Mechanistic Study on Ag^I-Catalyzed Oxidative Cross-Coupling/Cyclization between Terminal Alkynes and β-Enamino Esters...

A combined computational and experimental study was performed to elucidate the mechanism of the Ag^I-catalyzed oxidative cross-coupling/cyclization of terminal alkynes with β-enamino esters. The results indicated a more favorable Ag^I/Ag⁰-catalyzed radical mechanism (than cationic mechanism) which involves three key stages: (i) the initiation of radical species, (ii) the cyclization, and (iii) the formal 1,2-H shift. Meanwhile, the Ag^I species was found to be the active initiator for the delocalized nitrogen radical species generation, and Ag₂CO₃ acts as an effective oxidant to initiate the β-enamino ester radical formation. Furthermore, it was shown that the silver acetylide is the key intermediate in the title reaction and that the coordination of solvent dimethyl sulfoxide (DMSO) regulates the electronic properties of the Ag center better as compared with base 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), thereby enhancing the negative charge of the reaction sites and promoting the cyclization process. Finally, the DBU was revealed to play a key role in the final 1,2-H shift process through the formation of [DBU-H]⁺, acting as a proton shuttle to assist the proton migration process. The theoretical results provide key insights into the Ag^I/Ag⁰-catalyzed radical mechanism and guidelines for further development of Ag-catalyzed synthetic methods.