Mechanistic study on the side arm effect in a palladium/Xu-Phos-catalyzed enantioselective alkoxyalkenylation of γ-hydroxyalkenes.

Recently, the asymmetric bifunctionalization of alkenes has received much attention. However, the development of enantioselective alkoxyalkenylation has posed a considerable challenge and has lagged largely behind. Herein, we report a new palladium-catalyzed enantioselective alkoxyalkenylation reaction, using a range of primary, secondary, and tertiary γ-hydroxy-alkenes with alkenyl halides. By employing newly identified Xu-Phos (Xu8 and Xu9) with a suitable side-arm adjacent to the PCy2 motif, a series of allyl-substituted tetrahydrofurans were obtained in good yields with up to 95% ee. Besides (E)-alkenyl halides, (Z)-alkenyl halide was also examined and provided the corresponding (Z)-product as a single diastereomer, supporting a stereospecific oxidative addition and reductive elimination step. Moreover, deuterium labeling and VCD experiments were employed to determine a cis-oxypalladation mechanism. DFT calculations helped us gain deeper insight into the side-arm effect on the chiral ligand. Finally, the practicability of this method is further demonstrated through a gram-scale synthesis and versatile transformations of the products.

Construction of highly pure nanocrystalline heteropoly acid inside the channels of mesoporous material with the imprisoned hydrolyzation reaction of heteropoly acid etherate.

A novel technique was developed to prepare highly pure heteropoly acid (HPA) nanocrystals inside mesoporous SBA-15 by the imprisoned reaction of HPA etherate and water, which was utilized as a main driving force for the transportation of building-blocks. The transmission electron microscopy, X-ray diffraction, FT-IR spectrum and NMR characterizations unambiguously demonstrate that this method allows the highly pure heteropoly acid nanocrystals with intact Keggin-type structure controlled directionally self-assembly within mesopores of silica SBA-15. Such method may open up a new entry to the highly pure multicomponent nanocrystalline particles inside the cavity of the porous materials.

Assembly of heteropoly acid nanoparticles in SBA-15 and its performance as an acid catalyst.

Keggin-type 12-tungstophosphoric acid (TPA) nanocrystals have been assembled inside the pores of mesoporous silica through a vacuum impregnation method by using large-pore SBA-15 as a nanoreactor. The product was characterized by Brunauer-Emmet-Teller particle size distribution (BET-PSD), NMR and FT-IR spectroscopy, X-ray diffraction (XRD), tranmsission electron microscopy (TEM), differential thermal analysis (DTA) and FT-IR of adsorbed pyridine. The experimental results illustrate that the TPA nanocrystals are excellent Brønsted acid catalytic materials at room temperature.