Zinc-Catalyzed Dehydrogenative Silylation of Indoles

A unique Lewis acid/base system consisting of zinc triflate and pyridine was found to act as an effective catalyst for making an N(indolyl)-Si bond in a dehydrogenative fashion. Execution in a nitrile medium brings out the best performance of the Zn-pyridine system, which enables participation of flexible pieces of indoles and hydrosilanes, thereby giving diverse N-silylindoles in high to excellent yields. The Zn-pyridine system in the nitrile solvent is also applicable to the corresponding C-silylation in the case that the nitrogen atom of indoles has a substituent. Pyrrole, carbazole, arylamine, and thiophene substrates other than indoles undergo the dehydrogenative N-and/or Csilylation as well. Mechanistic studies showed that the role of the zinc Lewis acid is to activate the hydrosilane. The rate determining step of the present reaction was found to be involved in the stage of the indolyl-H bond cleavage, on the basis of kinetic isotope effect experiments. Kinetic studies indicated that the indole-based dehydrogenative N-silylation is first-order in indole, second-order in each of hydrosilane and zinc triflate, and positive and negative fractional orders in pyridine.