Synthesis and structural properties of para-diselenopyrazines

Recently, dithienopyrazines have emerged as promising building blocks in the field of materials science, showcasing their potential as hole-transport materials in organic electronic devices. Herein, we report the synthesis of its heavier analogues, the diselenopyrazines, along with an analysis of their optoelectronic and structural properties. In the acquired crystal structures, interesting molecular packing motifs suitable for potential device fabrication were observed. Additionally, short contact interactions are present in one of the investigated molecules.

Gold(I)-Catalyzed Intramolecular 7-endo-dig Cyclization of Triene-Yne Systems: New Access towards Azulenothiophenes

We report the direct synthesis of new azulene derivatives through gold-catalyzed cyclization reactions. A five-membered ring as backbone in the applied triene-yne substrates turned out to be crucial to induce the 7-endo-dig cyclization mode necessary to trigger azulene formation. The obtained targets are of high interest due to their potential applications in different fields, like organic materials, medicine or cosmetics. UV/Vis spectra and cyclic voltammetry were measured, based on these the electronic properties were determined. Short two or three step sequences towards the applied starting materials make this approach synthetically highly attractive.

Ratiometric fluorescent sensing of pyrophosphate with sp³-functionalized single-walled carbon nanotubes

Inorganic pyrophosphate is a key molecule in many biological processes from DNA synthesis to cell metabolism. Here we introduce sp3-functionalized (6,5) single-walled carbon nanotubes (SWNTs) with red-shifted defect emission as near-infrared luminescent probes for the optical detection and quantification of inorganic pyrophosphate. The sensing scheme is based on the immobilization of Cu2+ ions on the SWNT surface promoted by coordination to covalently attached aryl alkyne groups and a triazole complex. The presence of Cu2+ ions on the SWNT surface causes fluorescence quenching via photoinduced electron transfer, which is reversed by copper-complexing analytes such as pyrophosphate. The differences in the fluorescence response of sp3-defect to pristine nanotube emission enables reproducible ratiometric measurements in a wide concentration window. Biocompatible, phospholipid-polyethylene glycol-coated SWNTs with such sp3 defects are employed for the detection of pyrophosphate in cell lysate and for monitoring the progress of DNA synthesis in a polymerase chain reaction. This robust ratiometric and near-infrared luminescent probe for pyrophosphate may serve as a starting point for the rational design of nanotube-based biosensors.

Mn(III) O^N^O Complexes as Water-tolerant and Environmentally Benign Catalysts for Polyurethane Foam Synthesis

Polyurethanes are synthesized on industrial scale by the reaction of diisocyanates with diols in the presence of catalysts which are commonly based on tin complexes and amines. However, due to the toxicity and volatility of these tin catalysts and amines, there is the need to develop new catalysts that are more environmentally benign. Herein, we report the synthesis of O^N^O pincer-ligated Mn(III) and Fe(III) complexes that serve as suitable catalysts for urethane formation and are stable to hydrolysis as predicted by computations and observed experimentally. The O^N^O pincer scaffold is vital to the activity of these catalysts, simultaneously ensuring increased solubility in the reaction medium as well as providing a stable framework upon dissociation of co-ligands in the catalytic cycle. In silico mechanistic investigations for urethane formation show that the stabilization of active species in square-planar geometries enabled by these O^N^O ligands permit the simultaneous coordination of alcohol and isocyanate in suitable configuration at the metal center.

Cascade Reactions of Aryl-Substituted Terminal Alkynes Involving in Situ-Generated α-Imino Gold Carbenes

An efficient, highly selective and divergent synthetic method to construct 2-substituted indoles and aryl-annulated carbazoles via the intermolecular generation of α-imino gold carbenes from terminal alkynes or diynes in combination with sulfilimines is disclosed. Importantly, the tandem reaction is proposed to proceed through an intermolecular gold carbene generation/C-H annulation followed by the activation of a second alkyne leading to 6-endo-dig cyclization, which is significantly different from previous dual activation or 1,6-carbene shift approaches for diyne systems. In the case of ortho-alkynylaniline as starting material, an unexpected regioselective formation of the indole moiety via the intermolecular path, instead of intramolecular hydroamination was discovered. This reactivity paved the way for a one-pot synthesis of the 11H-indolo [3,2-c] quinoline scaffold by exploiting the formed amino indole for a subsequent Pictet-Spengler reaction with aldehydes. The photophysical properties of the carbazoles indicated good violet-blue emission with quantum yields up to 40 %.

Light-Induced Metal-Free Generation of Cyanocarbenes from Alkynyl Triazenes for the Synthesis of Nitrile Derivatives

The chemistry of alkynyl triazenes is an emerging field for organic chemists and especially acid-induced nucleophilic functionalizations, either directly, or after a prior reaction towards aromatic triazenes under extrusion of nitrogen, paved the way for fruitful strategies. In contrast, the chemical behavior of alkynyl triazenes upon irradiation with light is still unknown. Herein we present the first photoactivation of alkynyl triazenes that triggers an uncommon reactivity pattern involving the cleavage of the N1-N2 bond of the triazene moiety resulting in a unique approach to cyanocarbenes from a readily available, stable, and insensitive precursor. This allows to access various nitrile compounds without the use of a toxic cyanating agent by exploiting the reactivity pattern of carbenes. By variation of the reaction conditions and light sources, different substitution patterns can be obtained selectively in good yields under mild and metal-free conditions, thus introducing the alkynyl triazene unit as a photo accessible methylene nitrile synthon. Using this synthon, subclasses like α-alkoxynitriles, α-aminonitriles and α-cyanohydrazones become easily available. These exhibit synthetically valuable substitution patterns for the synthesis of pharmaceuticals, intermediates for total synthesis and amino acid synthesis.

Gold(I)-Mediated Cyclizations of Triene-Yne Systems: An Easy Access to Cyclopenta-Fused Anthracenes

The direct synthesis of cyclopenta-fused anthracenes (CP-anthracenes) through a gold(I)-catalyzed cyclization reaction of easily available triene-yne systems bearing a benzofulvene substructure, is reported. The targets are of great interest due to the potential use as organic materials and the preparation of these compounds is gaining huge importance. The applied starting materials are readily available through a three-step synthesis, which further contributes to the advantages of this route. In addition, UV-Vis and fluorescent spectra of the obtained CP-anthracenes were recorded.

Gold-Catalyzed Formal [4+2] Cycloaddition as Access to Antitumor-Active Spirocyclic Oxindoles from Alkynes and Isatin-Derived Ketimines

Due to its excellent bioactivity profile, which is increasingly utilized in pharmaceutical and synthetic chemistry, spirooxindole is an important core scaffold. We herein describe an efficient method for the construction of highly functionalized new spirooxindolocarbamates via a gold-catalyzed cycloaddition reaction of terminal alkynes or ynamides with isatin-derived ketimines. This protocol has a good functional group compatibility, uses readily available starting materials, mild reaction conditions, low catalyst loadings and no additives. It enables the transformation of various functionalized alkyne groups into cyclic carbamates. Gram-scale synthesis was achieved and DFT calculations verify the feasibility of the mechanistic proposal. Some of the target products exhibit good to excellent antiproliferative activity on human tumor cell lines. In addition, one of the most active compounds displayed a remarkable selectivity towards tumor cells over normal ones.

Gold-Catalyzed [3,3]-Sigmatropic Rearrangement of ortho-Alkynyl-S,S-diarylsulfilimines

Highly functionalized 5H-pyrrolo[2,3-b]pyrazine cores, carrying a diaryl sulfide moiety at the C-7 position, were obtained from a gold-catalyzed reaction using easily accessible ortho-alkynyl-substituted S,S-diarylsulfilimines as intramolecular nitrene transfer reagents for the first time. The reaction proceeds under mild conditions, providing excellent yields while tolerating a large variety of different substitution patterns. We provide experimental evidence for an intramolecular reaction mechanism, likely including an unprecedented gold-catalyzed amino sulfonium [3,3]-sigmatropic rearrangement.

Gold-Catalyzed Regio- and Stereoselective α-Acyloxy-β-Alkynylation of Ynol Ethers

Enol esters and conjugated enynes are valuable structural motifs for synthetic chemistry and material sciences. Herein, the synthesis of tetra-substituted enol ester 2-iodobenzoate derivatives was achieved in good yields at room temperature through a gold-catalyzed acyloxyalkynylation of sensitive ynol ethers with ethynylbenziodoxolones (EBXs), the latter acting as bifunctional reactants. The conversion is highly regioselective with a broad substrate scope. Mechanistically, an Au(III) species is the key intermediate of an Au(I)/Au(III) redox cycle. The reaction is synthetically useful and can easily be scaled up to gram scale.

An unexpected synthesis of azepinone derivatives through a metal-free photochemical cascade reaction

Azepinone derivatives are privileged in organic synthesis and pharmaceuticals. Synthetic approaches to these frameworks are limited to complex substrates, strong bases, high power UV light or noble metal catalysis. We herein report a mild synthesis of azepinone derivatives by a photochemical generation of 2-aryloxyaryl nitrene, [2 + 1] annulation, ring expansion/water addition cascade reaction without using any metal catalyst. Among the different nitrene precursors tested, 2-aryloxyaryl azides performed best under blue light irradiation and Brønsted acid catalysis. The reaction scope is broad and the obtained products underwent divergent transformations to afford other related compounds. A computational study suggests a pathway involving a step-wise aziridine formation, followed by a ring-expansion to the seven-membered heterocycle. Finally, water is added in a regio-selective manner, this is accelerated by the added TsOH.

Versatile access to nitrogen-rich π-extended indolocarbazoles via a Pictet–Spengler approach

A bidirectional Pictet-Spengler Reaction allows easy access to nitrogen-rich aromatics with seven fused rings. Photophysical measurements and computational methods show significant differences to parent N-heteropolycycles with fewer nitrogen atoms.

Substituted Cyclopentannulated Tetraazapentacenes

Brominated pentannulated dihydrotetraazapentacenes were prepared by gold- or palladium-catalyzed 5-endo-dig cyclization of TIPS-ethynylated dihydrotetraazaacenes (TIPS = triisopropylsilyl). Post-functionalization was demonstrated by Sonogashira alkynylation and Rosenmund-von Braun cyanation. Calculations predict these species to act as n-type semiconductors, which was verified for two derivates through characterization in organic field-effect transistors.

Entropy-Induced Selectivity Switch in Gold Catalysis: Fast Access to Indolo[1,2-a]quinolines

New N-heterocyclic compounds for organic functional materials and their efficient syntheses are highly demanded. A surprising entropy-induced selectivity switch in the gold-catalyzed intramolecular hydroarylation of 2-ethynyl N-aryl indoles was found and its exploitation led to straightforward syntheses of indolo[1,2-a]quinolines. Experimental and computational mechanistic investigations gave insight into this uncommon selectivity phenomenon and into the special reactivity of the indolo[1,2-a]quinolines. The high functional group tolerance of this methodology enabled access to a diverse scope with high yields. In addition, bidirectional approaches, post-functionalization reactions, and π-extension of the core structure were feasible. An in-depth study of the photophysical properties explored the structure-effect relationship for different derivatives and revealed a high potential of these compounds for future applications as functional materials.

Synthesis of Amide Enol 2-Iodobenzoates by the Regio- and Stereoselective Gold-Catalyzed Acyloxyalkynylation of Ynamides with Hypervalent Iodine Reagents

Multisubstituted alkenes are accessible by a gold-catalyzed acyloxyalkynylation of ynamides with ethynylbenziodoxolones (EBXs) with perfect atom-economy. The EBX reagents transfer both the carboxylate as well as the alkynyl entity. Overall, this cascade comprises the in situ generation of an alkynyl gold(III) species, a stereoselective C(sp)-C(sp²) bond formation, and a C-O coupling at the alkynyl position of the ynamides. This reaction proceeds under mild conditions and accepts a wide range of substrates. A number of tetrasubstituted amide enol 2-iodobenzoates bearing different functional groups were obtained in good to excellent yields. DFT calculations explain the observed regioselectivity. The synthetic potential of the reaction was further demonstrated by a number of selected follow-up transformations.

Synthesis of α-Ketoamides via Gold(I) Carbene Intermediates

α-Ketoamides have been found to be an important functional group in a broad spectrum of inhibitors such as the Corona virus and other viruses. Here we report an unprecedented gold-catalyzed 2-fold reaction of a bromoalkyne with anthranils. Hydrolysis of the initial product then directly leads to α-ketoamides. Water addition to the intermediate α-iminoimidoyl halides delivered α-ketoamides from a broad range of bromoalkynes.

Practical and modular construction of benzo[c]phenanthridine compounds

Here, we describe a general and modular strategy for the rapid assembly of benzo[c]phenanthridine (BCP) derivatives using homogeneous gold catalysis. Notably, in contrast to traditional methods based on the specially preformed substrates that have an inherent preference for the formation of this class of compounds with limited flexibility, this protocol is achieved via a selectively intramolecular cascade of a diazo-tethered alkyne and subsequently an intermolecular cyclization with a nitrile to facilitate the successive C-N and C-C bonds formation. This methodology uses readily available nitriles as the nitrogen source to deliver the products in good yield with excellent functional group compatibility. A preliminary anti-tumor activity study of these generated products exhibits high anticancer potency against five tumor cell lines, including HeLa, Mel624, SW-480, 8505C, LAN-1. Besides, we report a catalyst-controlled intermolecular cycloaddition/intramolecular insertion of the substrate with a fulvene to provide fused polycarbocycles containing a seven-membered ring.

Efficient Synthesis of Dipyrrolobenzenes and Dipyrrolopyrazines via Bidirectional Gold Catalysis: a Combined Synthetic and Photophysical Study

New N-heterocyclic fluorophores are sought-after compounds for organic electronic devices. Here, we report on a straightforward synthesis to access meta/para-dipyrrolobenzenes and para-dipyrrolopyrazines in high yields using a bidirectional gold-catalyzed cyclization strategy. The versatility of our reaction protocol was showcased by preparing dipyrroloarenes with different substituents, various functional groups, and in a multitude of substitution patterns. Furthermore, we showed that the dipyrroloarenes can be post-modified by N-alkylation to improve the solubility or bromination to yield precursors for further derivatization via cross-coupling. Investigation of the photophysical properties of the─mostly unprecedented─dipyrroloarenes identified strong blue emitters such as the diphenyl meta-dipyrrolobenzene with a quantum yield of 98%. Moreover, we showed that changes in the solvent polarity or interactions with Lewis acids such as borane can be used to fine-tune the photophysical properties of the fluorophores.

Dichotomy of platinum(II) and gold(III) carbene intermediates switching from N- to O-selectivity

Pt(II) and Au(III)-mediated intermolecular divergent annulations of benzofurazans and ynamides highlighted the N- to O-selectivity of tunable metal carbene intermediates. PtCl₂ with a bulky phosphite ligand resulted in the specific synthesis of six-membered quinoxaline N-oxides and successfully suppressed the in-situ deoxygenation of N-oxides. On the other hand, an unique gold(III) catalyst (2,6-di-MeO-PyrAuCl₃) led to the five-membered ring products, benzimidazoles. A broad scope of functional groups was well compatible, delivering better yields and selectivities in contrast to conventional gold(I) catalysts. The different behavior of presumed platinum(II) and gold(III) carbenes with respect to chemoselectivity was intensively examined by experiments and DFT calculations. A detailed mechanistic study, based on DFT calculations, revealed that the highly electrophilic carbocation-like gold(III) carbene triggers an oxophilic cyclization, followed by a cascade ring contraction and acyl migration. On the contrary, the Pt carbene species is less cationic, favoring the formation of the six-membered ring via N-attack.

Benzofurazan, a cyclic heterocycle, can form open-chain metal carbene species in the presence of suitable catalysts. Here the authors show divergent reactivity when using gold(III) and platinum(II) catalysts, and perform computational and experimental mechanistic studies to explain the differing reactivity

Gem-Diaurated Gold(III) Complexes: Synthesis, Structure, Aurophilic Interaction, and Catalytic Activity

We present a protocol to synthesize air stable gem-diaurated gold(III) compounds from 1,3-diketones in a single cycloauration step with tetrachloroauric acid. So far related species were only accessible from phosphonium bis(ylide) ligands which hold the two gold atoms in close proximity. Lacking such a constraint, our compounds show the longest Au-Au distances of all gem-diaurated carbons, ranging from 3.26 to 3.32 Å. Modeling based on results of CCSD(T) calculations shows no stabilization by aurophilic interactions for our gold(III) systems, compared to 9.1 kcal/mol for gold(I) gem-diauration. This demonstrates no aurophilic interactions are needed for the isolation of air stable gem-diaurated gold(III) complexes. We show the new gem-diaurated gold(III) compounds are active in the gold-catalyzed phenol synthesis and highly active in the cycloisomerization of an N-propargylcarboxamide; here, we obtained the so far highest known TON of over 2500 per gold atom with respect to the oxazole formation.

Hydrogenative Depolymerization of Polyurethanes Catalyzed by a Manganese Pincer Complex

Chemical recycling, in particular hydrogenative depolymerization, offers a promising way to utilize plastic waste. This report covers the manganese-catalyzed hydrogenation of polyurethane materials to the corresponding monomeric units. The key to success is a Mn pincer complex as a potent hydrogenation catalyst in combination with elevated temperatures (up to 200 °C) and appropriate solvents to ensure sufficient solubility of the polymers. A wide range of polyurethane samples of varying polyol and isocyanate compositions, some of which feature significant amounts of urea functionalities, are depolymerized, releasing polyetherols and diaminotoluene (TDA) in yields of up to 89 % and 76 %, respectively.

Gold-catalysed synthesis of phosphonate-substituted oxetan-3-ones – an easy access to highly strained HWE reagents

Gold-catalysed reaction of alkynyl phosphonates to attain alkenyl substituted oxetan-3-ones via subsequent HWE reaction.

Dual Gold/Silver Catalysis: Indolizines from 2-Substituted Pyridine Derivatives via a Tandem C(sp3)-H Alkynylation/Iminoauration

A dual gold/silver-catalyzed cascade C(sp³)-H alkynylation/iminoauration of 2-substituted pyridines with hypervalent iodine(III) reagents for the synthesis of indolizines is described. This novel reaction involves the formation of an alkynyl Au(III) species, a dual gold/silver-catalyzed C(sp³)-H functionalization, and a subsequent iminoauration process. A number of indolizines bearing diverse functionalities were prepared in good to excellent yield. Furthermore, a gram-scale reaction was efficiently conducted.

Hydroxylamine-mediated C-C amination via an aza-hock rearrangement

Despite the widespread use of anilines, synthetic challenges to these targets still exist. Selectivity is often an issue, when using the traditional nitration-reduction sequence or more modern approaches, including arene C-H aminations catalyzed by transition metals, photosensitizers, or electrodes. Accordingly, there is still a need for general methods to rapidly, directly access specific isomers of substituted anilines. Here, we report a simple route towards the synthesis of such motifs starting from benzyl alcohols, which are converted to anilines by the use of arylsulfonyl hydroxylamines, via an aza-Hock rearrangement. Good to excellent yields are observed. The method is applicable to various benzyl alcohol surrogates (such as ethers, esters, and halides) as well as simple alkylarenes. Functionalizations of pharmaceutically relevant structures are feasible under the reaction conditions. Over ten amination reagents can be used, which facilitates the rapid assembly of a vast set of compounds.

Modular Two-Step Access to π-Extended Naphthyridine Systems-Potent Building Blocks for Organic Electronics

Efficient synthetic approaches for the incorporation of nitrogen into polyaromatic compounds (PACs) in different patterns as stabilising moiety for π‐extended systems and modification tool for optoelectronic properties remain a challenge until today. Herein, we developed a new versatile pathway to napthyridine‐based PACs as non‐symmetric and regioisomeric pendant to pyrazine‐based PACs. A combination of a gold‐catalysed synthesis of 2‐aminoquinolines and the development of an in situ desulfonation and condensation of these precursors are the key steps of the protocol. The shape and type of attached functional groups of the PACs can be designed in a late stage of the overall synthetic procedure by the chosen anthranile and backbone of the ynamide introduced in the gold‐catalysed step. Single‐crystal X‐ray diffraction and the investigation of electronic properties of the compounds show the influence of the attached substituents. All naphthyridine‐based PACs show halochromic behaviour implying their use as highly sensitive proton sensor in non‐protic solvents.

Switchable Divergent Synthesis in Gold-Catalyzed Difunctionalizations of o-Alkynylbenzenesulfonamides with Aryldiazonium Salts

Gold-catalyzed difunctionalizations of o-alkynylbenzenesulfonamides with aryldiazonium salts are reported herein. Upon irradiation with the blue LEDs, benzosultam products were formed via aminoarylation accompanied by the release of N₂. Without irradiation, aryldiazonium salts were engaged as efficient electrophiles, facilitating electrophilic deaurations of the vinyl-Au(I) intermediates, followed by tautomerization to give the N-aryl-substituted α-imino (E)-hydrazones. The regioselectivities of 6-endo-dig and 5-exo-dig cyclizations were excellent.

Depolymerization of Technical-Grade Polyamide 66 and Polyurethane Materials through Hydrogenation

Chemical recycling provides a promising solution to utilize plastic waste. Here, a catalytic hydrogenative depolymerization of polyamide 66 (PA 66) and polyurethane (PU) was developed. The system employed Ru pincer complexes at high temperature (200 °C) in THF solution, and even technical-grade polymers could be hydrogenated with satisfactory yields under these conditions. A comparison of the system with some known heterogeneous catalysts as well as catalyst poisoning tests supported the homogeneity of the system. These results demonstrate the potential of chemical recycling to regain building blocks for polymers and will be interesting for the further development of polymer hydrogenation.

Phosphine-Catalyzed Vinylation at Low Acetylene Pressure

The vinylation of various nucleophiles with acetylene at a maximum pressure of 1.5 bar is achieved by organocatalysis with easily accessible phosphines like tri-n-butylphosphine. A detailed mechanistic investigation by quantum-chemical and experimental methods supports a nucleophilic activation of acetylene by the phosphine catalyst. At 140 °C and typically 5 mol % catalyst loading, cyclic amides, oxazolidinones, ureas, unsaturated cyclic amines, and alcohols were successfully vinylated. Furthermore, the in situ generation of a vinyl phosphonium species can also be utilized in Wittig-type functionalization of aldehydes.

Access to Indole-Fused Benzannulated Medium-Sized Rings through a Gold(I)-Catalyzed Cascade Cyclization of Azido-Alkynes

Because benzannulated and indole-fused medium-sized rings are found in many bioactive compounds, combining these fragments might lead to unexplored areas of biologically relevant and uncovered chemical space. Herein is shown that α-imino gold carbene chemistry can play an important role in solving the difficulty in the formation of medium-sized rings. Namely, phenylene-tethered azido-alkynes undergo arylative cyclization through the formation of a gold carbene intermediate to afford benzannulated indole-fused medium-sized tetracycles. The reactions allow a range of different aryl substitution patterns and efficient access to these otherwise difficult-to-obtain medium-sized rings. This study also demonstrates the feasibility of the semihollow-shaped C-dtbm ligand for the construction of a nine-membered ring.

Access to Indole-Fused Benzannulated Medium-Sized Rings through a Gold(I)-Catalyzed Cascade Cyclization of Azido-Alkynes

Invited for the cover of this issue is Hiroaki Ohno and co-workers at Kyoto University, Hokkaido University, and Heidelberg University. The image depicts a golden compass that guides the adventurer's way in an unknown chemical space. Read the full text of the article at 10.1002/chem.202101824.