Highly coordinated silicon compounds

Highly Coordinated Silicon Compounds -Hydrazino Groups as Intramolecular Donors... 

Keywords Aryllithiuiti Compounds / Chelating Substituents / Highly Coordinated Silicon Compounds / Sterically Hindered Silicon Centers... 

Summary The 2-dimethylaminomethyl-4,6-dimethylphenyl substituent 2 is introduced at silicon via the structurally characterized lithio compound 2-Li. In this manner 2-dimethylaminomethyl-4,6-dimethylphenyl substituted silanes, such as trichlorosilane 3, difluorosilane 4, and dihydridosilane 5 are available. The structural analyses of these silanes in the solid as well as in solution show 3 and 4 to be highly coordinated silicon compounds. 

The structure of 3 in solution at room temperature is dynamically pentacoordinated, showing a singlet for both the benzylic protons and the NMe2 group in the H NMR spectrum. A significant upheld shift in the SiNMR spectrum in comparison to mesityltrichlorosilane indicates a highly coordinated silicon compound (Table 1). 

In conclusion, in spite of its steric demand, the 2-dimethylaminomethyl-4,6-dimethylphenyl substituent is able to form highly coordinated silicon compounds. Investigations currently in progress are directed towards the synthesis of unsaturated silicon compounds stabilized by this substituent. 

J. Belzner, D. Schar, "Highly Coordinated Silicon Compounds - Hydrazino Groups as Intramolecular Donors", in Organosilicon Chemistry II From Molecules to Materials (Eds. N. Auner, J. Weis), VCH, Weinheim, 1996, p. 459. 

In some transformations, the choice of solvent represents a powerful adjuster bolt for the stereochemical outcome of the reaction. Divergent stereoselectivity in the reduction of glycid ketones 131 is observed when either diethyl ether or HMPA was used with trimethoxy silane as reducing reagent in the presence of catalytic amounts of lithium methoxide (Table 3.2) [49]. Highly coordinated silicon compounds show specific reactivities in comparison to tetra-coordinated silicon compounds [50]. 

Divalent silicon compounds (silylenes), one of the most interesting class of low-coordinated silicon compounds, had been known as highly reactive, short-lived transient species that resembled the carbon analogues (carbenes), until the recent... 

Six-coordinate silicon compounds result only when the silicon atom is bonded to highly electronegative atoms like fluorine, chlorine, oxygen, or nitrogen. A well-known example of this is the fluorosilicate anion SiF62, in which the fluorines are located in an octahedral array about silicon (322). The existence of (PhSiF5)2- in solution also has been confirmed by 19 F NMR spectroscopy (322). 

Compounds with High Coordination Numbers at Silicon Models for the Investigation of the Nucleophilic Substitution Reaction at Silicon Centers... 

The hexacoordinate silicon complexes discussed here are highly flexible compounds, like some other hypercoordinate silicon compounds,1 6 and undergo a variety of fluxional reorganization reactions, observable by temperature-dependent NMR spectroscopy. The dimethylamino-coordinated bis-chelates 30-38 are particularly suitable for an NMR study of ligand-site exchange processes (inter- or intramolecular), because of the pairs of diastereotopic V-methyl groups, which under certain... 

Both experimental data and theoretical calculations show that the transition of a trigonal bipyramidal into a square pyramidal structure for compounds of penta-coordinate silicon does not require high energy consumption. Up to now the attempts of experimental separation of diastereomers of organosilicon compounds with penta-coordinate silicon atom were unsuccessful. For spirocyclic bisacetylaceto-nates, XYSi(Acac)2, containing a hexa-coordinate silicon atom, however, geometric isomerism has been observed... 

The C vs P reactivity of phosphinomethanides towards silicon centers may be turned by the substitution pattern of both, the phosphinomethanide and the chlorosilane. Novel heterocycles, skeleton rearrangements and high coordinations numbers may be achieved. 9 and 14 potentially be precursors for a specific type of unsaturated silicon compounds, adding to the growing class of compoimds at the interface between organophosphorus and organosilicon chemistry. 

Silicon compounds are easily ionized, but as both these experimental and theoretical findings demonstrate, the resulting silyl cations are highly prone towards complexation. While silyl cations are thermodynamically stable, it is very difficult to obtain free SiR3 ions, not coordinated to any counterions or solvents in the environment. 

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