A novel method for selective generation of aryl radicals from diaryliodonium salts and iodanylidene malonates with sodium 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPONa) as a single-electron transfer (SET) reducing reagent is described. In the presence of various alkenes, aryl radicals formed after SET-reduction of hypervalent iodine compounds undergo alkene addition and the adduct radicals that are thus generated are efficiently trapped by the concomitantly generated TEMPO radical to eventually afford oxyarylated products in moderate to very good yields. The efficiency of aryl radical generation of various iodine(III) reagents is studied and the generation of an iodanylidene malonate aryl radical is also investigated by computational methods.

The utility of oxidative NHC catalysis for both the regioselective and chemoselective functionalization of carbohydrates is explored. Chiral NHCs allow for the highly regioselective oxidative esterification of various carbohydrates using aldehydes as acylation precursors. The transformation was also shown to be amenable to both cis/trans diol isomers, free amino groups, and selective for specific sugar epimers in competition experiments. Efficiency and regioselectivity of the acylation can be improved upon using two different NHC catalysts that act cooperatively. The potential of the method is documented by the regioselective acylation of an amino-linked neodisaccharide.

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.

Das Gebiet der Katalyse von Radikalreaktionen erfuhr in jüngster Zeit höchste Aufmerksamkeit. Viele interessante Radikalreaktionen wurden entdeckt und dann über Katalysezyklen erklärt. Diese Zyklen stellen allerdings selten den einzigen Weg vom Substrat zum Produkt dar. Die meisten Radikalreaktionen weisen intrinsische Ketten auf, die das Potential haben, das Produkt auch ohne Katalysator zu bilden. Wie wissen wir nun, ob eine in “katalytischen Mengen” zugegebene Verbindung einen Katalysator, einen Initiator oder gar etwas anderes darstellt? Im vorliegenden Aufsatz besprechen wir kritisch sowohl Katalysator-freie als auch katalytische Radikalreaktionen aus Sicht der Radikalchemie. Grundlagen der Kinetik und Thermodynamik von Radikalreaktionen werden genutzt, um die Probleme der Initiierung, Propagation und Inhibierung von Radikalreaktionen zu adressieren. Katalyse von Radikalreaktionen unterscheidet sich von anderen Gebieten der Katalyse in mehreren Aspekten. Wohingegen intrinsische Kettenreaktionen schwierig zu katalysieren sind, da die zugrundeliegenden individuellen Schritte so schnell ablaufen, bieten ineffiziente Kettenprozesse und nicht-Kettenprozesse verschiedene Möglichkeiten der Katalyse. Wir illustrieren Konzepte der Katalyse von Radikalreaktionen mit ausgewählten Beispielen aus der klassischen, aber auch der jüngsten Literatur.

A simple and efficient method for radical hydrodeiodination is reported. The novel approach uses electron catalysis. In situ generated Na-alcoholates are introduced as radical chain reducing reagents and reactions work with O₂ as cheap initiator. Hydrodeiodination works on aryl, alkenyl, alkynyl iodides and a tert-alkyl iodide also gets reduced applying the method. Albeit less general, the method is also applicable to the reduction of aryl bromides. The novel reagent is successfully used to conduct typical reductive radical cyclization reactions and mechanistic studies are reported.

Wir beschreiben eine neue und effiziente Methode zur Synthese von ortho-Trialkylstannylarylphosphanen durch Insertion von in situ erzeugten Arinen in die Sn-P-Bindung von stannylierten Phosphanen (R₃Sn-PR₂). Die simultan ablaufenden C-P- und C-Sn-Bindungsknüpfungen ergeben die stannylierten Produkte in hohen Ausbeuten. Diese lassen sich leicht in synthetisch wertvolle ortho-substituierte Arylphosphane überführen. Die Reaktion verläuft hoch effizient, mit guten Regioselektivitäten und ist praktisch in der Durchführung.

A simple and efficient method for radical hydrodeiodination is reported. The novel approach uses electron catalysis. In situ generated Na-alcoholates are introduced as radical chain reducing reagents and reactions work with O₂ as cheap initiator. Hydrodeiodination works on aryl, alkenyl, alkynyl iodides and a tert-alkyl iodide also gets reduced applying the method. Albeit less general, the method is also applicable to the reduction of aryl bromides. The novel reagent is successfully used to conduct typical reductive radical cyclization reactions and mechanistic studies are reported.

A novel and efficient approach to ortho-trialkylstannyl arylphosphanes by the reaction of arynes generated in situ with stannylated phosphanes (R₃SnPR₂) is described. Concurrent CP and CSn bond formation occurs with high yields, and stannylated products are easily transformed into valuable ortho-substituted arylphosphanes. The reaction features high efficiency, good regioselectivity, and excellent practicality.

The paper describes the addition of Mg-anilides and -thiolates to in situ generated benzyne with subsequent oxidative homocoupling of the adducts to give 2,2′-bis-substituted biphenyls in a one-pot process. Oxidative C–C bond formation was best achieved with a Cu catalyst using O₂ as oxidant. The sequence comprises two C–X and one C–C bond formation and the products were obtained in moderate yields. Some of the 2,2′-bis-substituted biphenyls have been characterized by X-ray analysis.

A novel method for the preparation of functionalized alternating copolymers is presented. Nitroxide-mediated polymerization of hexafluoroisopropyl acrylate with 7-octenyl vinyl ether provides the corresponding alternating polymer, which can be chemically modified using two orthogonal polymer-analogous reactions. A thiol–ene click reaction followed by amidation provides dual-functionalized alternating copolymers. The potential of this method is illustrated by the preparation of a small library (15 examples) of functionalized alternating copolymers.

Due to the great value of fluorinated arenes in agrochemistry, medicinal chemistry and materials science, development of methods for preparation of fluorinated arenes is of high importance. They can be either accessed by arene fluorination or by partial arene defluorination. However, the carbonfluorine bond belongs to the strongest σ-bonds, which renders CF activation highly challenging. Here it is shown that aryl and alkyl isonitriles efficiently activate the strong CF bond in perfluoroarenes by simple UV irradiation under mild conditions. Reactions proceed by formal direct insertion of the isonitrile into the CF bond without any transition metal. Activation occurs at arene CF bonds whereas aliphatic CF bonds remain unreacted. For selected perfluoroarenes CF activation occurs with high regioselectivity and resulting imidoyl fluorides are transformed into other valuable compounds. Theoretical studies give insights into the reaction mechanism.

Gold-Nanopartikel (AuNPs) erfahren zurzeit aufgrund ihrer interessanten physikochemischen Eigenschaften große wissenschaftliche Beachtung. Wir stellen hier eine einfache und kontrollierte lichtvermittelte Synthese von Gold-Nanopartikeln über Norrish-Typ-I-Reaktion in photoaktiven Polymeren vor. Diese funktionell konzipierten Polymere agieren sowohl als Reagentien zur photochemischen Reduktion von Goldionen als auch zur Stabilisierung der in situ hergestellten AuNPs. Durch geschickte Variation der Länge und der Zusammensetzung der photoaktiven Polymere kann die Größe der Gold-Nanopartikel in einem Bereich zwischen 1.5 nm und 9.6 nm gezielt eingestellt werden.

Ein kurzer und divergenter Syntheseweg mit hoch effizienten stereoselektiven Umsetzungen liefert Machaeriole und Cannabinoide aus einer gemeinsamen Vorstufe, der kommerziell erhältlichen (S)-Perillasäure. Schlüsselschritte des neuen Ansatzes sind eine stereospezifische Pd-katalysierte decarboxylierende Arylierung und eine Eintopf-Sequenz aus stereoselektiver Hydroborierung mit anschließender Oxidation bzw. Reduktion der Boran-Intermediate. Die divergente Strategie ließ sich überzeugend für je fünfstufige Synthesen von (+)-Machaeriol B, (+)-Machaeriol D sowie zugehörigen Analoga und für eine vierstufige Sequenz zu (+)-Δ⁸-THC und einem seiner Derivate heranziehen.

Es wird eine neue Methode zur Synthese funktionalisierter alternierender Copolymere vorgestellt. Die Nitroxid-vermittelte Polymerisation von Hexafluorisopropylacrylat mit 7-Octenylvinylether liefert das entsprechende alternierende Copolymer, welches mittels zweier orthogonaler polymeranaloger Reaktionen chemisch modifiziert werden kann. Eine Thiol-En-Klick-Reaktion mit anschließender Amidierung ermöglicht einen kombinatorischen Zugang zu doppelt funktionalisierten alternierenden Copolymeren. Das Potenzial dieser Methode wird anhand einer kleinen Bibliothek (15 Beispiele) funktionalisierter alternierender Copolymere demonstriert.

Es wird ein neuartiger und effizienter Zugang zu Allyloxyaminen über die Allylierung von Nitrosoarenen mit α-chiralen Allylboronaten vorgestellt. Die C-O-Bindungsknüpfung erfolgt mit hoher Stereospezifität, und die erhaltenen Allyloxyamine lassen sich leicht zu wertvollen chiralen Bausteinen wie Isoxazolidinen und Allylalkoholen umwandeln. Die Reaktion verläuft mit vollständiger Regioselektivität (O-Selektivität), hoher E/Z-Selektivität und mit exzellentem Chiralitätstransfer.

Gold nanoparticles (AuNPs) are subjects of broad interest in scientific community due to their promising physicochemical properties. Herein we report the facile and controlled light-mediated preparation of gold nanoparticles through a Norrish type I reaction of photoactive polymers. These carefully designed polymers act as reagents for the photochemical reduction of gold ions, as well as stabilizers for the in situ generated AuNPs. Manipulating the length and composition of the photoactive polymers allows for control of AuNP size. Nanoparticle diameter can be controlled from 1.5 nm to 9.6 nm.

Short and highly efficient stereoselective syntheses provide machaeriols and cannabinoids in a divergent approach starting from a common precursor, commercially available (S)-perillic acid. Key features of the novel strategy are a stereospecific palladium-catalyzed decarboxylative arylation and a one-pot sequence comprising a stereoselective hydroboration followed by oxidation or reduction of the corresponding intermediary boranes. The divergent approach is convincingly demonstrated by the five-step syntheses of (+)-machaeriol B, (+)-machaeriol D, and related analogues, and the four-step synthesis of (+)-Δ⁸-THC and an analogue.

A novel and efficient approach to allyloxyamines by the allylation of nitrosoarenes with α-chiral allylboronates is described. CO bond formation occurs with high stereospecificity and the product allyloxyamines are easily transformed into valuable chiral building blocks such as isoxazolidines and allylic alcohols. The reaction features complete regioselectivity (O-selectivity), high E/Z selectivity, and excellent chirality transfer.

The reaction of enals with β-diketones, β-ketoesters, and malonates bearing a β-oxyalkyl substituent at the α-position by oxidative NHC catalysis to provide highly substituted β-lactones is described. Reactions occur with excellent diastereo- and enantioselectivity. The organo cascade comprises two CC bond formations and one CO bond formation. Up to four contiguous stereogenic centers including two fully substituted stereocenters are formed in the cascade.

Controlled access to resveratrol-based natural products is offered by a novel, modular concept. A common building block readily available on a large scale serves as the starting material for the introduction of structurally important aryl groups by a Pd-catalyzed decarboxylative arylation and an oxidative Heck reaction with good yields and high stereoselectivity. The modular approach is convincingly documented by the successful synthesis of three racemic resveratrol-based natural products (quadrangularin A, ampelopsin D, and pallidol).

Ein neuartiges modulares Konzept für den kontrollierten Zugang zu racemischen Resveratrol-basierten Naturstoffen wird beschrieben. Ausgehend von nur einem leicht zugänglichen Baustein können durch palladiumkatalysierte decarboxylierende Arylierung und oxidative Heck-Reaktion strukturell entscheidende Arylreste variabel eingeführt werden. Die Schlüsselschritte laufen dabei in guter Ausbeute und mit hoher Selektivität ab. Das modulare Konzept wird am Beispiel der Totalsynthese von drei Resveratrol-basierten Naturstoffen (Quadrangularin A, Ampelopsin D und Pallidol) belegt.

The MgBr₂-catalyzed formal [3+2] cycloaddition of donor–acceptor activated cyclopropanes with nitrosoarenes offers a novel approach to various structurally diverse isoxazolidines. The reactions, which are experimentally easy to conduct, occur with complete stereospecificity and perfect control of regioselectivity. Product isoxazolidines can be readily transformed into α-amino lactones by reductive or decarboxylative NO cleavage and subsequent lactonisation, and the N-aryl bond cleavage is also possible under oxidative conditions.

The reaction of enals with β-diketones, β-ketoesters, and malonates bearing a β-oxyalkyl substituent at the α-position by oxidative NHC catalysis to provide highly substituted β-lactones is described. Reactions occur with excellent diastereo- and enantioselectivity. The organo cascade comprises two CC bond formations and one CO bond formation. Up to four contiguous stereogenic centers including two fully substituted stereocenters are formed in the cascade.

Die radikalische Carboiodierung verschiedener Arylamine wird vorgestellt. Aryldiazoniumsalze, in situ aus den entsprechenden Arylaminen erzeugt, reagieren dabei mit Bu₄NI zu Arylradikalen, welche 5-exo oder 6-exo-Cyclisierungen eingehen. Anschließende I-Abstraktion liefert die carboiodierten Produkte in guten bis exzellenten Ausbeuten. Bei Zugabe von TEMPO führt die Kaskade zu den entsprechenden carboaminoxylierten Produkten. Läuft die Reaktion in Gegenwart von PhTeTePh ab, so resultieren carbotellurierte cyclische Verbindungen.

This article describes the synthesis of bifunctional block copolymers (BCPs) of type 4 bearing orthogonally reactive backbone substituents such as 1,1,1,3,3,3-hexafluoroisopropoxycarbonyl groups as active esters and α-hydroxyalkylphenylketones (2-hydroxy-2-methyl-1-phenylpropan-1-one) as additional photoactive moieties via reversible addition fragmentation chain transfer (RAFT) polymerization. As monomers 1,1,1,3,3,3-hexafluoroisopropyl acrylate (HFIPA) and 2-hydroxy-2-methyl-1-(4-vinyl)phenylpropan-1-one (HAK) are used. Controlled radical polymerization provides BCPs p(HFIPA)-b-p(HAK) with molecular weights (M_n) ranging from 15,000 to 37,000 g mol⁻¹ and low molecular weight distributions (PDI = 1.2–1.4). The incorporated HFIPA and HAK moieties are used for sequential chemoselective postmodification of 4. The photoactive block of 4 can be functionalized through a nitroxide photoclick trapping reaction in the presence of functionalized nitroxides and the active ester moieties of the p(HFIPA)-block are readily thermally amidated using various amines. Chemically modified polymers are characterized by NMR, FTIR, and GPC methods which reveal a high degree of postfunctionalization, typically >95% for both orthogonal processes. The chemically modified polymers feature a narrow molecular weight distribution. The process is successfully applied to the synthesis of a small polymer library and also to the preparation of homo and block polynitroxides using 4-amino-TEMPO as amine component in the transamidation reaction. The polynitroxides obtained are characterized by cyclic voltammetry, FTIR, and UV–vis spectroscopy. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 52, 258–266

Radical carboiodination of various aryl amines is reported. Aryl diazonium salts, generated in situ from the corresponding aryl amines, are reacted with Bu₄NI to provide the corresponding aryl radicals which undergo 5-exo or 6-exo cyclization. Iodine abstraction eventually affords the carboiodinated products in good to excellent yields. If TEMPO is added, the cascade provides the cyclized carboaminoxylation products. Running the reaction in the presence of PhTeTePh affords the phenyltellurated cyclized products.

The MgBr₂-catalyzed formal [3+2] cycloaddition of donor–acceptor activated cyclopropanes with nitrosoarenes offers a novel approach to various structurally diverse isoxazolidines. The reactions, which are experimentally easy to conduct, occur with complete stereospecificity and perfect control of regioselectivity. Product isoxazolidines can be readily transformed into α-amino lactones by reductive or decarboxylative NO cleavage and subsequent lactonisation, and the N-aryl bond cleavage is also possible under oxidative conditions.

The paper describes a cooperative NHC (N-heterocyclic carbene) and ruthenium-based redox catalysis for the mild aerobic oxidative esterification of various aromatic and heteroaromatic aldehydes. The ruthenium(II) complex Ru(bpz)₃(PF₆)₂ (bpz=2,2′-bipyrazine) as catalyst is shown to be compatible with free NHCs. The NHC is used in these cascade reactions for the umpolung of the aldehyde to form the corresponding Breslow intermediate which in turn gets oxidized to an acylazolium ion by the ruthenium redox catalyst. Air is used as a terminal oxidant for oxidation (regeneration) of the ruthenium catalyst. In addition, we will show that in the absence of the ruthenium redox catalyst and alcohol, NHC-catalyzed aerobic oxidation of aldehydes delivers the corresponding acids in good to excellent yields. Mechanistic studies and DFT calculations supporting our suggested mechanisms are provided.

New types of photoactive homo and block copolymers bearing α−hydroxyalkylphenylketone (2-hydroxy-2-methyl-1-phenylpropan-1-one) moieties as backbone substituents are prepared using nitroxide-mediated radical polymerization (NMP). Such polymers can be readily activated via the Norrish-type I photoreaction to give polymeric acyl radicals. Photolysis in the presence of a persistent nitroxide, which serves as a C- radical trapping reagent, leads to chemically modified polymers conjugated with nitroxide moieties. The number-average molecular weight (M_n) of the prepolymers and the chemically modified polymers was determined by gel permeation chromatography (GPC). Structures were further confirmed by NMR spectroscopy and by attenuated total reflection (ATR) Fourier transform infrared (FTIR) spectroscopy.

This Concept article discusses the potential of oxidative carbene catalysis in synthesis and comprehensively covers pioneering studies as well as recent developments. Oxidative carbene catalysis can be conducted by using inorganic and organic oxidants. Applications in cascade processes, in enantioselective catalysis, and also in natural product synthesis are discussed.

TEMPO-Mediated oxidation of hydroxylamines (=hydroxyamines) and alkoxyamines to the corresponding oxime derivatives is reported (TEMPO=2,2,6,6-tetramethylpiperidin-1-yloxy radical; Scheme 2). These environmentally benign oxidations proceed in good to excellent yields (Table 1). For alkoxyamines, oxidation to the corresponding oxime ethers can be performed by using dioxygen as a terminal oxidant in the presence of 5–10 mol-% of TEMPO or 4-substituted derivatives thereof as a catalyst (Scheme 3 and Table 2). Importantly, benzyl bromides can directly be transformed to oxime ethers via in situ alkoxyamine formation by a nucleophilic substitution followed by TEMPO-mediated oxidation (Scheme 4 and Table 3).

The article describes unprecedented nitroxide-mediated radical polymerization of methyl and phenyl vinyl ketone (MVK and PVK) using a sterically highly hindered alkoxyamine as initiator/regulator. It is shown that controlled polymerization of PVK is far more difficult to achieve than controlled MVK polymerization. Whereas for MVK high conversion resulting in polyvinyl ketone with low polydispersity index is readily obtained, the PVK polymerization provides good results only in the presence of free nitroxide and styrene as additives. Vinyl ketone polymerizations are analyzed by ESI mass spectrometry. These MS studies provide insights into the problems associated with the controlled nitroxide-mediated polymerization of PVK. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012

Dieser Kurzaufsatz stellt jüngste Entwicklungen auf dem Gebiet radikalischer Trifluormethylierungen vor. Die Trifluormethylgruppe zählt zu den privilegierten Struktureinheiten in der Medizinalchemie. Viele Wirkstoffe und Wirkstoffkandidaten enthalten die Trifluormethylgruppe, und auch in Agrochemikalien findet sich oft eine CF₃-Einheit. Der vorliegende Kurzaufsatz befasst sich mit radikalischen Trifluormethylierungen von Alkenen und Arenen, wobei besonders auf die jüngsten Entwicklungen eingegangen wird; wichtige frühere Arbeiten werden jedoch ebenfalls diskutiert.

The synthesis of bifunctional mesoporous silica nanoparticles is described. Two chemically orthogonal functionalities are incorporated into mesoporous silica by co-condensation of tetraethoxysilane with two orthogonally functionalized triethoxyalkylsilanes. Post-functionalization is achieved by orthogonal surface chemistry. A thiol–ene reaction, Cu-catalyzed 1,3-dipolar alkyne/azide cycloaddition, and a radical nitroxide exchange reaction are used as orthogonal processes to install two functionalities at the surface that differ in reactivity. Preparation of mesoporous silica nanoparticles bearing acidic and basic sites by this approach is discussed. Particles are analyzed by solid state NMR spectroscopy, elemental analysis, infrared-spectroscopy, and scanning electron microscopy. As a first application, these particles are successfully used as cooperative catalysts in the Henry reaction.

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.

The communication reports on the metal-free 2,2,6,6-tetramethylpiperidine N-oxyl radical (TEMPO) catalyzed aerobic oxidation of various alcohols to aldehydes and ketones. A novel catalyst system that uses 1–4 mol% of TEMPO in combination with 4–6 mol% of aqueous hydroxylamine is introduced. No other additives are necessary and corrosive by-products are not formed during oxidation. Nitric oxide which is important for the catalytic cycle is generated in situ by reaction of the hydroxylamine with TEMPO. A catalytic cycle for the overall oxidation process is suggested.

An efficient mild one-pot protocol for the cross-coupling of nitrones and alkynyl-magnesium compounds using a catalytic amount of 2,2,6,6-tetramethylpiperidine-N-oxyl radical (TEMPO) as an environmentally benign organic oxidant and dioxygen as terminal oxidant is described. These coupling reactions can be performed without adding any transition metal on various N-tert-butylnitrones and alkynyl-Grignard reagents. Moreover, the alkynylated nitrone products are readily transformed to regioisomerically pure 3,5-disubstituted isoxazoles.

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 CC 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.

An efficient synthesis of N-phthalimidyl, benzamidyl, acetamidyl, carbamoyl, and ureayl derivatives of dihydropyridines and the application of these reagents as precursors for N-centered radicals are presented. These aminated dihydropyridines could be used in radical-transfer hydroamination reactions of various electron-rich as well as nonactivated olefins in the presence of thiols as polarity-reversal catalysts. These reactions worked without the aid of any transition metal. Steric and electronic effects exerted by the N-substitutents of the N-centered radicals are discussed. In contrast to most metal-catalyzed processes, the radical hydroamination delivered the opposite regioisomer with excellent anti-Markovnikov selectivity. Hydroamination products were obtained as protected amines that are readily isolated.

Nitroxide werden in der Synthese und in der Polymerchemie verwendet. In diesem Aufsatz werden zunächst Methoden zur Herstellung von Nitroxiden und deren physikalische Eigenschaften besprochen. Der folgende Abschnitt beschreibt die Verwendung von Nitroxiden als stöchiometrische und katalytische Oxidationsmittel in der organischen Synthese. Die Oxidation von Alkoholen und Carbanionen spielt dabei ebenso eine wichtige Rolle wie oxidative C-C-Bindungsknüpfungen. Anschließend widmet sich ein Abschnitt der Rolle von Nitroxiden als Abfangreagentien für C-zentrierte Radikale in der Radikalchemie. Nitroxide, die sich von Alkoxyaminen ableiten, fungieren als nützliche Ausgangsmaterialien für 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.

This article describes the application of nitroxide exchange reactions of surface-bound alkoxyamines as a tool for reversible chemical modification of self-assembled monolayers (SAMs). This approach is based on radical chemistry, which allows for introduction of various functional groups and can be used to reversibly introduce functionalities at surfaces. To investigate the scope of this surface chemistry, alkoxyamines with different functionalities were synthesized and were then applied to the immobilization of, for example, dyes, sugars, or biotin. Surface analysis was carried out by contact angle, X-ray photoelectron spectroscopy, and fluorescence microscopy measurements. The results show that this reaction is highly efficient, reversible, and mild and allows for immobilization of various sensitive functional groups. In addition, Langmuir–Blodgett lithography was used to generate structured SAMs. Site-selective immobilization of a fluorescent dye could be achieved by nitroxide exchange reactions.

Copper(I)-catalyzed [2+2] cycloadditions of various ketenes with 2-nitrosopyridine to afford synthetically highly valuable 1,2-oxazetidine-3-ones are shown to occur with good enantioselectivities. The thermal uncatalyzed process furnishes the unstable regioisomeric oxazetidinone. Density function theory (DFT) calculations give evidence that the reaction occurs via a concerted [2+2] cycloaddtion pathway.

The present communication reports on the synthesis of alkoxyamines by hydroboration of olefins with catecholborane and subsequent oxidation by using nitroxides. Oxidation occurs via a radical process and the intermediately formed C-radicals are trapped by the nitroxides to form the corresponding alkoxyamines. If functionalized nitroxides are used, the method allows incorporation of interesting functional moieties via this approach. The novel method is applied for chemical modification of cyclohexene as a test substrate. More importantly, it is also shown that the reaction sequence can be used for chemical modification of macromolecules containing multiple double bonds. This is documented by successful functionalization of poly(butadiene) and of perallylated polyglycerol.

Cu^I-catalyzed enantioselective nitroso-Diels–Alder reactions (NDA reactions) of 2-nitrosopyridine with various dienes are presented. The [CuPF₆(MeCN)₄]/Walphos-CF₃ catalyst system is best suited to catalyze the NDA reaction of various dienes by using 2-nitrosopyridine as a dienophile. In most of the cases studied, cycloadducts are obtained in quantitative yield with very good to excellent enantioselectivities. Based on DFT calculations, a model to explain the stereochemical outcome of the NDA reaction is presented. Finally, an efficient short synthesis of (−)-peracetylated conduramine A-1 by applying the enantioselective NDA reaction as a key step is described.

Oxidative homocoupling of aryl- and alkenylboronic acids was achieved with Wilkinson’s catalyst. Commercially available 2,2,6,6-tetramethylpiperidine-N-oxyl radical (TEMPO) was used as a stoichiometric oxidant.