Interest in catalysis by metal nanoparticles (NPs) is increasing dramatically, as reflected by the large number of publications in the last five years. This field, "semi-heterogeneous catalysis", is at the frontier between homogeneous and heterogeneous catalysis, and progress has been made in the efficiency and selectivity of reactions and recovery and recyclability of the catalytic materials. Usually NP catalysts are prepared from a metal salt, a reducing agent, and a stabilizer and are supported on an oxide, charcoal, or a zeolite. Besides the polymers and oxides that used to be employed as standard, innovative stabilizers, media, and supports have appeared, such as dendrimers, specific ligands, ionic liquids, surfactants, membranes, carbon nanotubes, and a variety of oxides. Ligand-free procedures have provided remarkable results with extremely low metal loading. The Review presents the recent developments and the use of NP catalysis in organic synthesis, for example, in hydrogenation and C--C coupling reactions, and the heterogeneous oxidation of CO on gold NPs.

Nanoparticles as Recyclable Catalysts: The Frontier between Homogeneous and Heterogeneous Catalysis

Over the past decade, the most significant, conceptual advances in the field of fluorination were enabled most prominently by organo- and transition-metal catalysis. The most challenging transformation remains the formation of the parent C-F bond, primarily as a consequence of the high hydration energy of fluoride, strong metal-fluorine bonds, and highly polarized bonds to fluorine. Most fluorination reactions still lack generality, predictability, and cost-efficiency. Despite all current limitations, modern fluorination methods have made fluorinated molecules more readily available than ever before and have begun to have an impact on research areas that do not require large amounts of material, such as drug discovery and positron emission tomography. This Review gives a brief summary of conventional fluorination reactions, including those reactions that introduce fluorinated functional groups, and focuses on modern developments in the field.

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Introduction of Fluorine and Fluorine-Containing Functional Groups

A wide array of forms of palladium has been utilized as precatalysts for Heck and Suzuki coupling reactions over the last 15 years. Historically, nearly every form of palladium used has been described as the active catalytic species. However, recent research has begun to shed light on the in situ transformations that many palladium precatalysts undergo during and before the catalytic reaction, and there are now many suggestions in the literature that narrow the scope of types of palladium that may be considered true “catalysts” in these coupling reactions. In this work, for each type of precatalyst, the recent literature is summarized and the type(s) of palladium that are proposed to be truly active are enumerated. All forms of palladium, including discrete soluble palladium complexes, solid-supported metal ligand complexes, supported palladium nano- and macroparticles, soluble palladium nanoparticles, soluble ligand-free palladium, and palladium-exchanged oxides are considered and reviewed here. A considerable focus is placed on solid precatalysts and on evidence for and against catalysis by solid surfaces vs. soluble species when starting with various precatalysts. The review closes with a critical overview of various control experiments or tests that have been used by authors to assess the homogeneity or heterogeneity of catalyst systems.

On the Nature of the Active Species in Palladium Catalyzed Mizoroki–Heck and Suzuki–Miyaura Couplings – Homogeneous or Heterogeneous Catalysis, A Critical Review

Transition-Metal-Catalyzed C−S, C−Se, and C−Te Bond Formation via Cross-Coupling and Atom-Economic Addition Reactions

The preparation, structure, and chemistry of hypervalent iodine compounds are reviewed with emphasis on their synthetic application. Compounds of iodine possess reactivity similar to that of transition metals, but have the advantage of environmental sustainability and efficient utilization of natural resources. These compounds are widely used in organic synthesis as selective oxidants and environmentally friendly reagents. Synthetic uses of hypervalent iodine reagents in halogenation reactions, various oxidations, rearrangements, aminations, C–C bond-forming reactions, and transition metal-catalyzed reactions are summarized and discussed. Recent discovery of hypervalent catalytic systems and recyclable reagents, and the development of new enantioselective reactions using chiral hypervalent iodine compounds represent a particularly important achievement in the field of hypervalent iodine chemistry. One of the goals of this Review is to attract the attention of the scientific community as to the benefits of...

Advances in Synthetic Applications of Hypervalent Iodine Compounds

Palladium nanoparticles (4 nm, fcc) were prepared through decomposition of [Pd2(dba)3] by H2 in the presence of a chiral xylofuranoside diphosphite. These particles catalyze the allylic alkylation of rac-3-acetoxy-1,3-diphenyl-1-propene with dimethyl malonate leading to an almost total conversion of the (R) enantiomer and almost no reaction with the (S). This gives rise to 97% ee for the alkylation product and a kinetic resolution of the substrate recovered with ca. 90% ee. This behavior was compared to that of a molecular catalyst at various dilutions, and the differences between the two systems are discussed. This is the first colloidal system shown to display such a high enantioselectivity besides the well-known Pt/cinchonidine system.

A case for enantioselective allylic alkylation catalyzed by palladium nanoparticles.

This Review deals with the asymmetric hydrogenation and asymmetric transfer hydrogenation of imines and some nitrogen-containing heteroaromatic compounds to afford amines. Hydrogenation and transfer hydrogenation are covered in separate sections. Particular attention is devoted to the substrates, since this reaction is highly substrate dependent. In this way the general structural trends in imine hydrogenation are presented, highlighting the more challenging substrates. Since hydrogenation of acyclic and cyclic (including heteroaromatic) imines have some different features they are also presented separately. The catalytic systems are classified according to the metal, and focus on the most successful catalytic systems and metal-ligand combinations. The catalytic cycles for these hydrogenation processes are also proposed.

Highlights of Transition Metal‐Catalyzed Asymmetric Hydrogenation of Imines

Synthesis of 2-substituted-benzothiazoles by palladium-catalyzed intramolecular cyclization of o-bromophenylthioureas and o-bromophenylthioamides

Carbohydrate derivative ligands in asymmetric catalysis

A series of novel diphosphite ligands derived from readily available D-(+)-glucose has been synthesized. These ligands have been applied to the Rh-catalyzed hydroformylation of vinyl arenes. Both excellent enantioselectivities (up to 91 %) and regioselectivities (up to 98.8 %) were achieved under mild conditions. The advantage of these ligands is that their modular natures allow facile, systematic variation in the configurations at the stereocenters [C(3), C(5)] at the ligand bridge and in the biphenyl substituents, enabling their effects on the stereoselectivity to be studied. Results show that the absolute configuration of the product is governed by the configuration at the stereogenic center C(3), while the level of the enantioselectivity is influenced by a cooperative effect between stereocenters C(3) and C(5). Replacement of the tert-butyl substituent by methoxy substituents at the para positions of the biphenyl moieties improved the enantioselectivities. We have characterized the rhodium complexes formed under CO/H2 by NMR techniques and in situ IR spectroscopy and have observed that there is a relationship between the structure of the [HRh(CO)2(PP)] species and their enantiodiscriminating performance in hydroformylation. Enantioselectivities were highest with ligands with a strong bis-equatorial coordination preference, while an equilibrium of species with bis-equatorial and equatorial-axial coordination modes considerably reduced the ee's.

Sergio Castillón

Papers

A case for enantioselective allylic alkylation catalyzed by palladium nanoparticles.

Highlights of Transition Metal‐Catalyzed Asymmetric Hydrogenation of Imines

Synthesis of 2-substituted-benzothiazoles by palladium-catalyzed intramolecular cyclization of o-bromophenylthioureas and o-bromophenylthioamides

Carbohydrate derivative ligands in asymmetric catalysis

Chiral diphosphites derived from D-glucose: new ligands for the asymmetric catalytic hydroformylation of vinyl arenes.