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Topic Review
Photooxidation
Photooxidation is an oxidation whose initiating or driving event is absorption of light by the substrate, a sensitizer, or a photocatalyst. The excited species can transfer energy to triplet oxygen to generate singlet oxygen, transfer an electron to produce radical ions and superoxide, or generate a semiconductor hole that removes an electron from an adsorbed substrate. A net increase in substrate oxidation state and a necessary photonic step define the concept; dark autoxidation and photochemical reactions with no oxidative change are excluded. In sensitized photooxygenation, singlet oxygen reacts with organic π systems through ene reactions, (4+2) cycloadditions, or (2+2) pathways, and it can oxidize sulfides and phosphines [1]. Other photooxidations proceed by electron transfer and radical chains rather than singlet oxygen [2]. Photooxidation is broader than photooxygenation because oxygen-atom incorporation is not required, and it differs from photoredox catalysis because catalyst turnover is not part of every photooxidation.
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  • 23 Sep 2026
Topic Review
Radical Reaction
A radical reaction is a chemical transformation in which an open-shell species bearing one or more unpaired electrons is an essential intermediate, chain carrier, or product-determining species. Radicals can be generated by homolysis, single-electron transfer, hydrogen-atom transfer, photolysis, thermolysis, or redox activation. Their characteristic elementary steps include addition to π bonds, atom transfer, fragmentation, rearrangement, substitution, combination, and disproportionation [1]. A chain radical reaction contains initiation, propagation, and termination, whereas a nonchain radical process can proceed through discrete photochemical or electrochemical generation and trapping of each radical. Within radical photochemistry, light creates the radical directly or through excited-state electron or energy transfer [2]. The defining criterion is mechanistic participation of an open-shell intermediate, not merely exposure to light or an oxidant. Radical reactions are distinguished from closed-shell ionic and concerted pericyclic reactions by their one-electron bond-making and bond-breaking steps.
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  • 23 Sep 2026
Topic Review
Polyketide
Within the context of plant-derived bioactive compounds, a polyketide is an organic natural product whose principal carbon framework is biosynthetically assembled by polyketide synthases through successive carbon–carbon bond-forming condensations of acyl-derived building blocks [1][2]. The growing carbon chain is typically generated through decarboxylative condensation reactions involving a starter acyl unit and extender units derived from malonyl or related coenzyme A substrates, producing intermediates containing characteristic carbonyl functionality [1][3]. In plants, type III polyketide synthases constitute a major enzymatic basis for this chemistry and generally operate as homodimeric enzymes that use CoA thioesters directly during iterative chain extension [3]. The resulting polyketide framework can undergo controlled cyclization, aromatization, reduction, oxidation, and other enzymatic transformations that establish its final connectivity, oxidation state, substitution pattern, and stereochemical features. The concept therefore encompasses natural-product structures defined by polyketide-chain assembly and the chemically elaborated molecular frameworks generated from that biosynthetic carbon skeleton [1][2][3].
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  • 28 Sep 2026
Topic Review
Platinum Group Metal Catalysis
Platinum group metal catalysis uses ruthenium, rhodium, palladium, osmium, iridium, or platinum as the regenerated catalytic center in a homogeneous or heterogeneous reaction. Molecular PGM catalysts operate through ligand-controlled organometallic steps such as oxidative addition, migratory insertion, β-hydride elimination, and reductive elimination. Heterogeneous PGM catalysts instead present metallic, oxidic, nanoparticle, single-atom, or supported sites at which reactants adsorb and undergo bond activation [1][2]. Accessible oxidation states, strong yet tunable substrate binding, and the ability to activate H–H, C–H, C–X, C=C, O–O, and related bonds are characteristic features of this catalyst family. The class is defined by the identity and turnover of the metal, not by a single reaction mechanism. It excludes stoichiometric use of PGM compounds and processes in which a PGM is only a support or spectator. PGM catalysis is narrower than noble-metal catalysis, which can also include gold and silver, and distinct from base-metal catalysis.
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  • 23 Sep 2026
Topic Review
Biooxidation
Biooxidation is oxidation catalyzed by an enzyme or a living biological system, resulting in net electron removal from an organic or inorganic substrate. Oxidases transfer reducing equivalents to molecular oxygen, dehydrogenases transfer them to cofactors such as NAD(P)+, oxygenases incorporate one or both atoms of O2 into a substrate, and peroxidases use peroxides as electron acceptors [1][2]. Whole-cell biooxidation can couple these enzyme reactions to cofactor regeneration and respiratory electron transport, while isolated-enzyme biooxidation contains only the catalytic protein and required cofactors. The class is defined by the biological catalyst and oxidative direction, not by a particular substrate or terminal acceptor. It therefore includes microbial oxidation of inorganic species and selective oxidation of organic functional groups, but excludes abiotic oxidation and biological transformations with no change in substrate oxidation state. Biooxidation is narrower than biotransformation generally and is the oxidative counterpart of enzymatic reduction.
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  • 23 Sep 2026
Topic Review
Macromolecular Chemistry
Macromolecular chemistry is the branch of chemistry concerned with the molecular constitution, synthesis, stereochemistry, transformation, and structural characterization of macromolecules and polymeric substances. A macromolecule is a molecule of high relative molecular mass whose structure substantially comprises multiple repetitions of units derived, actually or conceptually, from molecules of lower relative molecular mass, while a polymer is a substance composed of macromolecules [1]. The chemical description of a macromolecular system encompasses monomer-derived constitutional units, covalent connectivity, degree of polymerization, molar-mass distribution, chain-end structure, comonomer sequence, branching, crosslinking, molecular topology, and stereochemical organization such as tacticity [1][2]. Macromolecular chemistry also encompasses the chemical reactions by which these structural features are established or transformed, including chain-growth, step-growth, ring-opening, and other polymerization processes and subsequent reactions of polymer chains [2][3]. Its defining scope therefore centers on the relationship between molecular synthesis, repeated-unit constitution, chain architecture, stereochemical arrangement, and the chemical identity of high-molar-mass molecular systems [1][2][3].
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  • 27 Sep 2026
Topic Review
Base Catalysis
Acceleration of a chemical transformation by a species that accepts a proton or donates an electron pair to a substrate defines base catalysis. In general base catalysis a proton is partially transferred in the rate-determining step and the catalyst is regenerated; in specific base catalysis the active nucleophile is the lyate ion formed in a prior equilibrium. Lewis base catalysis is the complementary closed-shell process in which the catalyst binds a Lewis-acidic site (a carbonyl carbon, a silicon center, or a π-acidic metal) and thereby activates the substrate without net proton transfer [1]. Dual catalytic systems may combine a Lewis base with a photoredox cycle so that the two catalysts jointly generate the nucleophilic and electrophilic partners of a C–C bond-forming step [2]. In ribonucleic acid catalysis, general acid–base catalysis likewise proceeds by concerted proton transfer at the phosphodiester, underscoring that the defining elementary act is proton shuttling rather than metal redox chemistry [3]. The concept excludes stoichiometric use of a strong base as a reagent: the base must be turned over. It is complementary to acid catalysis, which activates by proton donation or Lewis-acid coordination [1].
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  • 23 Sep 2026
Topic Review
Biomimetic Catalysis
Catalytic systems designed to reproduce essential structural or mechanistic features of enzyme active sites constitute biomimetic catalysis. The catalyst is abiotic—typically a metal complex, a porous framework, or a designed organic host—yet it implements first-coordination-sphere ligands, second-sphere hydrogen bonds, or confinement that parallel those of a metalloenzyme. Porous metal–organic frameworks provide isolated, structurally defined sites that function as heterogeneous biomimetic catalysts [1]. Bioinspired framework catalysts span the continuum from immobilized enzymes to wholly synthetic mimics in which the framework itself is the catalyst [2]. Metalloporphyrin-containing MOFs illustrate biomimetic oxidation catalysis in which a heme-like iron center is held in a porous lattice [3]. The defining criterion is mechanistic or structural analogy to a biological catalyst, not merely high activity: a conventional solid acid or a simple metal salt is not biomimetic unless it is organized to copy an enzymatic binding or activation motif. The concept therefore lies between enzymatic catalysis and ordinary chemocatalysis, bounded by the requirement of an enzyme-inspired active-site design.
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  • 23 Sep 2026
Topic Review
Cyclic Carbonate Synthesis
Five-membered cyclic carbonates arise principally from cycloaddition of carbon dioxide to an epoxide, a transformation that inserts CO2 as the carbonyl and one ring oxygen of the heterocycle. Mechanistically, a Lewis acid activates the epoxide toward ring opening by a nucleophilic cocatalyst (halide or related anion); the resulting alkoxide captures CO2, and the carbonate anion displaces the leaving group to close the five-membered ring [1]. Bimetallic aluminium–salen complexes catalyze this sequence with two metal centers cooperating in epoxide activation and nucleophilic delivery [2]. One-component catalysts incorporate both the Lewis-acidic and the nucleophilic functions in a single molecular species, so that no external cocatalyst is required [3]. Alternative routes include oxidative carbonylation of diols and rearrangement of acyclic carbonates, but the epoxide–CO2 coupling is the defining contemporary construction. The product is a cyclic organic carbonate, distinct from polycarbonates (which may form if the intermediate alkoxide polymerizes) and from acyclic dialkyl carbonates.
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