Insight into Mechanistic Features of Ruthenium(II)–Pybox-Catalyzed C–H Amination

The mechanisms and controlling factors of intra- and intermolecular C–H bond amination catalyzed by cationic bis-imido complex [(Pybox)Ru(NSO 3 CH 2 CH 2 CH 2 R) 2 Cl] + ( 1 _ R, where R = H, Ph) were elaborated at the density functional level. It was shown that the cis _ 1 _ Ph isomer is slightly (2.7 (2.9) [3.6] kcal/mol) lower in energy than trans _ 1 _ Ph, and trans _ 1 _ Ph → cis _ 1 _ Ph isomerization proceeds via formation of the mono-imido complex cis -[(Pybox)Ru(Imd)Cl] + with a 31.0 (17.0) [9.0] kcal/mol energy barrier. The intramolecular α-, β-, and γ-C–H bond amination processes in trans_ 1 _ R are kinetically and thermodynamically feasible, while the required energy barrier decreases via α > β > γ for R = H, Ph. These reactions proceed via a C–H bond insertion pathway, except for the γ-C–H bond amination in trans_ 1 _ Ph, which proceeds via an H atom transfer mechanism. The H to Ph substitution on the C γ atom of the imido ligand only slightly reduces the required energy barriers for α- and β-C–H bond amination in trans_ 1 _ R . However, it dramatically reduces the γ-C–H bond amination barrier and switches the mechanism of the reaction from C–H bond insertion to H atom transfer . This dramatic effect is a result of the better electron-withdrawing nature of the Ph ring. Thus, by replacing the R ligand, located at the γ (as well as β) C position, one may control the rate (barrier height), mechanism, and product distribution of the C–H bond amination in trans_ 1 _ R . The cis _ 1 _ Ph isomer is found to be slightly less reactive than the trans _ 1 _ Ph isomer. The intermolecular methane C–H bond amination by trans _ 1 _ R cannot compete with the intramolecular reactions in trans _ 1 _ R . The intermolecular process becomes feasible only for C β H 3 C α H 2 Ph. The intermolecular C–H amination of the substrate C β H 3 C α H 2 Ph by cis _ 1 _ Ph seems as feasible as that for the trans _ 1 _ Ph isomer. Involvement of the monoimido intermediate of the bis-imido complex 1 _ Ph in intra- and intermolecular C–H amination is highly unlikely.