Intramolecularly Stabilized Phenylsilyl and Anthrylsilyl Cations

Abstract

Phenyl-substituted silylium cations with two −CH2Z substituents in the ortho positions (R2SiC6H3(CH2Z)2+, I) first synthesized by Corriu and co-workers provide useful model systems to investigate solvent coordination of silylium cations R2HSi+ (H is equivalent to the C6H5 group) in solution. By rotation of the side chains, group Z can be positioned in such a way that Si+−Z interactions lead to an internal solvation of the silylium cation thus competing for or even preventing external solvation of −R2Si+ in solution. For Z = NR2, the properties of type I cations resemble those of the strongly coordinated silylium cation−solvent complexes R‘2HSi(NR3)2+ as is reflected by complexation energies, interaction distances, and δ29Si NMR chemical shift values typical of pentacoordinated Si. Weakly coordinating solvents such as alkanes can be mimicked by using for type I cations the side chain −C(CH3)3. A −SiH2+ group interacts with the closest H atoms of the two adjacent methyl groups and, despite the fact that interactions are just electrostatic, I largely loses its silylium cation character as indicated by the calculated δ29Si value. More silylium cation character is retained when a 1,8-dimethyl-substituted anthryl ring rather than the phenyl ring is used as a template for simulating solvent coordination of silylium cations in solution. The silyl cation Me2SiC14H7(CH3)2+ (18) with the −SiMe2+ group at position 9 framed between the two methyl substituents has a δ29Si IGLO value of 187 ppm, which is just 150 ppm upfield from the corresponding value for Me2HSi+ in the gas phase (334 ppm). The synthesis of 18 and related cations should be an interesting target for experimental work on silylium cations in solution.