Metal-Free Three-Component Coupling Reaction of Ketones with Electron-rich Arenes and Selenium Dioxide for the Synthesis of α-Arylselanyl Ketones

Competing electrophilic substitution and oxidative polymerization of arylamines with selenium dioxide

This article describes the detailed analysis of the reaction between arylamines, such as aniline, o-anisidine, and methyl anthranilate, with selenium dioxide in acetonitrile. A systematic analysis of the reaction products with the help of 77Se NMR and single-crystal X-ray crystallography revealed that the reaction progress follows three major reaction pathways, electrophilic selenation, oxidative polymerization, and solvent oxidation. For aniline and o-anisidine, predominant oxidative polymerization occurred, leading to the formation of the respective polyaniline polymers as major products. For methyl anthranilate, the oxidative polymerization was suppressed due to the delocalization of amine lone pair electrons over the adjacent carboxylate function, which prompted the selenation pathway, leading to the formation of two of the isomeric diorganyl selenides of methyl anthranilate. The diaryl selenides were structurally characterized using single-crystal X-ray diffraction. Density functional theory calculations suggest that the highest occupied molecular orbital of methyl anthranilate was deeply buried, which suppressed the oxidative polymerization pathway. Due to solvent oxidation, oxamide formation was also noticed to a considerable extent. This study provides that utmost care must be exercised while using SeO2 as an electrophile source in aromatic electrophilic substitution reactions.

Selective Synthesis of Bis-Heterocycles via Mono- and Di-Selenylation of Pyrazoles and Other Heteroarenes

The insertion of selenium was achieved in the form of mono-selenides and di-selenides for the preparation of novel bis-heterocyclic compounds. This method is more general and provides scaffold diversity with high yields of products. The concentration-dependent mono- and di-selenylation reaction selectivity was achieved using SeO₂ as an efficient selenylating reagent.

Visible Light-Mediated C(sp2)-H Selenylation of Amino Pyrazole and Amino Uracils in the Presence of Rose Bengal as an Organophotocatalyst

Herein, we report an efficient methodology for the synthesis of alkyl, benzyl, and phenyl selenoethers of aminopyrazoles and aminouracils by C(sp²)-H functionalization in the presence of visible light and Rose Bengal as an organophotocatalyst. The reaction of amino pyrazole/iosothiazole/isoxazole or amino uracils with 0.5 equivalent of diphenyl/dibenzyl/diethyl diselenides in the presence of visible light in acetonitrile medium and a catalytic amount of Rose Bengal provided the corresponding phenyl, benzyl, or ethyl selenoethers in good to very good yields. We have also utilized some of the selenylated aminopyrazoles for the preparation of pyrazole-fused dihydropyrimidines tethered with arylselenoethers by a catalyst-free one-pot three-component reaction. The notable features of this methodology are metal-free reaction conditions, good to very good yields, use of an organic photocatalyst, and wide substrate scope; it is also applicable to gram-scale synthesis and provides selenoethers of medicinally important heterocycles such amio-pyrazole, isoxazole, isothiazole, and uracils.

Trifluoroacetic Acid-Mediated Oxidative Self-Condensation of Acetophenones in the Presence of SeO2: A Serendipitous Approach for the Synthesis of Fused [1,3]Dioxolo[4,5-d][1,3]dioxoles

A method for the synthesis of fused 1,3-dioxolanes was developed by self-condensation of glyoxal generated in situ by oxidation of acetophenones with SeO2 in the presence of trifluoroacetic acid. Three molecules of the glyoxal generated by oxidation of ketone with SeO2 condensed to form architecturally novel oxygen-containing heterocycles (3a-aryldihydro-[1,3]dioxolo[4,5-d][1,3] dioxole-2,5-diyl)bis(phenylmethanones). This reaction provides a unique methodology for the construction of four C-O bonds in a concerted fashion, generating highly embedded oxygen heterocycles from readily available ketones using affordable shelf reagents and simple reaction conditions.