Deciphering the Origin of Regioselectivity in Ru(II)-Catalyzed C–H Annulation of N-Chlorobenzamides with 1,3-Diynes

Understanding the reaction mechanism and origin of regioselectivity in transition metal-catalyzed C–H activation/annulation reactions with 1,3-diynes has remained an intriguing challenge. In this article, to establish the mechanism and decipher the origin of regioselectivity, we report a detailed computational density functional theory-based mechanistic investigation on the recently developed Ru(II)-catalyzed 4 + 2 annulation of N-chlorobenzamides with 1,3-diynes for the synthesis of 3-alkynylated isoquinolone derivatives. Our calculations reveal a redox-neutral pathway for the annulation reaction. The stepwise analysis of the reaction channels indicates the migratory insertion step and the concerted reductive elimination/oxidative addition of the Ru(p-cymene) moiety to form the N–C bond leading to the 3-alkynylated product to be the selectivity- and rate-determining steps, respectively. Finally, the distortion/interaction analysis using the activation-strain model suggests the steric effect as the determining factor for the observed regioselectivity for the formation of the 3-alkynylated product. Overall, the computationally obtained key insights into the catalytic mechanism and the origin of regioselectivity in the C–H activation/annulation reaction can be used as a guide to rationally design and develop novel transformation strategies for heterocycle synthesis.