Strain‐Modulated Reactivity: An Acidic Silane

Abstract

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 pKaDMSO 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.

An investigation into the Brønsted acidity of the perfluorinated alkoxy silanes {(F3C)3CO}3SiH and {(F6C5)3CO}2Si(Cl)H

“…However, there is growing evidence to suggest that the Brønsted acidity of H-silanes can be increased by introduction of electron-withdrawing substituents [1][2][3][4] or geometrical constraints. 5 One of the earliest examples of Brønsted acidic H-silanes is Cl 3 SiH. In the Benkeser reaction, Cl 3 SiH is deprotonated by tertiary amines to give the transient [Cl 3 Si] − ion in low concentrations, which then undergoes nucleophilic substitution reactions.…”

“…4 Recently, the bicyclic strain-modulated Brønsted acidic H-silane [TSMPSiH] + derived from a tris(2-skatyl)methylphosphonium scaffold was reported. 5 The enhanced Brønsted acidity results from charge compensation and relief of angular strain upon deprotonation.…”

Oxidative Additions of C−F Bonds to the Silanide Anion [Si(C2F5)3]−

“…They are also proficient at adding oxidatively to strong σ‐bonds such as C−X bonds (X=I, Br, Cl, F) affording neutral tetra‐ and pentacoordinated silicon(+IV) compounds (Figure 1). [1, 18, 19] A zwitterionic silanide was stated to show Lewis amphoteric behavior and to support both oxidation states +II and +IV at the silicon center therefore exhibiting an unusual reactivity [20] . We recently reported on the tris(pentafluoroethyl)silanide anion, [Si +II (C 2 F 5 ) 3 ] − , which due to its strong electron withdrawing C 2 F 5 groups reacts not only as a typical Lewis base but also as a Lewis acid [21] …”