Synthesis of 2,6‐Diarylphenyldimethylsilyl Cations: Polar‐π Distribution of Cation Character

The preparation of a series of planar chiral, ferrocenyl-substituted hydrosilanes as precursors of ferrocene-stabilized silicon cations is described. These molecules also feature stereogenicity at the silicon atom. The generation and (29)Si NMR spectroscopic characterization of the corresponding silicon cations is reported, and problems arising from interactions of the electron-deficient silicon atom and adjacent C(sp(3))-H bonds or aromatic π donors are discussed. These issues are overcome by tethering another substituent at the silicon atom to the ferrocene backbone. The resulting annulation also imparts conformational rigidity and steric hindrance in such a way that the central chirality at the silicon atom is set with complete diastereocontrol. These chiral Lewis acid catalysts were then tested in difficult Diels-Alder reactions, but no enantioinduction was seen.

Section: Resultsmentioning

confidence: 69%

Planar Chiral, Ferrocene‐Stabilized Silicon Cations

Schmidt

Klare

Fröhlich

et al. 2016

“…The Baldridge/Siegel team recently introduced a new class of silicon cations with various terphenyl substituents. The roof‐like structure not only provides kinetic stabilization through steric shielding of the silicon cation but also thermodynamic stabilization through fast oscillation of the cationic silicon atom between the π systems of the flanking arenes ( 2 ; Figure 1, middle left) 7. A similar effect was achieved by interaction with neutral halogen atoms ( 3 , middle right) 8.…”

Section: Introductionmentioning

confidence: 98%

The Family of Ferrocene‐Stabilized Silylium Ions: Synthesis, ²⁹Si NMR Characterization, Lewis Acidity, Substituent Scrambling, and Quantum‐Chemical Analyses

Müther

Hrobárik

Hrobáriková

et al. 2013

The purpose of this systematic experimental and theoretical study is to deeply understand the unique bonding situation in ferrocene-stabilized silylium ions as a function of the substituents at the silicon atom and to learn about the structure parameters that determine the (29)Si NMR chemical shift and electrophilicity of these strong Lewis acids. For this, ten new members of the family of ferrocene-stabilized silicon cations were prepared by a hydride abstraction reaction from silanes with the trityl cation and characterized by multinuclear (1)H and (29)Si NMR spectroscopy. A closer look at the NMR spectra revealed that additional minor sets of signals were not impurities but silylium ions with substitution patterns different from that of the initially formed cation. Careful assignment of these signals furnished experimental proof that sterically less hindered silylium ions are capable of exchanging substituents with unreacted silane precursors. Density functional theory calculations provided mechanistic insight into that substituent transfer in which the migrating group is exchanged between two silicon fragments in a concerted process involving a ferrocene-bridged intermediate. Moreover, the quantum-chemical analysis of the (29)Si NMR chemical shifts revealed a linear relationship between δ((29)Si) values and the Fe···Si distance for subsets of silicon cations. An electron localization function and electron localizability indicator analysis shows a three-center two-electron bonding attractor between the iron, silicon, and C'(ipso) atoms, clearly distinguishing the silicon cations from the corresponding carbenium ions and boranes. Correlations between (29)Si NMR chemical shifts and Lewis acidity, evaluated in terms of fluoride ion affinities, are seen only for subsets of silylium ions, sometimes with non-intuitive trends, indicating a complicated interplay of steric and electronic effects on the degree of the Fe···Si interaction.

“…[2b, 3, 4] The requirement for their beneficial use in preparative work is however ac lear controlo ver their reactivity. [5] In many cases, this was achieved by intramolecular coordination of weak Lewis bases (LB) to the cationic silicon center that results in tetra-coordination for the silicon center ( Figure 1). In saying that, the parallels to intramolecular Frustrated Lewis Pairs (FLPs)p opularized by Erker and Stephan become obvious.…”

Section: Introductionmentioning

confidence: 99%

An Experimental Acidity Scale for Intramolecularly Stabilized Silyl Lewis Acids

Künzler

Rathjen

Merk

et al. 2019

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 19F NMR chemical shifts and the 1J(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.