Functionalized Silyl Anions

Silyl Anions with Functional Groups (Hydrido)silyllithiums ... 

In contrast to triorganosilyl anions, functionalized silyl anions have been studied less extensively. Functional groups reported so far are hydrogen, chlorine, amino, alkoxy, and carbonyl groups. In addition to these species, silacyclopentadienide anions are also described in this section. Theoretical studies on the effect of functional groups are described in Section... 

Functionalizations via Silyl Hydride Functionalization and Hydrosilation A new general functionalization method based on the combination of living anionic polymerization and hydrosilation chemistry has been developed as illustrated in Scheme 7.26 [281]. First, a living polymeric organolithium compound is quantitatively terminated with chlorodimethylsilane to prepare the corresponding co-silyl hydride-functionalized polymer. The resulting co-silyl hydride-functionalized polymer can then react with a variety of readily available substituted alkenes to obtain the desired chain-end functionalized polymers via efficient regioselective transition-metal-catalyzed hydrosilation reactions [282-284]. 

Hirao et al. have further developed the above functionahzation reactions using DPE derivatives in an excellent procedure referred to as the chain-multi-functionalization of living anionic polymers [176]. For this purpose, a new DPE derivative, l,l-bis(3-terFbutyldimethylsilyloxymethylphenyl)ethylene (12). has been synthesized. This DPE is designed in such a way that the tert-butyldimethylsilyl ether acts as a protected functionality in a reaction with living anionic polymers, and is quantitatively transformed into a benzyl bromide (BnBr) or even chloride and iodide functions [176, 183]. As illustrated in Scheme 5.18,12 was first reacted with PSLi to introduce two silyl ether functionalities at the chain-end, followed by treatment with Me3SiCl/LiBr to transform into two BnBr fimctions as a result, a well-defined chain-end-(BnBr)2-functionalized PS was obtained. The same functionalized DPE-derived anion was then separately synthesized, and reacted with the above chain-end-(BnBr)2-functionalized PS. The four silyl ether functionalities thus introduced were transformed into four BnBr functions by the same treatment with MesSiCl/LiBr, and this resulted in a chain-end-(BnBr)4-functionalized PS. As the coupling and transformation reactions proceeded both cleanly and quantitatively, the same reaction sequence could be repeated four more times to successfiiUy introduce 8,16, 32, and 64 BnBr functions at the chain-ends (Scheme 5.19 Table 5.3) [184]. Furthermore, the same reaction sequence could be carried out with a-(BnBr)2-functionalized PMMA to afford a series of weU-defined chain-end-BnBr-multi-functionalized PMMAs with up to 16 BnBr functions [185]. 

The scope and utility of this methodology can be illustrated by its application to the facile s)mthesis of well-defined, -primary amine-functionalized polystyrene for which few other simple, efficient anionic functionalization procedures are available as shown in previous sections in this review. The second step in the development of this general functionalization procedure involves the reaction of the silyl hydride-functionalized polystyrene with substituted alkenes (see Scheme 21). To demonstrate the versatility and simplicity... 

The general two-step anionic ftmctionalization/hydrosila-tion methodology described herein has been shown to be applicable for the preparation of a wide variety of chain-end functional groups. This method couples the quantitative silyl hydride functionalization of well-defined polymeric organo-lithium compounds using dimethylchlorosilane with the second hydrosilation step in which the silyl hydride-functionalized polymer is added to a functionalized alkene in the presence of a platinum catalyst. Although it is a... 

Unfortunately this reaction is rather special and does not allow further functionalization at the silicon center. Therefore we decided to search for alternative starting materials with activated Si-C bonds, which can be cleaved under attractive conditions without any additives like HMPA. These precursors should furthermore allow a variety of functional groups at the silicon center and thus be useful building blocks for the field of synthetic chemistry. In general these tetraorganosilane precursors correspond to silyl anion and silyl dianion synthons of type B and D in Eqs. 3 and 4. 

In order to introduce groups into the silyl anion, which can be used for further functionalization, we also investigated reactions of hydride and amino-substituted bis(trimethylsilyl)silylpotassium compounds with the metallocenes. Both types of compounds underwent the transformation smoothly (Scheme 1). 

Quirk RP, Kim H, Polce MJ, Wesdemiotis C. Anionic synthesis of primary amine functionalized polystyrenes via hydrosUation of allylamines with silyl hydride functionalized polystyrenes. Macromolecules 2005 38 7895-7906. 

A new ABC star was also synthesized by modifying the above-mentioned procedure. An in-chain-SiCl-functionalized AB diblock copolymer (A PS and B PI) was first prepared by a procedure similar to that described above, followed by reacting the silyl chloride function with a dilithium agent, Li(CPh2CH2CH2CPh2)Li, to change the reaction site to 1,1-diphenylaIk-yllithium (Sioula et al, 1998). This new anionic reaction site was used to polymerize methyl methacrylate (MMA), yielding a well-defined ABC star composed of PS, PI, and PMMA (Mn = 77 000 g/mol, MJM = 1.06). 

Finally, two chapters are devoted to the most extreme of functionalization ionization. The generation of silyl cations and anions provides valuable synthons, e.g., for the further extension of the silicon scaffold. In chapter titled Silylium Ions of Volume 1, Thomas Muller (University of Oldenburg, Germany) summarizes the many applications of silylium cations in catalysis and discusses recent trends towards low valent silicon cation chemistry thus completing the full circle to other chapters that are more explicitly devoted to low valent silicon species. Finally, the chemistry of silyl anions is summarized in chapter titled Silyl Anions of Volume 2. 

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