Abstract
N−Me‐Benzothiazolium salts are introduced as a new family of Lewis acids able to activate Si−H σ bonds. These carbon‐centred Lewis acids were demonstrated to have comparable Lewis acidity towards hydride as found for the triarylboranes widely used in Si−H σ‐bond activation. However, they display low Lewis acidity towards hard Lewis bases such as Et3PO and H2O in contrast to triarylboranes. The N−Me‐benzothiazolium salts are effective catalysts for a range of hydrosilylation and dehydrosilylation reactions. Judicious selection of the C2 aryl substituent in these cations enables tuning of the steric and electronic environment around the electrophilic centre to generate more active catalysts. Finally, related benzoxazolium and benzimidazolium salts were found also to be active for Si−H bond activation and as catalysts for the hydrosilylation of imines.
Highlights
The use of hydridophilic main-group Lewis acids, B(C6F5)3, in SiÀH bond activation,[1] and more broadly in “frustrated Lewis pair” (FLP) chemistry,[2] has generated significant recent breakthroughs.[3]
The low oxophilicity of [2]+ indicated by this structure was confirmed by the lack of any evidence for binding of H2O to [2][BArCl]; no OÀH heterolytic cleavage was observed on addition of 2,6-lutidine and H2O to a DCM solution of [2][BArCl] (in contrast to what is observed with B(C6F5)3 and other Lewis acidic boranes).[22]
Okuda’s original observation of EtOH dehydrosilylation catalyzed by BPh3.[8b] the complete absence of benzyl alcohol (BnOH) dehydrosilylation using Na[B(Aryl)4] in MeCN indicates that B(Aryl)3 is not being formed by anion decomposition. These results combined confirm that under these conditions catalytic BnOH dehydrosilylation is initiated by the benzothiazolium salts and not by Lewis acidic boranes derived from anion decomposition.[32]
Summary
The use of hydridophilic main-group Lewis acids, B(C6F5)3, in SiÀH bond activation,[1] and more broadly in “frustrated Lewis pair” (FLP) chemistry,[2] has generated significant recent breakthroughs.[3]. One recent example from our group are N-methyl-acridinium salts which activate SiÀH bonds (Scheme 2) as indicated by SiÀH/SiÀD scrambling experiments but no silylium cations are observed in solution.[16] the Lewis acidity of the NÀMe-acridinium cation (1+) towards hydride is greater than that of B(C6F5)3.[17] This makes the conjugate organic hydride, N-Me-acridane
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