Asymmetric Arylative Dearomatization of β-Naphthols Catalyzed by a Chiral Phosphoric Acid

Intermolecular Asymmetric Arylative Dearomatization of 1-Naphthols

Arylative dearomatization forms quaternary stereocenters in cyclic systems with the concomitant introduction of an aromatic ring. Pd-catalyzed arylative dearomatization, which uses conditions analogous to cross-coupling, has emerged as a powerful method in an intramolecular context. But translating this from intramolecular cyclizations to an intermolecular process has proven extremely challenging: examples are scarce, and those that exist have not been rendered enantioselective, despite the potential for broad application in medicinal chemistry and natural product synthesis. We describe a strategy that utilizes attractive interactions between the ligand and substrate to overcome this challenge and promote intermolecular, highly enantioselective arylative dearomatization of naphthols using a broad range of aryl bromide electrophiles. Crucial to success is the use of the readily accessed sulfonated chiral phosphine sSPhos, which we believe engages in attractive electrostatic interactions with the substrate. Not only does sSPhos control enantioselectivity but it also drastically accelerates the reaction, most likely by facilitating the challenging palladation step that initiates dearomatization.

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.

Phosphine-catalysed transformations of ortho- and para-quinone methides

Organocatalytic methodologies for the derivatization of o-QM, p-QM and the analogous aza-QM have been recently developed and involve different catalytic systems such as phosphoric acids, thioureas, squaramides, NHC carbenes or chiral ammonium salts. Besides, phosphines, commonly used as ligands in metal-catalysed reactions, can be also used as organocatalysts. In this case, they are mainly involved as nucleophilic catalysts in reactions such as the Rauhut-Currier (RC) reaction. In this review, an analysis of the recent developments in racemic and enantioselective phosphine-catalysed transformations of o-QM, p-QM and aza-o-QM has been carried out.

Pd-Catalyzed intermolecular asymmetric allylic dearomatization of 1-nitro-2-naphthols with MBH adducts

An asymmetric allylic dearomatization reaction of 1-nitro-2-naphthol derivatives with Morita-Baylis-Hillman (MBH) adducts has been developed. By utilizing Pd catalyst derived from Pd(OAc)₂ and Trost ligand (R,R)-L1, the reaction proceeded smoothly in 1,4-dioxane at room temperature, affording substituted β-naphthalenones in good yields (up to 92%) and enantioselectivity (up to 90% ee). A range of substituted 1-nitro-2-naphthols and MBH adducts were found to be compatible under the optimized conditions. This reaction provides a convenient method for the synthesis of enantioenriched 1-nitro-β-naphthalenone derivatives.

Acetic Acid-Catalyzed Regioselective C(sp2)-H Bond Functionalization of Indolizines: Concomitant Involvement of Synthetic and Theoretical Studies

An atom economical and environmentally benign protocol has been developed for the regioselective C(sp²)-H bond functionalization of indolizines. The acetic acid-catalyzed cross-coupling reaction proceeds under metal-free conditions, producing a wide range of synthetically useful indolizine derivatives. The present protocol showed good functional group tolerance and broad substrate scope in good to excellent yields. Quantum mechanical investigation using density functional theory (DFT) has played a crucial role in understanding that acetic acid is the key player in determining the actual pathway as the catalyst and its ultrafast nature. Different pathways involving inter- and intramolecular proton transfer, with or without acetic acid, were investigated. Calculated results revealed that a proton shuttle mechanism is involved for the least energetic, most favorable acetic acid-catalyzed pathway. Furthermore, regioselectivity has also been explained theoretically.

Hydrogen Bond Assisted Central-to-Spiro Chirality Transfer and Central-to-Axial Chirality Conversion: Asymmetric Synthesis of Spirocycles

The asymmetric construction of chiral spiroenones bearing both axial and spiro-central chirality has been established for the first time by a central-to-spiro chirality transfer and a central-to-axial chirality conversion from chiral 2,3-diarylbenzoindolines. Mechanistic studies indicate the hydrogen bonds play important roles in this process, providing an efficient strategy for enantioselective construction of spirocyclic backbones via simultaneously controlling spiro-central and axial chirality in one operation.

Cu-Catalyzed Chemodivergent, Stereoselective Propargylic Dearomatization and Etherification of 2-Naphthols

The first stereoselective propargylic dearomatization of 2-naphthol derivatives is reported using a chiral Cu^II-L10 complex. The reaction shows chemodivergent reactivity and produced propargyl dearomatization and etherification product for differently substituted 2-naphthols. Both the reactions generate the desired products in high yields with excellent chemo- and stereoselectivities (up to 99% ee, dr = 9:1) by using only 2 mol % catalyst loading. Dearomatization products contain a contiguous all-carbon quaternary-tertiary stereocenter and a terminal alkyne functionality.

Asymmetric Dearomative (3+2)-Cycloaddition Involving Nitro-Substituted Benzoheteroarenes under H-Bonding Catalysis

In our studies, the organocatalytic 1,3-dipolar cycloaddition between 2-nitrobenzofurans or 2-nitrobenzothiophene and N-2,2,2-trifluoroethyl-substituted isatin imines has been developed. The reaction has been realized by employing bifunctional organocatalysis, with the use of squaramide derivative being crucial for the stereochemical efficiency of the process. The usefulness of the cycloadducts obtained has been confirmed in selected transformations, including aromative and non-aromative removal of the nitro group.

Regioselective benzoyloxylative dearomatization of naphthols by benzoyl peroxide under catalyst-free conditions

A direct regioselective benzoyloxylative dearomatization of both α- and β-naphthols by benzoyl peroxide under an air atmosphere, and radical inhibitor- and catalyst-free conditions at room temperature is described. The methodology provides a new efficient strategy for the construction of α-ketol derivatives bearing an oxo-quaternary carbon center from naphthols with good to excellent yields.

Catalytic asymmetric 1,4-type Friedel–Crafts (hetero)arylations of 1-azadienes: the highly enantioselective syntheses of chiral hetero-triarylmethanes

Strategies for achieving the direct catalytic asymmetric syntheses of benzofuran-containing hetero-triarylmethanes using a 1,4-type Friedel–Crafts (hetero)arylation reaction were 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.

Synthesis of chiral [2,3]-fused indolines through enantioselective dearomatization inverse-electron-demand Diels–Alder reaction/oxidation of indoles with 2-(2-nitrovinyl)-1,4-benzoquinone

Enantioselective dearomatization inverse-electron-demand Diels–Alder reaction/oxidation of indoles with 2-(2-nitrovinyl)-1,4-benzoquinone provided various novel enantioenriched [2,3]-fused indolines.

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.

Chiral Phosphoric Acid-Catalyzed Enantioselective Direct Arylation of Iminoquinones: A Case Study of the Model Selectivity

Chiral phosphoric acid (CPA)-catalyzed enantioselective arylation reactions have attracted immense attention recently. However, the preferential activation model in the stereodetermining step is controversial, and hence, the origin of enantioselectivity is still far from being understood. Two stereochemical models are provided on the basis of the asymmetric arylations of iminoquinones with naphthylamines (reaction 1) or naphthols (reaction 2) catalyzed by (R/S)-TRIP to explain the high enantioselectivity and the effect of CPAs scaffolds. Unexpectedly, our calculations reveal that substrate naphthylamines or naphthols prefer enantioselective aminal formation model II or 1,4-addition model I, respectively, which is the reverse of Tan's and Xu's model. The different noncovalent and steric interactions between catalysts and substrates are responsible for the observed model preference. Moreover, the enantioselectivity arises from distortion (reaction 1) and noncovalent interactions (reaction 2) that discriminate between the diastereomeric transition states. We further investigated the effect of SPINOL-based CPAs on the enantioselectivity and found that the more rigid skeleton and a smaller binding pocket lead to lower enantioselectivity as compared with that of BINOL-based CPA. The new insights into the reaction activation model rationalize the stereoselectivity outcome of direct asymmetric arylation reactions, and our general model can be extended to related transformations.

Chiral Phosphoric-Acid-Catalyzed Cascade Prins Cyclization

Asymmetric Prins cyclization of in situ generated quinone methides and o-aminobenzaldehyde has been developed with chiral phosphoric acid as an efficient catalyst. This unconventional method provides a facile access to diverse functionalized trans-fused pyrano-/furo-tetrahydroquinoline derivatives in excellent yield and with excellent diastereo- and enantioselectivities (up to 99% yield and 99% ee). Mechanistic studies suggested that the three adjacent tertiary stereocenters were constructed through the sequential formation of C-O, C-C, and C-N bonds.

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

Asymmetric dearomatization reactions have recently emerged as a powerful tool for the rapid build-up of the molecular complexity. Chiral three-dimensional polycyclic molecules bearing contiguous stereogenic centers can be synthesized from readily available planar aromatic feedstocks. Here we report that an intermolecular asymmetric dearomatization reaction of α-naphthols bearing a tethered nucleophile at the C4 position of the naphthol ring is achieved by a chiral phosphoric acid. The reaction proceeds via a highly chemo- and regioselective aminative dearomatization/Michael addition sequence, affording a wide array of functionalized cyclic ketones in good yields (up to 93%) with excellent enantioselectivity (up to >99% ee). The catalyst loading can be reduced to 0.1 mol%. Preliminary mechanistic investigations identify that the enantioselectivity is established in the dearomatization step, while the Michael addition is the rate-limiting step. A working model accounting for the origin of the stereochemistry is proposed based on DFT calculations.

Enantioselective [1,3] O-to-C rearrangement: dearomatization of alkyl 2-allyloxy/benzyloxy-1/3-naphthoates catalyzed by a chiral π-Cu(ii) complex

An unprecedented catalytic asymmetric [1,3] O-to-C rearrangement of alkyl 2-allyloxy/benzyloxy-1/3-naphthoates was realized under the catalysis of a chiral π-Cu(ii) complex (1-10 mol%). This dearomatization strategy provides facile access to highly functionalized β-naphthalenone derivatives bearing an all-carbon quaternary stereogenic center in high yield with excellent enantioselectivity. The π-cation interaction between the aromatic substituent of the ligand and the Cu(ii) center was proved by X-ray diffraction analysis and shown to be crucial for enantioselective control. Further preliminary mechanistic studies suggest that this intramolecular reaction proceeds through a contact ion pair intermediate.

Organocatalyzed Intermolecular Asymmetric Allylic Dearomatization of Both α- and β-Naphthols

The first highly stereoselective intermolecular catalytic asymmetric dearomatization (CADA) of α-naphthols through C-C formation and the first asymmetric allylic dearomatization of naphthols by chiral organocatalysis have been achieved. These new and complete atom-economic reactions provide enantioriched α- and β-naphthalenones bearing an all-carbon quaternary center.

Enantioselective Organocatalytic Sulfenylation of β-Naphthols

An enantioselective sulfenylation of β-naphthols has been developed for the first time using a newly synthesized cinchona-derived thiourea as the catalyst and N-(arylthio) succinimide (or phthalimide) as an electrophilic sulfur source. Various enantioenriched naphthalenones with an S-containing all-substituted stereocenter were prepared via a dearomatization strategy under mild reaction conditions.

Synthesis of benzannulated spiroketals with gold-catalyzed cycloisomerization/spiroketalization cascade

An unprecedented gold-catalyzed cascade reaction of alkynyl alcohol and quinone monoimines to diverse 5,5-benzannulated spiroketals has been successfully developed.

Dearomatization of naphthols using oxy-allyl cations: efficient construction of α-all-carbon quaternary center-containing 2-(2-oxocycloalkyl)cycloalkyl diketones

An efficient strategy for the dearomatization of both α- and β-naphthols using in situ generated oxy-allyl cations is reported.

Chiral N,N′-dioxide/Sc(OTf)3 complex-catalyzed asymmetric dearomatization of β-naphthols

A highly enantioselective and Z-selective dearomatization of β-naphthols with alkynones has been accomplished using a chiral N,N′-dioxide–Sc(OTf)₃ complex with excellent outcomes.