Silyl anions metal-free

Anionic metal-free initiation was successfully applied to both aliphatic and aromatic cyclic carbonates. This method is based on the reaction of a silyl ether with fluoride anions, for example, tetrabutyl ammonium fluoride (BU4NF) or tris (dimethylamino)sulfonium trimethylsilyl difluoride (TASF, [(CH3)2N]3 SSi(CH3)3p2), to produce an anion with a tetrabutyl ammonium or tris(dimethylamino)sulfonium counterion. The metal-free system is an efficient initiator for neopentyl carbonate polymerization. ... 

Several studies of the mechanism of the sodium coupling reaction have been carried out, and a mechanistic model has emerged which accounts for most of the experimental observations. The reaction chain appears to be anionic in nature. The first step is believed to be the slow reaction of the dichlorosilane with sodium to give an anion (equation 34). The chain-extending process is the reaction of polymer silyl anions with dichlorosilane molecules (equation 35), followed by rapid reduction of the chlorine-terminated polymer to the anion (equation 36). Reduction must take place at the surface of a sodium particle. As polymerization occurs, some molecules may break free from the sodium surface once in solution away from the metal, chain extension would cease. This portion may give rise to the low-molecular weight polymer. Other molecules probably become entangled at the sodium surface and so cannot break free. They therefore continue to react with fresh dichlorosilane, and eventually... 

A metal-free trimethylsilyl anion is formed from hexamethyldisilane by cleavage with TBAF in HMPA in equilibrium concentration, as revealed by H and F NMR analysis of the reaction mixture. Treatment of trisilane with TBAF provides Me3SiMe2Si /NBu4 and Me3SiF the fluoride anion attacks at the terminal silicon of trisilane [Eq. (21)] (59). Synthetic application of this species is described in Section VII. Other metal-free silyl anions are described in Section V. 

Silicon stabilizes an adjacent C- metal bond despite being more electropositive than hydrogen or carbon. The origins of this stabilization were discussed earlier in Section II.F. Whilst stabilization of the free anion is usually discussed, it should be remembered that the free anion is rarely involved. The stabilization of the a-silyl anionic species is illustrated by the ease and variety of methods available for generating a-silylcarbanions, namely ... 

This increase in stability caused by the change of the metal is in contrast to the proposed mechanism of racemization for metalated silanes. Since the rate-determining step of the racemization process is discussed in the literature [3a, 8-11] as the inversion of the free silyl anion, no drastic effect of transmetalation should be expected. 

A disadvantage is that solutions of silyl anions prepared from the halosilanes by one of these methods inevitably contain metal halides as by-product. Salt-free perarylated silyl anions can be prepared by the reaction of a disilane with lithium, sodium or potassium in polar solvents such as THF or DME (Table i)28b,29 Alternative procedures for the preparation of halide-free Ph3SiK utilize the reductive cleavage of the Si—C bond of Ph3Si—CMe2Ph by a sodium-potassium alloy in Et20 (equation 17), as well as via the reaction of PhsSiH with KH in DME . 

Initiators with nonmetal counterions. This class includes Group Transfer Polymerization (GTP) with silyl ketene acetals (silyl ester enolates) as initiators and metal-free anionic polymerization using initiators with, for example, tetrabutylammonium and phosphorous-containing counterions. This suppresses aggregation of ion pairs. 

When living poly(methyl methacrylate) (PMMA) prepared by group transfer polymerization (GTP) is used as a macroinitiator for the ROP of cyclic carbonates, a site transformation from the silyl ketene acetal (GTP-mechanism) to an alcoholate (anionic ROP-mechanism) with a metal-free counterion occurs (Scheme 12.5). The GTP of PMMA was initiated with l-methoxy-l-trimethylsilyloxy-2-methyl-l-propene (MTS) in combination with catalytic amounts of tetrabutyl ammonium cyanide in THF as solvent. Towards the end of the reaction, DTC is dissolved in the reaction mixture and lequiv. of fluoride anions (e.g. tris(dimethylamino) sulfonium difluorotrimethylsilicate TASF), with respect to the active species, is added. In this way, good yields of the respective block copolymers were obtained. A model experiment for this site transformation is the polymerization of DTC with MTS as the initiator and TASF as the desilylating agent. The fluoride anion promotes desilylation of the silyl ketene acetal with formation of an enolate, which reacts as a carbon-centered nucleophile with the carbonyl carbon of DTC, thereby... 

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