Synthesis of a new fluorophore: wavelength-tunable bisbenzo[f]isoindolylidenes

The creation of new functional molecules is a central task in chemical synthesis. Herein, we report the synthesis of a new type of fluorophore, bisbenzo[f]isoindolylidenes, from easily accessible dipropargyl benzenesulfonamides. Wavelength-tunable fluorophores emitting strong fluorescence of green to red light were obtained in this reaction. Late-stage modifications and incorporation of bioactive molecules into these fluorophores give rise to potential applications in biological studies. Detailed computational and experimental studies were conducted to elucidate the mechanism, and suggest a reaction sequence involving Garratt-Braverman type cyclization, isomerization, fragmentation, dimerization and oxidation.

Three-Component Synthesis of 2-Substituted Quinolines and Benzo[f]quinolines Using Tertiary Amines as the Vinyl Source

A metal-free method for the construction of 2-substituted quinolines and benzo[f]quinolines from aromatic amines, aldehydes, and tertiary amines has been demonstrated. Cheap and readily available tertiary amines acted as the vinyl source. A new pyridine ring was selectively formed via [4 + 2] condensation that was promoted by ammonium salt under neutral conditions and an oxygen atmosphere. This strategy provided a new route for the preparation of various quinoline derivatives with different substituents at the pyridine ring, which provides the possibility of further modification.

Facile Construction of New Hybrid Conjugation via Boron Cage Extension

Aromatic polycyclic systems have been extensively utilized as structural subunits for the preparation of various functional molecules. Currently, aromatics-based polycyclic systems are predominantly generated from the extension of two-dimensional (2D) aromatic rings. In contrast, polycyclic compounds based on the extension of three-dimensional (3D) aromatics such as boron clusters are less studied. Here, we report three types of boron cluster-cored tricyclic molecular systems, which are constructed from a 2D aromatic ring, a 3D aromatic nido-carborane, and an alkyne. These new tricyclic compounds can be facilely accessed by Pd-catalyzed B-H activation and the subsequent cascade heteroannulation of carborane and pyridine with an alkyne in an isolated yield of up to 85% under mild conditions without any additives. Computational results indicate that the newly generated ring from the fusion of the 3D carborane, the 2D pyridyl ring, and an alkyne is non-aromatic. However, such fusion not only leads to a ¹H chemical shift considerably downfield shifted owing to the strong diatropic ring current of the embedded carborane but also devotes to new/improved physicochemical properties including increased thermal stability, the emergence of a new absorption band, and a largely red-shifted emission band and enhanced emission efficiency. Besides, a number of bright, color-tunable solid emitters spanning over all visible light are obtained with absolute luminescence efficiency of up to 61%, in contrast to aggregation-caused emission quenching of, e.g., Rhodamine B containing a 2D-aromatics-fused structure. This work demonstrates that the new hybrid conjugated tricyclic systems might be promising structural scaffolds for the construction of functional molecules.

Copper-Catalyzed Oxidative C-H Annulation of Quinolines with Dichloroethane toward Benzoquinoliziniums Using an In Situ Activation Strategy

Described herein is a copper-catalyzed oxidative C-H annulation of quinolines with 1,2-chloroethane (DCE), providing a concise synthetic approach to benzoquinoliziniums. In this protocol, DCE not only serves as a solvent and an in situ activation agent of quinoline C2-H but also works as vinyl equivalents to constitute the six-membered azonia ring. Furthermore, the resultant benzoquinolizinium library exhibits good properties of binding to DNA and low cytotoxicity.

Facile Synthesis of 2-Methylnicotinonitrile through Degenerate Ring Transformation of Pyridinium Salts

Nucleophilic recyclization of pyridinium salts involving a CCN interchange ring transformation for the synthesis of 2-methylnicotinonitrile derivatives was herein developed. 3-Aminocrotononitrile (3-ACN) produced in situ from CH₃CN acted as a C-nucleophile, as well as the source of CH₃ and CN groups, which was supported by isotope-labeling and control experiments.

Synthesis of Aza[n]helicenes (n = 4-7) via Photocyclodehydrochlorination of 1-Chloro-N-aryl-2-naphthamides

A series of aza[n]helicenes (n = 4-7) was synthesized using a photocyclodehydrochlorination of 1-chloro-N-aryl-2-naphthamides as a general synthetic procedure introducing the nitrogen atom to the third ring of the helicene framework. The effect of the nitrogen presence in the helicene skeleton on the physicochemical properties of the prepared aza[n]helicenes was studied and compared to those of the parent carbo-analogues. The insertion of a nitrogen atom into the outer edge of the helicene molecule has a severe impact on certain physicochemical properties such as optical rotation, electrostatic potentials, and intermolecular interactions. On the other hand, some other properties such as UV/vis, fluorescence, and phosphorescence spectra remained almost unaffected when compared to the parent carbohelicenes. A nitrogen atom can be also used for further derivatization, which can lead to further modification of helicene properties, as manifested here in the fluorescence changes induced by protonation.

Ring-Expansion Strategy for α-Aryl Azahelicene Construction: Building Blocks for Optoelectronic Materials

An acid-mediated rapid synthesis of α-aryl azahelicenes via C-C bond cleavage of helical 9H-fluoren-9-ols is reported. The newly introduced aryl ring and pyridine moieties provide an excellent opportunity to further tune the properties of azahelicences: i.e., photoluminescence. The novel α-aryl azahelicenes showcase high circularly polarized luminescence (CPL) efficiencies (4.5 × 10^-3) as well as CPL brightness (B_CPL), reaching 7.39 M^-1 cm^-1, which indicates a potential application as chiral emitters.

Vinylation of α-Aminoazoles with Triethylamine: A General Strategy to Construct Azolo[1,5-a]pyrimidines with a Nonsubstituted Ethylidene Fragment

A new general synthesis of pharmaceutically important azolo[1,5-a]pyrimidines starting from widely available 3(5)-aminoazoles, aldehydes, and triethylamine is developed. The key is to enable the vinylation reaction that allows the in situ generation of elusive acyclic enamines and the subsequent annulation reaction to occur. This direct and practical strategy is capable of constructing a range of 5,6-unsubstituted pyrazolo[1,5-a]pyrimidines and [1,2,4]triazolo[1,5-a]pyrimidines. More importantly, this protocol provides a concise synthetic route to prepare the clinically used zaleplon.

Asymmetric systematic synthesis, structures, and (chir)optical properties of a series of dihetero[8]helicenes

A series of dihetero[8]helicenes have been systematically synthesized in enantiomerically enriched forms by utilizing the characteristic transformations of the organosulfur functionality. The synthetic route begins with assembling a ternaphthyl common synthetic intermediate from 2-naphthol and bissulfinylnaphthalene through an extended Pummerer reaction followed by facile multi-gram-scale resolution. The subsequent cyclization reactions into dioxa- and dithia[8]helicenes take place with excellent axial-to-helical chirality conversion. Dithia[8]helicene is further transformed into the nitrogen and the carbon analogs by replacing the two endocyclic sulfur atoms via SNAr-based skeletal reconstruction. The efficient systematic synthesis has enabled comprehensive evaluation of physical properties, which has clarified the effect of the endocyclic atoms on their structures and (chir)optical properties as well as the unexpected conformational stability of the common helical framework.