Ayako Taketoshi

Aerobic Oxidative Dehydrogenation of 2‐Substituted Imidazolines Promoted by a Cyclometalated Ruthenium Catalyst

Imidazole derivatives are chemically and pharmaceutically very important. The oxidative dehydrogenation of imidazolines is a general and reliable method of synthesizing 2-substituted imidazoles; metal salts serving as oxidants are generally employed. However, these reagents often suffer from limitations such as toxicity, explosibility, and harsh reaction conditions, and the use of large amounts of the oxidants produces serious disposal problems. Therefore, the development of a synthetic method employing aerial oxygen without the need for a co-oxidant is highly desired. In terms of catalytic oxidation, the oxidative dehydrogenation of amines and alcohols is promoted by their coordination to transition metal complexes, and there are a number of reports on the aerobic oxidation of alcohols and amines promoted by ruthenium catalysts. 5] However, to our knowledge, the transition-metal-catalyzed aerobic oxidation of imidazolines has, to date, not been achieved. We previously reported a pincer ruthenium complex incorporating a kNCN pincer ligand with imidazoline units that exhibited the oxidative dehydrogenation of the coordinated imidazoline unit with aerial oxygen to give an imidazole-ligated pincer complex. The s-donor character of the cyclometalated ligand was considered to assist the ruthenium-promoted aerobic oxidation of the coordinated imidazoline moiety. This observation persuaded us to explore the ruthenium-catalyzed oxidation of imidazolines with aerial oxygen. Several cyclometalated ruthenium complexes are known to be active catalysts in the base-cocatalyzed transfer hydrogenation of ketones, whereas no example of a cyclometalated ruthenium complex that efficiently catalyzes the oxidative dehydrogenation of 2substituted imidazolines has been reported. We report herein that a cyclometalated ruthenium complex, [RuCl(ppy)(tpy)][PF6] 1 a (ppy = 2-phenylpyridine; tpy = 2,2’:6’,2’’-terpyridine) is an excellent catalyst for the aerobic oxidative dehydrogenation of 2-substituted imidazolines, affording the corresponding imidazoles under mild conditions. In preliminary studies, the oxidation of 2-phenylimidazoline (2 a) using 1 a in methanol at 55 8C in air without an added oxidant was investigated, monitored by H NMR spectroscopy. The aerobic oxidation of 2 a was achieved by employing 1 a as the catalyst (Table 1, entry 1). The reaction proceeded much faster using molecular oxygen (101 325 Pa, balloon), whereas the same reaction under a nitrogen atmosphere gave only a

Molecular Design of Organic Superbases, Azacalix[3](2,6)pyridines: Catalysts for 1,2- and 1,4-Additions

The molecular design, characteristics, and catalytic activity of macrocyclic amino compounds, azacalix[3](2,6)pyridine derivatives, were studied. The introduction of an electron-donating group on the pyridine moiety and bridging amino phenyl group enabled the enhancement of the basicity of azacalix[3](2,6)pyridine up to pK(BH(+)) = 29.5 in CD(3)CN. These derivatives were shown to be efficient catalysts for 1,4-addition reactions of nitroalkanes or primary alcohols to α,β-unsaturated carbonyl compounds and 1,2-addition reactions of nitroalkanes to aromatic aldehydes.

Preparation of gold clusters on metal oxides by deposition-precipitation with microwave drying and their catalytic performance for CO and sulfide oxidation

Abstract Gold clusters and small nanoparticles supported on metal oxides could be prepared by deposition-precipitation followed by microwave irradiation as a drying method and then calcination. The drying method influenced the size of the Au particles. Au(III) was partly reduced during conventional oven drying, resulting in Au aggregates. In contrast, Au(III) was preserved during microwave drying owing to rapid and uniform heating, and the Au diameter was minimized to 1.4 nm on Al2O3. This method can be applied to several metal oxide supports having different microwave absorption efficiencies, such as MnO2, Al2O3, and TiO2. These catalysts exhibited higher catalytic activities for CO oxidation at low temperature and for selective aerobic oxidation of sulfide than those prepared by conventional methods.