Synthesis of catechols from phenols via Pd-catalyzed silanol-directed C-H oxygenation

A redox-neutral catechol synthesis

Ubiquitous tyrosinase catalyses the aerobic oxidation of phenols to catechols through the binuclear copper centres. Here, inspired by the Fischer indole synthesis, we report an iridium-catalysed tyrosinase-like approach to catechols, employing an oxyacetamide-directed C–H hydroxylation on phenols. This method achieves one-step, redox-neutral synthesis of catechols with diverse substituent groups under mild conditions. Mechanistic studies confirm that the directing group (DG) oxyacetamide acts as the oxygen source. This strategy has been applied to the synthesis of different important catechols with fluorescent property and bioactivity from the corresponding phenols. Finally, our method also provides a convenient route to ¹⁸O-labelled catechols using ¹⁸O-labelled acetic acid.

Catechols are common structural motifs in bioactive molecules and synthetic building blocks. Here the authors report a method to convert phenol derivatives into catechols via an iridium-catalysed redox-neutral C–H hydroxylation, giving diversely substituted products under mild conditions.

A meta-selective-C–H alkenylation of phenol-derivatives employing a traceless organosilicon template

A traceless organosilicon template-directed meta-selective-C–H alkenylation of phenols was realized with good yields and high selectivities.

Accessing Drug Metabolites via Transition-Metal Catalyzed C-H Oxidation: The Liver as Synthetic Inspiration

Can classical and modern chemical C-H oxidation reactions complement biotransformation in the synthesis of drug metabolites? We have surveyed the literature in an effort to try to answer this important question of major practical significance in the pharmaceutical industry. Drug metabolites are required throughout all phases of the drug discovery and development process; however, their synthesis is still an unsolved problem. This Review, not intended to be comprehensive or historical, highlights relevant applications of chemical C-H oxidation reactions, electrochemistry and microfluidic technologies to drug templates in order to access drug metabolites, and also highlights promising reactions to this end. Where possible or appropriate, the contrast with biotransformation is drawn. In doing so, we have tried to identify gaps where they exist in the hope to spur further activity in this very important research area.

Catalytic Reductive ortho-C-H Silylation of Phenols with Traceless, Versatile Acetal Directing Groups and Synthetic Applications of Dioxasilines

A new, highly selective, bond functionalization strategy, achieved via relay of two transition metal catalysts and the use of traceless acetal directing groups, has been employed to provide facile formation of C-Si bonds and concomitant functionalization of a silicon group in a single vessel. Specifically, this approach involves the relay of Ir-catalyzed hydrosilylation of inexpensive and readily available phenyl acetates, exploiting disubstituted silyl synthons to afford silyl acetals and Rh-catalyzed ortho-C-H silylation to provide dioxasilines. A subsequent nucleophilic addition to silicon removes the acetal directing groups and directly provides unmasked phenol products and, thus, useful functional groups at silicon achieved in a single vessel. This traceless acetal directing group strategy for catalytic ortho-C-H silylation of phenols was also successfully applied to preparation of multisubstituted arenes. Remarkably, a new formal α-chloroacetyl directing group has been developed that allows catalytic reductive C-H silylation of sterically hindered phenols. In particular, this new method permits access to highly versatile and nicely differentiated 1,2,3-trisubstituted arenes that are difficult to access by other catalytic routes. In addition, the resulting dioxasilines can serve as chromatographically stable halosilane equivalents, which allow not only removal of acetal directing groups but also introduce useful functional groups leading to silicon-bridged biaryls. We demonstrated that this catalytic C-H bond silylation strategy has powerful synthetic potential by creating direct applications of dioxasilines to other important transformations, examples of which include aryne chemistry, Au-catalyzed direct arylation, sequential orthogonal cross-couplings, and late-stage silylation of phenolic bioactive molecules and BINOL scaffolds.

The design of a readily attachable and cleavable molecular scaffold for ortho-selective C-H alkenylation of arene alcohols

We describe herein the design of a novel molecular scaffold that can induce facile oxidative olefinations when attached to alcohols. Benzylic, homo-, and bishomobenzylic alcohols are utilized. The scaffold can act as a protecting group for the alcohol in other transformations, and it is recoverable in excellent yield. The overall sequence can also be telescoped without purifications of intermediates, representing a net alcohol-based directed ortho-alkenylation.

A bentonite–gold nanohybrid as a heterogeneous green catalyst for selective oxidation of silanes

A highly efficient, green and recyclable heterogeneous bentonite–gold nanohybrid catalyst was developed for the selective oxidation of silanes.

Amino acid-promoted C–H alkylation with alkylboronic acids using a removable directing group

Palladium-catalyzed C–H alkylation reaction with alkylboronic acids has been developed using a removable pyridyldiisopropylsilyl directing group.

Dual C–H functionalization of N-aryl tetrahydroisoquinolines: a highly diastereoselective synthesis of dibenzo[a,f]quinolizines via visible-light induced oxidation and inverse electron-demand aza-Diels–Alder reaction

Dual C–H functionalization of amines with high diastereoselectivity is developedviavisible-light photocatalysis and inverse electron-demand aza-Diels–Alder reaction.

Sequential meta-C–H olefination of synthetically versatile benzyl silanes: effective synthesis of meta-olefinated toluene, benzaldehyde and benzyl alcohols

Template assisted sequential meta-C–H bis-olefination was achieved for synthetically versatile benzyl silanes with post synthetic modifications towards different useful functionalities.

A bio-inspired synthesis of oxindoles by catalytic aerobic dual C-H functionalization of phenols

We report a bio-inspired approach to the synthesis of oxindoles, which couples the energetic requirements of dehydrogenative C–N bond formation to the reduction of oxygen.

Nitrogen-containing heterocycles are fundamentally important to the function of pharmaceuticals, agrochemicals and materials. Herein, we report a bio-inspired approach to the synthesis of oxindoles, which couples the energetic requirements of dehydrogenative C–N bond formation to the reduction of molecular oxygen (O₂). Our method is inspired by the biosynthesis of melanin pigments (melanogenesis), but diverges from the biosynthetic polymerization. Mechanistic analysis reveals the involvement of Cu^II-semiquinone radical intermediates, which enable dehydrogenative carbon–heteroatom bond formation that avoids a catechol/quinone redox couple. This mitagates the deleterious polarity reversal that results from phenolic dearomatization, and enables a high-yielding phenolic C–H functionalization under catalytic aerobic conditions. Our work highlights the broad synthetic utility and efficiency of forming C–N bonds via a catalytic aerobic dearomatization of phenols, which is currently an underdeveloped transformation.

Structural and reactivity models for copper oxygenases: cooperative effects and novel reactivities

Dioxygen is widely used in nature as oxidant. Nature itself has served as inspiration to use O2 in chemical synthesis. However, the use of dioxygen as an oxidant is not straightforward. Its triplet ground-state electronic structure makes it unreactive toward most organic substrates. In natural systems, metalloenzymes activate O2 by reducing it to more reactive peroxide (O2(2-)) or superoxide (O2(-)) forms. Over the years, the development of model systems containing transition metals has become a convenient tool for unravelling O2-activation mechanistic aspects and reproducing the oxidative activity of enzymes. Several copper-based systems have been developed within this area. Tyrosinase is a copper-based O2-activating enzyme, whose structure and reactivity have been widely studied, and that serves as a paradigm for O2 activation at a dimetal site. It contains a dicopper center in its active site, and it catalyzes the regioselective ortho-hydroxylation of phenols to catechols and further oxidation to quinones. This represents an important step in melanin biosynthesis and it is mediated by a dicopper(II) side-on peroxo intermediate species. In the present accounts, our research in the field of copper models for oxygen activation is collected. We have developed m-xylyl linked dicopper systems that mimick structural and reactivity aspects of tyrosinase. Synergistic cooperation of the two copper(I) centers results in O2 binding and formation of bis(μ-oxo)dicopper(III) cores. These in turn bind and ortho-hydroxylate phenolates via an electrophilic attack of the oxo ligand over the arene. Interestingly the bis(μ-oxo)dicopper(III) cores can also engage in ortho-hydroxylation-defluorination of deprotonated 2-fluorophenols, substrates that are well-known enzyme inhibitors. Analysis of Cu2O2 species with different binding modes show that only the bis(μ-oxo)dicopper(III) cores can mediate the reaction. Finally, the use of unsymmetric systems for oxygen activation is a field that still remains rather unexplored. We envision that the unsymmetry might infere interesting new reactivities. We contributed to this topic with the development of an unsymmetric ligand (m-XYL(N3N4)), whose dicuprous complex reacts with O2 and forms a trans-peroxo dicopper(II) species that showed a markedly different reactivity compared to a symmetric trans-peroxo dicopper(II) analog. Nucleophilic reactivity is observed for the unsymmetric trans-peroxo dicopper(II) species against electrophilies such as H(+), CO2 and aldehydes, and neither oxygen atom transfer nor hydrogen abstraction is observed when reacting with oxygen atom acceptors (triphenyl phosphine, sulfides) and substrates with weak C-H bonds. Instead, electrophilic monooxygenase-like ortho-hydroxylation reactivity is described for these unsymmetric species upon reaction with phenolates. Finally, by using a second dinucleating unsymmetric ligand (L(N3N4)), we have described copper(I) containing heterodimetallic systems and explored their O2 binding properties. Site specific metalation led to the generation of dimeric heterometallic M'CuO2CuM' species from intermolecular O2 binding at copper sites.

Divergent Synthesis of 2-Aminofurans via Palladium-Catalyzed Acetoxylative, Alkoxylative, and Hydroxylative Cycloisomerization of Homoallenyl Amides

A fine-tunable transformation, including Pd-catalyzed acetoxylative, alkoxylative, and hydroxylative cycloisomerization of homoallenyl amides, has been realized with hypervalent iodine organic compounds as the oxidants, giving polysubstituted 2-aminofurans in promising yields at room temperature. The selective formation of three different types of products from the same starting materials makes this reaction particularly attractive and useful for organic synthesis.

The Toluene o-Xylene Monooxygenase Enzymatic Activity for the Biosynthesis of Aromatic Antioxidants

Monocyclic phenols and catechols are important antioxidant compounds for the food and pharmaceutic industries; their production through biotransformation of low-added value starting compounds is of major biotechnological interest. The toluene o-xylene monooxygenase (ToMO) from Pseudomonas sp. OX1 is a bacterial multicomponent monooxygenase (BMM) that is able to hydroxylate a wide array of aromatic compounds and has already proven to be a versatile biochemical tool to produce mono- and dihydroxylated derivatives of aromatic compounds. The molecular determinants of its regioselectivity and substrate specificity have been thoroughly investigated, and a computational strategy has been developed which allows designing mutants able to hydroxylate non-natural substrates of this enzyme to obtain high-added value compounds of commercial interest. In this work, we have investigated the use of recombinant ToMO, expressed in cells of Escherichia coli strain JM109, for the biotransformation of non-natural substrates of this enzyme such as 2-phenoxyethanol, phthalan and 2-indanol to produce six hydroxylated derivatives. The hydroxylated products obtained were identified, isolated and their antioxidant potential was assessed both in vitro, using the DPPH assay, and on the rat cardiomyoblast cell line H9c2. Incubation of H9c2 cells with the hydroxylated compounds obtained from ToMO-catalyzed biotransformation induced a differential protective effect towards a mild oxidative stress induced by the presence of sodium arsenite. The results obtained confirm once again the versatility of the ToMO system for oxyfunctionalization reactions of biotechnological importance. Moreover, the hydroxylated derivatives obtained possess an interesting antioxidant potential that encourages the use of the enzyme for further functionalization reactions and their possible use as scaffolds to design novel bioactive molecules.

General method for the synthesis of salicylic acids from phenols through palladium-catalyzed silanol-directed C-H carboxylation

A silanol-directed, palladium-catalyzed C-H carboxylation reaction of phenols to give salicylic acids has been developed. This method features high efficiency and selectivity, and excellent functional-group tolerance. The generality of this method was demonstrated by the carboxylation of estrone and by the synthesis of an unsymmetrically o,o'-disubstituted phenolic compound through two sequential C-H functionalization processes.

Reductive arene ortho-silanolization of aromatic esters with hydridosilyl acetals

The design and application of a single-pot, reductive arene C–H bond silanolization of esters for synthesis of ortho-formyl arylsilanols.

Transition metal-catalyzed C–H bond functionalizations by the use of diverse directing groups

In this review, a summary of transition metal-catalyzed C–H activation by utilizing the functionalities as directing groups is presented.

Palladium-catalyzed ortho-acyloxylation of N-nitrosoanilines via direct sp2 C–H bond activation

The palladium-catalyzed N-nitroso-directed ortho-acyloxylation of N-nitrosoanilines has been demonstrated via sp² C–H activation with PhI(OAc)₂ as the oxidant and Ac₂O/AcOH (1 : 1) or C₂H₅CO₂H as the reaction medium.

A biomimetic catalytic aerobic functionalization of phenols

The importance of aromatic C-O, C-N, and C-S bonds necessitates increasingly efficient strategies for their formation. Herein, we report a biomimetic approach that converts phenolic C-H bonds into C-O, C-N, and C-S bonds at the sole expense of reducing dioxygen (O2) to water (H2O). Our method hinges on a regio- and chemoselective copper-catalyzed aerobic oxygenation to provide ortho-quinones. ortho-Quinones are versatile intermediates, whose direct catalytic aerobic synthesis from phenols enables a mild and efficient means of synthesizing polyfunctional aromatic rings.

Arene C–H functionalisation using a removable/modifiable or a traceless directing group strategy

Recent advances in arene C–H functionalisation using a removable/modifiable or a traceless directing group strategy are reviewed.

Meta-selective C-H functionalization using a nitrile-based directing group and cleavable Si-tether

A nitrile-based template that enables meta-selective C-H bond functionalization was developed. The template is applicable to a range of substituted arenes and tolerates a variety of functional groups. The directing group uses a silicon atom for attachment, allowing for a facile introduction/deprotection strategy increasing the synthetic practicality of this template.

Twofold unsymmetrical C-H functionalization of PyrDipSi-substituted arenes: a general method for the synthesis of substituted meta-halophenols

Two-fold unsymmetrical functionalization of arenes, which includes sequential halogenation/oxygenation reactions, has been developed. This transformation allows for a general and efficient synthesis of substituted meta -halophenols from simple aryl iodides. Importantly, the PyrDipSi (2-diisopropylsilylpyrimidine) group can be easily removed or efficiently converted into valuable functionalities.