Tetravalent Spiroselenurane Catalysts: Intramolecular Se···N Chalcogen Bond-Driven Catalytic Disproportionation of H 2 O 2 to H 2 O and O 2 and Activation of I 2 and NBS.

Chalcogen-bonding interactions have recently gained considerable attention in the field of synthetic chemistry, structure, and bonding. Here, three organo-spiroselenuranes, having a Se(IV) center with a strong intramolecular Se···N chalcogen-bonded interaction, have been isolated by the oxidation of the respective bis (2-benzamide) selenides derived from an 8-aminoquinoline ligand. Further, the synthesized spiroselenuranes, when assayed for their antioxidant activity, show disproportionation of hydrogen peroxide into H 2 O and O 2 with first-order kinetics with respect to H 2 O 2 for the first time by any organoselenium molecules as monitored by 1 H NMR spectroscopy. Electron-donating 5-methylthio-benzamide ring-substituted spiroselenurane disproportionates hydrogen peroxide at a high rate of 15.6 ± 0.4 × 10 3 μM min -1 with a rate constant of 8.57 ± 0.50 × 10 -3 s -1 , whereas 5-methoxy and unsubstituted-benzamide spiroselenuranes catalyzed the disproportionation of H 2 O 2 at rates of 7.9 ± 0.3 × 10 3 and 2.9 ± 0.3 × 10 3 μM min -1 with rate constants of 1.16 ± 0.02 × 10 -3 and 0.325 ± 0.025 × 10 -3 s -1 , respectively. The evolved oxygen gas from the spiroselenurane-catalyzed disproportion of H 2 O 2 has also been confirmed by a gas chromatograph-thermal conductivity detector (GCTCD) and a portable digital polarographic dissolved O 2 probe. Additionally, the synthesized spiroselenuranes exhibit thiol peroxidase antioxidant activities for the reduction of H 2 O 2 by a benzenethiol co-reductant monitored by UV-visible spectroscopy. Next, the Se···N bonded spiroselenuranes have been explored as catalysts in synthetic oxidation iodolactonization and bromination of arenes. The synthesized spiroselenurane has activated I 2 toward the iodolactonization of alkenoic acids under base-free conditions. Similarly, efficient chemo- and regioselective monobromination of various arenes with NBS catalyzed by chalcogen-bonded synthesized spiroselenuranes has been achieved. Mechanistic insight into the spiroselenuranes in oxidation reactions has been gained by 77 Se NMR, mass spectrometry, UV-visible spectroscopy, single-crystal X-ray structure, and theoretical (DFT, NBO, and AIM) studies. It seems that the highly electrophilic nature of the selenium center is attributed to the presence of an intramolecular Se···N interaction and a vacant coordination site in spiroselenuranes is crucial for the activation of H 2 O 2 , I 2 , and NBS. The reaction of H 2 O 2 , I 2 , and NBS with tetravalent spiroselenurane would lead to an octahedral-Se(VI) intermediate, which is reduced back to Se(IV) due to thermodynamic instability of selenium in its highest oxidation state and the presence of a strong intramolecular N -donor atom.