Silicon hydride cations, stabilities

The internally stabilized silyl cation (36), formed by hydride transfer as shown, has a 2-silanorbomyl cation structure, and is not coordinated to die solvent or the counterion.78 NMR chemical shifts calculated on the basis of die bridged structure shown are in agreement with the experimental values. The autiiors describe it as free but internally re-stabilized .78 The silicon-stabilized oxonium ion (37) shows considerable stereoselectivity in its reactions (38) is the preferred product isomer by a 92 8 ratio, and (39) by 98 2.79... 

The formation of /3-silicon-stabilized cations from y-hydroxysilanes results from the normal pattern of pinacol rearrangements, where hydride and phenyl migration is common. Desilylation then also affords an alkene [110]. If the alkyl group is a ring residue, only hydride migration is observed (Eq. 64). 

FIGURE 57. Schematic description of the stabilization of a silicenium center and of a saturated silicon center by mono-, di- and tri-OH and SH substitutions, based on MP2/6-31 G //3-21 G calculations of appropriate isodesmic equations (e.g. equations 47 and 48 for the di-substituted systems). E° and E% are the dissociation energies of the neutral oxygen and sulfur substituted neutrals, respectively, to the corresponding cations and hydride ion. Subscripts I, II and III denote mono-, di- and tri-substitution, respectively. H3SiOH and H3SiSH are placed artificially at the same energy level. 

The BCS hydride-transfer reaction is a synthetic entry to a wide variety of cationic silicon species [4, 9, 14], which are stabilized either by intermolecular interaction with solvent molecules (Scheme 8a, c) [22, 27] or with the counteranion (Scheme 7b) [47] or by intramolecular interactions (Scheme 8e-g) [34, 41, 48]. Even silylium ions without a stabilizing attachment can be prepared according to this synthetic route (Scheme 8d) [21]. The examples brought together in Scheme 8 nicely demonstrate that the nature and in particular the reactivity of the generated stabilized silylium ion crucially depends on the substituents at the silicon atom and on the solvent and counteranion. 

The results presented in the previous paragraphs demonstrate that the extreme reactivity of three-coordinate silyl cations is significantly reduced in arene solvents by intermolecular polar-ir (p-it) interaction between the cation and the tr-system of the arene. Two model systems have been suggested and have been experimentally verified in which this p-n interaction is employed to modulate the cation character of the silicon atom intramolecularly and to control the reactivity of the silicon cation [34, 35]. Both classes of silyl cations, the cyclic disilyl-substituted arenium ions 71, 72 synthesized in our group [33,49,73] and the weta-terphenyl-substituted silyl cations 73 from the Siegel group [34, 74, 75], will be discussed here as examples for intramolecular stabilized silylated arenium ions. Both classes of silyl cations were synthesized by the standard BCS hydride-transfer reaction... 

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