Sterically encumbered Lewis acid and Lewis base combinations do not undergo the ubiquitous neutralization reaction to form "classical" Lewis acid/Lewis base adducts. Rather, both the unquenched Lewis acidity and basicity of such sterically "frustrated Lewis pairs (FLPs)" is available to carry out unusual reactions. Typical examples of frustrated Lewis pairs are inter- or intramolecular combinations of bulky phosphines or amines with strongly electrophilic RB(C(6)F(5))(2) components. Many examples of such frustrated Lewis pairs are able to cleave dihydrogen heterolytically. The resulting H(+)/H(-) pairs (stabilized for example, in the form of the respective phosphonium cation/hydridoborate anion salts) serve as active metal-free catalysts for the hydrogenation of, for example, bulky imines, enamines, or enol ethers. Frustrated Lewis pairs also react with alkenes, aldehydes, and a variety of other small molecules, including carbon dioxide, in cooperative three-component reactions, offering new strategies for synthetic chemistry.

Frustrated Lewis pairs: metal-free hydrogen activation and more.

Organocatalytic Reactions Enabled by N-Heterocyclic Carbenes.

The area of catalysis of radical reactions has recently flourished. Various reaction conditions have been discovered and explained in terms of catalytic cycles. These cycles rarely stand alone as unique paths from substrates to products. Instead, most radical reactions have innate chains which form products without any catalyst. How do we know if a species added in "catalytic amounts" is a catalyst, an initiator, or something else? Herein we critically address both catalyst-free and catalytic radical reactions through the lens of radical chemistry. Basic principles of kinetics and thermodynamics are used to address problems of initiation, propagation, and inhibition of radical chains. The catalysis of radical reactions differs from other areas of catalysis. Whereas efficient innate chain reactions are difficult to catalyze because individual steps are fast, both inefficient chain processes and non-chain processes afford diverse opportunities for catalysis, as illustrated with selected examples.

Catalysis of Radical Reactions: A Radical Chemistry Perspective.

This Minireview highlights recent developments in radical trifluoromethylation reactions. The trifluoromethyl group belongs to the privileged moieties in medicinal chemistry. Many drugs and drug candidates contain a trifluoromethyl substituent. Also in agrochemicals, the CF3 moiety often appears. The present article addresses the radical trifluoromethylation of alkenes and arenes mainly focussing on recent achievements. However, important earlier work in this field is also covered.

A "Renaissance" in radical trifluoromethylation

The generation of polymer brushes by surface-initiated controlled radical polymerization (SI-CRP) techniques has become a powerful approach to tailor the chemical and physical properties of interfaces and has given rise to great advances in surface and interface engineering. Polymer brushes are defined as thin polymer films in which the individual polymer chains are tethered by one chain end to a solid interface. Significant advances have been made over the past years in the field of polymer brushes. This includes novel developments in SI-CRP, as well as the emergence of novel applications such as catalysis, electronics, nanomaterial synthesis and biosensing. Additionally, polymer brushes prepared via SI-CRP have been utilized to modify the surface of novel substrates such as natural fibers, polymer nanofibers, mesoporous materials, graphene, viruses and protein nanoparticles. The last years have also seen exciting advances in the chemical and physical characterization of polymer brushes, as well as an ev...

Surface-Initiated Controlled Radical Polymerization: State-of-the-Art, Opportunities, and Challenges in Surface and Interface Engineering with Polymer Brushes

This Review describes the application of nitroxides to synthesis and polymer chemistry. The synthesis and physical properties of nitroxides are discussed first. The largest section focuses on their application as stoichiometric and catalytic oxidants in organic synthesis. The oxidation of alcohols and carbanions, as well as oxidative C-C bond-forming reactions are presented along with other typical oxidative transformations. A section is also dedicated to the extensive use of nitroxides as trapping reagents for C-centered radicals in radical chemistry. Alkoxyamines derived from nitroxides are shown to be highly useful precursors of C-centered radicals in synthesis and also in polymer chemistry. The last section discusses the basics of nitroxide-mediated radical polymerization (NMP) and also highlights new developments in the synthesis of complex polymer architectures.

Nitroxides: applications in synthesis and in polymer chemistry.

Nitroxide werden in der Synthese und in der Polymerchemie verwendet. In diesem Aufsatz werden zunachst Methoden zur Herstellung von Nitroxiden und deren physikalische Eigenschaften besprochen. Der folgende Abschnitt beschreibt die Verwendung von Nitroxiden als stochiometrische und katalytische Oxidationsmittel in der organischen Synthese. Die Oxidation von Alkoholen und Carbanionen spielt dabei ebenso eine wichtige Rolle wie oxidative C-C-Bindungsknupfungen. Anschliesend widmet sich ein Abschnitt der Rolle von Nitroxiden als Abfangreagentien fur C-zentrierte Radikale in der Radikalchemie. Nitroxide, die sich von Alkoxyaminen ableiten, fungieren als nutzliche Ausgangsmaterialien fur C-zentrierte Radikale, deren Einsatz in der Synthese und in der Polymerchemie beschrieben wird. Der letzte Abschnitt stellt die Grundlagen der Nitroxid-vermittelten Polymerisation (NMP) vor und zeigt neuere Entwicklungen beim Design komplexer Polymerarchitekturen auf.

Nitroxide: Anwendungen in der Synthese und in der Polymerchemie

The carbazole structural unit occurs in a large number of natural products that show interesting biological activity. In addition, the carbazole moiety has also found application as a building block in materials science. Although different valuable approaches to carbazoles have appeared, 3] owing to the importance of this substance class there is still demand for novel synthetic methods for its preparation. Some methods require the use of transition metals (TM). However, for biological studies, TMs have to be carefully removed from the final product. Also in the field of materials science, TM impurities might influence the physical properties of the targeted material. We therefore regard the development of transition-metal-free protocols for preparation of carbazoles as important. In Scheme 1 different approaches for the construction of the carbazole core are depicted. C N bond formation has been successfully used for the preparation of carbazoles and

Reactions of Arynes with Nitrosoarenes—An Approach to Substituted Carbazoles

Enantioselective cyclopropanation of enals by oxidative N-heterocyclic carbene catalysis

Cooperative effects can be observed in various research areas in chemistry; cooperative catalysis is well-established, the assembly of compounds on surfaces can be steered by cooperative effects, and supramolecular polymerization can proceed in a cooperative manner. In biological systems, cooperativity is observed in protein-protein, protein-lipid and protein-molecule interactions. Synergistic effects are relevant in frustrated Lewis pairs, organic multispin systems, multimetallic clusters and also in nanoparticles. However, a general approach to determine cooperativity in the different chemical systems is currently not known. In the present concept paper it is suggested that, at least for simpler systems that can be described at the molecular level, cooperativity can be defined based on energy considerations. For systems in which no chemical transformation occurs, determination of interaction energies of the whole system with respect to the interaction energies between all individual component pairs (subsystems) will allow determination of cooperativity. For systems comprising of chemical transformations, cooperativity can be evaluated by determining the activation energy of the synergistic system and by comparing this with activation energies of the corresponding subsystems that lack an activating moiety. For more complex systems, cooperativity is generally determined at a qualitative level.

Ludger Tebben

Papers

Nitroxides: applications in synthesis and in polymer chemistry.

Nitroxide: Anwendungen in der Synthese und in der Polymerchemie

Reactions of Arynes with Nitrosoarenes—An Approach to Substituted Carbazoles

Enantioselective cyclopropanation of enals by oxidative N-heterocyclic carbene catalysis

From Additivity to Cooperativity in Chemistry: Can Cooperativity Be Measured?