Synthesis of anionic hypervalent cyclic selenenate esters: relevance to the hypervalent intermediates in nucleophilic substitution reactions at the selenium(II) center.

The synthesis of a diaryl diselenide that contains 2,6-dicarboxylic acid groups, 2,2'-diselanediylbis(5-tert-butylisophthalic acid) (10), is described. Diselenide 10 undergoes intramolecular cyclization in methanol to form a cyclic selenenate ester, 5-tert-butyl-3-oxo-3H-benzo[c][1,2]oxaselenole-7-carboxylic acid (11). The cyclization reaction proceeds more rapidly in the presence of organic bases, such as pyridine, adenine, and 4,4'-bipyridine, to form pyridinium 5-tert-butyl-3-oxo-3H-benzo[c][1,2]oxaselenole-7-carboxylate (14), adeninium 5-tert-butyl-3-oxo-3H-benzo[c][1,2]oxaselenole-7-carboxylate (15), and 4,4'-bipyridiniumbis(5-tert-butyl-3-oxo-3H-benzo[c][1,2]oxaselenole-7-carboxylate) (16), respectively. However, 2,2'-diselanediyldibenzoic acid (22) does not undergo cyclization under similar conditions. Structural studies on cyclic selenenate esters 14-16 revealed that the Se···O (COO(-)) secondary distances (2.170, 2.075, and 2.176 Å) were significantly shorter than the corresponding Se···O distances (2.465, 2.472, and 2.435 Å) observed for the selenenate esters stabilized by the neutral donors (CHO, COOH, and COOEt). (1)H, (13)C, and (77)Se NMR spectroscopy of compounds 11 and 14-16 reveal that the aryl protons of compound 11 and the organic cations of compounds 14-16 exchange between the two carboxylate groups via a hypercoordinate intermediate. The corresponding hypercoordinate intermediate (14b, pyridinium selenuranide) for compound 14 was detected at low temperatures using (77)Se NMR spectroscopy. The presumed hypercoordinate intermediates in the carboxylate-exchange reactions at the selenium(II) center for a set of model reactions were optimized using DFT-B3LYP/6-311+g(d) calculations and their structural features compared with the X-ray structure of anionic selenenate esters 14-16.