Easy Synthesis of Functionalized Hetero[7]helicenes

Boron-containing helicenes as new generation of chiral materials: opportunities and challenges of leaving the flatland

Increased interest in chiral functional dyes has stimulated activity in the field of boron-containing helicenes over the past few years. Despite the fact that the introduction of boron endows π-conjugated scaffolds with attractive electronic and optical properties, boron helicenes have long remained underdeveloped compared to other helicenes containing main group elements. The main reason was the lack of reliable synthetic protocols to access these scaffolds. The construction of boron helicenes proceeds against steric strain, and thus the methods developed for planar systems have sometimes proven ineffective in their synthesis. Recent advances in the general boron chemistry and the synthesis of strained derivatives have opened the way to a wide variety of boron-containing helicenes. Although the number of helically chiral derivatives is still limited, these compounds are currently at the forefront of emissive materials for circularly-polarized organic light-emitting diodes (CP-OLEDs). Yet the design of good emitters is not a trivial task. In this perspective, we discuss a number of requirements that must be met to provide an excellent emissive material. These include chemical and configurational stability, emission quantum yields, luminescence dissymmetry factors, and color purity. Understanding of these parameters and some structure-property relationships should aid in the rational design of superior boron helicenes. We also present the main achievements in their synthesis and point out niches in this area, e.g. stereoselective synthesis, necessary to accelerate the development of this fascinating class of compounds and to realize their potential in OLED devices and in other fields.

Keto-enol equilibrium: stable tautomers of ortho-, meta-, and para-hydroquinones in large aromatics

The synthesis of [6]helicene para-quinone starting from the 1,4-dimethoxy-[6]helicene derivative is presented. The demethylation reaction with boron tribromide led to unexpected results. Instead of the expected para-hydroquinone, the diketone tautomeric form was isolated. In contrast to the 1,4-hydroquinone and 1,4-dihydroxynaphthalene, the stable tautomers for the [4] and [6]helicenes were the aromatic diketones. These experimental results were corroborated by calculations. Additional calculations showed that these tautomeric species were indeed the stable forms of 1,4 and 1,3-hydroquinones when present in larger aromatics, in drastic contrast with 1,2-dihydroxy-aromatics (known as catechol).

Resolution of a Configurationally Stable Hetero[4]helicene

We have developed an efficient chemical resolution of racemic hydroxy substituted dithia-aza[4]helicenes (DTA[4]H) 1(OH) using enantiopure acids as resolving agents. The better diastereomeric separation was achieved on esters prepared with (1S)-(−)-camphanic acid. Subsequent simple manipulations produced highly optically pure (≥ 99% enantiomeric excess) (P) and (M)-1(OH) in good yields. The role of the position where the chiral auxiliary is inserted (cape- vs. bay-zone) and the structure of the enantiopure acid used on successful resolution are discussed.

Enantioselective Synthesis of Dithia[5]helicenes and their Postsynthetic Functionalization to Access Dithia[9]helicenes

A highly enantioselective synthesis of 5,13-disubstituted dibenzo[d,d']benzo[1,2-b:4,3-b']dithiophenes is reported. Key for the successful assembly of these helical architectures is the last two successive Au-catalyzed intramolecular alkyne hydroarylation events. Specifically, the second cyclization is the enantiodetermining step of the whole process and provides the desired helicenes with excellent ee values when a TADDOL-derived 1,2,3-(triazolium)phosphonite moiety (TADDOL: α,α,α',α'-tetraaryl-1,3-dioxolane-4,5-dimethanol) is employed as an ancillary ligand. The absolute stereochemistry of the newly prepared structures has been determined by X-ray crystallography to be P; the optical properties of these heterohelicenes are also reported. A three-step procedure was subsequently developed that allows the transformation of the initially obtained dithia[5]helicenes into dithia[9]helicenes without erosion of the enantiopurity.

Facile Synthesis of Azahelicenes and Diaza[8]circulenes through the Intramolecular Scholl Reaction

Carbazole-based aza[7]helicenes and hetero[9]helicenes were successfully obtained via the intramolecular Scholl reaction of 3,6-bis(biphenyl-2-yl)carbazole congeners, while the reaction of 3,6-bis(naphthylphenyl)-appended carbazole gave a triple helicene via an unexpected simultaneous double aryl rearrangement. DFT calculations suggested that the rearrangement proceeded via an arenium cation intermediate. In addition, the reaction of methoxy-appended substrate gave an azahepta[8]circulene via the concurrent C-C bond formation. These helical dyes showed circularly polarized luminescence. The azahepta[8]circulene was further transformed into deeply saddle-distorted dibenzodiaza[8]circulenes as the first example of its solution-based synthesis and unambiguous structural determination.

Synthesis, crystal structures and properties of carbazole-based [6]helicenes fused with an azine ring

Novel carbazole-based [6]helicenes fused with an azine ring (pyridine, pyrazine or quinoxaline) have been prepared through a five-step synthetic sequence in good overall yields. Commercially available 2,3-dihaloazines were used as starting materials. To discern the effect of merging an azine moiety within a helical skeleton, the X-ray structures, UV–vis absorption and fluorescence spectra of the helicenes were investigated and compared to that of the parent carbazole-based [6]helicene (7H-phenanthro[3,4-c]carbazole).

Azahelicene-Fused BODIPY Analogues Showing Circularly Polarized Luminescence

Helical carbazole-based BODIPY analogues were readily synthesized via aza[7]helicenes. The structures of azahelicene-incorporated BF₂ dyes were elucidated by x-ray diffraction analysis. DFT calculations revealed that the π-conjugated system expanded from the helicene moiety to the BODIPY framework. The azahelicene-fused boron complexes showed the Cotton effects and the circularly polarized luminescence (CPL) in the visible region. Furthermore, an axially chiral binaphthyl group was attached to the helically chiral dyes, which enhanced the chiroptical properties.

Helicene-Based Chiral Auxiliaries and Chirogenesis

Helicenes are unique helical chromophores possessing advanced and well-controlled spectral and chemical properties owing to their diverse functionalization and defined structures. Specific modification of these molecules by introducing aromatic rings of differing nature and different functional groups results in special chiroptical properties, making them effective chiral auxiliaries and supramolecular chirogenic hosts. This review aims to highlight these distinct structural features of helicenes; the different synthetic and supramolecular approaches responsible for their efficient chirality control; and their employment in the chirogenic systems, which are still not fully explored. It further covers the limitation, scope, and future prospects of helicene chromophores in chiral chemistry.

Polycyclic Aromatic Hydrocarbons via Iron(III)-Catalyzed Carbonyl-Olefin Metathesis

Polycyclic aromatic hydrocarbons are important structural motifs in organic chemistry, pharmaceutical chemistry, and materials science. The development of a new synthetic strategy toward these compounds is described based on the design principle of iron(III)-catalyzed carbonyl-olefin metathesis reactions. This approach is characterized by its operational simplicity, high functional group compatibility, and regioselectivity while relying on FeCl3 as an environmentally benign, earth-abundant metal catalyst. Experimental evidence for oxetanes as reactive intermediates in the catalytic carbonyl-olefin ring-closing metathesis has been obtained.

Synthesis of a figure-eight azahelicene dimer with high emission and CPL properties

A bisbutadiyne bridged azahelicene dimer with a figure-eight shape, which exhibited both high fluorescence quantum yield and CPL properties, has been synthesized through the Glaser–Hay homo-coupling reaction with diethynylazahelicene.

The Use of (-)-Sparteine/Organolithium Reagents for the Enantioselective Lithiation of 7,8-Dipropyltetrathia[7]helicene: Single and Double Kinetic Resolution Procedures

The effect of organolithium reagent (RLi: R=nBu, iPr, sBu, tBu), solvent (diethyl ether, diethyl ether/THF and MTBE), and stoichiometry on the (-)-sparteine-mediated silylation of 7,8-dipropyltetrathia[7]helicene shows that, unusually, substantially more than 0.5 equivalent of RLi (R=iPr, sBu, tBu) and a large excess of (-)-sparteine (R=nBu, sBu) is often needed to achieve substantial conversions and good ee values. With nBuLi, however, just one equivalent of the organolithium reagent is sufficient to obtain high conversions. Our best results were obtained using the convenient tBuLi/(-)-sparteine adduct with which the need for a high (-)-sparteine/RLi ratio can be avoided. Single- and double-kinetic resolution (KR) procedures give enantiopure samples of 2-trimethylsilyl- and 2,13-di(trimethylsilyl)-7,8-dipropyltetrathia[7]helicene and two-step double-KR combining (-)-sparteine/sBuLi and chiral formamides affords the synthetically valuable 2-formyl-7,8-dipropyltetrathia[7]helicene. This is the first use of (-)-sparteine for the enantioselective lithiation of helicenes and the first report of tBuLi outperforming sBuLi in a (-)-sparteine-mediated procedure.

Synthesis of 3-(quinolin-2-yl)- and 3,6-bis(quinolin-2-yl)-9H-carbazoles

A simple and efficient synthesis of novel 3-(quinolin-2-yl)- and 3,6-bis(quinolin-2-yl)-9H-carbazoles, utilizing sodium ethoxide as a catalyst via a Friedländer condensation reaction between 3-acetyl-9-ethyl-9H-carbazole or 3,6-diacetyl-9-ethyl-9H-carbazole and β-aminoaldehydes or β-aminoketones is described. All of the title compounds were obtained in good yields of 52–72% and their structures were confirmed by IR, ¹H NMR, MS, and elemental analysis.

Molecular springs, muscles, rheostats, and precessing gyroscopes: from review to preview

The concepts of molecular springs and gyroscopes have existed for some time, and there have been numerous reports published about these fascinating topics. Here we describe our interest in this topic, reviewing our initial progress. This is not a complete story, rather an offering of synthetic strategies, interesting molecular structures, observations, and possibilities. In many cases, the "properties" component of the structure-property relationship for a given compound is the result of a computational prediction. Given the present state of theoretical chemistry, a computer's predictive power can far exceed that which can be presently accomplished by existing experimental analytical means. The theoretical results reported here allude to intriguing possibilities. Hopefully, this intrigue will help catalyze the development of definitive, albeit rather esoteric, single-molecule analytical experiments. The intentionally speculative nature of this review is intended to stimulate new challenges for and perspectives from those in related fields of interest; hopefully presenting a preview of what is to come.

Tetracyanoanthraquinodimethanes with a Chiral Amide Group: Preparation, Properties, and Charge-Transfer Photochirogenetic Reaction with 1,2-Dianisylacenaphthene-1,2-diol

[reaction: see text] A series of butterfly-shaped tetracyanoanthraquinodimethanes (TCNAQs) with a chiral amide auxiliary 1a-f were prepared from the corresponding anthraquinones. They are stronger acceptors than the unsubstituted derivative and undergo one-wave two-electron reduction. They form weak electron-donor-acceptor (EDA) complexes with the title pinacol 2. Upon charge-transfer excitation of these complexes, dihydro-TCNAQs 3 and 1,8-dianisoylnaphthalene 4 were efficiently formed, the latter of which is the product of a retropinacol reaction via 2+*. Partial enantiodifferentiation of rac-2 was realized during the photoreactions with 2-[(R)-1-phenylethylcarbamoyl]-TCNAQ 1ain CD3CN. Thus, optically active (S,S)-(+)-pinacol 2 (12.3% ee at 54% conversion; 21.5% ee at 70% conversion) was recovered from the photolyzates. This reaction represents a new and rare example of the pseudokinetic resolution of tert-alcohol accompanied by C-C bond fission. Significant differences in the association constants for the diastereomeric EDA complexes are responsible for the observed enantiodifferentiation.