Organocatalysis and C-H activation meet radical- and electron-transfer reactions

[Development of versatile oxidation systems based on the design of oxoammonium salts]

Organic nitroxyl radical catalysts have recently attracted great attention because they realize efficient alcohol oxidation under mild and environmentally benign conditions. A representative of this class is 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO). We have also developed 2-azaadamantane N-oxyls (AZADOs) as highly efficient oxidation catalysts. These nitroxyl radicals are generally oxidized by a cooxidant to generate oxoammonium salts, which are active species for alcohol oxidation. In the oxidation systems presented in this paper, we focus on the differences between these two species in terms of oxidation state and counter anion. Herein, the effects of a counter anion of an oxoammonium species on its reaction selectivity are shown. On the basis of the control of the counter anion, we have developed catalytic oxidative rearrangement of tertiary allylic alcohols to β-substituted α,β-unsaturated carbonyl compounds. Moreover, we have developed novel useful oxidation systems utilizing a catalytic oxoammonium salt; namely, a one-pot oxidation of primary alcohols to carboxylic acids and an aerobic alcohol oxidation.

Organocatalytic C-H activation reactions

Organocatalytic C-H activation reactions have recently been developed besides the traditional metal-catalysed C-H activation reactions. The recent non-asymmetric and asymmetric C-H activation reactions mediated by organocatalysts are discussed in this review.

Transition-metal-free synthesis of oxindoles by potassium tert-butoxide-promoted intramolecular α-arylation

Potassium tert-butoxide-mediated intramolecular α-arylations of fluoro- and chloro-substituted anilides provide oxindoles in DMF at 80 °C. In this manner, diversely substituted products have been obtained in moderate to high yields.

Metal- and chemical-oxidant-free C-H/C-H cross-coupling of aromatic compounds: the use of radical-cation pools

Pool and couple: A method for oxidative C-H/C-H cross-coupling has been developed using "radical-cation pools". Aromatic compounds react with aryl radical cations, which are generated and accumulated by low-temperature electrolysis (see scheme). This method avoids both the nonselective oxidation of substrates and oxidation of products and effects the C-H/C-H cross-coupling of aromatic compounds without metal complexes and chemical oxidants.

Transition-metal-free intramolecular N-arylations

N-Substituted phenoxazines and related aza analogs have been prepared from N-acetylated aryloxy anilides by transition-metal-free, base-catalyzed cyclization reactions. In the presence of a mixture of 10 mol % of N,N'-dimethylethylenediamine (DMEDA) and 2 equiv of K(2)CO(3) in toluene at 135 °C the products are obtained in high yields.

Use of dimethyl carbonate as a solvent greatly enhances the biaryl coupling of aryl iodides and organoboron reagents without adding any transition metal catalysts

The coupling reaction of aryl iodides with arylboronic acids to give biaryl compounds can be efficiently performed without adding a transition metal catalyst. The key to success is the use of dimethyl carbonate as a solvent. This finding provides a new strategy for constructing a biaryl linkage.