Siliconium triflate

The progress of elimination (at 65 °C, in vacuum sealed CDC13 solutions) was monitored by lH and 29Si NMR spectroscopy, and was found to depend on the counterion among the halides the order of reactivity is I> Br> Cl [50% elimination after 85, 130, and 175 min, respectively, from 93c(Cl), 93c(Br), and 93c(I)], whereas the siliconium triflate 93c(OTf) did not react. It is thus evident that nucleophilic attack of the anion on one of the 7V-methyl groups is involved in the rate determining step of the elimination. 

Table 1. Si chemical shifts for the equilibrium mixtures of 3 and the corresponding siliconium chlorides 6 at two temperatures, and for siliconium triflates (7).

Table 1. Selected bond lengths and angles from crystal structures of neutral binuclear hexacoordinate compex (5b), binuclear (8b) and mononuclear (9) ionic siliconium triflates.

The equilibrium reaction between neutral hexacoordinate chelates and pentacoordinate siliconium halide salts is discussed in Section III.A.4 (Eq. 17). This reaction can be driven completely to the ionic side by replacement of the chloro ligand by better leaving groups triflate and bromide (Eq. (18), listed again with compound labels see Section III.A.4.iv). The products of this counterion-exchange reaction are stable siliconium salts 90(OTf)-92(OTf), 90(Br)-92(Br), which no... 

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... 

The partial and reversible ionization of binuclear hexacoordinate silicon complexes 55a,c,j is described in Section III.A.5.ii. Like the mononuclear siliconium chloride salts, these can also form stable binuclear disiliconium salts (57a,c,j) by replacement of the chloride by other counterions, which are better leaving groups (triflate, bromide, or iodide, Eq. 37).69... 

The structural evidence for 97, 98 came from their characteristic 29Si chemical shifts (Table XXVIII), and a crystal structure analysis for 97a(OTf), the triflate salt derived from anion exchange with 97a (Fig. 56, Table XXIX). Table XXVIII shows that these two salts, the chloride and triflate, have the same 29Si chemical shifts, thus confirming the identical siliconium cation parts in both. In fact, from Table XXVIII it is also evident that the NMR spectra for the two siliconium... 

In the presence of bulky X ligands, a facile methyl halide elimination reaction is observed (Eq. 2) [3]. In this elimination the siliconium ion complex 2, with its two N->Si dative bonds, is converted into a neutral pentacoordinate complex 3, with only one remaining dative bond (Fig. 1, Table 1). The reaction is probably driven by partial release of steric interaction, caused by the removal of one of the A-methyl groups. This is indicated by a decrease in elimination rate in the presence of less bulky ligands, cyclohexyl and isobutyl, and the failure to observe elimination when X = methyl. The reactivity order of the halide ions follows their nucleophilicities F > Br > CF, while the less nucleophilic triflate ion does not react at all. 

Summary Aryl ligands carrying oxygen, phosphorus or sulfur donor atoms in the side chain are introduced at silicon. Here we report on the synthesis of silyl chlorides and silyl triflates. The interaction between the silicon centre and the donor atom are discussed on the basis of NMR and X-ray crystallographic data. In the reaction of bis[2-(methoxymethyl)phenyl]methylsilyl triflate 8 with nucleophiles, reactivity of oxonium as well as of siliconium ions is observed. 

Summary The first ionic dissociation of the Si-Cl bond in neutral hexacoordinate silicon complexes is reported. An equilibrium reaction between the ionic siliconium chloride and its neutral precursor (dissociation-recombination) is observed. The population ratio can be controlled by temperature or by replacement of the chloro ligand by a triflate group. The reaction enthalpy and entropy of the dissociation are both negative, suggesting that solvent organization facilitates dissociation at low temperature. 

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