DFT insight into Hashmi phenol synthesis catalyzed by Au single-walled nanotubes: mechanism and charge effect

The mechanism and charge effect of cycloisomerization of ω-alkynylfuran (Hashmi phenol synthesis) catalyzed by single-walled helical gold nanotubes (Au SWNTs) have been systematically investigated via density functional theory. Cycloisomerization of ω-alkynylfuran occurs by the 5-exo Friedel–Crafts-type (FCT) mechanism, namely 5-exo cyclization, furan ring opening, and ring closing of the dienone carbene–gold intermediate. The reactions with Au(6,0), Au(6,1), Au(6,2) and Au(6,3) SWNTs show low energy barriers along the 5-exo FCT path in acetonitrile solvent, but have different the rate-determining steps. From an energy perspective, the reaction rate-determining step catalyzed by Au(6,0) and Au(6,3) is the ring-closing of dienone carbine-gold intermediate, but that of Au(6,1) and Au(6,2) is the IM5 dissociation from the Au SWNTs, which can be attributed to the diversity of the d-band centers of the Au(6,m) SWNTs. The effect of the charge of the Au SWNTs on the catalytic activity was also investigated. Theoretical analysis shows a prominent charge effect, where the cationic Au(6,0), Au(6,3) SWNTs and anionic Au(6,1), Au(6,2) SWNTs are more favorable for the Hashmi phenol synthesis reaction. This results can be attributed to the Au(6,0) and Au(6,3) SWNTs with positive charge can reduce the adsorption energy of the substrate on the catalyst surface and decrease the energy barrier of the cyclization process and ring-closing step. Besides, the Au(6,1) and Au(6,2) with an anion could obviously decrease the dissociation energy of IM5 which is help for the Hashmi phenol reaction. Theoretical analysis shows that the structure and charge effects could influence the catalytic activity of Au(6,m) SWNTs toward Hashmi phenol synthesis. This work will provide insight into cycloisomerization of ω-alkynylfuran and valuable information for application of Au SWNTs in catalysis.