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.

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).

Selenocarboxylic acid salts such as alkali metal and ammonium sails are Luc must im portant starting compounds for the synthesis of selenocarboxylic acid derivatives. However, their synthesis has been lim ited to arom atic potassium [1] and piperidinium selenocarboxylates [2,3], because of the difficulties in the crystallization and purification of the aliphatic salts and of the limited availability of the starting bis(acyl) selenides. Recently we succeeded in the iso lation of a series of aliphatic bis(acyl) selenides [4], We now report the first isolation of simple potassium alkanecarboselenoates (2) and Se-alkyl alkanecarbo selenoates (3), via the bis(acyl) selenides. Results and DiscussionThe expected aliphatic potassium selenocarboxy lates (2) were isolated as crystals from the reaction of the corresponding aliphatic bis(acyl) selenides with potassium m ethanolate (eq. (1)

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A series of 0-triorganosilyl selenocarboxylates 2 are prepared by the reaction of sodium or potassium selenocarboxylate 1 with triorganosilyl chlorides. The selenone esters 2 are stable towards heat, but labile towards moisture, and are formed via Se-triorganosilyl selenocarboxylate 3. In the mass spectrometer, isomerization of 2 to its less stable selenol ester 3 takes place, resembling the Schonberg thione-thiol rearrangement.In contrast to thio-and dithiocarboxylic acid esters" -61, little has been known about the chemistry of selenocarboxylic esters such as selenol-(RCOSeR') and selenone esters (RCSeOR'), though they are interesting compounds spectroscopically and synthetically"'. In 1972, we found that potassium selenobenzoate reacts with chlorotrimethylsilane to give 0-trimethylsilyl selenobenzoateL8]. In this paper we report in detail on the synthesis and characterization of 0-triorganosilyl selenocarboxylates 2. Results and DiscussionThe expected 0-triorganosilyl selenocarboxylates 2 (here after called selenone esters) are readily obtained from the reaction of alkali metal selenocarboxylates 1 with triorganosily1 chlorides (eq. 1). For example, when trimethylsilyl chloride is added to a suspension of sodium 4-methylbenzenecarboselenoate (lo) in ether at 20°C, the color of the mixture gradually changes to purple red. After stirring for 2 h, the precipitate (NaC1) is filtered off, and the solvent and the excess trimethylsilyl chloride are evaporated under reduced pressure. Vacuum distillation of the residue gives 54% of 0-trimethylsilyl4-methylbenzenecarboselenoate (2 0 ) as a purple red liquid. Similarly, the reaction of other sodium selenocarboxylates 1 a -j, 1 -n, p with triorganosilyl or tertbutyldimethylsilyl chlorides afforded the corresponding 0-triorganosilyl selenocarboxylates 2a -n, p -w in moderate to good yields. The reaction with tert-butyldimethylsilyl chloride requires a reaction time longer than 5 h, presumably due to steric hindrance of the tert-butyldimethylsilyl group. The use of petroleum ether with boiling point below 30°C seems to be preferable for the preparation of aliphatic selenone esters 2 a -k. The structures of 2 have been established by mass, 'H-and l3C-NMR, electron, and IR spectra and microanalysis.In these selenone ester formation reactions, it is interesting whether the triorganosilyl group is directly attacked by the carbonyl oxygen atom of the sodium selenocarboxylates 1, or not. In order to clarify this, the reaction of sodium 2-methylbenzenecarboselenoate (1 n) with tert-butyldimethylsilyl chloride is monitored by I3C-NMR spectroscopy. The band at 6 = 221.2 (13C = 0) of the salt gradually decreases, and two new bands appear at 6 = 200.7 and 223.2, which were assigned to the carbonyl carbon resonances of Se-silyl esters 3 (R = 2-CH3C6H4, R;Si = tBuMe,Si) and to the 13C nucleus of the C=Se in the selenone ester 2n (R = 2-CH3-CsH4, R;Si = tBuMe,Si), respectively. The former band at 6 = 200.7 quickly decreases, and finally only the band at 6 = 223.2 remains. The 77Se-NMR si...