Chiral phosphoric acid catalyzed aminative dearomatization of α-naphthols/Michael addition sequence

Electrophile-Arene Affinity: An Energy Scale for Evaluating the Thermodynamics of Electrophilic Dearomatization Reactions

Rational design and development of organic reactions are lofty goals in synthetic chemistry. Quantitative description of the properties of molecules and reactions by physical organic parameters plays an important role in this regard. In this Article, we report an energy scale, namely, electrophile-arene affinity (EAA), for evaluating the thermodynamics of electrophilic dearomatization reactions, a class of important transformations that can rapidly build up molecular complexity and structural diversity by converting planar aromatic compounds into three-dimensional cyclic molecules. The acquisition of EAA data can be readily achieved by theoretically calculating the enthalpy changes (ΔH) of the hypothetical reactions of various (cationic) electrophiles with aromatic systems (taking the 1-methylnaphthalen-2-olate ion as an example in this study). Linear correlations are found between the calculated ΔH values and established physical organic parameters such as the percentage of buried volume %VBur (steric effect), Hammett's σ or Brown's σ+ (electronic effect), and Mayr's E (reaction kinetics). Careful analysis of the ΔH values leads to the rational design of a dearomative alkynylation reaction using alkynyl hypervalent iodonium reagents as the electrophiles.

Chiral Diselenophosphoric Acids for Ion Pair Catalysis: A Novel Approach to Enhance Both Proton Donating and Proton Accepting Properties

The activation of poorly reactive substrates via strong chiral acids is a central topic in asymmetric ion pair catalysis these days. Despite highly successful scaffolds such as N-triflylphosphoramides, these catalysts either lack C2-symmetry or provide multiple H-bond acceptor sites, leading to lower ee values for certain reactions. We present BINOL-based diselenophosphoric acids (DSA) as an extremely promising alternative. Using an intertwined approach of synthesis and NMR studies, we developed a synthetic approach to DSA with up to 98 % NMR yield. The obtained acids provide both very high proton donor and proton acceptor properties, a bifunctionality, which is key to catalytic applications. Indeed, first reactivity test proved the much higher acidity of DSA and its ability to initiate Mukaiyama-Mannich reaction and protodesilylation of silyl ethers. Together with their C2-symmetry, the single donor and single acceptor situation, the decreased tendency of self-association, and the straightforward synthesis with potential 3,3'-substitution, the DSA provide all features ideal for the further development of ion pair catalysis.

Organocatalytic Dearomative Spirocyclization Reaction of Enone-Tethered α-and β-Naphthols and Dearomatization Reaction of In Situ Generated Nitro-Olefin-Tethered α-Naphthols

Herein, we report a catalytic dearomative spirocyclization reaction of new substrates having aryl/alkyl enone tethered α- and β-naphthols and a dearomatization reaction of in situ generated nitro-olefin-tethered α-naphthols. The spirocarbocycles were obtained in moderate to good yields with high diastereoselectivities. A preliminary catalytic asymmetric variant was reported. A few applications such as hydrogenations and epoxidation reaction have also been demonstrated. Theoretical study has also been performed to understand high diastereoselectivity in the triethylamine catalyzed spirocyclization reaction.

Enantioselective Aminative Dearomatization of Indoles via Electrophilic 1,6-Addition of p-Quinone Diimides (p-QDIs)

Herein we report the first use of p-quinone diimide for the aminative dearomatization of 2,3-disubstituted indoles to furnish C3 aza-quaternary chiral indolenines. This approach, which proceeds via an electrophilic 1,6-addition of p-quinone diimide, allows the synthesis of an array of optically active aza-quaternary indolenines with high yields and excellent enantioselectivities. A one-pot approach of the same has also been established to further improve the synthetic accessibility of this protocol.

Organocatalytic Asymmetric Dearomative Spirocyclization/Oxa-Michael Addition Sequence: Synthesis of Polycyclic Tetralones

Herein, an organocatalytic asymmetric dearomative spirocyclization/oxa-Michael addition sequence with a newly designed substrate having two naphthol motifs has been developed. The reaction proceeds through in situ chiral vinylidene ortho-quinone methide (VQM) intermediate formation, dearomative spirocyclization of naphthol, and an oxa-Michael addition reaction. The densely functionalized tetralone products were formed in high yields with high diastereo- and enantioselectivities.

Asymmetric Brominative Dearomatization of 2-Naphthols Using a Cinchona Alkaloid-Based Organocatalyst

A cinchona alkaloid-based organocatalyst enables asymmetric brominative dearomatization of 2-naphthols, providing the corresponding bromonaphthalenones with high enantioselectivities. The first metal-free reaction can accommodate a variety of functional groups and give useful frameworks bearing a Br-containing tetrasubstituted stereogenic center.

Dearomatization/Spiroannulation of Halophenols Enables the Forging of Contiguous Quaternary Carbon Cyclohexadienones

A dearomatization/spiroannulation process has been successfully achieved between simple halophenols and α,β-unsaturated olefins under mild reaction conditions. This transformation addresses the chemoselectivity issue in the dearomatizative transformation of phenol scaffolds (6π-electron) caused by the SEAr process, enabling the construction of versatile cyclohexadienone frameworks containing contiguous quaternary all-carbon centers in high yields. Further studies have provided valuable insights into the process, revealing that debromination/spiroannulation occurs through the SRN1 pathway.

Catalytic asymmetric dearomatization of phenols via divergent intermolecular (3 + 2) and alkylation reactions

The catalytic asymmetric dearomatization (CADA) reaction has proved to be a powerful protocol for rapid assembly of valuable three-dimensional cyclic compounds from readily available planar aromatics. In contrast to the well-studied indoles and naphthols, phenols have been considered challenging substrates for intermolecular CADA reactions due to the combination of strong aromaticity and potential regioselectivity issue over the multiple nucleophilic sites (O, C2 as well as C4). Reported herein are the chiral phosphoric acid-catalyzed divergent intermolecular CADA reactions of common phenols with azoalkenes, which deliver the tetrahydroindolone and cyclohexadienone products bearing an all-carbon quaternary stereogenic center in good yields with excellent ee values. Notably, simply adjusting the reaction temperature leads to the chemo-divergent intermolecular (3 + 2) and alkylation dearomatization reactions. Moreover, the stereo-divergent synthesis of four possible stereoisomers in a kind has been achieved via changing the sequence of catalyst enantiomers.

Enantioselective Dearomatization of Substituted Phenols via Organocatalyzed Electrophilic Amination

Highly efficient and stereoselective dearomatization of substituted phenols was achieved via chiral phosphoric acid-catalyzed electrophilic para-amination with commercially available azodicarboxylates. This protocol readily afforded a series of chiral 2,5-cyclohexadienones bearing 4-aza-quaternary stereocenters with excellent yields and enantioselectivities (≤99% yield and >99% ee). Easy scale-up of this reaction to a gram scale and diverse derivatizations of the chiral products into α-tertiary amines and α-tertiary heterocycles derivatives well demonstrated the potential of this method.

Brønsted acid-catalyzed asymmetric dearomatization for synthesis of chiral fused polycyclic enone and indoline scaffolds

In the past two decades, substantial advances have been made on the asymmetric alkyne functionalization by the activation of inert alkynes. However, these asymmetric transformations have so far been mostly limited to transition metal catalysis, and chiral Brønsted acid-catalyzed examples are rarely explored. Here, we report a chiral Brønsted acid-catalyzed dearomatization reaction of phenol- and indole-tethered homopropargyl amines, allowing the practical and atom-economical synthesis of a diverse array of valuable fused polycyclic enones and indolines bearing a chiral quaternary carbon stereocenter and two contiguous stereogenic centers in moderate to good yields with excellent diastereoselectivities and generally excellent enantioselectivities (up to >99% enantiomeric excess). This protocol demonstrates Brønsted acid-catalyzed asymmetric dearomatizations via vinylidene-quinone methides.

Synthesis of spirocyclic 1,2-diamines by dearomatising intramolecular diamination of phenols

The stereocontrolled synthesis of complex spirotricyclic systems containing an embedded syn-1,2-diaminocyclohexane unit is reported, based upon a dearomatising oxidation of phenols bearing pendant ureas capable of acting as double nucleophiles. This complexity-generating transformation yields products with rich functionality suitable for application in the synthesis of potentially bioactive compounds.

Diastereoselective Synthesis of Dihydrobenzofuran-Fused Spiroindolizidines via Double-Dearomative [3 + 2] Cycloadditions

Spiroindolizidine oxindoles represent a kind of privileged scaffold in many biologically active natural alkaloids. 2,3-Dihydrobenzofuran derivatives exhibit significant bioactivities in a variety of pharmaceuticals. Herein, we assembled these two privileged fragments into a small molecule via double-dearomative [3 + 2] cycloadditions with pyridinium ylides and 2-nitrobenzofurans. This protocol features remarkable advantages including wide substrate scope, mild condition, high level of diastereoselectivities and yields. Thus, a collection of spiroindolizidine-fused dihydrobenzofurans/indolines were facilely produced efficiently.

Dearomatization [4+2] Cycloaddition of Nonactivated Benzene Derivatives

Dearomatization reactions have recently emerged as a powerful tool for the rapid buildup of molecular complexity. Here, an unparalleled thermal dearomatization [4+2] cycloaddition reaction between benzene derivatives and a 2H-phosphindole tungsten complex was reported. The unique reactivity of the in situ-generated 2H-phosphindole complex toward benzene was revealed by density functional theory calculations. We thus provide new insights into the dearomatization of nonactivated arenes and pave the way for the manipulation of the dearomatization for further applications.

Metal-free dearomatization reactions of naphthol-ynamides for the divergent and enantioselective synthesis of azaspirocycles

An efficient Brønsted acid (BA) catalyzed intramolecular dearomatization cyclization of naphthol-ynamides has been developed, enabling the practical and divergent synthesis of two azaspirocycles in high yields.

Dearomative Cyclopropanation of Naphthols via Cyclopropene Ring-Opening

The dearomatization of 2-naphthols represents a simple method for the construction of complex 3D structures from simple planar starting materials. We describe a cyclopropanation of 2-naphthols that proceeds via cyclopropene ring-opening using rhodium and acid catalysis under mild conditions. The vinyl cyclopropane molecules were formed with high chemoselectivity and scalability, which could be further functionalized at different sites. Both computational and experimental evidence were used to elucidate the reaction mechanism.

Dearomatization reaction of β-naphthols with disulfurating reagents

p-TsOH-catalyzed intermolecular dearomatization reactions of β-naphthols with disulfurating reagents were developed. Various β-naphthalenones bearing a quaternary carbon stereogenic center were obtained smoothly in good to excellent yields with high chemoselectivity in the presence of 5 mol% p-TsOH. This reaction features mild reaction conditions and excellent functional group tolerance.

Enantioselective Au(I)-catalyzed dearomatization of 1-naphthols with allenamides through Tethered Counterion-Directed Catalysis

The Tethered Counterion-Directed Catalysis (TCDC) approach has been applied to the enantioselective Au(I) catalyzed dearomatizations of 1-naphthols with allenamides. Stereocontrol is ensured by the intramolecular ion-pairing between the chiral gold-tethered phosphate and an iminium unit, that provides a rigid, well-defined chiral environment to the key electrophilic intermediate.

Transition-Metal-Free [3+2] Dehydration Cycloaddition of Donor-Acceptor Cyclopropanes With 2-Naphthols

A Brønsted acid-catalyzed domino ring-opening cyclization transformation of donor-acceptor (D-A) cyclopropanes and 2-naphthols has been developed. This formal [3+2] cyclization reaction provided novel and efficient access to the naphthalene-fused cyclopentanes in the absence of any transition-metal catalysts or additives. This robust procedure was completed smoothly on a gram-scale to afford the corresponding product with comparable efficiency. Furthermore, the synthetic application of the prepared product has been demonstrated by its transformation into a variety of synthetically useful molecules.

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.

Synthesis of Benzo-Fused Cyclic Ketones via Metal-Free Ring Expansion of Cyclopropanols Enabled by Proton-Coupled Electron Transfer

The metal-free ring expansion of cyclopropanols containing a pendant styrene moiety was successfully achieved using a proton-coupled electron transfer enabled by an organic photoredox catalyst. Through this, variants of 1-tetralone and 1-benzosuberone bearing a substituent at the benzylic position were selectively obtained through the regioselective ring closure of alkyl radical intermediates depending on the substitution pattern of the alkene moiety.

Asymmetric Intramolecular Dearomatization of Nonactivated Arenes with Ynamides for Rapid Assembly of Fused Ring System under Silver Catalysis

Arene dearomatization is a straightforward method for converting an aromatic feedstock into functionalized carbocycles. Enantioselective dearomatizations of chemically inert arenes, however, are quite limited and underexplored relative to those of phenols and indoles. We developed a method for diazo-free generation of silver-carbene species from an ynamide and applied it to the dearomatization of nonactivated arenes. Transiently generated norcaradiene could be trapped by intermolecular [4 + 2] cycloaddition, synthesizing polycycles with five consecutive stereogenic centers. This protocol constitutes the first highly enantioselective reaction based on the diazo-free generation of silver-carbene species. Mechanistic investigations revealed a dearomatization followed by two different classes of pericyclic reactions, as well as the origin of the chemo- and enantioselectivity.

Catalytic asymmetric dearomative [4 + 2] annulation of 2-nitrobenzofurans and 5H-thiazol-4-ones: stereoselective construction of dihydrobenzofuran-bridged polycyclic skeletons

An organocatalytic asymmetric dearomative [4 + 2] annulation of 2-nitrobenzofurans and 5H-thiazol-4-ones is developed for the construction of dihydrobenzofuran-bridged polycyclic skeletons with good results.

Enantioselective Desymmetrization of Bisphenol Derivatives via Ir-Catalyzed Allylic Dearomatization

Spirocyclic hexadienones with multiple stereogenic centers are frequently found in natural products but remain challenging targets to synthesize. Herein, we report the enantioselective desymmetrization of bisphenol derivatives via Ir-catalyzed allylic dearomatization reactions, affording spirocyclic hexadienone derivatives with up to three contiguous stereogenic centers in good yields (up to 90%) and excellent enantioselectivity (up to 99% ee). The high efficiency of this reaction is exemplified by the short reaction time (30 min), low catalyst loading (down to 0.2 mol %), and ability to perform the reaction on a gram-scale. The total syntheses of (+)-tatanan B and (+)-tatanan C were also realized using this Ir-catalyzed allylic dearomatization reaction as a key step.

Silver-Catalyzed para-Selective Amination and Aminative Dearomatization of Phenols with Azodicarboxylates in Water

An efficient silver-catalyzed para-selective amination and aminative dearomatization of phenols with azodicarboxylates was developed. It afforded the para-aminophenols or amino cyclohexadieneones from free phenols depending on whether it has a para-substituent. The reaction proceeded smoothly in water under simple and mild conditions, giving the highly selective products in good yields within a short reaction time. It also showed broad substrate scope and good functional group compatibility.

Atypical Dearomative Spirocyclization of β-Naphthols with Diazoacetamides Using a Silver Catalyst

A chemoselective dearomatization of the less reactive benzenoid unit in β-naphthol was developed. Spirocyclization with a reductant constructs a pivotal structure for drug candidates. One-pot oxidative conversion enabled the tandem dearomatization of β-naphthol, producing conjugated tetraenone variants. The potential utility of the product as an F^--selective anion sensor was also demonstrated. Theoretical studies revealed the intermediacy of silver-carbenoid species leading to chemoselective spirocyclization over arene cyclopropanation.

Activation of allylic esters in an intramolecular vinylogous kinetic resolution reaction with synergistic magnesium catalysts

Kinetic resolution (KR) of racemic starting materials is a powerful and practical alternative to prepare valuable enantiomerically enriched compounds. A magnesium-catalyzed kinetic resolution based on a designed intramolecular vinylogous Michael reaction is disclosed. Here we show a synergistic catalytic strategy based on the development of chiral ligands. Substrates containing linear allylic ester structures are designed and synthesized to construct key [6.6.5]-tricyclic chiral skeletons via this kinetic resolution process. Detailed mechanistic studies reveal a rational mechanism for the current intramolecular vinylogous KR reaction. The desired direct intramolecular asymmetric vinylogous Michael reaction of linear allylic esters is realized in high efficiency and enantioselectivity with the synergistic catalytic system.

Kinetic resolution allows to obtain enantioenriched compounds from racemic mixtures. Here, the authors report a synergistic magnesium catalyst promoting kinetic resolution of an intramolecular vinylogous Michael reaction to access [6.6.5]-tricyclic chiral skeletons.

Atroposelective synthesis of configurationally stable nonbiaryl N–C atropisomers through direct asymmetric aminations of 1,3-benzenediamines

An atroposelective synthesis of configurationally stable nonbiaryl N–C atropisomers through direct asymmetric aminations of 1,3-benzenediamines has been developed.

Chiral phosphoric acid-catalyzed asymmetric dearomatization reactions

We summarize in this review the recent development of chiral phosphoric acid (CPA)-catalyzed asymmetric dearomatization reactions.

Catalytic asymmetric multiple dearomatizations of phenols enabled by a cascade 1,8-addition and Diels-Alder reaction

A direct catalytic asymmetric multiple dearomatization reaction of phenols was disclosed, which provides expedient access to a series of architecturally complex polycyclic compounds bearing four stereogenic centers in high enantiopurity. The key to achieve such a transformation is the combination of a dearomative 1,8-addition of β-naphthols to para-quinone methides generated in situ from propargylic alcohols and a subsequent intramolecular dearomative Diels–Alder reaction. Noteworthily, this protocol enrichs not only the diversity of dearomatized products but also the toolbox of dearomatization strategies.

The first chiral phosphoric acid catalyzed asymmetric multiple dearomatizations of phenols for the synthesis of bridged polycyclic compounds are reported.

Atroposelective Synthesis of Biaryl Diamines and Amino Alcohols via Chiral Phosphoric Acid Catalyzed para-Aminations of Anilines and Phenols

A versatile method for atroposelective synthesis of chiral biaryl diamines and amino alcohols has been developed via para-amination of anilines and phenols with azodicarboxylates enabled by chiral phosphoric acid catalysis. Meanwhile, highly efficient kinetic resolution of the racemic biaryl anilines has also been realized through these reactions, giving selectivity factor up to 246. The gram-scale reaction and facile derivatizations of the chiral products well demonstrate the potential of these reactions in the development of novel chiral ligands and catalysts.

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Versatile methods for asymmetric synthesis of biaryl diamines and amino alcohols

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Atroposelective para-aminations of biaryl anilines and phenols

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Kinetic resolution of racemic biaryl anilines

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Facile transformations of chiral products