Residual Enantiomerism and Chromatographic Resolution of a 1,1,2‐Triarylalkene

Abstract

mixture consisting of unreacted 1 and two other germanium compounds. The mass spectrum of the major product (crude yield 50%, HPLC) showed it to contain a skeleton of eight germanium atoms. The X-ray structural analysis"O1 ( Fig. 1) confirmed this, showing that 2 is 4,8-dibromoocta-tert-butyltetracyclo[3.3.0.02~7.03~6]octagermane. Both enantiomers of the chiral molecule are present in pairs in the unit cell. Only four signals, instead of eight, were observed for the tert-butyl protons in the 'H NMR spectruml" of 2, which is in accord with the C, molecular symmetry. Not only does 2 exhibit exceptional thermal stability it is also air and moisture stable. n Br' Br Fig. I. Molecular structure of 2 (H atoms omitted). Selected bond lengths [pm] and angles ["I (standard deviations). The numbers marked with dashes indicate symmetry equivalent atoms.

Four Enantiomerization Routes of 1,2,2-Trimesitylvinyl Acetate. Enantioselective Liquid Chromatography of (E)- and (Z)-2-m-Methoxymesityl-1,2-dimesitylvinyl Acetates

“…In the present paper we extend our previous studies , of the different enantiomerization routes in 1,2,2-trimesitylvinyl acetate 3a by applying a similar labeling technique with a m -OMe group one at a time for each of the mesityl rings, thus generating 3b − d . In contrast with the work on 2a − d and 3a which applied only the DNMR technique, in the present work we achieved resolution of enantiomeric pairs of 3c and 3d by enantioselective LC and applied a stop-flow on-line chiroptical detection in order to follow the stereoisomerization processes.…”

“…They coalesce at 112.3 °C, giving a Δ G ⧧ of 18.8 kcal mol -1 . Barriers of 19.0 kcal mol -1 for the enantiomerization of 3a and the diastereomerization of 3b were obtained previously. ,

2 Isomerization routes of acetate 3c . D: A diastereomerization 3-ring flip process; E1, E2: two isomeric enantiomerization 2-ring flip processes.

…”

“…In C 6 N 5 NO 2 at room temperature 3c displays two MeO singlets in a 2:1 ratio, with Δν = 42.4 Hz. The diastereomerization barrier was evaluated by the DNMR method, similarly to that previously used. − On raising the temperature the two MeO signals are broadened and finally coalesce. In monitoring the DNMR behavior the OMe signal serves as a probe, giving the threshold 3-ring flip diastereomerization barrier of 19.7 kcal mol -1 at the coalescence temperature of 121.5 °C.…”

“…Liquid Chromatography Resolution and Stereoisomerization Barriers. To evaluate the different 2-ring flip enantiomerization barriers, which must be substantially higher than these threshold values, , a previously described technique 6,7 was used. Attempted resolutions of racemates of 3c and 3d were tried on different enantioselective LC columns.…”

“…In contrast with the work on 2a − d and 3a which applied only the DNMR technique, in the present work we achieved resolution of enantiomeric pairs of 3c and 3d by enantioselective LC and applied a stop-flow on-line chiroptical detection in order to follow the stereoisomerization processes. This resembles the previous study of 3b . This method was supplemented by using the DNMR technique in order to independently corroborate the measured values of the threshold barriers.…”