Synthesis and Facile Ring Expansion of Silylenecyclotetrasilane

Synopsis. A silylenecyclotetrasilane was synthesized and isolated at low temperature as marginally stable orange crystals. The silylenecyclotetrasilan...
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Organometallics 2010, 29, 1869–1872 DOI: 10.1021/om9010577

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Synthesis and Facile Ring Expansion of Silylenecyclotetrasilane Takeaki Iwamoto,* Yuichi Furiya, Hideki Kobayashi, Hiroyuki Isobe, and Mitsuo Kira Department of Chemistry, Graduate School of Science, Tohoku University, Aoba-ku, Sendai 980-8578, Japan Received December 10, 2009 Summary: Silylenecyclotetrasilane 4 synthesized as marginally stable orange crystals undergoes ring expansion to give cyclopentasilene 5 quantitatively with the activation parameters of ΔH q = 24.8 ( 1.0 kcal mol -1 and ΔS q = þ6.5 ( 3.2 cal mol -1 K -1. The activation parameters and DFT calculations support that the isomerization of 4 to 5 proceeds through stepwise double 1,2-silyl migrations. Since the first isolation of tetramesityldisilene by West et al.,1 various reactions of stable disilenes have been reported.2 One of the unique reactions of disilenes is an intramolecular double 1,2-migration of substituents over the silicon-silicon double bond (Scheme 1).3 West et al. have found that 1,2-diaryl migration between MesXylSidSiMesXyl and Mes2SidSiXyl2 (Mes =2,4,6-Me3C6H2, Xyl =2,6-Me2C6H3) proceeds intramolecularly through a 1,3-disilabicyclobutane-like transition state or intermediate in which two aryl groups are bridged across the Si-Si bond (dyotropic rearrangement).4,5 We have also observed 1,2-disilyl migration in acyclic tetrasilyldisilenes with an activation energy lower than those of the tetraaryldisilenes.6 Whereas various types of disilenes have been synthesized so far, these types of intramolecular migration reactions are still limited to a few acyclic disilenes. Recently we have synthesized the bicyclo[3.3.0]octasila-1(5)-ene 1 as a model for the Si(001) surface by the reductive dechlorination of the 1,1-dichlorocyclotetrasilane 2 (Scheme 2).7 Although a plausible mechanism can involve the initial formation of the bi(cyclotetrasilanylidene) 3 followed by its ring expansion to 1 via a 1,2-disilyl migration, such a ring expansion via a 1,2-migration of substituents over the SidSi double bond has never been elucidated. In the present paper, we wish to report the synthesis of a novel exocyclic *To whom correspondence should be addressed. E-mail: iwamoto@ m.tains.tohoku.ac.jp. (1) West, R.; Fink, M. J.; Michl, J. Science 1981, 214, 1343. (2) For recent comprehensive reviews of disilenes, see: (a) Okazaki, R.; West, R. Adv. Organomet. Chem. 1996, 39, 231. (b) Weidenbruch, M. In The Chemistry of Organic Silicon Compounds; Rappoport, Z., Apeloig, Y., Eds.; Wiley: Chichester, U.K., 2001; Vol. 3, p 391. (c) West, R. Polyhedron 2002, 21, 467. (d) Kira, M.; Iwamoto, T. Adv. Organomet. Chem. 2006, 54, 73. (3) For a recent review on silyl migrations, see: Kira, M.; Iwamoto, T. In The Chemistry of Organic Silicon Compounds, Rappoport, Z., Apeloig, Y., Eds., Wiley, Chichester, U.K., 2001,; Vol. 3, Chapter 16, p 853. (4) Yokelson, H.; Maxka, J.; Siegel, D. A.; West, R. J. Am. Chem. Soc. 1986, 108, 4239. Yokelson, H.; Siegel, D. A.; Millevolte, A. J.; Maxka, J.; West, R. Organometallics 1990, 9, 1005. (5) Reetz, M. T. Angew. Chem., Int. Ed. Engl. 1972, 11, 129. For a review on dyotropic rearrangement, see: Reetz, M. T. Adv. Organomet. Chem. 1977, 16, 33. (6) Kira, M.; Ohya, S.; Iwamoto, T.; Ichinohe, M.; Kabuto, C. Organometallics 2000, 19, 1817. Iwamoto, T.; Okita, J.; Kabuto, C.; Kira, M. J. Organomet. Chem. 2003, 686, 105. (7) Kobayashi, H.; Iwamoto, T.; Kira, M. J. Am. Chem. Soc. 2005, 127, 15376. r 2010 American Chemical Society

Scheme 1. 1,2-Migration of Substituents of Disilenes

Scheme 2. Plausible Mechanism of Formation of 1

Chart 1

disilene, the silylenecyclotetrasilane 4 (Chart 1), and its facile ring expansion to cyclopentasilene 5 via a 1,2-disilyl migration.8 Disilene 4 was synthesized selectively by the reductive coupling reaction of the 1,1-dibromocyclotetrasilane 6 and the 2,2-dichlorotrisilane 7 with potassium graphite at -40 °C in tetrahydrofuran (eq 1). The 1H NMR spectrum of the reaction mixture indicated the formation of disilene 4 as the sole product in 72% yield. Keeping the reaction mixture below 0 °C, removing the resulting salt by filtration, and recrystallizing from diethyl ether at -35 °C gave pure 4 as air-sensitive marginally stable orange crystals in 46% yield. The molecular structure of 4 was determined by 1H, 13C, and 29 Si NMR spectroscopy and X-ray analysis.9-11 Although the mechanism of the selective formation of disilene 4 is (8) For examples of base or Lewis acid induced ring expansion reaction of methylenecyclobutane to cyclopentene derivatives, see: Jiang, M.; Shi, M. Org. Lett. 2008, 10, 2239. Samuel, S. P.; Niu, T.; Erickson, K. L. J. Am. Chem. Soc. 1989, 111, 1429 and references cited therein. Published on Web 04/01/2010

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unclear, the combination of the starting materials 6 and 7 is important. When a mixture of 6 and the 2,2-dibromotrisilane (t-BuMe2Si)2SiBr2 was reduced with potassium graphite under similar conditions, a complex mixture including 4 and the unprecedented spiro[2.3]hexasilane 8 (Chart 2) was obtained.12

Chart 2

The molecular structure of 4 determined by X-ray analysis is shown in Figure 1.10 The geometry around the SidSi double bond is moderately trans-bent and twisted with bend angles θ of 16.9 and 15.0° for Si1 and Si2 atoms and a twist angle τ of 11.7°.13 The four-membered ring of 4 is almost planar, with the dihedral angle Si2-Si5-Si7-Si6 of 175.22(4)°. (9) Spectral data for 4: orange crystals; mp < 0 °C dec; 1H NMR (C6D6) δ 0.40 (s, 12 H, t-BuMe2Si), 0.42 (s, 36 H, SiMe3), 0.64 (s, 6 H, SiMe2), 1.08 (s, 18 H, t-Bu); 13C NMR (C6D6) δ -0.1 (t-BuMe2Si), 3.4 (SiMe3), 4.0 (SiMe2), 18.8 (C(CH3)3), 28.3 (C(CH3)3); 29Si NMR (C6D6) δ -58.1 (Si(SiMe3)2), -14.3 (SiMe2), -8.0 (SiMe3), 6.8 (t-BuMe2Si), 136.7 ((t-BuMe2Si)2SidSi), 162.2 ((t-BuMe2Si)2SidSi). (10) Crystal data for 4 (100 K): C26H72Si11, fw 693.83, monoclinic, space group P21/n, a = 13.572(1) A˚, b = 18.570(3) A˚, c = 17.526(2) A˚, β = 92.864(2)°, V = 4411.7(11) A˚3, Z = 4, Dcalcd = 1.045 Mg/m3, R1 = 0.0317 (I > 2σ(I)), wR2 = 0.0832 (all data), GOF = 1.082. (11) When the reaction mixture was warmed to room temperature, cyclopentasilene 5 was formed as a major product and obtained in 33% yield after recrystallization from Et2O. For details, see the Supporting Information. (12) Reduction of 6 and 2 equiv of (t-BuMe2Si)2SiBr2 with KC8 gave spiro[2.3]octasilane 8 in 69% yield as a major product. Experimental details and X-ray analysis of 8 are given in the Supporting Information. Spectral data for 8: pale yellow crystals; mp 244 °C dec; 1H NMR (C6D6) δ 0.32 (s, 12 H, t-BuMe2Si), 0.50 (s, 36 H, SiMe3), 0.66 (s, 12 H, t-BuMe2Si), 0.75 (s, 6 H, SiMe2), 1.15 (s, 36 H, t-Bu); 13C NMR (C6D6) δ 3.0 (tBuMe2Si), 3.5 (t-BuMe2Si), 4.9 (SiMe2), 5.5 (SiMe3), 21.1 (C(CH3)3), 30.0 (C(CH3)3); 29Si NMR (C6D6) δ -141.1 ((t-BuMe2Si)2Si), -120.5 (spiro Si), -65.3 (Si(SiMe3)2), -15.5 (SiMe2), -5.5 (SiMe3), 7.5 (t-BuMe2Si); UV-vis (hexane) λmax/nm (ε/M-1 cm-1) 248 (54 400), 259 (56 700), 286 (sh, 40 400), 320 (sh, 16 500). Anal. Calcd for C36H102Si14 (952.41): C, 47.92; H, 10.79. Found: C, 47.82; H, 10.61. (13) The bend angle θ is defined as an angle between the Si(sp3)-Si(sp2)-Si(sp3) plane and the axis through the Si(sp2)-Si(sp2) bond, and the twist angle τ is defined as the angle between the two axes that bisect the Si(sp3)-Si(sp2)-Si(sp3) angles as viewed along the SidSi axis. (14) Kira, M.; Maruyama, T.; Kabuto, C.; Ebata, K.; Sakurai, H. Angew. Chem., Int. Ed. Engl. 1994, 33, 1489. Sekiguchi, A.; Inoue, S.; Ichinohe, M.; Arai, Y. J. Am. Chem. Soc. 2004, 126, 9626. (15) The model compound 40 shown in Figure 4 is predicted to adopt an almost planar structure at the B3LYP/6-31G(d) level (see the Supporting Information). For examples of theoretical studies on the flat potential energy surface for trans-bending of disilenes, see: (a) Goldberg, D. E.; Hitchcock, P. B.; Lappert, M. F.; Thomas, K. M.; Thorne, A. J.; Haaland, A.; Schilling, B. E. R. J. Chem. Soc., Dalton Trans. 1986, 2387. (b) Olbrich, G. Chem. Phys. Lett. 1986, 130, 115. (c) Malrieu, J.-P.; Trinquier, G. J. Am. Chem. Soc. 1989, 111, 5916. (d) Trinquier, G.; Malrieu, J.-P. J. Phys. Chem. 1990, 94, 6184. (e) Trinquier, G. J. Am. Chem. Soc. 1990, 112, 2130. (f) Karni, M.; Apeloig, Y. J. Am. Chem. Soc. 1990, 112, 8589.

Figure 1. ORTEP drawing of silylenecyclotetrasilane 4. Thermal ellipsoids are shown at the 50% probability level. Hydrogen atoms are omitted for clarity. One t-BuMe2Si group involving the Si4 atom is disordered, with the site occupancy factors of major and minor silyl groups being 0.834(2) and 0.166(2), and only the major silyl group is shown. Selected bond lengths (A˚) and angles (deg): Si1-Si2 = 2.2061(8), Si1-Si3 = 2.3536(9), Si1-Si4 = 2.3755(9), Si2-S5 = 2.3722(8), Si2Si7 = 2.3625(8), Si5-Si6 = 2.3773(8), Si6-Si7 = 2.3646(8); Si2-Si1-Si3 = 113.47(3), Si2-Si1-Si4 = 124.42(3), Si3-Si1Si4 = 119.20(3), Si1-Si2-Si5 = 130.02(3), Si1-Si2-Si7 = 133.21(3), Si5-Si2-Si7 = 93.11(3), Si2-Si5-Si6 = 86.63(3), Si5-Si6-Si7 = 92.93(3), Si6-Si7-Si2 = 87.14(3); Si2-Si5Si7-Si6 = 175.22(4).

The SidSi double-bond distance of 4 is 2.2061(8) A˚, which is in the range of those of acyclic tetrasilyldisilenes (2.196(3)2.2598(18) A˚).6,14 The 1H NMR of disilene 4 in benzene-d6 shows a highly symmetric pattern containing two singlet signals due to 36 protons of SiMe3 groups and 6 protons of the SiMe2 moiety. The pattern suggests that disilene 4 adopts a trans-bent structure with a low barrier to planarity or a planar average structure in solution.15 The 29Si resonances of unsaturated silicon nuclei of 4 appear at 136.7 ((t-BuMe2Si)2Si) and 162.2 ppm (ring Si). As expected, disilene 4 undergoes thermal isomerization to give cyclopentasilene 5 quantitatively at 313 K in benzene-d6 (eq 2). Disilene 5 was isolated as air-sensitive orange crystals. The molecular structure of 5 was determined by NMR spectroscopy and X-ray analysis.16,17 In the solid state, the five-membered ring of 5 adopts an envelope conformation, in which the Si4 rests 0.75 A˚ outside of a mean plane defined by the remaining four silicon atoms Si1, Si2, Si3, and Si5 (Figure 2). The dihedral angle of the mean plane Si5-Si1Si2-Si3 and the plane Si3-Si4-Si5 is 33.4°. The distance of the SidSi double bond in 5 is 2.1926(6) A˚, which is close to 1 (2.180(3) A˚).7 While disilene 1 has a cis-bent SidSi double bond, the SidSi double bond of 5 adopts a slightly trans-bent (16) Spectral data for 5: yellow crystals; mp 207 °C; 1H NMR (C6D6) δ 0.38 (s, 36H, SiMe3), 0.45 (s, 12 H, t-BuMe2Si), 0.66 (s, 6 H, SiMe2), 1.08 (s, 18 H, t-Bu); 13C NMR (C6D6) δ -0.2 (t-BuMe2Si), 3.4 (SiMe3), 3.5 (SiMe2), 19.5 (C(CH3)3), 28.3 (C(CH3)3); 29Si NMR (C6D6) δ -104.8(Si(SiMe3)2), -23.7 (SiMe2), -9.5 (SiMe3), 5.9 (t-BuMe2Si), 150.2 (SidSi); UV-vis (hexane) λmax/nm (ε/103 M-1 cm-1) 448 (9.1), 335 (3.8), 284 (6.4); MS (EI, 70 eV) m/z (%) 692 (6, Mþ), 73 (100). Anal. Calcd for C26H72Si11 (693.79): C, 45.01; H, 10.46. Found: C, 44.87; H, 10.40. (17) Crystal data for 5 (120 K): C26H72Si11, fw 693.83, orthorhombic, space group P212121 (No. 19), a = 11.850(3) A˚, b = 16.464(4) A˚, c = 22.984(5) A˚, V = 4484.1(19) A˚3, Z = 4, Dcalcd = 1.028 Mg/m3, R1 = 0.0192 (I > 2σ(I)), wR2 = 0.0524 (all data), GOF = 1.072.

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Figure 2. ORTEP drawing of cyclopentasilene 5. Hydrogen atoms are omitted for clarity. Thermal ellipsoids are shown at the 50% probability level. Selected bond lengths (A˚) and angles (deg): Si1-Si2 = 2.1926(6), Si1-Si5 = 2.3449(6), Si1Si6 = 2.3619(6), Si2-Si3 = 2.3457(7), Si2-Si7 = 2.3591(6), Si3-Si4 = 2.3565(5), Si4-Si5 = 2.3654(6); Si2-Si1-Si5 = 110.75(2), Si2-Si1-Si6 = 127.29(1), Si5-Si1-Si6 = 121.67(2), Si1-Si2-Si3 = 110.799(17), Si1-Si2-Si7 = 128.41(2), Si3Si2-Si7 = 120.69(2), Si2-Si3-Si4 = 99.511(18), Si3-Si4Si5 = 109.63(2), Si4-Si5-Si1 = 98.77(2); Si5-Si1-Si2-Si3 = -3.85(2), Si4-Si5-Si3-Si2 = -147.70(2), Si4-Si3-Si5-Si1 = 145.51(2).

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Figure 3. Time course of the 1H NMR in the SiMe2 region during isomerization of 4 (solid circles) to 5 (open circles) at 313 K in C6D6. Scheme 3

geometry with bend angles θ of 5.34 and 3.12° for Si1 and Si2 atoms, respectively.

The first-order rate constants and the activation parameters for the isomerization of 4 to 5 were determined by monitoring the decrease of proton signals due to the SiMe2 unit of 4 (marked by solid circles in Figure 3) at various temperatures: ΔH q = 24.8 ( 1.0 kcal mol-1 and ΔS q = þ6.5 ( 3.2 cal mol-1 K-1.18 The positive ΔS q value suggests that the transition-state structure would not be restricted, and therefore any mechanisms involving simultaneous 1,2-silyl migration (dyotropic rearrangement) at the rate-controlling step, as observed for tetraaryldisilene (ΔS q = -36 cal mol-1 K-1),4 can be ruled out from the possible mechanism. Two possible stepwise paths for the isomerization of 4 to 5 can be envisaged (Scheme 3): (A) ring expansion giving cyclic silylene 9 followed by further migration of the t-BuMe2Si group and (B) migration of the t-BuMe2Si group, affording cyclotetrasilanylsilylene 10 followed by ring expansion.19 The thermal reaction of disilene 4 in ethyldimethylsilane, which is a typical trapping agent for silylene, gave only disilene 5 without formation of products resulting from the insertion of silylenes 9 or 10 into the Si-H bond of ethyldimethylsilane. Intramolecular migration of 4 to 5 would be much faster than the intermolecular Si-H insertion of the bulky silylenes.19 (18) An Eyring plot for the isomerization of 4 to 5 is shown in the Supporting Information. (19) Silyldisilene-disilanylsilylene rearrangements were observed by Sakurai and co-workers: (a) Sakurai, H.; Sakaba, H.; Nakadaira, Y. J. Am. Chem. Soc. 1982, 104, 6156. (b) Sakurai, H.; Nakadaira, Y.; Sakaba, H. Organometallics 1983, 2, 1484. (c) Sanji, T.; Mori, T.; Sakurai, H. J. Organomet. Chem. 2006, 691, 1169. See also: Ichinohe, M.; Kinjo, R.; Sekiguchi, A. Organometallics 2003, 22, 4621.

To distinguish the mechanisms, DFT calculations for the isomerization reactions of the model compound 40 to 50 , in which SiMe3 groups are replaced by SiH3 groups and t-BuMe2Si groups by SiMe3 groups, were carried out at the B3LYP/6-31G(d) level (Figure 4). Silylenecyclotetrasilene 40 is 11.0 kcal mol-1 higher in energy than cyclopentasilene 50 , mainly due to higher ring strain of the cyclotetrasilane ring.20 Two stepwise paths that correspond to paths A and B were predicted, while no concerted path connecting 40 and 50 was found. Silylene 100 in path B is 14.6 kcal mol-1 higher in energy than silylene 90 in path A, probably due to the higher steric and ring strain of 100 .20 In both stepwise paths, initial steps giving the silylenes 90 and 100 are the rate-controlling steps and the activation energy giving silylene 90 in path A (16.0 kcal mol-1) is 7.3 kcal mol-1 lower than that giving silylene 100 (23.3 kcal mol-1). These results suggested that the isomerization of 4 to 5 proceeds via path A rather than path B.21 (20) Inagaki et al. have shown that the strain energies (SEs) for the parent cyclotetrasilane, cyclopentasilene, and cyclopentasilane calculated at the B3LYP/6-311þþG(3df,2p)//B3LYP/6-31G(d) level are 12.9, 0.9, and 3.0 kcal mol-1, respectively (Naruse, Y.; Ma, J.; Inagaki, S. Tetrahedron Lett. 2001, 42, 6553). The energy difference between silylenecyclotetrasilene 40 and cyclopentasilene 50 (11.0 kcal mol-1) is roughly comparable to the difference in SE between cyclotetrasilane and cyclopentasilene (12.0 kcal mol-1), while the energy difference between 80 and 90 (14.6 kcal mol-1) is larger than the difference in SE between cyclotetrasilane and cyclopentasilane (9.9 kcal mol-1). (21) Although migration of the SiMe3 group on the cyclopentasilane ring to the divalent center of silylene 9 might be possible, steric repulsion around t-BuMe2Si groups greater than that around the SiMe3 groups would be responsible for the exclusive migration of the t-BuMe2Si group.

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SidSi bond dissociation reaction upon heating without isomerization reactions similar to the ring expansion of 4 to 5. In conclusion, we synthesized silylenecyclotetrasilane 4 selectively and disclosed the facile ring expansion reaction of 4 to cyclopentasilane 5 via stepwise 1,2-disilyl migrations. This ring expansion may be useful for the construction of unique unsaturated polycyclic silicon compounds.

Figure 4. Energy diagram for isomerization of silylenecyclotetrasilane 40 to cyclopentasilene 50 calculated at the B3LYP/ 6-31G(d) level.

The high migratory aptitude of trialkylsilyl groups compared with that of alkyl groups22 would make the isomerization kinetically feasible. Although the exocyclic tetraalkyldisilene 11 reported by Sakamoto et al. has highly strained disilacyclobutane moieties (Chart 3),23 it undergoes only a (22) Nagase, S.; Kudo, T. Organometallics 1984, 3, 1320. (23) Matsumoto, S.; Tsutsui, S.; Kwon, E.; Sakamoto, K. Angew. Chem., Int. Ed. 2004, 43, 4610. Tanaka, H.; Kwon, E.; Tsutsui, S.; Matsumoto, S.; Sakamoto, K. Eur. J. Inorg. Chem. 2005, 1235.

Acknowledgment. This work was supported in part by the Japan Science and Technology Agency (PRESTO (Synthesis and Control), T.I.) and by the Ministry of Education, Culture, Sports, Science, and Technology of Japan [Specially Promoted Research (No. 17002005, M.K. and T.I.) and a Grant-in-Aid for Scientific Research on Innovative Areas (No. 21108501, “pi-Space”, T.I.)]. Supporting Information Available: Text, figures, and tables giving details of preparation procedures, characterization data, X-ray analyses of 4 and 5, an Eyring plot for the thermal isomerization of 4 to 5, and theoretical calculations for model cyclic disilenes and CIF files giving the X-ray crystallographic data of 4, 5, and 8. This material is available free of charge via the Internet at http://pubs.acs.org.