Aerobic oxidation of aldehydes by visible light photocatalysis

An efficient and environmentally benign method for the oxidation of aldehydes to carboxylic acids has been developed. Singlet oxygen, generated by visible light in the presence of a Ru or Ir photocatalyst, reacted with aldehydes to give the corresponding carboxylic acids in excellent yields. The reaction is highly chemo-selective, in which only an aldehyde moiety is reactive even in the presence of other photo-oxidation active sites. This method is an example of an ideal green chemical reaction in the sense that molecular oxygen and visible light are key sources for the transformation.     

One-pot two-step conversion of aromatic carboxylic acids and esters to aromatic aldehydes via indium-catalyzed reductive thioacetalization and desulfurization

Described herein is that a new approach to a one-pot two-step conversion of aromatic carboxylic acids/esters to aromatic aldehydes, in which indium(III) iodide effectively catalyzes both the first reductive thioacetalization of carboxylic acids and a subsequent desulfurization of the in-situ formed thioacetal intermediates leading to aldehydes.     

A new oxidation of aldehydes to carboxylic acids

2-Hydroperoxyhexafluoro-2-propanol (HPHI) is a selective catalytic and stoichiometric reagent for the oxidation of aldehydes to acids under mildly basic conditions.     

TEMPO visible light photocatalysis: The selective aerobic oxidation of thiols to disulfides

TEMPO(2,2,6,6-tetramethylpipe ridine-1-oxyl) is well-established in orangocatalysis that usually work in synergy with transition-metal catalysis or semiconductor photocatalysis.Here,TEMPO was turned into a visible light photocatalyst to conduct the selective aerobic oxidation of thiols into disulfides.With O_2 as an oxidant,a mild and efficient protocol for the selective oxidation of thiols into disulfides including symmetrical and unsymmetrical ones with 5 mol% of TEPMO as a photocatalyst was developed at room temperature under the irradiation of 460 nm blue LEDs.It was found that a complex formed between TEMPO and thiols underpinned the visible light activity and disulfides were obtained in very high isolated yields.This work suggests that TEMPO takes diverse roles in for photocatalytic selective oxidative transformations with O_2 as the oxidant.     

Silver nitrate-catalyzed oxidation of aldehydes to carboxylic acids by H2O2

A variety of aromatic, aliphatic and conjugated aldehydes were converted to the corresponding carboxylic acid derivatives with 30% H₂O₂ as the oxidant in the presence of catalytic amounts of AgNO₃. The method described has wide range of applicabilities, does not involve cumbersome work-up, exhibits chemoselectivity and proceeds under mild reaction conditions, and the resulting products are obtained in good yields within reasonable time.     

Visible light photoredox catalysed amidation of carboxylic acids with amines

A visible-light promoted photoredox catalysed, green one-pot approach for the amidation of carboxylic acids with amines has been developed for the synthesis of diverse aliphatic and aromatic amides. The proposed strategy is extendable also to biologically active amides and could represent a low-cost alternative to the common synthetic pathways. The developed strategy may hold great potential for a comprehensive display of biologically interesting peptide synthesis and amino acid modification.     

CuCl catalyzed oxidation of aldehydes to carboxylic acids with aqueous tert-butyl hydroperoxide under mild conditions

Oxidation of aldehydes to the corresponding carboxylic acids can be performed highly efficiently at room temperature with 70% tert-butyl hydroperoxide (in water) in the presence of a catalytic amount of easily available ligand free CuCl in acetonitrile as solvent under very mild conditions. This oxidation protocol works well for various aldehydes including aliphatic aldehydes and aliphatic dialdehydes.     

Visible light photoredox catalysis: aerobic oxidation of perimidines to perimidinones

Aerobic oxidation of a series of 2,3-dihydro-1H-perimidines to the corresponding 4- and 6-perimidinones via visible light photoredox catalysis using Ru(bpy)₃²⁺ as a catalyst was reported. The scope and limitation of this oxidation were investigated and a possible photochemical mechanism was proposed.     

Fe3O4 nanoparticles/ethyl acetoacetate system for the efficient catalytic oxidation of aldehydes to carboxylic acids

A new methodology for the oxidation of aldehydes promoted by commercially available Fe₃O₄ nanoparticles (Fe₃O₄ NPs) activated by ethyl acetoacetate was developed. The use of ethyl acetoacetate as additive was crucial to achieve high reactivities. All reactions were realized under solvent free conditions, using air or tBuOOH as oxidants. Finally, the separation and reuse of the magnetically recoverable nanoparticles make this methodology very practical, simple and economical.     

Pseudohalide assisted aerobic oxidation of alcohols in the presence of visible-light

Pseudohalides are well known to do similar chemistry like halides. Thiocyanate, a pseudohalide acts like halides in many ways. Thiocyanate radicals (SCN) are generated from readily available thiocyanate salts using Rose Bengal through single electron transfer (SET) in the presence of visible light. Thiocyanate radicals abstract hydrogen like other halide radicals, so this aspect of chemistry was used for the oxidation of alcohols to their corresponding aldehydes using oxygen as the terminal oxidant. This method shows a broad scope and well tolerance towards various functional groups.     

Oxidation of benzoins to benzils in the presence of porphyrin sensitizers by air and sunlight or visible light

In this work, two new aerobic routes are introduced for the oxidation of a variety of benzoins to benzils by using molecular oxygen in the presence of porphyrins as sensitizers, using white light or sunlight in acetonitrile under mild conditions. The methods have a wide range of applications, do not involve cumbersome work-up, exhibit chemoselectivity and proceed under mild reaction conditions. The resulting products are obtained in good yields in a reasonable time.     

2D Bi2WO6/MoS2 as a new photo-activated carrier for boosting electrocatalytic methanol oxidation with visible light illumination

In this paper, a new two-dimensional (2D)/2D composite of Bi₂WO₆/MoS₂ was facile synthesized, and then was used as supporting material for depositing Pt nanoparticles. The as-synthesized Pt-Bi₂WO₆/MoS₂ was extended into photo-assisted electrocatalytic oxidation of methanol, which is a model anode reaction for direct methanol fuel cell. Compare with traditional electrocatalytic process, Pt-Bi₂WO₆/MoS₂ displays 1.5 times enhanced electrocatalytic performance on methanol oxidation with assistance of visible light irradiation and 2.2 times for commercial Pt/C. Besides, from the results of chronoamperometric and chronopotentiometry experiments, the stability of Pt-Bi₂WO₆/MoS₂ electrode is clearly improved under visible light irradiation. The synergistic effects of photo- and electro-catalytic in the heterojunction of Pt-Bi₂WO₆/MoS₂ in favor of the above enhancement. This research gives more insights in the fields of photo-assisted traditional electrocatalytic application by constructing of semiconductor heterojunction carrier.     

Sulfonated carbon catalyzed oxidation of aldehydes to carboxylic acids by hydrogen peroxide

Sulfonated carbon as a strong and stable solid acid catalyst exhibited excellent catalytic performance in various acid-catalyzed reactions. Here, sulfonated carbon, as catalyst for oxidation reaction, was prepared via the carbonization of starch followed by sulfonation with concentrated sulfuric acid. N₂ physisorption, X-ray diffraction, Fourier transform infrared spectroscopy, X-ray fluorescence and acid-base titration were used to characterize the obtained materials. The catalytic activity of sulfonated carbon was studied in the oxidation of aldehydes to carboxylic acids using 30 wt% H₂O₂ as oxidant. This oxidation protocol works well for various aldehydes including aromatic and aliphatic aldehydes. The sulfonated carbon can be recycled for three times without obvious loss of activity.     

Application of zirconium-modified silica gel as a stationary phase in the ion-exclusion chromatography of carboxylic acids II. Separation of aliphatic carboxylic acids with pyromellitic acid as eluent and with suppressed conductimetric detection

The application of zirconium-modified silica gels (Zr-Silica) as stationary phases for ion-exclusion chromatography with conductimetric detection (IEC–CD) for C₁–C₈ aliphatic carboxylic acids (formic, acetic, propionic, butyric, valeric, caproic, heptanoic and caprylic acids) was carried out using pyromellitic acid as the eluent. Zr-Silicas were prepared by the reaction of the silanol group on the surface of silica gel with zirconium tetrabutoxide [Zr(OCH₂CH₂CH₂CH₃)₄] in ethanol solution. An ASRS-Ultra anion self-regenerating suppressor in the K⁺ form was used for the enhancement of conductimetric detector response of these aliphatic carboxylic acids. A Zr-Silica adsorbed on 10 mg zirconium g⁻¹ silica gel was the most suitable stationary phase in IEC–CD for the separation of these aliphatic carboxylic acids. Excellently simultaneous separation and highly sensitive detection for these aliphatic carboxylic acids were achieved in 25 min by IEC–CD with the Zr-Silica column (250×4.6 mm I.D.) and a 0.2 mM pyromellitic acid containing 0.15% heptanol as the eluent.     

CuCl catalyzed selective oxidation of primary alcohols to carboxylic acids with tert-butyl hydroperoxide at room temperature

Direct oxidation of primary alcohols to the corresponding carboxylic acids is performed highly efficiently at room temperature with anhydrous tert-butyl hydroperoxide in the presence of a catalytic amount of easily available CuCl under ligand free conditions in acetonitrile. Benzylic alcohols are more reactive than aliphatic alcohols, and these benzylic alcohols are selectively oxidized to the corresponding acids in the presence of aliphatic alcohols such as 1-octanol and 1-decanol.     

A mild and efficient reaction for conversion of carboxylic acids into acid bromides with ethyl tribromoacetate/triphenylphosphine under acid-free conditions

Acid bromides were prepared efficiently from carboxylic acids with readily available ethyl tribromoacetate and triphenylphosphine at room temperature under neutral conditions. The present process is applicable to the preparation of various acid bromides from aromatic and aliphatic carboxylic acids. Aromatic carboxylic acids were found to be more reactive than aliphatic carboxylic acids under reaction conditions.     

A novel system for the synthesis of nitriles from carboxylic acids

A simple, mild and high yielding method for the conversion of various carboxylic acids to nitriles has been developed using diphosphorus tetraiodide in combination with ammonium carbonate at room temperature.     

A new direct synthesis of cinnamic acids from aromatic aldehydes and aliphatic carboxylic acids in the presence of sodium borohydride

Cinnamic acids have been prepared in 59-86% yields by a new direct synthesis from aromatic aldehydes and aliphatic carboxylic acids in the presence of sodium borohydride and N-methyl-2-pyrrolidinone (NMP) as solvent, at reflux (185-190°C), for 9-12 hours. Without sodium borohydride, this reaction is not possible.     

Aerobic oxidation under visible light irradiation of a fluorescent lamp with a combination of carbon tetrabromide and triphenyl phosphine

A combination of CBr₄–Ph₃P, a non-metal method, enables us to carry out aerobic photo-oxidation of alcohols and aromatic methyl groups to the corresponding carboxylic acids under irradiation of vis from a general-purpose fluorescent lamp. Aliphatic primary- and secondary alcohols, benzyl alcohols, and methyl groups at the aromatic nucleus generally afforded the carboxylic acids directly in good to high yield.     

Efficient esterification of carboxylic acids and phosphonic acids with trialkyl orthoacetate in ionic liquid

An operationally simple, inexpensive, efficient, and environmentally friendly esterification of various carboxylic acids, phosphonic acids, and phosphinic acids with triethyl orthoacetate or trimethyl orthoacetate under neutral conditions in a typical room temperature ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate, was successfully carried out to provide the corresponding ethyl esters or methyl esters in high yields.