Multielectron Reduction of Esters by a Diazabenzacenaphthenium Photoredox Catalyst.

A novel diazabenzacenaphthenium photocatalyst, N-BAP, with high photoredox abilities and visible-light absorption was designed and prepared in one step. Under visible-light irradiation, N-BAP promoted the four-electron reduction of esters in the presence of ammonium oxalate as a "traceless reductant" to generate carbinol anion intermediates that underwent protonation with water to give the corresponding alcohols. The resulting carbinol anions also exhibited nucleophilic reactivity under the photocatalytic conditions to undergo a 1,2-addition to a second carbonyl compound, affording unsymmetric 1,2-diols.

Photocatalytic Cross-Pinacol Coupling Promoted by Carbon Dioxide.

Cross-pinacol coupling of two different carbonyl compounds was achieved through successive one-electron transfer processes under photocatalytic conditions. In the reaction, an umpoled anionic carbinol synthon was generated in situ to react nucleophilically with a second electrophilic carbonyl compound. It was revealed that a CO2 additive promoted the photocatalytic generation of the carbinol synthon to suppress undesired radical dimerization. A wide variety of aromatic and aliphatic carbonyl substrates underwent the cross-pinacol coupling to afford the corresponding unsymmetric vicinal 1,2-diols, in which even a combination of carbonyl reactants with similar structures such as two aldehydes and two ketones were also well tolerated with high cross-coupling selectivity.

Surface Modification of a Supported Pt Catalyst Using Ionic Liquids for Selective Hydrodeoxygenation of Phenols into Arenes under Mild Conditions.

The selective and efficient removal of oxygenated groups from lignin-derived phenols is a critical challenge to utilize lignin as a source for renewable aromatic chemicals. This report describes how surface modification of a zeolite-supported Pt catalyst using ionic liquids (ILs) remarkably increases selectivity for the hydrodeoxygenation (HDO) of phenols into arenes under mild reaction conditions using atmospheric pressure H2 . Unmodified Pt/H-ZSM-5 converts phenols into aliphatic species as the major products along with a slight amount of arenes (10 % selectivity). In contrast, the catalyst modified with an IL, 1-butyl-3-methylimidazolium triflate, keeps up to 76 % selectivity for arenes even at a nearly complete conversion of phenols. The IL on the surface of Pt catalyst may offer the adsorption of phenols in an edge-to-face manner onto the surface, thus accelerating the HDO without the ring hydrogenation.

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 ß-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.

Detailed Structural Analysis of a Self-Assembled Vesicular Amphiphilic NCN-Pincer Palladium Complex by Using Wide-Angle X-Ray Scattering and Molecular Dynamics Calculations.

Wide-angle X-ray scattering experiments and all-atomistic molecular dynamics calculations were performed to elucidate the detailed structure of bilayer vesicles constructed by self-assembly of an amphiphilic palladium NCN-pincer complex. We found an excellent agreement between the experimental and calculated X-ray spectra, and between the membrane thickness determined from a TEM image and that calculated from an electron-density profile, which indicated that the calculated structure was highly reliable. The analysis of the simulated bilayer structure showed that in general the membrane was softer than other phospholipid bilayer membranes. In this bilayer assemblage, the degree of alignment of complex molecules in the bilayer membrane was quite low. An analysis of the electron-density profile shows that the bilayer assemblage contains a space through which organic molecules can exit. Furthermore, the catalytically active center is near this space and is easily accessible by organic molecules, which permits the bilayer membrane to act as a nanoreactor. The free energy of permeation of water through the bilayer membrane of the amphiphilic complex was 12 kJ mol-1 , which is much lower than that for phospholipid bilayer membranes in general. Organic molecules are expected to pass though the bilayer membrane. The self-assembled vesicles were shown to be catalytically active in a Miyaura-Michael reaction in water.

Instantaneous Click Chemistry by a Copper-Containing Polymeric-Membrane-Installed Microflow Catalytic Reactor.

The copper(I)-catalyzed Huisgen cycloaddition (azide-alkyne cycloaddition) is an important reaction in click chemistry that ideally proceeds instantaneously. An instantaneous Huisgen cycloaddition has been developed that uses a novel catalytic dinuclear copper complex-containing polymeric membrane-installed microflow device. A polymeric membranous copper catalyst was prepared from poly(4-vinylpyridine), copper(II) sulfate, sodium chloride, and sodium ascorbate at the interface of two laminar flows inside microchannels. Elucidation of the structure by XANES, EXAFS, and elemental analysis, as well as second-order Møller-Plesset perturbation theory (MP2) calculations and density functional theory (DFT) calculations assigned the local structure near Cu as a µ-chloro dinuclear Cu(I) complex. The microflow device promotes the instantaneous click reaction of a variety of alkynes and organic azides to afford the corresponding triazoles in quantitative yield.

A palladium-nanoparticle and silicon-nanowire-array hybrid: a platform for catalytic heterogeneous reactions.

We report the development of a silicon nanowire array-stabilized palladium nanoparticle catalyst, SiNA-Pd. Its use in the palladium-catalyzed Mizoroki-Heck reaction, the hydrogenation of an alkene, the hydrogenolysis of nitrobenzene, the hydrosilylation of an α,ß-unsaturated ketone, and the C-H bond functionalization reactions of thiophenes and indoles achieved a quantitative production with high reusability. The catalytic activity reached several hundred-mol ppb of palladium, reaching a TON of 2 000 000.

Electrophilic 1,4-Addition of Carbon Dioxide and Aldehydes to Enones.

An umpoled electrophilic 1,4-addition to enones was achieved under photocatalytic conditions. Various enones reacted with CO2 in the presence of an iridium photocatalyst and a benzimidazoline reductant under blue-light irradiation to give the corresponding γ-keto carboxylic acids. Aldehydes also coupled with enones under similar photocatalytic conditions to afford γ-keto alcohols (homoaldols) that were transformed into dihydrofurans and tetrahydrofurans through azeotropic posttreatments. Regioselective deuterium incorporation from D2O at the ß-position demonstrated that 1,4-addition takes place via homoenolate anions.

Self-assembled poly(imidazole-palladium): highly active, reusable catalyst at parts per million to parts per billion levels.

Metalloenzymes are essential proteins with vital activity that promote high-efficiency enzymatic reactions. To ensure catalytic activity, stability, and reusability for safe, nontoxic, sustainable chemistry, and green organic synthesis, it is important to develop metalloenzyme-inspired polymer-supported metal catalysts. Here, we present a highly active, reusable, self-assembled catalyst of poly(imidazole-acrylamide) and palladium species inspired by metalloenzymes and apply our convolution methodology to the preparation of polymeric metal catalysts. Thus, a metalloenzyme-inspired polymeric imidazole Pd catalyst (MEPI-Pd) was readily prepared by the coordinative convolution of (NH(4))(2)PdCl(4) and poly[(N-vinylimidazole)-co-(N-isopropylacrylamide)(5)] in a methanol-water solution at 80 °C for 30 min. SEM observation revealed that MEPI-Pd has a globular-aggregated, self-assembled structure. TEM observation and XPS and EDX analyses indicated that PdCl(2) and Pd(0) nanoparticles were uniformly dispersed in MEPI-Pd. MEPI-Pd was utilized for the allylic arylation/alkenylation/vinylation of allylic esters and carbonates with aryl/alkenylboronic acids, vinylboronic acid esters, and tetraaryl borates. Even 0.8-40 mol ppm Pd of MEPI-Pd efficiently promoted allylic arylation/alkenylation/vinylation in alcohol and/or water with a catalytic turnover number (TON) of 20,000-1,250,000. Furthermore, MEPI-Pd efficiently promoted the Suzuki-Miyaura reaction of a variety of inactivated aryl chlorides as well as aryl bromides and iodides in water with a TON of up to 3,570,000. MEPI-Pd was reused for the allylic arylation and Suzuki-Miyaura reaction of an aryl chloride without loss of catalytic activity.

Amphiphilic self-assembled polymeric copper catalyst to parts per million levels: click chemistry.

Self-assembly of copper sulfate and a poly(imidazole-acrylamide) amphiphile provided a highly active, reusable, globular, solid-phase catalyst for click chemistry. The self-assembled polymeric Cu catalyst was readily prepared from poly(N-isopropylacrylamide-co-N-vinylimidazole) and CuSO(4) via coordinative convolution. The surface of the catalyst was covered with globular particles tens of nanometers in diameter, and those sheetlike composites were layered to build an aggregated structure. Moreover, the imidazole units in the polymeric ligand coordinate to CuSO(4) to give a self-assembled, layered, polymeric copper complex. The insoluble amphiphilic polymeric imidazole Cu catalyst with even 4.5-45 mol ppm drove the Huisgen 1,3-dipolar cycloaddition of a variety of alkynes and organic azides, including the three-component cyclization of a variety of alkynes, organic halides, and sodium azide. The catalytic turnover number and frequency were up to 209000 and 6740 h(-1), respectively. The catalyst was readily reused without loss of catalytic activity to give the corresponding triazoles quantitatively.

Phenylboronic Ester-Activated Aryl Iodide-Selective Buchwald-Hartwig-Type Amination toward Bioactivity Assay.

In this study, a phenylboronic ester-activated aryl iodide-selective Buchwald-Hartwig-type amination was developed. When the reaction of aryl iodides and aryl/aliphatic amines using Ni(acac)2 is carried out in the presence of phenylboronic ester, the Buchwald-Hartwig-type amination proceeds smoothly to afford the corresponding amines in high yields. This reaction does not proceed in the absence of phenylboronic ester. A wide variety of aryl iodides can be applied in the presence of aryl chlorides and bromides, which remain intact during the reaction. The mechanistic studies of this reaction suggest that the phenylboronic ester acts as an activator for the amines to form the ″ate complex″. Chemical kinetics studies show that the reaction of aryl iodides, base, and Ni(acac)2 follows first-order kinetics, while that of amines and phenylboronic ester follows zero-order kinetics. The bioactivity screening of the corresponding products showed that some amination products exhibit antifungal activity.

Photocatalytic Carbinol Cation/Anion Umpolung: Direct Addition of Aromatic Aldehydes and Ketones to Carbon Dioxide.

We have developed a new photocatalytic umpolung reaction of carbonyl compounds to generate anionic carbinol synthons. Aromatic aldehydes or ketones reacted with carbon dioxide in the presence of an iridium photocatalyst and 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzimidazole (DMBI) as a reductant under visible-light irradiation to furnish the corresponding α-hydroxycarboxylic acids through nucleophilic addition of the resulting carbinol anions to electrophilic carbon dioxide.

Palladium membrane-installed microchannel devices for instantaneous Suzuki-Miyaura cross-coupling.

Instantaneous catalytic carbon-carbon bond-forming reactions were achieved in catalytic membrane-installed microchannel devices that have a polymeric palladium-complex membrane. The catalytic membrane-installed microchannel devices were provided inside the microchannels by means of coordinative and ionic molecular convolution at the interface between the organic and aqueous phases flowing laminarly, in which both non-crosslinked linear polymer ligands and palladium species dissolved. The palladium-catalyzed Suzuki-Miyaura reaction of aryl, heteroaryl, and alkenyl halides with arylboronic acids and sodium tetraarylborates was performed with the catalytic membrane-installed microchannel devices to give quantitative yields of biaryls, heterobiaryls, and aryl alkenes within 5 s of residence time in the defined channel region. These microchannel devices were applied to the instantaneous allylic arylation reaction of allylic esters with arylboron reagents under microflow conditions to afford the corresponding coupling products within 1 s of residence time.

Catalytic Reductive Alkylation of Amines in Batch and Microflow Conditions Using a Silicon-Wafer-Based Palladium Nanocatalyst.

We describe the development of the catalytic reductive alkylation of amines with aldehydes under the atmospheric pressure of H2 using a brush-like silicon-nanostructure-supported palladium nanoparticle composite (SiNS-Pd) as a silicon-wafer-based reusable heterogeneous catalyst. The present reaction of primary and secondary amines with various aliphatic and aromatic aldehydes in the presence of the catalyst (0.02-0.05 mol % Pd) gave the corresponding secondary and tertiary amines including Lomerizine and Aticaprant in a 68% quantitative yield without overalkylation. We also designed and fabricated a flow device equipped with SiNS-Pd for microflow reactions, which was applied to the gas-liquid-solid triphasic reaction system (i.e., H2 gas, a substrate solution, and a solid catalyst). A multigram-scale reaction of aniline and benzaldehyde was demonstrated to obtain N-benzylaniline (ca. 4 g/day), in which the internal volume of the flow channel was 43 µL, the residence time was approximately 1 s, and the turnover number (TON) reached 4.0 × 104 in a continuous 24 h run (1.7 × 103 h-1; 0.50 s-1).

Metallically gradated silicon nanowire and palladium nanoparticle composites as robust hydrogenation catalysts.

Heterogeneous catalysis of alkenes to alkanes is of great importance in chemical industry, but more efficient and reusable heterogeneous catalysts are still demanded. Here, we report a metallically gradated composite of a silicon nanowire array and palladium nanoparticles which are reused for the hydrogenation of an alkene. The catalyst promotes the hydrogenation of stilbene with atmospheric hydrogen (0.1 MPa) to give diphenylethane quantitatively. The recovered catalyst can be reused, and mediates the reaction without loss of yield more than one hundred times, whereas the stability of Pd/C degrades rapidly over 10 cycles of reuse. The catalyst allows the hydrogenation of a variety of alkenes, including tetra-substituted olefins. Structural investigation reveals that palladium nanoparticles are metallically gradated onto the silicon nanowire array under mild conditions by agglomeration of palladium silicide, as confirmed by XAFS and XPS together with argon-ion sputtering. This means of metal agglomeration immobilization may be applicable to the preparation of a variety of metal nanoparticle catalysts.

Activator-Promoted Aryl Halide-Dependent Chemoselective Buchwald-Hartwig and Suzuki-Miyaura Type Cross-Coupling Reactions.

Herein, we report the development of aryl halide-dependent chemoselective reactions, viz., the Buchwald-Hartwig type coupling reaction of an aryl iodide with an arylboronic acid and an aryl amine in the presence of a heterogeneous and reusable nickel catalyst and the Suzuki-Miyaura type coupling of an aryl chloride under similar conditions. Control experiments revealed that the presence of stoichiometric amounts of the phenylboronic acid/ester and aryl amine are essential for both reactions. NMR and XAFS studies suggested the formation of a boron-amine "ate" complex.

Second-Generation meta-Phenolsulfonic Acid-Formaldehyde Resin as a Catalyst for Continuous-Flow Esterification.

A second-generation m-phenolsulfonic acid-formaldehyde resin (PAFR II) catalyst was prepared by condensation polymerization of sodium m-phenolsulfonate and paraformaldehyde in an aqueous H2SO4 solution. This reusable, robust acid resin catalyst was improved in both catalytic activity and stability, maintaining the characteristics of the previous generation catalyst (p-phenolsulfonic acid-formaldehyde resin). PAFR II was applied in the batchwise and continuous-flow direct esterification without water removal and provided higher product yields in continuous-flow esterification than any other commercial ion-exchanged acid catalyst tested.

Development of an amphiphilic resin-dispersion of nanopalladium and nanoplatinum catalysts: design, preparation, and their use in green organic transformations.

An amphiphilic polymer resin-dispersion of nanoparticles of palladium was designed and prepared with a view toward use for catalysis in water. The amphiphilic polystyrene-poly(ethylene glycol) (PS-PEG) resin-dispersion of nanoparticles of palladium exhibited high catalytic performance in the hydrodechlorination of chloroarenes under aqueous conditions. The amphiphilic resin-supported nanopalladium and nanoplatinum particles also catalyzed aerobic oxidation of various alcohols including nonactivated aliphatic and alicyclic alcohols, which is one of the most fundamental and important yet immature processes in organic chemistry, in water under an atmospheric pressure of oxygen gas to form aldehydes, ketones, and carboxylic acids to meet green chemical requirements. Viologen polymer-supported nanopalladium catalyst realized alpha-alkylation of ketones with primary alcohols as the alkylating agents.

Asymmetric Suzuki-Miyaura coupling in water with a chiral palladium catalyst supported on an amphiphilic resin.

You oughta use water: Broad functional-group (FG) tolerance was observed for the title coupling of aryl halides (X = Cl, Br, I) and aryl boronic acids to give biaryl compounds with up to 94% ee. The chiral imidazoindole phosphine-palladium catalyst supported on an amphiphilic polystyrene-poly(ethylene glycol) (PS-PEG) resin could be recycled readily.

Oxidative cyclization of alkenols with oxone using a miniflow reactor.

A miniflow system for oxidative cyclization of alkenols with Oxone was developed. Thus, the oxidative cyclization of (Z)- and (E)-alkenols in i-PrOH with an aqueous solution of Oxone proceeded smoothly and safely in a PTFE tube without any exogenous catalytic species, and was subsequently quenched in a flow-reaction manner to afford the corresponding furanyl and pyranyl carbinols quantitatively within 5 or 10 min of residence time.