1-Methyl-2,3-dihydro-1H-benzimidazole-2-selone

A greener approach towards the synthesis of N-heterocyclic thiones and selones using the mechanochemical technique

This manuscript describes the synthesis of N-heterocyclic thiones and selones of a variety of imidazolium salts involving an eco-friendly and solventless ball-milling technique. The products have been isolated in almost quantitative yield, involving a minimum quantity of solvents only for the isolation of products by column chromatography, and in some cases for purification purposes. Both mono- and bisimidazolium salts afforded N-heterocyclic thiones and selones. The methodology is found to be superior in terms of reaction time, yield and energy efficiency as compared to conventional solution-state reactions.

Facile Access to 2-Selenoxo-1,2,3,4-tetrahydro-4-quinazolinone Scaffolds and Corresponding Diselenides via Cyclization between Methyl Anthranilate and Isoselenocyanates: Synthesis and Structural Features

A practical method for the synthesis of 2-selenoxo-1,2,3,4-tetrahydro-4-quinazolinone was reported. The latter compounds were found to undergo facile oxidation with H2O2 into corresponding diselenides. Novel organoselenium derivatives were characterized by the 1H, 77Se, and 13C NMR spectroscopies, high-resolution electrospray ionization mass spectrometry, IR, elemental analyses (C, H, N), and X-ray diffraction analysis for several of them. Novel heterocycles exhibited multiple remarkable chalcogen bonding (ChB) interactions in the solid state, which were studied theoretically.

Regulation of Tyrosinase Enzyme Activity by Glutathione Peroxidase Mimics

Hydrogen peroxide plays a crucial role in the melanogenesis process by regulating the activity of the key melanin-forming enzyme tyrosinase, responsible for the browning of fruits, vegetables, and seafood. Therefore, a molecule with dual activities, both efficient tyrosinase inhibition and strong hydrogen peroxide degrading ability, may act as a promising antibrowning agent. Herein, we report highly efficient selone-based mushroom tyrosinase inhibitors 2 and 3 with remarkable glutathione peroxidase (GPx) enzyme-like activity. The presence of benzimidazole moiety enhances the tyrosinase inhibition efficiency of selone 2 (IC₅₀ = 0.4 μM) by almost 600 times higher than imidazole-based selone 1 (IC₅₀ = 238 μM). Interestingly, the addition of another aromatic ring to the benzimidazole moiety has led to the development of an efficient lipid-soluble tyrosinase inhibitor 3 (IC₅₀ = 2.4 μM). The selenium center and the -NH group of 2 and 3 are extremely crucial to exhibit high GPx-like activity and tyrosinase inhibition potency. The hydrophobic moiety of the inhibitors (2 and 3) further assists them in tightly binding at the active site of the enzyme and facilitates the C═Se group to strongly coordinate with the copper ions. Inhibitor 2 exhibited excellent antibrowning and polyphenol oxidase inhibition properties in banana and apple juice extracts.

Coordination of 1-methyl-1,3-dihydro-2H-benzimidazole-2-selone to zinc and cadmium: Monotonic and non-monotonic bond length variations for [H(sebenzimMe)]2MCl2 complexes (M = Zn, Cd, Hg)

The reactions of 1-methyl-1,3-dihydro-2H-benzimidazole-2-selone, H(sebenzimMe), towards the zinc and cadmium halides, MX2 (M = Zn, Cd; X = Cl, Br, I), afford the adducts, [H(sebenzimMe)]2MX2, which have been structurally characterized by X-ray diffraction. The halide ligands of each of these complexes participate in hydrogen bonding interactions with the imidazole N-H moieties, although the nature of the interactions depends on the halide. Specifically, the chloride and bromide derivatives, [H(sebenzimMe)]2ZnX2 and [H(sebenzimMe)]2CdX2 (X = Cl, Br), exhibit two intramolecular N-H•••X interactions, whereas the iodide derivatives, [H(sebenzimMe)]2ZnI2 and [H(sebenzimMe)]2CdI2, exhibit only one intramolecular N-H•••I interaction. Comparison of the M-Se and M-Cl bond lengths of the chloride series, [H(sebenzimMe)]2MCl2 (M = Zn, Cd, Hg), indicates that while the average M-Cl bond lengths progressively increase as the metal becomes heavier, the variation in M-Se bond length exhibits a non-monotonic trend, with the Cd-Se bond being the longest. These different trends provide an interesting subtlety concerned with use of covalent radii in predicting bond length differences. In addition to tetrahedral [H(sebenzimMe)]2CdCl2, [H(sebenzimMe)]3,CdCl2•[H(sebenzim)Me]4CdCl2, which features both five-coordinate and six-coordinate coordinate centers, has also been structurally characterized. Finally, the reaction between CdI2 and H(sebenzimMe) at elevated temperatures affords the 1-methylbenzimidazole complex, [H(sebenzimMe)]-[H(benzimMe)]CdI2, a transformation that is associated with cleavage of the C-Se bond.

Bromination of bis-(pyridin-2-yl) diselenide in methyl-ene chloride: the reaction mechanism and crystal structures of 1H-pyridine-2-selenenyl dibromide and its cyclo-adduct with cyclo-pentene (3aSR,9aRS)-2,3,3a,9a-tetra-hydro-1H-cyclo-penta-[4,5][1,3]selenazolo[3,2-a]pyridinium bromide

1H-Pyridine-2-selenenyl dibromide, C5H5NSeBr2, 1, is a product of the bromination of bis-(pyridin-2-yl) diselenide in methyl-ene chloride recrystallization from methanol. Compound 1 is essentially zwitterionic: the negative charge resides on the SeBr2 moiety and the positive charge is delocalized over the pyridinium fragment. The C-Se distance of 1.927 (3) Å is typical of a single bond. The virtually linear Br-Se-Br moiety of 178.428 (15)° has symmetrical geometry, with Se-Br bonds of 2.5761 (4) and 2.5920 (4) Å, and is twisted by 63.79 (8)° relative to the pyridinium plane. The Se atom forms an inter-molecular Se⋯Br contact of 3.4326 (4) Å, adopting a distorted square-planar coordination. In the crystal, mol-ecules of 1 are linked by inter-molecular N-H⋯Br and C-H⋯Br hydrogen bonds, as well as by non-covalent Se⋯Br inter-actions, into a three-dimensional framework. (3aSR,(9aRS)-2,3,3a,9a-Tetra-hydro-1H-cyclo-penta[4,5][1,3]selenazolo[3,2-a]pyridinium-9 bromide, C10H12NSe+·Br-, 2, is a product of the cyclo-addition reaction of 1 with cyclo-pentene. Compound 2 is a salt containing a selenazolopyridinium cation and a bromide anion. Both five-membered rings of the cation adopt envelope conformations. The dihedral angle between the basal planes of these rings is 62.45 (11)°. The Se atom of the cation forms two additional non-covalent inter-actions with the bromide anions at distances of 3.2715 (4) and 3.5683 (3) Å, attaining a distorted square-planar coordination. In the crystal, the cations and anions of 2 form centrosymmetric dimers by non-covalent Se⋯Br inter-actions. The dimers are linked by weak C-H⋯Br hydrogen bonds into double layers parallel to (001).

Protolytic cleavage of Hg-C bonds induced by 1-methyl-1,3-dihydro-2H-benzimidazole-2-selone: synthesis and structural characterization of mercury complexes

Multinuclear ((1)H, (77)Se, and (199)Hg) NMR spectroscopy demonstrates that 1-methyl-1,3-dihydro-2H-benzimidazole-2-selone, H(sebenzim(Me)), a structural analogue of the selenoamino acid, selenoneine, binds rapidly and reversibly to the mercury centers of HgX2 (X = Cl, Br, I), while X-ray diffraction studies provide evidence for the existence of adducts of composition [H(sebenzim(Me))]xHgX2 (X = Cl, x = 2, 3, 4; X = I, x = 2) in the solid state. H(sebenzim(Me)) also reacts with methylmercury halides, but the reaction is accompanied by elimination of methane resulting from protolytic cleavage of the Hg-C bond, an observation that is of relevance to the report that selenoneine demethylates CysHgMe, thereby providing a mechanism for mercury detoxification. Interestingly, the structures of [H(sebenzim(Me))]xHgX2 exhibit a variety of different hydrogen bonding patterns resulting from the ability of the N-H groups to form hydrogen bonds with chlorine, iodine, and selenium.

2-Seleno-1-alkylbenzimidazoles and their Diselenides: Synthesis and Structural Characterization of a 2-Seleno-1-methylbenzimidazole Complex of Mercury

2-Seleno-1-methylbenzimidazole, H(sebenzimMe), can be synthesized from 1-methylbenzimidazole by sequential treatment with (i) BunLi, (ii) elemental selenium and (iii) HCl(aq). This method is also applicable to the synthesis of 2-seleno-1-t-butylbenzimidazole, H(sebenzimBut ). Single crystal X-ray diffraction and NMR spectroscopic data demonstrate that H(sebenzimMe) and H(sebenzimBut ) exist as the selone rather than the selenol tautomers, which is in accord with the results of density functional theory (B3LYP) calculations. The data also indicate that the selone is best represented as a C+-Se- zwitterion rather than as a C=Se doubly bonded species. Aerobic oxidation of H(sebenzimMe) and H(sebenzimBut ) in the presence of Et3N yields the diselenides, (sebenzimMe)2 and (sebenzimBut )2. In addition, H(sebenzimMe) reacts with HgCl2 to give [H(sebenzimMe)]2HgCl2, the first structurally characterized example of a 2-seleno-1-alkylimidazole mercury complex.