Strain-Modulated Reactivity: an Acidic Silane

Compounds of main-group elements such as silicon are attractive candidates for green and inexpensive catalysts. For them to compete with state-of-the-art transition-metal complexes, new reactivity modes must be unlocked and controlled, which can be achieved through strain. Using a tris(2-skatyl)methylphosphonium ([TSMPH₃]⁺) scaffold, we prepared the strained cationic silane [TSMPSiH]⁺. In stark contrast with the generally hydridic Si−H bond character, it is acidic with an experimental pK_a^DMSO within 4.7–8.1, lower than in phenol, benzoic acid, and the few hydrosilanes with reported pK_a values. We show that ring strain significantly contributes to this unusual acidity along with inductive and electrostatic effects. The conjugate base, TSMPSi, activates a THF molecule in the presence of CH-acids to generate a highly fluxional alkoxysilane via trace amounts of [TSMPSiH]⁺ functioning as a strain-release Lewis acid. This reaction involves a formal oxidation-state change from Si^II to Si^IV, presenting intriguing similarities with transition-metal-mediated processes.