Reaction of the (Dichloromethyl)oligosilanes R(Me3Si)2Si−CHCl2 (R = Me, Ph, Me3Si) with Organolithium Reagents and the Synthesis of Novel Kinetically Stabilized Silenes

Abstract

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).

Synthesis and Structural Characterization of Novel Silenes Stabilized by Intramolecular Coordination of a Dialkylamino Group

“…Treatment of 1 with organolithium compounds leads to the deprotonation of the dichloromethyl group, and this initiates a remarkable sequence of elimination, isomerization, and addition steps; the structures of the resultant final products depend on the groups R introduced by the organolithium derivatives used. [8,9] Our proposal for the mechanism of the whole reaction (molar ratio 1:RLi is 1:2) is outlined in Scheme 1. Loss of lithium chloride from the carbenoid 2 and subsequent silylcarbene ± silene rearrangement [9] of 3 afford the transient silene 4, which in the presence of an excess of the organolithium compound is immediately trapped to give the new carbenoids 5.…”

“…But when, for the reaction with 1, highly congested organolithium compounds RLi, such as 2,4,6-triisopropylphenyllithium or 2-tert-butyl-4,5,6-trimethylphenyllithium, were used, further addition of RLi to the silene double bond of 6 was prevented, and kinetically stabilized silenes could be synthesized and characterized. [8] The efficiency of the method with respect to the synthesis of SiC systems is best demonstrated by the reaction of 1 with dimethylamino-functionalized aryllithium derivatives. For that purpose, we have chosen and describe here the treatment of 1 with 8-dimethylamino-1-naphthyllithium, with 2-(dimethylaminomethyl)phenyllithium, and with 8-(dimethylaminomethyl)-1-naphthyllithium, respectively; the reaction leads to two novel intramolecularly donor-stabilized silenes and an unexpected silene resultant product.…”

“…The two dichloromethylsilanes are known to react with simple organolithium compounds, such as methyllithium or phenyllithium; the reaction follows the pattern discussed above (Scheme 1). [8] But as a result of their interaction with the dimethylamino-functionalized aryllithium compounds, we obtained complex mixtures of products. A silene or any resultant silene product could not be isolated.…”

“…[6] Recently we obtained 1-(8-dimethylamino-1-naphthyl)-1,2,2-tris(trimethylsilyl)silene (6 a) by the reaction of (dichloromethyl)tris(trimethylsilyl)silane (1) with 8-dimethylamino-1-naphthyllithium (molar ratio 1:2), and we succeeded in performing its X-ray structural analysis, which clearly characterized the compound as an intramolecularly donor-stabilized silene. [7] Meanwhile, the synthetic route applied for the preparation of 6 a proved to be a general method for the [ synthesis of donor-stabilized as well as of kinetically stabilized silenes, [8] and in the present paper we describe the generation of three novel silenes with strong intramolecular N 3 Si donor ± acceptor interactions; two of the silenes are indefinitely stable at room temperature, but the third one undergoes a rapid rearrangement by insertion of the SiC group into a CÀH bond to form a 1-silaacenaphthene.…”

Silylenes, Silenes, and Disilenes: Novel Silicon‐Based Reagents for Organic Synthesis?

“…5, [18] 6, [19] Scheme 4). [22] Increased stability is also realised with silenes influenced by reversed Si=C bond polarisation, that is, an Si dÀ =C d + instead of Si d + =C dÀ polarisation, effected by p-donor substituents on the carbon atom. This may be achieved either intermolecularly (7) [20,21] or intramolecularly (8).…”