Electronic effects of linker substitution on Lewis acid catalysis with metal-organic frameworks

Functionalized linkers can greatly increase the activity of metal-organic framework (MOF) catalysts with coordinatively unsaturated sites. A clear linear free-energy relationship (LFER) was found between Hammett σ(m) values of the linker substituents X and the rate k(X) of a carbonyl-ene reaction. This is the first LFER ever observed for MOF catalysts. A 56-fold increase in rate was found when the substituent is a nitro group.     

Unexpected four-membered over six-membered ring formation during the synthesis of azaheterocyclic phosphonates: experimental and theoretical evaluation

The cyclization of functionalized aminophosphonates is studied on both experimental and theoretical grounds. In a recently described route to phosphono-beta-lactams [Stevens C. V.; Vekemans, W.; Moonen, K.; Rammeloo, T. Tetrahedron Lett. 2003, 44, 1619], it was found that starting from an ambident allylic anion only four-membered rings were formed without any trace of six-membered lactams. New anion trapping experiments revealed that the gamma-anion is highly reactive in intermolecular reactions. Ab initio calculations predict higher reaction barriers for the gamma-anion due to restricted rotation about the C-N bond and due to highly strained transition states during ring closure. The sodium or lithium counterion, explicit dimethyl ether solvent molecules, and bulk solvent effects were properly taken into account at various levels of theory.     

"Click to chelate": synthesis and installation of metal chelates into biomolecules in a single step

Click chemistry has been employed for the assembly of novel and efficient triazole-based multidentate chelating systems while simultaneously attaching them to molecules of biological interest. The "click-to-chelate" approach offers a powerful new tool for the modification of (bio)molecules with metal chelators for potential diagnostic and therapeutic applications.     

K9CeP4S16, a new thio­phosphate of cerium with discrete [Ce(PS4)4]9− anions

The structure of nona­potassium cerium tetra­phos­phorus hexa­deca­sulfide, a zero‐dimensional material isostructural with Rb₉CeP₄Se₁₆, is reported.     

Synthesis of a new dextran-PEG-β-cyclodextrin host polymer using “Click” chemistry

The objective of this work was to synthesize a new type of dextran-based host polymer in which β-cyclodextrin (βCD) moieties are linked to the dextran backbone via flexible hydrophilic poly (ethylene glycol) (PEG) spacer. Short PEG-acrylate (M_n~375) was coupled with 5-hexynoic acid and grafted to the thiolated dextran using thiol-ene “click” chemistry. In the following step 6-monodeoxy-6-monoazido-βCD was “clicked” to the obtained alkyne terminated Dext-g-PEG copolymer (Dext-g-PEG-Hex-5-ynoate) using a copper (I)-catalyzed azide–alkyne cycloaddition. The reaction conditions and catalytic system were optimized. The synthesized Dext-g-PEG-βCD polymers have been characterized by ¹H-NMR spectroscopy and isothermal titration microcalorimetry and they show good binding properties comparing with conventional cross-linked pβCD.     

Combinatorial Approach to Chiral Tris‐ligated Carbophilic Platinum Complexes: Application to Asymmetric Catalysis

A straightforward methodology for the synthesis of libraries of chiral tris‐ligated cationic platinum complexes and their in situ evaluation as asymmetric carbophilic catalysts in a model domino hydroarylation/cyclization reaction of a 1,6‐enyne was developed. A catalyst‐generation process based on a combination of a monodentate and a bidentate phosphorus ligand allowed the formation of 108 chiral complexes. One‐pot screening of the stereoinduction obtained with this library in a test domino addition/cyclization reaction validated this approach and stressed the key role played by the monodentate ligand partner in obtaining high enantioselectivities. In the case of two challenging substrate/nucleophile combinations, the combinatorial approach resulted in a significant gain in enantioselectivity.     

Competitive reactions of organophosphorus radicals on coke surfaces

The efficacy of organophosphorus radicals as anticoking agents was subjected to a computational study in which a representative set of radicals derived from industrially relevant organophosphorus additives was used to explore competitive reaction pathways on the graphene-like coke surface formed during thermal cracking. The aim was to investigate the nature of the competing reactions of different organophosphorus radicals on coke surfaces, and elucidate their mode of attack and inhibiting effect on the forming coke layer by use of contemporary computational methods. Density functional calculations on benzene and a larger polyaromatic hydrocarbon, namely, ovalene, showed that organophosphorus radicals have a high propensity to add to the periphery of the coke surface, inhibiting methyl radical induced hydrogen abstraction, which is known to be a key step in coke growth. Low addition barriers reported for a phosphatidyl radical suggest competitive aptitude against coke formation. Moreover, organophosphorus additives bearing aromatic substituents, which were shown to interact with the coke surface through dispersive π-π stacking interactions, are suggested to play a nontrivial role in hindering further stacking among coke surfaces. This may be the underlying rationale behind experimental observation of softer coke in the presence of organophosphorus radicals. The ultimate goal is to provide information that will be useful in building single-event microkinetic models. This study presents pertinent information on potential reactions that could be taken up in these models.     

A Switchable Domino Process for the Construction of Novel CO2‐Sourced Sulfur‐Containing Building Blocks and Polymers

α‐Alkylidene cyclic carbonates (αCCs) recently emerged as attractive CO₂‐sourced synthons for the construction of complex organic molecules. Herein, we report the transformation of αCCs into novel families of sulfur‐containing compounds by organocatalyzed chemoselective addition of thiols, following a domino process that is switched on/off depending on the desired product. The process is extremely fast and versatile in substrate scope, provides selectively linear thiocarbonates or elusive tetrasubstituted ethylene carbonates with high yields following a 100 % atom economy reaction, and valorizes CO₂ as a renewable feedstock. It is also exploited to produce a large diversity of unprecedented functional polymers. It constitutes a robust platform for the design of new sulfur‐containing organic synthons and important families of polymers.