Silylenium cation

Radiation-Induced Polymerization. In 1956 it was discovered that D can be polymerized in the soHd state by y-irradiation (145). Since that time a number of papers have reported radiation-induced polymerization of D and D in the soHd state (146,147). The first successhil polymerization of cychc siloxanes in the Hquid state (148) and later work (149) showed that the polymerization of cycHc siloxanes induced by y-irradiation has a cationic nature. The polymerization is initiated by a cleavage of Si—C bond and formation of silylenium cation. 

Cypryk, M. Chojnowski, J. Kurjata, J. Tertiary Trisilyloxonium Ion and Silylenium Cation in Cationic Ring-opening Polymerization of Cyclic Siloxanes. In Organosilicon Chemistry VI From Molecules to Materials Auner, N., Weis, J., Eds. Wiley-VCH Weinheim, 2005 pp 85-92. 

Optimization of geometry for some silylenium cations was made (22,24). Such calculations predict that silylenium ions will adopt a planar structure in contrast to silyl anions, which are predicted to be pyramidal. It should be noted that theoretical studies also indicate a high ability of silicon to accommodate negative charge. The parent silyl anion H3Si was calculated to be more stable than its carbon analog by about 50 kcal/mol (24). This implies a remarkable affinity of silylenium ions toward electron-rich species. 

Aluminum trichloride catalyzed hydrosilation provides an interesting alternative to the free radical process. The addition of the silane was found to occur regioselectively at trisubstituted alkenes in a frans -fashion. Mechanistically it was concluded that the reaction proceeds via the addition of a silylenium cation to the multiple bond, generating the more stable (more substituted) carbocation, which abstracts a hydride from another molecule of hydrosilane (Scheme 2). ... 

Tertiary Trisilyloxonium Ion and Silylenium Cation in Cationic Ring-Opening Polymerization of Cyclic Siloxanes... 

Silylenium ions are more thermodynamically stable than carbenium ions, but are also much more reactive. Larger size of silicon atoms and longer bonds to substituents make them more accessible to nucleophilic attack than carbon. Thus, reactions at silicon are generally less sensitive to steric hindrance. Silicon is also more electropositive than carbon, and the positive charge in sSL is highly localized on silicon, while in =C it is largely dispersed over the substituents. This results in stronger electrostatic attraction forces between silylenium cation and electron-rich species [14]. 

The current chapter will neither cover perhydrogenated nor perhalogenated oligosilanes which have been the topic of a recent overview [9]. It will also omit compounds with multiple bmids between Si-atoms, sUylenes, silylenium cations, silyl animis, and sUylenoids. AU these will be treated in other chapters of this volume. 

When the counterion is complex, for example metal-halogen anions such as BF4-, the most electronegative portion of the counterion becomes attached to the silicon center. Because of this attachment, it is natural to consider the intermediacy of a silicenium cation (silylium or silylenium ion) intermediate in such reactions (Eq. 4). Bond energies derived from electron impact studies indicate that Eq. 4 is exothermic in the gas phase by about 8 kcal/mol.26,29 There seems little doubt that trivalent silicon-centered cationic species do exist in the gas phase30,31 or that processes similar to that shown in Eq. 4 do occur there.32,33... 

Calculations revealed that the parent structure of the silylenium ion H3Si+ is more stable than that of H3C+ by 41-73.5 kcal/mol. However AE tends to decrease strongly on replacement of the hydrogen atom, indicating that substituents, in particular it donors, are decidedly more effective in stabilizing carbenium ions than in stabilizing silylenium ions. Thus, the methyl cation is stabilized by an amino substituent by 93.8 kcal/mol but the silyl cation by only 38.3 kcal/mol (27). +... 

The greatly diminished stabilization of the silylenium compared to the carbenium structure by it substituents is connected with the known low effectiveness of 2p-3p (it) conjugation. Calculations were made on the =Si+ cation substituted with a phosphorus group with the hope of showing a superior 3p-3p (tt) conjugation of P—Si+ moiety (26,28). A smaller inductive destabilization by phosphorus than by electronegative substituents of the second period (N, O) was also expected. However, all... 

The transient existence of the silylenium ion has been considered in other reactions of chlorosilanes in the presence of Lewis acids. Guyot (147) successfully used the silyl chloride-silver salt system for initiation of the cationic polymerization of vinyl ethers [Eq. (35)]. Gel permeation... 

We use the term silylium ion for R3Si+, rather than silicenium ion, silylenium ion, or silyl cation, thus following IUPAC recommendations. See Nomenclature of Inorganic Chemistry G.J. Leigh, Ed. Blackwell Oxford, U.K., 1990, p. 106. 

With a bulkier substituent (2,4,6-trimethylphenyl = Mes), Mes3Si+ B(C6F5)3 is indicated as an isolated silylium in solution and very recently the cation is proved to be a completely free silylenium ion (equation 63). 

The hydride transfer from a hydrosilane to the carbenium ion leads to the transient formation of a complex of tricoordinate silicon cation with solvent. Lambert et al. showed that if a stable counterion of very low nucleophilicity is used, the complex of silylenium ion with solvent is a persistent species [12]. Tetrakis(pentafluorophenyl)borate was successfiilly explored for this purpose, and this system was later used by Olah ef al. [1] for the generation of trisilyloxonium ions derived from D4... 

Tertiary silyloxonium cation may be considered as the complex of the silylenium ion with siloxane. Thus, there is a question about the position of the equilibrium, in which the free silylenium ion would exist in much lower concentration than its complex, but, being much more reactive, could significantly contribute to propagation. 

In carbon chemistry, the term carbenium for trivalent, sextet, positively charged carbon was derived from carbene, for divalent, sextet, neutral carbon. Because divalent silicon is commonly called silylene, the appropriate term for trivalent, sextet, positively charged silicon is silylenium. The alternative but commonly used term carbocation for all positively charged carbon systems finds analogy in silyl cation (much less often, silico-cation). We will use both these naming systems in this discussion. The term siliconium, like carbonium, denotes the highest valency (pentavalency), as in SiH3. The term silicenium for the trivalent system has been discarded since silylene rather than silicene has been accepted as the appropriate term for the divalent state. 

Although considerable difficulties have been encountered in generating silylenium ions in solution, either as stable species or as reactive intermediates, such has not been the case in the gas phase. Numerous studies of silyl cations in the gas phase have been reported17"24, both by traditional mass spectrometric methods and by ion cyclotron resonance. 

An alternative proposal for the active propagating center involves the formation of the silicon-centered cation, the silylenium ion. In this proposal the initiation is similar to what has been proposed in the oxonium ion mechanism except that the positive charge is centered on the silicon to generate a silylenium ion (see Eq. 6.16). 

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