1016
Organometallics 1995, 14, 1016-1022
Synthesis and Structures of Extremely Hindered and Stable Disilenes Hiroyuki Suzuki, Norihiro Tokitoh, and Renji Okazaki" Department of Chemistry, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113, Japan
Jun Harada, Keiichiro Ogawa, and Shuji Tomoda Department of Chemistry, The College of Arts and Sciences, The University of Tokyo, 3-8-1 Komaba, Meguro-ku, Tokyo 153, Japan
Midori Goto Natural Institute of Materials and Chemical Research, 1-1 Higashi, Tsukuba, Ibaraki 305, Japan Received October 4, 1994@ Extremely hindered and stable disilenes Tbt(Mes)Si=Si(Mes)Tbt, ((2)-and (E)-6;Tbt = 2,4,6-tris[bis(trimethylsilyl)methyllphenyl,Mes = 2,4,6-trimethylphenyl)were synthesized by a reductive coupling reaction of the corresponding overcrowded dibromosilane Tbt(Mes)SiBr2 (4) with lithium naphthalenide in THF. The molecular structures of (2)-and (E)-6 were determined by X-ray crystallographic analysis at 120 K. Compound (21-6 crystallizes in the monoclinic space group P21h with a = 22.212(3) A, b = 13.368(2)A, c = 29.725(3) A, ,l? = 91.344(9)", V = 8824(2) Hi3, and 2 = 4. Compound (E)-64&Hs crystallizes in the monoclinic space group P21h with a = 18.130(3) A, b = 18.466(3) A, c = 28.522(2) A, ,6 = 95.976(9)", V = 9497(2) A3, and 2 = 4. The X-ray structures show remarkable p amidalization around silicon atoms and elongation of the Si-Si double bond (2.195(4) for (2)-6 and 2.228(2)A for (E)-6),the values of which are the longest ones reported so far for disilenes having carbon substituents on the silicon atoms. Both disilenes were found to be stable for weeks in the open air even in a microcrystalline form, and they were gradually oxidized to afford stereospecifically the (2)-and (E)-1,3,2,4-dioxadisiletane compounds 10. Compound (21-10crystallizes in the monoclinic space group P 2 h with a = 22.420(5) A, b = 13.458(6) A, c = 30.417(4) A, ,l? = 90.96(1)", V = 9176(4) A3, and 2 = 4. Reaction of Tbt(t-Bu)SiBrz ( 5 ) with lithium naphthalenide proceeded in a different way to afford the benzosilacyclobutene 11, which was most likely an intramolecular C-H insertion product of the intermediary silylene Tbt(t-Bu)Si: (12).
r
Introduction Since the first isolation of a stable disilene derivative, tetramesityldisilene, by West et al. in 1981,l introduction of bulky groups on silicon atoms has been an approved method for the synthesis of stable disilenes.2 Although several disilenes have been synthesized and characterized by X-ray crystallographic a n a l y ~ e scar,~ bon substituents used for kinetic stabilization are restricted to only five groups, i.e., mesityl, 2,6-diethAbstract published in Advance ACS Abstracts, January 15, 1995. (1) West, R.;Fink, M. J.;Michl, J. Science (Washington, D.C.) 1981, 214, 1343. (2)For reviews, see: (a)West, R. Pure Appl. Chem. 1984,56, 163. (b) West, R. Angew. Chem., Int. Ed. Engl. 1987, 26, 1201. (c) Tsumuraya, T.;Batcheller, S. A,; Masamune, S. Angew. Chem., Int. Ed. Engl. 1991,30,902. (3)(a)Fink, M. J.;Michalczyk, M. J.; Haller, K. J.; West, R.; Michl, J. J. Chem. SOC.,Chem. Commun. 1983, 1010. (b) Fink, M. J.; Michalczyk, M. J.; Haller, K. J.; West, R.; Michl, J. Organometallics 1984,3,793.(c) Masamune, S.;Murakami, S.; Snow, J. T.; Tobita, H.; Williams, D. J. Organometallics 1984, 3, 333. (d) Watanabe, H.; Takeuchi, K.; Fukawa, N.; Kato, M.; Goto, M.; Nagai, Y. Chem. Lett. 1987,1341.(e) Shepherd, B. D.; Powell, D. R.; West, R. Organometallics 1989,8,2664. (0 Shepherd, B. D.; Campana, C. F.; West, R. Heteroat. Chem. 1990,I , 1. (g) Archibald, R. S.; Winkel, Y.; Millevolte, A. J.; Desuer. J. M.: West. R. Organometallics 1992.11.3276.(h) Kira. M.: Makyama, T.;Kabuto, C.yEbata, K.; Sakurai, H.'Angew.' Chem.,'Znt: Ed. Engl. 1994,33,1489. @
ylphenyl, 2,4,6-triisopropylphenyl,tert-butyl, and l-adamantyl, and the investigation of the steric influence on structures and reactivities of disilenes is still insufficient. Recently we have developed a new steric protection group, 2,4,6-tris[bis(trimethylsilyl)methyllpheny14 (denoted as Tbt hereafter), and reported its high efficiency in the isolation of highly reactive chemical species such as novel cyclic poly~halcogenides,~ highly strained tin-containing small-ring compounds,6 and heavy congeners of ketones (silanethione and germanethi~ne).~ We became interested in the effect of the Tbt group on the stability of a disilene and undertook the investigation of its synthesis, molecular structure, (4)(a) Okazaki, R.;Unno, M.; Inamoto, N. Chem. Lett. 1987,2293. (b) Okazaki, R.;Unno, M.; Inamoto, N.; Yamamoto, G. Chem. Lett. 1989,493.(c) Okazaki, R.;Unno, M.; Inamoto, N. Chem. Lett. 1989, 791. (5)(a)Tokitoh, N.; Takeda, N.; Imakubo, T.; Goto, M.; Okazaki, R. Chem. Lett. 1992,1599.(b) Tokitoh, N.; Suzuki, H.; Matsumoto, T.; Matsuhashi, Y.; Okazaki, R.; Goto, M. J . Am. Chem. SOC.1991,113, 7047. (6)Tokitoh, N.; Matsuhashi, Y.; Okazaki, R. J . Chem. Soc., Chem. Commun. 1993,407. (7)(a)Suzuki, H.; Tokitoh, N.; Okazaki, R.; Nagase, S. J. Am. Chem. SOC.1994,116, 11578. (b) Tokitoh, N.; Matsumoto, T.; Manmaru, K.; Okazaki, R. J . Am. Chem. SOC.1993,115, 8855.
0276-733319512314-1016$09.00/00 1995 American Chemical Society
Organometallics, Vol. 14,No. 2, 1995 1017
Extremely Hindered and Stable Disilenes
Scheme 1 TbtBr-
1-BuLi
SiF,
P TbtSiF,
LAH
- Tbt
RLi
NBS/BPO
\
SiH,
Table 1. Experimental Crystallographic Data for (2)-6, (E)-6.C10Hsland (2)-10
TbtSiHB 1
empirical formula fw cryst size, mm
Tbt, SiBr,
/
d
/
temp, K cryst syst space group unit cell dimens
R
4; R = Mes 5; R = t-BU
2; R = Mes 3; R = t-Bu
(2)-6 C72H140Si14 1399.10 0.30 x 0.20 x 0.15 120 monoclinic P21h
a, A
SiMe, Tbt =
'
SiMe, Mes = SiMe,
Me,Si SiMe,
Scheme 2 Tbt\
Tbt LiNaph (2.2 eq) Tb: / SiBr, v Si=S[
/
22.212(3) 13.368(2) c, A 29.725(3) b deg 91.344(9) v,A3 8824(2) Z 4 density (calcd), g cm-3 1.053 scan type 20-w no. of obsd rflns 6430 data to param ratio 8.3 largest diff peak, e k3 1.7 largest diff hole, e A-3 2.3 R, % 10.1 Rw, % 10.3 b, A
,Mes
Tbt\
+
Si=S[
Med 4
and reactivities. The present paper delineates detailed accounts of the synthesis and unique structures of the highly hindered and stable disilenes Tbt(Mes)Si=Si(Mes)Tbt ((27-6,cis form; (E)-6, trans form).8
Results and Discussion Synthesis of Highly Hindered Dibromosilanes. Our first attempts to introduce Tbt and Mes groups onto a silicon atom employed nucleophilic substitution reactions of aryllithium reagents on tetrafluorosilane. The Tbt-substituted trifluorosilane TbtSiFs could be synthesized by the reaction of TbtLi, prepared from TbtBr and t-BuLi, with tetrafluor~silane.~ Although addition of MesLi to the trifluorosilane gave Tbt(Mes)SiFz,the yield was quite low and the purification was very difficult. The corresponding hydrosilane TbtSiH3 (1) was chosen, therefore, as an alternative precursor for nucleophilic substitution. Arylsilane 1 was synthesized by the reduction of trifluorosilane TbtSiF3 with lithium aluminum hydride (LAH)in THF (Scheme 1). Reaction of 1 with MesLi gave 2 in good yield (83%). By using the same procedure as for 2, dihydrosilane Tbt(t-Bu)SiH2 (3)was also obtained. Bromination of 2 and 3 with N-bromosuccinimide (NBS) afforded dibromosilanes 4 and 5, respectively. Synthesis of Disilenes. Disilenes Tbt(Mes)Si=Si(Mes)Tbt (6)were readily synthesized as a mixture of 2 and E isomers (ca. 40% in total) by a reductive coupling reaction of 4 with lithium naphthalenide in THF (Scheme 2). The reaction mixture was roughly purified by chromatography. It should be noted that the disilenes did not undergo any significant decomposition during the chromatography even in the air. Pure (2)-6(30%)was isolated as a lemon yellow microcrystalline compound by filtration of the concentrated pentane suspension of the chromatographed mixture of the disilenes, while a mixture of (E)-6 and naphthalene was obtained as an orange oil by concentration of the filtrate. ( 8 ) Preliminary report: Tokitoh, N.; Suzuki, H.; Okazaki, R.; Ogawa, K. J.Am. Chem. SOC.1993,115, 10428. (9) Unno, M. Ph.D. Thesis, The University of Tokyo, Tokyo, 1988.
(a-10
(E)-6cioH8 C8zH148Si14 1527.27 0.25 x 0.25 x 0.20 120 monoclinic P21k
C72H140Si1402 1431.10 0.50 x 0.20 x 0.20 297 monoclinic P21h
18.130(3) 18.466(3) 28.522(2) 95.976(9) 9497(2) 4 1.068 20-0 7950 10.3 1.6 1.3 8.0 8.1
22.420(5) 13.458(6) 30.417(4) 90.96(1) 9176(4) 4 1.036 20-w 3834 4.8 0.4 0.4 10.4 5.4
Table 2. Selected Bond Lengths (A) and Angles (deg) for (2M Si(1)-Si(2) Si(1)-C(7) Si(2)-C( 19)
2.195(4) 1.90(1) 1.899(9)
Si( 1)-C(l) Si(2)-C(13)
Si(2)-Si(l)-C(l) Si(2)-Si(l)-C(7) C(l)-Si(l)-C(7)
134.6(3) 109.7(3) 114.5(4)
Si( l)-Si(2)-C(l3) Si(l)-Si(2)-C(l9) C(13)-Si(2)-C(19)
1.90(1) 1.91(1) 136.3(3) 109.5(3) 113.5(4)
Careful recrystallization of this orange oil from benzene gave reddish orange crystals of (E)-6including naphthalene (ca. 10%). Although the molecular composition of (21-6 was certainly confirmed by both high-resolution FAB-MS and elemental analysis, it exhibited a very complicated 'H NMR spectrum, suggesting greater steric congestion around the silicon atoms than in its E isomer. The 29Si NMR spectrum of (E)-6measured in benzene-ds showed only one signal at 66.49 ppm in the sp2 silicon region, while that of (22-6 exhibited four peaks with roughly equal intensity at 56.16, 56.74, 57.12, and 58.12 ppm, most likely due to the existence of two or more conformational isomers on the NMR time scale. Crystal Structures of (2)and (E)-6. Thermal ellipsoid diagrams indicating the molecular structures of (2)-and (E)-6a t 120 K are shown in Figure 1, and the crystallographic data for these structures are summarized in Table 1. Selected bond lengths and angles are listed in Tables 2 (for (21-6)and 3 (for (E)-6*CioHs). The degree of pyramidalization at the silicon can be gauged by the angles formed by the CaTl-Si-Cavl plane and the silicon-silicon bond axis, 9.8(4) and 7.6(4)"for (2)-6and 9.4(3) and 14.6(3)"for (E)-6,respectively. The twist anglelo along the silicon-silicon axis of (21-6(14(1)")is larger than that of (E)-6(8.7(8)"),reflecting the severe steric repulsion of (21-6 between the two Tbt groups facing each other (Figure 2). The steric repulsion of (Z)-6is also reflected in the large differences between the Si-Si-Cnt angles (136.3(3),134.6(3)")and the SiSi-CMe, angles (109.5(3), 109.7(3)"). The Si-Si-Cnt (10)The twist angles are taken as half the difference of the angles subtended by the Cmt-Si-Si and Cmt,-Si-Si planes (26.4") and the CMeB-Si-Si and CMe,,-Si-Si planes (-2.4")for (Z)-6 and the CTbtSi-Si and CMes-Si-Si planes (25.9") and Cmt,-Si-Si and CMes,-SiSi planes (8.5")for (E)-6.
1018 Organometallics, Vol. 14, No. 2, 1995
Suzuki et al.
Figurel. ORTEP drawings of (21-6 (a) and (E)-6cloHs (b) with thermal ellipsoid plots (30%probability). Table 3. Selected Bond Lengths (A) and Angles (deg) for (E)-6.ClOH8 Si( 1)-Si(2) Si(l)-C(l) Si(l)-C(7) Si(2)-Si(l)-C(l) Si(2)-Si(l)-C(7) C(l)-Si(l)-C(7)
2.228(3) 1.922(7) 1.895(8) 130.0(2) 120.1(2) 108.8(3)
Si(2)-C(13) Si(2)-C( 19) Si(l)-Si(2)-C(l3) Si(l)-Si(2)-C(l9) C(13)-Si(2)-C(19)
164.3(5)'
1.927(7) 1.894(7)
2.4(5)'
132.2(2) 115.8(2) 109.2(3)
angles of (E)-6(130.0(2), 132.2(2)")are also larger, albeit not so much compared to (2)-6,than the Si-Si-CMes angles (120.1(2), 115.8(2)"). As for the silicon-silicon double-bond length, the value for (E)-6(2.228(3)A) is unexpectedly larger than that for (Z)-6(2.195(4)A). It is interesting that the steric repulsion of (21-6 is reflected in the twist angle and the bond angles around silicon atoms, whereas that of (E)-6is reflected mainly in the Si-Si double-bond length. Structural compariand (E)-6with previously reported disilenes sons of (2)are summarized in Table 4. The Si-Si double-bond
26.4(7)'
166.9(6)'
159.1(4)'
(4-6 8.5(4)'
166.5(4)'
Figure 2. Newman projections of (2)and (E)-6along the Si-Si axis showing the twist angles.
Organometallics, Vol.14,No. 2, 1995 1019
Extremely Hindered and Stable Disilenes
Table 4. Structural Comparisons in Hindered Disilenes disilene
Si=Si/8,
LC-Si-Cldeg
2.143(2)
112.1(2) 121.5(2)
2.160( 1)
x
OHC
ref
126.7(2) 121.2(1) 11931) 115.4(2)
3
12 14
3f
113.9(1) 126.8(1)
6.5(1)
18
3b
123.86(8) 122.77(8)
0
0
3b
2.140(3)
117.6(2)
117.6(2) 124.8(2)
10
0
3c
2.144
117.5
120.8 121.6
3
0
3d
2.138(2)
115.4(2)
123.4(1) 121.2(2)
0
2.8
3e
2.157(2)
0
0
3g
2.152(3)
0 0
0 5.4(0)
3g 3h
0.1(0)
3h
10.2(0)
3h
2.228(2)
115.3(1)
120.3(1) 124.1(1)
2.202( 1)
112.5(0)
122.9(0) 124.5(0)
8.9(1)
2.251(1)
114.9(0)
126.3(0) 117.5(0)
0
2.195(4)
113.5(4) 114.5(4)
136.3(3) 134.6(3) 109.5(3) 109.7(3)
2.228(3)
108.8(3) 109.2(3)
132.2(2) 130.0(2) 120.1(2) 115.8(2)
CHO
7
bent anglddeg
113.2(1)
@ ...'
0
twist anglddeg
2.143( 1)
lengths of 6 are much larger than those of other disilenes having carbon substituents on the silicon atoms, which varied within a range from 2.138to 2.160 A.3J1 The difference in bond angles between two kinds of the Si-Si-C bonds of 6 is also larger than the corresponding ones for other disilenes. While sterically 8,12b and hindered alkenes, for example,compounds 7,12a 9,12c relieve their strain mainly by twisting along the O
LC-Si-Sildeg
8
u "+; t-Bu
9
C-C axis (28.6' for 7,17' for 8,375"for 9) rather than (11)Very recently, Kira et al. reported the syntheses and structures of tetrakis(trialkylsily1)disilenes RZSi-SiRz (R = i-PrzMeSi, t-BuMezSi, i-P1-3Si).~~ The X-ray structures of these disilenes showed unusual elongation of the central Si-Si double-bond lengths (2.202(1)-2.251(1)8,) and pyramidalization around central silicon atoms. They ascribe these structural deformations to both the steric and electronic influence of trialkylsilyl substituents. (12) (a) Krebs, A.; Kaletta, B.; Nickel, W.-U.; Riiger, W.; Tikwe, L. Tetrahedron 1986, 42, 1693. (b) Garratt, P. J.; Payne, D.; Tocher, D. A. J . Og.Chem. 1990,55,1909. (c) Brooks, P. R.; Bishop, R.; Counter, J. A.; Tiekink, E. R. T. J. Org. Chem. 1994, 59,1365.
14(1)
8.7(8)
9.8(4) 7.6(4)
this work
14.6(3) 9.4(3)
this work
Scheme 3
by pyramidalization or elongation of the C-C double bond (1.5% for 7,0.5% for 8,2.2% for 9,compared to the normal C-C double-bond length of 1.337 A), disilenes 6 relieve their strain by changing the bond angles of Si-Si-Cmt and Si-Si-CMes and lengthening the SiSi double bond (the degree of lengthening is 2.2% for (22-6and 3.8% for 031-6 compared to the mean value of Si-Si double-bond lengths (2.147 A)). These characteristic deformations of 6 indicate the "soft" nature of the Si-Si double bond in comparison with the C-C double bond. Oxidation of 6. Both disilenes (2)and (E)-6were found to be quite stable in the open air in the solid state. Upon exposure to the air, a finely powdered sample of (22-6underwent very slow oxidation with a half-lifetime of approximately 40 days (Scheme 3). Disilenes previously synthesized have been reported to be very reactive toward atmospheric oxygen; for example, tetramesityldisilene is oxidized completely within a few minutes3b which is and (E)-1,2-bis(l-adamantyl)dimesityldisilene, considered to be so far the most stable disilene, is oxidized with a half-life time of about 2 days.3e The and (E)-6relative remarkable stability of disilenes (2)-
Suzuki et al.
1020 Organometallics, Vol. 14,No. 2, 1995
Figure 3. ORTEP drawing of (Z)-lO with thermal ellipsoid plots (30% probability).
-I:[, ,
Table 5. Selected Bond Lengths (A)and Angles (deg) for @)-lo Si( 1)-Si(2) Si( 1)-O( 1) Si( 1)-0(2) Si(1)-C(l) Si(1)-C(7) Si(2)-Si(l)-O(l) Si(2)-Si(l)-O(2) Si(2)-Si(l)-C( 1) Si(2)-Si(l)-C(7) O(1)-Si(1)-0(2) O(1)-Si(1)-C(1) O(l)-Si(l)-C(7) O(2)-Si(1)-C(1) O(2)-Si( 1)-C(7) C(l)-Si(l)-C(7) Si(1)-Si(2)-O(1)
2.395(7) 1.680(9) 1.698(9) 1.91(2) 1.87(1) 44.8(3) 44.9(3) 136.0(5) 109.5(5) 88.2(5) 113.2(6) 111.9(7) 117.7(7) 108.5(6) 114.5(7) 4433)
Si(2)-O( 1) Si(2)-0(2) Si(2)-C(37) Si(2)-C(43) Si(1)-Si(2)-0(2) Si(l)-Si(2)-C(37) Si(l)-Si(2)-C(43) O(l)-Si(2)-0(2) O( 1)-Si(2)-C(37) 0(1)-Si(2)-C(43) 0(2)-Si(2)-C(37) 0(2)-Si(2)-C(43) Si(l)-O(l)-Si(2) Si(l)-O(2)-Si(2)
1.687(9) 1.69l(9) 1.89(1) 1.90(2) 45.2(3) 138.2(5) 108.0(6) 88.1(5) 119.7(7) 107.4(6) 113.9(6) 111.1(7) 90.7(5) 89.9(5)
to the previously reported hindered disilenes is apparentlY due to the high steric demand ofthe Tbt S O U P . Expectedly,13 the oxidation of (21-6 resulted in the quantitative fOITllatiOn of the (2)-1,3,2,4-diOXadiSiletane (Z)-lO, the geometry of which was confirmed by X-ray crystdlo@'aPhic analysis. F i W e 3 shows an ORTEP hawing of(Zk10,and the CWStdlograPhic data for this structure are summarized in Table 1. Selected bond lengths and angles are listed in Table 5. The si-& distance of (Z)-lO (2.395(7) A) is similar to those of previously reported d i o ~ a d i s i l e t a n e s but , ~ ~ the ~ si-0 distances (1.679(9), 1.698(9), 1.687(9), and 1.691(9)& are SkhtlY longer. The central dioxadisiletane ring is folded, and the dihedral angle formed bY the two 0-Si-0 planes is 18.6", indicating steric repulsion between the two Tbt groups facing each other. Oxidation of (E)-6also proceeded stereospecificallyto give the (E)-1,3,2,4-dioxadisiletane (E)-10quantitatively. (13)It has been established that oxidation of a disilene proceeds stereospecifically to afford a 1,3,2,4-dioxadisiletane.(a) Michalczyk, M. J.; West, R.; Michl, J. J. Chem. SOC.,Chem. Commun. 1984,1525. (b) Michalczyk, M. J.;West, R.; Michl, J. J.Am. Chem. Soc. 1984,106, 821.(c) McKillop, K.L.; Gillette, G. R.; Powell, D. R.; West, R. J.Am. Chem. SOC.1992,114, 5203. (d) Sohn, H.; Tan, R. P.; Powell, D. R.; West, R. Organometallics 1994,13, 1390.
Scheme 4
Tbt\ SIBr, LiNaph (2.2
/ t-B u
eq)
tau
5
12
intramolecular C-H insertion
t-Bu\
/"
Me@
11
Reaction of 5 with Lithium Naphthalenide. Reduction of Tbt- and t-Bu-substituted dibromosilane 5 with lithium naphthalenide proceeded in a different
way. No coupling product, i.e., the corresponding disilene, was obtained. The product was benzosilacy,-Iobutene 1 1 1 4 (Scheme 4). The formation of 11 may be rationalized in terms of an intramolecular C-H insertion of intermediary silylene 12,15 generated by a-elimination of 5. No dimerization of 12 takes place, probably because the bulkiness oft-Bu group is greater than that ofMes group, which makes the dimerization difficult. Although there is a precedent that a photochemically generated mesityl-substituted silylene inserts into a C-H bond of an ortho mesityl methyl group to afford benzosilacyclobutene,16 we could not exclude rigorously the possibility of involvement of a silyl anion O r silyl radical species as an intermediate at present. (14)Although the structure of 11 was confirmed by X-ray crystallographic analysis, the refinement has not yet been converged with satisfactory agreement factors because of the inferiority of the single crystal used. The final crystallographic analysis of 11 will be reported elsewhere. (15)Thermolysis of Tbt-substituted diazomethane, Tbt(H)CNz, resulted in the formation of the corresponding benzocyclobutene derivative, which was considered to be an intramolecular C-H insertion product of intermediary carbene Tbt(H)C:.5a (16)Fink, M. J.; Puranik, D. B.; Johnson, M. P. J.Am. Chem. SOC. 1988,110, 1315.
Extremely Hindered and Stable Disilenes
Conclusion Extremely hindered and stable disilenes (2)-and (E)-6 were synthesized by a reductive couplingreaction of the corresponding overcrowded dibromosilane 4 with lithium naphthalenide in THF. The intermediate of this reaction is probably a divalent silicon species (silylene), the involvement of which is suggested by the occurrence of the intramolecular C-H insertion in 5, giving 11. X-ray crystallographic analysis of 6 revealed extremely distorted structures due to great steric congestion by Tbt and Mes groups. The extent of pyramidalization around silicon atoms and elongation of Si-Si bonds is much larger than those of distorted alkene analogs, indicating that the nature of a silicon-silicon double bond is very "soft" compared to a carbon-carbon double bond. (2)and (E)-6were found to be stable for weeks in the open air, owing to the effective steric protection by the Tbt group. This extremely bulky substituent influences the reactivities of disilenes 6 to such an extent that they undergo a facile thermal dissociation into the silylene Tbt(Mes)Si: under mild conditions.8 We are now investigating the reactivities of 6, and the results will be published elsewhere.
Experimental Section General Procedure. All melting points were uncorrected. All solvents used in the reactions were purified by the reported methods. THF was purified by distillation from benzophenone ketyl before use. All reactions were carried out under an argon atmosphere unless otherwise noted. Preparative gel permeation liquid chromatography (GPLC) was performed on an LC908 instrument with JAI gel 1H+2H columns (Japan Analytical Industry) and chloroform as solvent. Dry column chromatography (DCC) was performed with ICN silica DCC 60A. Flash column chromatography (FCC) was performed with silica gel BW 300 (Fuji Davison Chemical). The 'H NMR (500 MHz), 13C NMR (125 MHz), and 29SiNMR (53.5 MHz) spectra were measured in CDC13 and C6D6 with a Bruker AMas 500 or JEOL EX-270 spectrometer using CHCl3 or internal standard. The values of chemical shifts in 29SiNMR spectra are shown only for the central silicon atoms, since the trimethylsilyl groups around the 0 ppm region appear as complex multiplets due to the anisotropic effect of Tbt and Mes groups. Elemental analyses were performed by the Microanalytical Laboratory of the Department of Chemistry, Faculty of Science, The University of Tokyo. Preparation of { 2,4,6-Tris[bis(trimethylsilyl)methyllpheny1)silane (1). To a solution of trifluoro{2,4,6-tris[bis(trimethylsilyl)methyl]phenyl}silane(13.94 g, 21.87 mmol) in THF (120 mL) was added LAH (1.7 g, 44.7 mmol) at 0 "C, and the solution was heated under reflux for 10 h. After the reaction mixture was cooled to room temperature, the reaction was quenched with 1M HCI aqueous solution and the organic layer was separated. The aqueous layer was extracted with 50 mL of hexane, and the organic layer was dried with MgS04. Removal of the solvent quantitatively afforded 1 as a white solid, which was recrystallized from ethanol. 1: mp 163-165 "C; 'H NMR (CDC13) 6 0.00 (s, 18H), 0.02 (9, 18H), 0.03 (s, 18H), 1.32 (s, lH), 2.00 (br s, lH), 2.03 (br s, lH), 4.18 (s, 3H), 6.28 (br s, lH), 6.40 (br s, 1H); I3C NMR (CDC13) 6 0.45 (91, 0.64 (q), 0.73 (q), 30.37 (d), 30.60 (d), 30.69 (d), 119.99 (s), 121.51 (d), 126.31 (d), 144.89 (s), 151.61 (s), 151.83 (SI. Anal. Calcd for C27H&%7.0.5H20: C, 54.74; H, 10.72. Found: C, 54.84; H, 10.56. High-resolution EI-MS (70 eV): mlz observed 582.3212, calcd for C27H62Si7 582.3237. Preparation of {2,4,6-Tris[bis(trimethylsilyl)methyllpheny1)mesitylsilane (2). To a solution of MesLi, prepared from MesBr (1.9 mL, 12 mmol) and t-BuLi (1.59 M, 15 mL) at
Organometallics, Vol. 14,No. 2, 1995 1021 -78 "C in THF (40 mL), was added a THF solution (90 mL) of 1 (5.83 g, 10 mmol) at -78 "C. The solution was stirred for 10 h, during which time it was warmed t o room temperature. To this solution was added a saturated aqueous solution of NH4C1, and the organic layer was separated. The water layer was extracted with 50 mL of hexane, and the organic layer was dried with MgS04. After removal of the solvent, the residue was recrystallized from ethanol t o afford 2 (5.79 g, 83%) as colorless crystals. 2: mp 169-171 "C; 'H NMR (CDC13) 6 -0.07 (s, 18H), -0.02 (s, 18H), 0.04 (s, 18H), 1.30 (s, lH), 2.09 (br s, lH), 2.24 (s, 3H), 2.30 (br s, lH), 2.42 (s, 6H), 5.04 (s, 2H), 6.25 (br s, lH), 6.41 (br s, lH), 6.80 (s, 2H); 13CNMR (CDC13) 6 0.57 (91, 0.74 (q), 0.90 (91, 21.04 (q), 24.32 (q), 28.53 (d),28.63 (d), 30.47 (d), 122.30 (d), 123.54 (s), 127.09 (d), 128.49 (d), 129.26 (s), 139.25 (s), 144.21 (s), 144.35 (s), 151.62 (s), 151.78 (s). Anal. Calcd for C36&Si7: c , 61.63; H, 10.34. Found: C, 61.39; H, 10.32. Preparation of {2,4,6-Tris[bis(trimethylsilyl)methyllphenyl}-tert-butylsilane(3). To a THF solution (20 mL) of 1 was added t-BuLi (1.61 M, 0.6 mL) at -78 "C. The solution was stirred for 10 h, during which time it was warmed t o room temperqture. After removal of the solvent, the residue was chromatographed (DCC, hexane) to afford 3 (390 mg, 83%)as colorless crystals. 3: mp 211-213 "C; 'H NMR (CDC13) 6 0.04 (s, 54H), 1.12 (s, 9H), 1.32 (s, lH), 1.99 (br s, lH), 2.04 (br s, lH), 4.24 (9, 2H), 6.28 (br s, lH), 6.41 (br s, 1H); I3C NMR (CDCl3) 6 0.71 (q), 0.94 (q), 1.25 (q), 18.35 (s), 29.88 (d), 29.94 (q), 30.26 (d), 30.54 (d), 121.25 (d), 125.28 (s), 126.00 (d), 144.99 (s), 152.52 (s), 152.66 (SI. Anal. Calcd for C31H70Si7-1.5H20: C, 55.86; H, 11.04. Found: C, 56.07; H, 10.89. High-resolution EI-MS (70 eV): mlz observed 638.3858, calcd for C31H70Si7 638.3862. Preparation of {2,4,6-Tris[bis(trimethylsilyl)methyllpheny1)dibromomesitylsilane (4). A solution of 2 (8.36 g, 11.9 mmol), NBS (4.7 g, 26.4 mmol), and a catalytic amount of benzoyl peroxide in benzene (150 mL) was heated under reflux for 1h. After removal of the solvent, the crude reaction products were dissolved in hexane. The filtrate, after removal of the solvent, was purified by recrystallization from ethanol to afford 4 (8.25 g, 81%)as colorless crystals. 4: mp 221-223 "C; 'H NMR (CDCl3, 340 K) 6 0.04 (s, 36H), 0.08 (s, 18H), 1.37 (s, lH), 2.25 (s, 3H), 2.62 (s, 6H), 2.79 (br s, lH), 2.91 (br s, lH), 6.33 (br s, lH), 6.44 (br s, lH), 6.81 (s, 2H); I3C NMR (CDC13) 6 0.89 (q), 1.52 (91, 1.80 (91, 20.91 (91, 26.47 (q),27.93 (d), 28.05 (d), 30.89 (d), 123.50 (d), 123.94 (s), 128.71 (d), 130.74 (d), 132.32 (s), 140.62 (s), 142.85 (s), 146.70 (s), 152.10 (s), 152.54 (s). Anal. Calcd for C36H7&i7Br2-H20: C, 49.28; H, 8.27; Br, 18.21. Found: C, 49.39; H, 8.49; Br, 18.53. Preparation of {2,4,6-Tris[bis(trimethylsilyl)methyllpheny1)dibromo-tert-butylsilane(5). A solution of 3 (1.10 g, 1.67 mmol), NBS (593 mg, 3.33 mmol), and a catalytic amount of benzoyl peroxide in benzene (40 mL) was heated under reflux for 1h. After removal of the solvent, the crude reaction products were dissolved in hexane. The filtrate, after removal of the solvent, was purified by recrystallization from ethanol t o afford 5 (1.24 g, 93%) as colorless crystals. 5: mp 218-220 "C; 'H NMR (CDCl3) 6 0.06 (s, 18H), 0.07 (s, 18H), 0.09 (8, 18H), 1.23 (9, 9H), 1.34 (s, lH), 2.30 (s, lH), 2.40 (s, lH), 6.32 (br s, lH), 6.43 (br s, 1H); I3C NMR (CDC13) 6 0.94 (q), 1.82 (91, 2.17 (q), 27.72 (91, 28.10 (s), 30.36 (d), 30.41 (d), 30.84 (d), 120.86 (s), 123.51 (d), 128.83 (d), 146.83 (s), 153.67 (s), 154.36 (s); high-resolution EI-MS (70 eV) mlz observed 796.2095, calcd for C31H6&i779Brs1Br796.2053. Preparation of (Z)-1,2-Dimesityl-l,2-bis{2,4,6-tris[bis(trimethylsily1)methyllphenyl}disilene ((2) -6)and (E)l,2-dimesityl-1,2-bis{2,4,6-tris[bis(t~methylsilyl)methyllpheny1)disilene ((l3-6).To a solution of 4 (2.26 g, 2.63 mmol) in THF (120 mL) was added lithium naphthalenide (17 mL) prepared from lithium dispersion (196 mg, 28.2 mmol) and naphthalene (3.38 g, 26.4 mmol) in THF (45 mL) at -78 "C, and the mixture was stirred for 10 h, during which time it was warmed to room temperature. After removal of the
1022 Organometallics, Vol. 14, No. 2, 1995
Suzuki et al.
lithium naphthalenide (1.2 mL), prepared from lithium dispersolvent, pentane was added to the residue to precipitate sion (142 mg, 20.4 mmol) and naphthalene (2.42 g, 18.9 mmol) inorganic salts. The filtrate was passed through FCC with in THF (27 mL), a t -78 "C, and the mixture was stirred for pentane as eluent. The resulting solution, after evaporation 10 h, during which time it was warmed t o room temperature. of pentane, was subjected to sublimation in vacuo for 3 h t o After removal of the solvent, pentane was added to the residue remove naphthalene. To the residue was added a small to precipitate inorganic salts. After removal of the solvent, amount of hexane, and analytically pure (21-6 (489 mg, 27%) the residue was chromatographed (GPLC) to afford the l-tertwas obtained by filtration. The filtrate, after evaporation of butyl-2,2-bis(trimethylsilyl)-4,6-bis[bis(trimethylsilyl)methyllhexane, was purified by recrystallization from benzene to benzo-1-silacyclobutene (11; 139 mg, 80%)as colorless crystals. afford (E)-6 (122 mg, 10%). Removal of naphthalene by 11: mp 177-179 "C; 'H NMR (CDC13) 6 -0.01 (s, 9H), 0.016 exhaustive sublimation under reduced pressure gave almost (s, 9H), 0.019 (s, 9H), 0.04 (s, 18H), 0.08 (s, 9H), 1.19 (s, 9H), pure (E)-6 as orange crystals. Since they are not single 1.35 (s, lH), 1.45 (br s, lH), 4.63 (s, lH), 6.28 (br s, lH), 6.32 crystals of sufficient quality for X-ray diffraction, the single (br s, 1H); I3C NMR (CDC13) 6 0.34 (q), 0.48 (q), 0.56 (q), 0.72 crystal containing naphthalene (1:l)was used for the X-ray (q), 1.34 (q), 2.45 (q), 20.64 (SI, 26.34 (s), 28.67 (41, 30.70 (d), analysis. (2)-6yellow crystals, mp 193-195 "C dec; Y3i NMR 31.07 (d), 118.91 (d), 125.92 (d), 133.51 (s), 147.03 (s), 147.15 ( C 6 ~ s6) 56.16, 56.74, 57.12, 58.12; W (pentane) Amax 378 (6 (s), 156.08 (5). Anal. Calcd for C31H~Si7.2HzO: C, 55.84; H, 14 700), 403 (16 000) nm; FAB-MS mlz (relative intensity) 1396 10.58. Found: C, 55.84; H, 10.78. High-resolution EI-MS: (M+, 1.5%), 698 (18.91, 625 (9.2), 537 (18.81, 464 (21.1), 73 mlz observed 636.3704, calcd for C3&8Si7 636.3706. (100.0);high-resolution FAB-MS mlz observed 1396.7701,calcd X-ray Data Collection. Single crystals of (27-6, (E)for C72H140Si14 1396.7725. Anal. Calcd for C ~ Z H M O S C, ~M: WloH8, and (Z)-lOwere grown by the slow evaporation of their 61.81; H, 10.09. Found: C, 61.60; H, 9.86. As mentioned in saturated solutions in hexane, benzene, and ethyl acetate at the text, (2)-6 showed a very complicated 'H NMR spectrum room temperature, respectively. The intensity data for (21-6 at room temperature, the raw chart of which is gjven as and (E)-6CloHs were collected on a Rigaku AFC6A diffractosupplementary material together with its partial magnificameter with graphite-monochromated Cu K a radiation (1 = tion. Its 'H NMR spectrum at higher temperature could not 1.541 84 A) at 120 K using an Oxford Cryostream cooler. The be measured because of the ready thermal decomposition of intensity data for (2)-10were collected on a Rigaku AFC5R (2)-6into silylene Tbt(Mes)Si: ( E ) - 6 orange crystals, mp 132diffractometer with graphite monochromated Mo Ka radiation 142 "C dec; lH NMR (C&C&, 350 K) 6 -0.10 (br s, 36H), (1= 0.710 69 A). The structures of (21-6and (E)-6CroHswere 0.15 (s, 36H), 0.27 (br s, 36H), 1.44 (s, 2H), 2.10 (6, 6H), 2.42 solved by direct methods with SHELXS-8617and refined by (br s, 4H), 2.60 (br s, 6H), 3.06 (br s, 6H), 6.54 (br s, 4H), 6.71 the block-diagonal least-squares method using XTAL3.2.I8The (br s, 4H); 29SiNMR (CsD6) 6 66.49; W (pentane) A,, 368 ( E structure of (2)-10was solved by direct methods with SHEIXS12 000), 425 (11000), 460 (sh, 8800)nm; FAJ3-MS mlz (relative 8617and refined by the full-matrix least-squares method. All intensity) 1396 (M+,10.5%),698 (68.8),537 (84.8),464.3 (53.71, the non-hydrogen atoms (except for the naphthalene carbons 73 (100.0); high-resolution FAB-MS mlz observed 1396.7701, calcd for C72H140Si141396.7725. Anal. Calcd for C~ZHMOS~M:in the case of (E)-6CloH8) were refined anisotropically, and all hydrogen atoms were located by calculation. The final C, 61.81; H, 10.09. Found: C, 62.00; H, 9.45. cycles of the least-squares refinements were based on 6430 Oxidation of (2)and (E)-6. Finely powdered (22-6 was (for (2)-6),7950 (for (E)-WloH8),and 3834 (for (2)-10)observed exposed to the air for 6 months, during which time the yellow reflections (Z > 2olZl for (2)-6,(E)-6*CioHe,and (2)-10)and 775 crystals gradually became colorless. 'H NMR analysis showed ((2)-6),776 ((E)-WloHs),and 793 ((Z)-lO)variable parameters. the exclusive formation of (2)-2,4-dimesity1-2,4-bis{2,4,6-trisCrystal data for all the molecules are summarized in Table 1. [bis(trimethylsilyl)methyl]phenyl}-l,3,2,4-dioxadisiletane ((2)Full details of the crystallographic analysis of (2)-6, (E)10). Recrystallization from ethanol gave analytically pure (2)6CioH8, and (Z)-lO are given in the supplementary material. 10 as white crystals. (21-10: mp >300 "C; 'H NMR (CDCl3, 350 K) 6 -0.49 (s, 18H), 0.01 (5, 18H), 0.09 ( s , 54H), 0.32 (s, Acknowledgment. This work was supported by a 18H), 1.37 (br s, 2H), 2.00 (s, 6H), 2.16 (s, 6H), 2.19 (8, 2H), Grant-in-Aid for Scientific Research (No. 05236102) 2.63 (s, 6H), 2.78 (s, 2H), 6.27 (s, 2H), 6.39 (s, 4H), 6.60 (s, from the Ministry of Education, Science and Culture of 2H); 13CNMR (CDC13, 340 K) 6 0.57 (q),O.93 (91, 1.61 (q), 1.76 Japan. We are grateful to Dr. H. Imoto and Mr. K. (q), 2.85 (q),2.93 (q),3.38 (q),3.60 (91, 3.74 (91, 3.99 (q),20.82 Tsuge, the University of Tokyo, for the X-ray crystal(q), 22.51 (d), 22.75 (d), 25.95 (d), 26.53 (91, 26.93 (d), 27.16 lographic analysis of (23-10. We also thank Central (q),27.35 (q), 30.65 (d),31.46 (d), 123.99 (d),125.31(d), 127.09 Glass, Shin-etsu Chemical, and Tosoh Akzo Co., Ltd., (d), 127.78 (s), 129.85 (d), 129.98 (d), 130.74 (d), 134.61 (s), for the generous gifts of tetrafluorosilane, chlorosilanes, 136.59 (s), 143.66 (s), 144.51 (s), 144.62 (s), 144.98 (s), 151.32 and alkyllithiums, respectively. (s),152.08 (9); 29SiNMR (CDCl3) 6 -6.04, -5.90. Anal. Calcd for C72H14002Si14: C, 60.43; H, 9.86. Found: C, 60.22; H, 9.89. Supplementary Material Available: Tables giving crysBy using the same procedure as for (21-6, (E)-6 gave tal data, atomic coordinates, temperature factors, bond lengths quantitatively (E)-2,4-dimesityl-2,4-bis{2,4,6-tris[bis(trimethand angles, and torsion angles for (2)-6, (E)-6CioHs, and (2)ylsilyl)methyl]phenyl}-l,3,2,4-dioxadisiletane((E)-lO)as color10 and a figure showing the 'H NMR spectrum of (21-6 (69 less crystals: mp >300 "C; 'H NMR (CDC13, 350 K) 6 -0.28 pages). This material is contained in many libraries on (br s, 36H), 0.04 ( 5 , 36H), 0.08 (br s, 36H), 1.30 (s, 2H), 2.15 microfiche, immediately follows this article in the microfilm (s, 6H), 2.33 (br s, 4H), 2.66 (br s, 12H), 6.25 (br s, 2H), 6.31 version of the journal, and can be ordered from the ACS; see (br s, 2H), 6.61 (s, 4H); I3C NMR (CDC13, 340 K) 6 1.21 (91, any current masthead page for ordering information. 2.00 (q), 2.29 (q), 2.57 (91, 3.45 (41, 20.76 (41, 27.17 (91, 27.83 (d), 28.44 (d), 30.75 (d), 123.55 (d), 129.12 (s), 129.82 (d), 130.78 OM940771+ (d), 133.87 (s), 139.65 (s), 144.04 (s), 144.58 (s), 153.21 (5); (17)Sheldrick, G. M. SHELX-86, Program for Crystal Structure 29SiNMR (CDC13)6 -4.39. Anal. Calcd for C~ZHMOOZS~M: C, Determination. University of Gdttingen, Gottingen, Germany, 1986. 60.43; H, 9.86. Found: C , 60.20; H, 10.10. (18)Hall, S. R.; Flack, H. D.; Stewart, J. M. XTAL3.2, Program for Reaction of 6 with Lithium Naphthalenide. To a Crystal Structure Determination. Universities of Western Australia, Geneva, and Maryland, 1992. solution of 5 (217 mg, 0.272 mmol) in THF (10mL) was added
