Easy Access to Enantiomerically Pure Heterocyclic Silicon‐Chiral Phosphonium Cations and the Matched/Mismatched Case of Dihydrogen Release

Abstract

Phosphonium ions are widely used in preparative organic synthesis and catalysis. The provision of new types of cations that contain both functional and chiral information is a major synthetic challenge and can open up new horizons in asymmetric cation‐directed and Lewis acid catalysis. We discovered an efficient methodology towards new Si‐chiral four‐membered CPSSi* heterocyclic cations. Three synthetic approaches are presented. The stereochemical sequence of anchimerically assisted cation formation with B(C6F5)3 and subsequent hydride addition was fully elucidated and proceeds with excellent preservation of the chiral information at the stereogenic silicon atom. Also the mechanism of dihydrogen release from a protonated hydrosilane was studied in detail by the help of Si‐centered chirality as stereochemical probe. Chemoselectivity switch (dihydrogen release vs. protodesilylation) can easily be achieved through slight modifications of the solvent. A matched/mismatched case was identified and the intermolecularity of this reaction supported by spectroscopic, kinetic, deuterium‐labeling experiments, and quantum chemical calculations.

Structural and Electronic Effects on Phosphine Chalcogenide Stabilized Silicon Centers in Four-Membered Heterocyclic Cations

“…Other intriguing examples of nitrogen-, phosphorus-, oxygen-, and sulfur-stabilized silylium ions have stimulated promising applications in bond activation and catalysis . In recent years, the intramolecular stabilization of silylium ions has been further explored, particularly with regard to the generation of stereogenic Lewis acidic silicon centers with defined configurations. , …”

Hidden silylium-type reactivity of a siloxane-based phosphonium–hydroborate ion pair

“…17 We recently reported a methodology towards a new class of configurationally stable silicon-chiral heterocyclic cations, which is based on an anchimerically assisted cation formation by means of an incorporated phosphine sulfide function. 18 Herein, we draw an unexpected picture of silylium reactivity, which affects the common view of highly reactive silyl cations. Taking advantage of the intramolecular P + -S À stabilization of a cationic silicon center in combination with the chemical robustness and electronic flexibility of the Si-O-Si framework, we introduce a previously unknown concept of ion pair reactivity.…”

“…15 29 Si NMR signal upon hydride abstraction and intramolecular sulfur stabilization is much less pronounced here than in our previously described four-membered silyl phosphonium sulfides. 18 However, it must be taken into account that the 29 Si NMR chemical shift can be strongly dependent on the ring size, which can lead to significant deshielding in small ring systems compared to six-membered rings. 25 This suggests a strong Si-S-P interaction within the heterocyclic cation 7.…”

Two Faces of the Bi−O Bond: Photochemically and Thermally Induced Dehydrocoupling for Si−O Bond Formation

“…The elimination of benzene from a molecule containing an SÀ H and a SiÀ Ph moiety or from phenylsilane in the presence of a rare earth hydride complex have been reported. [32] This possibility could be ruled out, since no benzene was detected by 1 H NMR spectroscopy, when the reaction was performed in toluene under otherwise identical catalytic conditions (as in entry 5). Alternatively, TEMPO could act as a hydrogen atom acceptor to give TEMPOÀ H or the corresponding amine (2,2,6,6-tetramethylpiperidine) plus water.…”