Abstract
In this study, we investigate the influence of chiral and achiral cations on the enantiomerization of biphenylic anions in n-butylmethylether and water. In addition to the impact of the cations and solvent molecules on the free energy profile of rotation, we also explore if chirality transfer between a chiral cation and the biphenylic anion is possible, i.e., if pairing with a chiral cation can energetically favour one conformer of the anion via diastereomeric complex formation. The quantum-mechanical calculations are accompanied by polarizable MD simulations using umbrella sampling to study the impact of solvents of different polarity in more detail. We also discuss how accurate polarizable force fields for biphenylic anions can be constructed from quantum-mechanical reference data.
Highlights
Over the last 20 years, chiral ionic liquids (CILs) have been in the focus of a rapidly growing field of research [1,2,3,4]
Chirality transfer via ion pairing in asymmetric synthesis has been successfully realized in many high-impact studies [19,20,21,22,23], few examples that utilize ion aggregation in CILs for asymmetric synthesis exist
Since for axially chiral biphenyl derivatives the transition between enantiomers proceeds via rotation around the aryl-aryl bond, it is crucial that the corresponding torsional energy profile is represented correctly in the force field, which is not the case for many standard force fields of unsubstituted biphenyl [40,41]
Summary
Over the last 20 years, chiral ionic liquids (CILs) have been in the focus of a rapidly growing field of research [1,2,3,4]. The first synthesis of a CIL was reported in 1997 [5], it took seven more years until the first successful chirality transfer by a CIL in an asymmetric Baylis–Hillman reaction was realized by Pégot et al in 2004 [6]. CILs can be used either as reaction solvent [11,12,13] or incorporated into the catalytic system, for example, as chiral ligand [3,14,15,16] or chiral organocatalyst [17,18]. A better understanding of ion aggregation and chirality transfer in CILs is necessary to enable the rational design of new catalytically active ionic liquids (ILs). Computer simulations can be a valuable tool in this respect since they offer a molecular view on ion aggregation in solution
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