Synthesis of tetrahydroquinoline via hydrosilylation-transfer hydrogenation of quinoline with silane and water catalyzed by a switchable Lewis and Brønsted acid system

Employing a single catalyst in various forms within a reaction system to synergistically catalyzed reactions represent an efficient and convenient synthetic strategy. Using B(C₆F₅)₃ as a catalyst, the direct synthesis of tetrahydroquinoline was achieved through the catalyzed hydrosilylation and transfer hydrogenation of quinoline with silane in the presence of water. Through control experiments, it has been demonstrated that this reaction was catalyzed synergistically by both the Lewis acid and Brønsted acid. The Lewis acid B(C₆F₅)₃ catalyzes the hydrosilylation of quinoline, yielding a N-silylated-dihydroquinoline. Meanwhile, the Brønsted acid B(C₆F₅)₃·H₂O mediates the transfer hydrogenation of N-silylated-dihydroquinoline in the presence of quinoline and silane, forming the N-silylated-tetrahydroquinoline, ultimately yielding the desired product upon subsequent hydrolysis. Compared to traditional methods of reducing quinoline to tetrahydroquinoline using H₂, this reaction features mild conditions and high efficiency. Compared with other reaction systems for quinoline reduction using B(C₆F₅)₃ as the catalyst, silanes and/or boranes as hydride sources, and amines and/or indoles as proton sources, this method employs water as the proton source, exhibiting the advantages of being green and cost-effective. Moreover, this system demonstrates that in the presence of water, B(C₆F₅)₃ can switch between Lewis acid and Brønsted acid forms and exhibit a new mode of catalytic reaction.