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The title compound 6 was prepared by reductive coupling of [bromobis(trimethylsilyl)silyl]tris(trimethylsilyl)silylmethane (5) and chlorotrimethylsilane with lithium in THF. X-ray crystal structure analysis of 6 revealed the expected distortions of the molecular skeleton. Thus, the spatial demand of the two extended hemispherical (Me3Si)3Si groups forces a widening of the Si−C−Si angle at the central sp3 carbon atom to a value of 136°.

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Tris(trimethylsilyl)silylmagnesium bromide, obtained in situ from tris(trimethylsilyl)silyllithium and magnesium bromide, reacts with acetone, pivalaldehyde, or 2,4,6-trimethylbenzaldehyde to give the (1-hydroxyalkyl)tris(trimethylsilyl)silanes (Me3Si)3SiC(OH)Me2 ( l a ) , (Me3Si),CH(OH) tBu ( l b ) , and (Me,Si),SiCH(OH)Mes ( l c ) , resp. After deprotonation with methyllithium in ether at -78°C la-c eliminate trimethylsilanolate according to a modified Peterson mechanism to form transient silenes (Me3Si)zSi=CR1R2 (6a: R' = R2 = Me; 6b: R' = H, R2 = tBu; 6c: R' = H, R2 = Mes). In the absence of trapping agents these silenes dimerize, 6a leading to the linear dimer l-isopropenyl-2-isopropyl-l,l,2,2-tetrakis(trimethylsily1)disilane (7) and 6b giving the head-to-head cyclodimerization product (E)-3,4-di-tert-butyl-l,l,2,2-tetrakis(trimethylsilyl)-1,2-disilacyclobutane (E), whereas 6c in a very unusual cyclodimerization step affords (E)-1,2,3,8a-tetrahydro-1 -mesityl-5,7,8a-trimethyl-2,2,3,3-tetrakis(trimeth- The modification of the Peterson reaction in such a way that by elimination of trimethylsilanolate from deprotonated (1-hydroxyalkyl)tris(trimethylsilyl)silanes (1) Si=C systems, i.e. silenes, are formed is a promising route to this interesting class of unsaturated organosilicon compounds (eq. I). In 1988 we found that lithiated 1, obtained in situ by the reaction of tris(trimethylsilyl)silyllithium (2) with aliphatic ketones, is converted to give, depending on the reaction conditions, either (trimethylsiloxy)[bis(trimethylsilyl)silyllalkanes 3 or the polysilanes 4. Compounds 3 are formed by a 1,3-trimethylsilyl migration and protonation of the silanide by the enolizable ketone, and the products 4 are proposed to be the result of an addition of excess 2 to the double bond of transient silenes (obtained according to eq. 1) followed by a 1,3-Si, C-trimethylsilyl shift and protonation during hydrolytic workup [']. The disadvantage of this in situ method, with respect to the synthesis of silenes, is the enolizability of the ketones applied (only aliphatic ketones can be used, aromatic aldehydes and ketones do not undergo lithium silanide carbonyl addition) and the fact that under the conditions of an effective excess of 2 (i.e., when the ketone is dropped to the solution of 2) the reactive silene is trapped by the lithium silanide 2. In this paper we describe a method leading to isolated (1-hydroxyalkyl)tris(trimethylsilyl)silanes (1). In the presence of a base these alcohols are easily converted into transient silenes, which are characterized by various dimerization and addition reactions. The availability of pure, isolated hydroxyalkyl polysilanes 1 in the synthesis of silenes according to the Peterson concept offers the possibility of a free choice of the reaction medium and the base used to initiate the silanolate elimination. With respect to the significance of the solvent for the silene generation and its subsequent reactions, this is of particular importance. G e m .

The dichloromethyloligosilanes R1(Me3Si)2Si−CHCl2 (1a,b) (1a: R1 = Me; 1b: R1 = Ph), prepared by treatment of methylbis(trimethylsilyl)silane or phenylbis(trimethylsilyl)silane respectively, with chloroform and potassium tert‐butoxide, treated with the organolithium reagents R2Li (R2 = Me, Ph) to produce the intermediate organolithium derivatives R1R22Si−CLi(SiMe3)2 (10). Hydrolysis of 10 during the aqueous workup afforded the [bis(trimethylsilyl)methyl]silanes R1R22Si−CH(SiMe3)2 (2); quenching of the reaction mixture with chlorotrimethylsilane gave the [tris(trimethylsilyl)methyl]silanes R1R22Si−C(SiMe3)3 (11). The formation of 10 is discussed as proceeding through a remarkable series of isomerization processes involving the transient silenes R1R2Si=C(SiMe3)2 (7), which in the presence of an effective excess of R2Li are immediately trapped to give 10. By the use of sterically congested organolithium derivatives, the nucleophilic addition of R2Li to the Si=C bond of 7 can be prevented and kinetically stabilized silenes obtained. Thus, Ph(2,4,6‐iPr3C6H2)Si=C(SiMe3)2 (8b) was synthesized by the reaction of 1b with 2,4,6‐triisopropylphenyllithium (molar ratio 1:2). Similarly, (Me3Si)(2,4,6‐iPr3C6H2)Si=C(SiMe3)2 (8c) and (Me3Si)(2‐tBu‐4,5,6‐Me3C6H)Si=C(SiMe3)2 (9c) were prepared from (dichloromethyl)tris(trimethylsilyl)silane (1c) and 2,4,6‐triisopropylphenyllithium or 1c and 2‐tert‐butyl‐4,5,6‐trimethylphenyllithium (1:2), respectively, but due to difficulties in the separation of starting material and side products, 8b, 8c, and 9c were obtained in impure form only. Despite the steric congestion, the compounds are reactive and exhibit the usual behavior of silenes. Thus 8b, 8c, and 9c were chemically characterized by the reaction with water to give silanols, by the addition of methanol to give methoxysilanes and by formal [2+2] cycloadditions with benzaldehyde to afford stable 1,2‐oxasiletanes. 1‐Methyl‐1‐(2,4,6‐triisopropylphenyl)‐2,2‐bis(trimethylsilyl)silene (8a), produced as intermediate from 1a and 2,4,6‐triisopropylphenyllithium (1:2), proved to be unstable in the presence of excess aryllithium compound. Thus, only the addition product Me(2,4,6‐iPr3C6H2)2SiCH(SiMe3)2 (14) was isolated (after hydrolysis).

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A H A H aritr.syn-15 syn, syn-15Table 3. C')cloaddirion of allylsilane 2b and methyl propynoate (13) Reaction conditions 14. Yield [%I 15, Yield [%I 3 equiv 2b. -7X C --20 C. 19 h 4equi\2h. -78 t -2 5 C . 5 d 45 46 [a] 98 -3 e q u i v Z h . X C -4 0 C . 1 9 h 34 64 [a] [a] ( i i i r r . \ i +IS: \ w , . \ j~i i -l S = 3: 1. Traces of the Sakurai product and a cyclopentane dcriwtne were i i l w dcrected. tion at room temperature for fivedays. 14 and the bicyclo[2.2.0]hexane 15. the product of the domino reaction, were formed i n approximately equal amounts. Cycloaddition of 2 b with 13 in dichloromethane at reflux for 19 h provided 15 and 14 in yields of 64% and 34%, respectively. Bicyclo[2.2.0]hexane 15 was obtained as a mixture of the anti,sw and the s.vn,syn diastereomers in a 3:l ratio. The two diastereomers are easily distinguished by their I3C N M R spectra. The major isomer rmti.sjn-15 shows two signals for the aSi-CH2 groups, one anti to the methoxycarbonyl group at 6 =10.2 and one syn at 6 = 13.3. The minor isomer syrz.syn-15 is symmetrical and therefore exhibits only one signal for the aSi-CH, group in the region typical for the syn diastereoisomer (6 = 13.6). Thus, this product was assigned the syr,sjn configuration with both silylmethyl groups on the same side as the methoxycarbonyl group. Expcvinic~nral Procedure 15: A b o l u t i o n of methyl propynoate (13) (137 pL. 339 ing. 4.03 mmol) in dry dichloromethane ( 5 mL) was added to a stirred solution of titanium tetrachloride (490 pL. 840 mg. 4.43 inmol) in dry dichloromethane ( 5 mL) at room temperature. To this ini\ture was added a solution ofallyltriisopropylsilane (2b) (2.92 mL. 2.40 g.13.08 miiio) i n dry dichloromethane ( 5 mL). The reaction mixture was refluxed for 19 h and tlieii quenched by addition of an aqueous solution ofamrnonium chloride. The organic layer was separated, the aqueous layer was extracted three times with dichlorometh;rne. and the combined organic layers were dried over magnesium sulfatc. E\;iporatioii of the solvent m d flash chromatography (hexane:ether 1 5 : l ) of the rcsidue o n silica gel provided the bicyclo[2.2.0]hexane 15 (1.24 g. 64"/0) and the cyclobutene 14 (387 mg, 34%) both as colorless oils.

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