Mechanistic insights into rhodium-catalyzed C–H activation and annulation using vinyl acetate to access cinnolines and cinnolin-4(1H)-ones†

Transition metal-catalyzed C–H activation and annulation involving vinyl acetate remains a significant challenge, and the mechanistic process following vinyl insertion is ambiguous. Herein, we report the Rh(III)-catalyzed two-fold C–H activation and [4 + 2] annulation of N-aryl cyclic hydrazides with vinyl acetate to build 3,4-unsubstituted, 1,2-phthaloyl-protected cinnolines. Using prop-1-en-2-yl acetate as the C2 source, the tandem [4 + 2] annulation and methyl C(sp³)–H oxidation occurred to give 3-formyl cinnolines. Furthermore, The unusual K₂CO₃/O₂-mediated C4(sp²)–H oxidation/deprotection of the resulting 1,2-phthaloyl cinnolines enables an access to cinnolin-4(1H)-ones for the first time. The combination of control experiments, ESI-MS, and DFT calculations indicates there are two possible pathways after vinyl insertion: a thermodynamically preferred syn-OAc/H β-H elimination and a kinetically favored syn-Rh/H β-H elimination.