Aerobic oxidation of biomass-derived furfural to furoic acid was studied with an N-heterocyclic carbene as a homogeneous catalyst. Carbene species generated in situ on 1,3-bis(2,4,6-trimethylphenyl) imidazolium chloride with a strong organic base (1,8-diazabicyclo[5.4.0]undec-7-ene) was highly active and selective for the formation of furoic acid in dimethyl sulfoxide at 40 °C. This reaction initiates the formation of a Breslow intermediate between an N-heterocyclic carbene and a furfural molecule and the subsequent activation of molecular O2. While the active carbene catalyst promoted furfural dimerization to afford furoin as a side reaction, furoin was decomposed into the Breslow intermediate and furfural through a reverse reaction, which were then converted quantitatively to furoic acid. Kinetic studies revealed that the apparent activation energy for this furfural oxidation was only 20 kJ mol–1, which is significantly lower than that with a supported Au catalyst (30.4 kJ mol–1). The N-heterocyclic car...

Metal-Free and Selective Oxidation of Furfural to Furoic Acid with an N-Heterocyclic Carbene Catalyst

An efficient and green carbon disulfide surrogate via facile combination of potassium sulfide and chloroform has been developed. A variety of benzothiazine-thiones and benzothiazole-thiones were straightforwardly established along with the formation of five new chemical bonds in one pot from readily available starting materials, in which the widely used 2-mercaptobenzothiazole (MBT) was synthesized through this method in gram scale. Dichlorocarbene was demonstrated to be a carbon source intermediate from chloroform on the basis of control experimental studies. Meanwhile, potassium sulfide, as a trisulfur radical anion (S3˙−) source, was confirmed through UV-visible spectroscopy and an EPR study.

Green carbon disulfide surrogate via a combination of potassium sulfide and chloroform for benzothiazine-thione and benzothiazole-thione construction

Over the last three decades, consumption of total and saturated fat has steadily declined in Western diets as a proportion of calories intake. At the same time, omega (ω)-6 fatty acid intake has risen at the expense of ω-3 fatty acids, resulting in an ω-6/ω-3 ratio of 20:1 or higher. The observed changes in fatty acids ratio coincide with a significantly increased prevalence of coronary heart disease, hypertension, cancer, diabetes, obesity, rheumatoid arthritis, and autoimmune or neurodegenerative disorders. The low intake of ω-3 fatty acids may be attributed to their absence from the diet or lack of awareness about suitable dietary sources. A sustainable and cost-effective way of reaching a large population with essential ω-3 fatty acids is fortification of staple foods. A variety of food items enriched with ω-3 have entered the market in recent years, including beef, fish, dairy products, cereals, cereal bars, and infant formula. The present review discusses the role of ω-3 and ω-6 fatty acids, as well as their ratio, on human health. Additionally, it focuses on the latest developments regarding dietary sources, innovative technologies, and challenges of food fortification with ω-3 fatty acids. 

Futuristic food fortification with a balanced ratio of dietary ω-3/ω-6 omega fatty acids for the prevention of lifestyle diseases

The reaction paths and intermediate structures related to the formation of the Breslow intermediate and its oxidation along the oxidative/oxygenative lanes have been studied from a mechanistic point of view, with the support of gas-phase and computational studies. The results confirm the occurrence of a single-electron transfer from the Breslow intermediate to the molecular oxygen with formation of a radical couple that recombines either as a peroxide anion 7′ to afford the aldehyde-to-carboxylic acid product or as a hydroperoxy derivative 7″ that evolved into an electrophilic acyl azolium, opening to the aldehyde-to-ester conversion. Steric factors enter into determining the different reactivity. All of the intermediates of both catalytic paths have been observed and characterized under mass spectrometric conditions. In particular, for the imidazoline catalyst, the (+)ESI-MS/(MS) detection of the genuine Breslow intermediate was made possible in virtue of its limited reactivity. Mechanistic aspects of th...

Formation, Oxidation, and Fate of the Breslow Intermediate in the N-Heterocyclic Carbene-Catalyzed Aerobic Oxidation of Aldehydes.

Highly efficient asymmetric [3 + 2] cycloaddition reactions of thiazolium salts with a variety of electron-deficient 2-π components were achieved by chiral N,N′-dioxide/metal complex catalysts, providing a convenient route to access enantioenriched hydropyrrolo-thiazoles. The oxathiazine and sulfinate could be readily obtained by the oxidation of the hydropyrrolo-thiazole. Moreover, through ingenious collaboration of chiral catalysts with basic additives, the first cascade cycloaddition/rearrangement reaction with β,γ-unsaturated α-ketoesters was realized, and various pyrrolo-[1,4]thiazine derivatives were afforded in excellent results.

Enantioselective [3 + 2] cycloaddition and rearrangement of thiazolium salts to synthesize thiazole and 1,4-thiazine derivatives

Breslow intermediates that bear radical-stabilizing N substituents, such as benzyl, cinnamyl, and diarylmethyl, undergo facile homolytic C-N bond scission under mild conditions to give products of formal [1,3] rearrangement rather than benzoin condensation. EPR experiments and computational analysis support a radical-based mechanism. Implications for thiamine-based enzymes are discussed.

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Yi Xu

Papers

Radical [1,3] Rearrangements of Breslow Intermediates.

Delta-5-desaturase: A novel therapeutic target for cancer management.

Radical [1,3] Rearrangements of Breslow Intermediates.