively. The results show that the dihalogenation of 1 could be occurred step by step. S S Br Br TMSTMS S S TMSTMS Si Me Me S S TMSTMS XSi OH Me Me 1 4 5 X=Cl 6 X=Br 7 X=I a b c 3 8 X=Cl 3 X=Br 9 X=I S S TMSTMS X X de S S TMSBr Si Me Me Scheme 1. Synthetic route to 1, 3, 5-9. Reagents and conditions: (a) (i) 2.1 equiv n-BuLi in THF, –78˚C/2 h (ii) 1.1 equiv Me2SiCl2, 1, 81.0%; (b) 1.05 equiv NBS in DMF; (c) 1.05 equiv NXS (X = Cl, Br, I ) in DMF, 5, 39.3%; 6, 78.5%; 7, 71.2%; (d) 2.05 equiv NXS (X = Cl, Br, I ) in THF, 8, 83.4%; 3, 94.7%; 9, 43.5% (GC-MS); (e) 1.05 equiv NXS (X = Cl, Br, I ) in THF, 8, 86.2%; 3, 92.6%; 9, 44.7%. Copyright © 2011 SciRes. IJOC
C. Y. ZHAO ET AL. 164 2.2. X-Ray Structural Analysis of 7 The structure of 7 was confirmed by single crystal X-ray analysis1. There are two enantiomers of 7 packing in the crystal, and only one of two enantiomer molecules is shown in Figure 2 (All hydrogen atoms are omitted for clarity). The two linked thiophene rings are non-coplanar with the torsion angle (C4-C1-C8-C9) is 108.60˚ and the dihedral angle between them is 75.28˚. Iodine atom and silicon atom point away from each other with a distance of 5.173 Å (I1···Si2). There are short contacts including hydrogen bondings in the crystal packing. The distances of O1···H1, Si2···H1, H2A···H1, S2···H28C and O1···O2 are 2.035, 3.208, 2.274, 2.961 and 2.788 Å, respectively. 3. Experimental Section Synthesis of 1: To a solution of 3 (1.00 g, 2.13 mmol) in anhyd Et2O (25 mL), n-BuLi (2.43 M, 1.85 mL, 2.1 equiv) was added dropwise at –78˚C. After keeping at –78˚C for 2 h, Me2SiCl2 (0.31 mL, 2.56 mmol, 1.2 equiv) was add- ed, the reaction mixture was warmed slowly to ambient temperature, then 25 mL anhyd THF was added and re- fluxed at 70˚C - 80˚C overnight. The reaction mixture was quenched with H2O (40 mL), extracted with Et2O (3 × 40 mL) and then washed with sat. NaHCO3 (50 mL) and Figure 2. Crystallographic structure of 7. All hydrogen ato- ms are omitted for clarity. H2O (2 × 50 mL). After drying over MgSO4, the solvent was removed in vacuo. The residue was purified by column chromatography on silica gel with PE (60˚C - 90˚C) as eluent to yield 1 (0.63 g, 81.0%) as a white solid. mp 143˚C - 145˚C. 1H NMR (400 MHz, CDCl3): δ 7.39 (s, 2 H), 0.49 (s, 6 H), 0.35 (s, 18 H). 13C NMR (100 MHz, CDCl3 ): δ 153.6, 151.2, 141.7, 127.9, 0.0, –1.9. HRMS (TOF MS EI+) m/z calcd for [C16H26S2Si3] 366.0784, found 366.0788. IR (KBr): 3211 (C-H) cm–1. Synthesis of 6 from 1: To a solution of 1 (75.5 mg, 0.20 mmol) in DMF (10 mL), NBS (36.6 mg, 0.20 mmol) was added at ambient temperature for 1 h, the reaction mixture was quenched with H2O (15 mL), extracted with Et2O (3 × 20 mL), and then washed with H2O (3× 30 mL). After drying over MgSO4, the solvent was removed in vacuo. The residue was purified by column chromato- graphy on silica gel with CHCl3 as eluent to yield 6 (74.7 mg, 78.5%) as a white solid. From another one reaction on 56.3-mg scale of 1 in THF, 44.1 mg (78.3%) of 6 was obtained. mp 102˚C - 103˚C. 1H NMR (400 MHz, DM- SO-d6): δ 7.33 (s, 1 H), 7.24 (s, 1 H), 6.13 (s, 1 H), 0.31 (s, 9 H), 0.28 (s, 9 H), 0.04 (s, 6 H); 13C NMR (100 MHz, DMSO-d6): δ 144.2, 143.4, 142.8, 140.0, 139.9, 137.9, 137.3, 114.1, 1.4, –0.0, –0.5; HRMS (TOF MS EI+) m/z calcd for [C16H27BrOS2Si3] 461.9994, found 461.9995. IR (KBr): 3273 (O-H), 2956 (C-H), 839 (Si-O) cm–1. Synthesis of 5 from 1: 5 was prepared from 1 ac- cording to the same method used for 6. The reaction was taken on 47.4 mg scale of 1 in DMF, 22.2 mg (39.3%) of 5 was obtained. From another one reaction on 201.3 mg scale of 1 in THF, 127.3 mg (55.3%) of 5 were obtained, respectively. mp 93˚C - 95˚C. 1 H NMR (400 MHz, DMSO-d6): δ 7.35 (s, 1 H), 7.28 (s, 1 H), 6.16 (s, 1 H), 0.30 (s, 9 H), 0.28 (s, 9 H), 0.06 (s, 6 H); 13C NMR (100 MHz, DMSO-d6): δ 144.4, 143.5, 141.6, 137.4, 137.2, 137.1, 136.9, 128.8, 1.3, –0.1, –0.5. HRMS (TOF MS EI+) m/z calcd for [C16H27ClOS2Si3] 418.0500, found 418.0505. IR (KBr): 3273 (O-H), 2958 (C-H), 841 (Si-O) cm–1. Synthesis of 7 from 1: 7 was prepared from 1 accord- ing to the same method used for 6. The reaction was taken on 50.0 mg scale of 1 in DMF, 50.1 mg (71.2%) of 7 was obtained. From another one reaction on 27.0-mg scale of 1 in THF, 23.0-mg (61.2%) of 7 was obtained. mp 108˚C - 109˚C. 1H NMR (400 MHz, DMSO-d6): δ 7.31 (s, 1 H), 7.13 (s, 1 H), 6.10(s, 1 H), 0.31 (s, 9 H), 0.27 (s, 9 H), 0.01(s, 6 H). 13C NMR (100 MHz,CDCl3): δ 146.5, 145.9, 145.7, 145.6, 141.6, 137.7, 136.6, 81.0, 1.2, 0.0, –0.3. HRMS (TOF MS EI+) m/z calcd for [C16H27IOS2Si3] 509.9862, found 509.9856. IR (KBr): 3292 (O-H), 2954 (C-H), 843 (Si-O) cm–1. 1Crystal Data for 7: M = 1021.33, C32H54I2O2S4Si6 (two enantiomers) triclinic, space group P–1, a = 11.8196(11) Å, b = 15.3056(14) Å, c = 15.5471(15) Å, = 71.4980(10)˚, = 88.2930(10)˚, = 68.2770(10)˚, V = 2465.5(4) Å3, Z = 2, dcalcd = 1.376 g/cm3. A colorless crystal o dimensions 0.41 × 0.39 × 0.37 mm was used for measurement at 296 (2 K with the scan mode on a Bruker Smart APEX diffractometer with CCD detector using Mo-Ka radiation ( = 0.71073 Å). The data were corrected for Lorentz and polarization effects and absorption correc- tions were performed using SADABS program. The crystal structures were solved using the SHELXTL program and refined using full matrix least squares. The positions of hydrogen atoms were calculated theo- retically and included in the final cycles of refinement in a riding model along with attached carbons. The final cycle of fullmatrix least-squares refinement was based on 9111 independent reflections [I > 2σ(I)] and 431 variable parameters with R1 = 0.0379, wR2 = 0.1027. G. M. Shel- drick, SADABS; University of Göttingen: Germany, 1996. G. M. Shel- drick, SHELXTL Version 5.1; Bruker Analytical X-ray Systems, Inc., Madisonv/WI, 1997. Synthesis of 8 from 1: To a solution of 1 (55.2 mg, 0.15 mmol) in THF (8 mL), NCS (40.2 mg, 0.31 mmol) Copyright © 2011 SciRes. IJOC
165 C. Y. ZHAO ET AL. was added at ambient temperature and stirred overnight. The reaction mixture was quenched with H2O (15 mL), extracted with Et2O (3 × 20 mL), and then washed with H2O (3× 30 mL). After drying over MgSO4, the solvent was removed in vacuo. The residue was purified by column chromatography on silica gel with PE (60˚C - 90˚C) as eluent to yield 8 (47.6 mg, 83.4%) as a white solid. mp 60˚C - 61˚C. 1H NMR (400 MHz, CDCl3): δ 7.18 (s, 2 H), 0.32 (s, 18 H). 13C NMR (100 MHz, CDCl3): δ 138.2, 134.9, 132.8, 131.4, –0.3. HRMS (TOF MS EI+) m/z calcd for [C14H20Cl2S2Si2] 377.9922, found 377.9926. IR (KBr): 2956 (C-H) cm–1. Synthesis of 3 from 1: The preparation of 3 from 1 was made according to the same method used for making 8 from 1. From two reactions on a 51.5-mg and 46.7-mg scale of 1 in THF, 62.3 mg (94.7%) and 55.7 mg (93.4 %) of 3 were obtained, respectively. mp 79˚C - 80˚C (lit. mp 79˚C - 81˚C [13]). Synthesis of 8 from 5: To a solution of 5 (95.0 mg, 0.23 mmol) in THF (6 mL), NCS (36.3 mg, 0.27 mmol) was added at ambient temperature and stirred for 3 days. The reaction mixture was quenched with H2O (10 mL), extracted with Et2O (3 × 10 mL), and then washed with H2O (3× 30 mL). After drying over MgSO4, the solvent was removed in vacuo. The residue was purified by co- lumn chromatography on silica gel with PE (60-90˚C) as eluent to yield 8 (74.1 mg, 86.2%) as a white solid. Synthesis of 3 from 6: The preparation of 3 from 6 was made according to the same method used for making 8 from 5. From two reactions on a 95.3 mg and 61.6 mg scale of 6 in THF, 89.5 mg (92.6%) and 56.3 mg (90.1%) of 3 were obtained, respectively. Synthesis of 9 from 7: The preparation of 9 from 7 was made according to the same method used for making 8 from 5. From two reactions on a 98.0-mg and 45.1-mg scale of 7 in THF, 48.3 mg (44.7%) and 21.6 mg (43.5%) of 9 were obtained, respectively. mp 105˚C - 107˚C. 1H NMR (400 MHz, CDCl3): δ 7.06 (s, 2 H), 0.32 (s, 18 H). 13C NMR (100 MHz, CDCl3 ): δ 146.8, 143.9, 135.8, 131.4, –0.2. HRMS (ESI): m/z [M+ + H] calcd for C14H21I2S2Si2: 562.87071; found: 562.86951. IR (KBr): 2957 (C-H) cm–1. 4. Conclusions In summary, we have developed the synthetic method for making 1. In the presence of NXS (X = Cl, Br, I), a new type of ring opening reaction of DTS-2 is observed, which is important in theoretical understanding for the stability of π-conjugated systems containing silole rings. In addition, by using this ring opening reaction, two types of ring opened products, silanols (5 - 7) and di- halo[3,3’]bithiophenyls (8, 3 and 9) were obtained effi- ciently. The new findings are useful for application in organic synthesis. 5. 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