Current Organic Chemistry - Volume 17, Issue 12, 2013

This review summarizes the progress made in the last eight years on the synthesis and use in catalysis of polymer supported metal complexes obtained by co-polymerization of metal-containing monomers (MCMs) with suitable co-monomers and cross-linkers. These materials are divided into four important classes: microporous organic polymers, molecularly imprinted polymers (MIPs), macropouros organic polymers, polymers obtained by ring opening metathesis polymerization (ROMP). Although the MIPs have essentially a macroporous structure, they constitute a separate group of catalysts for their peculiar way of synthesis. The discussion focuses on the efficiency and reusability of all these potentially recoverable catalysts.

With the ever-increasing demand for environmentally benign synthetic routes that can lead to desirable organic products, photocatalytic techniques have become more appealing than conventional synthetic methods because the former make it possible selective chemical transformations of various organic reactions with the help of sunlight as the source of the energy for the reactions. At the heart of photocatalytic chemical transformations are materials called photocatalysts, without which the photocatalytic reactions either do not take place or occur only in a very sluggish manner. In this review, we highlight the recent research efforts toward the synthesis of new photocatalytic materials, especially at nanoscale, and their photocatalytic properties and photo-induced catalytic mechanisms when applied in selective organic transformations. In the discussion, particular attention is paid to two major types of photocatalysts: semiconductor- based photocatalysts and plasmonic metal-based photocatalysts. With regard to the photocatalytic reactions, emphasis is given to selective oxidation of alcohols and hydrocarbons and selective reduction of nitro compounds into value-added products and synthetic fuels (e.g., selective oxidation of alkanes or methane to alcohols or methanol).

Over the past decade, metal nanoparticles as homogeneous catalysts have attracted extensive attention for organic transformations. The metal nanoparticles as supports enhance catalyst loadings and facilitate the catalyst recovery. As active parts, metal nanoparticles stabilized by polymers and dendrimers usually witness drastically improved catalytic efficiency. Chiral molecular ligands modified metal nanoparticles endow chiral induction for asymmetric transformations. Consequently, the use of metal nanoparticles as supports or active parts provides a technically alternative approach over classical homogeneous catalysis, including hydrogenation, oxidation, carbon- carbon coupling reactions, asymmetric alkylation and asymmetric hydrosilylation etc. This brief review emphasizes on recent developments in this emerging area, covering types of stabilizers for preparing metal nanoparticles, catalytic efficiency, recyclability, enantioselectivity and catalytic mechanism.

Here we present an overview of immobilized chiral catalysts and some successful examples for enantioselective process. The focus will put on the last general achievements regarding on the immobilization methods of chiral catalysts in asymmetric catalytic reactions, and finally give a perspective of future development.

Heterogeneous catalysts based on supported metal nanoparticles are commonly used in many industrially relevant catalytic reactions. This review examines the existing literature of several types of the supported-metal nanoparticle heterogeneous catalysts. Specifically, this review includes: (i) a brief overview of the catalysts from the immobilization of metal nanoparticles on mesoporous silica materials, oxide and carbon nanotube (ii) an introduction of the existing strategies presented, e.g., the utilization of porous inorganic shell to cap metals nanoparticles to prevent aggregation during catalytic reaction, then (iii) a summary of the catalysts including metal nanoparticles encapsulated in hollow shells, which provide the thermal and chemical stability of metal nanoparticles during catalytic reactions.

Dendrimers are highly branched macromolecules with well-defined molecular structures and nanometer-scale dimensions. Dendrimer-supported catalysts behave homogeneously during the reaction, but most of them can be separated from the reaction medium via solvent precipitation. Their catalytic properties can be fine-tuned by adjusting the structure, size and shape of the dendrimers. In many cases, interesting dendrimer effects were observed, including increased or decreased the catalytic activity, stability and enantioselectivity. This review focuses on the recent decade of progress concerning the dendrimer-supported catalysts in the following reactions: (a) asymmetric hydrogenation and hydrosilylation; (b) asymmetric transfer hydrogenation; (c) carbon-carbon bond-forming reactions; (d) miscellaneous reactions.