Metal-Ligand Cooperation of Ir(III) Catalysts Bearing a β-Protic Pyrazole Chelate in Carboxylative Cyclization of Propargylic Amine with CO2.

Bifunctional iridium complexes having a β-protic C-N chelating pyrazole ligand catalyze the carboxylative cyclization of 1-methylamino-2-butyne under atmospheric CO2. This reaction affords a five-membered cyclic urethane, along with a six-membered regioisomer. Alkenyliridium(III) complexes as catalytic intermediate models are formed by stoichiometric reaction of related nonprotic pyrazole complexes with 1-methylamino-2-butyne and CO2 in the presence of Ag2O, although the protic pyrazole analog is too unstable to be detected. In particular, the isolable N-phenylpyrazole-ligated alkenyliridium(III) complex with a cyclic urethane framework is characterized by X-ray crystallography. The β-protic pyrazole ligand would be capable of accelerating protonolysis of the alkenyl carbon-iridium bond in the key intermediate, releasing the cyclic urethane product, and being responsible for the catalytic function.