Sterically Congested LLX-Type PNSi Tridentate Ligands for Iridium-Catalyzed C(sp3)-H Borylation.

Abstract

Iridium catalysts bearing LLX-type PNSi tridentate ligands (Ir/PNSi catalysts) were developed as new effective catalysts for the C(sp³)-H borylation of alkanes and alkyl groups that do not have directing groups. The Ir/PNSi catalysts were prepared in situ from [Ir(OMe)(cod)]₂ and 8-diarylphosphino-2-[(di-tert-butylhydrosilyl)methyl]quinolines, which are readily synthesized from commercially available 8-bromo-2-methylquinoline. The Ir/PNSi catalyst exhibited higher chemoselectivity than a standard Ir/Me₄phen and our previously reported Ir/NNSi catalysts. Thus, the Ir/PNSi catalysts promoted selective borylation of the sterically less crowded C(sp³)-H bonds in 2-methylheptane and dibutyldimethylsilane. The C(sp³)-H-selective borylation in the presence of C(sp²)-H bonds was also achieved with the Ir/PNSi catalysts in the reaction of 4-substituted alkylbenzenes. Such notable chemoselectivity may be due to the congested environment around the iridium center formed by the mer-tridentate coordination of the PNSi ligands.