Mechanism of the gold(i)-catalyzed synthesis of imidazo-pyrimidines and imidazo-pyrazines via [3 + 2] dipolar cycloaddition: a DFT study†
Density functional theory calculations have been carried out to study the mechanisms of gold(I)-catalyzed [3 + 2] cycloaddition reactions of pyridinium N-(heteroaryl)-aminides, as nucleophilic nitrenoids, with electron-rich alkynes, leading to imidazo-pyrimidines and imidazo-pyrazines. The calculation results suggest that the reaction mainly involves two plausible pathways leading to different products, since two carbon atoms of the asymmetric alkyne are attacked selectively by the aminide nitrogen atom. These reaction steps include C–C bond activation of the alkyne by the gold complex, a bimolecular nucleophilic attack, a closed-loop process and migration of the pyridine molecule to finally give cycloaddition products. The closed-loop process is the rate-limiting step for the whole catalytic reaction and the energy barrier shows that the pathway of the nucleophile onto the amide carbon atom of the alkyne with the electron-withdrawing group is favored. Through a detailed mechanistic investigation, we have explained the regioselectivity and the major/minor products observed by Davies et al. in gold(I)-catalyzed [3 + 2] cycloaddition reactions of nucleophilic nitrenoids with electron-rich alkynes.