Mechanistic Insights into Ru–S Complex-Catalyzed C–H Silylation and Borylation of N-Heteroarene: Distinct Bonding Interactions

The silylation and borylation of N-heteroarenes are essential processes for preparing key building blocks in organic synthesis. The Ru–S complex 1, [(PEt₃)Ru(DmpS)]⁺ (DmpS = 2,6-dimesitylphenyl thiolate), catalyzes both C–H silylation and borylation of N-heteroarenes. Herein, we performed a density functional study to investigate the mechanisms of 1 catalyzed C–H silylation of 1-methylindole using hydrosilanes and C–H borylation using dialkoxyhydroborane (HBpin) and dialkylhydroborane (9BBN). The mechanism involves four main steps: (i) Si–H/B–H activation, (ii) silyl/boryl transfer to 1-methylindole, (iii) proton abstraction to yield the silylated/borylated product, and (iv) H₂ elimination to regenerate complex 1. The rate-determining step is silyl/boryl transfer. Notably, upon B–H activation, the B–H bond of HBpin is fully cleaved, while the B–H bond of 9BBN remains partially intact. Moreover, instead of forming silylium/borenium ions, the Si–H and B–H activations lead to distinct Si–H/B–H-activated complexes: (i) thiosilane/thioborane-supported Ru–H complexes for hydrosilane and HBpin and (ii) a three-center two-electron Ru–H–B complex for 9BBN. Differences in bonding interactions affect the energy barriers in the silyl/boryl transfer. Insights into these electronic structures provide a foundation for designing metal–ligand cooperative catalysts for C–H silylation and borylation of N-heteroarenes.