Mechanism Of ω-Alkynylfuran Cycloisomerization By Gold Nanoparticles: A Theoretical Study

Due to the unique physicochemical properties and potential usages in several practical applications such as catalysis, nanotechnology and magnetic materials etc., nanosized gold clusters have attracted intensive research interests during the past decades. Nowadays, with the rapid development of modern nanotechnology, it has been found that gold nanoparticles have broad application prospects in the fields of catalysis, biology, environment, materials, medicine, and so on. The nanoparticles gold catalysts have good catalytic properties, which are related to the size, structure, shape, charge state and the carrier of gold nanoparticles. Because of the aurophilicity of alkynyl, gold complexs, nano gold clusters and gold/oxide catalyzed alkyne activation has gradually become a research hotspot.Phenols and their derivatives have extensive applications as versatile synthetic intermediates for the construction of natural products, polymers, and pharmaceuticals because of their high reactivity. The cycloisomerization of ω-alkynylfurans is an effective method to obtain one of the phenol derivative. In this thesis, we have employed density functional theory（DFT） to investigate the cycloisomerization of ω-alkynylfuran catalyzed by a series of nanosized gold nanoparticles. The thesis contains three parts:In the first part of thesis, the detailed reaction mechanism of the ω-alkynylfuran cycloisomerization catalyzed by medium-sized bare Au₃₈ nanoclusters was investigated by means of density functional theory at the Perdeω-Burke-Ernzerh of level of theory. Both the charge and size effects of the three-dimensional gold clusters are studied at the same level. The mechanistic studies show that the 5-exo Friedel-Crafts-type mechanism（5-exo FCT） is the most favorable mechanism, and could be selective to form phenol derivatives. Additionally, the solvation effect of acetonitrile apparently lowers the energy barrier of the 5-exo cyclization step and the ring-open of furan ring steps of the reaction. The positive charge on the gold clusters could enhance thecatalytic activity of the clusters. Suitable positive charges on the reaction sites are essential for the formation of phenol through the cycloisomerization of ω-alkynylfuran. The subnanometer gold clusters show high catalytic efficiency for the conversion of ω-alkynylfurans to phenol derivatives.In the second part of thesis, a detailed reaction mechanism of the triatomic gold cluster-catalyzed cycloisomerization of ω-alkynylfuran was systemically investigated via density functional theory at the TPSSh/def2-TZVP level. The computational results indicated that the 5-exo FCT mechanism is the most favorable mechanism to form the phenol derivatives. The strong interaction between the gold and vinyl fragments in the Friedel-Crafts adduct is essential for the priority of the 5-exo Friedel-Crafts-type mechanism. Then, the 5-exo FCT mechanism on the various planar gold clusters（Au4–10） was studied to clarify the size-effects of the planar gold clusters catalyzed ω-alkynylfuran cycloisomerization. The appropriate interactions between the alkyne group in the substrate and gold clusters play a key role for the 5-exo cyclization step. The energy barriers of the ring-closure of the dienone carbine-gold intermediate step show an interesting “odd–even” behavior respective to the number of gold atoms. The Au3 and Au4 clusters are the most active catalysts for the ω-alkynylfuran cycloisomerization to the phenol derivative. We also found that the active catalyst of the ω-alkynylfuran cycloisomerization catalyzed by the gold（I） complexes should be the gold（0） complexes of the in situ generation. The catalytic activity of the gold（0） complex is comparable with that of the planar gold clusters. These findings may guide the rational design of highly active gold catalysts for the ω-alkynylfuran cycloisomerization to phenol derivatives.In the third part of thesis, the Hashmi phenol synthesis reaction（the ω-alkynylfuran cycloisomerization） on the M₆@Au₃₂（M=Ag, Cu, Pd, Pt, Ru, and Rh） core-shell clusters were investigated. The 5-exo FCT mechanism of the ω-alkynylfuran cycloisomerization on the six core-shell nanoparticles were analyzed in details. The adsorption property and catalytic activity of the Au₃₈ nanocluster could be effectively tuned by the substitution the core Au atoms. The adsorption of alkynes is depended on the numbers of unoccupied and occupied d orbitals, orientation of d orbital and d-bonding down-shift of the M₆@Au₃₂ core-shell clusters. The core-shell nanoparticles could cause large changes in the interaction energy between the substrate and cluster fragments than that of Au₃₈, which is beneficial for facilitating the cyclization step andthe ring-closing of the dienone carbene-gold intermediate. The catalytic activity of Au₃₈ could be tuned by the substitution of the core gold atoms.Through the studies of structure and catalytic properties of a series of gold nanocluster systems, we obtained a deeper understanding of the structure of gold nanoclusters and the catalytic activity of gold nanoclusters for the ω-alkynylfuran cycloisomerization. We hope that the current results may promote our understanding of unique properties of nanogold clusers and lay some foundations for future experimental and theoretical studies in this field.