DFT Mechanistic Proposal of the Ruthenium Porphyrin-Catalyzed Allylic Amination by Organic Azides

Abstract

A DFT-based theoretical analysis describes the allylic amination of cyclohexene by 3,5(CF3)2phenylazide catalyzed by [Ru](CO) ([Ru]= Ru(TPP), TPP = dianion of tetraphenylporphyrin). The activation of an azide molecule (RN3) at the free ruthenium coordination site allows the formation of a monoimido complex [Ru](NR)(CO) with the eco-friendly dismissal of a N2 molecule. The monoimido complex can undergo a singlet→triplet interconversion to confer a diradical character to the RN ligand. Hence, the activation of the allylic C–H bond of cyclohexene (C6H10) occurs through a C–H···N interaction over the transition state. The formation of the desired allylic amine follows a “rebound” mechanism in which the nitrogen and carbon atom radicals couple to yield the organic product. The release of the allylic amine restores the initial [Ru](CO) complex and allows the catalytic cycle to resume by the activation of another azide molecule. On the singlet PES, the CO ligand may however be eliminated from the monoimido complex [Ru](NR)(CO)S, opening the way to an alternative catalytic cycle which also leads to allylic amine through comparable key steps. A second azide molecule occupies the vacant coordination site of [Ru](NR)S to form the bis-imido complex Ru(TPP)(NR)2, which is also prone to the intersystem crossing with the consequent C–H radical activation. The process continues until the azide reactant is present. The interconnected cycles have similarly high exergonic balances. Important electronic aspects are highlighted, also concerning the formation of experimentally observed byproducts.