Oxidation

Hypochlorites and perchlorates as oxidizing agents

Hypochlorites and perchlorates are inorganic chlorine-based oxidants that operate through distinct mechanisms and find application across synthetic and industrial chemistry. Hypochlorites such as NaOCl and Ca(OCl)₂are strong oxidants used in alcohol oxidations, epoxidations, and heteroatom oxidations, and are most effective in combination with catalysts such as TEMPO or metal porphyrins to achieve the desired selectivity. Perchlorates such asNaClO₄, Mg(ClO₄)₂, LiClO₄, and Fe(ClO₄)₃ are weaker oxidants but strong Lewis acids, used primarily as electrolyte additives, dehydrating agents, and activators in Lewis acid-catalyzedreactions; Fe(ClO₄)₃ also serves as a mild single-electron oxidant in radical and coupling reactions, while LiClO₄ is widely used in ether-based electrolyte systems and as a promoter in Diels-Alder cycloadditions and other pericyclic reactions.

Peroxides as oxidizing agents

Peroxide-based reagents such as H₂O₂, meta-chloroperoxybenzoic acid (mCPBA), cumene hydroperoxide, and dicumylperoxide are used as oxidants for epoxidations, Baeyer-Villiger oxidations, and heteroatom oxidations. mCPBA is favored for alkene epoxidation due to its selectivity, while cumene hydroperoxide and dicumylperoxide serve as radical initiators and oxidants in industrial processes such as propylene oxide production and polymer chemistry. H₂O₂ is particularly favored in catalytic systems for its low cost and water as the sole byproduct.

Chromates as oxidizing agents

Chromium(VI)-based reagents such asCrO₃, pyridinium chlorochromate (PCC), pyridinium dichromate (PDC), andK₂Cr₂O₇ are among the most widely used oxidants in organic synthesis, capable of oxidizing primary and secondary alcohols to aldehydes/ketones and carboxylic acids. PCC is particularly valued for its selectivity in stopping oxidation at the aldehyde stage, while PDC offers milder, neutral conditions suitable for acid-sensitive substrates like allylic and propargylic alcohols.

Sulfur oxides as oxidizing agents

Sulfur-based oxidants, including DMSO (activated via Swern, Pfitzner–Moffatt, or Parikh-Doeringconditions), SO₃, and sulfonyl peroxides, are used primarily for alcohol oxidations and heteroatom functionalization. Activated DMSO systems convert primary and secondary alcohols to aldehydes and ketones under mild conditions, with the choice of activating agent dictating temperature and substrate compatibility. Pyridinesulfurtrioxide complex (SO₃-py) is particularly effective for oxidizing primary alcohols to aldehydes without over-oxidation.

N-oxides as oxidizing agents

N-oxides such as N-methylmorpholine N-oxide (NMO) and trimethylamine N-oxide (TMANO) serve primarily as co-oxidants in catalytic oxidation systems, regenerating the active metal catalyst rather than acting as stoichiometric oxidants directly. TMANO is used as a co-oxidant in the Wacker-type oxidation and in osmium tetroxide-mediated dihydroxylation reactions. It enables syn-dihydroxylation of alkenes under mild aqueous conditions with excellent functional group tolerance. NMO is used as a co-oxidant in the Upjohn dihydroxylation process alongside catalytic OsO₄ and plays a central role in TEMPO-mediated alcohol oxidations where it acts as the terminal oxidant re-oxidizing TEMPO back to its active oxoammonium form. 8-EthylquinolineN-oxide is used in metal-catalyzedoxidative transformations where precise modulation of oxidant geometry and redox potential is required to achieve selectivity that is not accessible with simpler aliphatic N-oxide reagents.

Hypervalent iodine as oxidizing agents

Hypervalent iodine reagents are compounds in which iodine bears more than eight electrons in its valence shell, divided into two classes:trivalentλ³-iodanes (iodine (III)) andpentavalentλ⁵-iodanes (iodine(V)). These reagents operate under mild, metal-free conditions to oxidize alcohols to aldehydes or ketones, and to perform α-functionalization of carbonyl compounds, dearomatization, and C-heteroatom bond formation.

Dess-Martin Periodinane (DMP)and 2-Iodoxybenzoic acid(IBX)are pentavalent λ⁵-iodanes; DMP is favored for its selectivity and tolerance of sensitive functional groups, while IBX is notable for oxidizing alcohols directly in DMSO without side reactions.Togni's reagent (3,3-Dimethyl-1-(trifluoromethyl)-1,2-benziodoxole) and (diacetoxyiodo) benzene (PIDA) are trivalent λ³-iodanes used for electrophilic trifluoromethylation and oxidative functionalization of heteroatoms and aromatic systems, respectively.

Selected named oxidation reactions

Baeyer–Villiger oxidation

Baeyer–Villiger oxidation uses peracids, such as MCPBA, trifluoroperacetic acid, or a peroxide as an oxidizing agent to convert ketones into esters and aldehydes into formate esters. Modifications in 2004 by Brink, Arends, and Sheldon to the BV reaction have made it more sustainable by using hydrogen peroxide as the oxidant.¹

Jones oxidation

Jones oxidation is used for conversion of primary alcohol into a carboxylic acid or a secondary alcohol into a ketone, using chromium trioxide (CrO₃) dissolved in dilute sulfuric acid (known as Jones reagent) as a strong oxidizing agent in acetone as a solvent. Modifications to this reaction, such as Collins oxidation with the Collins reagent, are now prevalently used because of higher selectivity and milder conditions.

Oppenauer oxidation

Oppenauer oxidation is the selective oxidation of a secondary alcohol to the corresponding ketone using aluminum alkoxide (such as aluminum triisopropoxide, Al(OiPr)₃) as a Lewis acid catalyst in the presence of a sacrificial ketone (such as acetone or cyclohexanone) as a hydride acceptor under the principle of Le Chatelier's equilibrium shift.

Pinnick oxidation

Pinnick oxidation is a mild selective oxidation of an aldehyde to the corresponding carboxylic acid using sodium chlorite (NaClO₂) as the primary oxidant in the presence of a scavenger (such as 2-methyl-2-butene or resorcinol) to quench the hypochlorous acid byproduct and a phosphate buffer to maintain mildly acidic aqueous conditions.

Rubottom oxidation

Rubottom oxidation is theα-hydroxylation of carbonyls compounds via the reaction of silyl enol ethers and peroxyacids, which was first observed using MCPBA as the peroxyacid source. One notable example is the total synthesis of (-)-strictosidine reported by the Garg research group, wherein Rubottom oxidation is utilized to prepare the key α-siloxyketone intermediate.²

Kornblum oxidation

Kornblum oxidation uses dimethyl sulfoxide(DMSO)as both the solvent and the oxidizing agent in the presence sodium bicarbonate or triethylamine as a base to convert primary alkyl halide into corresponding aldehydes.

Ley–Griffith oxidation

Ley–Griffith oxidation is the stoichiometric/catalytic oxidation of a primary alcohol to the corresponding aldehyde or a secondary alcohol to the corresponding ketone using tetrapropylammonium perruthenate (TPAP) as a catalytic oxidant in the presence of N-methylmorpholineN-oxide (NMO) as a co-oxidant and molecular sieves as a drying agent.

Swern oxidation

Swern oxidation is a stoichiometric oxidation of a primary alcohol to the corresponding aldehyde or a secondary alcohol to the corresponding ketone using oxalyl chloride to activate dimethyl sulfoxide (DMSO) at low temperature (-78°C) in the presence of a base (such as triethylamine) to form carbonyl product.

Corey-Kim oxidation

Corey–Kim oxidation uses N-chlorosuccinimide and dimethyl sulfide as an oxidizing agent in triethylamine as a base to convert primary alcohol to the corresponding aldehyde or a secondary alcohol to the corresponding ketone. The primary advantage over the Swern oxidation is that the reaction can be run at temperatures at or above -25 °C.

Pfitzner-Moffat oxidation

Pfitzner–Moffatt oxidation converts primary alcohols to the corresponding aldehydes and secondary alcohols to ketones by activating DMSO with dicyclohexylcarbodiimide (DCC) at room temperature in the presence of a proton source (e.g., phosphoric acid or pyridinium trifluoroacetate).

Upjohn Dihydroxylation

Upjohn Dihydroxylation is a syn-dihydroxylation of alkenes to give vicinal diols using a catalytic amount of osmium tetraoxide (OsO₄)re-oxidized in situ byN-methylmorpholineN-oxide (NMO)as the co-oxidant, typically in aqueous acetone. It made osmium tetraoxide practical for large-scale use, and laid the conceptual groundwork for Sharpless Asymmetric Dihydroxylation.

Sharpless asymmetric dihydroxylation

Sharpless asymmetric dihydroxylation converts alkenes to enantiomerically enriched vicinal diols using a catalytic amount of osmium tetraoxide in the presence of a chiral cinchona alkaloid ligand (DHQD-PHALfor AD-mix-β orDHQ-PHALfor AD-mix-α) and potassium ferricyanide as the stoichiometric re-oxidant, in at-BuOH/H₂O mixture at 0 °C. The reaction delivers 90% ee across a wide range of mono-, di-, and trisubstituted alkenes, and earned K. Barry Sharpless a share of the 2001 Nobel Prize in Chemistry.

Sharpless asymmetric epoxidation

Sharpless asymmetric epoxidation is the transformation of an allylic alcohol into a chiral 2,3-epoxyalcohol, using titanium tetraisopropoxide Ti (OiPr)₄ as a Lewis acid catalyst in combination with a tartrate ester (diethyl or isopropyl tartrate, DET or DIPT) as a chiral ligand and tert-butyl hydroperoxide (TBHP) as the terminal oxidant.

Wacker-Tsuji oxidation

Wacker Oxidation is the palladium-catalyzed oxidation of ethylene to acetaldehyde using PdCl₂/CuCl₂in aqueous conditions with O₂as the terminal oxidant, originally developed as an industrial process in the 1950s–60s. Wacker-Tsuji Oxidation broadens the scope to higher terminal alkenes, converting them to methyl ketones(Markovnikov carbonyl product).In both cases, the key mechanistic step is nucleophilic attack of water on a Pd (II)-coordinated alkene, but Wacker-Tsujioxidation offers more synthetic utility beyond the industrial ethylene-to-acetaldehyde transformation.

Dess–Martin oxidation

Dess–Martin oxidation uses Dess–Martin periodinane (DMP), a hypervalent iodine(V) reagent, as a mild oxidizing agent to selectively oxidize primary alcohols into aldehydes and secondary alcohols into ketones at room temperature to give the carbonized product without over-oxidation of aldehydes to carboxylic acids.