Comparative Study of the Catalytic Amination of Benzylic C–H Bonds Promoted by Ru(TPP)(py)2 and Ru(TPP)(CO)

Abstract

AbstractA combined experimental and DFT‐based theoretical analysis elucidated the influence of the axial ligand L on the catalytic activity of Ru(porphyrin)L complexes in promoting the amination of benzylic C–H bonds by organic azides (RN3). Experimental data indicated that the catalytic activity of Ru(TPP)(CO) (1) (TPP = dianion of tetraphenylporphyrin) is comparable to that of Ru(TPP)(py)2 (2) (py = pyridine). DFT modelling disclosed that 2 can be regarded as a precatalyst that becomes active after the endergonic loss of one pyridine ligand to give the unsaturated species [Ru](py) (11) {[Ru] = Ru(porphine)}. This complex would react with RN3 to give the mono‐imido singlet complex [Ru](py)(NR)S (6S), which can be easily transformed into the triplet isomer 6T having diradical character at the imido N atom. The subsequent formation of the benzylic amine PhCH2NHR occurs through a radical homolytic activation of one C–H bond of the toluene substrate (PhCH3). Conversely, by staying on the singlet potential‐energy surface, 6S can undergo dissociation of the pyridine ligand to form [Ru](NR). This complex can activate another RN3 molecule to form the bis‐imido compound [Ru](NR)2, which is also catalytically active. At this point, the mechanism becomes independent of the nature of the original ligand L coordinated to [Ru].