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

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-diisopropylphenyl)-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 α 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.在温和条件下的甲醇体系中，IPr-金(I) [IPr = 1,3-双(2,6-二异丙基苯基)咪唑-2-亚基] 配合物可催化一系列炔丙基胺与二氧化碳（CO₂）发生羧基环化反应，生成(Z)-5-亚烷基-2-恶唑烷酮，且无需添加银盐、碱等助剂。对底物范围的考察表明，在炔烃末端引入芳香取代基会显著抑制该催化反应的性能。通过在二氧化碳气氛下用AuOH(IPr)处理甲基取代与苯基取代的炔丙基胺，证实了烯基金(I)配合物可作为催化中间体模型。对烯基金(I)配合物反应活性的对比研究清晰显示，α位连有烷基的烯基配体比连有苯基的烯基配体更易发生质子解。上述结果与动力学实验共同佐证了如下催化循环：氨基甲酸酯对结合于金中心的碳碳三键进行亲核进攻，随后发生脱金质子解，释放出环状氨基甲酸酯产物。