Cooperative Catalytic Activation of Si−H Bonds by a Polar Ru−S Bond: Regioselective Low-Temperature C−H Silylation of Indoles under Neutral Conditions by a Friedel−Crafts Mechanism

Abstract

Merging cooperative Si-H bond activation and electrophilic aromatic substitution paves the way for C-3-selective indole C-H functionalization under electronic and not conventional steric control. The Si-H bond is heterolytically split by the Ru-S bond of a coordinatively unsaturated cationic ruthenium(II) complex, forming a sulfur-stabilized silicon electrophile. The Wheland intermediate of the subsequent Friedel-Crafts-type process is assumed to be deprotonated by the sulfur atom, no added base required. The overall catalysis proceeds without solvent at low temperature, only liberating dihydrogen.

Electrophilic Activation of Silicon–Hydrogen Bonds in Catalytic Hydrosilations

“…Oestreich and coworkers have also proposed the involvement of electrophilic silicon species in ruthenium catalyzed hydrosilations and other transformations of silanes and organic substrates. [79] They invoke heterolytic activation of an Si-H bond across a Ru-S bond in the catalyst 36 (Scheme 42) to generate a metallasilylsulfonium group that acts as a reservoir of the electrophilic R3Si + cation. It is worth noting that this type of Si-H activation is reminiscent of Oro's mechanistic proposals (Schemes 36 and 37), invoking intermediary bases that serve as relays for the R3Si + cation.…”

Direct silylation reactions of inert C-H bonds via transition metal catalysis

“…The silyl group could be easily converted into the iodide functionality giving 2-iodoarylboronic acid derivatives, which was used as an efficient building block for the synthesis of ortho-linked oligoarenes via iterative Suzuki-Miyaura cross couplings [20c]. The group of Oestreich and Tatsumi [21] collaborated to disclose a C-H silylation reaction of indoles with the use of ruthenium catalyst containing a polar Ru-S bond (Scheme 20). The reaction featured excellent C3 regioselectivity, low temperature, lack of solvent, and liberating H 2 .…”

Rhodium‐Catalyzed Enantioselective Intramolecular CH Silylation for the Syntheses of Planar‐Chiral Metallocene Siloles

“…[13] As one of the most fruitful methods to construct organic silicons,aplethora of C À Hs ilylation protocols have been developed in the past decade.S urprisingly,w ith this large body of known catalysts and numerous commercial chiral ligands,enantioselective CÀ Hs ilylation remains largely elusive. [14,15] Kuninobu, Takai, and co-workers have pioneered the rhodium-catalyzed dehydrogenative silylation of aryl C À Hb onds,t hus affording ah ighly efficient protocol to access silafluorenes.…”