Experiments And Dft Computations Combine To Decipher Fe-Catalyzed Amidine Synthesis Through Nitrene Transfer And Nitrile Insertion

Multicomponent reactions are attracting strong interest as they contribute to the development of more efficient synthetic chemistry. Understanding their mechanism is thus an important issue to optimize their operation. However, it is also a challenging task owing to the complexity of the succession of molecular events involved. Computational methods have recently proven to be of utmost interest to help decipher some of these processes, and the development of integrated experimental and theoretical approaches thus appears as the most powerful means to understand these mechanisms at the molecular level. A good example is given by the synthesis of amidines which are important pharmaceutical compounds. Their synthesis requires the association of three components, often an alkyne, a secondary amine, and an organic azide as the nitrene precursor. We found that an alternative way is offered by an Fe-catalyzed combination of a hydrocarbon, a nitrile, and a nitrene which gives amidines in good yields under mild conditions. The efficiency of the transformation and the paucity of mechanistic information on these reactions prompted us to thoroughly investigate its mechanism. Several mechanistic scenarios were explored using experimental techniques, including radical trap and N-15 labeling studies, combined with density-functional theory (DFT) calculations of reaction profiles. This allowed us to show that the amidination reaction involves the trapping of an intermediate substrate cation by an Fe-released acetonitrile molecule pointing to a true multicomponent reaction occurring exclusively within the cage around the metal center. Moreover, the calculated energy barriers of the individual steps explained how amidination outweighs direct amination in these reactions. The perfect consistency between DFT results and specific experiments to validate them strongly supports these mechanistic conclusions and highlights the potency of this combined approach.