Mechanochemical Birch Reduction with Low Reactive Alkaline Earth Metals.

Birch reduction and similar dissolved metal-type transformations hold significant importance in the organic synthesis toolbox. Historically, the field has been dominated by alkali metal reductants. In this study, we report that largely neglected, low-reactive alkaline earth metals can become powerful and affordable reductants when used in a ball mill under essentially solvent-free conditions, in the presence of ethylenediamine and THF as liquid additives. Calcium can reduce both electron-deficient and electron-rich arenes, with yields of products similar to those obtained with lithium metal. Magnesium reveals enhanced reducing power, enabling the reduction of benzoic acids while keeping electron-rich aromatic moieties intact and allows for chemoselective transformations. The developed mechanochemical approach uses readily available and safer-to-handle metals, operates under air and ambient temperature conditions, and can be used for gram-scale preparations. Finally, we demonstrate that the developed conditions can be used for other dissolved metal-type reductive transformations, including reductive amination, deoxygenation, dehalogenation, alkene and alkyne reductions.

Natural pigments (anthocyanins and chlorophyll) and antioxidants profiling of European red and green gooseberry (Ribes uva-crispa L.) extracted using green techniques (UAE-citric acid-mediated extraction).

Green techniques to extract natural pigments are gaining prominence among consumers and food industries. This trend is predominantly due to the harmful effects imparted by commonly used synthetic dyes and the unwarranted stress created on our ecosystem. The objectives of this study were to obtain natural pigments (anthocyanins and chlorophyll) from Estonian-gown European green and red gooseberries by ultrasonic-assisted citric acid-mediated extraction method and perform antioxidant profiling (quantification via HPLC analysis). Green gooseberry extracts showed lower content of targeted compounds, with low concentrations of rutin (0.7-1.2 mg/L) and quercetin 3-glucoside (0.9-1.3 mg/L), while in the red gooseberry extracts, the amount was slightly higher (1.4-6.9 and 1.0-1.3 mg/L, respectively) with 0.6-6.8 mg/L cyanidin 3-glucoside and 0.32-0.35 mg/L peonidin 3 glucoside recorded. Further, the yield of anthocyanins ranged between 1.14-1.79 and 1.86-3.63 mg/100 g in green and red gooseberries, respectively. Total phenols ranged between 162-392 and 263-987 mg GAE/100 g in green and red gooseberry extracts, respectively. The DPPH free radicals scavenging activity showed 73-86% and 87-91% inhibition in both green and red gooseberry, respectively. Results showed significant improvements in pigment extraction with higher values obtained for targeted antioxidant compounds using conventional and UAE extraction (aqueous extract), thus confirming that green extractions are a reliable technique to obtain pigments of interest from natural sources. The results support consumers' demand and open up the avenue to explore pigments as natural colourants in food and cosmetics applications.

Mechanochemistry-Amended Barbier Reaction as an Expedient Alternative to Grignard Synthesis.

Organomagnesium halides (Grignard reagents) are essential carbanionic building blocks widely used in carbon-carbon and carbon-heteroatom bond-forming reactions with various electrophiles. In the Barbier variant of the Grignard synthesis, the generation of air- and moisture-sensitive Grignard reagents occurs concurrently with their reaction with an electrophile. Although operationally simpler, the classic Barbier approach suffers from low yields due to multiple side reactions, thereby limiting the scope of its application. Here, we report a mechanochemical adaptation of the Mg-mediated Barbier reaction, which overcomes these limitations and facilitates the coupling of versatile organic halides (e.g., allylic, vinylic, aromatic, aliphatic) with a diverse range of electrophilic substrates (e.g., aromatic aldehydes, ketones, esters, amides, O-benzoyl hydroxylamine, chlorosilane, borate ester) to assemble C-C, C-N, C-Si, and C-B bonds. The mechanochemical approach has the advantage of being essentially solvent-free, operationally simple, immune to air, and surprisingly tolerant to water and some weak Brønsted acids. Notably, solid ammonium chloride was found to improve yields in the reactions of ketones. Mechanistic studies have clarified the role of mechanochemistry in the process, indicating the generation of transient organometallics facilitated by improved mass transfer and activation of the surface of magnesium metal.