Akifumi Noujima

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.

Selective deoxygenation of epoxides to alkenes with molecular hydrogen using a hydrotalcite-supported gold catalyst: a concerted effect between gold nanoparticles and basic sites on a support.

Direct conversion of epoxides into the corresponding alkenes is an important reaction because it allows the use of oxirane rings as protecting groups for carbon–carbon double bonds. This transformation also occurs in the production of vitamin K in the human body and is useful for quantification of epoxide moieties in graphite epoxide or oxygenated carbon nanotubes. Traditionally, the deoxygenation of epoxides to alkenes has been conducted using stoichiometric amounts of reagents, which results in the production of large amounts of undesirable waste. To date, several catalytic deoxygenations using PPh3, Na/Hg, and NaBH4 as reductants have been reported. These catalysts, however, suffer from low activity, low atom efficiency, and tedious work-ups with moisturesensitive reaction conditions. An ideal “green” protocol for the catalytic deoxygenation of epoxides is the use of molecular hydrogen (H2) as a reducing reagent because, theoretically, water is the only by-product. However, the use of H2 often causes nonselective reduction of epoxides to yield alcohols and alkanes as by-products through hydrogenation of the epoxides and overhydrogenation of the desired alkenes, respectively. Although there are a few successful reports on the selective deoxygenation of epoxides using H2, selectivity for alkenes is restricted to low conversion levels and a limited range of substrates. 6] Therefore, the development of an efficient catalytic system for the selective deoxygenation of epoxides to the corresponding alkenes using H2 is highly desired. Recently, we discovered that heterogeneous gold and silver nanoparticle (NP) catalysts have high activities for the deoxygenation of various epoxides to alkenes with > 99% selectivity, using 2-propanol used as an environmentally friendly reductant. Furthermore, CO/H2O was found to work as an alternative reductant for the selective deoxygenation of epoxides to alkenes in water under mild reaction conditions. Herein, we demonstrate that gold NPs supported on hydrotalcite [HT: Mg6Al2(OH)16CO3·nH2O] (Au/HT) can act as a highly efficient heterogeneous catalyst for the deoxygenation of epoxides to alkenes with H2 used as an ideal reductant. Au/HT is applicable to various epoxides, and selectivities for alkenes are over 99% at high conversions. After the reaction, solid Au/HT can be easily recovered from the reaction mixture and reused with no decrease in its catalytic efficiency. The deoxygenation of styrene oxide (1a) using various inorganic-materials-supported Au NPs was carried out in toluene at 80 8C under 1 atm of H2 (Table 1). Among the Au NP catalysts tested, Au/HT exhibited the highest activity toward this deoxygenation to afford styrene (2a) in 95 % yield with a small amount of the overhydrogenated product ethylbenzene (3a ; Table 1, entry 1). Au/CeO2 and Au/Al2O3 also converted 1a, but selectivities for 2a were much lower than that of Au/HT (Table 1, entries 4 and 5). Interestingly, Au/TiO2 showed the highest selectivity for 2a, although the conversion of 1a was low (Table 1, entry 6). Au/SiO2 did not have any catalytic activity for this reaction (Table 1, entry 7). Notably, when the reaction temperature was lowered to 60 8C, Au/HT produced 2a as the sole product in quantitative yield with > 99% selectivity (Table 1, entry 2). Moreover, the carbon–carbon double bond of 2a was completely intact when the reaction time was prolonged (Table 1, entry 3). Next, various HT-supported metal NPs were examined in this reaction (Table 1, entries 8–13). Ag/HT, Ru/HT, Rh/ HT and Cu/HT did not function as catalysts (Table 1, entries 10–13). In the case of Pd/HT and Pt/HT, hydrogenation of 1a occurred to give 2-phenylethanol (4a), but no deoxygenated product was obtained (Table 1, entries 8 and 9). These results clearly revealed that the combination of Au NPs and HT had the best catalytic activity and selectivity toward the deoxygenation of epoxides to alkenes using H2. Scheme 1 shows the hydrogenation of 2a in the presence or absence of p-methylstyrene oxide (1b) using Au/HT or Au/ [*] A. Noujima, Dr. T. Mitsudome, Dr. T. Mizugaki, Prof. Dr. K. Jitsukawa, Prof. Dr. K. Kaneda Department of Materials Engineering Science Graduate School of Engineering Science Osaka University, 1-3, Machikaneyama Toyonaka, Osaka 560-8531 (Japan) Fax: (+ 81)6-6850-6260 E-mail: kaneda@cheng.es.osaka-u.ac.jp

Supported gold nanoparticles as a reusable catalyst for synthesis of lactones from diols using molecular oxygen as an oxidant under mild conditions

The oxidative lactonization of various diols using molecular oxygen as a primary oxidant can be efficiently catalyzed by hydrotalcite-supported Au nanoparticles (Au/HT). For instance, lactonization of 1,4-butanediol gave γ-butyrolactone with an excellent turnover number of 1400. After lactonization, the Au/HT can be recovered by simple filtration and reused without any loss of its activity and selectivity.

Gold Nanoparticle-Catalyzed Environmentally Benign Deoxygenation of Epoxides to Alkenes

We have developed a highly efficient and green catalytic deoxygenation of epoxides to alkenes using gold nanoparticles (NPs) supported on hydrotalcite [HT: Mg6Al2CO3(OH)16] (Au/HT) with alcohols, CO/H2O or H2 as the reducing reagent. Various epoxides were selectively converted to the corresponding alkenes. Among the novel metal NPs on HT, Au/HT was found to exhibit outstanding catalytic activity for the deoxygenation reaction. Moreover, Au/HT can be separated from the reaction mixture and reused with retention of its catalytic activity and selectivity. The high catalytic performance of Au/HT was attributed to the selective formation of Au-hydride species by the cooperative effect between Au NPs and HT.

Highly Efficient Etherification of Silanes by Using a Gold Nanoparticle Catalyst: Remarkable Effect of O2

O2 is acting! A nanosized hydroxylapatite-supported Au nanoparticle (NP) catalyst exhibited high activity under aerobic conditions, and various silyl ethers could be obtained from diverse combinations of silanes with alcohols. Moreover, O2 was found to act not as a stoichiometric oxidizing reagent, but as a non-consumed promoter, significantly boosting the catalytic activity of AuNPs.

Gold nanoparticle-catalyzed cyclocarbonylation of 2-aminophenols

Gold nanoparticles supported on hydrotalcite acted as a highly efficient catalyst for the cyclocarbonylation of 2-aminophenols to 2-benzoxazolinones without any additives. After the cyclocarbonylation, the hydrotalcite-supported gold nanoparticles could be reused without loss of catalytic efficiency.