Silylenium ions

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... 

There is some divergence in the nomenclature various names of these species are used, including silicenium, siliconium, and silylenium ion. According to Barton et al. (19), silylenium ion is the proper name since it is derived in a logical way from the name silylene in analogy to the carbenium ion originating from carbene. 

Quantum mechanical calculations of silylenium ion structures have been made mostly by ab initio methods. Although the results seem to depend to some extent on the procedure used, they provide important information concerning the structure and reactivity of silylenium ions. 

The comparative stability of a silylenium ion and the corresponding carbenium ion has been evaluated as the energy difference (AE) between products and substrates in the isodesmic Eq. (2) (20-22). 

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). +... 

Calculations of AE of the isodesmic Eq. (3) were executed to study the effect of substituents at silicon atom on the stability of the silylenium ion. 

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. 

Silylenium ions are common in gas-phase organosilicon chemistry, where they may be generated by various techniques including electron impact (29-33), photoionization (34-36), chemical ionization (37-45), collision-induced dissociation (25,46), and chemical-nuclear methods (15). Although this article is concerned with reactions in solution, a short account of gas-phase studies cannot be omitted, since they provide important information about chemical and physical properties of silylenium ions, which are so elusive in condensed phases. 

ICR mass spectrometry has been employed to study the kinetics and equilibria of the generation of silylenium ions by hydride transfer from silanes to carbenium ions (43,47). The kinetics of the formation of silylenium ions from fluoromethylsilanes [(CH3) SiF4 , n = 1-3] have also been investigated by this technique (48). Fluoride transfer was the dominant reaction in this system. 

Kinetic studies of silylenium ion formation by the fragmentation of a metastable parent ion produced by photoionization [Eq. (5)] have been... 

Silylenium ions are also produced as a result of combined chemical-nuclear decomposition of tritiosilanes [Eq. (6)] (15). An analogous method... 

Mass spectroscopy has been often used to study the chemical behavior of silylenium ions. Intramolecular rearrangement with elimination of a neutral molecule has often been observed (e.g., Ref. 50 and references cited therein). For example, l-phenyl-2-trimethylsilylethane transforms into the more stable phenyldimethylsilylenium ion [Eq. (7)] (51). Intermolecular rearrangements may occur as well (52). 

Kinetics of the reactions of Me3Si+ with water (57) and alcohol (31,32) were investigated by ICR mass spectrometry. Proposed mechanisms involve formation of an oxonium ion as the primary product, which in the case of the methanol adduct is eventually decomposed to methoxy-silylenium ion according to Eq. (9). Reactions of halide transfer to silylenium ion [Eqs. (10) and (11)] have been studied by FT mass spectrometry (59) and by tandem mass spectrometry (44). Hydride transfer... 

Gas-phase methods also constitute a source of important information on basic physical properties of silylenium ions. In particular, the thermochemical behavior is well characterized (30,33,34,47,61). Thermochemical data are applied for the evaluation of relative thermodynamic reactivities of silylenium ions in some systems. For example, affinities of R3Si+ and R3C+ toward various bases may be compared as the heterolytic dissociation energies of corresponding bonds [Eq. (12)] (47,61). It was shown that... 

Organosilicon chemists have long searched for a solution system in which silylenium ions are stable. Analogy to the carbenium ion has been a... 

Two reactions were explored in continuation of these attempts hydride transfer from silanes to a carbenium center and halide transfer from silanes to a Lewis acid. Media of low nucleophilicity and high ionization power were employed. Stable silylenium ions A and B were postulated to be formed by H- transfer from corresponding silyl hydride to triphenyl-methylium perchlorate in CH2C12 (71,72). However, soon after, and in view of results of additional experiments, this evidence was shown to be insufficient (19,73). 

Further attempts were made by Lambert and Schulz (17), who claimed to be able to generate stable silylenium ions as a result of H- transfer under conditions similar to those used elsewhere (71,72). Trisopropylmer-captosilyl hydride was used as the precursor [Eq. (13)]. Maximization of... 

In light of the great affinity of silylenium ions for electron-rich species, one could be also skeptical about the possibility of the existence of the tricoordinate Si+ species in solvents like acetonitrile (AN) and sulfolane. These solvents are known to have nucleophilic coordination ability. Gut-mann s donicity number (84), 14.1 and 14.8 for AN and sulfolane, respectively, is comparable to that for acetone, 17.0. Coordination of acetonitrile to silicon has been considered in some systems (85,86). The small equivalent conductance of triphenylsilyl perchlorate in CH2C12 may be explained by the domination of the covalent form (18). Consequently, the absence of the coordination of acetonitrile and sulfolane with the tricoordinate Si+ observed in CH2C12 (with 1 or 6 equiv of acetonitrile) may simply indicate that C104 coordinates to Si+ under these conditions more readily than acetonitrile and sulfolane [Eq. (15)]. The reverse situation in acetonitrile... 

D. Silylenium Ions as Intermediates in Reactions at the Silicon Center... 

The mechanism postulated involves formal hydride transfer leading to the formation of a silylenium ion intermediate [Eq. (19)]. It... 

The concept of the generation of silylenium ion by hydride abstraction with a stable carbenium ion has been explored in designing the synthesis of saturated and unsaturated cyclic silaethers (104). For this purpose the incipient silylenium ion is intramolecularly trapped by a properly located ethereal nucleophile, which leads to cyclization [Eq. (21)]. If the triphenyl-... 

The Lewis acid-induced 1,2 migration in (a-chloroalkyl)trialkylsilanes was first reported in 1947 (105). A mechanism involving the silylenium ion was proposed [Eq. (22)]. However, further studies did not confirm the... 

Observations considered as evidence in favor of silylenium ion intermediacy have been made in other systems. Most of them have already been reviewed (10). 

Sommer et al. (136) found that optically active silyl halides readily undergo racemization induced by halide ions. The reaction is related to halide ion exchange discovered by Allen and Modena (137). The first tandem kinetic studies of the racemization and halide exchange reaction led to conclusion that the reaction proceeds via a silylenium ion pair [Eq. (28)],... 

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