Palladium-Catalyzed Asymmetric [3 + 2] Annulation of Vinylethylene Carbonates with Alkenes Installed on Cyclic N-Sulfonyl Imines: Highly Enantio- and Diastereoselective Construction of Chiral Tetrahydrofuran Scaffolds Bearing Three Vicinal and Quaternary Stereocenters.

A multisubstituted tetrahydrofuran building block bearing three vicinal chiral carbon centers widely exists in a broad spectrum of bioactive natural products, and the development of efficient and convenient methods to establish this skeleton remains a challenging task. Herein, we have developed an efficient method for the construction of significant tetrahydrofuran scaffolds bearing three vicinal and α-quaternary chiral carbon stereocenters through Pd-catalyzed asymmetric [3 + 2] annulation of vinylethylene carbonates with alkenes installed on cyclic N-sulfonyl imines. A series of multisubstituted tetrahydrofuran derivatives are obtained in high efficiencies with excellent enantioselectivities and diastereoselectivities. Density functional theory (DFT) studies are accomplished to rationalize the stereocontrol of the annulation process and disclose that methanol could be applied to stabilize the reactive zwitterionic π-allylpalladium via the H-bond interaction.

The Studies on Structure and Stability of CaBn Clusters.

Calcium-boron systems have excellent properties of hardness, strength, and chemical stability, and we studied a series of CaBn clusters to investigate their structures and relative stability. The results showed the most stable structures of CaBn clusters are not planar. The B atoms tend to get together and form the planar ring to stabilize the structure, and the Ca atoms are coordinated to the periphery of the formations. The average binding energy (Eb), fragmentation energy (EF), second-order energy difference (Δ2E), adiabatic detachment energy (ADE), and adiabatic electron affinity (AEA) of the CaBn clusters were calculated to investigate the relative stability and the ability of removing or obtaining an electron. As shown by the results, EF and Δ2E values had obvious odd-even alteration as n increased, which indicated that the formations CaB4, CaB6, and CaB8 were more stable. The ADE values for CaBn clusters with even values of n were higher than those with odd values of n, which indicated CaBn clusters with even values of n had difficultly removing an electron. The AEA values of CaB3 and CaB7 were larger than the others, which meant CaB3 and CaB7 easily obtained an electron. These results provide a useful reference for understanding the formation mechanism and stability of the alkaline earth metal boride as well as guidance for synthesizing the CaBn clusters.