Thiocarboxylic acids, such as selenocarboxylic acids, exist predominantly in the thioxo form (RCSOH, thion acid) in polar solvents such as tetrahydrofuran (THF) at temperatures below −50 °C. Tellurocarboxylic acids (5) were observed for the first time by acidolysis of the corresponding cesium tellurocarboxylates with hydrogen chloride. The telluroic acids (6) exist predominantly in the telluroxo form (RCTeOH, telluron acid) in THF at temperatures below −70 °C. Telluron acids were reddish to blue violet for the aliphatics (R = alkyl) and dark green for the aromatics (R = aryl) and reacted with aryl isocyanates at −70 °C to give crystalline acyl carbamoyl tellurides in good yields.

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Carboxylic (RCOOH) and thiocarboxylic acids (RCOSH) are ubiquitous but important organic compounds with a great variety of use in chemistry.1 In contrast, the reactivity and structural features of selenocarboxylic acids (hereafter called selenoic acids) have little been studied mainly because of the lack of their general synthetic methods.2-4 In the previous synthetic efforts to attain such compounds, the isolation was hampered by the use of strong bases or water and resulted in decomposition.3 Recently, we have succeeded in the preparation of selenoic acid alkali metal salts, compounds which can be stored under an inert atmosphere.5 Herein, we report the first isolation of selenoic acids and their behavior in solution indicating the tautomeric equilibrium between selenoi and selenoxo forms of selenoic acids.The preparation of selenoic acids 2 was attained by the treatment of sodium selenocarboxylates l5b with an ether solution of hydrogen chloride under an Ar atmosphere (eq l).

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