The synthesis of cyclic disilene 10 was attempted by photolysis of 2-alkynyltrisilane 9. As a result, silylsilylene 14 was generated, along with the elimination of the alkynylsilane moiety, in the photolysis of 2-alkynyltrisilane 9, similar to the photolysis of alkynyldisilane. In addition, the product 14 trapped by triethylsilane underwent further photolysis to produce hydrosilylene 16.
Disilenes are the silicon analogue of carbon-carbon double-bond compounds, and the first isolation of tetramesityldisilene was reported by West et al. (1981). Since then, various types of disilenes have been synthesized, and their characters have been revealed in detail (for reviews, see Okazaki and West, 1996; Kira and Iwamoto, 2006; Lee and Sekiguchi, 2010). There remains an ever-increasing interest in the challenging study of the interaction of organic π-conjugated systems and the silicon-silicon double-bond units (Bejan and Scheschkewitz, 2007; Fukazawa et al., 2007; Kinjo et al., 2007; Sasamori et al., 2008; Iwamoto et al., 2009). In that field, we have succeeded in the synthesis and isolation of the first example of a silicon analogue of (E)-endiyne, (E)-1,2-dialkynyldisilenes 1, and revealed that conjugative interaction exists between the Si=Si and C≡C units (Sato et al., 2010).
Those compounds were synthesized by the reduction of dihalosilanes 2 (Scheme 1). In the reactions, however, only (E)-isomer was generated, and the formation of (Z)-isomers could not be detected. (Z)-1,2-Dialkynyldisilenes are expected to show through-space interaction between the two alkynyl groups, resulting in electronic properties different from those of (E)-isomers. To synthesize the (Z)-1,2-dialkynyldisilenes selectively, we attempted the reduction of compound 3, the two ethynylsilane moieties of which were linked by tether (for cyclic disilenes, see Sakurai et al., 2002; Tanaka et al., 2006; Abe et al., 2010), but the desired disilene 4 could not be synthesized (Scheme 2) (Mizuhata et al., 2015). The reaction afforded the cyclic 1,2-dihydrodisilane 5 in low yield, where the chlorine atoms on the silicon atoms were fully replaced by hydrogen atoms derived from solvent and/or benzylic protons of 3. This result suggests that hydrogen abstraction occurred by radical species generated in the stepwise reduction of chlorosilane moieties.
In this particular case, the reduction of the corresponding dichlorosilane, the most general method for the generation of a silylene (or a disilene), was found to be not appropriate. We then decided to adopt another synthetic method for a disilene or a silylene without using a reducing agent, i.e., the photolysis of the corresponding trisilane derivative. By using this method, many disilenes, including the first disilene, tetramesityldisilene, have been successfully synthesized (for reviews, see Okazaki and West, 1996; Kira and Iwamoto, 2006; Lee and Sekiguchi, 2010). The photolysis of linear-trisilanes giving disilenes proceeds via the initial formation of a silylene, followed by its dimerization (Ishikawa and Kumada, 1981). In addition, Apeloig et al. (1994) generated alkynylsilylenes 6–8 in a low-temperature glass matrix using photolysis of linear-trisilane bearing an alkynyl group, and its formation was suggested by the trapping reaction with triethylsilane (Et3SiH) (Scheme 3). Based on these results, we designed and synthesized compound 9, the 2-alkynyltrisilane moieties of which were linked, and attempted the photolysis of 9 to synthesize cyclic disilene 10 (Scheme 4). In this paper, we discuss the unexpected elimination of alkynylsilane rather than disilane in the photoreaction of 9.
The synthesis of the precursor, compound 9, was performed. 1,2-Bis(ethynyldimethylsilyl)benzene 11 was prepared by a reaction of 1,2-bis(bromodimethylsilyl)benzene (Cavelier-Frontin et al., 1991) with ethynylmagnesiumbromide in THF (11% yield) as a pale yellow viscous liquid. The synthesis of 9 was achieved by the reaction of the dilithio compound generated by the treatment of 11 with n-butyllithium followed by treatment with TipSi(SiMe3)2Cl (Tip=2,4,6-triisopropylphenyl) (Archibald et al., 1992) in 23% yield as a viscous liquid (Scheme 5). Compounds 9 and 11 were characterized by NMR spectroscopy together with elemental analysis.
To check the generation of silylene from 9 via the elimination of disilane described in Scheme 3, we performed the photolysis of compound 9 in the presence of Et3SiH, which is commonly used as a trapping reagent for silylenes. A n-hexane solution of precursor 9 and Et3SiH was placed in a quartz sealed tube and irradiated with a low-pressure mercury lamp at -78°C for 24 h (Scheme 6). As a result, compound 12 bearing trimethylsilylethynyl moieties and triethylsilyldihydrosilane 13 were obtained at 39% and 27%, respectively. Both compounds were characterized by NMR spectroscopy together with elemental analyses. The formation of compound 13 suggests the generation of silylsilylene 14 via the elimination of alkynylsilane (Scheme 6). That is, the generated silylene 14 was trapped by Et3SiH to afford trisilane 15, and it was photolyzed again to give hydrosilylene 16 in the next step. Silylene 16 was trapped by Et3SiH to afford dihydrosilane 13. Whereas hydrosilylenes are important intermediates in view of bearing a primitive substituent, hydrogen, there are few examples of the generation (Hong et al., 1994; Sasamori et al., 2007; Agou et al., 2012). As far as we know, only one example was reported for the generation of hydrosilylene by the photolysis of oligosilane (Hong et al., 1994). In that report, photolysis of MeSi(SiMe3)3 in the presence of Et3SiH afforded Me(H)Si(SiMe3)(SiEt3) mainly (36%), together with 1,1-dihydrodisilane, H2(Me)SiSiEt3, in 1.5% yield, which suggests the generation of hydromethylsilylene from the initial product, Me(H)Si(SiMe3)(SiEt3). In the example using compound 9, monohydrosilane 15 was not observed, and the yield of dihydrosilane 13 was much higher than that in the previous report. This result suggests that the Tip-substitution is effective for the generation of hydrosilylene. Actually, photoreaction of TipSi(SiMe3)2H (Archibald et al., 1992) in the presence of Et3SiH proceeded cleanly and afforded dihydrosilane 13 in 39% yield. When compound 9 was irradiated under the same conditions in the absence of Et3SiH, only a complicated mixture was obtained. Its 29Si NMR spectrum indicated the existence of (Z)-Tip(Me3Si)Si=Si(SiMe3)Tip (at 97.8 ppm) (Archibald et al., 1992), but no other signal was observed in the lower field region other than this signal.
In the photolysis of compound 9, alkynylsilane, not disilane, was eliminated from the central silicon atom of trisilane moieties. This result marked a sharp contrast with the results reported by Apeloig et al. (1994) described above. Neither the trapped compound of alkynylsilylene nor the compound bearing triethylsilylacetylene terminus were observed in these experiments.
A possible mechanism for the generation of 12 is proposed in Scheme 7. Compound 17 containing silacyclopropene moieties is generated, and then the photolysis of compound 17 may give silylene 14 and compound 12. A similar reaction mechanism is reported in the photoreaction of alkynyldisilane 18 in the presence of methanol by Ishikawa, Kumada, and their co-workers (Scheme 8) (Ishikawa et al., 1980; Ishikawa and Kumada, 1981). Silacyclopropene 19 could be isolated, and the following photolysis gave bis(trimethylsilyl)acetylene 20 and dimesitylsilylene 21, the generation of which was suggested by the isolation of 22. In this reaction, generation of 1-silaallene 23 was suggested as a side reaction by the formation of 24. It is likely due to this type of side-reaction that the yields of 12 and 13 were low, similar to the photolysis of 9.
In summary, silylsilylene 14 was observed to generate in the photolysis of 2-alkynyltrisilane 9 via the elimination of alkynylsilane rather than disilane. Judging from the results obtained in this work and those shown in Scheme 3, the elimination of alkynylsilane from 2-alkynyltrisilane is competitive with that of disilane, and the selectivity could change depending on the reaction conditions or small differences in the substituents. For the synthesis of dialkynyldisilene using photolysis, protection of the alkynyl moieties is considered to be necessary. In addition, hydrosilylene 16 was observed to generate via compound 15. The yield of the trapped compound, dihydrosilane 13, was still low, but much higher than the previous result, indicating this methodology possibly presents an alternative way to generate a hydrosilylene.
All experiments were performed under an argon atmosphere unless otherwise noted. All solvents were purified by standard methods and/or The Ultimate Solvent System (Pure Process Technology, Nashua, New Hampshire, USA) (Pangborn et al., 1996) prior to use. 1H-NMR (300 MHz), 13C-NMR (75 MHz), and 29Si-NMR (59 MHz) spectra were measured in C6D6 (Cambridge Isotope Laboratories, Inc., Tewksbury, Massachusetts, USA) with a JNM AL-300 spectrometer (JEOL resonance Inc., Tokyo, Japan). A signal due to C6D5H (7.15 ppm) was used as an internal standard in 1H-NMR, and a signal due to C6D6 (128 ppm) was used in 13C-NMR. Multiplicity of signals in 13C-NMR spectra was determined by DEPT technique. 29Si-NMR (59 MHz) spectra were measured in C6D6 with a JEOL JNM AL-300 spectrometer using the signal for SiMe4 (0 ppm) in C6D6 as an external standard. Gel permeation liquid chromatography (GPLC) was performed on an LC-908, LC-918, or LC-908-C60 (Japan Analytical Industry Co., Ltd., Tokyo, Japan) equipped with JAIGEL 1H and 2H columns (eluent: chloroform). All melting points were determined on a micro-melting-point apparatus (Yanaco, Kyoto, Japan). Elemental analyses were carried out at the Microanalytical Laboratory of the Institute of Chemical Research, Kyoto University. Preparative thin-layer chromatography (PTLC) was performed with Kieselgel 60 PF254 (Merck KGaA, Darmstadt, Germany, Art. No. 7747).
A solution of 1,2-bis(hydrodimethylsilyl)benzene (12.6 g, 64.8 mmol, Tokyo Chemical Industry Co., Ltd., Tokyo, Japan) in CCl4 (50 mL) was added to a solution of bromine (7.6 mL, 150 mmol, NACALAI TESQUE, Inc., Kyoto, Japan) in CCl4 (50 mL) and cooled by ice-bath while maintaining an argon atmosphere. Argon gas was bubbled through the mixture for a further 30 min before removing the solvent in vacuum. The residue was used for the next reaction without further purification. To that compound was added ethynylmagnesiumbromide solution (0.5 m THF solution, 370 mL, 185 mmol, Sigma-Aldrich Co. LLC, St. Louis, Missouri, USA) at room temperature, and the mixture was stirred at room temperature for 3 days. The reaction mixture was treated with aqueous sodium hydrogen carbonate (Wako Pure Chemical Industries, Ltd., Osaka, Japan), and the organic layer was extracted with n-hexane. After the removal of volatile substances, the residue was subjected to silica gel chromatography (eluent: n-hexane) to afford 11 (1.75 g, 7.22 mmol, 11%). 11: pale yellow viscous liquid; 1H-NMR (300 MHz, C6D6, rt, δ in ppm) 0.57 (s, 12H), 2.15 (s, 2H), 7.17–7.20 (m, 2H), 7.95–8.00 (m, 2H); 13C-NMR (75 MHz, C6D6, rt, δ in ppm) 2.1 (CH3), 90.4 (C), 96.6 (C), 128.9 (CH), 136.4 (CH), 142.3 (C); 29Si-NMR (59 MHz, C6D6, rt, δ in ppm) -59.2 (SiMe2); HRMS (EI) m/z calcd for C14H18Si2 ([M]+): 242.0947, found: 242.0946; Anal. calcd for C14H18Si2: C, 69.35; H, 7.48%, found: C, 69.34; H, 7.45%.
To a THF solution (10 mL) of 11 (328 mg, 1.35 mmol) was added n-butyllithium (1.60 m in n-hexane, 0.850 mL, 1.36 mmol, KANTO chemical co., Inc., Tokyo, Japan) at -78°C, and the mixture was stirred at -78°C for 30 min. The resulting solution was added to a THF solution (10 mL) of TipSi(SiMe3)2Cl (1.12 g, 2.71 mmol) at room temperature, and the mixture was stirred at room temperature for 12 h. The solvents were removed under reduced pressure. n-Hexane was added to the residue, and the resulting suspension was filtered through Celite® to remove inorganic salts. The reaction mixture was purified by GPLC (eluent: CHCl3) to afford 9 (308 mg, 0.309 mmol, 23%). 9: viscous liquid; 1H-NMR (300 MHz, C6D6, rt, δ in ppm) 0.37 (s, 36H), 0.69 (s, 12H), 1.18 (d, J=7.0 Hz, 12H), 1.34 (d, J=6.5 Hz, 24H), 2.76 (sept, J=7.0 Hz, 2H), 3.72 (sept, J=6.5 Hz, 4H), 7.14 (s, 4H), 7.26–7.32 (m, 2H), 8.26–8.32 (m, 2H); 13C-NMR (75 MHz, C6D6, rt, δ in ppm) 0.5 (CH3), 2.5 (CH3), 24.0 (CH3), 26.1 (CH3), 34.5 (CH), 35.2 (CH), 116.6 (C), 118.9 (C), 121.9 (CH), 126.3 (C), 128.8 (CH), 137.1 (CH), 142.6 (C), 149.9 (C), 156.5 (C); 29Si-NMR (59 MHz, C6D6, rt, δ in ppm) -12.1, -20.6, -68.4; Anal. calcd for C56H98Si8: C, 67.53; H, 9.92%, found: C, 67.48; H, 9.74%.
A solution of 9 (333 mg, 0.334 mmol) and triethylsilane (3.0 mL, 19 mmol, Sigma-Aldrich Co. LLC, St. Louis, Missouri, USA) in n-hexane (4 mL) contained in a quartz sealed tube was exposed to a low-pressure mercury lamp (SUV110PL-10 with UVB-110, SEN LIGHTS CORPORATION, Osaka, Japan) while immersed in a -78°C bath for 24 h. All of the volatile materials were removed under reduced pressure. The reaction mixture was separated by GPLC (eluent: toluene) to afford oligomeric products and the mixture of 12 and 13. The following PTLC (eluent: n-hexane) gave 12 (51 mg, 0.13 mmol, 39%) and 13 (62 mg, 0.18 mmol, 27%). 12: viscous liquid; 1H-NMR (300 MHz, C6D6, rt, δ in ppm) 0.16 (s, 18H), 0.66 (s, 12H), 7.17–7.20 (m, 2H), 7.99–8.02 (m, 2H); 13C-NMR (75 MHz, C6D6, rt, δ in ppm) 0.0 (CH3), 2.5 (CH3), 114.7 (C), 116.8 (C), 128.8 (CH), 136.3 (CH), 142.7 (C); 29Si-NMR (59 MHz, C6D6, rt, δ in ppm) -18.9, -21.4; HRMS (FAB) m/z calcd for C20H34Si4 ([M]+): 386.1738, found: 386.1737; Anal. calcd for C20H34Si4: C, 62.10; H, 8.86%, found: C, 61.97; H, 8.77%. 13: viscous liquid; 1H-NMR (300 MHz, C6D6, rt, δ in ppm) 0.72 (q, J=8.4 Hz, 6H), 0.98 (t, J=8.4 Hz, 9H), 1.20 (d, J=6.9 Hz, 6H), 1.33 (d, J=6.7 Hz, 12H), 2.77 (sept, J=6.9 Hz, 1H), 3.33 (sept, J=6.7 Hz, 2H), 4.52 (s, 2H), 7.12 (s, 2H); 13C-NMR (75 MHz, C6D6, rt, δ in ppm) 4.6 (CH2), 8.2 (CH3), 24.1 (CH3), 24.5 (CH3), 34.7 (CH), 35.3 (CH), 121.0 (CH), 124.7 (C), 150.4 (C), 155.7 (C); 29Si-NMR (119 MHz, C6D6, rt, δ in ppm) -5.7, -82.8; HRMS (DART) m/z calcd for C21H34Si4 ([M+H]+): 349.2741, found: 349.2725; Anal. calcd for C20H34Si4: C, 72.33; H, 11.56%, found: C, 72.08; H, 11.56%.
A solution of TipSi(SiMe3)2H (239 mg, 0.632 mmol) and triethylsilane (2.0 mL, 12 mmol) in n-hexane (6.0 mL) contained in a quartz sealed tube was exposed to a low-pressure mercury lamp while immersed in a -78°C bath for 24 h. All of the volatile materials were removed under reduced pressure. The following PTLC (eluent: n-hexane) gave 13 (86 mg, 0.25 mmol, 39%).
This work was supported by Grants-in-Aid for Scientific Research on Innovative Areas (Nos. 24109013, 23108707), Scientific Research (B) (Nos. 22350017, 25288021), Scientific Research (C) (No. 26410044), Young Scientist (B) (No. 21750042), Global COE Program (B09, ‘Integrated Materials Science’), and Excellent Graduate Schools from the Ministry of Education, Culture, Sports, Science and Technology of Japan.
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