Takao Osako

Highly efficient iron(0) nanoparticle-catalyzed hydrogenation in water in flow

Highly efficient catalytic hydrogenations are achieved by using amphiphilic polymer-stabilized Fe(0) nanoparticle (Fe NP) catalysts in ethanol or water in a flow reactor. Alkenes, alkynes, aromatic imines and aldehydes were hydrogenated nearly quantitatively in most cases. Aliphatic amines and aldehydes, ketone, ester, arene, nitro, and aryl halide functionalities are not affected, which provides an interesting chemoselectivity. The Fe NPs used in this system are stabilized and protected by an amphiphilic polymer resin, providing a unique system that combines long-term stability and high activity. The NPs were characterized by TEM of microtomed resin, which established that iron remains in the zero-valent form despite exposure to water and oxygen. The amphiphilic resin-supported Fe(0) nanoparticles in water and in flow provide a novel, robust, cheap and environmentally benign catalyst system for chemoselective hydrogenations.

Enantioselective carbenoid insertion into phenolic O-H bonds with a chiral copper(I) imidazoindolephosphine complex.

The enantioselective O-H carbenoid insertion reaction with a new chiral copper(I) imidazoindolephosphine complex has been developed. The chiral copper(I) complex catalyzed the insertion of carbenoids derived from α-diazopropionates into the O-H bonds of various phenol derivatives to give the corresponding α-aryloxypropionates with up to 91% ee.

Enantioposition-selective copper-catalyzed azide-alkyne cycloaddition for construction of chiral biaryl derivatives.

A highly enantioposition-selective copper-catalyzed azide-alkyne cycloaddition (CuAAC) of dialkynes bearing prochiral biaryls has been developed for the construction of 1,2,3-triazoles bearing axially chiral biaryl groups in up to 76% yield and up to 99% ee.

Aerobic flow oxidation of alcohols in water catalyzed by platinum nanoparticles dispersed in an amphiphilic polymer

We have developed a technique for the aqueous aerobic flow oxidation of alcohols in a continuous-flow reactor containing platinum nanoparticles dispersed on an amphiphilic polystyrene–poly(ethylene glycol) resin (ARP-Pt). Various primary and secondary alcohols including aliphatic, aromatic and heteroaromatic alcohols were efficiently oxidized within 73 seconds in a flowing aqueous system at 100–120 °C under 40–70 bar of the system pressure to give the corresponding carboxylic acids and ketones, respectively, in up to 99% yield. Benzaldehydes could be also prepared selectively from benzyl alcohols by conducting the flow oxidation under the standard conditions in the presence of triethylamine. Moreover, a practical gram-scale synthesis of surfactants was realized in the aqueous aerobic continuous flow oxidation for 36–116 hours. This aerobic flow oxidation system provides a safe, clean, green, rapid and efficient practical method for oxidizing alcohols.

Continuous-flow hydrogenation of olefins and nitrobenzenes catalyzed by platinum nanoparticles dispersed in an amphiphilic polymer

A method for the flow hydrogenation of olefins and nitrobenzenes in a continuous-flow reactor containing platinum nanoparticles dispersed on an amphiphilic polystyrene–poly(ethylene glycol) resin (ARP-Pt) was developed. The hydrogenation of olefins and nitrobenzenes was completed within 31 seconds in the continuous-flow system containing ARP-Pt, giving the corresponding hydrogenated products in up to 99% yield with good chemoselectivity. Moreover, long-term (63–70 h) continuous-flow hydrogenation of styrene and nitrobenzene produced more than ten grams of ethylbenzene and aniline, respectively, without significant loss of catalytic activity. The flow hydrogenation system provides an efficient and practical method for the chemoselective reduction of olefins and nitrobenzenes.

Aqueous Asymmetric 1,4-Addition of Arylboronic Acids to Enones Catalyzed by an Amphiphilic Resin-Supported Chiral Diene Rhodium Complex under Batch and Continuous-Flow Conditions

A rhodium-chiral diene complex immobilized on amphiphilic polystyrene-poly(ethylene glycol) (PS-PEG) resin (PS-PEG-diene*-Rh) has been developed. The immobilized rhodium-chiral diene complex (PS-PEG-diene*-Rh) efficiently catalyzed the asymmetric 1,4-addition of various arylboronic acids to cyclic or linear enones in water under batch conditions to give the corresponding β-arylated carbonyl compounds in excellent yields and with excellent enantioselectivity. The catalyst was readily recovered by simple filtration and reused 10 times without loss of its catalytic activity and enantioselectivity. Moreover, a continuous-flow asymmetric 1,4-addition in a flow reactor containing PS-PEG-diene*-Rh proceeded efficiently at 50 °C with retention of high enantioselectivity. Long-term continuous-flow asymmetric 1,4-addition during 12 h readily gave the desired product on a 10 g scale with high enantioselectivity.

Olefin Metathesis with Ruthenium–Carbene Supported on Iron Oxide

Significance: The Grubbs–Hoveyda ruthenium– carbene complex supported on ionic magnetic nanoparticles (Ru@IMNPs) was prepared by immobilization of imidazolium chloride 1 onto Fe3O4 2, anion exchange with NaPF6, and metathesis with ruthenium complex 3 (eq. 1). Ru@IMNPs catalyzed the ring-closing metathesis of dienes to give the corresponding cyclic olefins in 90–99% yield (eq. 2). The cross-metathesis of methyl acrylate with olefins using Ru@IMNPs also proceeded with high E-selectivity (eq. 3). Comment: In the ring-closing metathesis of N,Ndiallyl tosylamide, the catalyst was recovered magnetically and reused five times without significant loss of catalytic activity (6th run: 96% conversion), although ICP-MS analysis showed significant leaching of the ruthenium species into the product (a loss of 54% of the ruthenium content of the fresh catalyst) during the initial three runs of the recycling experiment. Results: Si(OMe)3 N N O O

Ullmann-Type Coupling of Phenols with Aryl Halides on Cuprian Zeolite USY

Significance: Copper(I)-exchanged zeolite USY (CuI-USY) catalyzed the Ullmann-type coupling of phenols with aryl iodides or bromides in the presence of cesium carbonate to give the corresponding diaryl ethers in up to 86% yield. In the reaction of 3,5-dimethylphenol with iodobenzene, the catalyst was recovered by simple filtration and reused four times without loss of catalytic activity. Comment: The authors have previously reported a Huisgen cycloaddition and a Glaser coupling with CuI-USY (Org. Lett. 2007, 9, 883; Eur. J. Org. Chem. 2009, 423). The catalytic activity of CuIUSY for the Ullmann-type coupling was superior to that of the other Cu(I) zeolites, such as CuI-MOR, CuI-β, or CuI-ZSM5. CuI-USY was ineffective for the reactions of 4-cyanoor 4-nitrophenols with phenyl halides. ICP-AES analysis revealed that no copper leached from the catalyst during the reaction. OH R1 X R2 + CuI-USY (10 mol% Cu) Cs2CO3 (2.0 equiv) PhMe or DMF 120–140 °C, 24 h 1.5 equiv O R1 R2

Silylation of Alcohol Derivatives with 1,2-Disilanes Catalyzed by Au/TiO2

Significance: Gold nanoparticles supported on titanium dioxide (Au/TiO2) catalyzed the silylation of water and primary, secondary, and tertiary aliphatic alcohols with 1,2-disilanes via Si–Si bond cleavage to give the corresponding silyl ethers in up to >99% yield (eq. 1). When tertiary benzylic alcohols were used for the reaction, the reduction proceeded to afford the corresponding alkanes as the major products (eq. 2). Comment: The authors previously reported the oxidative cycloaddition of 1,1,3,3-tetramethyldisiloxane to alkynes catalyzed by Au/TiO2 (J. Am. Chem. Soc. 2011, 133, 10426). The catalytic activity of Au/TiO2 for the silylation of water was superior to that of gold nanoparticles supported on other supports such as aluminum oxide (Al2O3) and zinc oxide (ZnO). Typical results: + Si Si R4 R4 R2 R2 R3 R3 Au/TiO2 (1 mol% Au) EtOAc, 25–55 °C 1–16 h Si Si R4 R4 R2 R2 R3 R3 O H H O

Asymmetric Ring-Opening of meso-Epoxides with a Chiral Metal-Organic Framework

1355 S . R E G A T I , Y . HE , M . T H I M M A I A H , P . L I , S . X I A N G, B . C H E N, * J . C . G . Z H A O * ( U N I V E R S I T Y OF TE X A S AT SA N A N T O N I O , US A A N D F UJ I A N N OR M A L U N I V E R S I T Y, F U Z H O U , P. R . O F C H I N A ) Enantioselective Ring-Opening of meso-Epoxides by Aromatic Amines Catalyzed by a Homochiral Metal-Organic Framework Chem. Commun. 2013, 49, 9836–9838.