Absolute Helical Arrangement of Stacked Benzene Rings: Heterogeneous Double-Helical Interaction Comprising a Hydrogen-Bonding Belt and an Offset Parallel Aromatic-Aromatic-Interaction Array

摘要

Compounds that form helical structures in the crystalline state or in solution have attracted much attention because living things utilize helical structures to store genetic information, and because compounds with a helical-ordered structure have provided new materials in solid- or liquid-crystal engineering. There are several types of helical structure: a single strand that folds helically owing to solvophobic effect or guest ligation, a double or multiple helix of strands derived from cation or anion ligation, or hydrogen bonding, and a helical super-structure formed by conformationally helical or discotic small molecules through columnar stacking supported by hydrogen bonds, for other examples see ref. In helical structures, a parallel aromatic-aromatic interaction is often a very important driving force to stabilize the structure, and sometimes hydrogen bonding or other weak intermolecular interactions assist the formation of the highly ordered helical arrangement. Such a helical structure is intrinsically chiral, that is, it has a right-handed helix (P-helix) or a left-handed helix (M-helix), which are enantiomeric. Asymmetric synthesis to produce one enantiomeric helix is important, especially in self-assembling systems,[5]-[7] in foldamers,[1a] or in polymers with a helical structure induced by catalytic chiral initiation.[10] There are generally two kinds of driving forces to determine which helicity a compound will adopt, they are, an internal or an external chiral source. For example, the helicity of DNA is derived from the internal chirality of the chiral carbon atom in the deoxyribose chain. On the other hand, one of oligophenylacetylenes prepared by Moore and co-workers folded into a single helical form upon ligation of a chiral ligand. In this context, absolute asymmetric assembly of an achiral molecule is especially intriguing. In the course of our investigation of spontaneous optical resolution, we found a discotic compound, tris(2-hydroxyethyl)-1,3,5-benzenetricarboxylate (1), which crystallized as chiral crystals in which the nearly planar molecules were helically arranged within individual single crystals. The helicity of the triester 1 was derived from the helical arrangement of the molecules, supported by an array of hydrogen bonding through a single moiety and offset aromatic stacking of the adjacent molecules.