Zinc-Catalyzed Dehydrogenative Silylation of Indoles

Abstract

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 ratedetermining 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.

A Drastic Effect of TEMPO in Zinc‐Catalyzed Stannylation of Terminal Alkynes with Hydrostannanes via Dehydrogenation and Oxidative Dehydrogenation

“…Besides cycle A as a major route, cycle B, where 3 is formed from 1 and 12 , may participate as only a minor pathway . At present, the exact role of pyridine is unclear, but could be to form an active species with Zn , as previously clarified in the zinc−pyridine−nitrile system . When viewed as a whole, interestingly, a unique mechanism is operative here; both processes of the dehydrogenation (−H 2 ) and the oxidative dehydrogenation (− H 2 O), which would be unlikely to coexist in general, are involved in a single reaction.…”

Silicon–Nitrogen Bond Formation via Heterodehydrocoupling and Catalytic N‐Silylation

“…Tsuchimoto and co‐workers screened various Lewis acid salts and bases for the N‐silylation of indoles, and found that Zn(OTf) 2 and C 5 H 5 N facilitated N‐silylation with a variety of substituted indoles and tertiary silanes (Figure 7). [50] This is related to their 2017 study with Zn(OTf) 2 and C 5 H 5 N as catalyst for N‐silylation with Ph 2 MeSiH [51] …”

Zinc/Indium Bimetallic Lewis Acid Relay Catalysis for Dehydrogenative Silylation/Hydrosilylation Reaction of Terminal Alkynes with Bis(hydrosilane)s

“…First up is the complexation of ZnX 2 with T to form a T ⋅⋅⋅ Zn II {[Zn II ]} complex, which most likely catalyzes the DS of 1 with 2 . Thus, [Zn II ] activates one H− Si bond of 2 to form 15 with a more electrophilic Si , to which T may coordinate for stabilization, as previously demonstrated . The nucleophilic terminal carbon of 1 then reacts with the electrophilic Si of 16 in a −H 2 fashion to yield 5 , and also to regenerate [Zn II ] and T .…”

“…To get insight into the reaction pathway, we explored whether, even in the co‐presence of InBr 3 , a T ⋅⋅⋅ Zn II complex could be formed (Table ), as in the DS of indoles catalyzed by the similar zinc−pyridine system . As shown in entries 1 and 2, the H α peak of T found at δ 8.61 in a 1 H NMR spectrum was upfield‐shifted to δ 8.29, upon measuring a mixture of T , Zn(NTf 2 ) 2 , and InBr 3 in a 7:3:1 ratio, which is the same as the relative amount of each used for the standard reaction conditions.…”