Optically Active Seleninic Acids: Optical Resolution and Stability

Abstract

Tricoordinate sulfur compounds are well known to have pyramidal structures, and many chiral tricoordinate sulfur compounds have already been isolated and clearly characterized. [1] Recently, chiral tricoordinate selenium and tellurium compounds, such as oxides, [2] onium salts, [3] ylides, [4] and imides, [5] have also been isolated, and their properties have been studied. [6±8] Chalcogenic acids are also tricoordinate and are considered to have pyramidal structures. However, no studies on optically active chalcogenic acids have been reported. There are at least two possible explanations for this: first, sulfinic acids readily undergo disproportionation [9] to give thiol sulfonates and sulfonic acids. Second, facile racemization of chalcogenic acids may occur via achiral chalcogenate anions with extrusion of a proton and/or by an intra-or intermolecular proton-transfer reaction. Seleninic acids do not disproportionate to the corresponding selenol selenonates and selenonic acids. Therefore, it may be possible to isolate optically active seleninic acids if we could suppress their racemization. We examined the optical resolution of areneseleninic acids by means of liquid chromatography on an optically active column and found that seleninic acid could be optically resolved, although the racemization was fast, especially at high concentrations, and also that bulky alkyl substituents on the benzene ring of areneseleninic acids were effective in retarding racemization. Here we describe the first optical resolution and the stability of seleninic acids.When the optical resolution of 2,4,6-trimethylbenzeneseleninic acid (2) was examined on an analytical scale by liquid chromatography on an optically active column packed with amylose carbamate derivative/silica gel (Daicel Chiralpak AS; 4.6 Â 250 mm), two peaks corresponding to he enantiomers were observed on the chromatogram, whereas only one peak was observed in the case of benzeneseleninic acid (1), as shown in Figure 1. This result shows that Figure 1. Chromatographic resolution of racemic seleninic acids 1 ± 5 on an optically active column packed with amylose carbamate derivative/silica gel (Daicel Chiralpak AS; 4.6 Â 250 mm) by HPLC on an analytical scale at 25 8C at a flow rate of 1.0 cm 3 min À1 . Eluent: a) hexane/2-propanol (85/15); b) hexane/2-propanol (95/5); c) hexane/2-propanol (90/10) (at 0 8C); d) hexane/2-propanol (98/2); e) hexane/2-propanol (98/2); f) hexane/2propanol (98/2). -J. Jun, unpublished results. [10] The X-ray data were collected on an Enraf-Nonius CAD4 automated diffractometer equipped with a Mo X-ray tube at room temperature. Crystal data of 1´(BF 4 ) 2 : triclinic, P1 Å (no. 2), Z 1, a 10.289(3), b 11.209(2), c 11.752(3) , a 96.81(2), b 112.58(2), g 111.33(2)8, V 1111.9(4) 3 , m 7.023 mm À1 , 1 calcd 2.032 g cm À3 , R1 0.0343, wR2 0.0912 for 3636 unique reflections(I b 2s(I)) and 268 variables. Crystal data of 2´(BF 4 ) 3 : tetragonal, I4 1 /acd (no. 142), Z 4, a 22.498(4), c 23.16(4) , V 11 724(21) 3 , m 5.324 mm À1 , d calcd 1.501 g cm ...