Taka H, Yamazaki Y, Shimizu T, Kamigata N
Department of Chemistry, Graduate School of Science, Tokyo Metropolitan University, Japan.
J Org Chem. 2000 Apr 7;65(7):2127-33. doi: 10.1021/jo991721n.
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.
分别使用手性柱通过液相色谱法对具有庞大取代基（rac-1a-d）和氨基（rac-2a-c）的动力学和热力学稳定的二芳基碲氧化物进行光学拆分，得到了稳定的对映体纯的碲氧化物。通过将其旋光率和圆二色光谱与硫或硒类似物的旋光率和圆二色光谱进行比较，确定了光学活性碲氧化物的绝对构型。研究了溶液中光学活性碲氧化物的外消旋化动力学，发现动力学和热力学稳定化对于防止碲氧化物的外消旋化非常有效。通过变温1H NMR测量，估计氨基与碲原子的分子内配位作用对碲氧化物的稳定化能约为5 kcal mol-1。通过使用H2(18)O的同位素实验研究了光学活性碲氧化物的外消旋化机理，结果表明光学活性碲氧化物通过非手性四配位水合物进行外消旋化。
