Aromatic ring strain in arylselenenyl bromides: role in facile synthesis of selenenate esters via intramolecular cyclization.

The synthesis and reactivity of 2,6-disubstituted arylselenium compounds derived from 2-bromo-5-tert-butylisophthalic acid (43) are described. The syntheses of bis(5-tert-butylisophthalic acid dimethyl ester)diselenide (46) and bis(5-tert-butylisophthalic acid diisopropyl ester)diselenide (47) have been achieved by the reaction of the corresponding ester precursors with disodium diselenide. Reduction of diselenide 46 with lithium aluminum hydride affords 2,2'-bis(5-tert-butylbenzene-1,3-dimethanol)diselenide (53). Diselenides 46 and 47 exhibit intramolecular Se...O interaction. Compound 53 does not show any intramolecular Se...O interaction. The anomalous Se...O nonbonded coordination observed in the single-crystal X-ray structures of compounds 46, 47 and 53 is compared and contrasted. The corresponding selenenyl bromides 54 and 55, derived from the reaction of diselenides 46 and 47 with bromine, are quite stable in the solid state. However, they undergo hydrolysis and subsequent intramolecular cyclization upon heating or after having been kept in solution over a period of time to give the corresponding selenenate esters 56 and 57. The X-ray crystallographic study and density functional theory calculations on 54 at the B3LYP/6-31G(d) level of theory indicate a significant distortion in planarity of the aromatic ring. Glutathione peroxidase-like activities of diselenides 46 and 47 and their selenenate esters 56 and 57 have been studied both by thiophenol and bioassay methods. The very low glutathione peroxidase-like activity of the diselenides (46 and 47) and their selenenate esters (56 and 57) in the thiophenol assay is attributed to the presence of the relatively strong Se...O intramolecular interaction in the selenenyl sulfide intermediates. The interaction retards the catalytic activity through both thiol exchange and an intramolecular cyclization reaction.