Tandem mass spectrometry for the analysis of self‐sorted pseudorotaxanes: the effects of Coulomb interactions

The increasing complexity of self‐assembled supramolecules generates the need for analytical techniques that can accurately elucidate their structures. Here, we explore the ability of tandem mass spectrometry to deliver structural information on a series of self‐sorted crown ether/ammonium pseudorotaxanes. Of these intertwined molecules, different charge states are accessible and the effects of Coulomb interactions on the fragmentation pattern can be examined. Three different cases can be distinguished: (1) one or more counterions are present in the complex and compete with the crown for binding to the ammonium ion. This destabilizes the supramolecular bond. (2) In multiply charged complexes, charge repulsion significantly alters the fragmentation behavior as compared with singly charged ions. (3) If guest and host are both charged, the supramolecular bond becomes very weak. The different charge states provide different pieces of information about the supramolecules under study. Although singly charged complexes provide data on the building block connectivity, the doubly charged analogs are more reliable with respect to complex stoichiometry. As there are several factors which may cause differences in the gas phase and solution behavior of supramolecules (the presence and absence of solvation, changes in the strength of non‐covalent interactions upon ionization), it is important to establish well understood correlations between the complexes' gas‐phase behavior and their solution structures. A more detailed understanding will help to characterize the structures of even more complex supramolecular architectures by mass spectrometry. Copyright © 2010 John Wiley & Sons, Ltd.     

A Supramolecular Hydrogen‐Bonded Network as a Diffusion Barrier for Metal Adatoms

Confined in a molecular corral : A supramolecular network changes the mechanism by which underpotential deposition (UPD) of copper proceeds on a gold electrode modified by a self‐assembled monolayer (SAM). Lateral diffusion of Cu adatoms is suppressed between adjacent cells of a network/SAM hybrid structure. Instead, UPD occurs by direct deposition into the SAM filled pores of the network, where the Cu adatoms are confined.

     

Application of a Supramolecular‐Ligand Library for the Automated Search for Catalysts for the Asymmetric Hydrogenation of Industrially Relevant Substrates

A procedure is described for the automated screening and lead optimization of a supramolecular‐ligand library for the rhodium‐catalyzed asymmetric hydrogenation of five challenging substrates relevant to industry. Each catalyst is (self‐) assembled from two urea‐functionalized ligands and a transition‐metal center through hydrogen‐bonding interactions. The modular ligand structure consists of three distinctive fragments: the urea binding motif, the spacer, and the ligand backbone, which carries the phosphorus donor atom. The building blocks for the ligand synthesis are widely available on a commercial basis, thus enabling access to a large number of ligands of high structural diversity. The simple synthetic steps enabled the scale‐up of the ligand synthesis to multigram quantities. For the catalyst screening, a library of twelve new chiral ligands was prepared that comprised substantial variation in electronic and steric properties. The automated procedures employed ensured the fast catalyst assembly, screening, and direct acquisition of samples for analysis. It appeared that the most selective catalyst was different for every substrate investigated and that small variations in the building blocks had a major impact on the catalyst performance. For two substrates, a catalyst was found that provided the product with outstanding enantioselectivity. The subsequent automated optimization of these two leads showed that an increase of catalyst loading, dihydrogen pressure, and temperature had a positive effect on the catalyst activity without affecting the catalyst selectivity.     

Fitting formula for the injection volume of a gas chromatograph for radio‐xenon sampling in the lower troposphere

GC is usually used for xenon concentration and radon removal in the International Monitoring System of the Comprehensive Nuclear‐Test‐Ban Treaty. In a gas chromatograph, the injection volume is defined to calculate the column capacity. In this paper, the injection volume was investigated and a fitting formula for the injection volume was derived and discussed subsequently. As a consequence, the xenon injection volume exponentially decreased with the column temperature increased, but exponentially increased as the flow rate increased.     

The metal flux: a preparative tool for the exploration of intermetallic compounds

This review highlights the use and great potential of liquid metals as exotic and powerful solvents (i.e. fluxes) for the synthesis of intermetallic phases. The results presented demonstrate that considerable advances in the discovery of novel and complex phases are achievable utilizing molten metals as solvents. A wide cross-section of examples of flux-grown intermetallic phases and related solids are discussed and a brief history of the origins of flux chemistry is given. The most commonly used metal fluxes are surveyed and where possible, the underlying principal reasons that make the flux reaction work are discussed.     

Identification of a Lead Candidate in the Search for Carbene‐Stabilised Homoaromatics

The effect of carbenes as Lewis donor groups on the homoaromaticity of mono‐ and bicyclic organic molecules is surveyed. The search for viable carbene‐stabilised homoaromatics resulted in a large amount of rejected candidates as well as nine promising candidates that are further analysed for their homoaromaticity by using a number of metrics. Of these, five appeared to show modest homoaromaticity, whereas another compound showed a level of homoaromaticity comparable with the homotropylium cation benchmark compound. Isoelectronic analogues and constitutional isomers of the lead compound were investigated, however, none of these showed comparable homoaromaticity. The implications of these calculations on the design of donor‐stabilised homoaromatics are discussed.     

Ionic interaction as a powerful driving force for the formation of heterobidentate assembly ligands

An ionic interaction has been used for the first time to assemble monophosphane ligands. NMR spectroscopy and X-ray studies show that cationic and anionic triphenylphosphane derivatives form ion pairs and subsequently act as a ligand in various transition-metal complexes. The position of the ionic functional groups allows both cis and trans coordination of the novel assembly ligand in square-planar transition-metal complexes.     

A novel green template for the synthesis of mesoporous silica

Mesoporous pure silicas and functionalized silica with a narrow pore size distribution centered at 3.8 nm were prepared by a novel template, amphiphilic dendritic polyglycerol. The resulting silica materials were characterized by electron microscopy; nitrogen adsorption; (1)H, (13)C, and (29)Si solid-state cross-polarization magic-angle spinning NMR spectroscopy. It was shown that the template could be completely removed from the pure and functionalized silica in an environmentally friendly way by means of a simple water extraction procedure. Furthermore, it was shown that these materials could be easily functionalized, for example, by employing aminopropyl groups. Thus, a new environmentally friendly pathway to this fascinating class of silica material has been opened.     

Continuous‐Flow Technology—A Tool for the Safe Manufacturing of Active Pharmaceutical Ingredients

In the past few years, continuous‐flow reactors with channel dimensions in the micro‐ or millimeter region have found widespread application in organic synthesis. The characteristic properties of these reactors are their exceptionally fast heat and mass transfer. In microstructured devices of this type, virtually instantaneous mixing can be achieved for all but the fastest reactions. Similarly, the accumulation of heat, formation of hot spots, and dangers of thermal runaways can be prevented. As a result of the small reactor volumes, the overall safety of the process is significantly improved, even when harsh reaction conditions are used. Thus, microreactor technology offers a unique way to perform ultrafast, exothermic reactions, and allows the execution of reactions which proceed via highly unstable or even explosive intermediates. This Review discusses recent literature examples of continuous‐flow organic synthesis where hazardous reactions or extreme process windows have been employed, with a focus on applications of relevance to the preparation of pharmaceuticals.     

Reversible Supramolecular Surface Attachment of Enzyme–Polymer Conjugates for the Design of Biocatalytic Filtration Membranes

To be used successfully in continuous reactor systems, enzymes must either be retained using filtration membranes or immobilized on a solid component of the reactor. Whereas the first approach requires large amounts of energy, the second approach is limited by the low temporal stability of enzymes under operational conditions. To circumvent these major stumbling blocks, we have developed a strategy that enables the reversible supramolecular immobilization of bioactive enzyme–polymer conjugates at the surface of filtration membranes. The polymer is produced through a reversible addition–fragmentation transfer method; it contains multiple adamantyl moieties that are used to bind the resulting conjugate at the surface of the membrane which has surface‐immobilized cyclodextrin macrocycles. This supramolecular modification is stable under operational conditions and allows for efficient biocatalysis, and can be reversed by competitive host–guest interactions.