Polyethylene Materials with Tunable Degradability by Incorporating In-Chain Mechanophores.

Polyolefins are recognized as fundamental plastic materials that are manufactured in the largest quantities among all synthetic polymers. The chemical inertness of the saturated hydrocarbon chains is crucial for storing and using polyolefin plastics, but poses significant environmental challenges related to plastic pollution. Here, we report a versatile approach to creating polyethylene materials with tunable degradability by incorporating in-chain mechanophores. Through palladium-catalyzed coordination/insertion copolymerization of ethylene with cyclobutene-fused comonomers, several cyclobutane-fused mechanophores were successfully incorporated with varied insertion ratios (0.35-26 mol %). The resulting polyethylene materials with in-chain mechanophores exhibit both high thermal stability and remarkable acid resistance. Upon mechanochemical activation by ultrasonication or ball-milling, degradable functional units (imide and ester groups) are introduced into the main polymer chain. The synergy of mechanochemical activation and acid hydrolysis facilitates the efficient degradation of high molecular weight polyethylene materials into telechelic oligomers.

Chemoselective Hydrogenolysis of Urethanes to Formamides and Alcohols in the Presence of More Electrophilic Carbonyl Compounds.

The development of methods for the chemical recycling of polyurethanes is recognized as an urgent issue. Herein, we report the Ir-catalyzed hydrogenolysis of the urethane C-O bond to produce formamides and alcohols, where both formamides and ester and amide functionalities are tolerated. The chemoselectivity observed is counterintuitive to the generally accepted electrophilicity order of carbonyl compounds. Hydrogenolysis of urea and isocyanurate, potential byproducts in the polycondensation process of polyurethanes, is also achieved alongside the selective degradation of polyurethanes themselves, which affords diformamides and diols. The time-course of the hydrogenative polyurethane degradation reveals that the bond cleavage occurs not from the terminal, but from any part of the polymer chain. The present catalysis offers a novel method for the recycling of polyurethane-containing polymer waste.

Stability of Medium-Ring Cyclic Unsaturated Carbonyl Compounds: Direct Access to Unsaturated Ketones with C-C Double Bonds at Distal Positions via Transfer Dehydrogenation of Alicyclic Ketones.

Medium-ring cyclic compounds have characteristic distortions. For alicyclic enones, conjugated enones are known to be less stable than unconjugated enones. In this study, the relative stability of cycloalkenones with varied C-C double bond positions with C6-C12 rings was theoretically investigated to reveal that C8-C12 cycloalkenones prefer the unconjugated isomer. Furthermore, direct access to distal unsaturated cycloalkenones has been accomplished by transfer dehydrogenation of cycloalkanones catalyzed by iridium complexes.

Mild Catalytic Degradation of Crystalline Polyethylene Units in a Solid State Assisted by Carboxylic Acid Groups.

Crystalline polyethylenes bearing carboxylic acid groups in the main chain were successfully degraded with a Ce catalyst and visible light. The reaction proceeds in a crystalline solid state without swelling in acetonitrile or water at a reaction temperature as low as 60 or 80 °C, employing dioxygen in air as the only stoichiometric reactant with nearly quantitative recovery of carbon atoms. Heterogeneous features of the reaction allowed us to reveal a dynamic morphological change of polymer crystals during the degradation.

Synthesis of Long-Chain Polyamides via Main-Chain Modification of Polyethyleneketones.

Long-chain polyamides (polyethyleneamides) were prepared from polyethylenes bearing in-chain carbonyl groups (polyethyleneketones) by the oxime formation and successive Beckmann rearrangement. (Diethylamino)sulfur trifluoride (DAST) was utilized as a promoter, which allowed mild conversion of the oxime group in spite of low solubility of the polymers. The polyethyleneamide exhibited different tensile property compared to a commercial HDPE.

Counterintuitive chemoselectivity in the reduction of carbonyl compounds.

The reactivity of carbonyl functional groups largely depends on the substituents on the carbon atom. Reversal of the commonly accepted order of reactivity of different carbonyl compounds requires novel synthetic approaches. Achieving selective reduction will enable the transformation of carbon resources such as plastic waste, carbon dioxide and biomass into valuable chemicals. In this Review, we explore the reduction of less reactive carbonyl groups in the presence of those typically considered more reactive. We discuss reductions, including the controlled reduction of ureas, amides and esters to aldehydes, as well as chemoselective reductions of carbonyl groups, including the reduction of ureas over carbamates, amides and esters; the reduction of amides over esters, ketones and aldehydes; and the reduction of ketones over aldehydes.

Synthesis of Novel Polymers with Biodegradability by Main-Chain Editing of Chiral Polyketones.

Although the use of biodegradable plastics is suitable for unrecoverable, single-use plastic, their high production cost and much lower variety compared to commodity plastics limit their application. In this study, we developed a new polymer with potential biodegradability, poly(ketone/ester), synthesized from propylene and carbon monoxide. Propylene and carbon monoxide are easily available at low costs from fossil resources, and they can also be derived from biomass. Using an atom insertion reaction to the main chain of the polymer, the main-chain editing of the polymer molecule proceeded with up to 89% selectivity for atom insertion over main-chain cleavage.

Tetrakispyrazolylethene: Protonation-Induced Emission.

Tetrakispyrazolylethene (1) was synthesized from pyrazole and hexachloroethane through a one-step substitution reaction. The increase of emission was detected both in solid and aqueous THF solution, compared with that in anhydrous THF. While the former originates from the crystal packing, the latter is attributed to the protonation-induced emission, independent of aggregation, based on the optical measurement under varying concentrations and particle-size distribution analysis.

Linear, Planar, Orbicular, and Macrocyclic Multinuclear Zinc (Meth)acrylate Complexes.

Zinc carboxylate complexes are widely utilized as artificial models of metalloenzymes and as secondary building units of PCPs/MOFs. However, the relationship between the structure of the monodentate carboxylato ligand and the molecular arrangement of multinuclear zinc carboxylate complexes is not fully understood because of the coordination flexibility of the Zn ion and carboxylato ligands. Herein, we report the structural analysis of a series of complexes derived from zinc (meth)acrylate which has a linear infinite chain structure. The molecular structure of µ4-oxido-bridged tetranuclear complexes [Zn4(µ4-O)(OCOR)6] revealed a distorted Zn4O core. Crystallization of zinc acrylate under aqueous conditions afforded a µ3-hydroxido-containing pentanuclear complex [Zn5(µ3-OH)2(OCOR)8] as the repeating unit of an infinite sheet-like structure in the solid state. It was also obtained by the hydrolysis of the µ4-oxido-bridged tetranuclear complex. In sharp contrast, the methacrylate analog retained the methacrylato ligands under aqueous crystallization conditions to form a macrocyclic dodecanuclear complex with methacrylato as the sole ligand.

Selective Hydrogenation of Aldehydes under Syngas Using CeO2-Supported Au Nanoparticle Catalyst.

Chemoselective hydrogenation of aldehydes to alcohols is of importance in synthetic chemistry. Here, we report a reusable CeO2-supported Au nanoparticle catalyst for the selective hydrogenation of aldehydes using syngas as the hydrogen source for which CO in syngas works as a site blocker to prevent side reactions. In particular, the hydrogenation of aldehydes with an easily reducible alkene, alkyne, or halogen moiety under syngas gave the corresponding alcohols with high selectivity, while the hydrogenation under pure hydrogen resulted in overreduction or dehalogenation. Of particular interest is that CO works as a site blocker but does not affect the hydrogenation rate significantly. A potential application of the present catalyst system was demonstrated by the conversion of terminal alkenes to alcohols via a one-pot hydroformylation/hydrogenation sequence.

Sustainable Copolymer Synthesis from Carbon Dioxide and Butadiene.

Carbon dioxide (CO2) has long been recognized as an ideal C1 feedstock comonomer for producing sustainable materials because it is renewable, abundant, and cost-effective. However, activating CO2 presents a significant challenge because it is highly oxidized and stable. A CO2/butadiene-derived δ-valerolactone (EVP), generated via palladium-catalyzed telomerization between CO2 and butadiene, has emerged as an attractive intermediate for producing sustainable copolymers from CO2 and butadiene. Owing to the presence of two active carbon-carbon double bonds and a lactone unit, EVP serves as a versatile intermediate for creating sustainable copolymers with a CO2 content of up to 29 wt % (33 mol %). In this Review, advances in the synthesis of copolymers from CO2 and butadiene with divergent structures through various polymerization protocols have been summarized. Achievements made in homo- and copolymerization of EVP or its derivatives are comprehensively reviewed, while the postmodification of the obtained copolymers to access new polymers are also discussed. Meanwhile, potential applications of the obtained copolymers are also discussed. The literature references were sorted into sections based on polymerization strategies and mechanisms, facilitating readers in gaining a comprehensive view of the present chemistry landscape and inspiring innovative approaches to synthesizing novel CO2-derived copolymers.

Hydrogen-Induced Formation of Surface Acid Sites on Pt/Al(PO3)3 Enables Remarkably Efficient Hydrogenolysis of C-O Bonds in Alcohols and Ethers.

The hydrogenolysis of oxygenates such as alcohols and ethers is central to the biomass valorization and also a valuable transformation in organic synthesis. However, a mild and efficient catalyst system for the hydrogenolysis of a large variety of alcohols and ethers with various functional groups is still underdeveloped. Here, we report an aluminum metaphosphate-supported Pt nanoparticles (Pt/Al(PO3)3) for the hydrogenolysis of a wide variety of primary, secondary, and tertiary alkyl and benzylic alcohols, and dialkyl, aryl alkyl, and diaryl ethers, including biomass-derived furanic compounds, under mild conditions (0.1-1 atm of H2, as low as 70 °C). Mechanistic studies suggested that H2 induces formation of the surface Brønsted acid sites via its cleavage by supported Pt nanoparticles. Accordingly, the high efficiency and the wide applicability of the catalyst system are attributed to the activation and cleavage of C-O bonds by the hydrogen-induced Brønsted acid sites with the assistance of Lewis acidic Al sites on the catalyst surface. The high efficiency of the catalyst implies its potential application in energy-efficient biomass valorization or fine chemical synthesis.

Nickel-Catalyzed C(sp3)-O Hydrogenolysis via a Remote-Concerted Oxidative Addition and Its Application to Degradation of a Bisphenol A-Based Epoxy Resin.

In this work, we developed a nickel-catalyzed transfer hydrogenolysis of 1-aryloxy-3-amino-2-propanols, which is a model compound of an amine-cured bisphenol A (BPA)-based epoxy resin. Mechanistic investigation revealed that the hydroxy group acts as the hydrogen donor to generate α-aryloxy ketone, which undergoes an unprecedented remote-concerted oxidative addition of the C(sp3)-O bond as suggested by DFT calculation. Successful application of this method was demonstrated by the degradation of a diamine-cured BPA-based epoxy resin, in which BPA was directly recovered from the resin.

Hexakispyrazolylethane: New Strategy for Stabilization of Hexaarylethane.

Hexakis(4-trimethylsilylpyrazol-1-yl)ethane was synthesized by the oxidative dimerization of tris(4-trimethylsilylpyrazol-1-yl)methane. Single-crystal X-ray structural analysis of hexakis(4-trimethylsilylpyrazol-1-yl)ethane showed that the ethane C-C bond (1.623(4) Å) is shorter than that in hexaphenylethane (1.67(3) Å). In solution, hexakis(4-trimethylsilylpyrazol-1-yl)ethane existed as a single species, contrastive that conventional hexaphenylethanes can keep the central C-C bond only by the aid of additional bridges between the two triarylmethyl units. Theoretical calculations indicated that the tris(pyrazol-1-yl)methyl radical, which is anticipated to be under equilibrium with hexakis(pyrazol-1-yl)ethane, is less stable than trityl radicals due to lack of delocalization of the radicals. Furthermore, introduction of pyrazole groups allowed additional bridging between the two triarylmethyl moieties through metal coordination to the adjacent N atoms: hexakis(4-trimethylsilylpyrazol-1-yl)ethane exhibited unique coordination to three Ag atoms affording a hexaarylethane analog bearing three N-Ag-N bridges.

Fluoroalkylated hypervalent sulfur fluorides: radical addition of arylchlorotetrafluoro-λ6-sulfanes to tetrafluoroethylene.

Fluorinated groups are essential hydrophobic groups in drug design. Combining a carbon-free tetrafluoro-λ6-sulfanyl (SF4) group with a polyfluoroalkyl group (RF) provides SF4RF groups, exhibiting high hydrophobicity with a short carbon chain. In this study, various aryltetrafluoro(polyfluoroalkyl)-λ6-sulfanes (ArSF4RF) were synthesized through the radical addition of arylchlorotetrafluoro-λ6-sulfanes (ArSF4Cl) to tetrafluoroethylene. In addition, quantification of hydrophobic constants (πPh) indicated that the SF4 group is considerably more hydrophobic than a difluoromethylene (CF2) group. Further transformation reactions revealed the stabilities and reactivities of these novel fluorinated groups. The high hydrophobicity and synthetic utility of the SF4RF group lead to the potential applications of the SF4RF group in the pharmaceutical field.

A dinuclear Rh(-i)/Rh(i) complex bridged by biphilic phosphinine ligands.

Bimetallic complexes have enabled precise control of catalysis by accumulating two discrete metal centres. In these complexes, bridging ligands are essential to combine multiple metals into one molecule. Among some bridging modes, an unsymmetric bridging mode will differentiate the electronic structures of the two metal centres. In this study, a dinuclear Rh(-i)/Rh(i) complex bridged by tridentate phosphine-phosphinine-phosphine ligands was prepared by reduction of the corresponding Rh(i) complex. Single-crystal X-ray analysis and DFT calculations suggest that the phosphinine ligands adopt an unsymmetric bridging mode wherein phosphinine accepts d-electrons from one Rh centre and, at the same time, donates lone pairs to the other Rh centre.

Boron Polycation Supported by Cyclic Bis(carbodiphosphorane).

A novel cyclic bis(carbodiphosphorane) ligand was prepared and investigated in coordination with group 13 elements, B, Al, and Ga. Al and Ga afforded dinuclear adduct where two metal centers were bridged by the bis(carbodiphosphorane) ligand. In contrast, the reaction with boron trichloride afforded a monomeric dicationic three-coordinate boron species composed of one boron moieties and one ligand. The structures of these products were determined by X-ray crystallography. In the dicationic boron compound, the sterically constrained cyclic structure enforced the boron center to acquire strained trigonal geometry with wide C-B-C angle of 140°. Furthermore, theoretical investigation with DFT and NBO suggested a significant contribution of tricationic two-coordinate boron resonance structure supported by two CDP ligands.

N-Fluorobenzenesulfonimide (NFSI) analogs with deprotectable substituents: synthesis of ß-fluoroamines via catalytic aminofluorination of styrenes.

The palladium-catalyzed aminofluorination of styrenes using novel N-fluorobenzenesulfonimide (NFSI) derivatives with deprotectable substituents, and the selective deprotection and transformation of the resulting products into amines under mild reaction conditions are herein disclosed.

Synthesis, Characterization, and Trapping of a Cyclic Diborylcarbene, an Electrophilic Carbene.

A carbene bearing two geminal boryl substituents, called diborylcarbene (DBC), has been predicted to be highly Lewis acidic in sharp contrast to the well-studied persistent carbenes stabilized by π-donating substituents. Studies on DBC have been limited to either the base-trapping or theoretical calculations. Herein, we developed chemical equivalents for DBC, namely, K/X-diborylcarbenoids 2X (X = F or Cl). Treatment of 2F with Al(C6F5)3 yielded [AlF(C6F5)3]--stabilized DBC 1-FAl, which showed a significant low-field shift of the carbenoid carbon from 169 ppm (doublet, coupling with 19F) to 242 ppm (singlet). The loss of halogen was also detected through electrospray ionization time-of-flight mass spectrometry analysis of 2X only in the presence of Al(C6F5)3. Generated DBC 1 from 1-FAl or 2Cl was successfully trapped with excess amounts of trialkylphosphines (PR3, R = Me or Et), which afforded the corresponding DBC-PR3 adducts. In addition, the Lewis acidity of DBC 1 was evaluated both experimentally and theoretically to reveal that 1 is one of the most Lewis acidic species among neutral molecules.

Chemoselectivity change in catalytic hydrogenolysis enabling urea-reduction to formamide/amine over more reactive carbonyl compounds.

The selective transformation of a less reactive carbonyl moiety in the presence of more reactive ones can realize straightforward and environmentally benign chemical processes. However, such a transformation is highly challenging because the reactivity of carbonyl compounds, one of the most important functionalities in organic chemistry, depends on the substituents on the carbon atom. Herein, we report an Ir catalyst for the selective hydrogenolysis of urea derivatives, which are the least reactive carbonyl compounds, affording formamides and amines. Although formamide, as well as ester, amide, and carbamate substituents, are considered to be more reactive than urea, the proposed Ir catalyst tolerated these carbonyl groups and reacted with urea in a highly chemoselective manner. The proposed chemo- and regioselective hydrogenolysis allows the development of a strategy for the chemical recycling of polyurea resins.