Dynamic helical cyclophanes with two quadruply-bridged planes arranged in an "obverse and/or reverse" relation

Chiral diversification through the assembly of achiral phenylacetylene macrocycles with a two-fold bridge

We demonstrate so-called "chiral diversification", which is a design strategy to create multiple chiral molecules through the assembly and double-bridging of achiral components. We used phenylacetylene macrocycles (PAMs) as an achiral element. In a molecule, two achiral rings of [6]PAM are stacked one above the other, or bound to each other mechanically. As an alternative, a single enlarged ring of [12]PAM was also assumed to be a doubled form of [6]PAM. In any case, one or two ring(s) are doubly-bridged by covalent bonds to exert chirality. Through intramolecular two-bond formation, these multiple chiral molecules were obtained as a set of products in one reaction. The dynamic chirality generated in molecules with either two helically-stacked rings of [6]PAM or a single helically-folded ring of [12]PAM was characterized by induced Cotton effects with the aid of an external chiral source. Thus, a chiral structure based on [12]PAM could be demonstrated as the first success. Alternatively, enantiomeric separation was achieved for molecules with two interlocked rings of [6]PAM to show remarkable chiroptical properties.

Supramolecular chirality transformation driven by monodentate ligand binding to a coordinatively unsaturated self-assembly based on C 3-symmetric ligands

Monodentate ligand binding is facilitated by supramolecular chirality transformations from propeller-shaped chirality into single-twist chirality by altering the self-assembly of C 3-symmetric chiral ligands. The C 3-symmetric chiral ligands (Im R 3Bz and Im S 3Bz) contain three chiral imidazole side arms (Im R  and Im S ) at the 1,3,5-positions of a central benzene ring. Upon coordination to zinc ions (Zn2+), which have a tetrahedral coordination preference, the C 3-symmetric chiral ligands assemble, in a stepwise manner, into a propeller-shaped assembly with a general formula (Im( R or S ) 3Bz)4(Zn2+)3. In this structure each Zn2+ ion coordinates to the three individual imidazole side arms. The resulting assembly is formally coordinatively unsaturated (coordination number, n = 3) and capable of accepting monodentate co-ligands (imidazole: ImH2) to afford a coordinatively saturated assembly [(ImH2)3(Im R 3Bz)4(Zn2+)3]. The preformed propeller-shaped chirality is preserved during this transformation. However, an excess of the monodentate co-ligand (ImH2/Zn2+ molar ratio of ∼1.7) alters the propeller-shaped assembly into a stacked dimer assembly [(ImH2) m (Im R 3Bz)2(Zn2+)3] (m = 4-6) with single-twist chirality. This switch alters the degree of enhancement and the circular dichroism (CD) pattern, suggesting a structural transition into a chiral object with a different shape. This architectural chirality transformation presents a new approach to forming dynamic coordination-assemblies, which have transformable geometric chiral structures.

Deciphering the helicity switching mechanism: a case study of the rigid three-tiered stacked architecture

Understanding the switching mechanism of helical molecular cages is critical in regulating their functions of asymmetric catalysis and enantioseparation. The helical inversion of a three-tiered stacked architecture was investigated by employing molecular dynamics simulations combined with free-energy calculations. A two-dimensional free-energy landscape characterizing the spinning processes of the top and bottom tiers around the z axis was determined using the extended adaptive biasing force method. The free-energy barrier in the least free-energy pathway was estimated to be 17.6 kcal mol-1, in excellent agreement with experimental measurements. Further analysis revealed that the barrier was caused by geometric deformation, weakening of π-π stacking between aromatic rings, and the re-orientation of polarized amine moieties. The present contribution takes a step toward understanding the dynamic helicity-based functions related to asymmetric reactions and optical resolution.

Linker-Directed Assembly of Twisted ortho-Phenylene-Based Macrocycles

o-Phenylene tetramers have been coassembled with linkers into macrocycles through imine condensation. Variation of linker connectivity and length allows both [1 + 1] and [2 + 2] macrocycles to be obtained, complementing (previously reported) [3 + 3] macrocycles. For the [1 + 1] macrocycles, linker length has a clear effect on o-phenylene geometry and macrocycle stability. For the [2 + 2] macrocycles, both homo- and heterochiral configurations are observed, suggesting limited communication of helix handedness in these systems.

Enhanced circular dichroism at elevated temperatures through complexation-induced transformation of a three-layer cyclophane with dualistic dynamic helicity

When two planes stacked one above the other are twisted, they provide a dynamic pair of helical conformations with (M)- or (P)-helicity. We designed a three-layer cyclophane that consists of two such dynamic pairs: the top and middle planes, and the middle and bottom planes. Hence, several global conformations could be created for the overall molecule, e.g., double-helical forms with a pair with the same helicity [(M,M) or (P,P)], and a meso-like form with a pair with a different helicity (M,P). These conformations dynamically interconvert to each other in solution. Chiroptical properties were given by the helical-sense preference of the double-helical forms, which was brought about through complexation with a chiral hydrogen-bonding guest. In terms of the conformational energy in a complexed state, when a desirable relationship between double-helical and meso-like forms was attained, complexation-induced circular dichroism was enhanced at elevated temperatures and decreased at lowered temperatures.

Dynamic or undynamic chirality generated by helical arrangement of a shape-persistent ring and rod doubly bridged in a molecule

Dynamic or undynamic chirality is generated in either threaded or unthreaded form of an assembly of phenylene–ethynylene-based ring and rod.