Yusuke Mikami

Design of a Silver–Cerium Dioxide Core–Shell Nanocomposite Catalyst for Chemoselective Reduction Reactions

The interaction of metals with ligands is the key factor in the design of catalysts and much effort has been devoted to the rational control of metal–ligand interactions in order to exploit catalytic properties. Quite sophisticated heterogeneous catalysts have been produced by controlling the size and shape of active metal species, and by screening and altering the composition of the supports. The supports can be considered as “macro ligands” for supported active metals, and the fine-tuning of the interactions between active metal species and supports is the most important factor through which high catalytic performance can be attained. Despite many intrinsic advantages of heterogeneous catalysts over homogeneous ones, such as their durability at high temperatures and reusability, the fine-tuning of metal–ligand interactions in heterogeneous catalysts is more difficult than in homogeneous catalysts, and remains a challenging objective. Our research group has recently reported that silver nanoparticles (AgNPs) on a basic support of hydrotalcite (Ag/HT) catalyzed the chemoselective reductions of nitrostyrenes and epoxides to the corresponding anilines and alkenes when using alcohols or CO/H2O as a reducing reagent while retaining the reducible C=C bonds. During the reductions, polar species of hydrides and protons were formed in situ at the interface of AgNPs/HT through a cooperative effect between the AgNPs and basic sites (BS) of HT, which were then exclusively active for the reduction of the polar functional groups (Figure 1). However, the use of H2 instead of alcohols or CO/H2O in our Ag catalyst system caused reductions of both the polar groups (nitro and epoxide) and the nonpolar C=C bonds. This nonselective reduction was due to the formation of nonpolar hydrogen species through the homolytic cleavage of H2 at the AgNPs surface, which is active for C=C bond reduction (Figure 2a).

Supported Gold and Silver Nanoparticles for Catalytic Deoxygenation of Epoxides into Alkenes

Some metal nanoparticles (NPs) have been shown to have unprecedented catalytic performance which far exceeds those of conventional metal complex catalysts. Especially, gold and silver NPs are known to exhibit outstanding catalytic ability in the aerobic epoxidation of propene and styrene, and in the industrial epoxidation of ethylene, respectively. Our recent research has focused on the catalytic potential of coinage-metal NPs under liquid phase conditions, wherein we found that gold, silver, and copper NPs supported on inorganic materials had unique catalytic properties for versatile organic synthesis such as the aerobic oxidation of alcohols, 9] oxidation of silanes into silanols using water, and hydration of nitriles. In the course of our study on the coinage-metal NP catalysis for the aerobic oxidation of alcohols, we envisioned that if epoxides could act as hydrogen accepters in place of molecular oxygen, the metal NPs could act as effective catalysts for reverse epoxidation, namely, deoxygenation of epoxides using alcohols as oxygen accepters (Scheme 1). Deoxygenation of epoxides into alkenes is important reaction in both organic synthesis and biological chemistry; for example, in the deprotection of oxirane rings and in the reproduction of vitamin K in the vitamin K cycle. Stoichiometric deoxygenations of epoxides have been carried out using a variety of reagents including phosphines, silanes, iodides, and heavy metals. However, these reagents are often toxic or employed in large excess, resulting in the production of undesired waste. In addition, most of these methods require special handling and harsh reaction conditions, which may affect other sensitive functional groups in the parent molecules. Several successful catalyst systems have appeared to date such as Re complexes with triphenylphosphine, a Fe complex with NaBH4, [18] and a Co complex with Na, but these systems suffer from the need for hazardous reductants, inert conditions, and display low catalytic activities (TOFs < 13 h , TONs< 20; TOF = turnover frequency, TON = turnover number), and low atom efficiencies. Therefore, the development of an efficient catalytic system for deoxygenation of epoxides remains of great importance. Herein, we discovered the intrinsic ability of gold and silver NPs in catalyzing deoxygenation of epoxides; gold and silver NPs supported on an inorganic material of hydrotalcite (HT; Au/HT and Ag/HT) allow highly efficient catalytic deoxygenation of epoxides into alkenes using alcohols. The selectivity for all the alkene products were over 99%, and excellent turnover number was achieved. To the best of our knowledge, this is the first report on the catalytic deoxygenation of epoxides using gold and silver NPs. The Au/HT and Ag/HT catalyst systems described herein offer a green protocol for removing oxygen from epoxides with the following advantages: 1) high catalytic activity and selectivity; 2) the use of safe and easy-to-handle catalysts and reducing reagents; 3) applicability to a wide range of epoxides; 4) a simple purification procedure owing to easy separation of the solid catalysts from the reaction mixtures; and 5) recyclebility of the catalysts without any loss in their efficiency. The discovery of this unique catalysis of gold and silver NPs will open new routes to selective functional transformations in organic synthesis. Scheme 1. The oxidation of alcohols using O2 versus deoxygenation of epoxides using alcohols.

OXIDANT-FREE LACTONIZATION OF DIOLS USING A HYDROTALCITE-SUPPORTED COPPER CATALYST

We newly synthesized well-crystallized hydrotalcite-supported copper nanoparticles, denoted as Cu/HT(c), which acted as a highly efficient heterogeneous catalyst for oxidant-free lactonization of various diols under liquid-phase conditions. The Cu/HT(c) catalyst could be recovered by simple filtration and reused without the significant loss of its activity and selectivity.