Divergent Synthesis of Chiral Covalent Organic Frameworks

Featuring the simultaneous generation of a library of compounds from a certain intermediate, divergent synthesis has found increasing applications in the construction of natural products and potential medicines. Inspired by this approach, presented herein is a general strategy to introduce functionality, in a divergent manner, into covalent organic frameworks (COFs). This modular protocol includes two stages of covalent assembly, through which functional COFs can be constructed by a three‐step transformation of a key platform molecule, such as 4,7‐dibromo‐2‐chloro‐1H‐benzo[d]imidazole (DBCBI). Constructed herein are four types of chiral COFs (CCOFs) from DBCBI by nucleophilic substitution, Suzuki coupling, and imine formation. The unique array of eight isoframework CCOFs allowed investigation of their catalytic performance and structure–activity relationship in an asymmetric amination reaction.     

Synthesis of Optically Pure Helicene Metallocenes

Two types of ruthenocenes and a ferrocene coordinated by rac ‐9H ‐cyclopenta[1,2‐c :4,3‐c ′]diphenanthrenyl anion(s), a [7]helicene with a cyclopentadienyl moiety at the center of its skeleton, were successfully synthesized: mono‐helicene ruthenocene 1 and its iron analogue 1_Fe with one [7]helicene ligand bound to the central metal, and bis‐helicene ruthenocene 2 with two [7]helicenes. Starting from a racemic mixture of the ligand precursor, rac ‐ 2 and meso ‐ 2 were obtained in a 7:3 ratio. Since the [7]helicene has a high racemization barrier, enantiomers of the complexes were isolated in their pure forms; they showed large optical rotations and intense circular dichroism (CD) responses.     

Individual (f,tA)‐ and (f,tC)‐Fullerene‐Based Nickel(II) Glycinates: Protected Chiral Amino Acids Directly Linked to a Chiral π‐Electron System

Stereoselective electrosynthesis of the first individual (^f,tA)‐ and (^f,tC)‐1,4‐fullerene derivatives with a non‐inherently chiral functionalization pattern is described, as well as the first example of an optically pure protected primary amino acid directly linked to the fullerene through only the chiral α‐amino‐acid carbon atom. An application of an auxiliary chiral nickel‐Schiff base moiety as derivatizing agent allowed separation of (^f,tA)‐ and (^f,tC)‐1,4‐fullerene derivatives using an achiral stationary phase, a separation which has never been done before.     

Chiral Amplification in Nature: Studying Cell‐Extracted Chiral Carotenoid Microcrystals via the Resonance Raman Optical Activity of Model Systems

Carotenoid microcrystals, extracted from cells of carrot roots and consisting of 95 % of achiral β‐carotene, exhibit a very intense chiroptical (ECD and ROA) signal. The preferential chirality of crystalline aggregates that consist mostly of achiral building blocks is a newly observed phenomenon in nature, and may be related to asymmetric information transfer from the chiral seeds (small amount of α‐carotene or lutein) present in carrot cells. To confirm this hypothesis, we synthesized several model aggregates from various achiral and chiral carotenoids. Because of the sergeant‐and‐soldier behavior, a small number of chiral sergeants (α‐carotene or astaxanthin) force the achiral soldier molecules (β‐ or 11,11′‐[D₂]‐β‐carotene) to jointly form supramolecular assemblies of induced chirality. The chiral amplification observed in these model systems confirmed that chiral microcrystals appearing in nature might consist predominantly of achiral building blocks and their supramolecular chirality might result from the co‐crystallization of chiral and achiral analogues.     

Gold Nanowire Chiral Ultrathin Films with Ultrastrong and Broadband Optical Activity

An ultrastrong and broadband chiroptical response is key but remains challenging for many device applications. A simple and cost‐effective bottom‐up method is introduced to fabricate large‐area long‐range ordered chiral ultrathin films with the Langmuir–Schaeffer technique using gold nanowires as building blocks. Significantly, as‐prepared ultrathin films display giant optical activity across a broad wavelength range covering visible and near infrared regions with an anisotropic factor of up to 0.285, which is the record value for bottom‐up techniques. Detailed experimental result and theoretical analysis disclose that such remarkable optical activity originates from birefringence and dichroism of the well‐aligned Au nanowire layers in the ultrathin films. The universality of this facile strategy for constructing chiral ultrathin films is further demonstrated with many other one‐dimensional nanomaterials.     

Enantioselective Synthesis of a Chiral Coordination Polymer with Circularly Polarized Visible Laser

Circular dichroism is known to be the feature of a chiral agent which has inspired scientist to study the interesting phenomena of circularly polarized light (CPL) modulated molecular chirality. Although several organic molecules or assemblies have been found to be CPL‐responsive, the influence of CPL on the assembly of chiral coordination compounds remains unknown. Herein, a chiral coordination polymer, which is constructed from achiral agents, was used to study the CPL‐induced enantioselective synthesis. By irradiation with either left‐handed or right‐handed CPL during the reaction and crystallization, enantiomeric excesses of the crystalline product were obtained. Left‐handed CPL resulted in crystals with a left‐handed helical structure, and right‐handed CPL led to crystals with a right‐handed helical structure. It is exciting that the absolute asymmetric synthesis of a chiral coordination polymer could be enantioselective when using CPL, and provides a strategy for the control of the chirality of chiral coordination polymers.     

Boron(III) Carbazosubphthalocyanines: Core‐Expanded Antiaromatic Boron(III) Subphthalocyanine Analogues

Condensation of 1,8‐diamino‐3,6‐dichlorocarbazole with a series of disubstituted 1,3‐diiminoisoindolines, followed by treatment with BF₃?OEt₂ led to the formation of the corresponding core‐expanded boron(III) subphthalocyanine analogues. These air‐stable π‐conjugated boron(III) carbazosubphthalocyanines possess two boron‐containing seven‐membered‐ring units and a 16 π‐electron skeleton, and represent the first examples of antiaromatic boron(III) subphthalocyanine analogues as supported by spectroscopic and theoretical studies. The molecular structure of one of these compounds was unambiguously determined by single‐crystal X‐ray diffraction analysis. In contrast to typical boron(III) subphthalocyanines, which adopt a cone‐shaped structure, the π skeleton of this compound is almost planar.     

Induction of Single‐Handed Helicity of Polyacetylenes Using Mechanically Chiral Rotaxanes as Chiral Sources

Effective induction of preferred‐handed helicity of polyacetylenes by pendant mechanically chiral rotaxanes is discussed. Polyacetylenes possessing optically active mechanically chiral rotaxanes in the side chains were synthesized by the polymerization of the corresponding enantiopure [2]rotaxane‐type ethynyl monomers prepared by the chiral‐phase HPLC separations. The CD Cotton effects revealed that the polyacetylenes took preferred‐handed helical conformations depending on the rotaxane chirality. The preferred‐handed helix was not disturbed by an additional chiral substituent on the rotaxane side chain. These results demonstrate the significance and utility of mechanically chiral rotaxanes for the effective construction of asymmetric fields.