Reactions of diselenoic acid esters with amines and X-ray crystal structure analyses of aromatic selenoamides^†

Aromatic diselenoic acid Se-methyl esters 1 react with amines at 0°C in tetrahydrofuran (THF) to yield selenoamides in moderate to good yields. The reaction course is highly dependent on the steric requirements of both starting materials. In the reactions of the ester 1a with 2-methylpiperidine and of the ester 1b with piperidine, the starting materials disappear within 1 hour with the liberation of black selenium, but the corresponding selenoamides are not produced. These results may be ascribed to the steric congestion caused by the formation of the selenoamide group from the tetrahedral intermediate 15. X-ray crystal structure analyses of the selenoamides 3 and 9 have been performed. The bond length of C(Se)N is shorter than a carbon nitrogen single bond. On the other hand, the CSe bond is longer than that of the ordinary carbon-selenium double bond. These results are indicative of the efficient delocalization of the electrons of nitrogen to the carbon–selenium double bond. The double bond character between the carbon attached to selenium and the nitrogen is also supported by the nitrogen atom showing sp² character. When a methyl group is introduced at the meta position of the aromatic ring, the deviation of the aromatic ring from the plane involving the carbon–selenium double bond and nitrogen atom becomes substantially large, perhaps due to the steric bulkiness of the selenium atom.