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Topic Review
Peptide Coupling Reaction
A peptide coupling reaction forms an amide bond between the carboxyl group of one amino-acid residue and the amino group of another. In conventional chemical coupling, the carboxylic acid is converted in situ or beforehand into a more electrophilic acyl species, after which nucleophilic attack by the amine, collapse of the tetrahedral intermediate, and loss of the activating group produce the peptide bond [1][2]. Carbodiimides, phosphonium salts, uronium reagents, active esters, acid halides, and mixed anhydrides are common activation classes [1]. The concept requires amide formation between amino-acid-derived partners and is therefore narrower than general amide synthesis, but it is not a transition-metal cross-coupling despite the shared word 'coupling'. Protecting groups or chemoselective activation normally control which amino and carboxyl groups react, while suppression of epimerization preserves residue configuration [2][3]. Peptide coupling is distinguished from native chemical ligation, which proceeds through a chemoselective capture and acyl-transfer sequence, and from ribosomal peptide-bond formation.
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Topic Review
Polyol Synthesis
Polyol synthesis is the chemical construction of an organic molecule bearing two or more hydroxyl groups, with control of their positions and, when relevant, relative and absolute configurations. The concept includes carbon–carbon bond-forming routes that generate alcohols, stereoselective reduction of hydroxy carbonyl compounds, epoxide opening, dihydroxylation, hydroboration–oxidation, and iterative homologation of oxygenated fragments. For 1,3-diols and extended 1,3-polyols, syn and anti relationships are commonly established by catalyst-controlled carbonyl addition or by diastereoselective reduction of β-hydroxy ketones [1][2]. Cascade strategies can create several carbon–carbon bonds and alcohol stereocenters in one sequence while remaining polyol synthesis because the polyhydroxy product is the target [3]. The term excludes the materials-science 'polyol process', in which a polyol is a solvent and reducing agent for metal nanoparticles rather than the compound being synthesized. It also excludes monohydric alcohol synthesis and mere extraction of naturally occurring polyols.
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Topic Review
Solid Acid Catalysis
Solid acid catalysis is the acceleration of a chemical transformation by Brønsted or Lewis acidic sites that are localized on an insoluble solid, so that the catalyst occupies a separate phase from the reactants [1][2]. In synthesis and catalytic reactions the concept is bounded by a heterogeneous acid—zeotype, heteropolyacid, sulfonated polymer, metal oxide, or related solid—and by protonation or coordinative activation of the substrate at that solid surface. Essential features are an accessible acidic site whose strength and local environment determine the elementary proton-transfer or hydride-transfer step, a porous or high-area architecture that presents those sites, and the absence of a dissolved molecular acid as the catalytic species [2][3]. Silver-exchanged zeolites and heteropolyacids illustrate how a cation associated with the solid acid can modify the protonic reactivity without converting the catalyst into a soluble acid [3]. The same solid-acid sites mediate carbonyl–nitrogen condensations and biomass-derived dehydrations when the substrate can adsorb at the protonic center [1][4]. The concept is distinguished from homogeneous Brønsted-acid catalysis by the phase-separated acid and by the additional constraints of pore size, site isolation, and adsorption equilibria. Conceptual limits exclude solid bases and exclude solids that function only as supports for a molecular acid that leaches into solution.
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Topic Review
Acid-Base Catalysis
Acid-base catalysis is the acceleration of a chemical reaction by proton transfer to or from the substrate in the rate-determining or product-determining step, so that an acid, a base, or a pair of acidic and basic sites stabilizes the transition state relative to the uncatalyzed path [1][2]. In catalysis and oxidation chemistry the concept is bounded by that proton-transfer elementary step as the catalytic event, and it includes both specific catalysis (by H+ or OH− of the medium) and general catalysis (by buffer acids and bases that are present in the transition state). Essential features are a donor and an acceptor of the proton, a defined timing of proton transfer relative to heavy-atom motion, and regeneration of the acid and base so that they are not consumed. Bifunctional acid-base catalysts place the two sites in a single molecular architecture, as in designed organocatalysts for the direct aldol reaction, so that enolate formation and carbonyl activation occur in one assembly [3]. The same proton-transfer logic operates at enzymatic active sites, including ribozymes and glycosidases, where nucleophilic attack is coupled to general acid-base assistance [1][4]. The concept is distinguished from nucleophilic catalysis, in which the catalyst forms a covalent adduct with the substrate, and from Lewis-acid catalysis, in which activation occurs by coordination of an electron pair rather than by proton transfer.
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Topic Review
Acidic Catalysis
Acidic catalysis is the acceleration of a transformation by protonation (or by a strongly hydrogen-bonding Brønsted acid) of a reactant, thereby increasing the electrophilicity of that reactant and lowering the barrier to nucleophilic attack or to rearrangement [1][2]. In catalysis and oxidation reactions the concept is bounded by an acidic species as the catalytic agent and by a mechanism in which the substrate is activated as an electrophile rather than as a nucleophile. Essential features are a Brønsted acid—molecular or solid—whose proton is transferred or strongly shared in the transition state, and turnover that returns the proton to the catalyst. Typical substrates are glycosyl donors, carbonyl compounds, and alkenes that become oxocarbenium, carboxonium, or carbocation-like intermediates upon protonation [1][2]. Sulfonated porous organic polymers illustrate a heterogeneous realization in which strongly acidic sulfonic groups are embedded in a hydrophobic framework that presents the proton to organic substrates [3]. The concept is distinguished from acid-base bifunctional catalysis by the absence of a necessary basic site in the catalytic cycle, and from Lewis-acid catalysis by the identity of the activating interaction as proton transfer rather than coordination to a vacant orbital. Conceptual limits exclude stoichiometric protonations that consume the acid and exclude oxidations in which acidity is incidental to a metal-centered redox event.
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Topic Review
Alcohol Oxidation
Alcohol oxidation is the conversion of a primary or secondary alcohol into the corresponding carbonyl compound—an aldehyde, ketone, or, under further oxidation of a primary alcohol, a carboxylic acid—by removal of hydrogen from the carbinol carbon and the hydroxyl group [1][2]. In catalysis and oxidation reactions the concept is bounded by that two-electron (or equivalent hydrogen-transfer) change at the alcohol carbon, and it excludes combustion of the entire molecule to CO2 as the defining event. Essential features are an oxidant or acceptor that takes up the extracted hydrogen or electrons, a catalyst that mediates the transfer, and a chemoselectivity that stops at a chosen oxidation level. Homogeneous acceptorless dehydrogenative oxidation releases H2 and leaves the carbonyl without a sacrificial oxidant [3]. Aerobic and electrocatalytic variants couple alcohol oxidation to O2 reduction or to anodic electron flow, often through a mediator that shuttles electrons and protons [2][4]. Heterogeneous gold clusters and plasmon-activated semiconductors provide surface pathways in which the alcohol adsorbs, loses hydrogen, and desorbs as the carbonyl [2]. The concept is distinguished from alkane C–H oxidation by the pre-existing C–O bond of the alcohol, and from oxidative esterification by termination at the carbonyl (or acid) rather than at an ester.
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Topic Review
Catalytic Oxidation
Catalytic oxidation is an oxidation in which a catalyst mediates electron and/or oxygen-atom transfer from an oxidant to a substrate and is regenerated in the catalytic cycle [1][2]. In catalysis and oxidation reactions the concept is bounded by a net increase in the oxidation state of the substrate and by turnover of the catalytic species, whether a metal surface, a molecular metal complex, or a mediator. Essential features are an oxidant (O2, H2O2, or an anodic equivalent), a catalyst that activates that oxidant or the substrate, and a sequence of electron-transfer steps that returns the catalyst to its resting state [2][3]. Heterogeneous variants include plasmon-enhanced oxidation at silver nanostructures and interface-confined ferrous centers that activate oxygen at a metal–oxide boundary [1][4]. Homogeneous biomimetic systems couple a metal catalyst to a coupled electron-transfer chain so that molecular oxygen or hydrogen peroxide can be used without stoichiometric metal consumption [3]. The concept is distinguished from stoichiometric oxidation by regeneration of the oxidant-activating species, and from catalytic reduction by the direction of electron flow. Conceptual limits exclude combustions in which the catalyst only ignites an uncontrolled radical chain and exclude oxygen-atom transfers that do not change the substrate’s formal oxidation state.
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Topic Review
Methane Oxidation
Methane oxidation is the conversion of CH4 to a more oxidized carbon product—methanol, CO, CO2, or biomass carbon—by transfer of oxygen atoms and/or electrons to the methane carbon. In catalysis and oxidation reactions the concept is bounded by methane as the substrate and by a net increase in the oxidation state of that carbon. Essential features are activation of the inert C–H bond, an oxidant (O2, a metal-oxo, nitrite, or a metal ion), and a pathway that determines whether the carbon stops at a partially oxidized product or proceeds to CO2. Anaerobic methane oxidation couples CH4 oxidation to denitrification or to the reduction of manganese or iron, so that oxygen in the formal sense need not be the terminal electron acceptor [1][2][3]. Aerobic methanotrophy uses O2 to hydroxylate methane at a metalloenzyme, including in extremely acidophilic Verrucomicrobia [4]. Nitrogen species can regulate the same oxidation in soils and sediments by competing at the methane-activating enzyme or by supplying an alternative electron acceptor [1][2]. The concept is distinguished from methane pyrolysis, which does not raise the carbon oxidation state by oxygen-atom or electron-acceptor coupling, and from Fischer–Tropsch chemistry, which builds C–C bonds from CO rather than oxidizing CH4. Conceptual limits exclude combustion descriptions that omit the catalytic or enzymatic C–H activation step as the defining event.
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Topic Review
Organocatalysis
Organocatalysis is the acceleration of a chemical transformation by a small organic molecule that is not a metal complex and is regenerated during the catalytic cycle [1][2]. In advanced synthetic organic chemistry the concept is bounded by the exclusive use of organic compounds—typically amines, phosphoric acids, N-heterocyclic carbenes, or phosphines—as the catalytic species, thereby excluding transition-metal catalysis and biocatalysis as the primary mode of activation [2][3]. Essential features are a defined activation mode (enamine, iminium, Brønsted-acid, nucleophilic, or carbene umpolung), turnover of the organic catalyst, and, in asymmetric organocatalysis, a chiral catalyst that differentiates prochiral faces of the substrate [2]. N-Heterocyclic carbenes operate by forming covalent Breslow-type intermediates that invert the polarity of carbonyl carbon atoms [1]. Phosphine organocatalysts add to electron-deficient π-systems to generate zwitterionic intermediates that mediate a range of annulations and substitutions [4]. Dual manifolds that merge a photoredox cycle with an organocatalytic cycle remain organocatalytic with respect to the bond-forming activation of the organic substrate [2]. The concept is distinguished from ligand-assisted metal catalysis, in which the organic molecule is not itself the catalytic center.
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Topic Review
Chemical Synthesis
Chemical synthesis is the deliberate preparation of a chemical substance from other substances through one or more controlled chemical reactions. A synthesis specifies starting materials, transformations, reaction conditions, and an isolation sequence that establishes the constitution and, when relevant, stereochemistry of the product. In multistep synthesis, intermediates are connected by an ordered route in which bond formation, bond cleavage, functional-group interconversion, and stereochemical control progressively produce the target structure. The concept includes small-molecule, macromolecular, and biomolecular construction when assembly occurs through chemically executed reactions; total chemical synthesis of proteins, for example, forms peptide segments and joins them by chemical ligation [1]. Parallel chemical synthesis applies the same criterion through repeated, spatially addressed reaction cycles [2]. Chemical synthesis is distinct from isolation of an existing substance, purely physical mixing or self-assembly without covalent transformation, and biosynthesis executed by a living organism's metabolic machinery.
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Topic Review
Photoredox Catalysis
Photoredox catalysis is a catalytic mode in which a light-absorbing catalyst, upon excitation, engages in single-electron transfer with an organic substrate or a sacrificial redox partner, thereby generating radical or radical-ion intermediates under conditions that would not produce those intermediates thermally [1][2]. In radical photochemical reactions the concept is bounded by a closed photoredox cycle: the photocatalyst must be restored to its original oxidation state after the productive electron transfer. Essential features are a chromophore with a sufficiently long-lived excited state, a pair of excited-state redox potentials that match the substrate, and a subsequent radical transformation—addition, coupling, or fragmentation—of the photogenerated intermediate [1][3]. Catalysts include transition-metal polypyridyl complexes, especially iridium and ruthenium species, and organic dyes that operate by analogous electron-transfer cycles [1][2]. Dual catalytic systems merge the photoredox cycle with a second cycle, such as organocatalysis or nickel catalysis, so that the radical intermediate is intercepted in a controlled bond-forming step [4]. The concept is distinguished from direct photochemistry of the substrate, in which no catalytic chromophore mediates the electron transfer, and from energy-transfer photocatalysis, in which the excited catalyst transfers energy rather than an electron.
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Topic Review
Alkene Synthesis
Construction of a carbon–carbon double bond is the defining operation of alkene synthesis in organic chemistry. The new C=C unit may arise by elimination (dehydrohalogenation, dehydration, or dehydrodecarboxylation), by olefination of a carbonyl or imine, or by stereodefined coupling of two carbon fragments. Photocatalytic dehydrodecarboxylation of carboxylic acids illustrates an elimination route in which loss of CO2 and a hydrogen equivalent produces the alkene [1]. Transition-metal-mediated sequences such as alkyne hydroboration followed by tandem Negishi–Suzuki coupling or organoborate migratory insertion furnish tri- and tetrasubstituted alkenes with high regio- and stereochemical definition [2]. Carbonyl-equivalent olefinations, including reactions of activated imines with nonstabilized phosphonium ylides, generate the double bond by formal C=X to C=C conversion with tunable alkene geometry [3]. Across these manifolds the conceptual core is formation of the olefinic π bond itself, not subsequent functionalization of an already present alkene. Regio- and stereoselectivity are intrinsic attributes of the bonding event, because substitution pattern and E/Z (or facial) outcome are fixed when the double bond is created.
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Topic Review
Ammoxidation
Ammoxidation is an oxidative nitrogen-incorporation process in which a hydrocarbon or heteroatom-substituted substrate reacts with ammonia and an oxidant, typically molecular oxygen, to furnish a nitrile or, under modified conditions, a primary amide. Classically, an alkene or alkylarene is converted to the corresponding nitrile at a mixed-metal oxide surface; the same formal transformation can be effected on alcohols and carboxylic acids with molecular catalysts. Iron single-atom catalysts mediate selective ammoxidation of alcohols to nitriles, coupling dehydrogenation with nitrogen transfer from ammonia [1]. Copper-catalyzed aerobic decarboxylative ammoxidation of phenylacetic acids yields primary amides rather than nitriles, showing that the nitrogenous oxidation level is catalyst- and substrate-dependent [2]. Carboxylic acid-modified metal oxides allow the product distribution between nitrile and amide to be tuned under aerobic conditions [3]. The defining stoichiometric feature is concurrent oxidation and C–N bond formation using NH3 as the nitrogen source, which distinguishes ammoxidation from simple amination (no net oxidation) and from aerobic oxidation that does not incorporate nitrogen.
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Topic Review
Antimicrobial Properties
Antimicrobial properties are the effects by which a material suppresses the growth of, or kills, microorganisms that come into contact with it, and they are reported as an inhibition zone, a minimum inhibitory concentration, or a reduction in viable count over a stated contact time. Oxide and metal nanoparticles dispersed in a polymer matrix act through several routes at once: release of metal ions, generation of reactive oxygen species, and direct contact with the cell envelope, so a hybrid of oxide nanoparticles in chitosan shows activity against both Gram-positive and Gram-negative strains [1]. For silver the released ion is the dominant agent, and the rate at which the particle dissolves oxidatively therefore controls how long the effect lasts [2]. Chitosan contributes its own polycationic action and, when combined with a plant extract, also carries antioxidant activity [3]. Bringing such particles into a food-contact film raises the question of how much silver migrates out of it [4]. Composite films of gelatin and chitosan nanoparticles extend the same approach to edible packaging [5].
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Topic Review
Epoxidation
Conversion of a carbon–carbon double bond into a three-membered cyclic ether (an oxirane or epoxide) is epoxidation. An electrophilic oxygen atom is delivered to the alkene from a peroxy acid, a metal–oxo or metal–peroxo species, a dioxirane, or an ylide, forming two new C–O bonds in one operation. Chromium- and manganese–salen complexes promote alkene epoxidation through high-valent metal–oxo intermediates whose ligand environment controls enantioselectivity [1]. Homogeneous and heterogeneous catalytic asymmetric epoxidations generalize the same oxygen-atom transfer to a prochiral alkene under chiral catalysis [2]. Ylide-based epoxidation constructs the oxirane by addition of a sulfur or related ylide to a carbonyl, which is a complementary C–C/C–O route to the same ring rather than direct oxidation of an alkene [3]. The defining product is the epoxide; allylic oxidation, dihydroxylation, and aziridination are related atom-transfer reactions that install different three-membered rings or different oxidation patterns. Ring-opening of an already formed epoxide is a subsequent reaction, not part of the epoxidation event.
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Topic Review
Fischer Synthesis
In the organic-synthesis taxonomy, Fischer synthesis denotes the Fischer indole synthesis, an acid-catalyzed conversion of an arylhydrazone into an indole. The arylhydrazone is normally formed by condensation of an arylhydrazine with an aldehyde or ketone. Dedicated reviews define the Fischer reaction as acid-promoted indole formation from an arylhydrazone [1]. Mechanistic studies describe tautomerization to an enehydrazine, carbon–carbon bond-forming rearrangement, cyclization, and ammonia elimination to establish the aromatic indole nucleus [2]. The nitrogen attached to the aryl group becomes the indole nitrogen, whereas the carbonyl substrate determines the substituents introduced into the five-membered ring. The process requires an arylhydrazone capable of enehydrazine formation and is therefore distinct from Madelung, Bartoli, and Larock indole syntheses, which use different precursors and bond-forming steps. Fischer glycosidation and Fischer esterification share the chemist's name but are separate reactions and are not included in this concept.
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Topic Review
Low Temperature Fischer-Tropsch Synthesis
The Fischer–Tropsch reaction converts synthesis gas (CO + H2) into hydrocarbons and water on a solid catalyst by repeated C–C coupling of C1 surface intermediates. Low-temperature Fischer–Tropsch (LTFT) synthesis is the regime, typically near 200–250 °C on cobalt or iron catalysts, that favors long-chain n-alkanes and waxes over the lighter, more olefinic slate of high-temperature Fischer–Tropsch operation. ε-Iron carbide is an active iron phase for this low-temperature conversion of syngas to hydrocarbons [1]. Product selectivity within the LTFT window can be shifted from paraffins toward α-olefins by modifying the catalyst environment without leaving the low-temperature manifold [2]. Plasma-synthesized nanocatalysts for CO hydrogenation under LTFT conditions instantiate the same C1 polymerization on a nanostructured metal surface [3]. The concept is bounded as catalytic CO hydrogenation with C–C chain growth at the lower-temperature FT branch; it is not methanol synthesis, not the methanol-to-olefins reaction, and not high-temperature FT, whose operating temperature, catalyst formulation, and carbon-number distribution differ.
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Topic Review
Nickel Catalysis
Nickel catalysis, within catalytic cross-coupling chemistry, is the use of nickel complexes to mediate carbon–carbon and carbon–heteroatom bond construction through organonickel intermediates that cycle among accessible oxidation states, commonly Ni(0)/Ni(II) and Ni(I)/Ni(III) [1]. The concept is bounded by elementary steps of oxidative addition, transmetalation or radical capture, and reductive elimination at a nickel center, and it excludes processes in which nickel functions only as a stoichiometric reductant. Essential features include the earth-abundant metal’s comparatively high electropositivity, its ready access to one-electron pathways, and its capacity to activate bonds that are reluctant toward palladium, including certain C–O, C–N, and C–Cl linkages [1][2]. Dual catalytic manifolds merge a photoredox cycle with a nickel cycle so that an alkyl radical is captured by nickel and then coupled with an aryl electrophile [2][3]. Nickel also promotes the Nozaki–Hiyama–Kishi addition of alkenylchromium reagents generated from alkenyl triflates [4]. The field is distinguished from palladium catalysis by the greater incidence of radical elementary steps and by a different functional-group activation profile.
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Topic Review
Nucleophilic Catalysis
Nucleophilic catalysis is a mode of catalysis in which a nucleophilic species adds reversibly to an electrophilic functional group, generating a covalently activated intermediate that undergoes subsequent transformation more rapidly than the uncatalyzed substrate [1][2]. In asymmetric synthesis and catalysis the concept is bounded by turnover: the nucleophilic catalyst must be released after the bond-forming event so that it is not consumed as a stoichiometric reagent. Essential features include a nucleophilic atom (commonly nitrogen, phosphorus, or carbon in an N-heterocyclic carbene or related heterocycle), a defined addition–elimination or addition–transfer sequence, and, in enantioselective variants, a chiral catalyst architecture that differentiates the faces of the activated intermediate [3]. Typical electrophiles are acyl donors, aldehydes, and imine-type carbonyl analogues. Anilinium and related nucleophiles accelerate oxime ligation and hydrazone formation by forming a more reactive iminium or Schiff-base intermediate that then undergoes transimination [1][2]. 4-(Dialkylamino)pyridines operate by nucleophilic addition to acylating agents to give an N-acylpyridinium ion whose reactivity and selectivity depend on the substitution pattern of the pyridine [4]. The concept is distinguished from general-base catalysis, in which the catalyst only abstracts a proton, and from Lewis-acid catalysis, in which activation occurs by coordination rather than by covalent addition.
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Topic Review
Peptide Synthesis
Peptide synthesis is the controlled, stepwise formation of amide bonds between amino acids to produce a defined linear or cyclic peptide sequence. In organic chemistry synthesis methods the concept encompasses both chemical assembly and, by extension, nonribosomal enzymatic assembly, but it is bounded by the production of a peptide as the target rather than of a folded protein by ribosomal translation. Solid-phase peptide synthesis anchors the C-terminal residue to an insoluble resin and adds N-protected amino acids iteratively, so that excess reagents are removed by filtration after each coupling [1]. The 9-fluorenylmethoxycarbonyl (Fmoc) and tert-butoxycarbonyl (Boc) protecting-group schemes define two principal chemical strategies; Fmoc chemistry uses base-labile Nα protection, whereas Boc chemistry uses acid-labile Nα protection and often employs in situ neutralization to improve the assembly of difficult sequences [2][3]. Nonribosomal peptide synthetases construct peptides on modular protein assembly lines that activate, thiolate, and condense amino acids without a messenger-RNA template [4]. The concept is distinguished from protein expression by the chemical or modular-enzymatic rather than ribosomal mechanism, and from simple amide-bond formation by the requirement for sequence control across multiple residues.
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