Cycloiridated Helicenes as Chiral Catalysts in the Asymmetric Transfer Hydrogenation of Imines

Double [8]Helicene Featuring a Dibenzo[g,p]chrysene Core: Synthesis and Chiroptical Response

Double [8]helicene 1, featuring a dibenzo[g,p]chrysene core, was synthesized via the Scholl reaction, and its structure was unambiguously confirmed by single-crystal X-ray diffraction analysis of its dicationic salt [1-Cl]2+·(SbCl6-)2. The compound exhibits red fluorescence with an emission maximum at 618 nm (λem) and a quantum yield of 16.2%, highlighting its potential in optoelectronic applications. Furthermore, circular dichroism (CD) and circularly polarized luminescence (CPL) measurements reveal notable chiroptical activity, with absorption and emission dissymmetry factors of |gabs| = 5.11 × 10-3 and |glum| = 7.1 × 10-4, respectively.

Rhenium coordination-induced conformational modulation in nitrogen-doped nanographene

The conformation of nanographene plays a pivotal role in determining its electronic, mechanical, and optical properties. Analyzing the conformation of nanographene is crucial because of its significant implications in various scientific and technological domains. In this work, we examined the changes in the structural conformation of nanographene upon coordination with a metal. We synthesized a series of nitrogen-doped nanographenes (2, 3, and 4) that differed by one C-C bond and their corresponding nanographene-Re complexes (2Re, 3Re, and 4Re). The conformations of the metal complexes were thoroughly analyzed using various spectroscopic techniques. A comparison of the X-ray crystal structures of nanographene before and after rhenium coordination revealed that rhenium coordination induced conformational changes in the nanographene moiety. Additionally, we assessed the catalytic activity of the nanographene-Re complexes toward the electrocatalytic hydrogen evolution reaction (HER). In the presence of a small amount of weak acid (1.5 mM acetic acid), the nanographene-Re complex exhibited hydrogen production at a low overpotential of 133 mV.

Synthesis and Chiroptical Properties of a Saddle-Shaped Quadruple Helicene with a Cyclooctatetrapyrrole Core

A quadruple helicene with a cyclooctatetrapyrrole core was synthesized via a simple Ullmann reaction. Its single-crystal structure exhibits a saddle shape. The compound shows fluorescence emission with λem = 575 nm and good chiroptical properties, including a dissymmetric absorption factor (|gabs|) of 5.84 × 10-3 and a dissymmetric emission factor (|glum|) of 1.21 × 10-2.

Asymmetric gem-Hydroboration and gem-Hydrogenation of Ynamides: A New Gateway to Chiral Fischer Carbene Complexes and their Catalytic Transformations

Ynamides, when reacted with H2 or HBpin in the presence of [Cp*RuCl]4, convert into chiral-at-metal Fischer carbenes by regioselective gem-hydrogenation or gem-hydroboration of the polarized triple bond, respectively. gem-Hydroboration concomitantly affords a carbogenic borylated stereocenter adjacent to the ruthenium carbene unit, the configuration of which can be controlled using an Evans auxiliary. These are the first examples of asymmetric gem-addition reactions to alkynes known in the literature; representative pianostool ruthenium carbene complexes formed by this unconventional route were characterized by crystallographic and spectroscopic means. They undergo various stoichiometric reactions and can even entertain catalytic asymmetric transformations, such as hydrogenative cyclopropanations or a novel type of borylative cycloisomerization.

Natural and Magnetic Circular Dichroism From the Infrared to the UV of a Hetero[4]helicene Radical Cation

Dithiabridged triarylamine hetero[4]helicenes are an interesting class of chiral compounds characterized by a high racemization barrier which makes them an attractive material for several applications. Their rich redox chemistry allows one to obtain radical cations stable at room temperature, that can be isolated in two configurationally stable enantiomers. While several studies have been reported on systems with transition metals possessing low-lying electronic states, the characterization of purely organic open-shell systems by VCD is still scarce. This may be due to the instability of radical-cation species and to the dark color assumed by the solution at the high concentration required by VCD. The oxidation of this molecular system shifts the first electronic transition towards the Near-IR (NIR) region, at approximately 1250 nm. By combining ECD, VCD, MCD, NIR-CD and computational methodologies rooted in density functional theory, we have investigated one molecule of this class of compounds and shed some light on the chiroptical properties accessible via redox-switch.

Cu-UiO-66 Catalyzed Synthesis of Imines via Acceptorless Dehydrogenative Coupling of Alcohols and Amines

Herein, the Cu-UiO-66 catalyst was developed for acceptorless dehydrogenative coupling (ADC) between alcohols and amines to produce imines. The Cu-UiO-66 catalyst was synthesized by installing Cu2+ onto Zr-oxo clusters in UiO-66, and the catalyst efficiently catalyzes the ADC reaction under mild and environmentally friendly conditions with excellent selectivity. Mechanistic studies reveal that the O2⋅- radicals and porosity of formed in Cu-UiO-66 participate cooperatively during the catalytic cycle. Meanwhile, the only by-product of the system is environmentally benign water. Cycling tests and hot filtration tests showed that the Cu-UiO-66 catalyst exhibited excellent stability and catalytic activity during the reaction. Importantly, the Cu-UiO-66 catalyst might provide a promising strategy for the ADC reaction between alcohols and amines to produce imines.

Stereocontrolled Synthesis of Chiral Helicene-Indenido ansa- and Half-Sandwich Metal Complexes and Their Use in Catalysis

Despite recent tremendous progress in the synthesis of nonplanar chiral aromatics, and helicenes in particular, their conversion to half-sandwich or sandwich transition metal complexes still lags behind, although they represent an attractive family of modular and underexplored chiral architectures with a potential catalytic use. In this work, starting from various chiral helicene-indene proligands, we prepared the enantio- and diastereopure oxa[6]- and oxa[7]helicene-indenido half-sandwich RhI and RhIII complexes and oxa[7]helicene-bisindenido ansa-metallocene FeII complex. To document their use, oxahelicene-indenido half-sandwich RhIII complexes were employed as chiral catalysts in enantioselective C-H arylation of benzo[h]quinolines with 1-diazonaphthoquinones to afford a series of axially chiral biaryls in mostly good to high yields and in up to 96 : 4 er. Thus, we developed stereocontrolled synthesis of chiral helicene-indenido ansa- and half-sandwich metal complexes, successfully demonstrated the first use of such helicene Cp-related metal complexes in enantioselective catalysis, and described an unusual sequence of efficient central-to-helical-to-planar-to-axial chirality transfer.

Highly Luminescent Aceno[6]helicenones by Intramolecular Radical Cyclization

Helicenes and helicenoid structures are promising candidates for future applications exploiting circularly polarized light. Ideal candidates should possess near-quantitative photoluminescence quantum yield, a high luminescence dissymmetry factor and an adjustable HOMO-LUMO gap. However, carbo[n]helicenes are poorly luminescent compounds and they absorb light mainly in the ultraviolet region. Here we show that the incorporation of a carbonyl group into helical scaffold significantly improves the fluorescence quantum yield and shifts the absorption to visible region. Although the carbonyl group is commonly considered as detrimental to efficient emission, fluorescence quantum yields up to Φ=0.43 were recorded. A straightforward synthetic approach to a highly luminescent tetraceno[6]helicenone and an aza analogue has been developed. The key step is a radical cyclization which is achieving dehydrative π-extension. The aza-analogue was incorporated as an emitter in organic light emitting diodes (OLEDs) and showed good performance.

Remote Catalytic C(sp3)-H Alkylation via Relayed Carbenoid Transfer upon Olefin Chain Walking

Transition metal carbenes have emerged as versatile intermediates for various types of alkylations. While reactions of metal carbene species with alkenes have been extensively studied, most examples focus on cyclopropanation and allylic C-H insertion. Herein, we present the first example of a catalytic strategy for the carbene-involved regioselective remote C-H alkylation of internal olefins by synergistically combining two iridium-mediated reactivities of olefin chain walking and carbenoid migratory insertion. The present method, utilizing sulfoxonium ylides as a bench-stable robust carbene precursor, was found to be effective for a series of olefins tethered with alkyl chains, heteroatom substituents, and complex biorelevant moieties. Combined experimental and computational studies revealed that reversible iridium hydride-mediated olefin chain walking proceeds to lead to a terminal alkyl-Ir intermediate, which then forms a carbenoid species for the final migratory insertion, resulting in regioselective terminal-alkylated products.

Pd-Catalyzed Dual C-H Activation/Cyclization: Convergent and Divergent Synthesis of 1-Azahelicenes

Herein, we report a convergent synthesis of 1-azahelicenes using easily available quinoline derivatives and cyclic diaryliodonium salts as starting materials. This reaction undergoes a palladium-catalyzed dual C-H activation/cyclization process to give facile access to a wide range of 1-aza[5]helicenes and 1-aza[6]helicenes with abundant functional groups (including F, Cl, Br, I, CF3, SeR, SR, and heteroaryl) in moderate to excellent yields, thereby providing new opportunities to fine-tune the properties of the helicene backbone. In addition, the obtained products could be further transformed into helicene-based Lewis base catalysts and redox switch materials easily. Notably, one selected 1-azahelicene shined bright yellow light by aggregation. These features enlarge the chemical space of 1-azahelicenes and inspire further utilization in other areas of research.

Organocatalytic hydrogen bond donor/Lewis base (HBD/LB) synthesis and chiroptical properties of thiabridged [5]helicenes

Thiabridged [5]helicenes are obtained from thioaryl-N-phthalimido benzo[a]phenothiazines using a hydrogen bond donor/Lewis base organocatalytic approach. Resolution of [5]helicenes using either (1S)-(-)-camphanic acid as a chiral auxiliary or CSP-HPLC is reported. Thiabridged [5]helicenes show an exceptional configurational stability with racemization energy barriers higher than 40 kcal mol-1. Electronic circular dichroism and TD-DFT calculations permit the assignment of the absolute configuration, demonstrating that the sign of optical rotation is not easily related to the M or P structure. Separated enantiomers show circularly polarized luminescence with high dissymmetry ratio.

Enantiopure [6]-Azairidahelicene by Dynamic Kinetic Resolution of a Configurationally Labile [4]-Helicene

A pair of enantiopure [6]-azairidahelicenes incorporating chirality at the metal center and on the helicenic ligand were synthesized by dynamic kinetic resolution (dkr) of a configurationally labile [4]-helicenic ligand (4-(2-pyridyl)-benzo[g]phenanthrene, L1H) using bis-cyclometalated chiral-at-metal only iridium(III) precursors as chiral inductors. The origin of the observed dkr is attributed to the different conformation and stability of diastereomeric reaction intermediates formed during the cyclometalation process. The isolated enantiomers exhibited circularly polarized phosphorescence (CPP), with |gphos| values of 1.8×10-3.

Crystallization-Induced Chirality Transfer in Conformationally Flexible Azahelicene Au(I) Complexes with Circularly Polarized Luminescence Activation

Flexible and twisted annulated π-systems exhibit numerous unique and desirable features, owing to their ability to display chirality. However, preventing their racemization due to the dynamic nature of their chirality remains a challenge. One promising approach to stabilize homochirality in such systems is chirality transfer from a chiral auxiliary to a moiety displaying dynamic chirality. Herein, we introduce a new approach for dynamic chirality stabilization in conformationally flexible azahelicene species via crystallization-induced intermolecular chirality transfer in Au(I) complexes featuring azahelicene (dibenzo[c,g]carbazole and benzo[c]carbazole) and enantio-pure chiral N-heterocyclic carbene (NHC) ligands with a complementary tailored shape. Crystallization of these azahelicene Au(I) complexes not only suppresses the dynamic chirality of the dibenzocarbazole species but also stabilizes their homochirality through the intermolecular conjunction between the chiral NHC and dibenzocarbazole ligands. In the Au(I) benzocarbazole complexes, the intermolecular conjunction and chirality transfer in the crystals induce chirality in the initially achiral benzocarbazole ligand. Furthermore, the crystallization of the studied complexes activates their circularly polarized luminescence (CPL) properties, which were suppressed in solution. Importantly, chirality transfer leads to significant CPL enhancement; the complexes that feature chirality transfer within the crystal structure exhibit luminescence dissymmetry factors 5 to 10 times higher than those of the complexes without chirality transfer.

Helicoselective Synthesis of Indolohelicenoids through Organocatalytic Central-to-Helical Chirality Conversion

We report the helicoselective and convergent construction of indolohelicenoids with excellent efficiency and stereocontrol. This reaction proceeds through a chiral-phosphoric-acid-catalyzed enantioselective cycloaddition and eliminative aromatization sequence, which can be finely controlled by adjusting the reaction temperature. Mechanistic studies reveal that the chiral phosphoric acid cooperatively serves as both a bifunctional and Brønsted acid catalyst, enabling one-pot central-to-helical chirality conversion. Additionally, the optical properties of the synthesized indolohelicenoids were characterized to explore their potential applications in organic photoelectric materials.

Controlling helicene's pitch by molecular tethering

We applied post-cyclization annulation to introduce a series of tethered S-shaped double [4]helicenes in which the intramolecular tether imposes a specific helical handedness. Introducing a tether and then shortening the tether length incrementally increase the pitch angle of [4]helicene, thus enabling a quantitative study of the effects of helicene's pitch on its electronic and (chiro)optical properties.

Straightforward Synthesis of Pentagon-Embedded Expanded [11]Helicenes for Radiative Cooling Property

Two new pentagon-embedded carbo[11]helicenes have been designed and synthesized in a three-step process, which are the first example of carbo[11]helicenes through the post-functionalization of twistacene. TD-DFT analyses indicate that both of them possess high enantiomerization barriers of 42.29 kcal/mol and 40.76 kcal/mol, respectively. They emit strong red fluorescence and can be chemically oxidized into stable cationic radicals upon addition of AgSbF6 evidenced by the bathochromic-shifted absorption spectra and the appearance of electronic paramagnetic resonance (EPR) signals. In addition, such helical derivatives can be chosen as radiative cooling materials in a glass model house, and the maxima of 5.4 °C for the former and 6.5 °C for the latter are found in the comparative tests, which might be caused by the NIR reflective response.

Iridium-Catalyzed Migratory Terminal C(sp3)-H Amidation of Heteroatom-Substituted Internal Alkenes via Olefin Chain Walking

Hydroamination facilitated by metal hydride catalysis is an appealing synthetic approach to access valuable nitrogen-containing compounds from readily available unsaturated hydrocarbons. While high regioselectivity can be achieved usually for substrates bearing polar chelation groups, the reaction involving simple alkenes frequently provides nonselective outcomes. Herein, we report an iridium-catalyzed highly regioselective terminal C(sp3)-H amidation of internal alkenes utilizing dioxazolones as an amino source via olefin chain walking. Most notably, this mechanistic motif of double bond migration to the terminal position operates not only with dialkyl-substituted simple alkenes including styrenes but also with heteroatom-substituted olefins such as enol ethers, vinyl silanes, and vinyl borons, thus representing the first example of the terminal methyl amidation of the latter type of alkenes through a nondissociative chain walking process.

Heterocyclic [9]Helicenes Exhibiting Bright Circularly Polarized Luminescence

We describe a concise synthetic strategy for the preparation of heterocyclic [9]helicenes and a simple preparative-scale protocol for the optical resolution of the resulting M- and P-enantiomers. The helicenes were characterized by single-crystal X-ray diffraction along with a range of spectroscopic and computational techniques. A fluorescence quantum yield of up to 65 % was observed, and the chiroptical properties of both M- and P-helicenes revealed large dissymmetry factors. The circularly polarized luminescence brightness reaches up to 17 M-1  cm-1 , as measured experimentally and verified computationally, which makes this the highest circularly polarized luminescence brightness among heterocyclic helicenes. We describe how chiroptical properties (both circular dichroism and circularly polarized luminescence) can be described and predicted using quantum chemical calculations. The synthetic approach also reveals by-products that originate from internal oxidation reactions, presumably mediated by the close proximity of the π-surfaces in the helicene structure.

Recent advances of Cp*Ir complexes for transfer hydrogenation: focus on formic acid/formate as hydrogen donors

Transfer hydrogenation reactions offer synthetically powerful strategies to deliver various hydrogenated compounds with the advantages of efficiency, atom economy, and practicability. On one hand, formic acid/formate function as promising hydrogen sources owing to their readily obtainable, inexpensive, and easy to handle nature. On the other hand, Cp*Ir complexes show high activities in transfer hydrogenation. This review highlights progress achieved for transfer hydrogenation of CO, CC, and CN bonds of a variety of unsaturated substrates, as well as amides focusing on Cp*Ir complexes as catalysts and formic acid/formate as hydrogen sources.

Enantioselective synthesis of chiral quinohelicenes through sequential organocatalyzed Povarov reaction and oxidative aromatization

Heterohelicenes are of increasing importance in the fields of materials science, molecular recognition, and asymmetric catalysis. However, enantioselective construction of these molecules, especially by organocatalytic methods, is challenging, and few methods are available. In this study, we synthesize enantioenriched 1-(3-indol)-quino[n]helicenes through chiral phosphoric acid-catalyzed Povarov reaction followed by oxidative aromatization. The method has a broad substrate scope and offers rapid access to an array of chiral quinohelicenes with enantioselectivities up to 99%. Additionally, the photochemical and electrochemical properties of selected quinohelicenes are explored.