Mechanism of Ruthenium Porphyrin Amination/Aziridination Catalysis Via Radical Intermediates

Insertion of an "RN" moiety into an organic skeleton is of particular interest for the attainment of aza-derivatives with intriguing pharmaceutical or biologic properties.[1] Amongst "RN" sources, organic azides are efficient in amination and aziridination sustainable processes, given the N2 release. In particular, transition metal porphyrins are excellent catalysts for the nitrene transfer into an organic skeleton with high chemio-, stereo-, and enantioselectivity.[2] A mechanism for the allylic and benzylic amination by aryl azides (ArN3) promoted by ruthenium(II) porphyrin complexes is proposed from experimental and theoretical approaches.[3] The role of the apical imido ligand at the intermediate [Ru](NR)(CO) complex has been ascertained by catalytic tests of the amination. Also, the DFT modeling with the analysis of Minimum Energy Crossing Point (MECP) has indicated the accessibility of [Ru](NR)(CO) to both the singlet and triplet ground state with the latter allowing a radical route for the C-H activation of the organic substrate. A similar behaviour is also possible starting from the bis imido [Ru](NR)2 and bis-pyridine [Ru](py)2 derivatives.[4] A drawing of the interconnected catalytic cycles based on [Ru](NR)(CO) and [Ru](NR)2 is shown. Catalytic aziridination processes of the olefins promoted by the same Ru(II)-porphyrin species have been also investigated.[5] Again, the triplet [Ru](NR)(CO) species is found to play a fundamental role, since the diradical RN ligand transfers spin density into one olefin carbon atom. The triplet N-C-C open chain, shown in the Figure, closes up affording a three-membered ring with return the singlet ground state. Other competitive processes are also energetically evaluated.