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Disilenyllithium from Tetrasila-1,3-butadiene: A Silicon Analogue of a Vinyllithium Masaaki Ichinohe, Kaori Sanuki, Shigeyoshi Inoue, and Akira Sekiguchi* Department of Chemistry, University of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan Received April 20, 2004 Summary: The reaction of dilithiosilane, (tBu2MeSi)2SiLi2 (2), with 1,1,2,2-tetrachloro-1,2-dimesityldisilane produced the tetrasila-1,3-butadiene derivative (tBu2MeSi)2SidSi(Mes)(Mes)SidSi(SiMetBu2)2 (3; Mes ) 2,4,6trimethylphenyl) as reddish purple crystals. The reaction of 3 with tBuLi in THF produced 1,1-bis(di-tert-butylmethylsilyl)-2-lithio-2-mesityldisilene (4) as red crystals, which is an sp2-type silyllithium species, by the reductive cleavage of the central Si-Si bond in 3. Since the isolation of tetramesityldisilene as the first stable SidSi doubly bonded compound in 1981,1 many stable disilenes have been prepared and characterized to date.2 However, the chemistry of the stable conjugated system of group 14 elements heavier than carbon is still not very well-known.2,3 In 1997, Weidenbruch et al. reported the synthesis and structure of hexakis(2,4,6triisopropylphenyl)tetrasilabuta-1,3-diene (1), which is the only known stable compound containing an Sid SiSidSi conjugated system.4 Very recently, we have also demonstrated the utility of 1,1-dilithiosilane and 1,1dilithiogermane in the preparation of a variety of doubly bonded derivatives of heavier group 14 elements.5-7 As a continuation of our work on the chemistry of unsaturated group 14 elements, we have examined the reaction of 1,1-dilithiosilane with 1,1,2,2-tetrachlorodisilane derivatives of the type RSiCl2-SiCl2R, with the hope of obtaining tetrasila-1,3-butadiene derivatives. And indeed, a novel tetrasila-1,3-butadiene derivative, (tBu2MeSi)2SidSi(Mes)(Mes)SidSi(SiMetBu2)2 (3; Mes ) 2,4,6trimethylphenyl) was synthesized by a new and different method from compound 1, a coupling reaction of R2SiLi2 with MesSiCl2SiCl2Mes. The reaction of 3 with tBuLi unexpectedly provided an easy and fast access to a disilenyllithium species, the first stable Si analogue of (1) West, R.; Fink, M. J.; Michl, J. Science 1981, 214, 1343. (2) For recent reviews on metallenes and dimetallenes of group 14 elements, see: (a) Weidenbruch, M. Eur. J. Inorg. Chem. 1999, 373. (b) Power, P. P. Chem. Rev. 1999, 99, 3463. (c) Escudie´, J.; Ranaivonjatovo, H.; Adv. Organomet. Chem. 1999, 44, 113. (d) Weidenbruch, M. In The Chemistry of Organic Silicon Compounds; Rappoport, Z., Apeloig, Y., Eds.; Wiley: New York, 2001; Vol. 3, Chapter 5. (e) Weidenbruch, M. Organometallics 2003, 22, 4348. (3) (a) For tetragermabutadiene derivatives, see: Scha¨fer, H.; Saak, W.; Weidenbruch, M. Angew. Chem., Int. Ed. 1999, 39, 3703. (b) For 1,2-disila-3-germacyclopenta-2,4-diene derivatives, see: Lee, V. Ya.; Ichinohe, M.; Sekiguchi, A. J. Am. Chem. Soc. 2000, 122, 12604. (4) Weidenbruch, M.; Willms, S.; Saak, W.; Henkel, G. Angew. Chem., Int. Ed. Engl. 1997, 36, 2503. (5) For dilithiosilane, see: (a) Sekiguchi, A.; Ichinohe, M.; Yamaguchi, S. J. Am. Chem. Soc. 1999, 121, 10231. (b) Ichinohe, M.; Arai, Y.; Sekiguchi, A.; Takagi, N.; Nagase, S. Organometallics 2001, 20, 4141. (6) For dilithiogermane, see: Sekiguchi, A.; Izumi, R.; Ihara, S.; Ichinohe, M.; Lee, V. Ya. Angew. Chem., Int. Ed. 2002, 41, 1598. (7) (a) Sekiguchi, A.; Izumi, R.; Lee, V. Ya.; Ichinohe, M. J. Am. Chem. Soc. 2002, 124, 14822. (b) Sekiguchi, A.; Izumi, R.; Lee, V. Ya.; Ichinohe, M. Organometallics 2003, 22, 1483.
a vinyllithium compound, which was isolated and fully characterized. The reaction of 2.5 equiv of bis(di-tert-butylmethylsilyl)dilithiosilane (2)5 with 1,1,2,2-tetrachloro-1,2-dimesityldisilane8 in dry THF proceeded immediately at room temperature to form the corresponding coupling product 1,1,6,6-hexa-tert-butyl-2,5-(di-tert-butylmethylsilyl)-3,4-dimesityl-1,6-dimethylhexasila-2,4-diene (3), as deep purple crystals in 11% isolated yield, which were separated by silica gel chromatography in a glovebox with degassed hexane as eluent (eq 1).9
The structure of 3 was determined by spectroscopic methods and finally confirmed by X-ray crystallography (Figure 1).10 The conformation of the tetrasilabutadiene moiety is highly twisted (the torsional angle Si1dSi2Si3dSi4 ) 72°), compared with 1 (the corresponding (8) Shibley, J. L.; West, R.; Tessier, C. A.; Hayashi, R. K. Organometallics 1993, 12, 3480. (9) Procedure for the synthesis of 3: bis(di-tert-butylmethylsilyl)dilithiosilane (2) was prepared by starting from 1,1-bis(di-tert-butylmethylsilyl)-2,3-bis(trimethylsilyl)silacyclopropene (300 mg, 0.58 mmol) with excess lithium in THF according to the published procedure.5 1,1,2,2-Tetrachloro-1,2-dimesityldisilane (102 mg, 0.23 mmol) was added to dilithiosilane 2, and then dry oxygen-free THF (7.0 mL) was introduced by vacuum transfer. The mixture was vigorously stirred at room temperature for 15 min, and the color of the reaction mixture immediately changed from reddish brown to dark purple. The solvent was evaporated, and the residue was subjected to column chromatography (silica gel, hexane: both materials were finely dried and degassed) in a glovebox. The reddish purple fraction was collected, followed by recrystallization from hexane to give deep purple crystals of 3 (25 mg) in 11% yield. Mp: 131 °C. 1H NMR (C6D6, δ): 0.31 (s, 6 H), 0.97 (s, 18 H), 1.10 (s, 6 H), 1.11 (s, 8 H), 1.29 (s, 18 H), 1.34 (s, 18 H), 1.85 (s, 6 H), 2.06 (s, 6 H), 2.98 (s, 6 H), 6.52 (s, 2 H), 6.72 (s, 2 H). 13C NMR (C D , δ): -2.1, 1.3, 21.2, 21.9, 22.2, 22.4, 22.6, 25.6, 28.0, 6 6 30.3, 30.4, 30.7, 31.3, 127.7, 128.3, 137.0, 139.0, 143.7, 145.3. 29Si NMR (C6D6, δ): 23.8, 71.5, 150.8. Anal. Calcd for C54H106Si8: C, 66.17; H, 10.90. Found: C, 66.10; H, 10.63; UV/vis (hexane): λmax/nm () 233 (sh, 18 800), 250 (sh, 10 600), 322(2400), 357 (3000), 413 (2300), 531 (3900). (10) Single crystals of compound 3 for the X-ray diffraction study were grown from the saturated toluene solution. Crystal data for 3‚ toluene at 120 K: C61H114Si8, fw ) 1072.24, triclinic, space group P1h , a ) 12.0630(5) Å, b ) 16.9760(6) Å, c ) 19.2960(5) Å, R ) 106.971(2)°, β ) 105.607(2)°, γ ) 102.579(2)°, V ) 3446.6(2) Å3, Z ) 2, Dcalcd ) 1.033 g cm-3. R ) 0.0811 (I > 2σ(I)), Rw ) 0.2306 (all data), GOF ) 1.081.
10.1021/om040056s CCC: $27.50 © 2004 American Chemical Society Publication on Web 05/20/2004
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Figure 1. Molecular structure of 3‚(toluene) with thermal ellipsoids drawn at the 30% level (toluene molecule and hydrogen atoms are omitted for clarity). Selected bond lengths (Å): Si1-Si2 ) 2.2003(12), Si1-Si5 ) 2.3952(11), Si1-Si6 ) 2.3914(11), Si2-Si3 ) 2.3376(11), Si2-C19 ) 1.905(3), Si3-Si4 ) 2.1983(12), Si3-C28 ) 1.901(3), Si4Si7 ) 2.4046(12), Si4-Si8 ) 2.3852(11). Selected bond angles (deg): Si2-Si1-Si5 ) 120.31(5), Si2-Si1-Si6 ) 116.20(5), Si5-Si1-Si6 ) 123.49(5), Si1-Si2-Si3 ) 131.80(5), Si1-Si2-C19 ) 122.33(10), Si3-Si2-C19 ) 104.92(10), Si2-Si3-Si4 ) 132.35(5), Si2-Si3-C28 ) 104.80(10), Si4-Si3-C28 ) 121.99(10), Si3-Si4-Si7 ) 120.41(5), Si3Si4-Si8 ) 115.31(5), Si7-Si4-Si8 ) 124.26(5). Selected torsional angle (deg): Si1-Si2-Si3-Si4 ) 72(2).
torsional angle is 51°).4 The four sp2 Si atoms are nearly planar: the sum of the terminal bond angles for Si1 and Si4 is 360.0°, and the sum of the internal bond angles for Si2 and Si3 is 359.1°. Each SidSi double bond is slightly twisted, and the dihedral angles between Si5Si1-Si6 and Si3-Si2-C19 and between Si2-Si3-C28 and Si7-Si4-Si8 are 14.6 and 16.6°, respectively. The SidSi double bond lengths are 2.2003(12) and 2.1983(12) Å, and the length of the central Si-Si single bond, which connects two SidSi double bonds, is 2.3376(11) Å. An increase in the torsional angle of SidSi-SidSi, twisting of each SidSi double bond, and elongation of the Si-Si bonds compared with 1 are mainly attributed to the steric hindrance of four tBu2MeSi groups on the terminal double bonded silicon atoms. The UV-vis spectrum of 3 shows the longest absorption peak at 531 nm, which is bathochromically shifted by ca. 100 nm relative to that of the related disilene ((tBu2MeSi)2Sid SiMes2, λmax 437 nm).5b Due to the extremely distorted structure of 3, it shows very unusual reactivity toward the reducing reagents. We have examined the reduction of 3 with various reducing reagents (Li metal, LiNp, and tBuLi). Among them, the reaction of 3 with tBuLi is relatively clean. Thus, 3 was allowed to react with 4 equiv of tBuLi in THF at -78 °C, and a rapid color change from purple to reddish brown occurred. The reaction mixture was warmed to room temperature slowly, and then the solvent was evaporated. The residue was recrystallized from hexane to give 1,1-bis(di-tert-butylmethylsilyl)-2lithio-2-mesityldisilene (4) as red crystals in 67% yield (eq 2).11 Disilenyllithium has been postulated as a key intermediate for the formation of 1 from tetrakis(2,4,6triisopropylphenyl)disilene; however, it was not isolated
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Figure 2. Molecular structure of 4 with thermal ellipsoids drawn at the 30% level. The lithium atom is disordered over two sites, Li1 and Li2, with occupancy factors of 0.5 each. One of the two sites for the lithium atoms (Li1) is represented, and the hydrogen atoms are omitted for clarity. Selected bond lengths (Å): Si1-Si2 ) 2.2092(7), Si1-Si3 ) 2.3716(7), Si1-Si4 ) 2.3685(7), Si2-Li1 ) 2.702(9), Si2-C19 ) 1.9243(19). Selected bond angles (deg): Si2-Si1-Si3 ) 113.43(3), Si2-Si1-Si4 ) 122.56(3), Si3-Si1-Si4 ) 123.78(3), Si1-Si2-C19 ) 109.79(6), Si1-Si2-Li1 ) 142.2(2), C19-Si2-Li1 ) 107.7(2).
and characterized.4 Disilene 4 is the first sp2-type silyllithium isolated and characterized in a pure form.12 The characteristic 29Si NMR signals for unsaturated silicon atoms are observed at 63.1 and 277.6 ppm and are assigned to the silyl-substituted sp2 silicon atom and the lithium-substituted silicon atom, respectively, by comparison with the calculated values for the model compound, (H3Si)2SidSi(Ph)Li.13 The formation of 4 can be rationalized by assuming an initial single-electrontransfer process involving intermediate formation of the anion radical of 3 and tert-butyl radical as a key radical pair, followed by the cleavage of the central Si-Si bond.14,15 The X-ray analysis shows that 4 has a monomeric structure solvated by THF molecules, and the lithium (11) Procedure for the synthesis of 4: crystals of the tetrasilabutadiene 3 (18 mg, 0.02 mmol) and tBuLi powder (5 mg, 0.08 mmol) were placed in a reaction tube. Dry oxygen-free THF (2.0 mL) was introduced by vacuum transfer, and the mixture was warmed from -78 °C to room temperature over 6 h. A rapid color change from purple to reddish brown was observed. The solvent was evaporated, followed by crystallization from hexane to give red crystals of 4 in 67% yield. 1H NMR (C7D8, δ): 0.22 (s, 3 H), 0.71 (s, 3 H), 1.32 (s, 18 H), 1.56 (s, 18 H), 2.31 (s, 3 H), 2.75 (s, 6 H), 6.93 (s, 2 H). 13C NMR (C7D8, δ): -3.8, -2.0, 21.4, 21.9, 22.0, 26.1, 30.7, 31.0, 127.1, 133.2, 139.8, 156.6. 29Si NMR (C7D8, δ): 15.1, 18.0, 63.4, 277.6. 7Li NMR (C7D8, δ): 0.31. (12) For Ar2GedGe(Ar)Li (Ar ) 2,6-diisopropylphneyl), see: Park, J.; Batcheller, S. A.; Masamune, S. J. Organomet. Chem. 1989, 367, 39. (13) The structure of the model compound, (H3Si)2SidSi(Ph)Li, was optimized at the B3LYP/6-31+G(d) level with Cs symmetry, and then NMR chemical shifts were calculated at the HF/6-311+G(2df,p) level. The calculated 29Si NMR chemical shifts are 139.1 and 282.5 ppm for the H3Si-substituted sp2 silicon atom and for the lithium/phenylsubstituted atom, respectively.
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atom is disordered over two sites, Li1 and Li2, with occupancy factors of 0.5 each (Figure 2: one of the two sites for the lithium atom (Li1) is represented).16 The geometry of the SidSi double bond is very similar to the appropriate part of 3. The Si1dSi2 double bond length is 2.2092(7) Å with a slight twist; the twisting
angle, which is defined by the angle between the two planes of Si3-Si1-Si4 and C19-Si2-Li1, is 15.2°. The disilenyllithium, representing a Si analogue of vinyllithium, would be very important for the synthesis of disilene derivatives. Further investigation into the reactivity of 4 is in progress.
(14) One of the reviewers has pointed out the possibility of nucleophilic cleavage of the Si-Si bond to give 4 and 1,1-bis(di-tertbutylmethylsilyl)-2-tert-butyl-2-mesityldisilene; however, this is unlikely. The 1H and 13NMR spectra of the reaction mixture of 3 with 4 equiv of tBuLi in THF-d8 showed the exclusive formation of 4 and isobutene as the sole side product. (15) The addition of LiH across the EdE′ bond (E, E′ ) Si, Ge) through single-electron transfer was observed in the reaction of 2- and 1-disilagermirenes with tBuLi; see: Lee, V. Ya.; Sekiguchi, A. Chem. Lett, 2004, 33, 84. (16) The single crystals of compound 4 for the X-ray diffraction study were grown from a saturated hexane solution in a glovebox. Crystal data of 4 at 120 K: C37H73LiO2.5Si4, fw ) 677.27, triclinic, space group P1h , a ) 11.7100(3) Å, b ) 12.2050(6) Å, c ) 16.2280(8) Å, R ) 72.562(2)°, β ) 81.609(3)°, γ ) 78.870(3)°, V ) 2161.47(16) Å3, Z ) 2, Dcalcd ) 1.041 g cm-3, R ) 0.0509 (I > 2σ(I)), Rw ) 0.1514 (all data), GOF ) 1.021.
Acknowledgment. This work was supported by a Grant-in-Aid for Scientific Research (Nos. 14078204 and 16205008) from the Ministry of Education, Science and Culture of Japan, and by the COE (Center of Excellence) program. Supporting Information Available: Tables giving the details of the X-ray structure determination, thermal ellipsoid plots, fractional atomic coordinates, anisotropic thermal parameters, bond lengths, and bond angles for 3 and 4. This material is available free of charge via the Internet at http://pubs.acs.org. OM040056S
