Elucidation of Substituent-Responsive Reactivities via Hierarchical and Asymmetric Assemblies in Crystalline p-Quinodimethane Derivatives

We propose a mechanism for substituent-responsive reactivities of p-quinodimethane derivatives with four ester groups through their hierarchical and asymmetric assembly modes. Four asymmetric 7,8,8-tris(methoxycarbonyl)-p-quinodimethanes with a 7-positioned ethoxycarbonyl (2 a(H)), 2'-fluoroethoxycarbonyl (2 b(F)), 2'-chloroethoxycarbonyl (2 c(Cl)), or 2'-bromoethoxycarbonyl (2 d(Br)) were synthesized and crystallized. 2 a(H), 2 b(F) and 2 d(Br) afforded only one shape crystal, while 2 c(Cl) did two polymorphic 2 c(Cl)-α and 2 c(Cl)-β. UV-irradiation induced topochemical polymerization for 2 a(H), no reactions for 2 b(F) and 2 c(Cl)-α, and [6+6] photocycloaddition dimerization for 2 c(Cl)-β and 2 d(Br). Such substituent-responsive reactivities and crystal structures were compared with those of the known symmetric 7,7,8,8-tetrakis(alkoxycarbonyl)-p-quinodimethanes such as 7,7,8,8-tetrakis(methoxycarbonyl)- (1 a(Me)-α and 1 a(Me)-β), 7,7,8,8-tetrakis(ethoxycarbonyl)- (1 b(Et)), and 7,7,8,8-tetrakis(bromoethoxycarbonyl)- (1 c(BrEt)). The comparative study clarified that the reactivities and crystal structures are classified into four types that link to each other. This linkage is understandable when we analyze the crystal structures through the following hierarchical and asymmetric assemblies; conformers, dimers, one dimensional (1D)-columns, two dimensional (2D)-sheets, and three dimensional (3D)-stacked sheets (3D-crystals). This supramolecular viewpoint is supported by intermolecular interaction energies among neighbored molecules with the density functional theory (DFT) calculation. Such research enables us to elucidate the substituent-responsive reactivities of the crystals, and reminds us of the selection of the right path in a so-called "maze game".

Nucleation kinetics of lithium phosphate precipitation

Fourth-order kinetics arises from the consecutive complexation leading to precipitation.

Homochiral and heterochiral associations observed in crystals of ArSO2-(Aib)5-OMe

We demonstrated two remarkably different types of network structures that consist of homochiral and heterochiral 1D chains of enantiomeric 3₁₀-helices in the crystals of ArSO2-(Aib)5-OMe.

Characterization of Hidden Chirality: Two-Fold Helicity in β-Strands

A β-strand is a component of a β-sheet and is an important structural motif in biomolecules. An α-helix has clear helicity, while chirality of a β-strand had been discussed on the basis of molecular twists generated by forming hydrogen bonds in parallel or non-parallel β-sheets. Herein we describe handedness determination of two-fold helicity in a zig-zag β-strand structure. Left- (M) and right-handedness (P) of the two-fold helicity was defined by application of two concepts: tilt-chirality and multi-point approximation. We call the two-fold helicity in a β-strand, whose handedness has been unrecognized and unclarified, as hidden chirality. Such hidden chirality enables us to clarify precise chiral characteristics of biopolymers. It is also noteworthy that characterization of chirality of high dimensional structures like a β-strand and α-helix, referred to as high dimensional chirality (HDC) in the present study, will contribute to elucidation of the possible origins of chirality and homochirality in nature because such HDC originates from not only asymmetric centers but also conformations in a polypeptide chain.

Exocyclic self-assembly behavior of carboxylic acid and lariat ether macrocyclic hosts: regulation by pendent arm

An exocyclic supramolecular building block through O–H⋯N hydrogen bonding interaction for the assembly of di-sidearm dibenzo-diazacrown ethers bearing a flexibility sidearm with PTA acid.