Effective Chirogenesis in a Bis(metallosalphen) Complex through Host-Guest Binding with Carboxylic Acids

摘要

Transfer of chiral information through supramolecular interactions (chirogenesis) has been observed in many natural systems including DNA and proteins, and is nowadays widely used in the development of smart artificial and biomimetic materials. The induction of chirality in bis(me-talloporphyrins) for example, has been successfully applied in assigning the absolute configuration of amines, diamines and aminoamides, aminoalcohols and epoxyalcohols, and diols by using a circular dichroism (CD) protocol. Effective chirality transfer with carboxylic acids, however, has proven to be highly difficult, and has only been achieved by using potassium carboxylate salts followed by tedious extractions,or by the addition of a huge excess of substrate to a metal-free host. The low efficiency of chiral induction with these previous methods is mainly due to the relatively weak host-guest interactions with carboxylic acid groups. To overcome this problem, we have designed a bis[Zn~(2+)(salphen)] complex 1 (salphen = N,N-phenylenebis-(salicylimine)), which, similar to 2,2'-biphenol units, exists in dynamic equilibrium between two chiral conformations (S and R enantiomers; see Scheme 1). We reasoned that the energy barrier of rotation increases upon binding of a ditopic ligand to the Lewis acidic Zn~(2+) centers. Herein, we demonstrate that 1 binds very strongly with acetic acid and that axial chirality can be effectively induced by exchange for chiral α-substituted carboxylic acids, with the practical advantage that substrate derivatization or use of excessive substrate is not required.