G. Royo

Pentacoordinate silicon compounds: Stereochemical non-rigidity of chelates formed by intramolecular ring-closure. Crystal structure of 8-dimethylamino-1-trifluorosilylnaphthalene☆

The application of dynamic NMR spectroscopy to the study of stereochemical non-rigidity in pentacoordinate chelated organosilicon compounds is described. It is shown that in the compounds Me2NCH(Me)C6H4SiXYZ, non-dissociative ligand permutation at silicon can be distinguished unambiguously from processes associated with rupture of the chelate ring and nitrogen inversion. The crystal and molecular structure of 8-Me2NC10H6SiF3 has been determined. Pentacoordination of the silicon atom is confirmed, with the donor nitrogen atom and a fluorine atom occupying axial sites in an overall trigonal bipyramidal geometry. The N → Si separation is 2.3 A (average of two distinct but closely related molecular conformations), which is less than the C1C8 distance in the naphthalene nucleus, indicating a substantial bonding interaction. NMR studies of the dynamic behaviour of the Me2N group, and where possible (19F, 1H) of the monodentate ligands in 8-dimethylamino-1-silylnaphthalene compounds, together with the results for the chelated benzylaminosilicon compounds, confirm that inversion of the absolute configuration at the silicon atom is not achieved by this process. The free energies of activation for non-dissociative ligand permutation at a silicon range from less than 7 kcal mol−1 [SiH3, Si(OR)3], which is below the limit of direct measurement, to 13 kcal mol−1 for Me2NCH(Me)C6H4SiF3; difunctional silicon chelate compounds (Cl, F, OR) display values from 9–12 kcal mol−1. These are comparable with those determined for fluxional processes in acyclic pentacoordinate silicon compounds.

Enhancement of SiH bond reactivity in pentacoordinated structures

Abstract The reactions of pentacoordinated silicon dihydrides with alcohols, carboxylic acids and carbonyl compounds have been studied. The dihydrides are markedly more reactive than the corresponding tetracoordinated species.

Problème de l'ion siliconium II. Comportement de vinylsilanes et d'éthylsilanes optiquement actifs☆

Abstract The resolution of vinylphenyl-1-naphthylmenthoxysilane is described. The reduction of this compound by LiAlH 4 gives different functional silanes of the ethyl, phenyl, 1-naphthyl series. However, the reduction by 3 LiAlH 4 /AlCl 3 gives silanes of the vinyl series. The stereochemistry of nucleophilic substitution reactions on silicon atom is discussed. These reactions have a high stereospecificity. These results and those obtained in hydrolysis and alcoholysis show that the Si + cation does not occur in these reactions in spite of the presence of a vinyl group.

Pentacoordinated silicon compounds. Intramolecular ring closure, site preferences of substituents and the stability of the resulting chelates

Dynamic NMR studies have been made of chelation in 2-(dimethylaminomethyl)phenylsilanes and 2-[1-(dimethylamino)ethyl]phenylsilanes with a wide range of substituents on the silicon atom. Temperature-dependent 19F spectra of compounds of the type Me2NCH2C6H4SiMeFX in which the geometry about the silicon atom is trigonal bipyramidal with the donor nitrogen atom axial, have established the preference of the substituent X for the axial site trans to the donor nitrogen atom, relative to that of the fluorine atom. Combined with other structural data this leads to te apicophilicity series: H < alkyl < aryl < OR,NR2 < F ≈ SR < Cl,OCOR. It is concluded that the apicophilicity is closely correlated with the ability of the bond trans to the donor atom to be stretched by electron donation to the central atom. The stability of the chelates has been investigated by dynamic 1H NMR spectroscopy of the potentially diastereotopic NMe2 groups. When the ligand that occupies the site trans to the donor nitrogen atom is electronegative, the stability is largely determined by the nature of that ligand, and increases with the apicophilicity. Intramolecular coordination is barely detectable in the case of an alkoxy group. Compounds containing only alkyl and aryl substituents in addition to the bidentate ligand are also not chelated, but dihydrogeno-alkyl (or -aryl) derivatives, in which the hydrogen atoms occupy the equatorial sites, and trihydrogeno derivatives, are relatively strongly coordinated.

Hydrogenosilanes: Pentacoordination and pseudorotation

Abstract A 1 H NMR study of pentacoordinated trihydrogenosilanes shows that there is only a very low energy barrier to isomerisation by pseudorotation at silicon. In contrast, this process is not detected over a large range of temperature for dihydrogeno silanes. The SiH bond has a high equatorial preference which stabilises the geometry of the trigonal bipyramidal structures.

Pentacoordinate silicon compounds: Pseudorotation at silicon

Abstract The variation with temperature of the 19 F and 1 H NMR spectra of pentacoordinate fluorosilanes which contain an intramolecular SiN coordination has been examined. A clear distinction between pseudorotation and ring opening-closing processes has been obtained for the first time and the corresponding activation energies determined.

Pentacoordinate silicon derivatives: relative apicophilicity of functional groups attached to a silicon atom

For bifunctional organosilanes o-(Me2NCH2)C6H4SiXFR the relative order of apicophilicity of substituent X versus the apicophilicity of F is determined as H, OR, NR2 < F < Cl, OCOC6H5X. The results show that the major trigonal bipyramidal conformer is not influenced by the electronegativity of the F atom.

Comportement chimique et stereochimique de fluoroalcoxysilanes chiraux : II. deplacement de l'atome de fluor avec inversion de configuration☆

Abstract The synthesis and chemica behaviour of two asymmetric fluoroalkoxysilanes (R1R2Si∗ (F)OR is described. Both react with some organometallic compounds (PhLi, PhCCL1, CH2CHCH2li, CH2CHCH2MgBr) giving selective and stereo-selective substitution of fluorine atoms. The stereochemistry of the reaction is predominant inversion of configuration. In both compounds the SiF bond is easily cleaved, the fluorine atom being displaced by organometallic reagents (PhLi, PhCCLi) with inversion, rather than the normal retention of configuration. We explain this chemical behaviour by the effect of p-electrons of oxygen atoms.

Five-co-ordinated silicon compounds: geometry of formation by intramolecular co-ordination. Crystal structure of 2-(dimethylaminomethyl)phenyl-1-naphthylsilane

Studies of a wide range of intramolecularly five-co-ordinated silicon compounds with chelating nitrogen donors have been made, with a view to obtaining further information on the details of nucleophilic substitution reactions at silicon. The crystal structure of 2-(dimethylaminomethyl)-phenyl-1 -naphthylsilane has been determined by single-crystal X-ray diffraction (R= 0.074 for 564 observed reflections). Crystals are tetragonal, space group I, with Z= 8 in a unit cell of dimensions a= 21.845(5) and c= 7.119(2)A. The central silicon atom shows essentially trigonal-bipyramidal co-ordination, with the axial positions occupied by the naphthyl group and the donor nitrogen atom, and the two hydrogen atoms occupying equatorial positions. Comparison with other structures reported in the literature shows that this geometry, always with axial disposition of the donor atom, is generally adopted by molecules of this type. Silicon-29, 19F, and 1H n.m.r. spectroscopic data for similar compounds with varying substituents on the silicon atom show that the same geometry is adopted by these intramolecularly co-ordinated species in solution.

Comportement chimique et stereochimique de fluoroalcoxysilanes chiraux : IV. Reaction avec les organomagnesiens☆

Abstract Saturated aromatic and vinylic organomagnesium compounds react with fluoroalkoxysilanes R 1 R 2 Si(F)OR with selective and stereoselective departure of the alkoxy group. The stereochemistry is retention of configuration. The selectivity is explained in terms of an electrophilic assistance of magnesium on oxygen, that governs the cleavage of the SiOR bond. When the Grignard reagent is more strongly solvated (THF, DME) competitive displacement of both functions is observed. The alkoxy group is always substituted with retention of configuration whereas fluorine is displaced with inversion of configuration. In the first case the stereochemistry is explained by an equatorial attack of the reagent. In the second case the results are explained by a simultaneous formation of two intermediates (equatorial and axial attack of the reagent).