Dendritic catalysts and dendrimers in catalysis

Emerging Strategies and Applications of Layer-by-Layer Self-Assembly

Layer-by-layer self-assembly is an approach to develop an ultrathin film on solid support by alternate exposure to positive and negative species with spontaneous deposition of the oppositely charged ions. This paper summarizes various approaches used for fabrication of layer-by-layer self-assembly as well as their utility to produce various devices. The layer-by-layer technique is basically used for formation of multilayer films. A variety of nanomaterials use it for the modification of films to enhance their resultant durability as well as strength. Studies have shown that many different types of materials can be used for fabrication of multilayers. Recently the layer-by-layer self-assembly technique has also been used for fabrication of gas sensors, hydrogen sensors and solar-based cells. Various methods, such as spin deposition, calcinations, and dry-transfer printing are being used for fabrication of thin films. In this review, the author summarizes the various interesting properties as well as fabrication strategies of layer-by-layer self-assembly.

Supramolecular catalysis. Part 2: artificial enzyme mimics

The design of artificial catalysts able to compete with the catalytic proficiency of enzymes is an intense subject of research. Non-covalent interactions are thought to be involved in several properties of enzymatic catalysis, notably (i) the confinement of the substrates and the active site within a catalytic pocket, (ii) the creation of a hydrophobic pocket in water, (iii) self-replication properties and (iv) allosteric properties. The origins of the enhanced rates and high catalytic selectivities associated with these properties are still a matter of debate. Stabilisation of the transition state and favourable conformations of the active site and the product(s) are probably part of the answer. We present here artificial catalysts and biomacromolecule hybrid catalysts which constitute good models towards the development of truly competitive artificial enzymes.

Cascade reactions in nanoreactors

In an attempt to mimic the biosynthetic efficiencies of nature and in a search for greener, more sustainable alternatives to nowadays ways of producing chemicals, one-pot cascade reactions have attracted a lot of attention in the past decade. Since most catalysts are not compatible with each other, compartmentalization techniques have often been applied to prevent catalyst inactivation. A various array of nanoreactors have been developed to meet the demand of having a site-isolated catalyst system, while maintaining the catalyst activity. Both multienzyme nanoreactors as well as enzyme/metal catalyst or organocatalyst systems have shown great potential in one-pot cascade reactions and hold promise for future developments in this field.

The aminocyclodextrin/Pd(OAc)2 complex as an efficient catalyst for the Mizoroki-Heck cross-coupling reaction

An aminocyclodextrin/Pd(OAc)2 complex is used as an efficient, reusable catalyst in the Mizoroki-Heck reaction of aryl halides/triflates with olefins to give carbon-carbon-coupled products in good to excellent yields. This simple, efficient catalytic system is applicable to a wide range of aryl and heteroaryl halides/triflates and olefins. This environmentally benign procedure is less hazardous, milder, uses a catalytic amount of ligand and Pd(OAc)2 , avoids an inert atmosphere, and catalyst recovery and reusability are achieved.

Nanostructured materials for applications in heterogeneous catalysis

In this review, a brief survey is offered on the main nanotechnology synthetic approaches available to heterogeneous catalysis, and a few examples are provided of their usefulness for such applications. We start by discussing the use of colloidal, reverse micelle, and dendrimer chemistry in the production of active metal and metal oxide nanoparticles with well-defined sizes, shapes, and compositions, as a way to control the surface atomic ensembles available for selective catalysis. Next we introduce the use of sol-gel and atomic layer deposition chemistry for the production and modification of high-surface-area supports and active phases. Reference is then made to the more complex active sites that can be created or carved on such supports by using organic structure-directing agents. We follow with an examination of the ability to achieve multiple functionality in catalysis via the design of dumbbells, core@shell, and other complex nanostructures. Finally, we consider the mixed molecular-nanostructure approach that can be used to develop more demanding catalytic sites, by derivatizing the surface of solids or tethering or immobilizing homogeneous catalysts or other chemical functionalities. We conclude with a personal and critical perspective on the importance of fully exploiting the synergies between nanotechnology and surface science to optimize the search for new catalysts and catalytic processes.

A highly-active and poison-tolerant Pt12 sub-nanocluster catalyst for the reductive amination of aldehydes with amines

A one-atom controlled platinum sub-nanocluster (under 1 nm) was synthesized using a phenylazomethine dendrimer template. This sub-nanocluster (SNC) catalyst exhibits a remarkable catalytic activity during reductive amination compared to the standard platinum nanoparticles of 2.2 ± 0.8 nm under mild conditions and a low catalyst loading. In addition, this catalyst has a remarkable poison-tolerance to amines without adding protic acid.

Selenium-containing polymers: promising biomaterials for controlled release and enzyme mimics

Although researchers have made great progress in the development of responsive polymeric materials for controlled drug release or diagnostics over the last 10 years, therapeutic results still lag behind expectations. The development of special materials that respond to physiological relevant concentrations, typically within the micromolar or nanomolar concentration regime, remains challenging. Therefore, researchers continue to pursue new biomaterials with unique properties and that respond to mild biochemical signals or biomarkers. Selenium is an essential element in human body with potential antioxidant properties. Because of selenium's electronegativity and atomic radius, selenium-containing compounds exhibit unique bond energy (C-Se bond 244 kJ mol⁻¹; Se-Se bond 172 kJ mol⁻¹). These values give the C-Se or Se-Se covalent bonds dynamic character and make them responsive to mild stimuli. Therefore, selenium-containing polymers can disassemble in response to changes under physiological relevant conditions. This property makes them a promising biomaterial for controlled release of drugs or synthetic enzyme mimics. Until recently, few researchers have looked at selenium-containing polymers as novel biomaterials. In this Account, we summarize our recent research on selenium-containing polymers and show their potential application as mild-responsive drug delivery vehicles and artificial enzymes. We begin by reviewing the current state of the art in the synthesis of selenium-containing main chain block copolymers. We highlight the dual redox and gamma-irradiation behaviors of diselenide-containing block copolymers assemblies, discussing the possibility of their use in a combination of chemotherapy and actinotherapy. We also describe the coordination of platinum with monoselenide containing block copolymers. Such structures offer the possibility of fabricating multidrug systems for cooperative chemotherapy. In addition, we summarize the methods for the covalent and noncovalent preparation of selenium-containing polymers with side chains, which highlight the opportunity to reversibly tune the amphiphilicity of selenium-containing polymers. Finally, we present strategies for the design of highly efficient selenium-containing dendritic polymers that can mimic enzymes. This field is still in its infancy period, and further research can only be limited by our imagination.

pH-tuned metal coordination and peroxidase activity of a peptide dendrimer enzyme model with a Fe(II)bipyridine at its core

Peptide dendrimer BP1 was obtained by double thioether bond formation between 5,5'-bis(bromomethyl)-2,2'-bipyridine and two equivalents of peptide dendrimer N1 (Ac-Glu-Ser)(8)(Dap-Glu-Ala)(4)(Dap-Amb-Tyr)(2)Dap-Cys-Asp-NH(2) (Dap = branching 2,3-diaminopropanoic acid, Amb = 4-aminomethyl-benzoic acid). At pH 4.0 BP1 bound Fe(ii) to form the expected tris-coordinated complex [Fe(II)(BP1)(3)] (K(f) = 2.1 × 10(15) M(-3)). At pH 6.5 a monocoordinated complex [Fe(II)(BP1)] was formed instead (K(f) = 2.1 × 10(5) M(-1)) due to electrostatic repulsion between the polyanionic dendrimer branches, as confirmed by the behavior of three analogues where glutamates were partially or completely replaced by neutral glutamines or positive lysines. [Fe(II)(BP1)] catalyzed the oxidation of o-phenylenediamine with H(2)O(2) with enzyme-like kinetics (k(cat) = 1.0 min(-1), K(M) = 1.5 mM, k(cat)/k(uncat) = 90 000) and multiple turnover, while Fe(2+) or [Fe(bipy)(3)](2+) were inactive. The labile coordination positions allowing coordination to H(2)O(2) and to the substrate are likely responsible for the enhanced peroxidase activity of the metallopeptide dendrimer.