Strain-Modulated Reactivity: An Acidic Silane

Area of Science:

• Organometallic Chemistry
• Catalysis
• Main-group Element Chemistry

Background:

• Main-group element compounds, particularly silicon, are promising for developing green and cost-effective catalysts.
• Achieving reactivity comparable to transition metals requires unlocking novel reaction pathways, often through molecular strain.
• The development of strained silicon compounds is crucial for expanding their catalytic applications.

Purpose of the Study:

• To synthesize and characterize a strained cationic silane, [TSMPSiH]+, using a tris(2-skatyl)methylphosphonium ([TSMPH3]+) scaffold.
• To investigate the unusual Si-H bond character and acidity of the synthesized silane.
• To explore the catalytic potential and reaction mechanisms involving the strained silane and its conjugate base.

Main Methods:

• Synthesis of the strained cationic silane [TSMPSiH]+.
• Experimental determination of the pKa in DMSO to quantify its acidity.
• Mechanistic studies to elucidate the role of ring strain, inductive, and electrostatic effects on acidity.
• Investigation of the reaction of the conjugate base (TSMPSi) with THF and CH-acids.

Main Results:

• The strained cationic silane [TSMPSiH]+ exhibits a remarkably acidic Si-H bond (pK a DMSO 4.7–8.1), significantly lower than typical hydrosilanes and even phenol or benzoic acid.
• Ring strain, alongside inductive and electrostatic effects, is identified as a major contributor to this unusual acidity.
• The conjugate base TSMPSi, in the presence of CH-acids, activates THF, leading to a fluxional alkoxysilane.
• The reaction proceeds via trace amounts of [TSMPSiH]+ acting as a strain-release Lewis acid, involving a formal Si(II) to Si(IV) oxidation state change.

Conclusions:

• Strained silicon compounds can exhibit unprecedented reactivity, challenging the conventional understanding of Si-H bond properties.
• The observed acidity and catalytic activity highlight the potential of strained main-group compounds as alternatives to transition metal catalysts.
• This work presents a novel reaction pathway with similarities to transition-metal-mediated processes, opening new avenues in catalysis.