Organic Chemicals from Wood: Selective Depolymerization and Dearomatization of Lignin via Aqueous Electrocatalysis

Replacing crude oil as the primary industrial source of carbon-based chemicals has become crucial for both environmental and resource sustainability reasons. In this scenario, wood arises as an excellent candidate, whilst depolymerization approaches have emerged as promising strategies to unlock the lignin potential as a resource in the production of high-value organic chemicals. However, many drawbacks, such as toxic solvents, expensive catalysts, high energy inputs, and poor product selectivity have represented major challenges to this task. Herein, we present an unprecedented approach using electrocatalysis for the simultaneous depolymerization and dearomatization of lignin in aqueous medium under ambient conditions. By employing water/sodium carbonate as a solvent system, we demonstrated a pathway for selectively depolymerizing lignin under reductive electrochemical conditions using carbon as an electrocatalyst. After reductive electrocatalysis, the presence of aromatic compounds was no longer detected via nuclear magnetic resonance (NMR) spectroscopy. Further characterization by NMR, FTIR spectroscopy, and mass spectrometry revealed the major presences of sodium levulinate, sodium 4-hydroxyvalerate, sodium formate, and sodium acetate as products. By achieving a complete dearomatization, valuable aliphatic intermediates with enhanced reactivity were selectively obtained, opening new avenues for further synthesis of many different organic chemicals, and contributing to a more sustainable and circular economy.

Interrupted SNAr-Alkylation Dearomatization

Dearomatizations provide powerful synthetic routes to rapidly assemble substituted carbocycles and heterocycles found in a plethora of bioactive molecules. Harnessing the advantages of dearomatization typically requires vigorous reagents because of the difficulty in disrupting the stable aromatic core. A relatively mild dearomatization strategy is described that employs lithiated nitriles or isocyanides in a simple SNAr-type addition to form σ-complexes that are trapped by alkylation. The dearomatizations are diastereoselective and efficient and rapidly install two new carbon-carbon bonds, one of which is a quaternary center, as well as nitrile, isocyanide, and cyclohexadiene functionalities.

Silver-Catalyzed Dearomative Skeletal Editing of Indazoles by Donor Carbene Insertion

Given the prevalence of heterocyclic scaffolds in drug-related molecules, converting these highly modular heterocyclic scaffolds into structural diversified and dearomatized analogs is an ideal strategy for improving their physicochemical and pharmacokinetic properties. Here, we described an efficient method for silver carbene-mediated dearomative N-N bond cleavage leading to skeletal hopping between indazole and 1,2-dihydroquinazoline via a highly selective single-carbon insertion procedure. Using this methodology, a series of dihydroquinazoline analogues with diarylmethylene-substituted quaternary carbon centers were constructed with excellent yields and good functional group compatibility, which was further illustrated by the late-stage diversification of important pharmaceutically active ingredients. DFT calculations indicated that the silver catalyst not only induces the formation of the silver carbene, but also activates the diazahexatriene intermediate, which plays a crucial role in the formation of the C-N bond.

Krumholzibacteriota and Deltaproteobacteria contain rare genetic potential to liberate carbon from monoaromatic compounds in subsurface coal seams

Biogenic methane in subsurface coal seam environments is produced by diverse consortia of microbes. Although this methane is useful for global energy security, it remains unclear which microbes can liberate carbon from the coal. Most of this carbon is relatively resistant to biodegradation, as it is contained within aromatic rings. Thus, to explore for coal-degrading taxa in the subsurface, this study reconstructed relevant metagenome-assembled genomes (MAGs) from coal seams by using a key genomic marker for the anaerobic degradation of monoaromatic compounds as a guide: the benzoyl-CoA reductase gene (bcrABCD). Three MAGs were identified with this genetic potential. The first represented a novel taxon from the Krumholzibacteriota phylum, which this study is the first to describe. This Krumholzibacteriota MAG contained a full set of genes for benzoyl-CoA dearomatization, in addition to other genes for anaerobic catabolism of monoaromatics. Analysis of Krumholzibacteriota MAGs from other environments revealed that this genetic potential may be common, and thus, Krumholzibacteriota may be important organisms for the liberation of recalcitrant carbon in a broad range of environments. Moreover, the assembly and characterization of two Syntrophorhabdus aromaticivorans MAGs from different continents and a Syntrophaceae sp. MAG implicate the Deltaproteobacteria class in coal seam monoaromatic degradation. Each of these taxa are potential rate-limiting organisms for subsurface coal-to-methane biodegradation. Their description here provides some understanding of their function within the coal seam microbiome and will help inform future efforts in coal bed methane stimulation, anoxic bioremediation of organic pollutants, and assessments of anoxic, subsurface carbon cycling and emissions.IMPORTANCESubsurface coal seams are highly anoxic, oligotrophic environments, where the main source of carbon is "locked away" within aromatic rings. Despite these challenges, many coal seams accumulate biogenic methane, implying that the coal seam microbiome is "unlocking" this carbon source in situ. For over two decades, researchers have endeavored to understand which organisms perform these processes. This study provides the first descriptions of organisms with this genetic potential from the coal seam environment. Here, we report metagenomic insights into carbon liberation from aromatic molecules and the degradation pathways involved and describe a Krumholzibacteriota, two Syntrophorhabdus aromaticivorans, and a Syntrophaceae MAG that contain this genetic potential. This is also the first time that the Krumholzibacteriota phylum has been implicated in anaerobic dearomatization of aromatic hydrocarbons. This potential is identified here in numerous MAGs from other terrestrial and marine subsurface habitats, implicating the Krumholzibacteriota in carbon-cycling processes across a broad range of environments.

Synthesis of Benzofuro[3,2-b]indol-3-one Derivatives via Dearomative (3 + 2) Cycloaddition of 2-Nitrobenzofurans and para-Quinamines

An efficient dearomative (3 + 2) cycloaddition of para-quinamines and 2-nitrobenzofurans has been developed. This reaction proceeds smoothly under mild conditions and affords a series of benzofuro[3,2-b]indol-3-one derivatives in good to excellent yields (up to 98%) with perfect diastereoselectivities (all cases > 20:1 dr). The scale-up synthesis and versatile derivatizations demonstrate the potential synthetic application of the protocol. A plausible reaction mechanism is also proposed to account for the observed reaction process. This work represents the first instance of the N-triggered dearomative (3 + 2) cycloaddition of 2-nitrobenzofurans.

Dearomatization of 3-Aminophenols for Synthesis of Spiro[chromane-3,1'-cyclohexane]-2',4'-dien-6'-ones via Hydride Transfer Strategy-Enabled [5+1] Annulations

The Sc(OTf)3-catalyzed dearomative [5+1] annulations between readily available 3-aminophenols and O-alkyl ortho-oxybenzaldehydes were developed for synthesis of spiro[chromane-3,1'-cyclohexane]-2',4'-dien-6'-ones. The "two-birds-with-one-stone" strategy was disclosed by the dearomatization of phenols and direct α-C(sp3)-H bond functionalization of oxygen through cascade condensation/[1,5]-hydride transfer/dearomative-cyclization process. In addition, the antifungal activity assay and derivatizations of products were conducted to further enrich the utility of the structure.

Dearomative Intramolecular Diels-Alder/Sulfur Extrusion Reaction of Thiophenes with Alkynes Using peri-Substituted Naphthalene as a Tether

Dearomative intramolecular Diels-Alder/sulfur extrusion reaction of thiophenes with alkynes successfully afforded fluoranthenes in moderate to excellent yields. The proximity of both reactive sites fixed at the peri-position of naphthalene would play an important role in the progress of this reaction. Tri(o-tolyl)phosphine effectively suppressed the side reactions as a sulfur scavenger.

Diversification of Simple Arenes into Complex (Amino)cyclitols

Highly oxygenated cyclohexanes, including (amino)cyclitols, are featured in natural products possessing a notable range of biological activities. As such, these building blocks are valuable tools for medicinal chemistry. While de novo synthetic strategies have provided access to select compounds, challenges including stereochemical density and complexity have hindered the development of a general approach to (amino)cyclitol structures. This work reports the use of arenophile chemistry to access dearomatized intermediates which are amenable to diverse downstream transformations. Practical guidelines were developed for the synthesis of natural and non-natural (amino)cyclitols from simple arenes through a series of strategic functionalization events.

Visible-Light-Promoted Tandem Skeletal Rearrangement/Dearomatization of Heteroaryl Enallenes

Access to complex three-dimensional molecular architectures via dearomatization of ubiquitous aryl rings is a powerful synthetic tool, which faces, however, an inherent challenge to overcome energetic costs due to the loss of aromatic stabilization energy. Photochemical methods that allow one to populate high-energy states can thus be an ideal strategy to accomplish otherwise prohibitive reaction pathways. We present an original dearomative rearrangement of heteroaryl acryloylallenamides that leads to complex fused tricycles. The visible-light-promoted method occurs under mild conditions and tolerates a variety of functional groups. According to DFT modeling used to rationalize the outcome of the cascade, the reaction involves a sequential [2+2] allene-alkene photocycloaddition, which is followed by a selective retro- [2+2] step that paves the way for the dearomatization of the heteroaryl partner. This scenario is original with respect to the reported photochemical reactivity of similar substrates and thus holds promise for ample future developments.

Brook Rearrangement as Trigger for Dearomatization Reaction: Synthesis of Non-Aromatic N-Heterocycles

The [1,2]-Brook rearrangement stands as a potent technique for constructing complex molecules. In this study, we showcase its power in the dearomatization of aromatic N-heterocycles. Through a concise four-step process that integrates lithiation, nucleophilic addition, Brook rearrangement and dearomatization reaction, we demonstrate a versatile strategy for generating diverse non-aromatic N-heterocycles which exhibit ambident reactivities. Various acyl silanes, halo-pyridines, and quinolines have been explored within this context. The synthetic utility of this methodology is demonstrated through the construction of complex architectures.

Concise Synthesis of (±)-Fortuneicyclidins and (±)-Cephalotine B Enabled by Pd-Catalyzed Dearomative Spirocyclization

Total syntheses of C11-oxygenated Cephalotaxus alkaloids, fortuneicyclidins A and B, and cephalotine B, were achieved. The key for the synthesis is a Pd-catalyzed dearomative spirocyclization of bromofurans with N-tosylhydrazones, followed by acid-mediated tandem transformation to construct the tetracyclic skeleton with the C11-oxygen functional group. Chemo-selective and catalytic functional group conversions of the tetracyclic intermediate completed the synthesis of fortuneicyclidins and cephalotine B in 8 and 9 steps, respectively.

Total Synthesis of Altemicidin: A Surprise Ending for a Monoterpene Alkaloid

Monoterpene alkaloids encompass distinct chemical diversity and wide-ranging bioactivity. Their compact complexity has made them popular as synthetic targets and has inspired many distinct strategies and tactics in the field of heterocyclic chemistry. This article documents the evolution of a synthetic program aimed at accessing the unusual sulfonamide-containing natural product altemicidin, which was generally believed to be a monoterpene alkaloid throughout our entire synthetic investigations but has recently been found to originate through an unexpected and quite disparate biosynthetic pathway. By leveraging a pyridine dearomatization/cycloaddition strategy, we developed a concise pathway to the 5,6-fused bicyclic azaindane core and, after significant experimentation, an ultimate synthesis of altemicidin itself. Tactics to productively manipulate the multiple functional groups present on this highly polar scaffold proved challenging but were eventually realized via several carefully orchestrated and chemoselective transformations-investments that paid dividends in the form of significantly shorter chemical synthesis. Surprisingly, the bond-forming logic between our presumed abiotic synthetic strategy to this alkaloid class and its subsequently identified biosynthetic pathway is eerily similar.

Dearomatization of C6 Aromatic Hydrocarbons by Main Group Complexes

The dearomatization reaction is a powerful method for transformation of simple aromatic compounds to unique chemical architectures rapidly in synthetic chemistry. Over the past decades, the chemistry in this field has evolved significantly and various important organic compounds such as crucial bioactive molecules have been synthesized through dearomatization. In general, photochemical conditions or assistance by transition metals are required for dearomatization of rigid arenes. Recently, main-group elements, especially naturally abundant elements in the Earth's crust, have attracted attention as they have low toxicity and are cost-effective compared to the late transition metals. In recent decades, a variety of low-valent main-group molecules, which enable the activation of stable aromatic compounds under mild conditions, have been developed. This minireview highlights the developments in the chemistry of dearomatization of C6 aromatic hydrocarbons by main-group compounds leading to the formation of seven-membered EC6 (E=main-group elements) ring or cycloaddition products.

2H-Phosphindole-Enabled Dearomatization and [4+2] Cycloaddition of (Hetero)Arenes

The heavier main group multiple bonds offer an effective tool for small molecule activation. Transient 2H-phosphinidole working as a reactive phosphadiene system undergoes phospha-Diels-Alder reaction with a wide range of non-activated aromatic carbocycles and heterocycles, including naphthalene, anthracene, phenanthrene, furan, thiophene, pyrrole, pyridine, and benzo-fused heterocycles, affording concise access to a range of polycyclic fused rings feature with phosphorus at the bridgehead. These results demonstrate that non-activated (hetero)arenes are capable of acting as 2π systems in [4+2] cycloaddition with highly reactive 2H-phosphindole complex.

Enantioselective Dearomative [4+2] Cycloaddition Reaction of 1-Naphthols with In-Situ Generated ortho-Quinone Methides

We disclose a catalytic, enantioselective dearomative reaction of non-functionalized 1-naphthols, which poses a synthetic challenge to organic chemists because of the relative ease of rearomatization via the elimination of a proton. In this work, the direct dearomatization of non-functionalized 1-naphthols was achieved through a chiral phosphoric acid (CPA) catalyzed enantioselective dearomative [4+2] cycloaddition reaction with in-situ generated ortho-quinone methides (o-QMs). The reported convergent method allows the use of readily available simple 1-naphthols without pre-functionalization, furnishing a variety of naphthopyran derivatives in good yields (up to 96 %) and moderate to excellent enantioselectivities (up to >99 % ee) under mild reaction conditions. The observed regio-, diastereo-, and enantioselectivities are the keys to the success of the current strategy utilizing o-QM as a diene surrogates, in combination with CPA catalysis.

NMR-Verified Dearomatization of 5,7-Substituted Pyrazolo[1,5-a]pyrimidines

Tetrahydropyrazolo[1,5-a]pyrimidine (THPP) is an attractive scaffold for designing biologically active compounds. The most obvious way to obtain such compounds is to reduce pyrazolopyrimidines with complex hydrides, because the pyrimidine ring is reduced in the preference over the pyrazole ring. The presence of substituents at positions five and seven of pyrazolo[1,5-a]pyrimidines complicates the set of reaction products but makes it more attractive for medicinal chemistry because four possible stereoisomers can be formed during reduction. However, the formation of only syn-isomers has been described in the literature. This article is the first report on the formation of anti-configured isomers along with syn-isomers in the reduction of model 5,7-dimethylpyrazolo[1,5-a]pyrimidine, which was confirmed by NMR. The bicyclic core in the syn-configuration was shown to be conformationally stable, which was used to estimate the long-range interproton distances using NOESY data. At the same time, long-range dipole-dipole interactions corresponding to a distance between protons of more than 6 Å were first registered and quantified. In turn, the bicyclic core in the trans-configuration represents a conformationally labile system. For these structures, an analysis of conformations observed in solutions was carried out. Our results indicate the significant potential of trans-configured tetrahydropyrazolo[1,5-a]pyrimidines for the development of active small molecules. While possessing structural lability due to the low energy of the conformational transition, they have the ability to adjust to the active site of the desired target.

Recent Progresses in the Catalytic Stereoselective Dearomatization of Pyridines

1,2- and 1,4-dihydropyridines and N-substituted 2-pyridones are very important structural motifs due to their synthetic versatility and vast presence in a variety of alkaloids and bioactive molecules. In this article, we gather and summarize the catalytic and stereoselective synthesis of partially hydrogenated pyridines and pyridones via the dearomative reactions of pyridine derivatives up to mid-2023. The material is fundamentally organized according to the type of reactivity (electrophilic/nucleophilic) of the pyridine nucleus. The material is further sub-divided taking into account the nucleophilic species when dealing with electrophilic pyridines and considering the reactivity manifold of pyridine derivatives behaving as nucleophiles at the nitrogen site. The latter more recent approach allows for an unconventional entry to chiral N-substituted 2- and 4-pyridones in non-racemic form.

Intermolecular Dearomative 1,2-Amination/Carbonylation via Nucleophilic Addition of Simple Amines to Arene π-Bonds

The incorporation of the privileged amino functionality is of paramount importance in organic synthesis. In contrast to the well-developed amination methods for alkenes, the dearomative amination of arenes is largely underexplored due to the inherently inert reactivity of arene π-bonds and selectivity challenges. Herein, we report an intermolecular dearomative aminofunctionalization via direct nucleophilic addition of simple amines to chromium-bound arenes. This multicomponent 1,2-amination/carbonylation reaction enables rapid access to complicated alicyclic compounds containing amino and amide functionalities from benzene derivatives under CO-gas-free conditions, which also represents the first application of nitrogen-based nucleophiles in η6-coordination-induced arene dearomatizations.

Copper-Catalyzed Asymmetric Dearomative [3+2] Cycloaddition of Nitroheteroarenes with Azomethines

Catalytic asymmetric dearomative [3+2] cycloaddition of α-imino γ-lactones with either 3-nitroindoles or 2-nitrobenzofurans by using a chiral copper complex as the catalyst was developed. A wide range of structurally diverse polyheterocyclic compounds containing spirocyclic-fused butyrolactone-pyrrolidine-indoline and butyrolactone-pyrrolidine-dihydrobenzofuran skeletons could be smoothly obtained with excellent results (>99:1 dr and 98% ee). The potential synthetic applications of this methodology were also demonstrated by the scale-up experiment and by the diverse transformations of one product. This method is characterized by high asymmetric induction, wide functional group tolerance and scalability, and attractive product diversification.

Homogeneous Dearomative Hydrogenation with a Co/P4 N2 Catalyst: A Nucleophilic Approach

Arene hydrogenation is the most straightforward method to prepare carbo- and heterocycles. However, the high resonance energy prevents aromatic substrates from hydrogenation. Herein the homogeneous, nucleophilic hydrogenation of less electron-rich arenes and heteroarenes is reported. The Co(P₄ N₂ )H species that has been demonstrated to be a strong hydride donor could deliver a hydride ion to the cyano (hetero)arene substrates. Deuterium labeling experiments supported a Michael-type reaction pathway. Theoretical analyses have been conducted to investigate the hydricity of the catalytically active CoH species and the electrophilicity of the arene substrates. An outlook for the synthesis of more challenging substituted benzenes was proposed based on the in silico modification of the CoH species.

Asymmetric Fluorofunctionalizations with Carboxylate-Based Phase-Transfer Catalysts

Fluorine is an attractive element in the field of pharmaceutical and agrochemical chemistry due to its unique properties. Considering the chiral environment in nature, where enantiomers often show different biological activities, the introduction of fluorine atom(s) into organic molecules to make chiral fluorinated compounds is an important subject. Herein, we describe the story of the development of our chiral carboxylate-based phase-transfer catalysts and their applications for asymmetric fluorocyclizations of alkenes bearing a carboxylic acid, an amide, and an oxime as an internal nucleophile with a dicationic fluorinating reagent, Selectfluor. We also describe dearomative fluorinations of indole derivatives, 2-naphthols, and resorcinols.

Development of Transition-Metal-Catalyzed Dearomatization Reactions

To develop dearomatization reactions based on a nucleophilic activation of phenols, naphthols, and indoles, ipso-Friedel-Crafts-type C-alkylation must be selectively promoted over competitive O- or N-alkylation reactions. Resolving this chemoselectivity issue is essential for developing this class dearomatization reaction. We found that various dearomatization reactions could be developed using appropriately designed aromatic substrates with an electrophilic moiety for intramolecular reactions. This review describes the transition-metal-catalyzed dearomatization reactions developed by our group. π-Allylpalladium species, η3-propargylpalladium species, alkynes activated by Au(I) species, and silver carbene species could be applied as electrophiles in our reaction system, which provided access to a wide variety of dearomatized products from planar aromatic compounds in a highly chemoselective manner.

Chiral Anthranyl Trifluoromethyl Alcohols: Structures, Oxidative Dearomatization and Chiroptical Properties

Chiral trifluoromethyl alcohol groups were introduced at the hindered ortho positions of 9,10-diphenylanthracenes to investigate their effects on the physical properties and reactivity towards oxidative dearomatization. In such compact structures, the position in different quadrants and the preferred orientation of the -CH(OH)CF₃ groups were determined by the relative and absolute configurations of each stereoisomer, respectively. As a consequence, the stereochemistry governs the organization of the H-bonded molecules in single crystals (homochiral dimers vs ribbon), whereas in chlorinated solvents, they all behave as discrete compounds. Concerning their reactivity, the stereospecific dearomative oxidation of these molecules leads to 9,10-bis-spiro-isobenzofuran-anthracenes, when using organic single-electron transfer oxidants. The chiroptical properties of the alcohols and the corresponding dearomatized products were compared and showed an important modulation of the intensity.

Base-Induced Dehydrogenative and Dearomative Transformation of 1-Naphthylmethylamines to 1,4-Dihydronaphthalene-1-carbonitriles

Solvothermal treatment of 1-naphthylmethylamine with potassium hydride (KH) or n-butyllithium (n-BuLi)-potassium t-butoxide (t-BuOK) in THF induces unusual two consecutive β-hydride eliminations to form 1-naphthonitrile and KH. The freshly generated KH is hydridic enough to undergo dearomative hydride addition to the resultant 1-naphthonitrile regioselectively at the C4 position to afford α-cyano benzylic carbanion, which could be functionalized by a series of electrophiles, liberating the corresponding 1,4-dihydronaphthalene-1-carbonitriles having a quaternary carbon center.

Novel [FeFe]-Hydrogenase Mimics: Unexpected Course of the Reaction of Ferrocenyl α-Thienyl Thioketone with Fe3(CO)12

The influence of the substitution pattern in ferrocenyl α-thienyl thioketone used as a proligand in complexation reactions with Fe₃(CO)₁₂ was investigated. As a result, two new sulfur–iron complexes, considered [FeFe]-hydrogenase mimics, were obtained and characterized by spectroscopic techniques (¹H, ¹³C{¹H} NMR, IR, MS), as well as by elemental analysis and X-ray single crystal diffraction methods. The electrochemical properties of both complexes were studied and compared using cyclic voltammetry in the absence and in presence of acetic acid as a proton source. The performed measurements demonstrated that both complexes can catalyze the reduction of protons to molecular hydrogen H₂. Moreover, the obtained results showed that the presence of the ferrocene moiety at the backbone of the linker of both complexes improved the stability of the reduced species.

Hydroamination of Dihapto-Coordinated Benzene and Diene Complexes of Tungsten: Fundamental Studies and the Synthesis of γ-Lycorane

Reactions are described for complexes of the form WTp(NO)(PMe₃)(η²-arene) and various amines, where the arene is benzene or benzene with an electron-withdrawing substituent (CF₃, SO₂Ph, SO₂Me). The arene complex is first protonated to form an η²-arenium species, which then selectively adds the amine. The resulting η²-5-amino-1,3-cyclohexadiene complexes can then be subjected to the same sequence with a second nucleophile to form 3-aminocyclohexene complexes, where up to three stereocenters originate from the arene carbons. Alternatively, 1,3-cyclohexadiene complexes containing an ester group at the 5 position (also prepared from an arene) can be treated with acid followed by an amine to form trisubstituted 3-aminocyclohexenes. When the amine is primary, ring closure can occur to form a cis-fused bicyclic γ-lactam. Highly functionalized cyclohexenes can be liberated from the tungsten through oxidative decomplexation. The potential utility of this methodology is demonstrated in the synthesis of the alkaloid γ-lycorane. An enantioenriched synthesis of a lactam precursor to γ-lycorane is also described. This compound is prepared from an enantioenriched version of the tungsten benzene complex. Regio- and stereochemical assignments for the reported compounds are supported by detailed 2D-NMR analysis and 13 molecular structure determinations (SC-XRD).

Asymmetric Direct/Stepwise Dearomatization Reactions Involving Hypervalent Iodine Reagents

A remarkable growth in hypervalent iodine-mediated oxidative transformations as stoichiometric reagents as well as catalysts has been well-documented due to their excellent properties, such as mildness, easy handling, high selectivity, environmentally friendly nature, and high stability. This review aims at highlighting the asymmetric oxidative dearomatization reactions involving hypervalent iodine compounds. The present article summarizes asymmetric intra- and intermolecular dearomatization reactions using chiral hypervalent iodine reagents/catalysts as well as hypervalent iodine-mediated dearomatization reactions followed by desymmetrization.

Strain-Release-Driven Friedel-Crafts Spirocyclization of Azabicyclo[1.1.0]butanes

The identification of spiro N-heterocycles as scaffolds that display structural novelty, three-dimensionality, beneficial physicochemical properties, and enable the controlled spatial disposition of substituents has led to a surge of interest in utilizing these compounds in drug discovery programs. Herein, we report the strain-release-driven Friedel-Crafts spirocyclization of azabicyclo[1.1.0]butane-tethered (hetero)aryls for the synthesis of a unique library of azetidine spiro-tetralins. The reaction was discovered to proceed through an unexpected interrupted Friedel-Crafts mechanism, generating a highly complex azabicyclo[2.1.1]hexane scaffold. This dearomatized intermediate, formed exclusively as a single diastereomer, can be subsequently converted to the Friedel-Crafts product upon electrophilic activation of the tertiary amine, or trapped as a Diels-Alder adduct in one-pot. The rapid assembly of molecular complexity demonstrated in these reactions highlights the potential of the strain-release-driven spirocyclization strategy to be utilized in the synthesis of medicinally relevant scaffolds.

A Striking Mode of Activation of Carbon Disulfide with a Cooperative Bis(silylene)

The reactivity of the 1,4-substituted bis(silylenyl)terphenylene 1, 1,4-[ortho-(LSi)C₆ H₄ ]₂ C₆ H₄ , (L=RC(NtBu)₂ , R=Ph, Mes) towards CS₂ is reported. It results in a dearomatization of the phenylene ring, affording the 1,3-substituted cyclohexadiene derivative 2. According to DFT calculations, a transient silene containing a Si=C bond capable of π(C=C) addition at the aromatic phenylene ring is a key intermediate. In contrast, addition of CS₂ to the biphenyl-substituted mono-silylene ortho-(LSi)C₆ H₄ -C₆ H₅ 3 leaves the aromatic π-system intact and forms, in a [1+2] cycloaddition reaction, the corresponding thiasilirane 4 with a three-membered SiSC ring. Further experimental studies led to the isolation of the novel mesoionic five-membered Si₂ S₂ C heterocycle 6, which reacts with CS₂ under C-C bond formation. All isolated new compounds were fully characterized and their molecular structures determined by single-crystal X-ray diffraction analyses.

Palladium-Catalyzed Dearomative Methoxyallylation of 3-Nitroindoles with Allyl Carbonates

Herein we report a Pd-catalyzed dearomative methoxyallylation of 3-nitroindoles with readily available allyl carbonates. Good yields (up to 86 %) and diastereoselectivity (up to >20:1 dr) are obtained for a wide range of substrates. The compatibility of gram-scale synthesis and the relatively low catalyst loading (down to 1 mol % of [Pd]) enhance the practicality of this method. The kinetic experiments indicate that the rate-determining step of this reaction is the nucleophilic attack of the alkoxide anion.

Nitropyridines as 2π-Partners in 1,3-Dipolar Cycloadditions with N-Methyl Azomethine Ylide: An Easy Access to Condensed Pyrrolines

1,3-Dipolar cycloaddition reactions of 2-substituted 5-R-3-nitropyridines and isomeric 3-R-5-nitropyridines with N-methyl azomethine ylide were studied. The effect of the substituent at positions 2 and 5 of the pyridine ring on the possibility of the [3+2]-cycloaddition process was revealed. A number of new derivatives of pyrroline and pyrrolidine condensed with a pyridine ring were synthesized.

Silver-Catalyzed Asymmetric Dearomatization of Electron-Deficient Heteroarenes via Interrupted Barton-Zard Reaction

Herein we report a catalytic asymmetric dearomatization reaction of electron-deficient heteroarenes with α-substituted isocyanoacetates through an interrupted Barton-Zard reaction. A range of optically active pyrrolo[3,4-b]indole derivatives was obtained in good yields (up to 97 %) with high stereoselectivities (up to >20:1 dr and 97 % ee), using a catalytic system consisting of a cinchona-derived amino-phosphine and silver oxide. This reaction features wide substrate scope and mild conditions, and provides a new strategy for developing asymmetric dearomatization reactions.

Dearomatizing Amination Reactions

Dearomatization reactions allow the direct synthesis of structurally complex sp³ -rich molecules from readily available "flat" precursors. Established dearomatization processes commonly involve the formation of new C-C bonds, whereas methods that enable the introduction of C-N bonds have received less attention. Because of the privileged position of nitrogen in drug discovery, significant recent methodological efforts have been directed towards addressing this deficiency. Consequently, a variety of new processes are now available that allow the direct preparation of sp³ -rich amino-containing building blocks and scaffolds. This review gives an overview of C-N bond forming dearomatization reactions, particularly with respect to scaffold assembly processes. The discussion gives historical context, but the main focus is on selected methods that have been reported recently.

Hypervalent-Iodine(III)-Mediated Tandem Oxidative Dearomatization/Aziridination of Phenolic Amines: Synthesis of Functionalized Unactivated Aziridines

A new hypervalent-iodine(III)-mediated tandem reaction involving oxidative dearomatization and in situ aziridination of phenolic amines is described, providing a mild and effective method for the assembly of structurally interesting and synthetically useful aziridines. Importantly, the densely functionalized aziridines resulting from this unprecedented tandem reaction offer a platform for expeditious access to architecturally diverse aza-heterocycles through transformations initiated by selective ring-opening of aziridines.

Palladium-Catalyzed Three-Component Coupling Reaction via Benzylpalladium Intermediate

Transition-metal catalyzed multi-component reactions have captured the attention of researchers in organic synthesis and drug synthesis due to their advantages of simple operation, easy availability of raw materials and without separation of intermediates. Among the multi-component reactions, the three-component processes have been developed into effective organic procedures. This personal account reviews our and other group's studies on the development of three-component coupling reaction for the rapid construction of two new chemical bonds simultaneously via benzylpalladium intermediates. Catalyst-switched three-component reactions of benzyl halides, activated olefins, and allyltributylstannane were successfully conducted to produce the corresponding benzylallylation products. Activation and conversion of carbon monoxide and carbon dioxide via π-benzylpalladium intermediates provide access to a wide range of unsaturated ketones and esters with excellent functional group tolerance. Meanwhile, other methods to produce benzylpalladium intermediates, including Heck insertion of alkenes into arylpalladium complexes, the oxidative addition of benzyl carbonate to palladium complexes and palladium-carbene migratory insertion, were also highlighted.

Dearomatization of Unactivated Arenes via Catalytic Hydroalkylation

The dearomative cyclization of linear amides to complex spirocyclic butyrolactams has been enabled by photoredox catalysis through a reductive radical-polar crossover mechanism. This mechanism operates with precision on unactivated aromatic substrates to give a wide range of 1,4-hydroalkylation products. This method utilizes a simple organic catalyst/reductant pair to deliver products in a highly flexible manner with respect to substitution, and the products can be further functionalized under simple conditions to afford a collection of motifs. The mechanistic analysis performed here outlines the salient features of this strategy, which were applied to prepare a collection of complex scaffolds including the anticonvulsive agent gabapentin.

A Solvent-free, Catalyst-free Formal [3+3] Cycloaddition Dearomatization Strategy: Towards New Fluorophores for Biomolecules Labelling

A general, sustainable dearomatization reaction for nitrogen-containing heterocycles was developed. Under solvent free conditions and without catalyst, the biorenewable methyl coumalate (MC) reacted as an efficient C₃ partner to convert nine types of basic aromatic rings into their pyrido[1,2-a] fused derivatives in good to excellent yields. The fluorescence properties of some of the products were harnessed to conjugate fluorescent tags to bovine serum albumin (BSA) and immunoglobulin G.

Enantio- and Diastereoselective Nucleophilic Addition of N-tert-Butylhydrazones to Isoquinolinium Ions through Anion-Binding Catalysis

A highly enantio- and diastereoselective thiourea-catalyzed dearomatization of isoquinolines employing N-tert-butylhydrazones as neutral α-azo carbanions and masked acyl anion equivalents has been developed. Experimental and computational data supports the generation of highly ordered complexes wherein the chloride behaves as a template for the catalyst, the hydrazone reagent, and the isoquinolinium cation, providing excellent stereocontrol in the formation of two contiguous stereogenic centers. The ensuing selective and high-yielding transformations provide appealing dihydroisoquinoline derivatives.

Photoinduced Dearomatizing Three-Component Coupling of Arylphosphines, Alkenes, and Water

A unique photoinduced reaction that couples a triarylphosphine, an alkene, and water to produce 2-(cyclohexa-2,5-dienyl)ethylphosphine oxide is reported herein. The alkene inserts into a C(aryl)-P bond of the arylphosphine, the aryl ring is dearomatized into the cyclohexadienyl ring, and the phosphorus is oxidized. The three components are all readily available, and their intermolecular coupling significantly increases molecular complexity. The products formed are applicable to the Wittig olefination.

Dearomatizing [4+1] Spiroannulation of Naphthols: Discovery of Thermally Activated Delayed Fluorescent Materials

Disclosed here is a palladium-catalyzed direct [4+1] spiroannulation of ortho-C-H bonds of naphthols with cyclic diaryliodonium salts to construct spirofluorenyl naphthalenones (SFNP) under mild reaction conditions. This spiroannulation directly transforms the hydroxy group into a carbonyl group, and also tolerates reactive functional groups such as the halo groups, which provide an opportunity to rapidly assemble structurally new thermally activated delayed fluorescent (TADF) materials that feature a carbonyl group with an adjacent spirofluorenyl unit as the acceptor. As an illustrated example, the OLED device utilizing the assembled DMAC-SFNP as the host material exhibits a low turn-on voltage of 2.5 V and an ultra-high external quantum efficiency of 32.2 %. This work provides inspiration for structurally new TADF materials, and also displays the potential of C-H activation as a synthetic strategy for the innovation of optoelectronic materials.

Flavoprotein monooxygenases: Versatile biocatalysts

Flavoprotein monooxygenases (FPMOs) are single- or two-component enzymes that catalyze a diverse set of chemo-, regio- and enantioselective oxyfunctionalization reactions. In this review, we describe how FPMOs have evolved from model enzymes in mechanistic flavoprotein research to biotechnologically relevant catalysts that can be applied for the sustainable production of valuable chemicals. After a historical account of the development of the FPMO field, we explain the FPMO classification system, which is primarily based on protein structural properties and electron donor specificities. We then summarize the most appealing reactions catalyzed by each group with a focus on the different types of oxygenation chemistries. Wherever relevant, we report engineering strategies that have been used to improve the robustness and applicability of FPMOs.

Rapid Access to Dispirocyclic Scaffolds Enabled by Diastereoselective Intramolecular Double Functionalization of Benzene Rings

Here we describe the diastereoselective synthesis of (5r,8r)-1,9-diazadispiro[4.2.4⁸ .2⁵ ]tetradecatrienes via domino double spirocyclization of N-arylamide derivatives. This reaction can serve as a fast way to synthesize diazadispirocycles, which are found in the core structures of bioactive natural products. Product diversification via Suzuki-Miyaura cross coupling and application to the synthesis of 1-oxa-9-azadispiro[4.2.4⁸ .2⁵ ]tetradecatrienes were also conducted.

Lewis Acid Promoted Dearomatization of Naphthols

Two-step dearomative functionalization of naphthols promoted by Lewis acids and copper(I) catalysis was developed. Initially, Lewis acid complexation inverted the electronic properties of the ring and established an equilibrium with the dearomatized counterpart. Subsequent trapping of the dearomatized intermediate with organometallics as well as organophosphines was demonstrated and provided the corresponding dearomatized products.

Total Synthesis of (±)-Phaeocaulisin D

The tropone sesquiterpene phaeocaulisin D, isolated from the rhizomes of Curcuma phaeocaulis, has previously been shown to inhibit nitric oxide production in macrophages. A total synthesis of phaeocaulisin D was accomplished by using an intramolecular cyclization-dearomatization as a key step. The highlights of the synthesis are effective formation of the 5-7 fused tropone system, and selective methylation of a late-stage intermediate.

Computational Study on the Synergic Effect of Brønsted Acid and Hydrogen-Bonding Catalysis for the Dearomatization Reaction of Phenols with Diazo Functionality

Catalytic dearomative transformations of phenol variants via an ipso-Friedel-Crafts reaction could provide a straightforward method for the rapid assembly of functionalized spiromolecules as versatile synthetic scaffolds. We previously reported a dearomative spirocyclization reaction by merging Brønsted acid and hydrogen-bonding catalysis. However, it was unclear how the reaction proceeded and how the synergic effect was triggered. Described herein are the computational studies used to elucidate the reaction mechanism. Such calculations indicated that the applied catalysts, maleic acid and Schreiner's thiourea, work cooperatively. The synergic effect enabled the chemoselectivity to interconvert between phenol dearomatization and O-H insertion, which is a major side reaction. This investigation also revealed that not only does the Schreiner's thiourea catalyst serve as a hydrogen bonding donor, but the sulfur atom in thiourea possesses a general base function. The dual functional support of the thiourea along with maleic acid would thus realize the chemoselective prioritization of dearomatization over the O-H insertion reaction under mild conditions.

Allylic and Allenylic Dearomatization of Indoles Promoted by Graphene Oxide by Covalent Grafting Activation Mode

The site-selective allylative and allenylative dearomatization of indoles with alcohols was performed under carbocatalytic regime in the presence of graphene oxide (GO, 10 wt % loading) as the promoter. Metal-free conditions, absence of stoichiometric additive, environmentally friendly conditions (H₂ O/CH₃ CN, 55 °C, 6 h), broad substrate scope (33 examples, yield up to 92 %) and excellent site- and stereoselectivity characterize the present methodology. Moreover, a covalent activation model exerted by GO functionalities was corroborated by spectroscopic, experimental and computational evidences. Recovering and regeneration of the GO catalyst through simple acidic treatment was also documented.

Tunable Electrochemical C-N versus N-N Bond Formation of Nitrogen-Centered Radicals Enabled by Dehydrogenative Dearomatization: Biological Applications

Herein, an environmentally friendly electrochemical approach is reported that takes advantage of the captodative effect and delocalization effect to generate nitrogen-centered radicals (NCRs). By changing the reaction parameters of the electrode material and feedstock solubility, dearomatization enabled a selective dehydrogenative C-N versus N-N bond formation reaction. Hence, pyrido[1,2-a]benzimidazole and tetraarylhydrazine frameworks were prepared through a sustainable transition-metal- and exogenous oxidant-free strategy with broad generality. Bioactivity assays demonstrated that pyrido[1,2-a]benzimidazoles displayed antimicrobial activity and cytotoxicity against human cancer cells. Compound 21 exhibited good photochemical properties with a large Stokes shift (approximately 130 nm) and was successfully applied to subcellular imaging. A preliminary mechanism investigation and density functional theory (DFT) calculations revealed the possible reaction pathway.

Synthesis and Facile Dearomatization of Highly Electrophilic Nitroisoxazolo[4,3-b]pyridines

A number of novel 6-R-isoxazolo[4,3-b]pyridines were synthesized and their reactions with neutral C-nucleophiles (1,3-dicarbonyl compounds, π-excessive (het)arenes, dienes) were studied. The reaction rate was found to be dependent on the nature of the substituent 6-R. The most reactive 6-nitroisoxazolo[4,3-b]pyridines are able to add C-nucleophiles in the absence of a base under mild conditions. In addition, these compounds readily undergo [4+2]-cycloaddition reactions on aromatic bonds C=C(NO₂) of the pyridine ring, thus indicating the superelectrophilic nature of 6-NO₂-isoxazolo[4,3-b]pyridines.

Nucleophilic Addition Reaction with Dearomatization of Naphthalene Ring

Various aromatic lactones have been synthesized and their regioselectivity (1,2-addition vs. 1,4- or 1,6-addition) investigated in reactions with organolithium species, particularly n-BuLi and sec-BuLi. The regioselectivity varied greatly depending on various factors, such as the bulkiness of both substrates and organolithium species, and types of solvent and cosolvent. In particular, 1,4-addition with dearomatization occurred preferentially using sec-BuLi as the nucleophile in tetrahydrofuran (THF) with hexamethylphosphoramide (HMPA) or N,N'-dimethylpropyleneurea (DMPU) as cosolvent. For sec-BuLi, the reaction was estimated to proceed through a single-electron transfer mechanism.

Oxidative Dearomative Cross-Dehydrogenative Coupling of Indoles with Diverse C-H Nucleophiles: Efficient Approach to 2,2-Disubstituted Indolin-3-ones

The oxidative, dearomative cross-dehydrogenative coupling of indoles with various C-H nucleophiles is developed. This process features a broad substrate scope with respect to both indoles and nucleophiles, affording structurally diverse 2,2-disubstituted indolin-3-ones in high yields (up to 99%). The oxidative dimerization and trimerization of indoles has also been demonstrated under the same conditions.