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 pKa DMSO within 4.7-8.1, lower than in phenol, benzoic acid, and the few hydrosilanes with reported pKa 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 SiII to SiIV , presenting intriguing similarities with transition-metal-mediated processes.
Keywords: acidity; silanes; silicon; strained molecules; zwitterions.
© 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.