Controlling Chemoselectivity in Ruthenium(II)-Induced Cyclization of Aniline-Functionalized Alkynes

The cyclization of heteroatom-functionalized alkynes induced by d⁶-transition-metal centers has traditionally been associated with the vinylidene pathway. However, recent evidence suggests that d⁶-transition-metal centers can also activate alkynes through non-vinylidene pathways. In this study, we conducted a comprehensive experimental and theoretical investigation into the reactions between the Ru(II) complex [Ru([9]aneS3)(bpy)(OH₂)]²⁺ and 2-alkynylanilines. Our study revealed that the selectivity between the vinylidene and non-vinylidene pathways can be tuned by reaction temperature, substrate, and solvent polarity. This strategic control allows for the preferential formation of either C2- or C3-metalated indole zwitterion complexes. Additionally, we identified a rare decyclization mechanism that enables the conversion of C2-metalated indoles to C3-metalated indoles, underscoring the significance of product stability in these pathways. Overall, this work demonstrates practical approaches to control the preference between vinylidene and non-vinylidene pathways, which is crucial for the design of new catalysts and metalated heterocyclic complexes.