On the Origin of the Non-Planarity in Biarylsilyloxonium Ions.

The structure determination of a series of biphenylsilyloxonium cations through NMR and XRD studies, supported by DFT calculations was carried out, allowing to get insights into the origin of the oxygen pyramidalization in biphenyl and binaphthylsilyl oxoniums. Low barrier of inversion in the oxygenyl series points toward a key role of the biaryl axis in the inversion of configuration at the oxygen center.

Chiral Chalcogenyl-Substituted Naphthyl- and Acenaphthyl-Silanes and Their Cations.

Cyclic silylated chalconium borates 13[B(C6 F5 )4 ] and 14[B(C6 F5 )4 ] with peri-acenaphthyl and peri-naphthyl skeletons were synthesized from unsymmetrically substituted silanes 3, 4, 6, 7, 9 and 10 using the standard Corey protocol (Chalcogen Ch=O, S, Se, Te). The configuration at the chalcogen atom is trigonal pyramidal for Ch=S, Se, Te, leading to the formation of cis- and trans-isomers in the case of phenylmethylsilyl cations. With the bulkier tert-butyl group at silicon, the configuration at the chalcogen atoms is predetermined to give almost exclusively the trans-configurated cyclic silylchalconium ions. The barriers for the inversion of the configuration at the sulfur atoms of sulfonium ions 13 c and 14 a are substantial (72-74 kJ mol-1 ) as shown by variable temperature NMR spectroscopy. The neighboring group effect of the thiophenyl substituent is sufficiently strong to preserve chiral information at the silicon atom at low temperatures.

An Experimental Acidity Scale for Intramolecularly Stabilized Silyl Lewis Acids.

A new NMR-based Lewis acidity scale is suggested and its application is demonstrated for a family of silyl Lewis acids. The reaction of p-fluorobenzonitrile (FBN) with silyl cations that are internally stabilized by interaction with a remote chalcogenyl or halogen donor yields silylated nitrilium ions with the silicon atom in a trigonal bipyramidal coordination environment. The 19 F NMR chemical shifts and the 1 J(CF) coupling constants of these nitrilium ions vary in a predictable manner with the donor capability of the stabilizing group. The spectroscopic parameters are suitable probes for scaling the acidity of Lewis acids. These new probes allow for the discrimination between very similar Lewis acids, which is not possible with conventional NMR tests, such as the well-established Gutmann-Beckett method.

Silyl Chalconium Ions: Synthesis, Structure and Application in Hydrodefluorination Reactions.

The synthesis of two series of silylated chalconium borates, 9 and 10, which are based on the peri-naphthyl and peri-acenaphthyl framework, is reported (chalcogen (Ch): O, S, Se, Te). NMR investigations of the selenium- and tellurium-containing precursor silanes 3 d-f and 8 d, f revealed a significant through-space J-coupling between the chalcogen nuclei and the Me2 SiH group. Experimental and computational results typify the synthesized cations 9 and 10 as chalconium ions. The imposed ring strain weakens the Si-Ch linkage compared to acyclic chalconium ions. This attenuation of the Si-Ch bond strength is more pronounced in the acenaphthene series. Surprisingly, the Si-O bonds in oxonium ions 9 a and 10 a are the weakest Si-Ch linkage in both series. The synthesized silyl chalconium borates are active in hydrodefluorination reactions of alkyl fluorides with silanes. A cooperative activation of the silane by the Lewis acidic (silicon) and by the Lewis basic side (chalcogen) is suggested.