Aerobic oxidative Heck/dehydrogenation reactions of cyclohexenones: efficient access to meta-substituted phenols

A para- to meta-isomerization of phenols

Phenols and their derivatives are ubiquitous in nature and critically important industrial chemicals. Their properties are intimately linked to the relative substitution pattern of the aromatic ring, reflecting well-known electronic effects of the OH group. Because of these ortho-, para-directing effects, meta-substituted phenols have historically been more difficult to synthesize. Here we describe a procedure to transpose phenols that hinges on a regioselective diazotization of the corresponding ortho-quinone. The procedure affords the meta-substituted phenol directly from its more common and accessible para-substituted isomer, and demonstrates good chemoselectivity that enables its application in late-stage settings. By changing the electronic effect of the OH group and its trajectory of hydrogen bonding, our transposition can be used to diversify natural products and existing chemical libraries, and potentially shorten the length and cost of producing underrepresented arene isomers.

Tandem dehydrogenation-olefination-decarboxylation of cycloalkyl carboxylic acids via multifold C-H activation

Dehydrogenation chemistry has long been established as a fundamental aspect of organic synthesis, commonly encountered in carbonyl compounds. Transition metal catalysis revolutionized it, with strategies like transfer-dehydrogenation, single electron transfer and C-H activation. These approaches, extended to multiple dehydrogenations, can lead to aromatization. Dehydrogenative transformations of aliphatic carboxylic acids pose challenges, yet engineered ligands and metal catalysis can initiate dehydrogenation via C-H activation, though outcomes vary based on substrate structures. Herein, we have developed a catalytic system enabling cyclohexane carboxylic acids to undergo multifold C-H activation to furnish olefinated arenes, bypassing lactone formation. This showcases unique reactivity in aliphatic carboxylic acids, involving tandem dehydrogenation-olefination-decarboxylation-aromatization sequences, validated by control experiments and key intermediate isolation. For cyclopentane carboxylic acids, reluctant to aromatization, the catalytic system facilitates controlled dehydrogenation, providing difunctionalized cyclopentenes through tandem dehydrogenation-olefination-decarboxylation-allylic acyloxylation sequences. This transformation expands carboxylic acids into diverse molecular entities with wide applications, underscoring its importance.

Pd-Catalyzed Hydroboration of Vinylarenes with B2pin2

A palladium(II)-catalyzed Markovnikov hydroboration of aryl alkenes with readily available bis(pinacolato)diboron (B2pin2) is reported. The reaction proceeded with low catalyst loading (0.5 mol %) in the absence of N- or P-containing ligands, affording the products in up to 90% yield. Trifluoracetic acid serves as the hydrogen source, enabling the synthesis of benzylic boronic esters under mild ambient conditions.

Divergent Total Syntheses of Lycorine Alkaloids via a Sequential C-H Functionalization Strategy

A Pd-catalyzed one-pot sequential C-H functionalization strategy was utilized to prepare four lycorine alkaloids and one pseudo-lycorine alkaloid from the common intermediate 4. By switching the followed oxidative conditions of air, DMSO/H2O/I2, and DMSO/O2, based on the Pd(PPh3)4/K2CO3/toluene catalytic system, three key intermediates 12a, 12b, and 12c with different substitution patterns could be obtained in a well-controlled manner. As a result, four natural products γ-lycorane, hippadine, anhydrolycorinone, and anhydrolycorine as well as a pseudo-lycorine alkaloid Δ(4a,10b)-6-oxodihydrolycorine were successfully synthesized within 10 steps through this divergent route.

Protocol for the preparation and application of CeO2-CuO catalysts in the decarboxylative oxidation reaction of benzoic acids to phenols

Phenolic chemicals are important building blocks in chemical and material industries. In this protocol, we describe the preparation of CeO2-CuO catalysts and the application in the decarboxylative oxidation reaction of benzoic acids to phenols. Furthermore, we describe how to modify the basic sites of CeO2-CuO catalysts by CO2 treatment to increase the selectivity of phenol and the regeneration process of used catalyst. For complete details on the use and execution of this protocol, please refer to Du et al. (2023).1.

Zwitterionic Aqua Palladacycles with Noncovalent Interactions for meta-Selective Suzuki Coupling of 3,4-Dichlorophenol and 3,4-Dichlorobenzyl Alcohol in Water

The site-selective reaction of substrates with multiple reactive sites has been a focus of the current synthetic chemistry. The use of attractive noncovalent interactions between the catalyst and substrate is emerging as a versatile approach to address site-selectivity challenges. Herein, we designed and synthesized a series of palladacycles, to control meta-selective Suzuki coupling of 3,4-dichlorophenol and 3,4-dichlorobenzyl alcohol. Noncovalent interactions directed zwitterionic aqua palladacycles catalyzed meta-selective Suzuki couplings of 3,4-dichloroarenes bearing hydroxyl in water have been developed. Experiments and density functional theory (DFT) calculations demonstrated that the electrostatic interactions play a critical role in meta-selective coupling of 3,4-dichlorophenol, while meta-selective coupling of 3,4-dichlorobenzyl alcohol arises due to the hydrogen-bonding interactions.

Iron-Catalyzed Gamma-Gamma Dimerization of Siloxydienes

We report the oxidative dimerization reaction of siloxydienes derived from simple enones that creates a new gamma-gamma (γ-γ) C-C bond using catalytic iron and benzoyl peroxide as the terminal oxidant in acetonitrile solvent at ambient temperature. The reaction shows a broad substrate scope including cyclic and acyclic siloxydienes derived from ketones, aldehydes, and esters, which are converted to 1,8-dicarbonyl compounds under mild catalytic reaction conditions in 19-89 % yield across 30 examples. The method is suitable for the coupling of sterically demanding carbon centers, including the formation of vicinal quaternary centers. Conceptually, the dienol ether serves as a precursor to a conjugated radical cation, which undergoes highly site selective γ-dimerization reactions. The γ-γ dimerization strategy is applied to the synthesis of a bioactive analogue of honokiol.

Selective synthesis of meta-phenols from bio-benzoic acids via regulating the adsorption state

Phenols are important building blocks widely applied in many fields. The pronounced orientational effect of the phenolic hydroxyl group makes achieving selective synthesis of meta-phenols challenging. Accessing meta-phenols needs lengthy synthetic sequences. Herein, we first developed a heterogeneous CO2-mediated CeO2-5CuO catalyst for decarboxylative oxidation of benzoic acids with a more than 80% selectivity to meta-phenols. This technology is based on a traceless directing group relay method. The CeO2-CuO catalysts with different Ce/Cu ratios exhibited controllable reaction selectivity between decarboxylation and decarboxylative oxidation. Spectroscopy experiments and computational studies showed the adsorption state of benzoic acid was found to be crucial for subsequent reaction pathways. The moderate adsorption on CO2-mediated CeO2-5CuO catalyst contributes to the distinct selectivity of phenol. Furthermore, the paddlewheel intermediate facilitates the synthesis of meta-phenols from benzoic acids. This traceless directing group method would promote the development of useful one-pot meta-substituted phenols from bio-based benzoic acids.

meta-Selective C-H arylation of phenols via regiodiversion of electrophilic aromatic substitution

Electrophilic aromatic substitution is among the most widely used mechanistic manifolds in organic chemistry. Access to certain substitution patterns is, however, precluded by intrinsic and immutable substituent effects that ultimately restrict the diversity of the benzenoid chemical space. Here we demonstrate that the established regioselectivity of electrophilic aromatic substitution can be overcome simply by diverting the key σ-complex intermediate towards otherwise inaccessible substitution products. This 'regiodiversion' strategy is realized through the development of a general and concise method for the meta-selective C-H arylation of sterically congested phenols. Consisting of a Bi(V)-mediated electrophilic arylation and a subsequent aryl migration/rearomatization, our process is orthogonal to conventional C-H activation and cross-coupling approaches, and does not require prefunctionalization of the substrate. Mechanistically informed applications in synthesis showcase its utility as a versatile and enabling route to highly functionalized, contiguously substituted aromatic building blocks that defy synthesis via existing methods.

(NHC)Pd(II) hydride-catalyzed dehydroaromatization by olefin chain-walking isomerization and transfer-dehydrogenation

Transition-metal-catalyzed homogeneous dehydrogenation and isomerization are common organic molecular activation reactions. Palladium hydrides are good olefin isomerization catalysts but are usually short-lived species under redox-active dehydrogenation conditions. Here, we show that Pd-H in the presence of an N-heterocyclic carbene ligand and an alkene regulator enables transfer-dehydroaromatization, avoiding the homo-disproportionation pathway. The desired product is obtained with up to 99:1 selectivity, and the exo-to-endo olefin isomerization can be carried out in one pot. In contrast to previously reported methods that rely on the efficient removal of Pd-H, the approach reported herein benefits from the steric effects of the N-heterocyclic carbene and the choice of alkene to regulate the competing reactivity of allylic C‒H activation and hydropalladation. This method circumvents the challenges associated with tedious olefin separation and a low exo-to-endo olefin isomerization ratio and expands the scope to include challenging endo- and exo-cyclic olefins under mild, neutral, and oxidant-free conditions. Overall, herein, we provide a strategy to synthesize (hetero)aromatic compounds via chemoselective dehydrogenation of cyclic alkenes over ketones and the dehydrogenative Diels-Alder reaction of a cyclic enamine.

Aromatic compounds can be prepared via dehydrogenation of cyclic compounds. Here the authors report the dehydroaromatization of endocyclic and exocyclic olefins via chain-walking isomerization and transfer-dehydrogenation catalyzed by palladium N-heterocyclic carbene complexes in the presence of alkenes as sacrificial reagents.

Stereoselective [2+2]-Cycloadditions of chiral allenic ketones and alkenes: Applications towards the synthesis of benzocyclobutenes and endiandric acids

Cyclobutanes are important motifs that have found utility in many contexts. Prior work has demonstrated an enantioselective isomerization/stereoselective [2 + 2] as a means to access bicyclo [4.2.0] octanes. Herein, the utility of this method is demonstrated towards the synthesis of benzocyclobutenes and a key intermediate towards the endiandric acids.

Simple and Practical Conversion of Benzoic Acids to Phenols at Room Temperature

Phenols are important organic molecules because they have found widespread applications in many fields. Herein, an efficient and practical approach to prepare phenols from benzoic acids via simple organic reagents at room temperature is reported. This approach is compatible with various functional groups and heterocycles and can be easily scaled up. To demonstrate its synthetic utility, bioactive molecules and unsymmetrical hexaarylbenzenes have been prepared by leveraging this transformation as strategic steps. Mechanistic investigations suggest that the key migration step involves a free carbocation instead of a radical intermediate. Considering the abundance of benzoic acids and the utility of phenols, it is anticipated that this method will find broad applications in organic synthesis.

Synthesis of conjugated dienes via palladium-catalysed aerobic dehydrogenation of unsaturated acids and amides

A Pd(II)-catalyzed direct aerobic dehydrogenation of γ,δ-olefinic acids and amides has been demonstrated. The present protocol dehydrogenates the least acidic amides and acids, thus replacing the traditional enolate strategy for dehydrogenation. A broad spectrum of conjugated dienamides and dienoic acids were produced in good to excellent yields. A possible reaction mechanism was proposed and supported by deuterium labelling studies.

Palladium-Catalyzed Aerobic α,β-Dehydrogenation of Aliphatic Amides

A Pd(II)-catalyzed α,β-dehydrogenation of substituted aliphatic amides assisted by a reusable bis-chelating 8-aminoquinoline ligand is demonstrated. Broad spectra of β-substituted including olefin-substituted aliphatic amides are well tolerated. The present protocol efficiently dehydrogenates the less acidic aliphatic amides via the chelation-assisted β-C-H bond activation and replaces the traditional enolate-based strategy.

Visible-light-driven C(sp2)-H arylation of phenols with arylbromides enabled by electron donor-acceptor excitation

We have developed a catalyst-free visible-light-driven C(sp²)-H arylation of unprotected phenols with arylbromides to give 2-arylated phenols. This reaction proceeds through the excitation of an electron donor-acceptor complex between a phenolate and an arylbromide, electron transfer, and debrominative C(sp²)-C(sp²) coupling.

Insertion of ammonia into alkenes to build aromatic N-heterocycles

Ammonia is one of the most abundant and simple nitrogen sources with decent stability and reactivity. Direct insertion of ammonia into a carbon skeleton is an ideal approach to building valuable N-heterocycles for extensive applications with unprecedented atom and step economy. Here, we show an electrochemical dehydrogenative method in which ammonia is inserted directly into alkenes to build aromatic N-heterocycles in a single step without the use of any external oxidant. This new approach achieves 98–99.2% atom economy with hydrogen as the only byproduct. Quinoline and pyridine with diverse substitutions are readily available. In this work, electrochemistry was used to drive a 4-electron oxidation reaction that is hard to access by other protocols, providing a parallel pathway to nitrene chemistry. In a tandem transformation that included three distinct electrochemical processes, the insertion of ammonia further showcased the tremendous potential to manipulate heterocycles derived from Hantzsch ester to diazine via pyridine and pyrrole.

Aromatic heterocycles containing nitrogen are ubiquitous in biologically relevant small molecules. Here the authors show an unorthodox methodology for their synthesis, by inserting the nitrogen atom into a carbon ring, with ammonia in electrochemical conditions.

Synthesis of meta-(Indol-3-yl)phenols from Indoles and Cyclohexenone via Palladium(II)-Catalyzed Oxidative Heck Reaction and Dehydrogenative Aromatization in a One-Step Sequence

Facile construction of a meta-(indol-3-yl)phenol framework with a wide substrate scope (a total of 25 compounds) via a palladium(II)-catalyzed oxidative Heck reaction and dehydrogenative aromatization in a one-step sequence is reported. This methodology affords a novel route for the privileged structures that are challenging to access via a direct link between indole and phenol, in a highly efficient and atom-economical manner.

Rhodium-Catalyzed Dealkenylative Arylation of Alkenes with Arylboronic Compounds

The C-C bond formation reaction represents a fundamental and important transformation in synthetic chemistry, and exploring new types of C-C bond formation reactions is recognized as appealing, yet challenging. Herein, we disclose the first example of rhodium-catalyzed dealkenylative arylation of alkenes with arylboronic compounds, thereby providing an unconventional access to bi(hetero)aryls with excellent chemoselectivity. In this method, C(aryl)-C(alkenyl) and C(alkenyl)-C(alkenyl) bonds in various alkenes and 1,3-dienes can be cleaved via a hydrometalation and followed by β-carbon elimination pathway for Suzuki-Miyaura reactions. Furthermore, a series of novel organic fluorescent molecules with excellent photophysical properties has been efficiently constructed with this protocol.

Palladium mediated one-pot synthesis of 3-aryl-cyclohexenones and 1,5-diketones from allyl alcohols and aryl ketones

One-pot synthesis of Robinson annulated 3-aryl-cyclohexenones from allyl alcohols and ketones using palladium is reported. Long chain aliphatic or aryl substitutions at the C1 position of allyl alcohol result in the formation of 1,5-diketone products. This simple one-pot method avoids the use of highly electrophilic vinyl ketones.

Pd-Catalyzed tandem C–C/C–O/C–H single bond cleavage of 3-allyloxybenzocyclobutenols

A Pd-catalyzed skeletal rearrangement of 3-allyloxybenzocyclobutenols was achieved, which involved tandem C–C/C–O bond cleavage and C–H allylic substitution.

Silver(I)-Catalyzed Enyne Cyclization/Aromatization of Alkyne-Tethered Cyclohexadienones to Access Meta-Substituted Phenols

Herein we report a highly regioselective silver(I)-catalyzed intramolecular annulation of alkyne-tethered cyclohexadienones to access meta-substituted phenols with enone functionality, which are difficult to synthesize from conventional methods. The reaction proceeds via intramolecular 1,6-enyne cyclization followed by aromatization and subsequent oxetene ring rearrangement. This strategy has also been compatible with a wide range of C-tethered cyclohexadienones to afford indanes in high yields. The unique functionality of products allows further transformations to expand the diversity.

Palladium-Catalyzed Regioselective Coupling Cyclohexenone into Indoles: Atom-Economic Synthesis of β-Indolyl Cyclohexenones and Derivatization Applications

Herein, we report a palladium-catalyzed dehydrogenative cross-coupling of indoles with cyclic enones to give β-indolyl cyclic enones under mild and neutral reaction conditions. The key to the success is to explore a mild condition, which ensures the indole C-H activation and subsequent syn β-hydride elimination through rapid enolization isomerization of Pd(II)-enolate while suppressing other undesired side reactions. Synthetic utility has also been demonstrated in the flexible transformation of the coupling products to meta-phenols and benzo[a]carbazoles.

Synthesis of unsymmetrically substituted triarylamines via acceptorless dehydrogenative aromatization using a Pd/C and p-toluenesulfonic acid hybrid relay catalyst

An efficient and convenient procedure for synthesizing triarylamines based on a dehydrogenative aromatization strategy has been developed. A hybrid relay catalyst comprising carbon-supported Pd (Pd/C) and p-toluenesulfonic acid (TsOH) was found to be effective for synthesizing a variety of triarylamines bearing different aryl groups starting from arylamines (diarylamines or anilines), using cyclohexanones as the arylation sources under acceptorless conditions with the release of gaseous H₂. The proposed reaction comprises the following relay steps: condensation of arylamines and cyclohexanones to produce imines or enamines, dehydrogenative aromatization of the imines or enamines over Pd nanoparticles (NPs), and elimination of H₂ from the Pd NPs. In this study, an interesting finding was obtained indicating that TsOH may promote the dehydrogenation.

An efficient and convenient procedure for synthesizing triarylamines based on a dehydrogenative aromatization strategy has been developed.

Palladium-catalyzed aerobic synthesis of ortho-substituted phenols from cyclohexanones and primary alcohols

An aerobic palladium catalyzed site-specific synthesis of ortho-substituted phenols from cyclohexanones and primary alcohols via an oxidation/aldol/dehydration/aromatization process has been developed.

Rhodium(III)-Catalyzed Meta-Selective C-H Alkenylation of Phenol Derivatives

Rhodium(III)-catalyzed remote meta-selective-C-H alkenylation of phenol derivatives has been developed using a traceless organosilicon template as the directing group. This transformation proceeds smoothly with good yields and high meta-selectivities toward a series of phenol and alkene substrates. In addition, this protocol provides an effective strategy for late-stage transformations of various meta-alkenylated aromatic compounds.

Copper-Catalyzed Dehydrogenative Diels-Alder Reaction

A practical and effective copper-catalyzed dehydrogenative Diels-Alder reaction of gem-diesters and ketone with dienes has been established. The active dienophiles were generated in situ via a radical-based dehydrogenation process, which reacted with a wide variety of dienes to afford various polysubstituted cyclohexene derivatives in good to excellent yields.