Cobalt-based catalysis for carboxylative cyclization of propargylic amines with CO2 at atmospheric pressure

Abstract A cobalt-bicyclic guanidine catalytic system consisting of CoBr2 and 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) for the carboxylative cyclization of terminal propargylic amines with CO2 was firstly developed in this work to produce 2-oxazolinones efficiently. The existence of induction period urged us to understand the reaction mechanism of cobalt catalysis. Investigation on the roles of CoBr2 and TBD was conducted using control experiments and density functional theory (DFT) calculation. TBD presumably acts as a base to activate propargylic amine for favorable CO2 capture, and a ligand to coordinate with CoBr2 via forming bulkier CoBr2(TBD) in the same time. This bulkier complex can enhance the O-nucleophility of the in situ formed carbamate intermediate and then promote subsequent intramolecular cyclization to generate 2-oxazolinone, accounting for the high activity of cobalt catalysis. This protocol enables the synthesis of various 2-oxazolinones from propargylic amines and CO2 under atmospheric pressure in good to excellent yields, representing a simple, cost-effective and practical route for CO2 fixation to 2-oxazolinones under mild conditions.