Wacker-Type Oxidation Using an Iron Catalyst and Ambient Air: Application to Late-Stage Oxidation of Complex Molecules

Reversal of the Regioselectivity of Iron-Promoted Hydrogenation and Hydrohalogenation of gem-Difluoroalkenes

The reaction regioselectivity of gem-difluoroalkenes is dependent on the intrinsic polarity. Thus, the reversal of the regioselectivity of the addition reaction of gem-difluoroalkenes remains a formidable challenge. Herein, we described an unprecedented reversal of regioselectivity of hydrogen atom transfer (HAT) to gem-difluoroalkenes triggered by Fe-H species for the formation of difluoroalkyl radicals. Hydrogenation of the in situ generated radicals gave difluoromethylated products. Mechanism experiments and theoretical studies revealed that the kinetic effect of the irreversible HAT process resulted in the reversal of the regioselectivity of this scenario, leading to the formation of a less stable α-difluoroalkyl radical regioisomer. On basis of this new reaction of gem-difluoroalkene, the iron-promoted hydrohalogenation of gem-difluoroalkenes for the efficient synthesis of aliphatic chlorodifluoromethyl-, bromodifluoromethyl- and iododifluoromethyl-containing compounds was developed. Particularly, this novel hydrohalogenation of gem-difluoroalkenes provided an effect and large-scale access to various iododifluoromethylated compounds of high value for synthetic application.

Enantioselective total synthesis of (‒)-lucidumone enabled by tandem prins cyclization/cycloetherification sequence

The Ganoderma meroterpenoids are a growing class of natural products with architectural complexity, and exhibit a wide range of biological activities. Here, we report an enantioselective total synthesis of the Ganoderma meroterpenoid (‒)-lucidumone. The synthetic route features several key transformations, including a) a Cu-catalyzed enantioselective silicon-tethered intramolecular Diels-Alder cycloaddition to construct the highly functionalized bicyclo[2.2.2]octane moiety; b) Brønsted acid promoted tandem O-deprotection/Prins cyclization/Cycloetherification sequence followed by oxidation to install concurrently the tetrahydrofuran and the fused indanone framework; c) Fleming-Tamao oxidation to generate the secondary hydroxyl; d) an iron-catalyzed Wacker-type oxidation of hindered vinyl group to methyl ketone.

Chemical and Redox Noninnocence of Pentane-2,4-dione Bis(S-methylisothiosemicarbazone) in Cobalt Complexes and Their Application in Wacker-Type Oxidation

Cobalt complexes with multiproton- and multielectron-responsive ligands are of interest for challenging catalytic transformations. The chemical and redox noninnocence of pentane-2,4-dione bis(S-methylisothiosemicarbazone) (PBIT) in a series of cobalt complexes has been studied by a range of methods, including spectroscopy [UV-vis, NMR, electron paramagnetic resonance (EPR), X-ray absorption spectroscopy (XAS)], cyclic voltammetry, X-ray diffraction, and density functional theory (DFT) calculations. Two complexes [CoIII(H2LSMe)I]I and [CoIII(LSMe)I2] were found to act as precatalysts in a Wacker-type oxidation of olefins using phenylsilane, the role of which was elucidated through isotopic labeling. Insights into the mechanism of the catalytic transformation as well as the substrate scope of this selective reaction are described, and the essential role of phenylsilane and the noninnocence of PBIT are disclosed. Among the several relevant species characterized was an unprecedented Co(III) complex with a dianionic diradical PBIT ligand ([CoIII(LSMe••)I]).

Markovnikov-Selective Cobalt-Catalyzed Wacker-Type Oxidation of Styrenes into Ketones under Ambient Conditions Enabled by Hydrogen Bonding

The replacement of palladium catalysts for Wacker-type oxidation of olefins into ketones by first-row transition metals is a relevant approach for searching more sustainable protocols. Besides highly sophisticated iron catalysts, all the other first-row transition metal complexes have only led to poor activities and selectivities. Herein, we show that the cobalt-tetraphenylporphyrin complex is a competent catalyst for the aerobic oxidation of styrenes into ketones with silanes as the hydrogen sources. Remarkably, under room temperature and air atmosphere, the reactions were exceedingly fast (up to 10 minutes) with a low catalyst loading (1 mol %) while keeping an excellent chemo- and Markovnikov-selectivity (up to 99 % of ketone). Unprecedently high TOF (864 h-1 ) and TON (5,800) were reached for the oxidation of aromatic olefins under these benign conditions. Mechanistic studies suggest a reaction mechanism similar to the Mukaiyama-type hydration of olefins with a change in the last fundamental step, which controls the chemoselectivity, thanks to a unique hydrogen bonding network between the ethanol solvent and the cobalt peroxo intermediate.

Synthesis of spirooxindoles via formal acetylene insertion into a common palladacycle intermediate

A palladium-catalyzed spirocyclization reaction is reported, which is proposed to arise via insertion of an oxabicycle into a palladacycle, formed from carbocyclization and a C-H functionalization sequence. Mechanistic studies suggest the insertion is diastereoselective and a post-catalytic retro-Diels-Alder step furnishes an alkene, wherein the oxibicycle has served as an acetylene surrogate. Aryl iodides and carbamoyl chlorides were compatible as starting materials under the same reaction conditions, enabling the convergent and complementary synthesis of spirooxindoles, as well as other azacycles. These spirooxindoles allowed further transformations that were previously unaccessible.

Two Copper-Containing Polyoxometalate-Based Metal-Organic Complexes as Heterogeneous Catalysts for the C-H Bond Oxidation of Benzylic Compounds and Olefin Epoxidation

Using a one-pot assembly method, two novel copper-containing Keggin-type polyoxometalates (POMs)-based metal-organic complexes, that is, [Cu^II₂(pbba)₂NO₃^-(H₂O)₂(PW₁₂O₄₀)]·3H₂O [PW₁₂-Cu-pbba, H₂pbba = 1,1'-(1,4-phenylene-bis(methylene))-bis(pyridine-3-carboxylic acid)] and [Cu^II₂(pbba)₂(H₂O)₂(GeW₁₂O₄₀)]·3H₂O (GeW₁₂-Cu-pbba), were successfully synthesized. These two complexes are isostructural, differing only in their POM components. They are applicable as heterogeneous catalysts for the C-H bond oxidation of benzylic compounds and olefin epoxidation under mild conditions, with oxygen as the oxidant and isobutyraldehyde as the coreductant. The catalytic activity of PW₁₂-Cu-pbba was superior to that of GeW₁₂-Cu-pbba. Under the optimal conditions, PW₁₂-Cu-pbba catalyzed the oxidation of indane into 1-indanone with an 81% yield and >99% selectivity within 48 h. As heterogeneous catalysts, both complexes demonstrated excellent recoverability and high stability and could be stably reused five times without significant activity loss.

Synthesis of functionalized malononitriles via Fe-catalysed hydrogen atom transfers of alkenes

Described herein is a practical and convenient approach that enabled radical-mediated conjugate addition of unreactive alkenes to electron-deficient alkenes leading to a broad range of substituted malononitriles. These reactions are believed to proceed by Fe-catalysed hydrogen atom transfer (HAT) onto the alkenes affording carbon-centered radical intermediates with Markovnikov selectivity, followed by the capture of electron-deficient alkenes. We explored this synthesis approach under mild conditions with high efficiency and broad substrate scope and the utility is highlighted by the further synthetic transformations of the obtained substituted malononitriles.

Synthesis of 2-hydroxytetrahydrofurans by Wacker-type oxidation of 1,1-disubstituted alkenes

1,1-Disubstituted alkenes feature high steric hindrance, which renders their Wacker-type oxidation difficult. We demonstrate the stereoselective synthesis of 2-hydroxytetrahydrofurans via the Wacker-type oxidation of 3-methyl-3-buten-1-ols by using a PdCl₂(MeCN)₂/NO/BQ catalyst system under 1 atm O₂ in H₂O or H₂O/DMF.

SOMOphilic Alkynylation of Unreactive Alkenes Enabled by Iron-Catalyzed Hydrogen Atom Transfer

We report an efficient and practical iron-catalyzed hydrogen atom transfer protocol for assembling acetylenic motifs into functional alkenes. Diversities of internal alkynes could be obtained from readily available alkenes and acetylenic sulfones with excellent Markovnikov selectivity. An iron hydride hydrogen atom transfer catalytic cycle was described to clarify the mechanism of this reaction.

Mechanistic Studies on the Hexadecafluorophthalocyanine-Iron-Catalyzed Wacker-Type Oxidation of Olefins to Ketones*

The hexadecafluorophthalocyanine-iron complex FePcF₁₆ was recently shown to convert olefins into ketones in the presence of stoichiometric amounts of triethylsilane in ethanol at room temperature under an oxygen atmosphere. Herein, we describe an extensive mechanistic investigation for the conversion of 2-vinylnaphthalene into 2-acetylnaphthalene as model reaction. A variety of studies including deuterium- and ¹⁸ O₂ -labeling experiments, ESI-MS, and ⁵⁷ Fe Mössbauer spectroscopy were performed to identify the intermediates involved in the catalytic cycle of the oxidation process. Finally, a detailed and well-supported reaction mechanism for the FePcF₁₆ -catalyzed Wacker-type oxidation is proposed.

Aerobic Direct Dioxygenation of Terminal/Internal Alkynes to α-Hydroxyketones by an Fe Porphyrin Catalyst

We herein report a new synthetic method for the preparation of α-hydroxyketones by the dioxygenation of alkynes. The reaction proceeds at room temperature under the action of Fe porphyrin and pinacolborane under air as a green oxidant to produce α-hydroxyketones. The mild reaction conditions allow chemoselective oxidation with functional group tolerance. Terminal alkynes in addition to internal alkynes are applicable, affording unsymmetrical α-hydroxyketones that are difficult to obtain by any reported dioxygenation of unsaturated C-C bonds.

Photoinduced Acetylation of Anilines under Aqueous and Catalyst-Free Conditions

A green and efficient visible-light induced functionalization of anilines under mild conditions has been reported. Utilizing nontoxic, cost-effective, and water-soluble diacetyl as photosensitizer and acetylating reagent, and water as the solvent, a variety of anilines were converted into the corresponding aryl ketones, iodides, and bromides. With advantages of environmentally friendly conditions, simple operation, broad substrate scope, and functional group tolerance, this reaction represents a valuable method in organic synthesis.

Iron-catalyzed domino decarboxylation-oxidation of α,β-unsaturated carboxylic acids enabled aldehyde C-H methylation

A practical and general iron-catalyzed domino decarboxylation-oxidation of α,β-unsaturated carboxylic acids enabling aldehyde C-H methylation for the synthesis of methyl ketones has been developed. This mild, operationally simple method uses ambient air as the sole oxidant and tolerates sensitive functional groups for the late-stage functionalization of complex natural-product-derived and polyfunctionalized molecules.

Iron-Catalyzed Wacker-type Oxidation of Olefins at Room Temperature with 1,3-Diketones or Neocuproine as Ligands*

Herein, we describe a convenient and general method for the oxidation of olefins to ketones using either tris(dibenzoylmethanato)iron(III) [Fe(dbm)₃ ] or a combination of iron(II) chloride and neocuproine (2,9-dimethyl-1,10-phenanthroline) as catalysts and phenylsilane (PhSiH₃ ) as additive. All reactions proceed efficiently at room temperature using air as sole oxidant. This transformation has been applied to a variety of substrates, is operationally simple, proceeds under mild reaction conditions, and shows a high functional-group tolerance. The ketones are formed smoothly in up to 97 % yield and with 100 % regioselectivity, while the corresponding alcohols were observed as by-products. Labeling experiments showed that an incorporated hydrogen atom originates from the phenylsilane. The oxygen atom of the ketone as well as of the alcohol derives from the ambient atmosphere.

Formate-driven catalysis and mechanism of an iridium-copper complex for selective aerobic oxidation of aromatic olefins in water

A hetero-dinuclear IrIII-CuII complex with two adjacent sites was employed as a catalyst for the aerobic oxidation of aromatic olefins driven by formate in water. An IrIII-H intermediate, generated through formate dehydrogenation, was revealed to activate terminal aromatic olefins to afford an Ir-alkyl species, and the process was promoted by a hydrophobic [IrIII-H]-[substrate aromatic ring] interaction in water. The Ir-alkyl species subsequently reacted with dioxygen to yield corresponding methyl ketones and was promoted by the presence of the CuII moiety under acidic conditions. The IrIII-CuII complex exhibited cooperative catalysis in the selective aerobic oxidation of olefins to corresponding methyl ketones, as evidenced by no reactivities observed from the corresponding mononuclear IrIII and CuII complexes, as the individual components of the IrIII-CuII complex. The reaction mechanism afforded by the IrIII-CuII complex in the aerobic oxidation was disclosed by a combination of spectroscopic detection of reaction intermediates, kinetic analysis, and theoretical calculations.

Total Synthesis of ent-Plagiochianin B

An enantioselective total synthesis of plagiochianin B is described that employs (+)-3-carene as its point of departure and delivers the enantiomer of the natural product. Key features of the synthesis include a palladium-mediated regioselective oxidative cleavage of an olefin residing on a pyridine derived from a 6π-azatriene electrocyclization.

Organic synthesis with the most abundant transition metal–iron: from rust to multitasking catalysts

The promising aspects of iron in synthetic chemistry are being explored for three-four decades as a green and eco-friendly alternative to late transition metals. This present review unveils these rich iron-chemistry towards different transformations.

Hydrogenation of Alkenes via Cooperative Hydrogen Atom Transfer

Radical hydrogenation via hydrogen atom transfer (HAT) to alkenes is an increasingly important transformation for the formation of thermodynamic alkane isomers. Current single-catalyst methods require stoichiometric oxidant in addition to hydride (H^-) source to function. Here we report a new approach to radical hydrogenation: cooperative hydrogen atom transfer (cHAT), where each hydrogen atom donated to the alkene arrives from a different catalyst. Further, these hydrogen atom (H•) equivalents are generated from complementary hydrogen atom precursors, with each alkane requiring one hydride (H^-) and one proton (H⁺) equivalent and no added oxidants. Preliminary mechanistic study supports this reaction manifold and shows the intersection of metal-catalyzed HAT and thiol radical trapping HAT catalytic cycles to be essential for effective catalysis. Together, this unique catalyst system allows us to reduce a variety of unactivated alkene substrates to their respective alkanes in high yields and diastereoselectivities and introduces a new approach to radical hydrogenation.

Manganese/Copper Co-catalyzed Electrochemical Wacker-Tsuji-Type Oxidation of Aryl-Substituted Alkenes

A manganese/copper co-catalyzed electrochemical Wacker-Tsuji-type oxidation of aryl-substituted alkenes has been developed. The process involves the use of 5 mol % MnBr₂ and 7.5 mol % CuCl₂, in 4:1 acetonitrile/water in an undivided cell at 60 °C, with 2.8 V constant applied potential. α-Aryl ketones are formed in moderate to excellent yields, with the advantages of avoidance of palladium as a catalyst and any external chemical oxidant in an easily operated, cost-effective procedure.

Iron-Catalyzed Direct Cross-Coupling of Ethers and Thioether with Alcohols for the Synthesis of Mixed Acetals

An iron-catalyzed direct O-alkylation of alcohols via α-C(sp3)–H activation of ethers and a thioether has been established that tolerates cyclic and acyclic ethers and alcohols containing aromatic N-heterocyclic moieties, providing an efficient and green method for the synthesis of mixed acetals with good to excellent yields. The robustness of this protocol is demonstrated by the late-stage oxidation of a structurally complex natural product.

Oxidation of Alkenes by Water with H2 Liberation

Oxidation by water with H₂ liberation is highly desirable, as it can serve as an environmentally friendly way for the oxidation of organic compounds. Herein, we report the oxidation of alkenes with water as the oxidant by using a catalyst combination of a dearomatized acridine-based PNP-Ru complex and indium(III) triflate. Compared to traditional Wacker-type oxidation, this transformation avoids the use of added chemical oxidants and liberates hydrogen gas as the only byproduct.

Recent advances in Wacker oxidation: from conventional to modern variants and applications

Recent developments in the well-known Wacker oxidation process from conventional to modern variants and applications to natural products' synthesis are compiled in this review.

Visible-light-induced aerobic C3–H fluoroalkoxylation of quinoxalin-2(1H)-ones with fluoroalkyl alcohols

A novel and efficient method of visible-light-induced C3–H fluoroalkoxylation of quinoxalin-2(1H)-ones with fluoroalkyl alcohols is developed. This approach uses readily available fluoroalkyl alcohols as fluoroalkoxylation reagents and displays a wide substrate scope, providing the fluoroalkoxylated products in moderate to good yields. Compared with the previous method, such a transformation uses oxygen as an oxidant, which avoids the utilization of plenty of PhI(TFA)₂. In addition, this strategy also gives a practical tool for the rapid synthesis of histamine-4 receptor antagonist and new N-containing bidentate ligands. A radical mechanism was suggested according to the results of control experiments.

A visible-light-induced aerobic C3–H fluoroalkoxylation of quinoxalin-2(1H)-ones with fluoroalkyl alcohols was reported, providing a green access to fluoroalkoxylated quinoxalin-2(1H)-ones.

Isopropanol as a hydrogen source for single atom cobalt-catalyzed Wacker-type oxidation

The first example of Wacker-type oxidation catalyzed by a single atom cobalt catalyst under dioxygen using isopropanol as the hydrogen source was established.

Synthesis of 7,3'-Epoxy-8,4'-Oxyneolignane-1’-Carboxylic Acid from Natural Eusiderin A and its Activity Against Trichophyton mentagrophytes

Background:

Eusiderin A is a neolignan derivate, which makes up the majority of the secondary metabolite of Eusideroxylon zwageri. It has been reported as a potent biopesticide and antifungal agent. Previous studies on the oxidation of terminal methylene of the allylic chain in Eusiderin A have been able to produce primary alcohol, pinacol, and an aldehyde which demonstrated strong activity against plant pathogenic fungi, therefore activity against dermal fungi needs to be studied.

Objective:

The current study aims to improve the hydrophilicity of Eusiderin A via oxidation of the allylic chain in order to derive a potent antifungal property.

Methods:

Transformation of Eusiderin A has been achieved by using the Wacker Oxidation Method in combination with the α-Hydroxylation-Ketone Method to produce 7,3’-epoxy-8,4’-oxyneolignane-1’- carboxylic acid. The structure of the 7,3’-epoxy-8,4’-oxyneolignane-1’-carboxylic acid was identified from spectroscopy data. The in vitro antifungal activity study was performed using the paper disc diffusion method against Trichophyton mentagrophytes.

Results:

New molecule of natural Eusiderin A through the oxidation of the allylic chain to increase the hydrophilicity of Eusiderin A has been designed. Based on the observed UV, IR, 1H and 13C-NMR, and MS spectra, it can be stated that the 7,3’-epoxy-8,4’-oxyneolignane-1’-carboxylic acid has been formed. At a concentration of 50 ppm, this compound showed antifungal activity against Trichophyton mentagrophytes.

Conclusion:

It can be concluded that the 7,3’-epoxy-8,4’-oxyneolignane-1’-carboxylic acid is a potent antifungal agent as it is able to inhibit the Trichophyton mentagrophytes colonies growth.

Concise gram-scale synthesis of Euphorikanin A skeleton through a domino ring-closing metathesis strategy

Euphorikanin A is a diterpenoid possessing a highly congested and unprecedented 5/6/7/3-fused tetracyclic ring skeleton. To access the challenging chemical structure of Euphorikanins, an efficient total synthetic approach is described. The stereoselective synthesis of the core structure of Euphorikanin A has been achieved from a simple dienyne building block, and a domino ring-closing metathesis (RCM) strategy was used for the gram-scale synthesis of the highly strained Euphorikanin A core. This paves the way for the synthesis of structurally diverse Euphorikanins.

Bio-inspired iron-catalyzed oxidation of alkylarenes enables late-stage oxidation of complex methylarenes to arylaldehydes

It is a long-standing challenge to achieve efficient and highly selective aerobic oxidation of methylarenes to benzaldehydes, owing to overoxidation problem stemming from the oxidizability of benzaldehyde far higher than the toluene under usual aerobic conditions. Herein we report a bio-inspired iron-catalyzed polymethylhydrosiloxane-promoted aerobic oxidation of methylarenes to benzaldehydes with high yields and selectivities. Notably, this method can tolerate oxidation-labile and reactive boronic acid group, which is normally required to be transformed immediately after its introduction, and represents a significant advance in the area of the chemistry of organoboronic acids, including the ability to incorporate both aldehyde and ketone functionalities into unprotected arylboronic acids, a class that can be difficult to access by current means. The robustness of this protocol is demonstrated on the late-stage oxidation of complex bioactive molecules, including dehydroabietic acid, Gemfibrozil, Tocopherol nicotinate, a complex polyol structure, and structurally complex arylboronic acids.

Oxidation of toluenes to benzaldehydes is usually accompanied by overoxidation products. Here, the authors report an iron-catalysed aerobic oxidation of methylarenes to benzaldehydes with high yields and selectivities, even in presence of boronic acid groups and in complex natural products and drugs.

Iron(ii) tetrafluoroborate complexes of new tetradentate C-scorpionates as catalysts for the oxidative cleavage of trans-stilbene with H2O2

Iron(ii) complexes of two new tetradentate C-scorpionate ligands are characterized. Both catalyze stilbene cleavage using either H₂O₂ or a O₂/photocatalyst oxidant.

Thiyl radical promoted iron-catalyzed-selective oxidation of benzylic sp3 C–H bonds with molecular oxygen

A ligand-free iron-catalyzed method for the oxygenation of benzylic sp³ C–H bonds by molecular oxygen (1 atm) using a thiyl radical as a cocatalyst has been developed.

Iron-catalyzed reductive cyclization reaction of 1,6-enynes for the synthesis of 3-acylbenzofurans and thiophenes

A facile synthesis of 3-acylbenzofurans and thiophenes via iron(ii)-catalyzed reductive cyclization of 1,6-enynes has been developed.

Iron-catalyzed oxidative C–C(vinyl) σ-bond cleavage of allylarenes to aryl aldehydes at room temperature with ambient air

The iron-catalyzed C−C single bond cleavage and oxidation of allylarenes without the assistance of heteroatoms/directing groups to produce aryl aldehydes is disclosed.

Total Synthesis of Asperphenins A and B

The first total synthesis of asperphenins A and B has been accomplished in a concise, highly stereoselective fashion from commercially available materials (15 steps, 9.7% and 14.2% overall yields, respectively). The convergent route featured the judicious choice of protecting groups, fragment assembly strategy and a late-stage iron-catalyzed Wacker-type selective oxidation of an internal alkene to the corresponding ketone.