Isolation of Stable Enantiomerically Pure Telluroxides and Their Stereochemistry

Abstract

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.

Optically active telluronium imides: Optical resolution, absolute configuration, and mechanism of racemization

“…Mesityl 2,4,6-triisopropylphenyltelluronium Ntoluene-4 -sulfonimide (1) was synthesized by reacting the corresponding telluroxide, which was prepared by oxidation of telluride with tert-butyl hypochlorite [13], with toluene-4-sulfonamide in the presence of molecular sieves in 97% yield, as shown in Scheme 1. Similarly, bulkier telluronium Ntoluene-4-sulfonimide 2 was obtained in 89% yield.…”

“…These results indicate that telluronium imides and telluroxides exist in equilibrium through hydrolysis of the imides and imination of the oxides. We previously clarified that optically active telluroxides readily racemize in the presence of a trace amount of water remaining in the solvent despite careful purification [13,16]. Therefore, the mechanism involving the corresponding telluroxides, which are formed in situ by hydrolysis of the telluronium imides, is the most plausible for the racemization of telluronium imides.…”

“…When optically active telluronium imide (R)-(+)-4 was stored in acetonitrile (3.3 × 10 −5 M) containing a large excess (65 equiv) of 1 N NaOH solution, Cotton effects corresponding to optically active telluroxide (R)-(+)-9 [13] appeared immediately with a decrease in that of (R)-(+)-4 ( Fig. 3), whereas no Cotton effect for the telluroxide was observed when (R)-(+)-4 was hydrolyzed under neutral conditions, probably because racemization of the resulting telluroxide occurred more rapidly than hydrolysis of the imide under neutral conditions.…”

“…Since the stereochemistry of the resulting telluroxide formed under basic conditions was R [13,16], the hydrolysis of telluronium imides proceeds with a retention of stereochemistry, at least under basic conditions. This result differs from that with sulfonium imide: sulfonium imides are known to be hydrolyzed with an inversion of stereochemistry under basic conditions [17].…”

“…Telluranes 10 and 11 exist in equilibrium. Elimination of amide from the tellurane yields telluroxide 12 with a retention of stereochemistry, which then racemizes via an achiral hydrate [13,16]. Imination of the resulting racemized telluroxide gives the racemic telluronium imide.…”