Environmentally Friendly γ‐MnO2 Hexagon‐Based Nanoarchitectures: Structural Understanding and Their Energy‐Saving Applications

Although about 200,000 metric tons of γ‐MnO₂ are used annually worldwide for industrial applications, the γ‐MnO₂ structure is still known to possess a highly ambiguous crystal lattice. To better understand the γ‐MnO₂ atomic structure, hexagon‐based nanoarchitectures were successfully synthesized and used to elucidate its internal structure for the present work. The structural analysis results, obtained from the hexagon‐based nanoarchitectures, clearly show the coexistence of akhtenskite (ε‐MnO₂), pyrolusite (β‐MnO₂), and ramsdellite in the so‐called γ‐MnO₂ phase and verified the heterogeneous phase assembly of the γ‐MnO₂ state, which violates the well‐known “De Wolff” model and derivative models, but partially accords with Heuer's results. Furthermore, heterogeneous γ‐MnO₂ assembly was found to be a metastable structure under hydrothermal conditions, and the individual components of the heterogeneous γ‐MnO₂ system have structural similarities and a high lattice matches with pyrolusite (β‐MnO₂). The as‐obtained γ‐MnO₂ nanoarchitectures are nontoxic and environmentally friendly, and the application of such nanoarchitectures as support matrices successfully mitigates the common problems for phase‐change materials of inorganic salts, such as phase separation and supercooling‐effects, thereby showing prospect in energy‐saving applications in future “smart‐house” systems.     

Efficient Noble‐Metal‐Free γ‐Fe2O3@NiO Core–Shell Nanostructured Photocatalysts for Water Oxidation

Flowerlike noble‐metal‐free γ‐Fe₂O₃@NiO core–shell hierarchical nanostructures have been fabricated and examined as a catalyst in the photocatalytic oxidation of water with [Ru(bpy)₃](ClO₄)₂ as a photosensitizer and Na₂S₂O₈ as a sacrificial electron acceptor. An apparent TOF of 0.29 μmols^?1 m^?2 and oxygen yield of 51 % were obtained with γ‐Fe₂O₃@NiO. The γ‐Fe₂O₃@NiO core–shell hierarchical nanostructures could be easily separated from the reaction solution whilst maintaining excellent water‐oxidation activity in the fourth and fifth runs. The surface conditions of γ‐Fe₂O₃@NiO also remained unchanged after the photocatalytic reaction, as confirmed by X‐ray photoelectron spectroscopy (XPS).     

Manganese‐Catalyzed β‐Methylation of Alcohols by Methanol

We report an earth‐abundant‐metal‐catalyzed double and single methylation of alcohols. A manganese catalyst, which operates at low catalyst loadings and short reaction times, mediates these reactions efficiently. A broad scope of primary and secondary alcohols, including purely aliphatic examples, and 1,2‐aminoalcohols can be methylated. Furthermore, alcohol methylation for the synthesis of pharmaceuticals has been demonstrated. The catalyst system tolerates many functional groups among them hydrogenation‐sensitive examples and upscaling is easily achieved. Mechanistic investigations are indicative of a borrowing hydrogen or hydrogen autotransfer mechanism involving a bimetallic K‐Mn catalyst. The catalyst accepts hydrogen as a proton and a hydride from alcohols efficiently and reacts with a chalcone via hydride transfer.     

Manganese‐Catalyzed anti‐Selective Asymmetric Hydrogenation of α‐Substituted β‐Ketoamides

A Mn‐catalyzed diastereo‐ and enantioselective hydrogenation of α‐substituted β‐ketoamides has been realized for the first time under dynamic kinetic resolution conditions. anti‐α‐Substituted β‐hydroxy amides, which are useful building blocks for the synthesis of bioactive molecules and chiral drugs, were prepared in high yields with excellent selectivity (up to >99 % dr and >99 % ee) and unprecedentedly high activity (TON up to 10000). The origin of the excellent stereoselectivity was clarified by DFT calculations.     

Hierarchical β‐Mo2C Nanotubes Organized by Ultrathin Nanosheets as a Highly Efficient Electrocatalyst for Hydrogen Production

Production of hydrogen by electrochemical water splitting has been hindered by the high cost of precious metal catalysts, such as Pt, for the hydrogen evolution reaction (HER). In this work, novel hierarchical β‐Mo₂C nanotubes constructed from porous nanosheets have been fabricated and investigated as a high‐performance and low‐cost electrocatalyst for HER. An unusual template‐engaged strategy has been utilized to controllably synthesize Mo‐polydopamine nanotubes, which are further converted into hierarchical β‐Mo₂C nanotubes by direct carburization at high temperature. Benefitting from several structural advantages including ultrafine primary nanocrystallites, large exposed surface, fast charge transfer, and unique tubular structure, the as‐prepared hierarchical β‐Mo₂C nanotubes exhibit excellent electrocatalytic performance for HER with small overpotential in both acidic and basic conditions, as well as remarkable stability.     

Heterogeneously Porous γ‐MnO2‐Catalyzed Direct Oxidative Amination of Benzoxazole through CH Activation in the Presence of O2

Oxidative amination of azoles through catalytic C? H bond activation is a very important reaction due to the presence of 2‐aminoazoles in several biologically active compounds. However, most of the reported methods are performed under homogeneous reaction conditions using excess reagents and additives. Herein, we report the heterogeneous, porous γ‐MnO₂‐catalyzed direct amination of benzoxazole with wide range of primary and secondary amines. The amination was carried under mild reaction conditions and using molecular oxygen as a green oxidant, without any additives. The catalyst can easily be separated by filtration and reused several times without a significant loss of its catalytic performance. Of note, the reaction tolerates a functional group such as alcohol, thus indicating the broad applicability of this reaction.     

Ultrathin Hexagonal Hybrid Nanosheets Synthesized by Graphene Oxide‐Assisted Exfoliation of β‐Co(OH)2 Mesocrystals

In the present study, we report the synthesis of a high‐quality, single‐crystal hexagonal β‐Co(OH)₂ nanosheet, exhibiting a thickness down to ten atomic layers and an aspect ratio exceeding 900, by using graphene oxide (GO) as an exfoliant of β‐Co(OH)₂ nanoflowers. Unlike conventional approaches using ionic precursors in which morphological control is realized by structure‐directing molecules, the β‐Co(OH)₂ flower‐like superstructures were first grown by a nanoparticle‐mediated crystallization process, which results in large 3D superstructure consisting of ultrathin nanosheets interspaced by polydimethoxyaniline (PDMA). Thereafter, β‐Co(OH)₂ nanoflowers were chemically exfoliated by surface‐active GO under hydrothermal conditions into unilamellar single‐crystal nanosheets. In this reaction, GO acts as a two‐dimensional (2D) amphiphile to facilitate the exfoliation process through tailored interactions between organic and inorganic molecules. Meanwhile, the on‐site conjugation of GO and Co(OH)₂ promotes the thermodynamic stability of freestanding ultrathin nanosheets and restrains further growth through Oswald ripening. The unique 2D structure combined with functionalities of the hybrid ultrathin Co(OH)₂ nanosheets on rGO resulted in a remarkably enhanced lithium‐ion storage performance as anode materials, maintaining a reversible capacity of 860 mA h g^?1 for as many as 30 cycles. Since mesocrystals are ubiquitous and rich in morphological diversity, the strategy of the GO‐assisted exfoliation of mesocrystals developed here provides an opportunity for the synthesis of new functional nanostructures that could bear importance in clean renewable energy, catalysis, photoelectronics, and photonics.     

NaBArF4‐Catalyzed Oxidative Cyclization of 1,5‐ and 1,6‐Diynes: Efficient and Divergent Synthesis of Functionalized γ‐ and δ‐Lactams

An efficient NaBAr^F₄‐catalyzed oxidative cyclization of readily available 1,5‐ and 1,6‐diynes has been developed. Importantly, this transition metal‐free oxidative catalysis proceeds via a presumable Lewis acid‐catalyzed S_N2’ pathway, which is distinct from the relevant oxidative rhodium and gold catalysis. This method leads to the facile and practical construction of a diverse range of synthetically useful γ‐ and δ‐lactams in mostly good to excellent yields with broad substrate scope.     

Cascade Mn‐Mediated γ‐Alkylation/oxa‐Michael Addition of Enones with 1,3‐Dicarbonyls

A radical‐based strategy for regioselective γ‐C?C bond formation/oxa‐conjugate addition, forming the tetrahydrobenzofuran core common to many bioactive natural products is described. The technique utilizes readily available enone derivatives and 1,3‐dicarbonyl compounds as coupling partners in an oxidative formal [3+2] cycloaddition mediated by Mn^III. The transformation delivers polycyclic products in good yields and proceeds with complete regiocontrol and excellent stereoselectivity. Sterically encumbered substrates are notably well‐tolerated and bond formation occurs readily to form neopentyl and all‐carbon quaternary centers in good yields. Several stereo‐ and chemoselective transformations of the products are described.     

Tandem IBX‐Promoted Primary Alcohol Oxidation/Opening of Intermediate β,γ‐Diolcarbonate Aldehydes to (E)‐γ‐Hydroxy‐α,β‐enals

A tandem IBX‐promoted oxidation of primary alcohol to aldehyde and opening of intermediate β,γ‐diolcarbonate aldehyde to (E)‐γ‐hydroxy‐α,β‐enal has been developed. Remarkably, the carbonate opening delivered exclusively (E)‐olefin and no over‐oxidation of γ‐hydroxy was observed. The method developed has been extended to complete the stereoselective total synthesis of both (S)‐ and (R)‐coriolides and d ‐xylo‐ and d ‐arabino‐C‐20 guggultetrols.     

Spontaneous Nanobelt Formation by Self‐Assembly of β‐Benzyl GABA

We present the formation of a nanobelt by self‐assembly of β‐benzyl GABA (γ‐aminobutyric acid). This simple γ‐amino acid building block self‐assembled to form a well‐defined nanobelt in chloroform. The nanobelt showed distinct optical properties due to π–π interactions. This new‐generation self‐assembled single amino acid may serve as a template for functional nanomaterials.     

Manganese(I)‐Catalyzed β‐Methylation of Alcohols Using Methanol as C1 Source

Highly selective β‐methylation of alcohols was achieved using an earth‐abundant first row transition metal in the air stable molecular manganese complex [Mn(CO)₂Br[HN(C₂H₄PⁱPr₂)₂]] 1 ([HN(C₂H₄PⁱPr₂)₂]=MACHO‐ⁱPr). The reaction requires only low loadings of 1 (0.5 mol %), methanolate as base and MeOH as methylation reagent as well as solvent. Various alcohols were β‐methylated with very good selectivity (>99 %) and excellent yield (up to 94 %). Biomass derived aliphatic alcohols and diols were also selectively methylated on the β‐position, opening a pathway to “biohybrid” molecules constructed entirely from non‐fossil carbon. Mechanistic studies indicate that the reaction proceeds through a borrowing hydrogen pathway involving metal–ligand cooperation at the Mn‐pincer complex. This transformation provides a convenient, economical, and environmentally benign pathway for the selective C?C bond formation with potential applications for the preparation of advanced biofuels, fine chemicals, and biologically active molecules     

The Role of Alkali Metal in α‐MnO2 Catalyzed Ammonia‐Selective Catalysis

The unexpected phenomenon and mechanism of the alkali metal involved NH₃ selective catalysis are reported. Incorporation of K⁺ (4.22 wt %) in the tunnels of α‐MnO₂ greatly improved its activity at low temperature (50–200 °C, 100 % conversion of NO_x vs. 50.6 % conversion over pristine α‐MnO₂ at 150 °C). Experiment and theory demonstrated the atomic role of incorporated K⁺ in α‐MnO₂. Results showed that K⁺ in the tunnels could form a stable coordination with eight nearby O atoms. The columbic interaction between the trapped K⁺ and O atoms can rearrange the charge population of nearby Mn and O atoms, thus making the topmost five‐coordinated unsaturated Mn cations (Mn_5c, the Lewis acid sites) more positive. Therefore, the more positively charged Mn_5c can better chemically adsorb and activate the NH₃ molecules compared with its pristine counterpart, which is crucial for subsequent reactions.     

Efficient Oxidation of 1,2‐Diols into α‐Hydroxyketones Catalyzed by Organotin Compounds

Get selective! A selective oxidation of 1,2‐diols to α‐hydroxyketones catalyzed by organotin compounds has been developed (see scheme). Invaluable chemo‐ and stereoselectivity were found in the reaction. The catalytic system has been achieved by electrochemical and chemical oxidation.

     

Carnation‐like CuO Hierarchical Nanostructures Assembled by Porous Nanosheets for Nonenzymatic Glucose Sensing

Carnation‐like CuO hierarchical nanostructures assembled by ultrathin porous nanosheets were successfully fabricated via a facile solvothermal route followed with heat treatment. As‐prepared CuO nanostructures exhibited excellent catalytic activity toward glucose oxidation in the absence of any enzymes. Under the optimized conditions, the CuO‐based enzymeless glucose sensor showed high sensitivity of 3.15 mA mM^?1 cm^?2, low limit of detection (98 nM, S/N=3), good reproducibility, excellent selectivity and long‐time stability. The superb nonenzymatic glucose sensing performance of the CuO hierarchical nanostructures was attributed to the highly catalytically active sites at the edges and basal planes of the CuO nanosheets, facile transportation of analytes through the abundant mesopores and macropores, robust and stable hierarchical structure. Moreover, the CuO‐based enzymeless glucose sensor showed high accuracy and reliability in comparison with clinical glucometer for quantitative determination of glucose in human blood serum samples.