Copper(II)-mediated oxidation of 1,2-dioxime to furoxan

1,2-Dioximes undergo oxidative transformation mediated by copper(II) ions in acetonitrile to form the corresponding furoxans in high yields. A series of 1,2-dioximes including aliphatic, aromatic, and heterocyclic dioximes were oxidized using these mild conditions.     

Efficient palladium(II)-catalyzed homocoupling of thiazole-4-carboxylic or oxazole-4-carboxylic derivatives

An efficient Pd(OAc)₂-catalyzed homocoupling of thiazole-4-carboxylic or oxazole-4-carboxylic derivatives is described. It represents a facile and practical methodology to prepare bis-5,5′-thiazole (oxazole)-4,4′-dicarboxylic derivatives in good to excellent yields. This protocol tolerates a series of substitutions on the thiazole (oxazole) rings, including alkyl, carbonyl, and electron-withdrawing/donating group substituted phenyl groups.     

Copper(II) catalyzed selective oxidation of primary alcohols to aldehydes with atmospheric oxygen

Copper(II) complex 1 selectively catalyzes the oxidation of primary alcohols to aldehydes in high yields by atmospheric oxygen in the presence of TEMPO. This procedure does not require an additive and the catalyst 1 is recyclable without loss of activity.     

Cu(II) 2-quinoxalinol salen catalyzed oxidation of propargylic,benzylic, and allylic alcohols using tert-butyl hydroperoxide in aqueous solutions

The synthesis of carbonyl compounds by oxidation of alcohols is a key reaction in organic synthesis. Such oxidations are typically conducted using catalysts featuring toxic metals and hazardous organic solvents. Considering green and sustainable chemistry, a copper(II) complex of sulfonated 2-quinoxalinol salen (sulfosalqu) has been characterized as an efficient catalyst for the selective oxidation of propargylic, benzylic, and allylic alcohols to the corresponding carbonyl compounds in water when in combination with the oxidant tert-butyl hydroperoxide. The reactions proceed under mild conditions (70 °C in water) to produce yields up to 99% with only 1 mol % of catalyst loading. This reaction constitutes of a rare example of propargylic alcohol oxidation in water, and it makes this process greener by eliminating the use of hazardous organic solvents. Excellent selectivity was achieved with this catalytic protocol for the oxidation of propargylic, benzylic, and allylic alcohols over aliphatic alcohols. The alcohol oxidation is thought to go through a radical pathway.     

Cu(II)-catalyzed oxidation of sulfides

A variety of sulfides and disulfides were converted into the corresponding sulfoxide derivatives with 70% t-BuOOH (water) as the oxidant in the presence of catalytic quantity of CuBr₂. The method described does not involve cumbersome work-up, has wide range of applicabilities, exhibits chemoselectivity, and proceeds under mild reaction conditions, and the resulting products are obtained in good yields within reasonable time.     

Copper(I)-catalyzed diastereoselective formation of oxazolines and N-sulfonyl-2-imidazolines

A simple and short method for the reaction of methyl isocyanoacetate with aldehydes and N-sulfonylimines is presented. The reaction is catalyzed by copper(I) complexes and proceeds with excellent yields and high diastereoselectivities.     

Selective copper(II)-catalyzed aerobic oxidative cleavage of aromatic gem-disubstituted alkenes to carbonyl compounds under neutral and mild conditions

Three copper(II) catalytic systems, CuCl₂·2H₂O, CuCl₂·2H₂O+phenanthroline, and [Cu(μ-Cl)Cl(phen)]₂ were used to cleave alkenes to their corresponding carbonyl compounds under aerobic and neutral conditions. [Cu(μ-Cl)Cl(phen)]₂ shows enhanced selectivity over the other two catalytic systems. The oxidative cleavage reactions were carried out in mixed H₂O/THF solvent system under oxygen (4 atm) at 60°C. The real oxidant is 2-hydroperoxytetrahydrofuran, which is generated in situ in the process through the reaction between THF and oxygen catalyzed by copper(II). The cleavage reactions are selective for aromatic gem-disubstituted alkenes. Aromatic internal alkenes are slow to be oxidized, and both aliphatic terminal and internal alkenes are inert to oxidative cleavage. Free radical scavenger 2,2,6,6, tetramethylpiperidinyl-1-oxyl (TEMPO) deactivates the reaction indicating the involvement of free radical path in the reaction mechanism.     

Copper(II)-catalyzed enantioselective conjugate addition of nitro esters to 2-enoyl-pyridine N-oxides

A highly enantioselective Michael addition of nitro esters to 2-enoyl-pyridine N-oxides was developed by using chiral copper catalysts. The Michael addition products can be obtained in high yields and with up to 96% ee.     

Platinum(II)-catalyzed addition of alcohols to alkynes

A simple, high-yielding synthesis of acetals from the platinum(II)-catalyzed addition of alcohols to alkynes is described. The regioselectivity of the method and its mechanism are also discussed.     

Kinetics of alkaline chloramine-T oxidation of arginine monohydrochloride with and without catalytic action of Cu(II) ion

The kinetics of uncatalysed and Cu(II) catalysed oxidation of arginine monohydrochloride was investigated. Both reactions follow a singular order dependence each in oxidant and substrate. An inverse order dependence is reported with the alkali concentration. A plot of observed rate constant versus Cu(II) concentrations Cu(II)2.0×10^–5 M is linear; from the intercept the rate constant for the uncatalysed pathway was calculated. However, at high copper ion concentrations i.e. Cu(II)>2.0×10^–5 M a fixed value of rate constant was found for all catalyst concentrations. Added neutral salts show an insignificant effect on the reaction rate. Mechanisms were proposed for both cases and rate expressions were derived by applying steady state assumptions.

---

Die Kinetik der alkalischen Chloramin-T-Oxidation von Arginin-monohydrochlorid mit und ohneCu(II) als Katalysator

Zusammenfassung Die Oxidation erfolgt sowohl mit als auch ohne Cu(II)-Katalysator in erster Ordnung bezüglich des Oxidationsmittels und des Substrats; inverse Ordnung wird bezüglich der Alkalikonzentration beobachtet. Bis zu einer Cu(II)-Konzentration von2×10^–5 M ist die Geschwindigkeitskonstante der Katalysatorkonzentration proportional; darüber wird eine konstantbleibende Geschwindigkeit beobachtet, die nun von der Cu(II)-Konzentration unabhängig ist. Neutralsalze haben keinen Effekt auf die Geschwindigkeitskonstante. Es wird für den katalysierten und unkatalysierten Reaktionsablauf ein Mechanismus vorgeschlagen und ein mathematischer Ansatz präsentiert.

     

Copper(II) chloride-catalyzed Glaser oxidative coupling reaction in polyethylene glycol

Polyethylene glycol was shown to be an environmentally benign reaction medium for the copper(II) salt -catalyzed Glaser coupling reaction of terminal alkynes. In particular, oxygen as the sole oxidant worked very well for arylacetylenes even in short reaction time. The product was easily separated by extraction and the catalytic system could be reused four times without significant loss of reactivity.     

Copper(II)-catalyzed oxidation of d-alpha-tocopherol by oxygen in aqueous solution

alpha-Tocopherol (alpha-Toc) was solubilized in aqueous solutions using 13 solubilizing agents and the products of oxidation by oxygen in the presence and the absence of Cu(II) were analyzed by HPLC. In the presence of Cu(II), the oxidation was accelerated and 5-formyl-7,8-dimethyltocol and alpha-tocoquinone were the major oxidation products. Their yields greatly increased in the presence of Cu(II). The yields and the rates of formation of the products were dependent on the properties of solubilizing agents and other conditions as well as the presence of Cu(II) or other metal ions. It is suggested that slight changes in the structure of the solubilizing agents affect the course of the reaction.     

Copper(II) Schiff base complexes derived from 2,2′-dimethyl-propandiamine: Synthesis, characterization and catalytic performance in the oxidation of styrene and cyclooctene

Two new mononuclear copper(II) complexes ([CuL¹]·CHCl₃ (1) and [CuL²] (2)) have been prepared by the reaction of two ONNO type Schiff base ligands, ([bis(2-hydroxy-propiophenone)2,2′-dimethylpropan-diamine] (H₂L¹) and [bis(5-bromosalicylaldehyde)2,2′-dimethyl-propandiamine] (H₂L²)) with Cu(OAc)₂·H₂O in 1:1 molar ratios. The complexes have been characterized by elemental analyses, IR and UV-Vis spectroscopy. The structures have been confirmed by X-ray single crystal analysis at 100 K. The Cu(II) atom in 1 is coordinated equatorially by a N₂O₂ donor set of the tetradentate, dinegative Schiff-base (L¹)²⁻ in a distorted square planar arrangement. While in [CuL²] (2), the Cu(II) ion possesses an additional weak intermolecular contact with one bromine atom of the ligand, thus the coordination sphere of 2 can be described as strongly distorted square pyramidal. The catalytic performance of the prepared copper complexes for the oxidation of styrene and cyclooctene with tert-butyl hydroperoxide has been evaluated.     

Copper mediated oxidation of amides to imides by Selectfluor

The combination of Selectfluor and copper(I) bromide has shown a strong oxidation ability, readily oxidizing amides into the corresponding imides in acetonitrile at room temperature in less than 1 h. This transformation under mild conditions gives good to excellent chemical yields. A possible reaction mechanism is proposed.     

Copper(I) oxide and benzoic acid ‘on water’: a highly practical and efficient catalytic system for copper(I)-catalyzed azide-alkyne cycloaddition

A simple combination of Cu₂O and PhCO₂H ‘on H₂O’ has been developed as a highly practical and efficient catalytic system for copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC). It not only provides a further evidence for the strategy of carboxylic acids-promoted CuAAC, but also offers significant advantages to CuAAC because Cu₂O is one of the most stable and cheapest Cu(I) sources; PhCO₂H is one of the structurally simplest bidentate ligands; and water is the most ‘green’ solvent.     

Copper(I)-Catalyzed Asymmetric Allylation of Ketones with 2-Aza-1,4-Dienes

Catalytic asymmetric allylation of ketones under proton-transfer conditions is a challenging issue due to the limited pronucleophiles and the electrophilic inertness of ketones. Herein, a copper(I)-catalyzed asymmetric allylation of ketones with 2-aza-1,4-dienes (N-allyl-1,1-diphenylmethanimines) is disclosed, which affords a series of functionalized homoallyl tertiary alcohols in high to excellent enantioselectivity. Interestingly, N-allyl-1,1-diphenylmethanimines work as synthetic equivalents of propanals. Upon the acidic workup, a formal asymmetric β-addition of propanals to ketones is achieved. An investigation on KIE effect indicates that the deprotonation of N-allyl-1,1-diphenylmethanimines is the rate-determining step, which generates nucleophilic allyl copper(I) species. Finally, the synthetic utility of the present method is demonstrated by the asymmetric synthesis of (R)-boivinianin A and (R)-gossonorol.     

Copper (II)-catalyzed regio- and stereoselective addition of H/P(O)R2 to alkynes

Cu(acac)₂ is the new universal catalyst for β-E regio- and stereoselective syn-addition of the H–P(O)-bond of diphenylphosphine oxide, H-phosphinates, dialkylphosphites to various alkynes in the synthesis of P(O)-containing alkenes. Without additives and ligands Cu(II)-compounds showed better results than CuI or Ni(acac)₂. The catalytic system developed is tolerant to typical organic functional groups present in the alkynes and to the nature of different substituents in the H–P(O)-compounds.     

Copper(I)-catalyzed hydrophosphination of styrenes

Hydrophosphination of styrenes has been accomplished with metal salts for the first time. (CuOTf)₂·toluene complex is the catalyst of choice, but CuCl can also be used. “In-situ” EPR and NMR studies suggest Cu(I) as the catalytically active metal species, giving exclusively the anti-Markovnikov product. Phosphine oxides or β-ketophosphine oxides can be prepared in one-pot by oxidation with molecular oxygen.     

Copper(II)-catalyzed preparation of alkylindium compounds and applications in cross-coupling reactions both in aqueous media

An efficient water-based method for the synthesis of alkylindium compound in the presence of a catalytic amount of cheap and readily available CuSO₄·5H₂O (10 mol%) was developed. The thus-generated alkylindium compounds effectively underwent palladium-catalyzed cross-coupling reactions with a myriad of aryl halides in aqueous media, leading to the cross-coupled products in modest to high yields. The mildness of the formed alkyl organometallics allowed the tolerance to various important functional groups incorporated in both substrates of alkyl iodides and aryl halides.