Siliconium compounds

Keywords Nucleophilic Substitution / Siliconium Compounds / Silicon Halides... 

The signal set of modification I is very similar to the spectrum of cationic complexes such as 2-OTf (compare Figs, lb and Ic), thus identifying this modification as a pentacoordinated siliconium compound. Moreover, addition of 1 equiv. ClAlMej to a solution of 4 in CDCI3, thereby transforming the chloride of 4 into the less nucleophilic Me2AlCl3 " anion, reduces the complex H NMR spectrum (Fig. lb) to that of cationic complexes (compare Fig. Ic). 

Keywords ionic dissociation, equilibrium reaction, siliconium compounds, neutral hexacoordinate complexes, negative entropy... 

Compound 86 exists in two diastereomeric forms due to the chiral silicon centers, which are evident in the various NMR spectra (Table XXV). The crystal structure of 86 shows a molecular inversion center, and hence belongs to the meso diastereomer. The geometry around silicon in the crystal is a distorted TBP and the chloride is more than 7 A away from silicon, in accord with a pentacoordinate siliconium chloride structure. The dative Si bonds are exceptionally short in 86 (1.802 and 1.807 A) relative to those in other pentacoordinate complexes.2,3,1115... 

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... 

For several of the siliconium salts crystal structure analyses were obtained, confirming the pentacoordination and the ionic nature of the compounds (well separated cations and anions). The crystal structures for 90a(OTf), 90c(OTf), 91a(OTf), 91a(AlCl4), and 93a(OTf) are depicted in Figs. 47-51, respectively. Further structural support is found in the 29Si NMR chemical shifts (Table XXVI). A remarkable observation in Table XXVI is the nearly equal 29Si chemical shifts of siliconium salts sharing the same silicon complex, but with different anions [e.g. 91a(OTf), 91a(Br), and 91a(AlCl4)] the equal shifts are the evidence that the siliconium cations are essentially independent of... 

Consequently, the ionization process is a specific phenomenon caused by the lability of the Si-Br or Si-I bonds and the affinity of silicon for oxygen not all racemization processes involve ionic 1 1 adducts. Moreover, the case of the tin compounds shows that the ability to undergo racemization is connected with the ability to increase coordination number. Thus, since silicon, germanium, and tin compounds show similar conductimetric and kinetic behavior, we may conclude that racemization and nucleophilic substitution are both activated by nucleophiles and involve a coordination extension process as shown in Scheme 67. However, existence of the siliconium ion 209 cannot be excluded in the case of bromosilanes. 

In summary, the silanorbomyl cations 5 and 25 are stable compounds in arene solvents at ambient temperature, lacking direct coordination to solvent or counterion. They are stabilized by intramolecular Jt-interaction and can be therefore regarded as an almost symmetrically bridged p-silyl-substituted carbocation with siliconium ion character. 

The ability of silicon to extend its coordination sphere in organic compounds has been intensively studied in the last decade by Corriu et al. [1-4] and some other groups [5,6]. In these investigations mainly the 2-(dimethylaminomethyl)phenyl ligand A (Fig. 1) was used [1-6], The results obtained promoted the understanding of nucleophilic substitution processes at silicon furthermore, reactive silicon compounds with a silicon-element double bond [2] or species containing siliconium ion have been stabilized with this kind of ligand [4,6],... 

Novel Pentacoordinate Siliconium Complexes Stabilized by Oxygen and Nitrogen Donors Highly Sensitive and Unusual Equilibrium between Ionic Penta- and Neutral Hexacoordinate Compounds ... 

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