Mechanistic Aspects of the Carboxylative Cyclization of Propargylamines and Carbon Dioxide Catalyzed by Gold(I) Complexes Bearing an N-Heterocyclic Carbene Ligand

摘要

The carboxylative cyclization of a range of propargylic amines using carbon dioxide (CO2) is promoted by IPr-gold(I) (IPr = 1,3-bis(2,6-diisopropyl-phenyl)-imidazol-2-ylidene) complexes to afford (Z)-5-alkylidene-2-oxazolidones in methanol under mild conditions, even in the absence of additives such as silver salts and bases. Investigation of the substrate scope shows that the catalytic performance is markedly retarded by the introduction of aromatic substituents at the alkyne terminus. The formation of alkenylgold(I) complexes as catalytic intermediate models is demonstrated by the treatment of methyl- and phenyl-substituted propargylamines with AuOH(IPr) under a CO2 atmosphere. A comparison of the reactivity of the alkenylgold(I) complexes clearly indicates that the alkenyl ligand attached to an alkyl group at the alpha position is more susceptible to protonolysis compared with that attached to a phenyl group. These results and kinetic experiments corroborate a catalytic cycle that involves the nucleophilic attack of carbamate at the C-C triple bond bound to the Au center and its subsequent protodeauration to release the cyclic urethane products.