Showing papers on "Surface modification published in 2022"

Organometallic-functionalized interfaces for highly efficient inverted perovskite solar cells

Abstract: Further enhancing the performance and stability of inverted perovskite solar cells (PSCs) is crucial for their commercialization. We report that the functionalization of multication and halide perovskite interfaces with an organometallic compound, ferrocenyl-bis-thiophene-2-carboxylate (FcTc2), simultaneously enhanced the efficiency and stability of inverted PSCs. The resultant devices achieved a power conversion efficiency of 25.0% and maintained >98% of their initial efficiency after continuously operating at the maximum power point for 1500 hours under simulated AM1.5 illumination. Moreover, the FcTc2-functionalized devices passed the international standards for mature photovoltaics (IEC61215:2016) and have exhibited high stability under the damp heat test (85°C and 85% relative humidity). Description Organometallics stabilizing perovskites Perovskite solar cells with inverted (p-i-n) structure can have greater stability and lifetimes than conventional n-i-p structures but usually have somewhat lower power conversion efficiencies (PCEs). Li et al. report that an organometallic compound, ferrocenyl-bis-thiophene-2-carboxylate, can stabilize a multication perovskite layer of an inverted perovskite solar cells. After 1500 hours of maximum power point operation, 98% of the 25.0% PCE was maintained. The solar cell also exhibited high stability in damp heat tests. —PDS Functionalizing interfaces with an organometallic compound created an efficient and stable inverted perovskite solar cell.

Removal of metal ions using a new magnetic chitosan nano-bio-adsorbent; A powerful approach in water treatment

Abstract: In this study, a magnetic chitosan/Al2O3/Fe3O4 (M-Cs) nanocomposite was developed by ethylenediaminetetraacetic acid (EDTA) functionalization to enhance its adsorption behavior for the removal of Cd(II), Cu(II) and Zn(II) metal ions from aqueous solution. The results revealed that the EDTA functionalization of M-Cs increased its adsorption capacity ~9.1, ~5.6 and ~14.3 times toward Cu, Cd and Zn ions. The maximum adsorption capacity followed the order of Cd(II) > Cu(II) > Zn(II) and the maximum adsorption efficiency was achieved at pH of 5.3 with the removal percentage of 99.98, 93.69 and 83.81 %, respectively, for the removal of Cu, Cd and Zn ions. The metal ions adsorption kinetic obeyed pseudo-second-order equation and the Langmuir isothermal was found the most fitted model for their adsorption isothermal experimental data. In addition, the thermodynamic study illustrated that the adsorption process was exothermic and spontaneous in nature.

Surface Modification of 2D Photocatalysts for Solar Energy Conversion

Abstract: 2D materials show many particular properties, such as high surface‐to‐volume ratio, high anisotropic degree, and adjustable chemical functionality. These unique properties in 2D materials have sparked immense interest due to their applications in photocatalytic systems, resulting in significantly enhanced light capture, charge‐transfer kinetics, and surface reaction. Herein, the research progress in 2D photocatalysts based on varied compositions and functions, followed by specific surface modification strategies, is introduced. Fundamental principles focusing on light harvesting, charge separation, and molecular adsorption/activation in the 2D‐material‐based photocatalytic system are systemically explored. The examples described here detail the use of 2D materials in various photocatalytic energy‐conversion systems, including water splitting, carbon dioxide reduction, nitrogen fixation, hydrogen peroxide production, and organic synthesis. Finally, by elaborating the challenges and possible solutions for developing these 2D materials, the review is expected to provide some inspiration for the future research of 2D materials used on efficient photocatalytic energy conversions.

Cellulose Nanopaper: Fabrication, Functionalization, and Applications

Abstract: Cellulose nanopaper has shown great potential in diverse fields including optoelectronic devices, food packaging, biomedical application, and so forth, owing to their various advantages such as good flexibility, tunable light transmittance, high thermal stability, low thermal expansion coefficient, and superior mechanical properties. Herein, recent progress on the fabrication and applications of cellulose nanopaper is summarized and discussed based on the analyses of the latest studies. We begin with a brief introduction of the three types of nanocellulose: cellulose nanocrystals, cellulose nanofibrils and bacterial cellulose, recapitulating their differences in preparation and properties. Then, the main preparation methods of cellulose nanopaper including filtration method and casting method as well as the newly developed technology are systematically elaborated and compared. Furthermore, the advanced applications of cellulose nanopaper including energy storage, electronic devices, water treatment, and high-performance packaging materials were highlighted. Finally, the prospects and ongoing challenges of cellulose nanopaper were summarized.

Cellulose Nanopaper: Fabrication, Functionalization, and Applications

Abstract: Cellulose nanopaper has shown great potential in diverse fields including optoelectronic devices, food packaging, biomedical application, and so forth, owing to their various advantages such as good flexibility, tunable light transmittance, high thermal stability, low thermal expansion coefficient, and superior mechanical properties. Herein, recent progress on the fabrication and applications of cellulose nanopaper is summarized and discussed based on the analyses of the latest studies. We begin with a brief introduction of the three types of nanocellulose: cellulose nanocrystals, cellulose nanofibrils and bacterial cellulose, recapitulating their differences in preparation and properties. Then, the main preparation methods of cellulose nanopaper including filtration method and casting method as well as the newly developed technology are systematically elaborated and compared. Furthermore, the advanced applications of cellulose nanopaper including energy storage, electronic devices, water treatment, and high-performance packaging materials were highlighted. Finally, the prospects and ongoing challenges of cellulose nanopaper were summarized.

A review on experimental chemically modified activated carbon to enhance dye and heavy metals adsorption

Abstract: Effective and low-cost removal of dye and heavy metals from wastewater still is a great challenge for researchers. Adsorption using activated carbon is widely used in removing these toxic pollutants. Physical, chemical, and biological modifications have been studied for improving activated carbon adsorption performance. Literature suggests that chemical modified activated carbon showed maximum adsorption capacity towards dye and heavy from aqueous solution. Chemical modifications, including acid, base, and impregnation, are studied extensively due to reagent availability, easy modification, and tuning facilities of surface functional groups. However, systematic documentation of chemical modifications on activated carbon is required for dye and heavy metals removal efficiency improvement from wastewater. This review focused on the up to date experimental chemically modified activated carbon that showed improved adsorption capacity towards dye and heavy metals from aqueous solution. The available experimental data recommends that an appropriate treatment strategy of a chemical modification process enhanced dye and heavy metals adsorption capacity of the modified activated carbon. Optimum modification process developed textural or surface functional groups properties of modified activated carbon that improved adsorption or binding capacity toward adsorbate or a particular species. In addition, the adsorption capacity of modified and corresponding activated carbon is compared. • Experimental works of dye and heavy metals adsorption are summarized. • Changed of physiochemical properties of AC after chemical modification are mentioned. • Optimized treatment parameters of chemical modification are focused. • Compared adsorption performance between modified and corresponding AC adsorbents. • Systematic documentation help in tailoring surface functional groups of modified AC.

A Perspective on Late-Stage Aromatic C–H Bond Functionalization

Abstract: Late-stage functionalization of C–H bonds (C–H LSF) can provide a straightforward approach to the efficient synthesis of functionalized complex molecules. However, C–H LSF is challenging because the C–H bond must be functionalized in the presence of various other functional groups. In this Perspective, we evaluate aromatic C–H LSF on the basis of four criteria—reactivity, chemoselectivity, site-selectivity, and substrate scope—and provide our own views on current challenges as well as promising strategies and areas of growth going forward.

Copper-catalyzed radical relay in C(sp3)-H functionalization.

Abstract: Radical-involved transition metal (TM) catalysis has greatly enabled new reactivities in recent decades. Copper-catalyzed radical relay offers enormous potential in C(sp3)-H functionalization which combines the unique regioselectivity of hydrogen atom transfer (HAT) and the versatility of copper-catalyzed cross-coupling. More importantly, significant progress has been achieved in asymmetric C-H functionalization through judicious ligand design. This tutorial review will highlight the recent advances in this rapidly growing area, and we hope this survey will inspire future strategic developments for selective C(sp3)-H functionalization.

Recent advances in transition-metal-catalyzed carbene insertion to C–H bonds

Abstract: C-H functionalization has been emerging as a powerful method to establish carbon-carbon and carbon-heteroatom bonds. Many efforts have been devoted to transition-metal-catalyzed direct transformations of C-H bonds. Metal carbenes generated in situ from transition-metal compounds and diazo or its equivalents are usually applied as the transient reactive intermediates to furnish a catalytic cycle for new C-C and C-X bond formation. Using this strategy compounds from unactivated simple alkanes to complex molecules can be further functionalized or transformed to multi-functionalized compounds. In this area, transition-metal-catalyzed carbene insertion to C-H bonds has been paid continuous attention. Diverse catalyst design strategies, synthetic methods, and potential applications have been developed. This critical review will summarize the advance in transition-metal-catalyzed carbene insertion to C-H bonds dated up to July 2021, by the categories of C-H bonds from aliphatic C(sp3)-H, aryl (aromatic) C(sp2)-H, heteroaryl (heteroaromatic) C(sp2)-H bonds, alkenyl C(sp2)-H, and alkynyl C(sp)-H, as well as asymmetric carbene insertion to C-H bonds, and more coverage will be given to the recent work. Due to the rapid development of the C-H functionalization area, future directions in this topic are also discussed. This review will give the authors an overview of carbene insertion chemistry in C-H functionalization with focus on the catalytic systems and synthetic applications in C-C bond formation.

Nitrogen-Centered Radicals in Functionalization of sp2 Systems: Generation, Reactivity, and Applications in Synthesis.

Abstract: The chemistry of nitrogen-centered radicals (NCRs) has plentiful applications in organic synthesis, and they continue to expand as our understanding of these reactive species increases. The utility of these reactive intermediates is demonstrated in the recent advances in C-H amination and the (di)amination of alkenes. Synthesis of previously challenging structures can be achieved by efficient functionalization of sp2 moieties without prefunctionalization, allowing for faster and more streamlined synthesis. This Review addresses the generation, reactivity, and application of NCRs, including, but not limited to, iminyl, aminyl, amidyl, and aminium species. Contributions from early discovery up to the most recent examples have been highlighted, covering radical initiation, thermolysis, photolysis, and, more recently, photoredox catalysis. Radical-mediated intermolecular amination of (hetero)arenes can occur with a variety of complex amine precursors, generating aniline derivatives, an important class of structures for drug discovery and development. Functionalization of olefins is achievable in high anti-Markovnikov regioselectivity and allows access to difunctionalized structures when the intermediate carbon radicals are trapped. Additionally, the reactivity of NCRs can be harnessed for the rapid construction of N-heterocycles such as pyrrolidines, phenanthridines, quinoxalines, and quinazolinones.

Carbon Dots in Bioimaging, Biosensing and Therapeutics: A Comprehensive Review

Abstract: Carbon dots (CDs), comprising crystalline graphitized carbon cores and polymer surface groups, are currently attracting a lot of interest in biological fields owing to their fluorescent properties, high photostability, biocompatibility and low toxicity. In addition, the easy preparation and functionalization of CDs stimulate the development of CDs‐based composite materials with specific functions. Presently, the biological applications of CDs are growing at a remarkable speed, justifying the need for up‐to‐date review articles that capture recent progress in this blossoming field. In this review, breakthroughs in the synthesis, modification, optical properties, toxicology and biocatalytic platforms of CDs are described. Further, recent research related to bioimaging, biosensing, drug delivery, antibacterial, anticancer (photothermal therapy, photodynamic therapy and synergistic therapy) and antiviral therapies involving CDs are discussed in detail. Finally, a perspective on the prospects and challenges of CDs in the fields of biomedicine and biotechnology is provided.

Additive-mediated intercalation and surface modification of MXenes.

Abstract: 2D carbides and nitrides of transition metals, also known as MXenes, are an emerging class of 2D nanomaterials that have shown excellent performances and broad application prospects in the fields of energy storage, catalysis, sensing, electromagnetic shielding, electronics and photonics, and life sciences. This unusual diversity of applications is due to their superior hydrophilicity and conductivity, high carrier concentration, ultra-high volumetric capacitance, rich surface chemistry, and large specific surface area. However, it is difficult to make MXenes with the desired surface functional groups that deliver high reactivity and high stability, because most MXenes are extracted from ceramics (MAX phase) by an etching process, where a large number of metal atoms are inevitably exposed on the surface, with other anions and cations embedded uncontrollably. The exposed metal atoms and implanted ions are thermodynamically unstable and readily react with trace oxygen or oxygen-containing groups to form the corresponding metal oxides or degrade chemically, resulting in a sharp decline in activity and loss of excellent physicochemical properties. The addition of certain synergistic additives during the intercalation and chemical modification of surface functional groups under non-hazardous conditions can result in stable and efficient MXene-based materials with exceptional optical, electrical, and magnetic properties. This review discusses several such methods, mainly additive-mediated intercalation and chemical modification of the surface functional groups of MXene-based materials, followed by their potential applications. Finally, perspectives are given to discuss the future challenges and promising opportunities of this exciting field.

Intercalation‐Activated Layered MoO ₃ Nanobelts as Biodegradable Nanozymes for Tumor‐Specific Photo‐Enhanced Catalytic Therapy

Abstract: The existence of natural van der Waals gaps in layered materials allows them to be easily intercalated with varying guest species, offering an appealing strategy to optimize their physicochemical properties and application performance. Herein, we report the activation of layered MoO3 nanobelts via aqueous intercalation as an efficient biodegradable nanozyme for tumor-specific photo-enhanced catalytic therapy. The long MoO3 nanobelts are grinded and then intercalated with Na+ and H2 O to obtain the short Na+ /H2 O co-intercalated MoO3-x (NH-MoO3-x ) nanobelts. In contrast to the inert MoO3 nanobelts, the NH-MoO3-x nanobelts exhibit excellent enzyme-mimicking catalytic activity for generation of reactive oxygen species, which can be further enhanced by the photothermal effect under a 1064 nm laser irradiation. Thus, after bovine serum albumin modification, the NH-MoO3-x nanobelts can efficiently kill cancer cells in vitro and eliminate tumors in vivo facilitating with 1064 nm laser irradiation.

Surface and Interface Engineering of Zn Anodes in Aqueous Rechargeable Zn-Ion Batteries.

Abstract: Rechargeable zinc-ion batteries (ZIBs) have shown great potential as an alternative to lithium-ion batteries. The ZIBs utilize Zn metal as the anode, which possesses many advantages such as low cost, high safety, eco-friendliness, and high capacity. However, on the other hand, the Zn anode also suffers from many issues, including dendritic growth, corrosion, and passivation. These issues are largely related to the surface and interface properties of the Zn anode. Many efforts have therefore been devoted to the modification of the Zn anode, aiming to eliminate the above-mentioned problems. This review gives a comprehensive summary on the mechanism behind these issues as well as the recent progress on Zn anode modification with focus on the strategies of surface and interface engineering, covering the design and application of both the Zn anode supports and surface protective layers, along with abundant examples. In addition, the promising research directions and perspective on these strategies are also presented.

Advanced Functional Liquid Crystals

Abstract: Liquid crystals have been intensively studied as functional materials. Recently, integration of various disciplines has led to new directions in the design of functional liquid‐crystalline materials in the fields of energy, water, photonics, actuation, sensing, and biotechnology. Here, recent advances in functional liquid crystals based on polymers, supramolecular complexes, gels, colloids, and inorganic‐based hybrids are reviewed, from design strategies to functionalization of these materials and interfaces. New insights into liquid crystals provided by significant progress in advanced measurements and computational simulations, which enhance new design and functionalization of liquid‐crystalline materials, are also discussed.

Diversification of aliphatic C–H bonds in small molecules and polyolefins through radical chain transfer

Abstract: The ability to selectively introduce diverse functionality onto hydrocarbons is of substantial value in the synthesis of both small molecules and polymers. Herein, we report an approach to aliphatic carbon–hydrogen bond diversification using radical chain transfer featuring an easily prepared O-alkenylhydroxamate reagent, which upon mild heating facilitates a range of challenging or previously undeveloped aliphatic carbon–hydrogen bond functionalizations of small molecules and polyolefins. This broad reaction platform enabled the functionalization of postconsumer polyolefins in infrastructure used to process plastic waste. Furthermore, the chemoselective placement of ionic functionality onto a branched polyolefin using carbon–hydrogen bond functionalization upcycled the material from a thermoplastic into a tough elastomer with the tensile properties of high-value polyolefin ionomers. Description A clean break for C–H bonds Carbon–hydrogen (C–H) bonds are ubiquitous in pharmaceuticals and plastics but are difficult to transform. Fazekas et al. report a versatile reagent that strips hydrogen without immediately trapping the carbon. Heating or photolysis of the reagent produces a pair of radicals, one of which rapidly cleaves a C–H bond while the other remains comparatively inert. A wide variety of other radical sources can then intercede to form carbon–halogen, carbon–carbon, and carbon–sulfur bonds. A two-step upcycling sequence that added imidazolium groups to postconsumer polyethylene foam produced a potentially valuable ionomer. —JSY A radical reagent strips hydrogen atoms from organic molecules and plastics to enable their broad functionalization.

Forging C−heteroatom bonds by transition-metal-catalyzed enantioselective C–H functionalization

Abstract: Transition-metal-catalyzed enantioselective C–H activation provides an efficient and atom-economic access to valuable chiral molecules from readily available feedstocks. The generation of minimized waste and the availability of unprecedented disconnection have significantly impacted organic synthesis. Considering the academic and industrial importance of heteroatom-containing chiral molecules, it is surprising that enantioselective C–H activation/C–heteroatom (C–X) bond-forming reactions are far less investigated than the C–C formation counterparts. In this review, we summarize the advances in C–X bond-forming asymmetric C–H activation proceedings through C–H metalation, organized based on the utilized catalytic systems. Direct C–H functionalization has recently emerged as one of the most efficient strategies to access structurally complex molecules from readily accessible feedstocks in an atom- and step-economic manner. In particular, enantioselective C–H activation has garnered increasing attention by enabling chemists to efficiently assemble valuable chiral compounds by asymmetrically manipulating C–H bonds into useful functionalities. Apart from the extensively studied C–C bond formation, very few endeavors have been focused on the C–X formation analogs. Motivated by the utility of the latter approach in constructing academically and industrially important heteroatom-containing chiral compounds, we provide herein an overview on C–X forming asymmetric C–H activation reactions proceeding through C–H metalation. The advancements are organized according to the employed catalytic systems, which include Pd(II) catalysis, group-9 CpxM(III) catalysis, monovalent group-9 metal catalysis, and multi-boryl/silyl Ir(III) catalysis, with emphasis on the design philosophy, mechanism, and mode of enantiocontrol. Direct C–H functionalization has recently emerged as one of the most efficient strategies to access structurally complex molecules from readily accessible feedstocks in an atom- and step-economic manner. In particular, enantioselective C–H activation has garnered increasing attention by enabling chemists to efficiently assemble valuable chiral compounds by asymmetrically manipulating C–H bonds into useful functionalities. Apart from the extensively studied C–C bond formation, very few endeavors have been focused on the C–X formation analogs. Motivated by the utility of the latter approach in constructing academically and industrially important heteroatom-containing chiral compounds, we provide herein an overview on C–X forming asymmetric C–H activation reactions proceeding through C–H metalation. The advancements are organized according to the employed catalytic systems, which include Pd(II) catalysis, group-9 CpxM(III) catalysis, monovalent group-9 metal catalysis, and multi-boryl/silyl Ir(III) catalysis, with emphasis on the design philosophy, mechanism, and mode of enantiocontrol. One of the central goals of modern organic chemistry is to construct complex molecules from readily accessible and abundant feedstocks. Transition metal-catalyzed C–H activation reactions hold great potential by making use of ubiquitous but otherwise inert C–H bonds as synthetic handles. During the past two decades, numerous novel methods have been developed in this vibrant research area, enabling the highly efficient and selective transformation of abundant and simple hydrocarbons into higher-value products in an atom and step-economic fashion.1Bergman R.G. Organometallic chemistry: C–H activation.Nature. 2007; 446: 391-393https://doi.org/10.1038/446391aGoogle Scholar, 2Lyons T.W. Sanford M.S. Palladium-catalyzed ligand-directed C–H functionalization reactions.Chem. Rev. 2010; 110: 1147-1169https://doi.org/10.1021/cr900184eGoogle Scholar, 3Daugulis O. Roane J. Tran L.D. Bidentate, monoanionic auxiliary-directed functionalization of carbon–hydrogen bonds.Acc. Chem. Res. 2015; 48: 1053-1064https://doi.org/10.1021/ar5004626Google Scholar, 4He J. Wasa M. Chan K.S.L. Shao Q. Yu J.-Q. Palladium-catalyzed transformations of alkyl C–H Bonds.Chem. Rev. 2017; 117: 8754-8786https://doi.org/10.1021/acs.chemrev.6b00622Google Scholar, 5Gandeepan P. Müller T. Zell D. Cera G. Warratz S. Ackermann L. 3d Transition Metals for C–H Activation.Chem. Rev. 2019; 119: 2192-2452https://doi.org/10.1021/acs.chemrev.8b00507Google Scholar These methods have also enabled the late-stage functionalization of structurally complex molecules and provided a range of unprecedented disconnections for retro-synthetic analysis, which has had a great impact on organic synthesis.6McMurray L. O'Hara F. Gaunt M.J. Recent developments in natural product synthesis using metal-catalysed C–H Bond functionalisation.Chem. Soc. Rev. 2011; 40: 1885-1898https://doi.org/10.1039/C1CS15013HGoogle Scholar, 7Wencel-Delord J. Glorius F. C–H Bond activation enables the rapid construction and late-stage diversification of functional molecules.Nat. Chem. 2013; 5: 369-375https://doi.org/10.1038/nchem.1607Google Scholar, 8Abrams D.J. Provencher P.A. Sorensen E.J. Recent applications of C–H functionalization in complex natural product synthesis.Chem. Soc. Rev. 2018; 47: 8925-8967https://doi.org/10.1039/C8CS00716KGoogle Scholar In particular, the development of asymmetric C–H functionalization reactions has allowed the efficient and diverse construction of valuable chiral molecules. Several approaches have been developed, including radical involved hydrogen atom abstraction,9Milan M. Bietti M. Costas M. Enantioselective aliphatic C–H bond oxidation catalyzed by bioinspired complexes.Chem. Commun. (Camb). 2018; 54: 9559-9570https://doi.org/10.1039/C8CC03165GGoogle Scholar,10Wang F. Chen P. Liu G. Copper-catalyzed radical relay for asymmetric radical transformations.Acc. Chem. Res. 2018; 51: 2036-2046https://doi.org/10.1021/acs.accounts.8b00265Google Scholar metallocarbene or metallonitrene insertion,11Davies H.M.L. Manning J.R. Catalytic C–H functionalization by metal carbenoid and nitrenoid insertion.Nature. 2008; 451: 417-424https://doi.org/10.1038/nature06485Google Scholar and C–H metalation that generates a well-defined metal-carbon bond.12Zheng C. You S.-L. Recent development of direct asymmetric functionalization of inert C–H bonds.RSC Adv. 2014; 4: 6173-6214https://doi.org/10.1039/C3RA46996DGoogle Scholar, 13Newton C.G. Wang S.-G. Oliveira C.C. Cramer N. Catalytic enantioselective transformations involving C–H bond cleavage by transition-metal complexes.Chem. Rev. 2017; 117: 8908-8976https://doi.org/10.1021/acs.chemrev.6b00692Google Scholar, 14Saint-Denis T.G. Zhu R.-Y. Chen G. Wu Q.-F. Yu J.-Q. Enantioselective C(sp3)‒H bond activation by chiral transition metal catalysts.Science. 2018; 359eaao4798https://doi.org/10.1126/science.aao4798Google Scholar, 15Loup J. Dhawa U. Pesciaioli F. Wencel-Delord J. Ackermann L. Enantioselective C–H activation with earth-abundant 3d transition metals.Angew. Chem. Int. Ed. Engl. 2019; 58: 12803-12818https://doi.org/10.1002/anie.201904214Google Scholar, 16Liao G. Zhou T. Yao Q.-J. Shi B.-F. Recent advances in the synthesis of axially chiral biaryls via transition metal-catalysed asymmetric C–H functionalization.Chem. Commun. (Camb). 2019; 55: 8514-8523https://doi.org/10.1039/C9CC03967HGoogle Scholar, 17Yoshino T. Matsunaga S. Chiral carboxylic acid assisted enantioselective C–H activation with achiral CpxMIII (M = Co, Rh, Ir) catalysts.ACS Catal. 2021; 11: 6455-6466https://doi.org/10.1021/acscatal.1c01351Google Scholar, 18Vyhivskyi O. Kudashev A. Miyakoshi T. Baudoin O. Chiral catalysts for Pd0-catalyzed enantioselective C–H activation.Chemistry. 2021; 27: 1231-1257https://doi.org/10.1002/chem.202003225Google Scholar Despite the robustness of the latter approach, most efforts have been focused on the construction of C–C bonds, leaving the C–heteroatom (C–X) bond formation far less studied.12Zheng C. You S.-L. Recent development of direct asymmetric functionalization of inert C–H bonds.RSC Adv. 2014; 4: 6173-6214https://doi.org/10.1039/C3RA46996DGoogle Scholar, 13Newton C.G. Wang S.-G. Oliveira C.C. Cramer N. Catalytic enantioselective transformations involving C–H bond cleavage by transition-metal complexes.Chem. Rev. 2017; 117: 8908-8976https://doi.org/10.1021/acs.chemrev.6b00692Google Scholar, 14Saint-Denis T.G. Zhu R.-Y. Chen G. Wu Q.-F. Yu J.-Q. Enantioselective C(sp3)‒H bond activation by chiral transition metal catalysts.Science. 2018; 359eaao4798https://doi.org/10.1126/science.aao4798Google Scholar, 15Loup J. Dhawa U. Pesciaioli F. Wencel-Delord J. Ackermann L. Enantioselective C–H activation with earth-abundant 3d transition metals.Angew. Chem. Int. Ed. Engl. 2019; 58: 12803-12818https://doi.org/10.1002/anie.201904214Google Scholar, 16Liao G. Zhou T. Yao Q.-J. Shi B.-F. Recent advances in the synthesis of axially chiral biaryls via transition metal-catalysed asymmetric C–H functionalization.Chem. Commun. (Camb). 2019; 55: 8514-8523https://doi.org/10.1039/C9CC03967HGoogle Scholar, 17Yoshino T. Matsunaga S. Chiral carboxylic acid assisted enantioselective C–H activation with achiral CpxMIII (M = Co, Rh, Ir) catalysts.ACS Catal. 2021; 11: 6455-6466https://doi.org/10.1021/acscatal.1c01351Google Scholar, 18Vyhivskyi O. Kudashev A. Miyakoshi T. Baudoin O. Chiral catalysts for Pd0-catalyzed enantioselective C–H activation.Chemistry. 2021; 27: 1231-1257https://doi.org/10.1002/chem.202003225Google Scholar This is striking when considering the ubiquity of heteroatom-containing chiral compounds in natural products, pharmaceuticals, agrochemicals, and materials, as well as versatile chiral building blocks. One of the major challenges in C–X forming asymmetric C–H activation reactions is that the enantocontrol in asymmetric C–H activation might be dramatically influenced by the large number of competitive coordination anions. These anions could be in situ generated by oxidants/electrophilic functionalization reagents (e.g., PhI(OAc)2, Ac2O/I2, N-fluorobenzenesulfonimide (NFSI) etc.), which are typically required in large amount for C–X formations. As a result, previously established catalytic systems are limited to the construction of only a few types of C–X bonds from specific C–H bonds. Nevertheless, recent research endeavors on this topic have led to the assembly of optically active products bearing various types of chirality (such as central, axial, and planar chirality) by manipulating otherwise inert C–H bonds into a range of C–X bonds (such as C–B, C–N, C–O, C–F, C–Si, C–Ge, and C–I bonds) in an enantioselective fashion. In this tutorial review, we try to provide an overview of the advancements in C–X bond-forming catalytic asymmetric C–H activation proceeding through C–H metalation. Asymmetric C–H functionalization reactions via outer-sphere mechanism9Milan M. Bietti M. Costas M. Enantioselective aliphatic C–H bond oxidation catalyzed by bioinspired complexes.Chem. Commun. (Camb). 2018; 54: 9559-9570https://doi.org/10.1039/C8CC03165GGoogle Scholar, 10Wang F. Chen P. Liu G. Copper-catalyzed radical relay for asymmetric radical transformations.Acc. Chem. Res. 2018; 51: 2036-2046https://doi.org/10.1021/acs.accounts.8b00265Google Scholar, 11Davies H.M.L. Manning J.R. Catalytic C–H functionalization by metal carbenoid and nitrenoid insertion.Nature. 2008; 451: 417-424https://doi.org/10.1038/nature06485Google Scholar and Pd-catalyzed asymmetric allylic C–H functionalizations proceeding through [(π-allyl)Pd] intermediates19Wang P.-S. Gong L.-Z. Palladium-catalyzed asymmetric allylic C–H functionalization: mechanism, stereo- and regioselectivities, and synthetic applications.Acc. Chem. Res. 2020; 53: 2841-2854https://doi.org/10.1021/acs.accounts.0c00477Google Scholar will not be included. The achievements are organized according to the catalytic systems characterized by transition metals and chiral ligands. These include Pd(II) catalysis, group-9 CpxM(III) catalysis, monovalent group-9 metal catalysis and multi-boryl/silyl Ir(III) catalysis (Figure 1). Emphasis would be placed on the mechanisms and modes of chiral induction. We believe that such a classification would be beneficial for a better understanding of the design philosophy, application, and limitations of each catalytic system. Pd(II)-catalyzed C–H functionalization is arguably the most well-investigated reaction in the field of C–H activation.2Lyons T.W. Sanford M.S. Palladium-catalyzed ligand-directed C–H functionalization reactions.Chem. Rev. 2010; 110: 1147-1169https://doi.org/10.1021/cr900184eGoogle Scholar, 3Daugulis O. Roane J. Tran L.D. Bidentate, monoanionic auxiliary-directed functionalization of carbon–hydrogen bonds.Acc. Chem. Res. 2015; 48: 1053-1064https://doi.org/10.1021/ar5004626Google Scholar, 4He J. Wasa M. Chan K.S.L. Shao Q. Yu J.-Q. Palladium-catalyzed transformations of alkyl C–H Bonds.Chem. Rev. 2017; 117: 8754-8786https://doi.org/10.1021/acs.chemrev.6b00622Google Scholar However, enantioselective C–H functionalization based on this strategy is predominantly focused on C–C bond formation. A formidable challenge lies in that Pd(II)-catalyzed C–H activation/C–X bond formation generally requires the use of excess strong oxidants (e.g., PhI(OAc)2, BzO-NR2) and/or inorganic bases. These components could generate large amount of coordinating counter anions that compete with chiral ligands during the enantio-determining step, resulting in racemic background reaction and eroded chiral induction. To date, only a handful of examples of Pd(II)-catalyzed asymmetric C–H activation/C–X formation reactions have been developed. In 2008, Yu and co-workers reported the first Pd(II)-catalyzed enantioselective C–H alkylation of both C(sp2)–H and C(sp3)–H in pyridine-containing compounds using mono-N-protected amino acid (MPAA) as chiral ligands.20Shi B.-F. Maugel N. Zhang Y.-H. Yu J.-Q. PdII-catalyzed enantioselective activation of C(sp2)-H and C(sp3)-H bonds using monoprotected amino acids as chiral ligands.Angew. Chem. Int. Ed. Engl. 2008; 47: 4882-4886https://doi.org/10.1002/anie.200801030Google Scholar This has triggered the prosperity of MPAA enabled Pd(II)-catalyzed asymmetric C–H activation reactions, including several C–X formation variants.21Shao Q. Wu K. Zhuang Z. Qian S. Yu J.-Q. From Pd(OAc)2 to chiral catalysts: the discovery and development of bifunctional mono-N-protected amino acid ligands for diverse C–H functionalization reactions.Acc. Chem. Res. 2020; 53: 833-851https://doi.org/10.1021/acs.accounts.9b00621Google Scholar In 2013, Wang, Yu, and co-workers reported a Pd(II)-catalyzed enantioselective intramolecular C(sp2)–H activation/C–O forming reaction enabled by N-Boc-Ile-OH ligand (MPAA-1) based on desymmetrization strategy (Figure 2A).22Cheng X.-F. Li Y. Su Y.-M. Yin F. Wang J.-Y. Sheng J. et al.Pd(II)-catalyzed enantioselective C–H activation/C–O bond formation: synthesis of chiral benzofuranones.J. Am. Chem. Soc. 2013; 135: 1236-1239https://doi.org/10.1021/ja311259xGoogle Scholar The reaction was carried out in the presence of excess PhI(OAc)2 oxidant, representing the first example of enantioselective C–H functionalization through Pd(II)/Pd(IV) catalysis. The remarkably high reactivities and enantioselectivities could be predominantly ascribed to two facts: (1) the competitive coordination of acetate anion that hampers the enantioselectivity was minimized because the coordinating ability of such monodentate component is relatively lower than the bidentate MPAA-type chiral ligands; (2) MPAA-1 was used in relatively high loading (40 mol % of ligand for 5 mol % of Pd(OAc)2 catalyst) in most examples to compete over excess acetate. Various α-quaternary carbon-containing biarylacetic acids were compatible, furnishing chiral γ-lactones in moderate to good yields with high enantioselectivities (89% to 96% ee). MPAA ligands have also enabled the enantioselective C–H iodination based on both desymmetrization and kinetic resolution strategies (Figure 2B). In 2013, the Yu group pioneered an enantioselective C–H iodination through desymmetrization of trifluoromethanesulfonyl-protected diarylmethylamines.23Chu L. Wang X.-C. Moore C.E. Rheingold A.L. Yu J.-Q. Pd-catalyzed enantioselective C–H iodination: asymmetric synthesis of chiral diarylmethylamines.J. Am. Chem. Soc. 2013; 135: 16344-16347https://doi.org/10.1021/ja408864cGoogle Scholar In this mild iodination protocol (30°C), iodine was used as both the iodination reagent and the sole oxidant (Figure 2B; Equation 1). The high degree of chiral induction was ascribed to the judicious choice of N-Bz-Leu-OH (MPAA-2) as ligand and the combination of CsOAc and Na2CO3. The influence of competing coordination anions in inorganic bases was minimized by the increased ligand loading (40 mol %) and the addition of DMSO co-solvent (15 equiv). DMSO was proposed to suppress the racemic background reaction by sequestering the small portion of chiral ligand free Pd species. The protocol has further enabled the kinetic resolution of racemic benzylamines, affording highly enantioenriched iodinated products and substrates in up to 244 selectivity factors (s factor, Figure 2B; Equation 2).24Chu L. Xiao K.-J. Yu J.-Q. Room-temperature enantioselective C–H iodination via kinetic resolution.Science. 2014; 346: 451-455https://doi.org/10.1126/science.1258538Google Scholar Meanwhile, You and co-workers developed the kinetic resolution of quinoline-N-oxides through Pd(II)-catalyzed asymmetric C–H iodination, using MPAA-3 as chiral ligand (Figure 2B; Equation 3, s factors, 4.1 to 27).25Gao D.-W. Gu Q. You S.-L. Pd(II)-catalyzed intermolecular direct C–H Bond iodination: an efficient approach toward the synthesis of axially chiral compounds via kinetic resolution.ACS Catal. 2014; 4: 2741-2745https://doi.org/10.1021/cs500813zGoogle Scholar This method features a rare example of constructing axial chirality through C–H activation/C–X formation. Mechanistic studies in related work indicate that MPAA acts as a bidentate dianionic ligand.21Shao Q. Wu K. Zhuang Z. Qian S. Yu J.-Q. From Pd(OAc)2 to chiral catalysts: the discovery and development of bifunctional mono-N-protected amino acid ligands for diverse C–H functionalization reactions.Acc. Chem. Res. 2020; 53: 833-851https://doi.org/10.1021/acs.accounts.9b00621Google Scholar,26Cheng G.-J. Yang Y.-F. Liu P. Chen P. Sun T.-Y. Li G. et al.Role of N-acyl amino acid ligands in Pd(II)-catalyzed remote C–H activation of tethered arenes.J. Am. Chem. Soc. 2014; 136: 894-897https://doi.org/10.1021/ja411683nGoogle Scholar,27Cheng G.-J. Chen P. Sun T.-Y. Zhang X. Yu J.-Q. Wu Y.-D. A combined IM-MS/DFT study on [Pd(MPAA)]-catalyzed enantioselective C–H activation: relay of chirality through a rigid framework.Chemistry. 2015; 21: 11180-11188https://doi.org/10.1002/chem.201501123Google Scholar The amidate moiety in MPAA participates in the concerted-metalation deprotonation (CMD) process, while the side chain of the amino acid provides the source of chirality and affects the bite angle. The enantioselective C–H cleavage was proposed to proceed through a transition state with minimized repulsion between the substrate and amino acid side chain (Figure 2C, TS-2A). Based on the structure of MPAA, Yu and colleagues developed a class of bidentate acetyl-protected aminomethyl oxazoline (APAO) ligands for Pd(II)-catalyzed asymmetric C(sp3)–H arylation, alkenylation, and alkynylation.28Wu Q.-F. Shen P.-X. He J. Wang X.-B. Zhang F. Shao Q. et al.Formation of α-chiral centers by asymmetric β-C(sp3)-H arylation, alkenylation, and alkynylation.Science. 2017; 355: 499-503https://doi.org/10.1126/science.aal5175Google Scholar Beyond these Pd(II)/Pd(IV) process, they further applied APAOs to the asymmetric C(sp3)–H borylation that underwent Pd(II)/Pd(0) catalytic cycle (Figure 3).29He J. Shao Q. Wu Q. Yu J.-Q. Pd(II)-catalyzed enantioselective C(sp3)-H borylation.J. Am. Chem. Soc. 2017; 139: 3344-3347https://doi.org/10.1021/jacs.6b13389Google Scholar A range of borylated cyclobutanecarboxylic amides were obtained in good yields with excellent enantioselectivities (up to 99.8% ee). Interestingly, α-alkyl substitution at the amide could slightly enhance the enantioselectivity. Utilization of ligands lacking a stereogenic center on either the oxazoline motif or the side chain only led to decreased reactivity and enantioselectivity. The crucial role of both chiral centers on the APAO ligand backbone was explained by an enantiocontrol mode shown in Figure 3B. This protocol was also expanded to include the desymmetrization of other cyclic systems (e.g., cyclopropane and cyclohexane), as well as gem-methyl groups of acyclic aliphatic amides. Crucial to this success is the adjustment of the steric hindrance and relative configuration of the two chiral carbons within the ligands. The enantioenriched borylation products could be readily transformed into fluoro-, hydroxyl-, and aryl-containing compounds without erosion of chirality. Enantioselective functionalization of unbiased methylene C(sp3)–H bonds has been a longstanding challenge in asymmetric synthesis.30Chen G. Gong W. Zhuang Z. Andrä M.S. Chen Y.-Q. Hong X. et al.Ligand-accelerated enantioselective methylene C(sp3)-H bond activation.Science. 2016; 353: 1023-1027https://doi.org/10.1126/science.aaf4434Google Scholar,31Yan S.-B. Zhang S. Duan W.-L. Palladium-catalyzed asymmetric arylation of C(sp3)-H bonds of aliphatic amides: controlling enantioselectivity using chiral phosphoric amides/acids.Org. Lett. 2015; 17: 2458-2461https://doi.org/10.1021/acs.orglett.5b00968Google Scholar In 2018, Shi and co-workers reported the first strongly coordinating bidentate directing group (DG) enabled Pd(II)-catalyzed enantioselective arylation of unbiased methylene C–H bonds.32Yan S.-Y. Han Y.-Q. Yao Q.-J. Nie X.-L. Liu L. Shi B.-F. Palladium(II)-catalyzed enantioselective arylation of unbiased methylene C(sp3)-H bonds enabled by a 2-pyridinylisopropyl auxiliary and chiral phosphoric acids.Angew. Chem. Int. Ed. Engl. 2018; 57: 9093-9097https://doi.org/10.1002/anie.201804197Google Scholar The key to the success was the combination of their previously established 2-pyridinylisopropyl (PIP) DG with non-C2 symmetric chiral phosphoric acid ligands.32Yan S.-Y. Han Y.-Q. Yao Q.-J. Nie X.-L. Liu L. Shi B.-F. Palladium(II)-catalyzed enantioselective arylation of unbiased methylene C(sp3)-H bonds enabled by a 2-pyridinylisopropyl auxiliary and chiral phosphoric acids.Angew. Chem. Int. Ed. Engl. 2018; 57: 9093-9097https://doi.org/10.1002/anie.201804197Google Scholar They further identified 3,3′-disubstituted BINOLs as a type of more readily accessible and efficient chiral ligands, which found its broad application in highly enantioselective alkynylation, alkenylation/aza-Wacker cyclization, and inter-/intramolecular arylation of unbiased methylene C(sp3)–H bonds.33Han Y.-Q. Ding Y. Zhou T. Yan S.-Y. Song H. Shi B.-F. Pd(II)-catalyzed enantioselective alkynylation of unbiased methylene C(sp3)-H bonds using 3,3’-fluorinated-BINOL as a chiral ligand.J. Am. Chem. Soc. 2019; 141: 4558-4563https://doi.org/10.1021/jacs.9b01124Google Scholar,34Zhang Q. Shi B.-F. 2-(Pyridin-2-yl)isopropyl (PIP) amine: an enabling directing group for divergent and asymmetric functionalization of unactivated methylene C(sp3)-H bonds.Acc. Chem. Res. 2021; 54: 2750-2763https://doi.org/10.1021/acs.accounts.1c00168Google Scholar Very recently, the groups of Shi35Zhou T. Jiang M.-X. Yang X. Yue Q. Han Y.-Q. Ding Y. et al.Synthesis of chiral β-lactams by Pd-catalyzed enantioselective amidation of methylene C(sp3)–H bonds.Chin. J. Chem. 2020; 38: 242-246https://doi.org/10.1002/cjoc.201900533Google Scholar and Chen36Tong H.-R. Zheng W. Lv X. He G. Liu P. Chen G. Asymmetric synthesis of β-lactam via palladium-catalyzed enantioselective intramolecular C(sp3)–H bonds.ACS Catal. 2020; 10: 114-120https://doi.org/10.1021/acscatal.9b04768Google Scholar independently developed Pd(II)-catalyzed intramolecular enantioselective C(sp3)–H amidation using 3,3′-disubstituted BINOL ligands enabled by bidentate DGs, streamlining the synthesis of a series of chiral β-lactams (Figure 4). Judicious choice of Pd(II) catalyst, iodoarene oxidants and bidentate DGs was crucial for the remarkable reaction performance. Although Chen’s choice of 8-aminoquinoline-derived DGs delivered good yields and high enantioselectivities exclusively for benzylic methylene C(sp3)–H bonds, both benzylic and unbiased methylene C(sp3)–H bonds are compatible with Shi’s system using PIP DG. The improved enantioselectivity for aliphatic substrates in the latter case might be a result of steric communication between the gem-dimethyl moiety of PIP DG and the backbone of the chiral ligands (Figure 4C).34Zhang Q. Shi B.-F. 2-(Pyridin-2-yl)isopropyl (PIP) amine: an enabling directing group for divergent and asymmetric functionalization of unactivated methylene C(sp3)-H bonds.Acc. Chem. Res. 2021; 54: 2750-2763https://doi.org/10.1021/acs.accounts.1c00168Google Scholar Transient directing group (TDG) has recently emerged as an appealing strategy to improve the overall atom- and step-economy of transition-metal-catalyzed C–H activation. The reversible installation and removal of TDG allows the use of native substrates without the tedious installation and removal of external DGs. Additionally, the utilization of chiral TDG has brought new opportunities to catalytic asymmetric C–H activation,37Liao G. Zhang T. Lin Z.-K. Shi B.-F. Transition metal-catalyzed enantioselective C–H functionalization via chiral transient directing group strategies.Angew. Chem. Int. Ed. Engl. 2020; 59: 19773-19786https://doi.org/10.1002/anie.202008437Google Scholar,38Lapuh M.I. Mazeh S. Besset T. Chiral transient directing groups in transition-metal-catalyzed enantioselective C–H Bond functionalization.ACS Catal. 2020; 10: 12898-12919https://doi.org/10.1021/acscatal.0c03317Google Scholar as exemplified by the pioneering work of Yu and co-workers on enantioselective C–H arylation of 2-alkyl-benzaldehydes with L-tert-leucine as TDG.39Zhang F.-L. Hong K. Li T.-J. Park H. Yu J.-Q. Functionalization of C(sp3)-H bonds using a transient directing group.Science. 2016; 351: 252-256https://doi.org/10.1126/science.aad7893Google Scholar Based on the same strategy, they further established the enantioselective fluorination of benzylic C(sp3)–H bonds (Figure 5).40Park H. Verma P. Hong K. Yu J.-Q. Controlling Pd(IV) reductive elimination pathways enables Pd(II)-catalysed enantioselective C(sp3)-H fluorination.Nat. Chem. 2018; 10: 755-762https://doi.org/10.1038/s41557-018-0048-1Google Scholar This was particularly challenging due to the sluggish C(sp3)–F reductive elimination (RE) from Pd(IV) intermediates. The chemoselectivity and enantioselectivity were improved by increasing the side-chain bulkiness of TDG and utilizing C6F5CO2H in the place of acetic acid. More importantly, the replacement of chiral amino acid TDGs with the corresponding diethylamides generated a cationic Pd(IV) species, which strongly favors C(sp3)–F RE over undesired C–O formation pathway. The opposite absolute configuration of the enantioenriched C–O formation by-product with the fluorinated product was explained by two distinct mechanisms. The former was supposed to be generated via SN2-type RE, whereas the latter was a result of inner-sphere RE. A series of ortho- or meta-occupied o-alkyl benzaldehydes bearing electron-deficient substituents were fluorinated in moderate to good yields and high enantioselectivities (up to 99% ee). Importantly, the resulting ortho-fluorinated product could be transformed into diverse C–N, C–O, and C–S bonds, which greatly enriched the synthetic diversity of this protocol. Ever since the independent elegant work by the Cramer41Ye B. Cramer N. Chiral cyclopentadienyl ligands as stereocontrolling element in asymmetric C–H functionalization.Science. 2012; 338: 504-506https://doi.org/10.1126/science.1226938Google Scholar and Rovis42Hyster T.K. Knörr L. Ward T.R. Rovis T. Biotinylated Rh(III) complexes in engineered streptavidin for accelerated asymmetric C–H activation.Science. 2012; 338: 500-503https://doi.org/10.1126/science.1226132Google Scholar groups in 2012, enantioselective C–H functionalization based on CpxM(III) catalysis have been extensively studied.17Yoshino T. Matsunaga S. 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Emerging porous organic polymers for biomedical applications

Abstract: Porous organic polymers (POPs) have emerged as a new class of multifunctional porous materials and received tremendous research attention from both academia and industry. Most POPs are constructed from versatile organic small molecules with diverse linkages through strong covalent bonds. Owing to their high surface area and porosity, low density, high stability, tunable pores and skeletons, and ease of functionalization, POPs have been extensively studied for gas storage and separation, heterogeneous catalysis, biomedicine, sensing, optoelectronics, energy storage and conversion, etc. Particularly, POPs are excellent platforms with exciting opportunities for biomedical applications. Consequently, considerable efforts have been devoted to preparing POPs with an emphasis on their biomedical applications. In this review, first, we briefly describe the different subclasses of POPs and their synthetic strategies and functionalization approaches. Then, we highlight the state-of-the-art progress in POPs for a variety of biomedical applications such as drug delivery, biomacromolecule immobilization, photodynamic and photothermal therapy, biosensing, bioimaging, antibacterial, bioseparation, etc. Finally, we provide our thoughts on the fundamental challenges and future directions of this emerging field.