Oxidative Homokupplung von Aryl-, Alkenyl- und Alkinyl-Grignard-Reagentien mit TEMPO und Sauerstoff

Arynes as Radical Acceptors: TEMPO-Mediated Cascades Comprising Addition, Cyclization, and Trapping

The application of arynes as radical acceptors is described. The stable radical TEMPO (2,2,6,6-tetramethyl piperidine 1-oxyl) is shown to add to various ortho-substituted benzynes generating the corresponding aryl radicals which engage in 5-exo or 6-endo cyclizations. The cyclized radicals are eventually trapped by TEMPO. The introduced method provides ready access to various dihydrobenzofurans, oxindoles, and sultones by a conceptually novel approach.

Transition-Metal-Free Oxidative Cross-Coupling of Tetraarylborates to Biaryls Using Organic Oxidants

Readily prepared tetraarylborates undergo selective (cross)-coupling through oxidation with Bobbitt's salt to give symmetric and unsymmetric biaryls. The organic oxoammonium salt can be used either as a stoichiometric oxidant or as a catalyst in combination with in situ generated NO2 and molecular oxygen as the terminal oxidant. For selected cases, oxidative coupling is also possible with NO2 /O2 without any additional nitroxide-based cocatalyst. Transition-metal-free catalytic oxidative ligand cross-coupling of tetraarylborates is unprecedented and the introduced method provides access to various biaryl and heterobiaryl systems.

Syntheses, Properties, and Catalytic Activities of Metal(II) Complexes and Free Bases of Redox-Switchable 20π, 19π, and 18π 5,10,15,20-Tetraaryl-5,15-diazaporphyrinoids

In spite of significant advances in redox-active porphyrin-based materials and catalysts, little attention has been paid to 20π and 19π porphyrins because of their instability in air. Here we report the meso-modification of 5,10,15,20-tetraarylporphyrin with two nitrogen atoms, which led to redox-switchable 20π, 19π, and 18π 5,10,15,20-tetraaryl-5,15-diazaporphyrinoids (TADAPs). Three kinds of metal(II) complexes and free bases of TADAP were prepared by the metal-templated annulation of the corresponding metal-bis(dipyrrin) complexes. The inductive and resonance effects of the meso-nitrogen atoms on the aromatic, optical, electrochemical, and magnetic properties of the entire TADAP π-systems were assessed by using various spectroscopic measurements and density functional theory calculations. The aromaticity and π-π* electronic transition energies of the TADAPs varied considerably, and were shown to be dependent on the oxidation states of the π-systems. In contrast to the isoelectronic 5,10,15,20-tetraarylporphyrin derivatives, the 20π and 19π TADAPs were chemically stable under air. In particular, the 19π TADAP radical cations were extremely stable towards dioxygen, moisture, and silica gel. This reflected the low-lying singly occupied molecular orbitals of their π-systems and the efficient delocalization of their unshared electron spin. The capability of MgTADAP to catalyze aerobic biaryl formation from aryl Grignard reagents was demonstrated, which presumably involved a 19π/20π redox cycle.

Nitroxyl-Radical-Catalyzed Oxidative Coupling of Amides with Silylated Nucleophiles through N-Halogenation

A nitroxyl-radical-catalyzed oxidative coupling reaction between amines with an N-protecting electron-withdrawing group (EWG) and silylated nucleophiles was developed to furnish coupling products in high yields, thus opening up new frontiers in organocatalyzed reactions. This reaction proceeded through the activation of N-halogenated amides by a nitroxyl-radical catalyst, followed by carbon-carbon coupling with silylated nucleophiles. Studies of the reaction mechanism indicated that the nitroxyl radical activates N-halogenated amides, which are generated from N-EWG-protected amides and a halogenation reagent, to give the corresponding imines.

Nitroxides: applications in synthesis and in polymer chemistry

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.

Stereoselective palladium-catalyzed carboaminoxylations of indoles with arylboronic acids and TEMPO

Indoles are not indolent: Various indoles react with arylboronic acids chemodivergently. C-H arylation of free indole and N-methylindole gives the corresponding C(2)-arylated indoles A whereas N-acylated, N-benzoylated, and N-Boc-protected indoles provide the corresponding arylcarboaminoxylated products B with excellent diastereoselectivity in good to excellent yields.