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(NSO3CH2CH2CH2R)(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 alpha-, beta-, and gamma-C-H bond amination processes in trans_1_R are kinetically and thermodynamically feasible, while the required energy barrier decreases via alpha > beta > gamma for R = H, Ph. These reactions proceed via a C-H bond insertion pathway, except for the gamma-C-H bond amination in trans_1_Ph, which proceeds via an H atom transfer mechanism. The H to Ph substitution on the C-gamma atom of the imido ligand only slightly reduces the required energy barriers for alpha- and beta-C-H bond amination in trans_1_R. However, it dramatically reduces the gamma-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 gamma (as well as beta) 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 (CH3CH2Ph)-H-beta-H-alpha. The intermolecular C-H amination of the substrate (CH3CH2Ph)-H-beta-H-alpha 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.