Switchable C-H Silylation of Indoles Catalyzed by a Thermally Induced Frustrated Lewis Pair

It is a challenging task to achieve regioselective C-H silylation of indoles with Ph2SiH2, as bis(indol-3-yl)-substituted silanes are always obtained along with the C3-silylated indoles. Without adding any additive, we developed an Al(C6F5)(3)-based catalyst system for the synthesis of C3-selective silylated indoles under mild conditions and propose here the reaction mechanism on the basis of systematic studies including detailed experimental data, characterization of key reaction intermediates, and isotope-labeled experiments. The adduct 8a generated from the coordination of indoline with Al(C6F5)3 at room temperature could activate silanes such as 2a to form a thermally induced frustrated Lewis pair (FLP) under heated conditions, which could efficiently catalyze the regioselective C-H silylation. The direct use of 8a as catalyst enabled us to successfully inhibit the formation of different indoline byproducts without the specific requirement of indole substrates and any additive, achieving up to 99% yields of C3-silylated indoles. More importantly, this Al(C6F5)(3)-based thermally induced FLP catalyst system realized the precise control of C-H silylation process by switching the external thermal stimuli on/off.