ion from a primary OH group of glyc-erol. 223,231 A similar mechanism was proposed manyyears ago for alcohol oxidation on Pt/C, involving asecond step, the transfer of a hydride ion to the Ptsurface (Scheme 11). 8,87,237 We consider it more feasible that the rate-deter-mining step is the cleavage of the C-H bond at theR-carbon atom. A similar mechanism is now generallyaccepted for Au electrodes (Scheme 12). 238 Despite thestructural differences between Au nanoparticles andan extended Au electrode surface, there are alsosimilarities, such as the critical role of aqueousalkaline medium and the absence of deactivation dueto decomposition products (CO and C x H y frag-ments). 239,240 An important question is the nature of active siteson Au nanoparticles. Electrooxidation of ethanol onAu nanoparticles supported on glassy carbon re-quired the partial coverage of Au surface by oxides. 241 Another analogy might be the model proposed for COoxidation. 219,242,243 According to this suggestion, theactive site consists of an ensemble of metallic Auatoms and a cationic Au

Oxidation of Alcohols with Molecular Oxygen on Solid Catalysts

Treatment of a stoichiometric hydroxyapatite (HAP), Ca10(PO4)6(OH)2, with PdCl2(PhCN)2 gives a new type of palladium-grafted hydroxyapatite. Analysis by means of powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), energy-dispersive X-ray (EDX), IR, and Pd K-edge X-ray absorption fine structure (XAFS) proves that a monomeric PdCl2 species is chemisorbed on the HAP surface, which is readily transformed into Pd nanoclusters with a narrow size distribution in the presence of alcohol. Nanoclustered Pd0 species can effectively promote the alcohol oxidation under an atmospheric O2 pressure, giving a remarkably high turnover number (TON) of up to 236,000 with an excellent turnover frequency (TOF) of approximately 9800 h(-1) for a 250-mmol-scale oxidation of 1-phenylethanol under solvent-free conditions. In addition to advantages such as a simple workup procedure and the ability to recycle the catalyst, the present Pd catalyst does not require additives to complete the catalytic cycle. The diameters of the generated Pd nanoclusters can be controlled upon acting on the alcohol substrates used. Oxidation of alcohols is proposed to occur primarily on low-coordination sites within a regular arrangement of the Pd nanocluster by performing calculations on the palladium crystallites.

Hydroxyapatite-Supported Palladium Nanoclusters: A Highly Active Heterogeneous Catalyst for Selective Oxidation of Alcohols by Use of Molecular Oxygen

Asymmetric Organocatalytic Cyclization and Cycloaddition Reactions

Mg-Al mixed oxides as highly active acid-base catalysts for cycloaddition of carbon dioxide to epoxides

New developments in catalytic alcohol oxidations for fine chemicals synthesis

Halolactonization is one of the fundamental transformations in synthetic organic chemistry. This reaction provides synthetically useful products, which can be employed as synthetic intermediates for divergent transformations. A catalytic asymmetric version of this transformation would be very attractive. However, though a number of attempts to develop catalytic asymmetric halolactonization reactions have been made, and several related enantioselective halocyclizations have been developed, these reactions are still under development. Recently, highly enantioselective halolactonization reactions with organocatalysts were reported. Borhan and co-workers reported an enantioselective chlorolactonization of 4-substituted 4-pentenoic acids in the presence of hydroquinidine 1,4-phthalazinediyl diether ((DHQD)2PHAL), [4a] and Tang and co-workers reported an enantioselective bromolactonization of conjugated Z enynes with a bifunctional cinchona-alkaloid catalyst bearing a urea moiety. However, the former reaction was limited to chlorolactonization; bromoand iodolactonization were not successful, although Br and I are generally more readily transformed into various functional groups than Cl. The latter reaction is limited to particular substrates, such as conjugated Z enynes. Therefore, the development of a novel efficient method for catalytic asymmetric halolactonization is still important. During the preparation of this manuscript, Veitch and Jacobsen reported a tertiary-amine-catalyzed enantioselective iodolactonization. Herein, we present our study on organocatalytic asymmetric halolactonization. By using the structurally unique C3-symmetric trisimidazoline 1a, we developed a novel asymmetric bromolactonization of 5-substituted 5-hexenoic acids. Our working hypothesis for the development of the enantioselective bromolactonization is shown in Scheme 1. We assumed that if the alkenyl carboxylic acid and an appropriate chiral amine could form an ion pair, a chiral environment would be created. At the same time, the carboxylic acid should be activated. Bromolactonization would then proceed enantioselectively, because the olefin and the two possible bromonium intermediates would be in equilibrium in the presence of the brominating reagent, and the activated carboxylic acid, which would be in a chiral environment, should react preferentially with one of the two bromonium ions. This approach is different from recent successful approaches, which mainly involved the creation of chiral environments around the halo cations. The key to this hypothesis was the appropriate choice of a chiral amine that would have a good interaction with carboxylic acids. We envisioned that the C3-symmetric trisimidazoline 1a (Scheme 2), which we developed recently as a new organocatalyst entry, could be suitable for our working hypothesis, because an interesting interaction of the trisimidazoline derived from ethylenediamine with carboxylic acids led to the formation of 1:3 complexes in the field of material sciences (Scheme 2).

Asymmetric Bromolactonization Catalyzed by a C3‐Symmetric Chiral Trisimidazoline

Heterogeneous oxidation of allylic and benzylic alcohols catalyzed by ru-al-mg hydrotalcites in the presence of molecular oxygen

A method for conducting enantioselective bromolactonization reactions of trisubstituted alkenoic acids, using the C(3)-symmetric trisimidazoline 1 and 1,3-dibromo-5,5-dimethyl hydantoin as a bromine source, has been developed. The process generates chiral δ-lactones that contain a quaternary carbon. The results of studies probing geometrically different olefins show that (Z)-olefins rather than (E)-olefins are favorable substrates for the process. The method is not only applicable to acyclic olefin reactants but can also be employed to transform cyclic trisubstituted olefins into chiral spirocyclic lactones. Finally, the synthetic utility of the newly developed process is demonstrated by its application to a concise synthesis of tanikolide, an antifungal marine natural product.

Tomoyo Matsushita

Papers

Asymmetric Bromolactonization Catalyzed by a C3‐Symmetric Chiral Trisimidazoline

Heterogeneous oxidation of allylic and benzylic alcohols catalyzed by ru-al-mg hydrotalcites in the presence of molecular oxygen

C3‐Symmetric Trisimidazoline‐Catalyzed Enantioselective Bromolactonization of Internal Alkenoic Acids

Kinetic resolution of β-substituted olefinic carboxylic acids by asymmetric bromolactonization.

Facile preparation of oxazole-4-carboxylates and 4-ketones from aldehydes using 3-oxazoline-4-carboxylates as intermediates.