Contributing to Biochemistry and Optoelectronics: Pyrrolo[1',2':2,3]imidazo[1,5-a]indoles and Cyclohepta[4,5]pyrrolo[1,2-c]pyrrolo[1,2-a]imidazoles via [3+2] Annulation of Acylethynylcycloalka[b]pyrroles with Δ1-Pyrrolines

Available pyrrolylalkynones with tetrahydroindolyl, cycloalkanopyrrolyl, and dihydrobenzo[g]indolyl moieties, acylethynylcycloalka[b]pyrroles, are readily annulated with Δ¹-pyrrolines (MeCN/THF, 70 °C, 8 h) to afford a series of novel pyrrolo[1',2':2,3]imidazo[1,5-a]indoles and cyclohepta[4,5]pyrrolo[1,2-c]pyrrolo[1,2-a]imidazoles functionalized with an acylethenyl group in up to an 81% yield. This original synthetic approach contributes to the arsenal of chemical methods promoting drug discovery. Photophysical studies show that some of the synthesized compounds, e.g., benzo[g]pyrroloimidazoindoles, are prospective candidates for TADF emitters of OLED.

Tandem Palladium/Copper-Catalyzed Decarboxylative Approach to Benzoimidazo- and Imidazophenanthridine Skeletons

A protocol for a tandem Pd/Cu-catalyzed intermolecular cross-coupling cascade between o-bromobenzoic acids and 2-(2-bromoaryl)-1H-benzo[d]imidazoles or the corresponding imidazoles is presented. The protocol provides conceptually novel and controlled access to synthetically useful N-fused (benzo)imidazophenanthridine scaffolds with high efficiency, a broad substrate scope, and excellent functional group compatibility.

Synthesis of Imidazo[1,2-f]phenanthridines by Recyclable Magnetic MOF-Catalyzed Coupling and Cyclization of 2-(2-Bromoaryl)imidazoles with Cyclohexane-1,3-diones Followed by Aromatization

2-(2-Bromoaryl)imidazoles react with cyclohexane-1,3-diones in the presence of a catalytic amount of recyclable Fe₃O₄@SiO₂@MOF-199 and a base to give the corresponding C-C coupled and cyclized products 6,7-dihydroimidazo[1,2-f]phenanthridin-8(5H)-ones in high yields. The magnetic MOF catalyst could be easily recovered and reused four times without any significant loss of catalytic activity. The coupled and cyclized scaffolds were aromatized to imidazo[1,2-f]phenanthridines in high yields by a one-pot sequential procedure including reduction, dehydration, and oxidation. The present protocol could be applied to the synthesis of Zephycandidine A, which is known to exhibit anti-tumor activity.

Synthesis of indenophenanthridine via a [4+2] annulation strategy: a “turn-off’’ Fe3+ ion sensor, practical application in live cell imaging and reversible acidochromism studies

An efficient protocol has been developed for the synthesis of a novel fluorescent probe, 1,2-disubstituted-indeno[1,2,3-gh]phenanthridine, derived from a series of α-oxo-ketene dithioacetals (OKDTAs) and indenoquinoline under essential conditions via a [4+2] annulation in excellent yield.

A Photochemical Intramolecular C-N Coupling Toward the Synthesis of Benzimidazole-Fused Phenanthridines

Herein, we report a direct photochemical dehydrogenative C-N coupling of unactivated C(sp²)-H and N(sp²)-H bonds. The catalysts or additive-free transformation of 2-([1,1'-biphenyl]-2-yl)-1H-benzo[d]imidazole to benzo[4,5]imidazo[1,2-f]phenanthridine was achieved at ∼350 nm of irradiation via ε-hydrogen abstraction. DFT calculations helped to understand that the N-H···π interaction was essential for the reaction to proceed at a lower energy than expected.

Regioselective intramolecular sp2 C–H amination: direct vs. mediated electrooxidation

We reported an electrochemical intramolecular C–H amination approach to construct benzimidazole-fused phenanthridines.

Palladium-Catalyzed Synthesis of Fluorescent Benzo[4,5]imidazo[1,2-a]pyridines through Annulation Reaction of Benzimidazoles and Alkynyl Bromides with Internal Alkynes

An efficient synthesis of fused azapolycycles based on (benz)imidazole and pyridine scaffolds has been developed. In all cases, the first nucleophilic addition of (benz)imidazoles to alkynyl bromides in tert-pentyl alcohol can proceed in a stereoselective manner to provide (Z)-N-(1-bromo-1-alken-2-yl)benzimidazoles at 110 °C. Sequentially, these adducts containing alkenyl bromide can undergo Pd-catalyzed intermolecular C-H annulation in the presence of internal alkynes in dimethylacetamide, affording fluorescent (benz)imidazole-fused pyridines in good to high yields. These compounds generally exhibit blue or green fluorescences (454-503 nm for solution states and 472-506 nm for solid states), and the fluorescence quantum yields remained in 0.19-0.89 and 0.02-0.74 for solution and solid states, respectively.

Cascade π-Extended Decarboxylative Annulation Involving Cyclic Diaryliodonium Salts: Site-Selective Synthesis of Phenanthridines and Benzocarbazoles via a Traceless Directing Group Strategy

A novel cascade π-extended decarboxylative annulation (PEDA) involved with cyclic diaryliodonium salts is described. Via fine-tuning of the reaction conditions, the Pd(II)-catalyzed site-selective N1/C2 or C2/C3 annulation of commercially available indole-2-carboxylic acids can be achieved, affording valuable phenanthridines or benzocarbazoles, respectively. The key strategy is the carboxylic acid functionality being employed as both a traceless directing group for the ortho C-N or C-C coupling and a reactive group for the cascade π-extended decarboxylative annulation in a highly step economical manner.

C-N Coupling via Antiaromatic Endocyclic Nitrenium Ions

Herein, we report a C-N coupling reaction via antiaromatic endocyclic nitrenium ions. The nitrenium ion intermediate was generated by a combination of iodine(III) reagent PhI(OCOCF₃)₂ and the N-H center of benzimidazole units at ambient laboratory conditions. Metal-free synthesis of benzimidazole-fused phenanthridine derivatives was achieved in good to excellent yields.

Palladium-catalyzed tandem one-pot synthesis of π-expanded imidazoles through a sequential Heck and oxidative amination reaction

An efficient palladium-catalyzed route for tandem one-pot synthesis of π-expanded imidazoles from 2-vinyl imidazoles and aryl halides is described.

Synthesis of 1,3-Azaphospholes with Pyrrolo[1,2- a]quinoline Skeleton and Their Optical Applications

A facile synthesis of 1,3-azaphospholes with a pyrrolo[1,2- a]quinoline skeleton has been described. These new annulated 1,3-azaphospholes exhibit good photoelectric performance and can be used as the emitting dopant in organic light-emitting diodes (OLEDs) and dye for bioimaging.

Quinoxaline as an integrated directing group in palladium-catalyzed ortho-C–H bond arylation of the aryl unit of 2-arylquinoxalines

ortho-Directed arylation of the C(sp²)–H bond of 2-arylquinoxalines with a broad range of aryl bromides was achieved using phosphine-free Pd(OAc)₂.

Unveiling the photophysical and morphological properties of an acidochromic thiophene flanked dipyrrolopyrazine-based chromophore for optoelectronic application

A series of dipyrrolopyrazine (DPP) based chromophores featuring thiophene and varied donor (N,N-dimethylamine, NH₂, OMe) and acceptor (CF₃, CN, NO₂) appendages have been synthesized. The structures and properties of the chromophores were investigated by absorption spectroscopy, electrochemistry, differential scanning calorimetry, and thermogravimetric analysis. X-ray crystallographic analysis indicates a planar geometry for the molecule 7g. Surface morphological studies reveal the formation of microrods and nanorods. The acidochromic behavior of the chromophore which shows a prominent red-shift in the absorption spectra owing to the protonation of the pyrazine segment provides a valuable opportunity to assess the sensory response. Acid dependent spectral changes could be successfully applied to detect pH in biological fluids and acid impurities in solvents.

A series of dipyrrolopyrazine (DPP) based chromophores featuring thiophene and varied donor (N,N-dimethylamine, NH₂, OMe) and acceptor (CF₃, CN, NO₂) appendages have been synthesized.

A Chemoselective N-Arylation Reaction of 2-Aminopyridine Derivatives with Arynes

A chemoselective N-arylation reaction of 2-aminopyridine derivatives with arynes in good to excellent yields has been described. The N-arylation products could be further applied to the facile construction of benzoisoquinuclidines and isoquinuclidines as well as pyrido[1,2-a]benzimidazoles.

Regioselective synthesis of dipyrrolopyrazine (DPP) derivatives via metal free and metal catalyzed amination and investigation of their optical and thermal properties

Pyrazine is an important molecular scaffold employed in organic optoelectronic materials.

Copper(ii) catalyzed synthesis of novel helical luminescent benzo[4,5]imidazo[1,2-a][1,10]phenanthrolines via an intramolecular C–H amination reaction

Novel helical luminescent benzoimidazophenanthrolines were prepared using a Cu(ii) catalyzed C–H amination reaction.

Rh(iii)-catalyzed sequential C–H activation and annulation: access to N-fused heterocycles from arylazoles and α-diazocarbonyl compounds

Valuable N-fused heterocycles were successfully obtained via Rh(iii)-catalyzed sequential C–H activation and annulation.

Asymmetric hydroarylation of vinyl ethers catalyzed by a hydroxoiridium complex: azoles as effective directing groups

Asymmetric hydroarylation of vinyl ethers using azoles as directing groups was realized by use of a hydroxoiridium/chiral phosphine catalyst.

Applications of Palladium-Catalyzed C-N Cross-Coupling Reactions

Pd-catalyzed cross-coupling reactions that form C–N bonds have become useful methods to synthesize anilines and aniline derivatives, an important class of compounds throughout chemical research. A key factor in the widespread adoption of these methods has been the continued development of reliable and versatile catalysts that function under operationally simple, user-friendly conditions. This review provides an overview of Pd-catalyzed N-arylation reactions found in both basic and applied chemical research from 2008 to the present. Selected examples of C–N cross-coupling reactions between nine classes of nitrogen-based coupling partners and (pseudo)aryl halides are described for the synthesis of heterocycles, medicinally relevant compounds, natural products, organic materials, and catalysts.

How To Reach Intense Luminescence for Compounds Capable of Excited-State Intramolecular Proton Transfer?

Photoinduced intramolecular direct arylation allows structurally unique compounds containing phenanthro[9',10':4,5]imidazo[1,2-f]phenanthridine and imidazo[1,2-f]phenanthridine skeletons, which mediate excited-state intramolecular proton transfer (ESIPT), to be efficiently synthesized. The developed polycyclic aromatics demonstrate that the combination of five-membered ring structures with a rigid arrangement between a proton donor and a proton acceptor provides a means for attaining large fluorescence quantum yields, exceeding 0.5, even in protic solvents. Steady-state and time-resolved UV/Vis spectroscopy reveals that, upon photoexcitation, the prepared protic heteroaromatics undergo ESIPT, converting them efficiently into their excited-state keto tautomers, which have lifetimes ranging from about 5 to 10 ns. The rigidity of their structures, which suppresses nonradiative decay pathways, is believed to be the underlying reason for the nanosecond lifetimes of these singlet excited states and the observed high fluorescence quantum yields. Hydrogen bonding with protic solvents does not interfere with the excited-state dynamics and, as a result, there is no difference between the occurrences of ESIPT processes in MeOH versus cyclohexane. Acidic media has a more dramatic effect on suppressing ESIPT by protonating the proton acceptor. As a result, in the presence of an acid, a larger proportion of the fluorescence of ESIPT-capable compounds originates from their enol excited states.

Pd(0)-catalyzed domino C-N coupling/hydroamination/C-H arylation reactions: efficient synthesis and photophysical properties of azaindolo[1,2-f]phenanthridines

A series of new 7- and 4-azaindolo[1,2-f]phenanthridines were synthesized by a domino Pd(0)-catalyzed reaction, which involves three sequential steps: C-N coupling, hydroamination, and intramolecular C-H arylation. The products show promising fluorescence properties with high quantum yields (12-65%).