Reactions of Silenes: A New Silene to Silene Thermal Rearrangement

Sep 1, 1995 - Kaarina K. Milnes, Michael C. Jennings, and Kim M. Baines. Journal of the American Chemical Society 2006 128 (7), 2491-2501. Abstract | ...
0 downloads 0 Views 902KB Size
Download PDF
Organometallics 1996, 14,4359-4365

4359

Reactions of Silenes: A New Silene to Silene Thermal Rearrangement Paul Lassacher, Adrian G. Brook,* and Alan J. Lough Lash Miller Chemical Laboratories, University of Toronto, Toronto M5S 1A1, Canada Received May 9, 1995@ Photolysis of the acylsilane (Me3Si)TipSiCOAd at room temperature gave rise to two geometric isomers of (Me3Si)TipSi=C(OSiMe3)Ad which a r e less reactive t h a n previously reported silenes. The formation of the phenylacetylene adducts of these silenes required heating t o 90 "C, and no reaction was observed with (trimethylsilyllacetylene, styrene, or 2,4-hexadiene. The formation of their methanol adducts required heating to 100 "C and proved to be a stereospecific process. If the original silene was heated at 120 "C for 5 days, in hopes of eliminating hexamethyldisiloxane and forming a d y n e , a new rearranged silene (Tip)MeSi=C(SiMe2OSiMes)Adwas formed, which proved to be more reactive, giving adducts with phenylacetylene and methanol. A crystal structure of the silacyclobutene formed from this rearranged silene with phenylacetylene unequivocally confirmed its proposed structure. Heating of the rearranged silene to 150 "C resulted in its complete decomposition to unidentified products, and extended photolysis gave no change. The silenes (Me3Si)RSi-C(OSiMe3)Ad (R = Mes, Tip) reacted with benzophenone to give 1,2-siloxetanes but did not react with acetone. If E-Me3Si(Mes)Si=C(OSiMe3)Adwas heated together with styrene, a [2 21 cycloaddition product was formed. The cophotolysis of (Me3C)(Me3Si)2SiCOAdwith phenylacetylene or (trimethylsily1)acetylene gave rise t o silene cycloadducts, each a mixture of stereoisomers, suggesting t h a t the intermediate silene also existed a s a mixture of geometric isomers.

+

It is well established that the 1,3-shift of a trimethylsilyl group from silicon to oxygen in acylpolysilanes under photochemical or thermal conditions gives rise to ~ i l e n e s . l - Many ~ of these silenes are in equilibrium with their head to head dimers and/or readily revert to their parent acyl silane^.'*^ Silenes of the family (Me3Si)RSi=(OSiMes)Ad have been made, and with R = t-Bu only a single geometric isomer of a relatively stable silene was initially reported to have been f ~ r m e d . ~ When R = Mes, a mixture of relatively stable geometric isomers was obtained and characterized for the first time.6 Unlike previously described silenes these species did not revert to their parent acylsilanes on heating but instead slowly decomposed. We speculated that this decomposition might involve the elimination of hexamethyldisiloxane from the silenes, which would lead to the formation of compounds containing the hitherto unknown silicon-carbon triple bond, as shown in Scheme 1. Also, by introduction of increasingly sterically hindered R and R groups into the silenes, it was hoped that the elimination process would be facilitated and ~~~

~

Abstract published in Advance ACS Abstracts, July 15, 1995. (1)Brook, A. G.; Harris, J. W.; Lennon, J.; El Sheik, M. J. Am. Chem. SOC.1979, 101, 83. (2) Brook, A. G.; Nyburg, S. C.; Abdesaken, F.; Gutekunst, B.; Gutekunst, G.; Kallury, R., K. M. R.; Poon, Y. C . ;Chang, Y. M.; WongNg, W. J. Am. Chem. SOC.1982, 104, 5667. (3) Brook, A. G.; Vorspohl, K.; Ford, R. R.; Hesse, M.; Chatterton, W. J. Organometallics 1987, 6 , 2128. (4) Brook, A. G.; Nyburg, S. C.; Reynolds, W. F.; Poon, Y. C.; Chang, Y. M.; Lee, J . S.; Picard, J . P. J . Am. Chem. SOC.1979, 101, 6750. (5) Baines, K. M.; Brook, A. G.; Ford, R. R.; Lickiss, P. D.; Saxena, A. K.; Chatterton, W. J.; Sawyer, J. F Behnam, B. A. Organometallics 1998, 8, 693. (6) Brook, A. G.; Baumegger, A,; Lough, A. J. Organometallics 1992, 11, 3088. @

- Scheme 1

/ R' ,Si=C\ R OSiMe,

Me,Si,

heat

+

RSECR

Me,SiOSiMe,

Scheme 2

TpSi(SiMe,),

MeLVAdCOCI

0

hv

Tip-Si-C-Ad Me,h

Me3Si, ,Si=C TP

/

Ad

Tp,

\

OSiMe,

Me&/

/

Ad

Si=C\

OSiMe,

rarod:1 2a.b

the stability of the triple bonded species would be increased. To test this rationale, the 2,4,6-triisopropylphenyl (Tip) group was introduced into the adamantoylsilane 1, as shown in Scheme 2. Upon photolysis under mild conditions a t room temperature in deuteriobenzene, the new silene geometric isomers 2a,b were formed. Numerous attempts to separate and crystallize these silenes from various solvents have not been successful t o date, but the silenes have been fully characterized by lH, 13C,and 29SiNMR and MS spectroscopy, as well as by their phenylacetylene adducts 3a,b and their methanol adducts 4a,b (Scheme 3). Thus, when phenylacetylene was added to a 2:l mixture of silenes 2a,b (as established by 'H NMR spectroscopy),a 2:l mixture of the related l-silacyclobut2-enes 3a,b was formed after the solution was heated a t 90 "C for 12 h. These findings agree with the results of previous experiments,6 in which it has been shown

0276-7333/95/2314-4359$09.00/00 1995 American Chemical Society

4360 Organometallics, Vol. 14, No. 9, 1995

Lassacher et al. Scheme 3

lip I Me,Si-SiI Me0

H 1 C- Ad 1 OSiMe,

(Ma,

12hr

,Si=C,

1"C

pd osiMe,

H=Ph

W C , 12 hr

TiP

2 isomrs 2 1

2a,b

that the reaction of phenylacetylene with silenes is highly regio- and stereospecific. However, if (trimethylsily1)acetylene was used a s the reagent, no reaction occurred, even after heating of the solution for a prolonged time. Reaction of methanol with a 2:l mixture of silenes 2a,b did not immediately occur a t room temperature and proved to be a stereospecific process. If the reaction mixture was kept a t room temperature for 16 h, the major silene isomer reacted exclusively to give a single methanol adduct 4a whereas the minor silene isomer remained unreacted, as shown by 'H and 29Si NMR spectroscopy. The preparation of pure 4a was achieved by quenching the reaction mixture with silica gel at this time, followed by chromatography and recrystallization. To obtain a 2:l mixture of the methanol adducts 4a,b the reaction mixture had to be kept a t 100 "C for 12 h , a t which time NMR spectroscopy showed the silenes had completely reacted. Attempted separation of this mixture of isomers has not been successful and so 4b could not be obtained free of 4a. Even if the acylsilane 1 was cophotolyzed with methanol, most of the silenes remained unreacted after photolysis and the reaction mixture had to be treated as above to obtain the adducts 4a,b. The reaction of silenes with methanol has generally been observed to be a nonstereospecific p r ~ c e s s , ~ and the bulkiness of the Tip group is probably the reason that the reaction is stereospecific in this case. In the case of the mesityl-substituted silenes the reaction with methanol has been shown to be nonstereospecific6 CID (collision-induced dissociation) MS experiments with silenes 2a,b showed a small peak a t mlz = 378, which could be attributed to the hoped-for silyne, as well as a peak at mlz = 162, attributable to hexamethyldisiloxane. These products could arise from the elimination reaction according t o Scheme 1. The relative intensity of the signal attributed to the triple-bonded species is higher in the case of the Tip-substituted silenes than with the mesityl-substituted silene analogs. However, the intensity of their peaks compared t o the molecular ion indicates that this reaction pathway is not very important under mass spectrometric conditions. If the silenes 2a,b were heated, either in deuterioxylene or neat, a t 120 "C for 5 days, no evidence was found for formation of a silyne, but a remarkable rearrangement t o a new silene 6 occurred. This reaction could be followed easily by lH NMR spectroscopy and gave a 95%yieId of a single geometric isomer of 6 together with a small amount of unidentified byproducts. A possible mechanism for the formation of 6 is shown in Scheme 4. The initially formed silenes 2a,b either sequentially or concertedly must exchange the location of their Me3( 7 ) Kiro, M.; Maroyama, T.; Sakurai, H. J . A m . Chem. SOC.1991,

113, 3986.

F P psi&, Me,Si T i T - A d ttC=C--Ph

2 isomels, 23

4a,b

.

3a,b

Scheme 4

2a,b

1

1 ,J-Me$iO shiR

TP,

p d ,Si=C 'SiMe,OSiMe, Me 6

Si and MeaSiO groups. Then a 1,3-methyl shift from sp3-hybridized to sp2-hybridized silicon occurs, followed by a 1,3-shift of a trimethylsiloxy group from sp3hybridized to sp2-hybridized silicon yielding 6. Other MesSiO-substituted silenes have been shown t o undergo similar rearrangements involving MesSi and MesSiO interchange, as well a s 1,3-Me and 1,3-MesSiO migrations under photochemical condition^,^ but this was the first time that shifts of MesSi and MesSiO groups which are attached to the silicon-carbon double bond of silenes of this type have been observed to occur thermally. However, thermal 1,3-shifts of methyl groups in silenes with quite different structures have been observed by both E a b o d and Wiberg,g and AunerlO has observed 1,3-shifts of siloxy groups even a t low temperatures. If silene 6 was photolyzed for a n extended period, no further reaction occurred, but if it was heated to temperatures above 150 "C, complete decomposition took place. The position of the 29SiNMR signal of the sp2-hybridized silicon atom of 6 a t 108.0 ppm lies in between the positions of signals of silenes of the family MezSi=, which usually give resonance signals a t 126144 ~ p m , ~and J l those of the families (MesSi)2Si= and (MeaSi)RSi=, which give signals a t 51-73 The 13Cchemical shift of the sp2-hybridizedcarbon a t 127.3 ppm follows the same trend with the comparable ranges 77-118 ppm and 190-214 ppm, respectively, for the sp2-hybridized carbon atom in the families of silenes discussed above. When phenylacetylene was added to silene 6 a t room temperature, a single adduct 8 was formed (Scheme 51, whose geometry is shown by the structure diagram in (81 Eaborn, C.; Happer, D. A,; Hitchcock, P. B.; Hopper, S. P.; Safa, K. D.; Washburne, S. S.; Walton, D. A. R. J . Organomet. Chem. 1980, 186, 309.

(9) Wiberg, N.; Kopf, H. Chem. Ber. 1987,120, 653. (101 Ziche, W.; Auner, N.; Behm, J. Organometallics 1992,11,3805. (11)Wiberg, N.; Wagner, G.; Mueller, G. Angew. Chem., Int. Ed. Eng. 1985,24, 229. (12) Brook, A. G.; Abdesaken, F.; Gutekunst, G.; Plavac, N. Organometallics 1982, I , 994.

Reaction of Silenes

Organometallics, Vol. 14, No. 9, 1995 4361 Scheme 5 Tip

Ad

Me-SI-1 .

I

I C-SiMe,OSiMe,

li:Si=C/Ad

H

Me

1

Me0 0

l i p Ad I Ci8.-SiMe20SiMe3

\SiMe,OSiMe,

I a h Me4iI

H-C=C-

Table 2. Atomic Coordinates ( x lo4) and Equivalent Isotropic Displacement Parameters (A2 x 10s) for l a X

2071(1) 3166(1) 3769(1) 3320(1) 2224(2) 1757(2) 1577(2) 1857(2) 2174(2) 1818(2) 1055(2) 1893(2) 716(2) 789(2) 1550(2) 1081(2) 1909(2) 3389(2) 3807(2) 4555(2) 4025(2) 3209(2) 2771(2) 1528(2) 881(2) 654(2) 1066(3 1701(2) 1932(2) 1625(2) 1994(2) 1669(2) 982(2) 626(2) 930(2) 2766(2) 3175(2) 3003(2) 655(2) 1041(2) -110(2) 501(2) -274(2) 669(2)

Figure 1. Table 1. Summary of’crystal Data, Details of Intensity Collection, and Refinement Parameters

;: A8

C,

v, A3

z

Dcalcr Mg m-3 p(Mo Ka),mm-’ F(OO0) (0 scan width, deg range 0 collcd, deg index ranges

tot. no. refls no. obsd data [I > 2s(Z)1 R [ I > 2a(Z)] wR2 (all data) goodness of fit largesffmean hio params refined madmin density in AF map, e1A3

Ph

7

6

c1421

empirical formula M, cryst size, mm cry$ class, space group

I

C~oHszOSis 643.17 0.35 x 0.25 x 0.32 orthorhombic, Pbca 19.268(3) 18.712(3) 20.726(3) 7473(2) 8 1.143 0.156 528 0.53 3.0-22.5 h , -10 to 12; k, -20 to 20; 1, -22 to 2 3869 2536 0.038 0.084 0.866 0.00110.000 411 0.1771-0.205

Figure 1. Tables 1 and 2 show the details of the X-ray structure determination and the atomic coordinates and thermal parameters in 7. On the assumption that silene 6 reacted with phenylacetylene suprafacially the geometry of the adduct 7 indicates that the geometry of silene 6 must be E. This is most surprising because it had been anticipated that the preferred geometry would be that where there was minimal steric interaction between the bulky Tip and Ad groups, as would exist in the Z isomer. If silene 6 was treated with methanol, the reaction proved to be stereospecific, as in the case of 2 above, and a single adduct 8 was formed (geometry unknown). The steric bulk of the Tip group, and the fact that it is not freely rotating, as shown by the ‘Hand I3C NMR data, suggest that the silene 6 is locked in a conformation which allows addition of methanol from only one direction. When the silenes 2a,b were treated with benzophenone, only a single isomeric adduct of the siloxetane 5 was formed. The fact that no second isomer could be detected in this reaction again seems to be due to the

a

Y

8606(1) 9006(1) 9183(1) 9183(1) 8756(2) 9420(2) 9413(2) 8200(2) 7454(2) 6903(2) 6860(2) 7124(2) 7588(2) 7804(2) 7851(2) 8138(2) 8399(2) 9848(2) 8311(2) 9721(2) 8255(2) 9593(2) 8843(2) 9970(2) 10277(2) 10813(2) 11055(2) 10739(2) 10205(2) 7766(2) 7112(2) 6494(2) 6473(2) ‘7116(2) 7755(2) 7076(2) 7090(2) 6427(2) 5774(2 5429(2) 5831(2) 8437(2) 8324(2) 8853(2)

Ueq)

2

3334(1) 4432(1) 5854(1) 5191(1) 4280(1) 4147(2) 3524(2) 4746(1) 4669(1) 5097(2) 4916(2) 580l(2) 5019(2) 5724(2) 5896(2) 4591(1) 5466(1) 4005(2) 4193(2) 5766(2) 6041(2) 6479(2) 2745(1) 4618(2) 4538(2) 4943(2) 5438(2) 5534(2) 5130(2 3019(1) 2986(1) 2787(1) 2597(1) 2606(1) 2813(1) 3122(2) 2494(2) 3518(2) 2396(2) 1835(2) 2232(2) 2780(2 2816(2) 2178(2)

Ueq) is defined as one-third of the trace of the orthogonalized

U, tensor. Scheme 6 (Me,Si)RSi=C(OSiMe,)Ad

Ph,C=O

R OSiMe, I I Me,Si-Si-C-Ad

I

/

0-CPh

5 R=Tp 9 R=Mes

bulkiness of the Tip group, because when the mesitylsubstituted silenes reacted with benzophenone, two isomers of 9 were formed (Scheme 6). The ring silicon atoms of these cycloaddition products showed resonances a t 35-39 ppm in their 29SiNMR spectra. This is the region where the ring silicon atoms of known siloxetanes and disiloxetanes (both of which have silicon and oxygen atoms in a four-membered ring)

Lassacher et al.

4362 Organometallics, Vol. 14,No. 9, 1995 Scheme 8

Scheme 7 Me,Si,

,Ad Si=C, + Mes' OSiMe,

CH,=CHPh

-

yes BSiMe3 Me,Si-Si-C-Ad

I I H,C-CHP

h

10

are known to resonate.13 The proposed structure is also confirmed by the resonance frequencies of the ring carbon atoms and by all other spectroscopic methods. It was remarkable to find that neither the silenes 2a,b nor the related mesityl-substituted silenes reacted with acetone at room temperature. Even after 2 weeks at 40 "C a mixture of 2a,b and acetone in deuteriobenzene showed only the original silene signals in their NMR spectra. Heating of a solution of the Mes silenes for 16 h resulted in the formation of at least 5 products (as shown by lH NMR spectroscopy) which could not be separated. The stereospecific cycloaddition reactions which have been described above led us to examine the stereoselectivity of the reactions of some other unsaturated molecules with E-MesSi(Mes)Si=C(OSiMes)Ad.This silene could be easily obtained by heating a mixture of 2 and E isomers of the silene to 120 "C for 5 h, at which time NMR spectroscopy showed all the 2 isomer had decomposed, but more than 95% of the E isomer remained. It has already been shown6 that only one isomeric silacyclobutene is formed if the E-silene is treated with (trimethylsilyl)acetylene.6 The reaction of the E-silene with benzophenone also gave rise to only one isomer (geometry unknown) of the siloxetane 9, isolated in 85%yield, showing that this reaction is also highly stereospecific. Heating E-(MesSi)MesSi=C(OSiMes)Adwith styrene cleanly gave rise to only one isomer (geometry unknown) of the [2 21 cycloaddition product, the silacyclobutane 10 (Scheme 7). 2,4-Hexadiene did not react at all with this silene, suggesting that the electronic properties of phenyl groups activate double bonds toward reactions with silenes. The fact that acetylenes have been shown to react in a highly regio- and stereospecific manner with silenes led us to examine their reactions with the silene (MeaSi)(Me&)Si=C(OSiMe3)Ad which was previously reported to have been formed as a single geometric isomer based on NMR e v i d e n ~ e .When ~ the parent acylsilane of this silene was cophotolysed with either phenyl- or (trimethylsilyl)acetylenes, the formation of two isomeric adducts, l l a , b and 12a,b respectively, was observed by NMR spectroscopy, suggesting strongly that the intermediate silene must also have existed as a pair of geometric isomers, in contrast to the previous observation~ (Scheme ~ 8). In summary, it has been found that when very bulky groups such as Tip are attached to the sp2-hybridized silicon atom of a silene, the additions of methanol, benzophenone, styrene, and phenylacetylene are stereospecific reactions, only one isomeric product being formed from one of the silene geometric isomers. The related mesityl silene also reacted stereospecifically with

+

(13)(a) Brook, A. G.; Chatterton, W. J.; Sawyer, J. F.; Hughes, D. W.; Vorspohl, K. Organometallics 1987, 6, 1246. (b) Michalczyk, M. J.; West, R.; Michl, J. J. Chem. SOC.,Chem. Commun. 1984, 1525.

7

Me, i 0 I1 Me3C-Si-C-Ad Me,ki

[

+

HC-R

-b Me,C

#

OSiMe,

I

M e 3 c % s i = c : / 7 + Me3Si~Si-C-Ad Me3& HC= R

' !

2 isomers l l a , b R = Ph 12a,b R = SiMe,

benzophenone, styrene, and phenylacetylene. The bulky groups greatly reduce the reactivity of the silenes toward the above reagents, and in some cases no reaction occurs at all. Attempts to thermally induce the loss of hexamethyldisiloxane from these silenes to form a silyne failed, and a clean thermal silene-to-silene rearrangement was observed under the conditions employed. Experimental Section

All experiments were performed with oven-dried glassware under nitrogen using standard inert-atmosphere and vacuumline techniques. All reaction solvents were dried and distilled prior to use: diethyl ether and tetrahydrofuran were dried over sodium benzophenoneketyl, hexanes were dried over sodiumpotassium alloy, and benzene was dried over lithium aluminum hydride. Photolyses were carried out using three 100-W PAR 38 mercury spot lamps, whose output is mainly at 360 nm and longer wavelengths. All lH NMR spectra were obtained on a Gemini 200 spectrometer, and a Varian XL400 machine was used to record 13C and 29Si spectra. The spectra were run in CsDs unless otherwise specified. Where necessary, APT and DEPT pulse sequences were used in obtaining 13C spectra t o allow unambiguous assignment of signals. Most 29Sispectra were obtained using the DEPT pulse sequence. All mass spectra were run on a VG 70-2509 mass spectrometer operating in the electron impact (E11mode for both lowand high-resolution mass spectra. Elemental analyses were performed by Galbraith Laboratories Inc., Knoxville, TN. Melting points are uncorrected. Synthesis of (2,4,6-Triisopropylphenyl)bis(trimethylsily1)adamantoylsilane (1). Compound 1 was prepared by the cleavage of (Tip)Si(SiMe&in THF with 1equiv of MeLi, the resulting red solution being added at -78 "C t o 1equiv of AdCOCl in THF. After the mixture was stirred at -78 "C for 1 h and then at room temperature for 5 h; workup gave 43% of 1 after recrystallization from methanol. Mp: 128-130 "C. Anal. Calcd for C32H470Si3: C, 71.04; H, 10.43. Found: C, 70.72; H, 10.06. lH NMR: 6 0.42 (18 H, s, MesSi), 1.16 (6 H, d, p-Me),1.31(12 H, d, o-Me) 1.50-1.95 (15 H, m, Ad), 2.73 (1 H, m, p-CHI, 3.25(2 H, m, o-CHI, 7.12 (2 H, s, CH Tip). I3C NMR (6): 2.32 (SiMe3); 23.96 (p-CH3);26.80 (O-CH3);28.77 (Ad CHI; 34.29 (p-CH);36.08 (o-CH);36.95, 38.96 (Ad CH2); 52.41 (q-Ad);122.16 (Tip CHI; 130.23, 149.91, 155.68 (q-Tip);249.99 (C=O). 29SiNMR: 6 -13.67 (Me&), -45.53 (central Si).IR: 1610 cm-' (C=O). MS [miz (%)I (EI): 540 (5.3,M+); 525 (12.1, M+ - Me); 499 (M+ - z-Pr);467 (53, Mf - SiMes);377 (69, M+ - COAd); 303 (100). Photolysis of Acylsilane 1. Formation of Silenes 2a,b. A solution of 0.4 g (0.74 mmol) of acylsilane 1 in 0.6 mL of deuteriobenzene was photolyzed for 1 h at about 10 "C in a sealed NMR tube under nitrogen. At this time the 'H, 13C, and 29SiNMR spectra showed no evidence of the starting material and revealed the presence of 2 sets of signals,

Reaction of Silenes

Organometallics, Vol. 14,No. 9, 1995 4363

get the pure major isomer, the reaction mixture was not heated attributed to silenes 2a,b present in the ratio 2:l. The spectra but was kept at room temperature for 16 h, at which point showed no change when stored a t room temperature for NMR spectroscopy showed that only silene 2a had reacted. months or even when heated to 60 "C for 1week. Numerous The mixture was quenched with silica gel, chromatographed attempts to crystallize the silenes from various solvents have (using hexanesethyl acetate = 1OO:l as eluent) and recrystalfailed t o date. Data for 2a (major isomer) are as follows. 'H lized from acetone to give pure 4a. Data for 4a are as follows. NMR: 6 -0.11,0.30 (each 9 H, s, SiMe3); 1.21, 1.30, 1.43 (each Mp: 85-87 "C. 'H NMR: 6 0.29, 0.35 (each 9 H, s, SiMe3); 6 H, d, Me); 1.52-2.10 (15 H, m, Ad); 2.79 (1 H, m, p-CHI; 1.18 (6 H, d, p-Me); 1.42 (12 H, d, o-Me); 1.50-1.95 (15 H, m, 3.75 (2 H, m, o-CH); 7.07 (2 H, s, Tip CH). The coupling Ad); 2.65 (1H, m, p-CH); 3.32 (3 H, s, OMe); 3.73 (3 H, m, constants J of all the doublets in this and all the following HCAd and o-CHI; 7.20 (2 H, s, Tip H). 13C NMR 6 1.33,1.76 compounds always lay in the range 6.2 and 6.95 Hz, and their (SiMe3); 23.99,26.12,26.61 (Me); 28.96 (Ad CH); 33.09 (0-CH); magnitude was not specific to 0-or p-CH3 groups. I3C NMR 34.44 (p-CH); 37.41, 41.17 (Ad CHz); 38.45 (q-Ad); 52.24 (06 1.22 (Measi); 2.59 (OSiMe3);24.41, 25.10, 25.80 (Me); 29.39 Me); 79.99 (HCAd); 122.27 (Tip CHI; 130.23, 150.55, 156.85 (Ad CHI; 35.06 (p-CH); 38.13 (o-CH); 37.72, 42.73 (Ad CHz); (Tip C). 29SiNMR 6 15.39 (OSiMe3);1.42 (Si-OMe); -16.01 43.66 (q-Ad); 121.08 (Tip CH); 131.90, 150.85, 155.51 (q-Tip); (SiMea). HRMS: M+ - H calcd for C33H5902Si3 571.3823, 195.86 (C sp2). 29SiNMR: 6 40.33 (Si sp2); 12.51 (OSiMe3); found 571.3831; M+ - Me, calcd for C32H5702Si3 557.3666; -13.09 (SiMe3). Data for 2b (minor isomer) are as follows. found 557.3649. MS [mlz (%)I (EI): 571 (0.5, M' - HI; 557 lH NMR: 6 0.28, 0.39 (each 9 H, s, SiMe3); 1.14, 1.31, 1.44 (14, M+ - Me); 499 (27, M+- SiMe3); 467; 335 (100, MesSiSi(each 6 H, d, Me); 1.52-2.10 (15 H, m, Ad with 2a); 2.75 (1H, (OMe)Tip+). Anal. Calcd for C33H&Si3: C, 69.16; H, 10.55. m, p-CH overlapping with 2a);4.07 (2 H, m, o-CHI; 7.05 (2 H, Found: C, 68.99; H, 10.94. Data for 4b (together with 4a) s, Tip CH). 13C NMR: 6 1.52 (Measi); 2.38 (OSiMe3); 24.26, are as follows. 'H NMR: 6 0.18, 0.46 (each 9 H, s, SiMe3); 24.43, 25.30 (Me); 29.25 (Ad CHI; 34.80 (p-CHI; 38.80 (0-CH); 1.21 (6 H, d, p-Me); 1.40 (12 H, d, o-Me); 1.50-1.90 (15 H, m, 37.55, 42.15 (Ad CHz); 45.16 (q-Ad); 121.22 (Tip CHI; 133.64, Ad); 2.65 (1H, m, p-CH); 3.46 (3 H, s, OMe); 3.73 (3 H, m, 154.92, 155.59 (q-Tip); 191.04 (sp2C). 29SiNMR 6 43.31 (Si p-CH); 3.73 (1H, s, HCAd); 7.22 (2 H, s, Tip H). 13CNMR: 6 sp2); 12.95 (OSiMe3); -16.17 (SiMe3). CID-MS [mlz (%)I of 1.33, 1.76 (SiMe3); 23.99, 26.12, (Me); 29.01 (Ad CHI; 33.09 isomers (linked scan (BB) done in first field free region (FFR) (0-CH);34.50 (p-CH);37.41,41.37 (Ad CHz); 38.38 (q-Ad);52.35 using COz as collision gas, 80% transmission): 525 (27, M+ (0-Me);83.47 (HCAd); 122.08 (Tip CHI; 131.84,150.31,156.85 Me); 498 (62, M+ - C3H6); 467 (97, M+ - SiMes), 391 (331,339 (Tip C). 29SiNMR: 6 15.18 (OSiMe3);-1.78 (Si-OMe); -16.40 (511, 135 (100, Ad). (SiMe3). Synthesis of the Silacyclobutenes3a,b. A slight excess Synthesis of the Siloxetane 5. After 400 mg of the of dry phenylacetylene was added to a mixture of silenes 2a,b acylsilane 1 (0.74 mmol) in 0.6 mL of C6D6 was photolyzed for prepared as above. After heating of the sample to 60 "C, 13C 1h, 135 mg of benzophenone (0.75 mmol) dissolved in 0.4 mL NMR spectroscopy showed the presence of 2 adducts in the of C6D6 was added and the reaction mixture was kept at 50 ratio 2: 1. Several recrystallizations from various solvents did "C for 1 h, at which time NMR spectroscopy showed no not change the ratio of the products, but chromatography on remaining starting material. The solvent was removed, and silica gel (using hexanes as eluent) gave rise to pure 3a the resulting white foam was recrystallized twice from Et20 together with 3b, slightly contaminated with 3a. Data for 3a to give 0.42 g of 5 (yield = 80%). The spectroscopic data (major isomer, recrystallized from acetone) are as follows. showed no evidence of the formation of a second isomer. Data Mp: 152-155 "C. lH NMR: 6 -0.01,0.34 (each 9 H, s, SiMe3); for 5 are as follows. Mp: 125-128 "C. lH NMR: 6 -0.12, 1.24 (6 H, d, p-Me); 1.26, 1.32, 1.34, 1.55 (each 3 H, d, o-Me); 0.47 (each 9 H, s, SiMes); 1.07, 1.24, 1.36, 1.54 (each 3 H, d, 1.55-2.15 (15 H, m, Ad); 2.81, 3.42, 3.66 (each 1H, m, CH); Me); 1.26 (6 H, d, p-Me); 1.55- 2.20 (15 H, m, Ad); 2.81, 3.14, 7.02 (1H, s, ring CH); 7.12-7.20 and 7.66-7.69 (7 H, m, arom 4.02 (each 1 H, m, CHI; 6.95-8.05 (12 H, m, arom H). 13C H). 13CNMR: 6 1.43,3.16 (SiMe3);24.16 (p-Me);22.84,26.26, NMR: 6 2.17,4.45 (SiMe3);24.11 (2 C, Me), 22.95,24.42,27.90, 26.74, 27.39 (0-Me) 29.41 (Ad CHI; 32.88, 34.84, 36.48 (CH); 28.46, (Me); 29.22 (Ad CH); 31.83, 34.73, 35.27 (CH); 37.04, 37.58,39.70 (Ad CHz);41.30 (q-Ad);98.90 (ring C-Ad); 121.14, 38.66 (Ad CHz);42.30 (q-Ad);99.68, 111.13 (q-ring C) 120.48, 121.40 (Tip-CH); 127.27, 127.81, 129.11 (Ph CHI; 131.30 (Ph 122,12 (Tip CH); 126.73,127.02,126.84,127.65,129.90,130.54 ipso); 142.18, 150.39, 154.98, 155.71 (q-Tip); 144.06 (ring (Ph CH); 145.18, 146.63, 150.72, 153.84, 157.91 (arom q-C). =CHI; 166.90 (ring =CPh). 29SiNMR 6 7.57 (OSiMe3);-9.68 (ring Si); -16.21 (SiMe3). Anal. Calcd for C ~ O H ~ Z OC,S ~ ~ : 29SiNMR: 6 39.03 (ring Si); 4.01 (OSiMe3); -16.69 (SiMe3). HRMS: Calcd for C45H6602Si3, 722.4371; found, 722.4377. MS 74.72; H, 9.73. Found: C, 74.58; H, 9.54. HRMS: Calcd, [mlz (%)I (EI): 722 (6.6, M+);707 (2, M+ - Me); 649 (83, M+ 642.4109; found, 642.4098. MS [mlz (%)I (EI): 642 (5, M+); SiMe3);402 (100, (MesSiO)AdC=CPhz'). 569 (52, M+ - SiMe3); 319 (42); 277 (90); 245 (75); 135 (100, Ad); 73 (97, SiMe3). Data for 3b (minor isomer, together with Synthesis of the Silene 6. The silenes 2a,b, prepared as 3a) are as follows. IH NMR 6 0.34,0.49 (each 9 H, s, SiMe3); above, were heated to 120-125 "C for 5 days, either in 1.21 (6 H, d, p-Me); 1.32, 1.34, 1.41, 1.50 (each 3 H, d, o-Me); deuterioxylenes or neat. The reaction was monitored via 'H 1.40-1.90 (15 H, m, Ad); 2.78, 3.39, 3.84 (each 1 H, m, CHI; NMR spectroscopy, and after 5 days 95% of 6 was formed, together with Some unidentified byproducts. Numerous at6.89 (1H, s, ring CH); 7.10-7.30 and 7.80-7.84 (7 H, m, arom tempts to crystallize this new silene from various solvents H). 13CNMR: 6 1.57,4.23 (SiMea);23.08,23.73,24.05, 26.31, failed, but the spectroscopic data for 6 and its reaction products 26.85, 28.32 (Me); 29.26 (Ad CHI; 31.42, 31.99, 34.68 (CH); 7 and 8 confirm the proposed structure. Data for 6 are as 37.15,39.64 (Ad CH2);41.59 (q-Ad);97.83 (ring C-Ad); 120.99, follows. lH NMR: 6 0.25 (9 H, s, SiMes); 0.57 (6 H, s, SiMez); 122.03 (Tip-CH); 127.20, 127.81, 128.33 (Ph CHI; 132.42 (Ph 0.94 (3 H, s, SiMe); 1.14, 1.26, 1.42 (each 6 H, d, Me); 1.52ipso); 141.74, 151.10, 154.32, 155.78 (q-Tip); 139.63 (ring 2.10 (15 H, m, Ad); 2.72 ( l H , m, p-CHI; 3.69 (2 H, m, o-CH); =CH); 159.98 (ring =C-Ph). 29Si NMR: 6 3.27 (OSiMe3); 7.04 (2 H, s, Tip CH). 13C NMR: 6 2.62 (3 C, Me&); 4.92 (1 -12.63 (ring Si); -20.10 (SiMe3). C, SiMe); 8.86 (2 C, SiMez); 23.85, 24.09,25.84 (each 2 C, Me); Formation of the Methanol Adducts 4a,b. A solution 30.11 (Ad CH); 34.73 (P-CH); 39.53 (o-CH); 37.29, 47.80 (Ad of 0.4 mg of acylsilane 1 in 0.6 mL of deuterotoluene was CHz);40.98 (q-Ad);121.49 (Tip CH); 127.29 (sp2C); 138.76 (Tip photolyzed in a capped NMR tube for 1 h, at which point no C-ipso); 151.38, 151.88 (q-Tip). 29SiNMR (also using 29Si{1H) further starting material was present. To this was added 0.05 coupled spectra) (6): 108.08 (quartet, SiMe); 1.90 (septet, mL of dry methanol, and the reaction mixture had to be SiMez); 5.53 (m, SiMe3). MS [mlz (%)I (EI):540 (11,M+);525 maintained at 100 "C for 24 h before a quantitative formation (13, M+ - Me); 467 (100, M+ - SiMe3); 337 (26, M+ - Tip). of the methanol adducts was obtained. Chromatography and HRMS: M+, calcd for C32H560Si3 540.3639, found 540.3612; recrystallization from acetone yielded a 2 . 5 1 4a,b mixture of M+ - Me, calcd for C31H530Si3525.3404, found 525.3612 isomers which could not be separated by these methods. To

4364 Organometallics, Vol. 14,No. 9, 1995

Synthesis of the Silacyclobutene 7. To a solution of silene 6 in deuteriotoluene (prepared as above) was added a slight excess of phenylacetylene. The reaction mixture was kept a t 90 "C for 16 h. At this time NMR spectroscopy showed the reaction to be complete. The product was purified by chromatography using hexanes as eluent and subsequent recrystallization from acetone. Figure 1 shows a view of 7. Mp: 112-114 "C. lH NMR: 6 0.30 (9 H, s, SiMe3); 0.51 (3 H, s, SiMe); 0.69, 0.76 (each 3 H, s, SiMez); 1.19 (6 H, d, p-Me); 1.32-1.46 (4 d, o-Me, overlapping with Ad); 1.40-2.20 (Ad); 2.78, 3.28,4.14(each 1H, m, CHI; 6.80(1 H, s, ringCH); 7.107.30 and 7.90-8.00 (7 H, m, Tip and Ph-CH). NMR (in CDC13): 6 2.21 (Me&); 4.80, 5.15, 8.12 (SiMe); 22.73, 23.83, 23.88, 24.11, 26.74, 27.10 (Me); 29.25 (Ad CHI; 32.74, 34.12, 35.84 (CHI; 36.71, 42.26 (Ad CH2); 40.20 (q-Ad); 57.94 (ring C-Ad); 136.48 (ring =CH) 163.89 (ring =CPh); 121.11, 121.58 (Tip CH); 127.28, 127.43, 127.53 (Ph CHI; 135.31 (Ph ipso); 142.15, 149.91, 153.66, 154.55 (q-Tip). 29Si NMR 6 6.86 (OSiMe3);5.28 (SiMe2); -9.70 (ring Si). HRMS: M+, calcd for C&&Si3 642.4108; found 642.4132. MS [mlz (%)I (EI): 642 (5, M+); 627 (4, M' - Me); 335 (100, TipSiMe(OSiMea)+); 147 (75, Me3SiOSiMe2+);73 (43, Me3Si+). Synthesis of the Methanol Adduct 8. To a solution of silene 6 (prepared as above) was added a slight excess of dry methanol, resulting in an exothermic reaction and the disappearance of the yellow color. After 1 h a t room temperature the solvent and excess methanol were removed under vacuo, and the resulting white foam was purified by chromatography on silica gel (using hexanes:ethyl acetate = 1OO:l as eluent) and subsequent recrystallization from acetone. Data for 8 are as follows. Mp: 94-97 "C. lH NMR: 6.0.22 (9 H, s, SiMe3); 0.51, 0.53 (each 3 H, s, SiMez); 0.91 ( 3 H, s, SiMe); 1.18, 1.37, 1.39 (each 6 H, d, Me); 1.59-2.15 (15 H, m, Ad); 2.75 (1H, m, p-CH); 3.09 (3 H, s, OMe); 1.53 ( l H , s, Ad-C-H 3.70 (2 H, m, o-CH); 7.19 (2 H, s, Tip CH). NMR: 6 2.42 (MesSi); 5.33 (SiMe); 6.05, 6.17 (SiMez); 23.99, 24.02, 26.36 (Me); 29.39 (Ad CH); 32.00, 34.48, 35.92 (CH); 36.59 (q-Ad); 37.07, 45.47 (Ad CH2);49.59 (OMe) 122.16 (Tip CH); 132.57, 150.04, 155.58 (qTip). 29Si NMR (also using 29Si{1H}coupled spectra) (6): 9.49 (OSiMe3); 7.15 (SiMez); 6.30 (SiOMe). HRMS: M+, calcd for C33H6002Si3572.3901; found 572.3883; M+ - Me, calcd for C32H5702Si3 557.3666; found 557.3653. MS [mlz (%)I (EI): 572 (2, M+); 557 (30, M' - Me); 541 (7, M+ - OMe); 369 (34, M+ - Tip); 277 (100, TipSi(OMe)Me+);147 (34, Me3SiOSiMez+); 73 (17, SiMes'). Anal. Calcd for C33H6002Si3: C, 69.16; H, 10.55. Found: C, 68.86; H, 10.78. Synthesis of the Siloxetane 9. A solution of 415 mg of mesitylbis(trimethylsily1)adamantoylsilane in 0.7 mL of deuterioxylenes was photolyzed for 50 min. At this time 'H NMR spectroscopy showed complete conversion to silene geometric isomers in the ratio 1.4:l. This solution was heated to 120 "C for 5 h, a t which time the minor isomer was completely decomposed but more than 90% of the major isomer remained undestroyed. A solution of 165 mg of benzophenone in 0.3 mL of C& was added a t room temperature, and NMR spectroscopy showed all the silene to be reacted after 30 min. Recrystallization from hexanes yielded pure 9 (85%). The 29Si and some of the NMR data of a second isomer could be obtained when the reaction was carried out without destroying the minor silene isomer, but this product could not be separated from the major isomer, and most 'H and 13C NMR signals overlapped. Data for 9 are as follows. Mp: 145-147 "C. 'H NMR: 6 -0.22,0.39 (each 9 H, s, SiMe3); 1.60-2.2 (15 H, m, Ad); 2.06, 2.43, 2.90 (each 3 H, s, CH3); 6.63, 6.77 (each 1H, s, Mes-HI; 6.90-7.30 and 7.71-8.02 (10 H, m, Ph-H).W NMR: 6 1.09, 2.99 (SiMe3); 21.07, 24.42, 28.09 (CH3);29.29 (Ad CHI; 37.21, 39.74 (Ad CH2); 41.56 (q-Ad); 100.13, 109.73 (ring C); 126.44, 126.95 (Mes CHI; 142.34, 145.96, 136.01, 146.70 (q-Mes); 127.2-132.5 (Ph C, partial overlapping). 29Si NMR: 6 36.25 (ring Si); 3.27 (OSiMes); -15.85 (SiMe3). HRMS: calcd for C3gH540~Si3~638.3422; found, 638.3450. MS [miz (%)I (EI): 638 (3, M+); 565 (74, M+ - SiMes); 402 (100,

Lassacher et al. Ad(Me3SiO)C=CPhzC). Anal. Calcd for C3gHs402Si3: C, 73.29; H, 8.52. Found: C, 72.43; H, 8.54. Assignable data for the second isomers are as follows. I3C NMR: 6 -0.45,4.99 (SiMes) 94.04, 106.09 (ring C). 29Si NMR: 6 34.53 (ring Si); 5.77 (OSiMe3); -14.88 (SiMes). Synthesis of the Styrene Adduct 10. A solution of 420 mg of mesityl-bis(trimethy1silyl)adamantoylsilanein 0.7 mL of deuterioxylenes was photolyzed for 1 h. The sealed NMR tube was then heated to 120 "C for 5 h a t which time NMR spectroscopy showed that just the major isomer remained and the minor isomer had decomposed completely. After the addition of 0.10 mL of styrene the solution was heated to 100 "C for 1 h. Chromatography (using hexanes as eluent) and subsequent recrystallization from ethanol gave 150 mg of pure 10. Mp: 105-108 "C. lH NMR (all NMR data in CDC13) (6): -0.27, 0.13 (each 9 H, s, SiMe3); 1.4-2.0 (17 H, m, Ad and CH2 ring); 2.24, 2.32, 2.46 (each 3 H, s, Mes CH3); 3.58 (1H, m, CH ring); 6.79 (2 H, s, Mes CHI; 7.11-7.39 (5 H, m, Ph H). I3C NMR: 6 0.19,2.94 (SiMes); 21.17,24.72,26.17 (Mes CH3); 21.99 (CHzring); 28.81 (Ad CHI; 37.22, 39.35 (Ad CH2);40.54 (q Ad); 48.52 (CH ring); 100.37 (q ring C); 125.53, 127.92, 129.08 (each 1C, arom CH); 127.62,129.67 (each 2 C, Ph CHI; 131.29 (Ph ipso); 138.49, 143.35, 144.74, 146.01 (q-C Mes). 29Si NMR: 6 4.44 (OSiMe3); -10.70 (ring Si); -15.43 (SiMe3). MS [m/z(%)I (EI): 560 (2, M+);545 (4, M+ - Me); 487 (33, M+ - SiMes); 456 (Mes(SiMe3)Si=C(OSiMes)Ad+ - H). HRMS: Calcd for C33H490Si3(MI - Me), 545.3091; found, 545.3066. Synthesis of the Silacyclobutenes lla,b. A solution of 0.2 g of bis(trimethylsily1)trt-butyladamantoylsilane(0.5 mmol) and a slight excess of phenylacetylene in 0.6 mL of deuteriobenzene was photolyzed for 1 h. 'H NMR spectroscopy showed no remaining starting material but the formation of two isomeric adducts in the ratio 2.75:1, which could not be separated by chromatography or crystallization from various solvents. Data for l l a (major isomer) are as follows. 'H NMR: 6 0.38, 0.39 each 9 H, s, SiMe3); 1.24 (9 H, s, CH3); 1.55-2.10 (15 H, m, Ad); 6.57 (1H, s, HC=); 7.08-7.23 and 7.69-7.76 (5 H, m, Ph H). 13C NMR: 6 3.06, 4.82 (SiMe3); 22.95 (q-C);30.09 (CH3);29.36 (Ad CH); 37.27,40.39 (Ad CH2); 39.99 (q-Ad); 98.69 (ring C-Ad); 127.60, 127.65, 128.49 (Ph HI; 138.03 (ring HC=); 141.98 (Ph ipso); 168.01 (ring PhC=). 2gSi NMR 6 4.61 (OSiMe3); 1.84 (ring Si); -18.57 (SiMes). Data for l l b (minor isomer) are as follows. lH NMR: 6 -0.18,O.Ol (each 9 H, s, SiMe3); 1.16 (9 H, s, CH3); 1.55-2.10 (Ad, overlapping with lla); 6.61 (1 H, s, HC=); 6.90-7.30 and 7.79-7.82 (Ph H, overlapping with lla). I3C NMR: 6 0.88, 1.07 (SiMes); 21.39 (q-C); 29.52 (CH3); 28.15 (Ad CHI; 37.28, 41.01 (Ad CH2); 40.00 (q-Ad, overlapping with lla); 139.7 (Ph ipso); 162.95 (PhC=); the other signals overlapped with lla. NMR 6 13.52 (OSiMe3);-13.67 (ring Si); -19.21 (SiMe3). MS (both isomers together) [mlz (9611 (EI): 497 (2, M+); 482 (4, M+ - Me); 424 (10, M+ - SiMe3); 339 (75, M+ - SiCMe3SiMes); 135 (95, Ad); 73 (100, SiMes). Synthesis of the Silacyclobutenes 12a,b. The same procedure as above was used, except that (trimethylsily1)acetylene was the reagent employed. The isomers, formed in the ratio 2:1, were not separable. Data for 12a are as follows. IH NMR: 6 0.30, 0.33, 0.36 (each 9 H, s, SiMes); 1.21 (CH3); 1.67-2.08 (15 H, m, Ad); 7.20 (ring HC=). 13CNMR: 6 1.50, 2.89, 4.77 (SiMes); 22.35 (q-C); 29.38 (Ad CHI; 30.19 (CH3); 37.32,40.10 (Ad CHZ);39.39 (q-Ad); 100.34 (ring C-Ad); 157.29 (ring HC=); 179.92 (TMS-C=). 29SiNMR: 6 11.03 (OSiMe3); 3.46 (ring Si); -12.73, -19.42 (SiMe3). HRMS (together with 12b): Calcd for C26H520Si4, 492.3095; found 492.3087. Data for 12b are as follows. 'H NMR: 6 0.35,0.31,0.30 (each 9 H, s, SiMes); 1.95 (9 H, s, CH3); 1.67-2.08 (Ad overlapping with 12a); 7.11 (ring HC=). 13C NMR: 6 1.02, 2.33, 3.89 (SiMes); 25.72 (q-C);29.82 (Ad CHI; 30.66 (CH3);37.21, 40.38 (Ad CH2); 40.25 (q-Ad); 101.68 (ring C-Ad); 153.10 (ring HC=); 172.41 (ring MesSiC=). 29SiNMR 6 5.75 (OSiMe3); 5.43 (ring Si); -12.17, -15.80 (SiMe3). MS (together with 12a [mlz (9611 (EI): 492 (3, M+); 477 (7, M+ - Me); 435 (100, M+ - CMe3);

Organometallics, Vol. 14, No. 9, 1995 4365

Reaction of Silenes 419 (15, M+ - SiMe3); 147 (70, MesSiOSiMez+); 135 (82, Ad); 73 (86, SiMes). X-ray Structural Determination. The cyclobutene ring A in 7 (see Figure 1) forms a least-squares plane with maximum deviation of 0.011(2)A from the plane for atom C(2). The atom C(21) in ring B is almost in the plane formed by ring A (with a deviation of only 0.021(5)A for C(21) from the fitted plane of ring A), and the aromatic ring B is twisted by 33.4(2)" from the plane of ring A. The geometry of the molecule is affected by both bond and angle strain. Angles which are noticably large for tetrahedral geometry are C(3)-Si(l)-C(31), C(l)-Si(l)--C(31), and C(4)C(l)-Si(2), which are 121.3(2), 121.12(12), and 119.2(2)", respectively, and the angle C(21)-C(2)-C(l) of 127.4(3)" is larger than expected for sp2 orbital hybridization. The distances Si(l)-C(l) = 2.002(3) A, Si(l)-C(31) = 1.907(3)A, and Si(B)-C(l) = 1.900(3)A are significantly lengthened for C-Si bonds, and C(l)-C(4) = 1.585(4) A is also noticably long. These bond length and angle distortions appear to be caused by a combination of the effects of cyclobutene ring strain and the steric bulk of the substituent groups present. Intensity data for the crystal structure were collected on a Siemens P4 diffractometer at 173 K, using graphite-monochromated Mo Ka radiation (1 = 0.710 73 A). The o scan technique was applied with variable scan speeds ranging from 3 to 30"/min. The intensities of 3 standard reflections measured every 97 reflections showed no intensity decay. No correction was made for absorption. The structure was solved by direct methods. Non-hydrogen atoms were refined anisotropically by full-matrix least-squares, using all data, to minimize I w ( F o 2- FC2I2, where w-l = 02(F2) (0.0367P)2and P = (FO2- 2Fc2)/3. Hydrogen atoms were positioned on geometric grounds (C-H = 0.96 A), and an overall hydrogen atom thermal parameter was refined to a value of 0.042(1) A2. Crystal data, data collection, and leastsquares parameters are listed in Table 1. The atomic coordinates are listed in Table 2, and some important bond lengths and bond angles are listed in Table 3. All calculations were

+

Table 3. Bond Lengths (A)and Bond Angles (deg) for Comnound 7 Si(l)-C(l) Si(l)-C(3)

Ring Lengths C(l)-C(2) 2.002(3) 1.827(3) C(2)-C(3)

1.559(4) 1.338(4)

Si(1)- C(31) Si(2)- C( 1)

Other Lengths 1.907(3) C(l)-C(4) 1.900(3)

1.585(4)

C(l)-Si(l)-C(B) Si(l)-C(l)-C(2)

Ring Angles 75.58(13) C(l)-C(2)-C(3) 81.5(2) C(2)-C(3)-Si(l)

108.2(3) 94.6(2)

C(3)-Si(l)-C(31) C(l)-Si(l)--C(31) C(4)-C(l)-Si(2)

Other Angles 121.3(2) C(21)-C(2)-C(l) 121.12(12) Si(2)-O-Si(3) 119.2(2)

127.4(3) 155.7(2)

performed using SHELXTL-PC14and SHELXL-93I5on a 48666 personal computer. Figure 1 is a view of the molecule showing the crystallographic labeling scheme.

Acknowledgment. P.L. is grateful to the Austrian Science Foundation for an "Erwin Schroedinger"scholarship. This research was supported by the Natural Sciences and Engineering Research Council of Canada. Supporting Information Available: Tables of bond lengths and bond angles, anisotropic displacement parameters, and hydrogen atom coordinates and thermal parameters (5 pages). Ordering information is given on any current masthead page. OM950336V (14)Sheldrick, G. M. SHELXTLPC, Siemens Analytical X-ray Instruments Inc., Madison, WI. (15)Sheldrick, G. M. SHELXL-93, Program for Crystal Structure Refinement, University of Gottingen, Germany.