2-((Dimethylamino)methyl)phenyl alkyl (or aryl) selenoxides, configurationally stabilized by intramolecular coordination of an amino group to the selenium atom, were optically resolved into their enantiomeric isomers by means of high-performance liquid chromatography using an optically active column packed with amylosecarbamate derivative/silica gel. This is the first example of the isolation of optically pure selenoxides without bulky substituents and also the first isolation of optically pure alkyl aryl selenoxides. The absolute configuration of the (-)-isomers could be assigned to be the S-form by comparison of their specific rotations, circular dichroism spectra, and behavior on the optically active column with those of the sulfur analogue, prepared by Andersen's method. Racemization of the optically active selenoxides was accelerated not only in acidic solution but also in basic media. This result indicates there are two different mechanisms for their racemization in acidic and basic media. The stabilization energy of the selenoxides by the intramolecular coordination of an amino group to the selenium atom was estimated to be ca. 3 kcal mol(-)(1) on the basis of variable-temperature (1)H NMR measurements.

Ever since an optically active enantiomeric selenoxide was obtained for the first time by Davis and co-workers in 1983, 1,2 there have been several reports on the isolation of optically active selenoxides 3 by various approaches, such as optical resolution of diastereomeric mixtures, 4 chromatographic resolution of enantiomeric mixtures using an optically active column, 5 complexation with a chiral ligand, 6 and asymmetric oxidation of selenides. 7,8 Optically active selenoxides have also been extensively studied as key intermediates in asymmetric synthesis. 3,8a-d,9,10 Optically active telluroxides have also been recently reported as transient key intermediates in asymmetric reactions by Uemura and co-workers. 11 However, optically active telluroxides have not yet been isolated, perhaps because telluroxides undergo racemization Via a hydrate, 12 formed by the addition of water, much faster than selenoxides. 13 We report here the first isolation of optically active telluroxides by means of optical resolution using an optically active column. Their stereochemistry and configurational labilities are also examined.Mesityl 2,4,6-triisopropylphenyl telluroxide (3) was subjected to several optically active columns 14 using high-performance liquid chromatography at an analytical scale. Two peaks corresponding to each enantiomer of 3 were observed separately when an optically active column such as AS, AD, OB, or OJ was used, and the best separation was obtained on a column packed with amylose carbamate derivative/silica gel (AS). Using the AS column, 2,4,6-tri-tert-butylphenyl mesityl telluroxide (1) and 2,4,6-tri-tert-butylphenyl phenyl telluroxide (2) were also resolved into two peaks corresponding to the enantiomers, as shown in Figure 1, while telluroxide 4, which has less bulky substituents, showed only one peak. Racemic telluroxide 2 was resolved into its enantioisomers better than 1. Deterioration of asymmetric recognition for 1 is probably due to steric hindrance around the telluroxide moiety or to the similarity of the bulkiness of the two aryl groups on the tellurium atom of 1. The chromatogram of telluroxide 3 showed an unusual shape which indicated that racemization was occurring in the column. These results show that a substituent more bulky than a 2,4,6-triisopropylphenyl group is needed to inhibit racemization.We attempted to resolve the racemic telluroxides 1 and 2 into their optical isomers at a preparative scale using mediumpressure liquid chromatography with the same type of column. 15 In the optical resolution of telluroxide 2, the first eluted enantiomer had a positive optical rotation {[R] D 123.0 (c 0.16, MeCN)} while the second enantiomer had a negative optical rotation {[R] D -68.8 (c 0.23, MeCN)}. However, the optical purities corresponding to the specific rotations could not be determined because racemization occurred readily. 16 Optically pure telluroxide (+)-1 17 was finally obtained (20 mg) from the (1) Davis, F. A.; Billmers, J. M.; Stringer, O. D. Tetrahedron Lett. 1983, 24, 3191. (2...

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Optical resolution of kinetically and thermodynamically stabilized diaryl telluroxides possessing bulky substituents (rac-1a-d) and amino group (rac-2a-c), respectively, by liquid chromatography using optically active columns yielded stable enantiomerically pure telluroxides. The absolute configurations of the optically active telluroxides were determined by comparing their specific rotations and CD spectra with those of sulfur or selenium analogues. The kinetics for the racemization of optically active telluroxides in solution was studied, and it was found that kinetic and thermodynamic stabilization were very effective preventing the racemization of telluroxides. The stabilization energy of telluroxides by intramolecular coordination of the amino group to the tellurium atom was estimated to be ca. 5 kcal mol-1 by variable temperature 1H NMR measurement. The mechanism for the racemization of optically active telluroxides was studied by an isotope experiment using H2(18)O, and the results indicated that optically active telluroxides underwent racemization via an achiral tetracoordinated hydrate.

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