2630
Organometallics 1995, 14, 2630-2632
Preparation of (Pentamethyldisilany1)lithium and Two Isomers of (Heptamethyltrisilany1)lithium. Structural Characterization of (CH&Si2Li1 Akira Sekiguchi,* Masato Nanjo, Chizuko Kabuto, and Hideki Sakurai*li Department of Chemistry and Organosilicon Research Laboratory, Faculty of Science, Tohoku University, Aoba-ku, Sendai 980-77, Japan Received January 18, 1995@ Summary: (Pentamethyldisilany1)lithium(Me~SiSiMezLi, la) and two isomers of (heptamethyltrisilany1)lithium (Me[MezSilzSiMe&i, 1b; Me[Me3SilzSiLi, le) were prepared and isolated as highly inflammable colorless crystals by the reaction of the corresponding bis(oligosilany1)mercury species and lithium in toluene. The tetrameric structure of silyllithium la was established by X-ray difraction. The reaction of la with chlorosilanes gave the expected substitution product quantitatively i n hexane, whereas a n electron-transfer reaction occurred with benzyl chloride to form clecamethyltetrasilane and bibenzyl. Silyl anions are useful in some applications not only in organosilicon chemistry but also in organic synthesis.2 Gilman pioneered the preparation of phenylsubstituted silyl anion^,^ and later trialkylsilyl anions were i n t r ~ d u c e d .More ~ recently, a l k ~ x y and - ~ aminosubstituted6 silyl anions have been r e p ~ r t e d .Despite ~ the large number of these reports on monosilyl anions, (pentamethyldisilany1)lithium(MesSiSiMezLi, la),the simplest oligosilanyllithium with an Si-Si bond, had + Current address: Department of Industrial Chemistry, Faculty of Science and Technology, Science University of Tokyo, Noda, Chiba 278,Japan. Abstract published in Advance ACS Abstracts, May 1, 1995. (1)Chemistry of Organosilicon Compounds. 314. (2)(a)Fleming, I. In Comprehensive Organic Chemistry;Barton, D., Ollis, W. D., Eds.; Pergamon Press: Oxford, U.K, 1979;Vol. 3,pp 664669. (b) Lambert, J . B.; Schulz, W. J., J r . In The Chemistry oforganic Silicon Compounds;Patai, S., Rappoport, Z., Eds.; Wiley: Chichester, U.K., 1989;pp 1007-1010. (3)Gilman, H.; Lichtenwalter, G. D. J . Am. Chem. SOC.1958,80, 608. (4)EtsSiLi: (a) Vyazankin, N. S.; Razuvaev, G. A,; Gladyshev, E. N.; Korneva, S. P. J. Organomet. Chem. 1967,7,353. (b)Vyazankin, N. S.; Gladyshev, E. N.; Korneva, S. P.; Razuvaev, G. A. Zh. Obshch. Chem. Abstr. 1967,66,76050. (c) Vyazankin, Khim. 1966,36,2025; N. S.; Gladyshev, E. N.; Arkhangel'skaya, E. A.; Razuvaev, G. A.; Korneva, S. P. Izv. Akad. Nauk SSSR, Ser. Khim. 1968,2081;Chem. Abstr. 1969,70,334. MesSiNa: (d) Sakurai, H.; Okada, A.; Kira, M.; Yonezawa, K. Tetrahedron Lett. 1971,1511. (e) Sakurai, H.; Okada, A.; Umino, H.; Kira, M. J . Am. Chem. SOC.1973,95,955.(0 Sakurai, H.; Kondo, F. J. Organomet. Chem. 1975, 92, C46. MesSiK: (g) Sakurai, F.; Kira, M.; Umino, H. Chem. Lett. 1977, 1265. MesSiLi: (h)Hengge, E.; Holtschmidt, N. Monatsh. Chem. 1968,99, 340. (i) Still, W. C. J . Org. Chem. 1976,41,3063. (5) Watanabe, H.; Higuchi, K.; Goto, T.; Muraoka, T.; Inose, J.; Kageyama, M.; Iizuka, Y.; Nozaki, M.; Nagai, Y. J.Organomet. Chem. 1981,218,27. (6)Tamao, K.; Kawachi, A,; Ito, Y. J . Am. Chem. SOC.1992,114, 3989. (7)Theoretical studies on silyl anions: (a) Hopkinson, A. C.; Lien, M. H. Tetrahedron 1981,37, 1105. (b) Magnusson, E. Tetrahedron 1985,41,2945.(c) Schleyer, P. v. R.; Clark, T. J. Chem. Soc., Chem. Commun. 1986,1371.(d) Schleyer, P. v. R.; Reed, A. E. J.Am. Chem. SOC.1988,110,4453. (e) Hopkinson, A. C.; Rodriquez, C. F. Can. J. Chem. 1990,68, 1309. NMR studies on silyl anions: (0 Olah, G. A.; Hunadi, R. J . J . Am. Chem. SOC.1980,102, 6989. (g) Buncel, E.; Venkatachalam, T. K.; Eliasson, B.; Edlund, U. J . Am. Chem. Soc. 1985, 107, 303. (h) Edlund, U.;Lejon, T.; Venkatachalam, T. K.; Buncel, E. J. Am. Chem. SOC.1986,107,6408. (i) Edlund, U.;Lejon, T.; F'yykko, P.; Venkatachalam, T. K.; Buncel, E. J. Am. Chem. SOC. 1987,109,5982. @
Scheme 1 R' MenSiSiR'MeH Li / toluene r.t. , 12 h
a: R'
-
R2 = Me;
t
-BupHg / heptane 95 'c,
R'Me2SiSiR2MeLi 1 b: R'
-
-
+
(R'Me2SiSiR2Me)2Hg 2 Hg
SiMeg, R2 = Me; c: R' = Me, R2 = SiMe3
never been isolated and characterized, although its formation as a byproduct in the course of the preparation of (trimethylsily1)lithium (Me3SiLi)4ifrom hexamethyldisilane and methyllithium has been noted.* Silyl anions readily undergo scrambling and redistribution reactions, presumably by electron-transfer reaction^.^ We report herein the first successful isolation and full characterization of (pentamethyldisilany1)- and two isomeric (heptamethyltrisilany1)lithiumcompounds, together with the crystal structure of la and its reactions. For the preparation of (pentamethyldisilany1)lithium (la),we have adopted a strategy based on the lithiummercury exchange reaction.1° Thus, bidpentamethyldisilany1)mercu-y (2aP was subjected to the Li-Hg exchange reaction with excess lithium metal in toluene t o give colorless crystals of l a that could be recrystallized from pentane a t 0 "C. Like most other organolithium compounds, l a was highly inflammable in air (Scheme 1).l2 (Pentamethyldisilany1)lithium(la) is tetrameric in the solid state, as determined by X-ray difYraction.13The crystal structure of la possesses a crystallographic 2-fold axis, the ORTEP drawing of l a being shown in (8)(a) Hudrlik, P. F.; Waugh, M. A.; Hudrlik, A. M. J. Organomet. Chem. 1984,271,69. (b) Hudrlik, P. F.; Hudrlik, A. M.; Yimenu, T.; Waugh, M. A.; Nagendrappa, G. Tetrahedron 1988,44,3791.(c) Gong, L.; Leung-Toung, R.; Tidwell, T. T. J. Org. Chem. 1990,55,3634. (d) Nadler, E. B.; Rappoport, Z. Tetrahedron Lett. 1990,31,555. Matyjaszewski et al. reported the spectroscopic observation of PhMezSiSiMezLi as a mixture with PhMezSiLi in THF: (e) Ruehl, K. E.; Davis, M. E.; Matyjaszewski, K. Organometallics 1992,11,788.Allred et al. have reported the preparation of SyMell- anions: (DAllred, A. L.; Smart, R. T.; Van Beek, D. A., Jr. Organometallics 1992,11,4225. (9)(a)Carberry, E.; West, R. J . Am. Chem. Soc. 1969,91,5440.(b) Sakurai, H.; Okada, A. J. Organomet. Chem. 1972,35,C13. (10)Et6GezLi has been prepared by the reaction of (Et6GedzHg and Li. Bravo-Zhivotovskii, D. A.; Pigaev, S. D.; Vyazankina, 0. A,; Vyazankin, N. S. Izv.Akad.Nauk SSSR, Ser. Khim. 1984,2414;Chem. Abstr. 1985,102,132171. (11)Compound 2a was prepared as an air- and light-sensitiveyellow oil by heating a mixture of pentamethyldisilane and di-tert-butylmercury in heptane at 95 "C for 6 h 'H NMR (C&, 6) 0.21 ( 8 , 18 H), 0.41(s,12 H); 13CNMR (c&, 6) -0.14,0.64;zgsi NMR ('&De, 6) -5.74, 34.9. (12)Compound la: lH NMR (C7D8, 6) 0.22 ( 6 , 9 HI, 0.33(s, 6 H); 13C NMR (C,Da, 6) -2.51,-0.56 29SiNMR (C7D8, 6) -79.6 to -81.4 (m), -12.4;6Li NMR (C7D8, 6) 2.11;'H NMR (THF-d8, 6) -0.17 ( 6 , 9 H), -0.04 (s,6 H); 13CNMR (THFda, 6) -0.15,1.31;29SiNMR (THFds, 180 K, 6) -74.9(t,J = 18.8Hz), -10.6;6Li NMR (THF-de, 6) 0.58.
0276-733319512314-2630$09.00/0 0 1995 American Chemical Society
Communications
Organometallics, Vol. 14, No. 6, 1995 2631 Table 1. Reaction of MesSiSiMezLi (la) with Electrophiles" reagent
D2O MeaSiCl MezSiClz Cl(SiMe2)&1 PhCHzCl 0 2
products
yield (%)b
MesSizD Me(SiMe2)sMe Me(SiMe2)aMe Me(SiMe2)loMe PhCHzSizMe5 Me(SiMe&Me PhCHzCHzPh MesSizH (Me5Siz)zO Me(SiMe2)rMe
100 100 100 100 42
36 42
31 18 19
Aliquots of a stock solution of la (ca. 0.1 M) in hexane were used for the reactions. Determined by GLC. a
sWT4-
1
si3
c6
Figure 1. ORTEP drawing of la. Selected bond lengths (A):Lil-Li2: 2.749(7),Lil-Lil' = 2.795(7), Lil-Li2' = 2.783(7),Li2-Lil' = 2.783(7),Li2-Li2' = 2.819(8),Li1'Li2' = 2.749(7), Sil-Lil = 2.662(4),Sil-Li2 = 2.683(6), Sil-Lil' = 2.687(5),Si3-Lil= 2.715(6),Si3-Li2 = 2.700(6), Si3-Li2' = 2.650(5), Sil-Si2 = 2.348(0), Sil-C1 = 1.944(2),Sil-C:! = 1.919(4),Si3-Si4 = 2.344(0),Si3-C6 = 1.944(4), Si3-C7 = 1.928(4). Selected bond angles (deg): Si2-Sil-C1 = 101.9(1),Si2-Sil-C2 = 103.6(1), C1-Sil-C2 = 102.5(1),Si4-Si3-C6 = 101,8(1),Si4-Si3C7 = 102.6(1),C6-Si3-C7 = 101.6(1). Figure 1. The lithium atoms are arranged such that a tetrahedron with an average Li-Li distance of 2.780 A is defined.14 The Li-Li distance of the framework is somewhat longer than that of typical alkyllithium tetramers (Li-Li = 2.55-2.56 A).15 Substitution of carbon in tetrameric alkyllithium with silicon should lead to expansion of the lithium core due to the increased Li-Si bond length compared to that of Li-C. The pentamethyldisilanyl group caps each face of the tetrahedron with the three nearly equal Li-Si distances of 2.683 A (average). The equal length of the Li-Si bonds implies that each silicon participates equally in bonding to the three lithium atoms. The C-Si bond distance of 1.934 A (av) is considerably longer than the normal one (1.88 A). However, the Si-Si distance of 2.346 & . (av) is quite normal, suggesting no dative bonding exists as observed for (tris(trimethylsily1)silyl)(13) A single crystal (0.4 x 0.3 x 0.25 mm) of l a was sealed in a glass capillaly tube for data collection. Diffraction data were collected at 200 K on a Rigaku Denki AFCdR diffractometer with a rotating anode (45 kV, 200 mA) with graphite-monochromatized Mo KO. radiation (1 = 0.710 69 A). A total of 3852 reflections with 28 = 3-62" were collected. Crystal data: molecular formula SiaCzoHsoLir, M , = 553.2, orthorhombic; a = 9.354(2)A, b = 17.998(4)A, c = 22.881(5)A, V = 3852.1(16) A3, space group Abu2, Z = 4, D,= 0.954 g/cm3. The final R factor was 0.033 (R, = 0.035) for 2769 reflections with F, > 3dFo). (14) The hexameric structure of (LiSiMe& found by X-ray diffraction, was reported; see: (a) Schaaf, T. F.; Butler, W.; Glick, M. D.; Oliver, J. P. J . Am. Chem. Soc. 1974,96,7593. (b) Ilsley, W. H.; Schaaf, T. F.; Glick, M. D.; Oliver, J . P. J . Am. Chem. SOC.1980, 102, 3769. (15) (a)Dietrich, H. Acta CrystuZZogr. 1963,16, 681. (b) Weiss, E.; Lucken, E. A. C. J . J . Orgunomet. Chem. 1964,2, 197. (c) Weiss, E.; Hencken, G. J . Orgunomet. Chem. 1970,21, 265. (d) Dietrich, H. J . Organomet. Chem. 1981, 205, 291. (e) Weiss, E.; Lambertsen, T.; Schubert, B.; Cockcroft, J. K.; Wiedenmann, A. Chem. Ber. 1990,123, 79.
lithium.16 Of particular interest is the fact that the angles about silicon are significantly contracted, the sum of the angles being only 307.0'. The Si-Si bond distance of l a is in close agreement with the calculated value of 2.393 A for H3SiSiH2- at the 6-31G* 1 e ~ e l .In l~ addition, the calculation shows significant contraction of the angles about silicon (95" for H-Si-H). The unique structural feature of l a is also elucidated by its NMR spectra. The 29SiNMR resonance of the silicon attached to lithium was observed at -74.9 ppm as a triplet signal with equal intensities (J(6Li-29Si)= 18.8 Hz) in THF-dg at 180 K.12 Thus, the silicon atom is coupled only to one lithium atom, indicating that l a is monomeric in THF.ls Unusually small values of other coupling constants, J(29Si-29Si) = 19.1 Hz and J(13C29Si) = 9.5 Hz, suggest localization of the negative charge on the silicon atom attached t o the lithium. In hydrocarbon solvents such as toluene and benzene, the 29SiNMR spectrum differs from that in THF, appearing a t -79.6 to -81.4 ppm as an unresolved multiplet in toluene-d8.12 These results suggest that l a is aggregated in hydrocarbons, and a tetrameric structure may be involved. Due to the lesser ionic and more covalent character of the species in toluene than in the case for THF, the coupling constants J(29Si-29Si)= 43.2 Hz and J(13C-29Si) = 15.9 Hz are much larger than those in THF. By the same procedure, two isomeric (heptamethyltrisilanyl)lithiums, l b and IC,were prepared (Scheme l).19 As the anionic silicon centers are substituted by the trimethylsilyl group, a major contribution of stabilizing the ionic character comes from the trimethylsilyl groups, which is indicated by the lack of scalar coupling between silicon and lithium. Thus, the 29Si NMR spectrum of ICshowed a singlet signal a t -133.8 ppm in THF-d8 a t 180 K. Like la, however, l b showed the triplet signal at -62.7 ppm (J= 18.6 Hz) in THF-ds at 180 K. (16) The Si-Si bond distances of {[LiSi(SiMe3)&3DME} and (Me3Si)sSiLi(THF)s,determined by X-ray diffraction methods, were reported to be 2.342 and 2.330-2.331 A: (a) Becker, G.; Hartmann, H.-M.; Munch, A.; Riffel, H. 2.Anorg. AZZg. Chem. 1986, 530, 29. (b) Heine, A.; Herbst-Irmer, R.; Sheldrick, G. M.; Stalke, D. h o g . Chem. 1993, 32, 2694. (c) Dias, H. V. R.; Olmstead, M. M.; Ruhlandt-Senge, K.; Power, P. P. J . Organomet. Chem. 1993,462, 1. (17) Damewood, J . R., Jr.; Hadad, C. M. J . Phys. Chem. 1988,92, 33. (18) Other factors to be considered are the slow exchange of Li under the conditions of the NMR study and steric bulkiness due to an extra SiMez unit. (19) l b : 'H NMR (C7D8, 6) 0.36 (s,6 H), 0.18 (s,6 H), 0.15 (s,9 HI; 7Li NMR (C7D8, 6) 2.21; 13C NMR (C7D8, 6) -0.49, -1.21, -4.19; 29Si NMR (C7D8, 6) -14.2, -40.4, -73.5 (m). IC: 'H NMR (C7D8, 6) 0.36 (9, 3 H), 0.27 (6,36 H); 7Li NMR (C7Ds,6)2.18; 13C NMR (C7Ds, 6) 2.8, -1.6; Z9Si NMR (C7D8, 6 ) -8.3, -137 (m).
Communications
2632 Organometallics, Vol. 14, No. 6, 1995 The present X-ray and NMR data for la-c suggest that the delocalization of the negative charge is much smaller than for the carbanions due to the Si-Si distance being longer than that of Si-C.20 As a result, the oligosilanyl anions exhibit high reactivity owing to the substantial localization of the negative charge. On reaction of la with chlorosilanes,substitution reactions occurred even in hexane to give the corresponding oligosilanes quantitatively,whereas an electron-transfer reaction occurred with benzyl chloride to result in the formation of decamethyltetrasilaneand bibenzyl (Table 1). In these reactions, no dodecamethylcyclohexasilane formed; thus, no scrambling reaction occurred.
Further applications of (oligosilany1)lithiumspecies in organosilicon synthesis will be reported soon.21 Acknowledgment. We are grateful for the financial support of the Ministry of Education, Science and Culture of Japan (Specially Promoted Research No. 02102004 and the Grant-in-Aid for Scientific Research on Priority Area of Reactive Organometallics No. 05236102). M.N. thanks the Japan Society for Promotion of Science for the Fellowship for Japan Junior Scientists. SupplementaryMaterial Available: Text giving details of the X-ray experiment, figures giving additional views, and tables of atomic parameters, anisotropic temperature factors, and distances and angles for la (8 pages). Ordering information is given on any current masthead page. OM950035L
(20) It is known that (Me3Si)aSiLi is a stable silyl anion: Gilman, H.; Smith, C. L. J. Orgumnet. Chem. 1968,14, 91.
(21)Sekiguchi, A.; Nanjo, M.; Kabuto, C . ; Sakurai, H. J.Am. Chem. SOC.,in press.
