Self-assembly of large multi-component systems is a common strategy for the bottom-up construction of discrete, well-defined, nanoscopic-sized cages. Icosahedral or pseudospherical viral capsids, built up from hundreds of identical proteins, constitute typical examples of the complexity attained by biological self-assembly. Chemical versions of the so-called 5 Platonic regular or 13 Archimedean semi-regular polyhedra are usually assembled combining molecular platforms with metals with commensurate coordination spheres. Here we report novel, self-assembled cages, using the conical-shaped carboxylic acid derivatives of calix[4]arene and calix[5]arene as ligands, and the uranyl cation UO22+ as a metallic counterpart, which coordinates with three carboxylates at the equatorial plane, giving rise to hexagonal bipyramidal architectures. As a result, octahedral and icosahedral anionic metallocages of nanoscopic dimensions are formed with an unusually small number of components.
Squaramides are presented as modular, easy to optimize, and effective catalysts for the conversion of epoxides and carbon dioxide into cyclic organic carbonates (COCs). The catalytic potential of these squaramides, in combination with a suitable halide nucleophile, is particularly noted when internal epoxides are examined as substrates, and their transformation into disubstituted COCs marks a rare case of an effective organocatalyst for these challenging conversions. Control experiments support the mechanistic view that the squaramides are predominantly involved in the stabilization of intermediate oxo and carbonato anions which, after their formation, are able to displace a bromide nucleophile from an initially formed 1:1 assembly comprising the squaramide host.
AlIII complexes of amino‐tris(phenolate) ligand scaffolds have been prepared to attain highly Lewis acidic catalysts. Combination of the aforementioned systems with ammonium halides provides highly active catalysts for the synthesis of organic carbonates through addition of carbon dioxide to oxiranes with initial turnover frequencies among the highest reported to date within the context of cyclic carbonate formation. Density functional theory (DFT) studies combined with kinetic data provides a rational for the relative high activity found for these AlIII complexes, and the data are consistent with a monometallic mechanism. The activity and versatility of these AlIII complexes has also been evaluated against some state‐of‐the‐art catalysts and the combined results compare favorably in terms of catalyst construction, stability, activity, and applicability.
1 September 2016 in Journal of the American Chemical Society
Significant progress has been observed in recent years in the synthesis of allylic amines, which are important building blocks in synthetic chemistry. Most of these processes are effective toward the preparation of allylic amines, with limited potential to introduce three or four different substituents on the olefinic unit in a stereocontrolled fashion. Therefore, the discovery of a mild and operationally simple protocol allowing such challenging stereoselective synthesis of multisubstituted allylic amines remains an inspiring target. Herein, we report the first general and practical methodology for the stereoselective synthesis of tri- and tetrasubstituted allylic amines based on Pd-catalyzed conversion of allyl surrogates readily obtained from cyclic vinyl carbonates. These rare conversions are characterized by excellent stereoselectivity, operational simplicity, mild reaction conditions, and wide scope in reaction partners. DFT studies were performed to rationalize the stereocontrol in these allylic amine formation reactions, and evidence is provided that the formation of a six-membered palladacyclic intermediate leads toward the formation of (Z)-configured allylic amine products.