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‐3 H ‐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‐3 H ‐benzo[ c ][1,2]oxaselenole‐7‐carboxylate ( 14 ), adeninium 5‐ tert ‐butyl‐3‐oxo‐3 H ‐benzo[ c ][1,2]oxaselenole‐7‐carboxylate ( 15 ), and 4,4′‐bipyridiniumbis(5‐ tert ‐butyl‐3‐oxo‐3 H ‐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 ( 14 b , 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 .