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

Article abstract of DOI:10.1002/ejic.201500656

A 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 (6_S), which can be easily transformed into the triplet isomer 6_T 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, 6_S 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]. The catalytic activity of Ru(TPP)(py)₂ (2) (TPP = dianion of tetraphenylporphyrin, py = pyridine) in the amination of C-H bonds by organic azides (RN₃) was experimentally compared to that of Ru(TPP)(CO) (1). A computational analysis corroborates similar radical mechanisms with a triplet imido intermediate, in spite of the electronic differences of the apical ligands.