Metal-free oxidation of aromatic carbon-hydrogen bonds through a reverse-rebound mechanism

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.

Direct C-H Hydroxylation of N-Heteroarenes and Benzenes via Base-Catalyzed Halogen Transfer

Hydroxylated (hetero)arenes are valued in many industries as both key constituents of end products and diversifiable synthetic building blocks. Accordingly, the development of reactions that complement and address the limitations of existing methods for the introduction of aromatic hydroxyl groups is an important goal. To this end, we apply base-catalyzed halogen transfer (X-transfer) to enable the direct C-H hydroxylation of mildly acidic N-heteroarenes and benzenes. This protocol employs an alkoxide base to catalyze X-transfer from sacrificial 2-halothiophene oxidants to aryl substrates, forming SNAr-active intermediates that undergo nucleophilic hydroxylation. Key to this process is the use of 2-phenylethanol as an inexpensive hydroxide surrogate that, after aromatic substitution and rapid elimination, provides the hydroxylated arene and styrene byproduct. Use of simple 2-halothiophenes allows for C-H hydroxylation of 6-membered N-heteroarenes and 1,3-azole derivatives, while a rationally designed 2-halobenzothiophene oxidant extends the scope to electron-deficient benzene substrates. Mechanistic studies indicate that aromatic X-transfer is reversible, suggesting that the deprotonation, halogenation, and substitution steps operate in synergy, manifesting in unique selectivity trends that are not necessarily dependent on the most acidic aryl position. The utility of this method is further demonstrated through streamlined target molecule syntheses, examples of regioselectivity that contrast alternative C-H hydroxylation methods, and the scalable recycling of the thiophene oxidants.

Synthesis of meta-carbonyl phenols and anilines

Phenols and anilines are of extreme importance for medicinal chemistry and material science. The development of efficient approaches to prepare both compounds has thus long been a vital research topic. The utility of phenols and anilines directly reflects the identity and pattern of substituents on the benzenoid ring. Electrophilic substitutions remain among the most powerful synthetic methods to substituted phenols and anilines, yet in principle achieving ortho- and para-substituted products. Therefore, the selective preparation of meta-substituted phenols and anilines is the most significant challenge. We herein report an efficient copper-catalyzed dehydrogenation strategy to exclusively synthesize meta-carbonyl phenols and anilines from carbonyl substituted cyclohexanes. Mechanistic studies indicate that this transformation undergoes a copper-catalyzed dehydrogenation/allylic hydroxylation or amination/oxidative dehydrogenation/aromatization cascade process.

Late-stage modification of bioactive compounds: Improving druggability through efficient molecular editing

Synthetic chemistry plays an indispensable role in drug discovery, contributing to hit compounds identification, lead compounds optimization, candidate drugs preparation, and so on. As Nobel Prize laureate James Black emphasized, "the most fruitful basis for the discovery of a new drug is to start with an old drug"1. Late-stage modification or functionalization of drugs, natural products and bioactive compounds have garnered significant interest due to its ability to introduce diverse elements into bioactive compounds promptly. Such modifications alter the chemical space and physiochemical properties of these compounds, ultimately influencing their potency and druggability. To enrich a toolbox of chemical modification methods for drug discovery, this review focuses on the incorporation of halogen, oxygen, and nitrogen-the ubiquitous elements in pharmacophore components of the marketed drugs-through late-stage modification in recent two decades, and discusses the state and challenges faced in these fields. We also emphasize that increasing cooperation between chemists and pharmacists may be conducive to the rapid discovery of new activities of the functionalized molecules. Ultimately, we hope this review would serve as a valuable resource, facilitating the application of late-stage modification in the construction of novel molecules and inspiring innovative concepts for designing and building new drugs.

Strategic application of C-H oxidation in natural product total synthesis

The oxidation of unactivated C-H bonds has emerged as an effective tactic in natural product synthesis and has altered how chemists approach the synthesis of complex molecules. The use of C-H oxidation methods has simplified the process of synthesis planning by expanding the choice of starting materials, limiting functional group interconversion and protecting group manipulations, and enabling late-stage diversification. In this Review, we propose classifications for C-H oxidations on the basis of their strategic purpose: type 1, which installs functionality that is used to establish the carbon skeleton of the target; type 2, which is used to construct a heterocyclic ring; and type 3, which installs peripheral functional groups. The reactions are further divided based on whether they are directed or undirected. For each classification, examples from recent literature are analysed. Finally, we provide two case studies of syntheses from our laboratory that were streamlined by the judicious use of C-H oxidation reactions.

Deoxyfluorination of phenols for chemoselective 18F-labeling of peptides

The challenge of forming C-18F bonds is often a bottleneck in the development of new 18F-labeled tracer molecules for noninvasive functional imaging studies using positron emission tomography (PET). Nucleophilic aromatic substitution is the most widely employed reaction to functionalize aromatic substrates with the radioactive fluorine-18 but its scope is restricted to arenes containing electron-withdrawing substituents. Furthermore, many protic functional groups are incompatible with basic fluoride anions. Peptide substrates, which are highly desirable targets for PET molecular imaging, are particularly challenging to label with fluorine-18 because they are densely functionalized and sensitive to high temperatures and basic conditions. To expand the utility of nucleophilic aromatic substitution with fluorine-18, we describe two complementary procedures for the radiodeoxyfluorination of bench-stable and easy-to-access phenols that ensure rapid access to densely functionalized electron-rich and electron-poor 18F-aryl fluorides. The first procedure details the synthesis of an 18F-synthon and its subsequent ligation to the cysteine residue of Arg-Gly-Asp-Cys in 10.5 h from commercially available starting materials (189-min radiosynthesis). The second procedure describes the incorporation of commercially available CpRu(Fmoc-tyrosine)OTf into a fully protected peptide Lys-Met-Glu-(CpRu-Tyr)-Leu via solid-phase peptide synthesis and subsequent ruthenium-mediated uronium deoxyfluorination with fluorine-18 followed by deprotection, accomplished within 7 d (116-min radiosynthesis). Both radiolabeling methods are highly chemoselective and have conveniently been automated using commercially available radiosynthesis equipment so that the procedures described can be employed for the synthesis of peptide-based PET probes for in vivo imaging studies according to as low as reasonably achievable (ALARA) principles.

Biomimetic design of an α-ketoacylphosphonium-based light-activated oxygenation auxiliary

The biomimetic design of a transition metal complex based on the iron(iv)-oxo porphyrin π-cation radical species in cytochrome P450 enzymes has been studied extensively. Herein, we translate the functions of this iron(iv)-oxo porphyrin π-cation radical species to an α-ketoacyl phosphonium species comprised of non-metal atoms and utilize it as a light-activated oxygenation auxiliary for ortho-selective oxygenation of anilines. Visible light irradiation converts the α-ketoacyl phosphonium species to the excited state, which acts as a transiently generated oxidant. The intramolecular nature of the process ensures high regioselectivity and chemoselectivity. The auxiliary is easily removable. A one-pot protocol is also described.

Concise Total Syntheses of the 6-7-5 Hamigeran Natural Products

Herein, we report the total syntheses of four hamigeran natural products featuring a 6-7-5 tricyclic carbon skeleton. We utilized a palladium-catalyzed intramolecular cyclopropanol ring opening cross-coupling to build the central seven-membered ring and a series of oxidations including a challenging aromatic C-H oxidation to introduce the peripheral functionalities. This approach enabled us to achieve the first total syntheses of hamigeran C (14 steps), debromohamigeran I (12 steps), and hamigeran I (13 steps). Our synthesis also resulted in hamigeran G in 13 steps, which is significantly shorter than the previously reported one (24 steps, longest linear sequence).

Iron-Catalyzed Hydrobenzylation: Stereoselective Synthesis of (-)-Eugenial C

Metal-hydride hydrogen atom transfer (MHAT) has emerged as a useful tool to form quaternary carbons from alkenes via hydrofunctionalization. Methods to date that cross-couple alkenes with sp3 partners rely on heterobimetallic catalysis to merge the two cycles. Here, we report an iron-only cross-coupling via putative MHAT/SH2 steps that solves a key stereochemical problem in the synthesis of meroterpenoid eugenial C and obviates the need for nickel. The concise synthesis benefits from a conformationally locked o,o'-disubstituted benzyl bromide and a locally sourced chiral pool terpene coupling partner.

Regiodivergent and Enantioselective Hydroxylation of C-H bonds by Synergistic Use of Protein Engineering and Exogenous Dual-Functional Small Molecules

It is a great challenge to optionally access diverse hydroxylation products from a given substrate bearing multiple reaction sites of sp³ and sp² C-H bonds. Herein, we report the highly selective divergent hydroxylation of alkylbenzenes by an engineered P450 peroxygenase driven by a dual-functional small molecule (DFSM). Using combinations of various P450BM3 variants with DFSMs enabled access to more than half of all possible hydroxylated products from each substrate with excellent regioselectivity (up to >99 %), enantioselectivity (up to >99 % ee), and high total turnover numbers (up to 80963). Crystal structure analysis, molecular dynamic simulations, and theoretical calculations revealed that synergistic effects between exogenous DFSMs and the protein environment controlled regio- and enantioselectivity. This work has implications for exogenous-molecule-modulated enzymatic regiodivergent and enantioselective hydroxylation with potential applications in synthetic chemistry.

4,4′-(Butane-1,4-diyl)bis(4-methyl-1,2-dioxolane-3,5-dione)

Over the past decades, studies of cyclic diacyl peroxides have shown superior or even fundamentally new reactivity compared to their acyclic counterparts in various reactions. Previously, the scope of cyclic diacyl peroxides was limited to the mono peroxy compounds. The first doubled cyclic diacyl peroxide is presented herein. The diperoxide was characterized by NMR spectroscopy, mass spectrometry, and IR spectroscopy. The structure of 4,4′-(butane-1,4-diyl)bis(4-methyl-1,2-dioxolane-3,5-dione) was confirmed by X-ray diffraction analysis. The novel diperoxide was prepared in a 55% overall yield in three steps from dibromobutane and diethyl methylmalonate.

Activation of O-Electrophiles via Structural and Solvent Effects: SN2@O Reaction of Cyclic Diacyl Peroxides with Enol Acetates

The reactions of O-electrophiles, such as organic peroxides, with carbon nucleophiles are an umpolung alternative to the common approaches to C-O bond formation. Nucleophilic substitution at the oxygen atom of cyclic diacyl peroxides by enol acetates with the following deacylation leads to α-acyloxyketones with an appended carboxylic acid in 28-87% yields. The effect of fluorinated alcohols on the oxidative functionalization of enol acetates by cyclic diacyl peroxides was studied experimentally and computationally. Computational analysis reveals that the key step proceeds as a direct substitution nucleophilic bimolecular (S_N2) reaction at oxygen (S_N2@O). CF₃CH₂OH has a dual role in assisting in both steps of the reaction cascade: it lowers the energy of the S_N2@O activation step by hydrogen bonding to a remote carbonyl and promotes the deacylation of the cationic intermediate.

Unified Total Syntheses of Benzenoid Cephalotane-Type Norditerpenoids: Cephanolides and Ceforalides

Detailed herein are our synthetic studies toward the preparation of the C₁₈- and C₁₉-benzenoid cephalotane-type norditerpenoids. Guided by chemical network analysis, the core structure of this natural product family was constructed in a concise manner using an iterative cross-coupling, followed by a formal inverse-electron-demand [4 + 2] cycloaddition. Initial efforts to functionalize an alkene group in the [4 + 2] cycloadduct using a Mukaiyama hydration and a subsequent olefination led to the complete C₁₈-carbon framework. While effective, this approach proved lengthy and prompted the development of a direct alkene difunctionalization that relies on borocupration to advance the cycloadduct to the natural products. Late-stage peripheral C-H functionalization facilitated access to all of the known cephanolides in 6-10 steps as well as five recently isolated ceforalides in 8-13 steps.

Site-Selective C-H Arylation of Diverse Arenes Ortho to Small Alkyl Groups

Catalytic systems for direct C-H activation of arenes commonly show preference for electronically activated and sterically exposed C-H sites. Here we show that a range of functionally rich and pharmaceutically relevant arene classes can undergo site-selective C-H arylation ortho to small alkyl substituents, preferably endocyclic methylene groups. The C-H activation is experimentally supported as being the selectivity-determining step, while computational studies of the transition state models indicate the relevance of non-covalent interactions between the catalyst and the methylene group of the substrate. Our results suggest that preference for C(sp² )-H activation next to alkyl groups could be a general selectivity mode, distinct from common steric and electronic factors.

Intermetallic Copper-Based Electride Catalyst with High Activity for C-H Oxidation and Cycloaddition of CO2 into Epoxides

Inorganic electrides have been proved to be efficient hosts for incorporating transition metals, which can effectively act as active sites giving an outstanding catalytic performance. Here, it is demonstrated that a reusable and recyclable (for more than 7 times) copper-based intermetallic electride catalyst (LaCu0.67 Si1.33 ), in which the Cu sites activated by anionic electrons with low-work function are uniformly dispersed in the lattice framework, shows vast potential for the selective C-H oxidation of industrially important hydrocarbons and cycloaddition of CO2 with epoxide. This leads to the production of value-added cyclic carbonates under mild reaction conditions. Importantly, the LaCu0.67 Si1.33 catalyst enables much higher turnover frequencies for the C-H oxidation (up to 25 276 h-1 ) and cycloaddition of CO2 into epoxide (up to 800 000 h-1 ), thus exceeding most nonnoble as well as noble metal catalysts. Density functional theory investigations have revealed that the LaCu0.67 Si1.33 catalyst is involved in the conversion of N-hydroxyphthalimide (NHPI) into the phthalimido-N-oxyl (PINO), which then triggers selective abstraction of an H atom from ethylbenzene for the generation of a radical susceptible to further oxygenation in the presence of O2 .

Antiviral Profiling of C-18- or C-19-Functionalized Semisynthetic Abietane Diterpenoids

Viral infections affect several million patients annually. Although hundreds of viruses are known to be pathogenic, only a few can be treated in the clinic with available antiviral drugs. Naturally based pharmacotherapy may be a proper alternative for treating viral diseases. Several natural and semisynthetic abietane-type diterpenoids have shown important antiviral activities. In this study, a biological evaluation of a number of either C-18- or C-19-functionalized known semisynthetic abietanes against Zika virus, Dengue virus, Herpes virus simplex type 1, and Chikungunya virus are reported. Semisynthetic abietane ferruginol and its analogue 18-(phthalimid-2-yl)ferruginol displayed broad-spectrum antiviral properties. The scale-up synthesis of this analogue has been optimized for further studies and development. This molecule displayed an EC₅₀ between 5.0 and 10.0 μM against Colombian Zika virus strains and EC₅₀ = 9.8 μM against Chikungunya virus. Knowing that this ferruginol analogue is also active against Dengue virus type 2 (EC₅₀ = 1.4 μM, DENV-2), we can conclude that this compound is a promising broad-spectrum antiviral agent paving the way for the development of novel antivirals.

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.

HFIP in Organic Synthesis

1,1,1,3,3,3-Hexafluoroisopropanol (HFIP) is a polar, strongly hydrogen bond-donating solvent that has found numerous uses in organic synthesis due to its ability to stabilize ionic species, transfer protons, and engage in a range of other intermolecular interactions. The use of this solvent has exponentially increased in the past decade and has become a solvent of choice in some areas, such as C-H functionalization chemistry. In this review, following a brief history of HFIP in organic synthesis and an overview of its physical properties, literature examples of organic reactions using HFIP as a solvent or an additive are presented, emphasizing the effect of solvent of each reaction.

Electrochemical aromatic C-H hydroxylation in continuous flow

The direct hydroxylation of arene C–H bonds is a highly sought-after transformation but remains an unsolved challenge due to the difficulty in efficient and regioselective C–H oxygenation and high reactivity of the phenolic products leading to overoxidation. Herein we report electrochemical C–H hydroxylation of arenes in continuous flow for the synthesis of phenols. The method is characterized by broad scope (compatible with arenes of diverse electronic properties), mild conditions without any catalysts or chemical oxidants, and excellent scalability as demonstrated by the continuous production of 1 mol (204 grams) of one of the phenol products.

The direct hydroxylation of arene C–H bonds is a highly sought-after transformation but with little literature precedent. Herein the authors report a scalable electrochemical C–H hydroxylation of arenes in continuous flow for the synthesis of phenols.

Carboxylate as a Non-innocent L-Ligand: Computational and Experimental Search for Metal-Bound Carboxylate Radicals

We show that the carboxylate radical acts as an L-ligand with certain high-spin transition metal centers. Such coordination preserves the O-radical character needed for C-H activation via hydrogen atom transfer. Capture of the new C-radical by the metal and subsequent reductive elimination leads to formal C-H acyloxylation. Decarboxylation of the RCO₂ radical is minimized through hybridization effects introduced by spiro-cyclopropyl moiety.

TEMPO-mediated late stage photochemical hydroxylation of biaryl sulfonium salts

The late stage photochemical hydroxylation of biaryl sulfonium salts was enabled with a TEMPO derivative as a simple oxygen source, in metal free conditions. The scope and mechanism of this exceptionally simple synthetic method, which constructs important arylated phenols from aromatic C-H bonds, are herein discussed.

A Perspective on Late-Stage Aromatic C-H Bond Functionalization

Late-stage functionalization of C–H bonds (C–H LSF) can provide a straightforward approach to the efficient synthesis of functionalized complex molecules. However, C–H LSF is challenging because the C–H bond must be functionalized in the presence of various other functional groups. In this Perspective, we evaluate aromatic C–H LSF on the basis of four criteria—reactivity, chemoselectivity, site-selectivity, and substrate scope—and provide our own views on current challenges as well as promising strategies and areas of growth going forward.

C-H Acyloxylation of Polycyclic Aromatic Hydrocarbons

The C-H acyloxylation of polycyclic aromatic hydrocarbons (PAHs) is described. This reaction constructs aryl acyloxylate scaffolds from PAHs with equimolar hypervalent iodine compounds under mild reaction conditions. Interestingly, the blue light irradiation accelerated this transformation. Additionally, the synthesis of structurally new symmetric and unsymmetric diaroyloxylated fluoranthenes was accomplished with a ruthenium photoredox catalyst.

Stereoselective Synthesis of (±)-Cephanolide B

A concise and stereoselective total synthesis of (±)-cephanolide B was achieved in 15 steps. The key steps in the synthesis were as follows: (i) an intermolecular Diels-Alder reaction followed by lactonization to form the oxabicyclo[2.2.2]octane DE ring; (ii) a tandem reaction, featuring an intramolecular Pauson-Khand reaction, a 6π-electrocyclization, and an oxidative aromatization by O₂, to construct the ABC-tricyclic rings (6-5-6); and (iii) a phthaloyl peroxide-mediated arene oxygenation to install the C-13 phenol group.

A Chiral-Pool-Based Strategy to Access trans-syn-Fused Drimane Meroterpenoids: Chemoenzymatic Total Syntheses of Polysin, N-Acetyl-polyveoline and the Chrodrimanins

trans-syn-Fused drimane meroterpenoids are unique natural products that arise from contra-thermodynamic polycyclizations of their polyene precursors. Herein we report the first total syntheses of four trans-syn-fused drimane meroterpenoids, namely polysin, N-acetyl-polyveoline, chrodrimanin C, and verruculide A, in 7-18 steps from sclareolide. The trans-syn-fused drimane unit is accessed through an efficient acid-mediated C9 epimerization of sclareolide. Subsequent applications of enzymatic C-H oxidation and contemporary annulation methodologies install the requisite C3 hydroxyl group and enable rapid generation of structural complexity to provide concise access to these natural products.

Decarboxylative Hydroxylation of Benzoic Acids

Herein, we report the first decarboxylative hydroxylation to synthesize phenols from benzoic acids at 35 °C via photoinduced ligand-to-metal charge transfer (LMCT)-enabled radical decarboxylative carbometalation. The aromatic decarboxylative hydroxylation is synthetically promising due to its mild conditions, broad substrate scope, and late-stage applications.

Iron-catalyzed arene C-H hydroxylation

[Figure: see text].

Stereoelectronic power of oxygen in control of chemical reactivity: the anomeric effect is not alone

Although carbon is the central element of organic chemistry, oxygen is the central element of stereoelectronic control in organic chemistry. Generally, a molecule with a C-O bond has both a strong donor (a lone pair) and a strong acceptor (e.g., a σ*_C-O orbital), a combination that provides opportunities to influence chemical transformations at both ends of the electron demand spectrum. Oxygen is a stereoelectronic chameleon that adapts to the varying situations in radical, cationic, anionic, and metal-mediated transformations. Arguably, the most historically important stereoelectronic effect is the anomeric effect (AE), i.e., the axial preference of acceptor groups at the anomeric position of sugars. Although AE is generally attributed to hyperconjugative interactions of σ-acceptors with a lone pair at oxygen (negative hyperconjugation), recent literature reports suggested alternative explanations. In this context, it is timely to evaluate the fundamental connections between the AE and a broad variety of O-functional groups. Such connections illustrate the general role of hyperconjugation with oxygen lone pairs in reactivity. Lessons from the AE can be used as the conceptual framework for organizing disjointed observations into a logical body of knowledge. In contrast, neglect of hyperconjugation can be deeply misleading as it removes the stereoelectronic cornerstone on which, as we show in this review, the chemistry of organic oxygen functionalities is largely based. As negative hyperconjugation releases the "underutilized" stereoelectronic power of unshared electrons (the lone pairs) for the stabilization of a developing positive charge, the role of orbital interactions increases when the electronic demand is high and molecules distort from their equilibrium geometries. From this perspective, hyperconjugative anomeric interactions play a unique role in guiding reaction design. In this manuscript, we discuss the reactivity of organic O-functionalities, outline variations in the possible hyperconjugative patterns, and showcase the vast implications of AE for the structure and reactivity. On our journey through a variety of O-containing organic functional groups, from textbook to exotic, we will illustrate how this knowledge can predict chemical reactivity and unlock new useful synthetic transformations.

Redox-Neutral Selenium-Catalysed Isomerisation of para-Hydroxamic Acids into para-Aminophenols

A selenium-catalysed para-hydroxylation of N-aryl-hydroxamic acids is reported. Mechanistically, the reaction comprises an N-O bond cleavage and consecutive selenium-induced [2,3]-rearrangement to deliver para-hydroxyaniline derivatives. The mechanism is studied through both 18 O-crossover experiments as well as quantum chemical calculations. This redox-neutral transformation provides an unconventional synthetic approach to para-aminophenols.

Redox-Neutrale Selen-katalysierte Isomerisierung von para-Hydroxamsäuren zu para-Aminophenolen

Über eine Selen‐katalysierte para‐Hydroxylierung von N‐Arylhydroxamsäuren wird berichtet. Mechanistisch verläuft diese über N‐O‐Bindungsbruch und nachfolgende Selen‐induzierte [2,3]‐Umlagerungen um para‐Hydroxylanilinderivate zu erzeugen. Der Mechanismus wurde sowohl mittels 18O‐Überkreuzungsexperimenten als auch quantenchemischen Berechnungen untersucht. Diese redox‐neutrale Transformation ermöglicht einen ungewöhnlichen synthetischen Zugang zu para‐Aminophenolen.

Total Synthesis of the Cephalotaxus Norditerpenoids (±)-Cephanolides A-D

Concise syntheses of the Cephalotaxus norditerpenoids cephanolides A-D (8-14 steps from commercial material) using a common late-stage synthetic intermediate are described. The success of our approach rested on an early decision to apply chemical network analysis to identify the strategic bonds that needed to be forged, as well as the efficient construction of the carbon framework through iterative Csp²-Csp³ cross-coupling, followed by an intramolecular inverse-demand Diels-Alder cycloaddition. Strategic late-stage oxidations facilitated access to all congeners of the benzenoid cephanolides isolated to date.

Electro-oxidative C(sp2)–H/O–H cross-dehydrogenative coupling of phenols and tertiary anilines for diaryl ether formation

The selective electrochemical oxidation of tertiary anilines in the presence of phenolic reactants leads to diaryl ether products.

Recent advances in reactions using diacyl peroxides as sources of O- and C-functional groups

This review summarizes recent advances in reactions utilizing diacyl peroxides as O- and C-sources, with examples illustrating how the reactivity of diacyl peroxides in organic reactions can be controlled.

Oxidative α-acyloxylation of acetals with cyclic diacyl peroxides

Selective functionalization of the non-activated acetal α-position with formal retaining of the acetal fragment was realized using cyclic diacyl peroxides.

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.

Selective Dealkenylative Functionalization of Styrenes via C-C Bond Cleavage

As a readily available feedstock, styrene with about 25 million tons of global annual production serves as an important building block and organic synthon for the synthesis of fine chemicals, polystyrene plastics, and elastomers. Thus, in the past decades, many direct transformations of this costless styrene feedstock were disclosed for the preparation of high-value chemicals, which to date, generally performed on the functionalization of styrenes through the allylic C-H bond, C(sp²)-H bond, or the C=C double bond cleavage. However, the dealkenylative functionalization of styrenes via the direct C-C single bond cleavage is so far challenging and still unknown. Herein, we report the novel and efficient C-C amination and hydroxylation reactions of styrenes for the synthesis of valuable aryl amines and phenols via the site-selective C(Ar)-C(alkenyl) single bond cleavage. This chemistry unlocks the new transformation and application of the styrene feedstock and provides an efficient protocol for the late-stage modification of substituted styrenes with the site-directed dealkenylative amination and hydroxylation.

Product-oriented Direct Cleavage of Chemical Linkages in Lignin

Lignin is one of the most important biomacromolecules in the plant biomass and the largest renewable source of aromatic building blocks in nature. Selectively producing value-added chemicals from the catalytic transformation of renewable lignin is of strategic significance and meet sustainability targets owing to the excessive consumption of non-renewable petroleum resource, but remains a long-term challenge owing to the complexity of lignin structure. This Minireview provides a summary and perspective of the extensive research that provides insight into selectively catalytic transformations of lignin and its derived monomers via directed scissor of chemical linkages (C-O and C-C bonds) with product-oriented targets. Furthermore, some challenges and opportunities of lignin catalytic transformation are provided based on existing problems in this field for readers to discuss future research directions.