Siliconium ion

About this time, it was postulated that the transition state was a trivalent siliconium ion followed by the rapid addition of water. In 1982, this view was supported by Schmidt et al. [15], They studied the hydrolysis of tetra-alkoxy-silanes and various alkoxyalkylsilanes. It was noted that the alkyl-containing silanes reacted significantly faster than the analogous alkoxysilanes. They attributed this result to the stabilizing effect that the electron donating character of the alkyl group (+1 effect) would have on a siliconium ion. 

If we assume that the reaction mechanism for the hydrolysis goes through a siliconium ion and that its formation is the rate-determining step, the following should be the rate expression of the forward reaction ... 

Since the siliconium ion will then quickly react with water, the H+ concentration remains constant and we can evaluate the expression ... 

Smith [26] studied the hydrolysis of methylmethoxysilanes and determined the reactions to be first order in both the substrate and the acid. He is one of the few to suggest a mechanism other than the formation of a siliconium ion. He suggested that a five-coordinate reaction intermediate in which the entering and the leaving groups attach to the same face of the silicon ... 

This was taken to indicate that solvolysis of the two substrates does not give the same cyclic siliconium ion intermediate 32, but that one (if not both) has an open fi-... 

However, the studies of Fujiyama and Munechika55 on the solvolyses of 2-(aryldimethylsilylethyl) chlorides 33 revealed aryl substituent effects which were consistent with the formation of a cyclic siliconium ion 34. 

The formation of these products can be explained by consideration of the reaction mechanism, which is most usefully pictured as proceeding via a cyclic siliconium ion 128 (equation 87). 

Attack of a nucleophile on the /1-silyl carbocation 127 or the cyclic siliconium ion 128 leads to desilylation and formation of the Sakurai product. When nucleophilic attack is disfavoured by steric hindrance at the silicon, competing intramolecular attack by the enolate becomes important. This 5-exo-tet cyclization gives the trimethylsilylcyclopentane product with high stereospecificity, the trimethylsilyl group having undergone a 1,2 shift. 

It has recently been found that replacement of ketones by esters in this reaction leads to the production of cyclobutanes, sometimes as the major product, arising from intramolecular attack of the enolate at the secondary carbon of the siliconium ion (equation 88)154. 

The experimental dependency of the /J-silyl effect on 0 in solvolysis reactions is sketched in Figure 764. Obviously, it differs from that anticipated for a k mechanism with rate-determining formation of siliconium ion or from the cosine-squared function expected for the pure hyperconjugative stabilization model. Apparently, the /J-silyl effect is operative in the solvolysis of both the syn- and rmt/ -periplanar conformations. The rate acceleration in the latter might be ascribed to a more favourable geometry for the (T-anchimeric assistance. 

Further support comes from calculation of 1 Jch spin-spin coupling constants at IGLO-DFPT/BIII level. The data were interpreted as giving strong evidence for intramolecularly TT-stabilized silanorbomyl cation structure lacking coordination to solvent or counterion. The species can also be regarded as an almost symmetrically bridged /J-silyl carbocation with siliconium ion character. 

A special case of reversible ionization of a hexacoordinate silicon complex has been described as a novel tautomeric equilibrium.41 It differs from the formation of siliconium-ion salts in that the positive charge resides on nitrogen, in a dimethylammonium cation, and not on silicon. The transsilylation of lg with 12 in equimolar concentrations leads to the pentacoordinate zwitterionic complex 13 (Eq. (10), Section II.B.5). However, when the molar ratio was 2 1, respectively, an equilibrium mixture of tautomers (58, 59) was obtained, as shown in Eq. (21). The same mixture was also obtained when a second mole-equivalent of lg was added to 13. 

We make a distinction between two types of ionic pentacoordinate complexes those which are in dynamic equilibrium with neutral hexacoordinate complexes have been dealt with in Sections III.A.4, III.A.5.ii, and III.B.2. The second group includes those pentacoordinate siliconium-ion salts which are formed as such and are stable and do not equilibrate (to a noticeable extent) with their hypothetical neutral hexacoordinate counterparts. The present section discusses this group of persistent salts of pentacoordinate silicon cations. 

Stable siliconium-ion salts can also be prepared without exchange of ions, in the case of the bulky -butyl ligand, directly from the TMS-hydrazide and t-BuSiCl3 (Eq. 35).66 The t-butylsiliconium chlorides 93(0) are the only known stable chloride salts of this kind. The chloride can still be exchanged with other anions, as shown in Eq. (18), to produce 93a,c,j bromide, iodide, and triflate.83... 

However, in the case of more weakly interacting counterion such as TPFPB the NMR data are in line with the formation of a pentacoordinated siliconium ion. [124] It was concluded that the stability of the pentacoordinated ion strongly depends on solvent and counterions. Specifically, methanol seems to stabilize the tetracoordinated form extensively. [124]... 

There are comparatively few cationic complexes, the most important being the octahedral /3-diketonates and tropolonates of Si and Ge such as [Ge(acac)3]+ and Si trop3+. Siliconium ions can also be formed by reactions such as... 

Intramolecularly coordinated silyl cations are known since the pioniering studies of Corriu [5] and Willcott [6] both using a tridentate substituent at silicon. Siliconium ions bearing the bidentate PTMH or DMBA substituent may be obtained via different pathways (Scheme 3). 

Most purines are sufficiently volatile for direct injection into the mass spectrometer without chemical pretreatment. However, trimethylsilyl derivatives (IMS) have been used for the cnhsnCviTisnt of volatility. Xhc tri ss spcctrci of the TMS dcrivstivcs of the two msjor purine bases are given in a publication by McCloskey et al. In each case, loss of a TMS methyl radical generates a stable siliconium ion (M-15). 

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