Preparation and Structural Study of the Enantiomers of α,α‘-Bis(trifluoromethyl)-9,10-anthracenedimethanol and Its Perdeuterated Isotopomer, Highly Effective Chiral Solvating Agents

Abstract

Enantiopure forms of alpha,alpha'-bis(trifluoromethyl)-9,10-anthracenedimethanol and the corresponding perdeuterated isotopomers were prepared. The conformational study was carried out by (1)H NMR, and the absolute configuration was determined by the X-ray study of the crystallized diastereoisomeric carbamate derivative. This compound was tested as a chiral solvating agent (CSA). The results showed very good discrimination for several racemic mixtures that improved other classical methods. The study of diastereomeric complexes was carried out by determination of the stoichiometry of the complex and the binding constant of the equilibrium.

Molecular tweezers for enantiodiscrimination in NMR: Di‐(R,R)‐1‐[10‐(1‐hydroxy‐2,2,2‐trifluoroethyl)‐9‐anthryl]‐2,2,2‐trifluoroethyl benzenedicarboxylates

“…At low temperatures the internal rotation process is frozen. 18 Previous studies of other esters of ABTE 1 have shown that at these low temperatures it is possible to observe and unambiguously assign separate NMR signals for frozen cisoid and transoid conformers 18 from which the cisoid/transoid ratios could be measured 18 by integration. This might also be applied to the three new isomers 2-4, assuming independent behavior for both anthracene rings of each compound.…”

“…18 Assuming that the main conformation of the benzylic bond keeps the CÀ ÀCF 3 bond perpendicular with respect to the anthracene plane, two low energy conformations for each substituted anthracenic group are possible. In the cisoid conformation, both CF 3 groups are on the same face of the anthracenic group, while in the transoid one the CF 3 groups are on opposite faces Figure 1.…”

“…Both have a hydrogen bond acceptor or donor group and an aromatic group that allows p interactions. The discrimination achieved was compared with the enantiodiferentation obtained with compound 1, which has been shown to be a highly effective CSA 18 and can be used as a reference.…”

“…The excellent enantiodifferentiation of 6 with 3 proves that the latter is an optimal CSA, because other chiral solvating agents as 1 give poorer results. 18 All three new CSAs undergo conformational changes in the process of wrapping around the substrate. This results in the most favorable association complex for each enantiomer, and the resulting pair of optimized complexes have different geometries.…”

“…[12][13][14][15][16] Recently, we have developed a chiral molecular tweezers which could be used as a chiral solvating agent and shows a high performance with several enantiomeric mixtures. 17 The muconate of di-a,a 0 -(bistrifluoromethyl)-9,10-anthracenedimethanol (ABTE) 18 1 features two anthracene rings separated by a spacer which defines a chiral cavity. The geometry, volume, and general characteristics of this cavity depend on the nature of spacer (the dicarboxylic acid), the relative position of the ABTE groups, their dimensions and the presence and/or position of the two hydroxyl functions, which allow the formation of internal hydrogen bonds.…”