Regioselective Hydration of Alkynes by Iron(III) Lewis/Brønsted Catalysis

1-Butyl-3-methylimidazolium tetrafluoroborate as suitable solvent for BF3: the case of alkyne hydration. Chemistry vs electrochemistry

In order to replace the expensive metal/ligand catalysts and classic toxic and volatile solvents, commonly used for the hydration of alkynes, the hydration reaction of alkynes was studied in the ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate (BMIm-BF4) adding boron trifluoride diethyl etherate (BF3·Et2O) as catalyst. Different ionic liquids were used, varying the cation or the anion, in order to identify the best one, in terms of both efficiency and reduced costs. The developed method was efficaciously applied to different alkynes, achieving the desired hydration products with good yields. The results obtained using a conventional approach (i.e., adding BF3·Et2O) were compared with those achieved using BF3 electrogenerated in BMIm-BF4, demonstrating the possibility of obtaining the products of alkyne hydration with analogous or improved yields, using less hazardous precursors to generate the reactive species in situ. In particular, for terminal arylalkynes, the electrochemical route proved to be advantageous, yielding preferentially the hydration products vs the aldol condensation products. Importantly, the ability to recycle the ionic liquid in subsequent reactions was successfully demonstrated.

FeIII -Based Eutectic Mixtures as Multi-task and Reusable Reaction Media for Efficient and Selective Conversion of Alkynes into Carbonyl Compounds

An efficient, simple and general protocol for the selective hydration of terminal alkynes into the corresponding methyl ketones has been developed by using a cheap, easy-to-synthesise and sustainable FeIII -based eutectic mixture [FeCl3  ⋅ 6H2 O/Gly (3 : 1)] as both promoter and solvent for the hydration reaction, working: i) under mild (45 °C) and bench-type reaction conditions (air); and ii) in the absence of ligands, co-catalysts, co-solvents or toxic, non-abundant and expensive noble transition metals (Au, Ru, Pd). When the final methyl ketones are solid/insoluble in the eutectic mixture, the hydration reaction takes place in 30 min, and the obtained methyl ketones can be isolated by simply decanting the liquid FeIII -DES, allowing the direct isolation of the desired ketones without VOC solvents. By using this straightforward and simple isolation protocol, we have been able to recycle the FeIII -based eutectic mixture system up to eight consecutive times. Furthermore, the FeIII -eutectic mixture is able to promote the selective and efficient formal oxidation of internal alkynes into 1,2-diketones, with the possibility of recycling this system up to three consecutive times. Preliminary investigations into a possible mechanism for the oxidation of the internal alkynes seem to indicate that it proceeds through the formation of the corresponding methyl ketones and α-chloroketones.

Fe(III)-Catalyzed Tandem Cyclization of Phenylpropiolamides with 3-Formylchromones for the Construction of 2-Pyridones

A highly efficient and atom-economic iron(III)-catalyzed three-component heteroannulation reaction between phenylpropiolamides, 3-formylchromones, and water is described for the construction of diversely multifunctionalized 2-pyridones. This protocol allows rapid access to a variety of 2-pyridones bearing an ortho-hydroxybenzoyl and a benzoyl scaffold under operationally simple conditions. The synthetic utility of the synthesized 2-pyridone scaffolds is demonstrated by transformation into biologically interesting complex heterocycles.

Autotandem Catalysis: Inexpensive and Green Access to Functionalized Ketones by Intermolecular Iron-Catalyzed Amidoalkynylation/Hydration Cascade Reaction via N-Acyliminium Ion Chemistry

Iron-based catalysts were applied in cascade-type reactions for the synthesis of different carbonyl compounds. The reactions proceeded by a new iron-catalyzed cascade of alkynylation/hydration by using both the σ- and π-Lewis acid properties of iron salts. The alkynylation reactions of several endo and exocyclic acetoxylactams were achieved with three different catalysts including FeCl 3 .6H 2 O, FeCl 3 , and Fe(OTf) 3 showing the efficiency of σ-Lewis acidity of iron (III) salts in catalyzing the alkynylation reaction. We also demonstrated that the reaction sequence could be shortened by the direct use of hydroxylactams, leading to an environmentally friendly protocol, avoiding the need to perform unnecessary lengthy steps. A combination of the hard/soft iron Lewis acid properties was then used to implement an unprecedented tandem intermolecular alkynylation/intramolecular hydration sequence allowing expedient access to a new carbonyl structures from trivial materials.

Synthesis of 2-Aryl Acetophenones via Hydrobromination and Oxy-isomerization of (o-Arylethynyl)benzyl Alcohols

Hydrobromination and oxy-isomerization of (o-arylethynyl)benzyl alcohols to yield brominated aryl ketones were achieved with bromotrimethylsilane. The substrate scope suggested that vinyl carbocations, stabilized by the conjugated aryl groups, are the reaction intermediates. 1H-Isochromene was also detected by ¹H NMR, and an isolated 1H-isochromene was converted to the product when retreated with TMSBr. The formation of 1H-isochromene is equivalent to a 6-endo-dig cyclization and contrasts with the corresponding reactions under basic conditions, in which the 5-exo-dig process dominated.

Synthesis of functionalised 2,3-dihydroquinolin-4(1H)-ones vs. quinoline or N-alkenylindole derivatives through sequential reactions of 2-alkynylanilines with ketones

This study describes diversity-oriented synthesis of 2,2,3-substituted-2,3-dihydroquinolin-4(1H)-ones vs. functionalised quinoline or N-alkenylindole derivatives through Brønsted acid mediated or Lewis acid catalyzed sequential reactions of 2-alkynylanilines with ketones. In particular, a series of challenging quinolin-4-one derivatives are prepared with good functional group tolerance in an atom-economical fashion by using p-toluenesulfonic acid monohydrate as a promoter of the reaction of ketones with 2-alkynylanilines in EtOH at reflux, while the same starting materials give the corresponding 4-substituted quinolines in toluene at 110 °C both in the presence of p-toluenesulfonic acid monohydrate as the promoter and FeCl3 as the catalyst. The divergent formation of N-alkenylindole derivatives occurs by switching to the use of ZnBr2 as the catalyst under the same reaction conditions. Conversely, only 4-methylsubstituted quinoline derivatives were isolated by reacting 2-ethynylanilines and/or 2-trimethylsylilanilines with ketones in all examined cases.

Combined Photoredox and Iron Catalysis for the Cyclotrimerization of Alkynes

Successful combinations of visible-light photocatalysis with metal catalysis have recently enabled the development of hitherto unknown chemical reactions. Dual mechanisms from merging metal-free photocatalysts and earth-abundant metal catalysts are still in their infancy. We report a photo-organo-iron-catalyzed cyclotrimerization of alkynes by photoredox activation of a ligand-free Fe catalyst. The reaction operates under very mild conditions (visible light, 20 °C, 1 h) with 1-2 mol % loading of the three catalysts (dye, amine, FeCl2 ).

[3,3]-Acyloxy Rearrangement-Triggered Regioselective Hydration of δ-Acetoxy-α,β-Alkynoates/Halo Alkynes

Herein, we report a simple, efficient, highly regioselective, and broad-scope hydration method that is facilitated by an unusual interception of an electrophilic intermediate by water generated via acetate group participation during [3,3]-acyloxy rearrangement. Various carboxylate-directing groups including acetate, acrylates, pivalates, benzoate or its derivatives, and those derived from bioactive natural products were successfully implemented to direct the regioselective hydration for various functionalized δ-acyloxy-β-ketoester synthesis. The reaction pathway was further confirmed by ¹⁸O labeling experiments, and to the best of our knowledge, this is the first report of hydration through an electrophilic intermediate generated during [3,3]-acyloxy rearrangement. Synthetic application includes the synthesis of a modifiable C5-carbon chain, five-, six-, and seven-membered heterocycles, and natural product diversification.

Photoinduced direct hydration of dipyridylacetylenes in acidic aqueous solution

The straightforward and catalyst-free photoinduced hydration reaction of dipyridylacetylenes in acidic aqueous solution was achieved upon UV irradiation at room temperature.

Deep eutectic solvent-catalyzed Meyer-Schuster rearrangement of propargylic alcohols under mild and bench reaction conditions

The Meyer-Schuster rearrangement of propargylic alcohols into α,β-unsaturated carbonyl compounds has been revisited by setting up an atom-economic process catalyzed by a deep eutectic solvent FeCl3·6H2O/glycerol. Isomerizations take place smoothly, at room temperature, under air and with short reaction times. The unique solubilizing properties of the eutectic mixture enabled the use of a substrate concentration up to 1.0 M with the medium being recycled up to ten runs without any loss of catalytic activity.

Metal-Free Carbonyl-Assisted Regioselective Hydration of Alkynes: An Access to Dicarbonyls

Metal-free regioselective hydration of o-alkynylaldehydes with the assistance of neighboring carbonyl oxygen is disclosed. The developed protocol provides a facile route to synthesize a series of multisubstituted carbonyl containing scaffolds that enable the potential application toward the synthesis of highly diversified 5-azaindoles. γ-Carbolines and 2,8-diazacarbazoles can also be accessed directly without isolating the dicarbonyl compounds. The developed methodology is operationally simple and environment-friendly, tolerates a wide variety of functional groups, and is applicable toward large scale synthesis.

Hydration of phenylacetylene on sulfonated carbon materials: active site and intrinsic catalytic activity

A series of sulfonated carbon materials (sulfonated glucose-derived carbon, carbon nanotubes, activated carbon and ordered mesoporous carbon, denoted as Sglu, SCNT, SAC and SCMK, respectively) were synthesized and applied as acid catalysts in phenylacetylene (PA) hydration reactions. The sulfonic acid groups (-SO3H) were identified to be the only kind of active sites and were quantified with XPS and a cation exchange process. Mechanistic studies revealed that the catalytic PA hydration reaction follows pseudo first order reaction kinetics. Sglu exhibits a higher reaction rate constant (k) and lower apparent activation energy (E a) in the hydration reactions than SCNT catalysts. NH3-temperature programmed desorption measurement results revealed that the relatively high catalytic activity of Sglu was attributed to both the stronger acidity and larger number of -SO3H active sites. This work exhibited the performance of carbon materials without any extra acidic additives in PA hydration reaction and investigated the intrinsic catalytic activity by kinetics. The present work provides the possibility for acid catalytic applications of carbon materials, which sheds light on the environmentally friendly and sustainable production strategy for aldehyde ketone compounds via the catalytic alkyne hydration reactions.

Cobaloxime-catalyzed hydration of terminal alkynes without acidic promoters

Cobaloxime (Co(dmgBF₂)₂·2H₂O), an inexpensive first-row transition-metal complex, catalyzed hydration of terminal alkynes gave the corresponding methyl ketones in good to excellent yields under neutral conditions (additional protic acids and silver salts are not required). A wide range of functional groups, such as allyl ether, benzyl ethers, carboxylic esters, imides, amides, nitro, and halogens, were tolerated. The mild reaction conditions together with the inexpensive feature and easy availability of the catalyst well address the current challenges in the field of alkyne hydration.

Gold(I)-Catalyzed Cyclization/Carbonylation Cascade Reaction of 1,6-Diynes: An Access to β,γ-Unsaturated Ketones

A gold-catalyzed cyclization/carbonylation cascade reaction of 1,6-diynes is reported. The reaction goes through 6-exo-dig and 6-endo-dig cyclizations in sequence, followed by hydration to provide the β,γ-unsaturated ketones with moderate to high yields under mild reaction conditions. This is the first example of intercepting the postulated 1,3-oxazine vinylgold intermediate with another pendant alkyne, which not only verifies the proposed mechanism but also provides the ketone products with cyclized 1,2,3,6-tetrahydropyridine or 3,6-dihydro-2H-pyran frameworks from corresponding diynes.

Visible-light photoredox catalyzed hydroacylation of electron-deficient alkenes: carboxylic anhydride as an acyl radical source

Simple aromatic carboxylic anhydrides have been employed for a novel acyl Michael addition via visible light photoredox catalysis.

Bimetallic nanosized solids with acid and redox properties for catalytic activation of C-C and C-H bonds

A new approach is presented to form self-supported bimetallic nanosized solids with acid and redox catalytic properties. They are water-, air- and H2-stable, and are able to activate demanding C-C and C-H reactions. A detailed mechanistic study on the formation of the Ag-Fe bimetallic system shows that a rapid redox-coupled sequence between Ag+, O2 (air) and Fe2+ occurs, giving monodisperse Ag nanoparticles supported by O-bridged diatomic Fe3+ triflimides. The system can be expanded to Ag nanoparticles embedded within a matrix of Cu2+, Bi3+ and Yb3+ triflimide.

Brønsted Acid-Mediated Hydrative Arylation of Unactivated Alkynes

The Brønsted acid-mediated reaction of unactivated alkynes with aryl sulfoxides leads to simultaneous hydration and intermolecular C-C bond formation. This solvent- and metal-free transformation directly delivers α-arylated carbonyl compounds as the products of a formal hydrative arylation in an atom-economical manner. The products bear useful synthetic handles for further functionalization.

One step microwave-assisted synthesis of nanocrystalline WOx–ZrO2 acid catalysts

One-step microwave synthesis allows one to tune WO_x species and ZrO₂ polymorphs showing different activities in acid reactions.

An environmentally benign hydration of alkynes catalyzed by gallic acid/tannic acid in water

A gallic acid catalyzed hydration strategy from alkynes under mild conditions has been developed.

Highly Regioselective Iodination of Arenes via Iron(III)-Catalyzed Activation of N-Iodosuccinimide

An iron(III)-catalyzed method for the rapid and highly regioselective iodination of arenes has been developed. Use of the powerful Lewis acid, iron(III) triflimide, generated in situ from iron(III) chloride and a readily available triflimide-based ionic liquid allowed activation of N-iodosuccinimide (NIS) and efficient iodination under mild conditions of a wide range of substrates including biologically active compounds and molecular imaging agents.

Visible light promoted hydration of alkynes catalyzed by rhodium(III) porphyrins

Visible light promoted hydration of a wide scope of alkynes to ketones catalyzed by rhodium(III) porphyrin complexes was described. The key intermediate β-carbonyl alkyl was observed and independently synthesized. The rate of photolysis is over two orders of magnitude faster than that of the thermal process.