A new, efficient preparation has been devised for potassium ferrate(VI) (K(2)FeO(4)). The ability of this high-valent iron salt for oxidizing organic substrates in nonaqueous media was studied. Using benzyl alcohol as a model, the catalytic activity of a wide range of microporous adsorbents was ascertained. Among numerous solid supports of the aluminosilicate type, the K10 montmorillonite clay was found to be best at achieving quantitative formation of benzaldehyde, without any overoxidation to benzoic acid. The roles of the various parameters (reaction time and temperature, nature of the solvent, method of preparation of the solid reagent) were investigated. The evidence points to a polar reaction mechanism. The ensuing procedure was applied successfully, at room temperature, to oxidation of a series of alcohols to aldehydes and ketones, to oxidative coupling of thiols to disulfides, and to oxidation of nitrogen derivatives. At 75 degrees C, the reagent has the capability of oxidizing both activated and nonactivated hydrocarbons. Toluene is turned into benzyl alcohol (and benzaldehyde). Cycloalkanes are also oxidized, in significant (30-40%) yields, to the respective cycloalkanols (and cycloalkanones). Thus, potassium ferrate, used in conjunction with an appropriate heterogeneous catalyst, is a strong and environmentally friendly oxidant.

N‐Heterocyclic carbenes (NHCs) form stable zwitterionicadducts with a range of heteroallenes, ketenes, and allenes. Although the first representatives of this class of inner salts were first investigated as far back as the 1960s, they have enjoyed a sustained interest from the chemical community over the years. Depending on the nature of their anionic moiety, NHC betaines display a very broad palette of reactivities and have found applications in various fields of organic synthesis and catalysis. In this Microreview, the synthesis, properties, and reactivity of NHC betaines are surveyed. The NHCs under consideration include ylidenes derived from imidazole, benzimidazole, imidazoline, thiazole, or triazole, and the heteroallenes investigated so far are carbon dioxide, carbon disulfide, isocyanates, isothiocyanates, and their selenium analogues. A historical background is provided for each type of adduct under consideration, but emphasis is placed mainly on developments that have appeared in the literature within the past few years. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

Five imidazol(in)ium-2-dithiocarboxylates bearing cyclohexyl, mesityl, or 2,6-diisopropylphenyl substituents on their nitrogen atoms were prepared from the corresponding Nheterocyclic carbenes (NHCs) by reaction with carbon disulfide. They were characterized by IR, UV/Vis, and NMR spectroscopy, and by thermogravimetric analysis. Their molecular structures were determined by X-ray diffraction. For the sake of comparison, tricyclohexylphosphonium dithiocarboxylate was also examined. The data acquired were scrutinized to evaluate their usefulness for assessing the steric and elec-

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New 1,3‐diarylimidazol(in)ium chlorides bearing phenyl, 1‐naphthyl, 4‐biphenyl, 2‐tolyl, 2,6‐dimethylphenyl, and 3,5‐dimethylphenyl substituents were synthesized. They were combined with [RuCl2(p‐cymene)]2 and potassium tert‐butoxide or sodium hydride to generate the corresponding ruthenium‐N‐heterocyclic carbene complexes in situ. Catalyst precursors derived from imidazol(in)ium salts bearing the 2,4,6‐trimethylphenyl (mesityl) and the 2,6‐diisopropylphenyl groups were also prepared. The catalytic activity of all these species in the photoinduced ring‐opening metathesis polymerization of cyclooctene was investigated. The C4‐C5 double bond in the imidazole ring of the N‐heterocyclic carbene ligands was not crucial to achieve high catalytic efficiencies. The presence or the absence of alkyl groups on the ortho positions of the phenyl rings had a more pronounced influence. Blocking all the ortho positions was a requisite for obtaining efficient catalysts. Failure to do so probably results in the ortho‐metallation of the carbene ligand, thereby altering the coordination sphere of the ruthenium active centers.

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where L = 1,3-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene (3a) or 1,3-bis(2,4,6-trimethylphenyl)-4,5-dichloroimidazolin-2-ylidene (3b), were isolated in high yields upon heating a toluene solution of [RuCl 2 A C H T U N G T R E N N U N G (p-cymene)] 2 with 1 equivalent of carbene ligand under an ethylene atmosphere. They were characterized by NMR and TGA. Their catalytic activity was investigated in the atom transfer radical polymerization of vinyl monomers. In the polymerization of methyl methacrylate, complex 3a displayed faster reaction rates than 3b and the related phosphine-based complex 2a (L = tricyclohexylphosphine), although control was more effective with the latter catalyst. When n-butyl acrylate or styrene served as monomer, a major shift of reactivity was observed between complex 2a that promoted controlled radical polymerization, and complexes 3a or 3b that favored metathetical coupling. Further homocoupling experiments with various styrene derivatives confirmed the outstanding aptitude of complex 3a (and to a lesser extent of 3b) to catalyze olefin metathesis reactions. Contrary to monometallic ruthenium-arene complexes of the [RuCl 2 A C H T U N G T R E N N U N G (p-cymene)(L)] type, the new homobimetallic species did not require the addition of a diazo compound or visible light illumination to initiate the ring-opening metathesis of norbornene or cyclooctene. When a,w-dienes were exposed to 3a or 3b, a mixture of cycloisomerization and ring-closing metathesis products was obtained in a non-selective way. Addition of a terminal alkyne co-catalyst enhanced the metathetical activity while completely repressing the cycloisomerization process. Thus, quantitative conversions of diethyl 2,2-diallylmalonate and N,N-diallyltosylamide were achieved within 2 h at room temperature using 2 mol % of catalyst precursor 3 a and 6 mol % of phenylacetylene.