Chiral Handle Research Articles

Controlling the Transition of Nanospheres to Superhelices in Aqueous Media by Using a "Smart" Pyrazine Building Block.

For the development of nature-inspired chiral nanostructures, controlling the chirality communication along the assembling pathways in aqueous media is a crucial task. Herein, we designed and synthesized pyrazine-based derivative Pyra-Met with an L-methionine group as a chiral handle. Due to the presence of flexible side chains in the Pyra-Met building block, its interactions with the solvent molecules in the assembled state are restricted, which leads to the generation of nanospheres in aqueous media. The Pyra-Met nanospheres selectively interact with Au3+ ions to generate dynamic helical fibers of the Pyra-Met:Au3+ ensemble with 'hidden' chirality. Finally, superhelices with the desired helical sense are achieved in the presence of L/D-methionine via a co-assembly process. This work presents a feasible strategy to achieve the desired handedness of supramolecular assemblies for better understanding of chirality translation in supramolecular chemistry and it highlights the importance of fine tailoring of building block to control the morphology of the assembled architecture.

Controlling the Transition of Nanospheres to Superhelices in Aqueous Media by Using a “Smart” Pyrazine Building Block

AbstractFor the development of nature‐inspired chiral nanostructures, controlling the chirality communication along the assembling pathways in aqueous media is a crucial task. Herein, we designed and synthesized pyrazine‐based derivative Pyra‐Met with an L‐methionine group as a chiral handle. Due to the presence of flexible side chains in the Pyra‐Met building block, its interactions with the solvent molecules in the assembled state are restricted, which leads to the generation of nanospheres in aqueous media. The Pyra‐Met nanospheres selectively interact with Au3+ ions to generate dynamic helical fibers of the Pyra‐Met:Au3+ ensemble with ‘hidden’ chirality. Finally, superhelices with the desired helical sense are achieved in the presence of L/D‐methionine via a co‐assembly process. This work presents a feasible strategy to achieve the desired handedness of supramolecular assemblies for better understanding of chirality translation in supramolecular chemistry and it highlights the importance of fine tailoring of building block to control the morphology of the assembled architecture.

Sui Generis Helicene-Based Supramolecular Chirogenic System: Enantioselective Sensing, Solvent Control, and Application in Chiral Group Transfer Reaction.

A novel dioxa[6]helicene-based supramolecular chirogenic system (1) as a specific chiral recognition host for enantiopure trans-1,2-cyclohexanediamine (2) is reported. Host 1 with an inherent free phenolic group and a (1S)-camphanate chiral handle on the opposite terminal rings of the helicene chromophore acted as an efficient turn on fluorescent sensor for S,S-2 with an excellent enantioselective factor, α = KSS/KRR = 6.3 in benzene. This specific host–guest interaction phenomenon is found to be solvent-dependent, which leads to an enantioselective chiral (camphanate) group transfer to the diamine guest molecule. In the case of R,R-2, the de value is up to 68% even at room temperature. Intriguingly, the induced helicity in dioxa[6]helicene diol 6, upon supramolecular hydrogen-bonding interactions, is of opposite sense with positive helicity for S,S-2 and negative helicity for R,R-2, as shown by circular dichroism spectroscopy and in combination with theoretical calculations. This chiral supramolecular system is found to be an excellent host–guest pair for enantiomeric recognition of 2, based on their electronic and steric factors.

Self-assembled nanostructures of optically active phthalocyanine derivatives. Effect of central metal ion on the morphology, dimension, and handedness

Four optically active {(2,3,9,10,16,17,23,24-octa[(S)-2-methylbutoxy]} phthalocyanine derivatives with different central metal ions, namely (S)-H2Pc(β-OC5H11)8 (1), (S)-ZnPc(β-OC5H11)8 (2), (S)-CuPc(β-OC5H11)8 (3), and (S)-NiPc(β-OC5H11)8 (4) have been synthesized and their self-assembly behaviors systematically investigated by electronic absorption and circular dichroism (CD) spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) techniques, and X-ray photoelectron spectroscopy (XPS). Metal free phthalocyanine together with its zinc and copper congeners self-assembles into right-handed helical arrangements in a stack of phthalocyanine chromophores according to the CD spectroscopic result, while the nickel complex does so into the left-handed helical arrangements. These helical arrangements, acting as elemental primary structures, further pack in a hierarchical fashion into either highly ordered right-handed fibrous nanostructures with average 300 μm length, 4.8 μm width, and 4.4 μm helical pitch for 1 and 300 μm length, 2.4 μm width, and 1.8 μm helical pitch for 2 but left-handed fibrous nanostructures with average 4 μm length, 0.48 μm width, and 0.12 μm helical pitch for 3 and 300 μm length, 2 μm width, and 1.2 μm helical pitch for 4, clearly revealing the effect of central metal ion on the distance and relative orientation of neighboring phthalocyanine chromophores and in turn the supramolecular chirality, morphology, dimension, and handedness of the self-assembled nanostructures. The present result not only represents a unique phenomenon in the self-assembly of phthalocyanine compounds but more importantly denotes the transcription and amplification of molecular chirality to supramolecular helicity with different helical bias during the self-assembly processes without changing the chiral handles attached to the phthalocyanine chromophore.

Nucleoside-Assisted Self-Assembly of Oligo(p-phenylenevinylene)s at Liquid/Solid Interface: Chirality and Nanostructures

The formation of DNA nucleoside-assisted π-conjugated nanostructures was studied by means of scanning tunneling microscopy (STM) and force field simulations. Upon adsorption of the achiral oligo(p-phenylenevinylene) (OPV) derivative at the liquid/solid interface, racemic conglomerates with mirror related rosettes are formed. Addition of the DNA nucleosides D- and L-thymidine, which act as "chiral handles", has a major effect on the supramolecular structure and the expression of chirality of the achiral OPV molecules. The influence of these "chiral handles" on the expression of chirality is probed at two levels: monolayer symmetry and monolayer orientation with respect to the substrate. This was further explored by tuning the molar ratio of the building blocks. Molecular modeling simulations give an atomistic insight into the monolayer construction, as well as the energetics governing the assembly. Thymidine is able to direct the chirality and the pattern of OPV molecules on the surface, creating chiral lamellae of π-conjugated dimers.

ChemInform Abstract: Ionic Chiral Handle-Induced Solid State Asymmetric Synthesis: Origin of the Asymmetric Induction Elucidated Through Absolute Configuration Correlation Studies.

Abstract Irradiation of crystals of two salts formed between prochiral, photochemically reactive carboxylic acids and non-absorbing, optically active amines is shown to lead to optically active photoproducts in > 95% enantiomeric excess, a process in which the ammonium ion has acted as an “ionic chiral handle.” Through a combination of X-ray crystallography and chemical correlations, the absolute configurations of the photoproducts and the salts from which they are derived were determined. This allowed the absolute steric course of the photorearrangements to be charted and the structural factors responsible for the high enantioselectivity to be identified.

Detection of zinc ions under aqueous conditions using chirality assisted solid-state fluorescence of a bipyridyl based fluorophore

A pyrrole end-capped bipyridyl ligand incorporating a chiral handle exhibited high solid-state emission when compared to the achiral analogue 1b and to the racemic molecule 1c which allowed the design of a reusable fluorescent probe for the selective detection of Zn2+ under aqueous conditions.

Coiled-coil gel nanostructures of oligo(p-phenylenevinylene)s: gelation-induced helix transition in a higher-order supramolecular self-assembly of a rigid pi-conjugated system.

Left-handed-helical nanoropes are formed from higher-order self-assemblies of chiral aggregates during the gelation of a rigid π-conjugated oligo(p-phenylenevinylene) with remote chiral handles (see picture; R=C12H25). The concentration- and temperature-dependent CD-signal changes observed for these assemblies are indicative of a helix transition. This process is reminiscent of the folding and unfolding of helical biological macromolecules. Supporting information for this article is available on the WWW under http://www.wiley-vch.de/contents/jc_2002/2004/z53874_s.pdf or from the author. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Chiral Polymorphism: A Scanning Tunneling Microscopy Study

Physisorbed monolayer films of a chiral terephthalic acid derivative have been imaged on highly oriented pyrolytic graphite (HOPG) at the solution−substrate interface using scanning tunneling microscopy. The molecule comprises a nonchiral aromatic moiety and a chiral handle. It is found to form several two-dimensional polymorphs. The packing arrangement of the molecule is primarily directed by the aromatic part, whereas the chiral part may assume different conformations on the graphite surface. The high-resolution data confirm that for all polymorphs molecular chirality is transferred to the two-dimensional adlayer structure in an enantiospecific way. This enantiospecificity is attributed to adsorbate−substrate interactions.

Properties and Reactions of Ring-Deactivated Deuterated Hydroxymethylpyrroles

Chlorination and mesylation at the hydroxymethyl position of deuterium-labeled N-substituted 2-(hydroxymethyl)pyrroles has been shown to occur with some involvement of a highly reactive azafulvenium species. An electron-withdrawing camphorsulfonyl substituent on nitrogen provides a chiral handle for direct analysis of the stereochemical outcome of the reactions by 1H NMR.

Ionic chiral handle-induced asymmetric synthesis in a solid state Norrish type II photoreaction

Irradiation of crystals of salts formed between 4-(2,4,6-triisopropylbenzoyl)benzoic acid ( 1a) and various optically active amines is shown to lead to optically active Norrish type II photoproducts in low to moderate enantiomeric excesses, a process in which the ammonium ion has acted as an “ionic chiral handle.” X-ray crystallography reveals that the salts crystallize with two independent, mirror image-related molecules of the benzophenone moiety in the asymmetric unit, and this is suggested to be the source of the low ees observed.

Ionic chiral handle-induced solid state asymmetric synthesis: Origin of the asymmetric induction elucidated through absolute configuration correlation studies

Irradiation of crystals of two salts formed between prochiral, photochemically reactive carboxylic acids and non-absorbing, optically active amines is shown to lead to optically active photoproducts in > 95% enantiomeric excess, a process in which the ammonium ion has acted as an “ionic chiral handle.” Through a combination of X-ray crystallography and chemical correlations, the absolute configurations of the photoproducts and the salts from which they are derived were determined. This allowed the absolute steric course of the photorearrangements to be charted and the structural factors responsible for the high enantioselectivity to be identified.

Asymmetric synthesis of?-substituted?-amino acids using a?-alanine derivative with two chiral handles

Enantiomerically pureα-substitutedβ-amino acids have been synthesized, the key step being the diastereoselective alkylation of aβ-alanine derivative with two chiral handles (1S,2R,5S)-menthyl N-[(1S,2S,5S)-2-hydroxypinan-3-ylidene]β-alaninate.

Synthesis and Crystal Structure of a Novel Organic Ion-Complex Crystal for Second-Order Nonlinear Optics

The novel organic ion-complex crystal composed of protonated merocyanine and p-toluenesulfonate anion, i.e., 1-methyl-4-(2-(4-hydroxyphenyl)vinyl)pyridinium 4-toluenesulfonate (MC-PTS), was synthesized for second-order nonlinear optics. Crystal structure analysis revealed that MC-PTS crystallized in the space group of P1, i.e., the most desired space group for waveguide applications where molecular dipoles are perfectly aligned in one direction. It was also pointed out that the tetrahedral sulfonate anion plays the role of a chiral handle to give noncentrosymmetric space groups.

Asymmetric Synthesis of L-[beta-11C]Alanine Using A Glycine Derivative with Two Chiral Handles, (1R,2S,3R)-8-Phenylmenthan-3-yl N-[(1R,2R,5R)-2-Hydroxypinan-3-ylidene]glycinate.

Asymmetric Synthesis of L-[beta-11C]Alanine Using A Glycine Derivative with Two Chiral Handles, (1R,2S,3R)-8-Phenylmenthan-3-yl N-[(1R,2R,5R)-2-Hydroxypinan-3-ylidene]glycinate.

Chiral handle-induced diastereoselectivity in an organic photorearrangement: Solution versus solid state results

The degree of asymmetric induction in the di-π-methane photorearrangement due to the presence of a remote chiral handle was found to vary with the nature and location of the chiral handle and with the medium in which the reaction was carried out.

Study of a model system for asymmetric induction in [2C + 2C] photoannelation reactions

Photocycloaddition of cyclopent-2-enone and (S)-(+)-2-methyl-1-(2-methylbutoxy)-1-trimethylsilyl-oxyprop-1-ene leads both to oxetanes and cyclobutanes. The asymmetric induction, experimentally determined, in the head-to-tail [2C + 2C] cyclobutane adducts amounts to about 30%, but the intrinsic induction is nearly quantitative. The absolute configuration of the predominant enantiomer conforms to that expected from the sterically preferred reaction mode. The chiral handle can easily be recovered and recycled.