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

Directing-Group-Assisted Non-Strained Ether C-O Bond Homolysis Mediated by Low-Valent Titanium

Ether C-O bonds are typical constituents of organic molecules that are seldom regarded as reactive functional groups except when highly strained. With the assistance of appropriate directing groups, low-valent titanium was found to homolytically cleave non-strained C-O bonds. In particular, a newly designed catechol monoether directing group rendered a route toward the activation of non-benzylic C(sp3)-O bonds. This method has been applied to conventional radical addition reactions to alkenes.

Visible-light-driven oxidation of organosilanes by a charge-transfer complex

Direct oxidation of organosilanes is one of the most straightforward ways to access silanols. Herein, we describe a novel photo-induced strategy for oxidation of organosilanes to access silanols, promoted by a photoactive charge-transfer complex (CTC) between sodium benzenesulfinate and molecular O₂. A streamlined sequence transformation of organosilanes to silyl ethers was also readily achieved. This developed protocol represents the first example of CTC-based oxidation of organosilanes, offering a facile approach to access a series of silanol and silyl ether products.

Versatile Synthetic Platform for 1,2,3-Triazole Chemistry

1,2,3-Triazole scaffolds are not obtained in nature, but they are still intensely investigated by synthetic chemists in various fields due to their excellent properties and green synthetic routes. This review will provide a library of all synthetic routes used in the past 21 years to synthesize 1,2,3-triazoles and their derivatives using various metal catalysts (such as Cu, Ni, Ru, Ir, Rh, Pd, Au, Ag, Zn, and Sm), organocatalysts, metal-free as well as solvent- and catalyst-free neat syntheses, along with their mechanistic cycles, recyclability studies, solvent systems, and reaction condition effects on regioselectivity. Constant developments indicate that 1,2,3-triazoles will help lead to future organic synthesis and are useful for creating molecular libraries of various functionalized 1,2,3-triazoles.

Ligand-Enabled C-H Hydroxylation with Aqueous H2O2 at Room Temperature

With the large number of Pd(II)-catalyzed C-H activation reactions of native substrates developed in the past decade, the development of catalysts to enable the use of green oxidants under safe and practical conditions has become an increasingly important challenge. Notably, the compatibility of Pd(II) catalysts with sustainable aqueous H2O2 has been a long-standing challenge in catalysis including Wacker-type oxidations. We report herein a bifunctional bidentate carboxyl-pyridone (CarboxPyridone) ligand that enables room-temperature Pd-catalyzed C-H hydroxylation of a broad range of benzoic and phenylacetic acids with an industry-compatible oxidant, aqueous hydrogen peroxide (35% H2O2). The scalability of this methodology is demonstrated by a 1000 mmol scale reaction of ibuprofen (206 g) using only a 1 mol % Pd catalyst loading. The utility of this protocol is further illustrated through derivatization of the products and synthesis of polyfluorinated natural product coumestan and pterocarpene from phenol intermediates prepared using this methodology.

Palladium-Catalyzed Organic Reactions Involving Hypervalent Iodine Reagents

The chemistry of polyvalent iodine compounds has piqued the interest of researchers due to their role as important and flexible reagents in synthetic organic chemistry, resulting in a broad variety of useful organic molecules. These chemicals have potential uses in various functionalization procedures due to their non-toxic and environmentally friendly properties. As they are also strong electrophiles and potent oxidizing agents, the use of hypervalent iodine reagents in palladium-catalyzed transformations has received a lot of attention in recent years. Extensive research has been conducted on the subject of C—H bond functionalization by Pd catalysis with hypervalent iodine reagents as oxidants. Furthermore, the iodine(III) reagent is now often used as an arylating agent in Pd-catalyzed C—H arylation or Heck-type cross-coupling processes. In this article, the recent advances in palladium-catalyzed oxidative cross-coupling reactions employing hypervalent iodine reagents are reviewed in detail.

Cobalt-catalyzed highly selective hydroxylation of organohydrosilanes and hydrosiloxanes

The highly selective and scalable dehydrogenative hydroxylation of hydrosilanes, featuring a low loading of the Earth-abundant cobalt catalyst, water as the green oxidant, and good generality for various hydrosilanes, is reported.

Decoding Directing Groups and Their Pivotal Role in C-H Activation

Synthetic organic chemistry has witnessed a plethora of functionalization and defunctionalization strategies. In this regard, C-H functionalization has been at the forefront due to the multifarious applications in the development of simple to complex molecular architectures and holds a brilliant prospect in drug development and discovery. Despite been explored tremendously by chemists, this functionalization strategy still enjoys the employment of novel metal catalysts as well metal-free organic ligands. Moreover, the switch to photo- and electrochemistry has widened our understanding of the alternative pathways via which a reaction can proceed and these strategies have garnered prominence when applied to C-H activation. Synthetic chemists have been foraging for new directing groups and templates for the selective activation of C-H bonds from a myriad of carbon-hydrogen bonds in aromatic as well as aliphatic systems. As a matter of fact, by varying the templates and directing groups, scientists found the answer to the challenge of distal C-H bond activation which remained an obstacle for a very long time. These templates have been frequently harnessed for selectively activating C-H bonds of natural products, drugs, and macromolecules decorated with multiple C-H bonds. This itself was a challenge before the commencement of this field as functionalization of a site other than the targeted site could modify and hamper the biological activity of the pharmacophore. Total synthesis and pharmacophore development often faces the difficulty of superfluous reaction steps towards selective functionalization. This obstacle has been solved by late-stage functionalization simply by harnessing C-H bond activation. Moreover, green chemistry and metal-free reaction conditions have seen light in the past few decades due to the rising concern about environmental issues. Therefore, metal-free catalysts or the usage of non-toxic metals have been recently showcased in a number of elegant works. Also, research groups across the world are developing rational strategies for directing group free or non-directed protocols that are just guided by ligands. This review encapsulates the research works pertinent to C-H bond activation and discusses the science devoted to it at the fundamental level. This review gives the readers a broad understanding of how these strategies work, the execution of various metal catalysts, and directing groups. This not only helps a budding scientist towards the commencement of his/her research but also helps a matured mind searching out for selective functionalization. A detailed picture of this field and its progress with time has been portrayed in lucid scientific language with a motive to inculcate and educate scientific minds about this beautiful strategy with an overview of the most relevant and significant works of this era. The unique trait of this review is the detailed description and classification of various directing groups and their utility over a wide substrate scope. This allows an experimental chemist to understand the applicability of this domain and employ it over any targeted substrate.

Engineered Pseudomonas putida for biosynthesis of catechol from lignin-derived model compounds and biomass hydrolysate

Catechol is an industrially relevant chemical with myriad applications. Its production via chemical route suffers from several drawbacks the major being a non-green and nonselective route. Currently, bio-based products using biocatalyst are gaining attention due to the growing environmental and health hazards concerns over the use of petroleum-derived feedstock. Lignocellulosic biomass serves as a promising feedstock. Lignin valorization is the demand of the current scenario which is complicated task by its complexity, heterogeneity and diversity of lignin structures posing limitations toward lignin valorization via chemical routes. There are several microorganisms that possess the ability to metabolize lignin monomers via their central metabolic pathways and this paves the way to the synthesis of a number of products. Pseudomonas putida KT2440 is one such organism and was chosen for genetic manipulations for catechol biosynthesis using lignin-derived model compounds and biomass hydrolysate stream comprising of various lignin monomers. Catechol production was engineered by diverting various lignin monomers and addressing the identified metabolic bottlenecks particularly vanillic acid accumulation toward catechol biosynthesis. The engineered strain could convert the model lignin monomers as well as monomers in the biomass hydrolysates to catechol and vanillic acid in more than 60% and 90% molar yields, respectively.

Benzene-Free Synthesis of Multisubstituted Catechol via Oxidative Dearomatic Reorganization

A benzene-free synthesis of multisubstituted catechol via an oxidative dearomatic reorganization is reported. This reaction tolerated a wide spectrum of functionalities, which could be applied in the synthesis of an electron-deficient arene-conjugated catechol that is difficult to access via biomimetic oxidative coupling. In addition, a diversification-oriented transformation that leveraged the versatile catechol afforded a series of functionality-rich products.

Recent developments in synthesis of catechols by Dakin oxidation

In recent years, Dakin oxidation has evolved primarily around the conversion of ortho- and para-hydroxy benzaldehydes and acetophenones to dihydric phenols, which are found in nature and complex organic materials.

Synthesis of a Gold-Metal Oxide Core-Satellite Nanostructure for In Situ SERS Study of CuO-Catalyzed Photooxidation

This work reports on an assembling-calcining method for preparing gold-metal oxide core-satellite nanostructures, which enable surface-enhanced Raman spectroscopic detection of chemical reactions on metal oxide nanoparticles. By using the nanostructure, we study the photooxidation of Si-H catalyzed by CuO nanoparticles. As evidenced by the in situ spectroscopic results, oxygen vacancies of CuO are found to be very active sites for oxygen activation, and hydroxide radicals (*OH) adsorbed at the catalytic sites are likely to be the reactive intermediates that trigger the conversion from silanes into the corresponding silanols. According to our finding, oxygen vacancy-rich CuO catalysts are confirmed to be of both high activity and selectivity in photooxidation of various silanes.

Hypervalent Iodine Reagents in Palladium-Catalyzed Oxidative Cross-Coupling Reactions

Hypervalent iodine compounds are valuable and versatile reagents in synthetic organic chemistry, generating a diverse array of useful organic molecules. Owing to their non-toxic and environmentally friendly features, these reagents find potential applications in various oxidative functionalization reactions. In recent years, the use of hypervalent iodine reagents in palladium-catalyzed transformations has been widely studied as they are strong electrophiles and powerful oxidizing agents. For instance, extensive work has been carried out in the field of C–H bond functionalization via Pd-catalysis using hypervalent iodine reagents as oxidants. In addition, nowadays, iodine(III) reagents have been frequently employed as arylating agents in Pd-catalyzed C–H arylation or Heck-type cross-coupling reactions. In this review, recent advancements in the area of palladium-catalyzed oxidative cross-coupling reactions using hypervalent iodine reagents are summarized in detail.

Halodule pinifolia (Seagrass) attenuated lipopolysaccharide-, carrageenan-, and crystal-induced secretion of pro-inflammatory cytokines: mechanism and chemistry

OBJECTIVES

The study aimed to explore the anti-inflammatory effect, underlying mechanism, and chemistry of Halodule pinifolia extract.

METHODS

The ethyl acetate (EHP) and methanol (MHP) extracts of Halodule pinifolia were screened for pro-inflammatory cytokine inhibition effect under various in vitro (LPSand crystal-induced inflammation) and in vivo models (LPS-induced endotoxaemia model, carrageenan-induced paw oedema model, and oxalate-induced renal nephropathy model of inflammation). The effect of EHP on the expression of inflammatory markers using western blot analysis (in vitro) was investigated. Chemical constituents of bioactive EHP were isolated through chromatography and characterised using NMR spectroscopy. Furthermore, EHP was standardised for rosmarinic acid, vanillic acid, and ethyl protocatechuate using HPLC. Also, total phytosterols, phenolic, and flavonoid content of EHP were determined by UV spectroscopy.

KEY FINDINGS

EHP was comparatively more effective than MHP in inhibiting cytokines secretions under LPS-induced in vitro models. Furthermore, EHP was screened under endotoxaemia in vivo model, EHP (250 mg/kg) reduced plasma IL-6, TNF-α, and IL-1β levels by 88.3%, 78.2%, and 74.5%, respectively. In the carrageenan-induced oedema model, EHP (200 mg/kg) reduced paw volume and release of TNF-α (69.3%) and IL-1β (43.1%). EHP (200 mg/kg) further controlled renal nephropathy by inhibiting plasma IL-1β and BUN levels. Also, a significant reduction of mRNA expressions of TNF-α and IL-1β and KIM-1 in renal tissues was observed. Through western blot, EHP was identified to regulate the expression of pro-form as well as mature-form of IL-1β and caspase-1. EHP constituted rosmarinic acid (RA), vanillic acid (VA), ethyl protocatechuate (EP), sitosterol, stigmasterol, campesterol, and dihydrobrassicasterol. It was determined that 4.6 mg/g of RA, 2.92 mg/g of VA, 0.76 mg/g of EP, 21.7 mg/g of total phenolics, 29.8 mg/g of total flavonoids, and 48.2 mg/g of total phytosterols were present in dry EHP. The presence of anti-inflammatory constituents such as RA, VA, and PE in EHP corroborated the in vitro and in vivo anti-inflammatory activity of EHP.

CONCLUSION

The anti-inflammatory property of EHP and its action through attenuation of pan-cytokines suggest that it can be developed into an oral pharmaceutical drug.

Silanol: a bifunctional group for peptide synthesis and late-stage functionalization

Chemical modification of a specific amino acid residue on peptides represents an efficient strategy to improve their pharmacokinetics and facilitates the potential to achieve post-synthetic diversification of peptides. Herein, we reported the first Pd-catalyzed late-stage ortho-olefination of Tyr residues on peptides with high chemo- and site-selectivity, by employing the easily attached and removable silanol as a bifunctional protecting group and directing group. Up to hexapeptides with variation on amino acid sequences or locations of the Tyr residue and different olefins were compatible with this protocol, which enriched the chemical toolbox for late-stage modification via C(sp2)-H functionalization. Furthermore, the orthogonal protection strategies of Tyr were also developed and could be applied to SPPS.

Cobalt-Catalyzed C-H Acetoxylation of Phenols with Removable Monodentate Directing Groups: Access to Pyrocatechol Derivatives

An efficient cobalt-catalyzed C-H acetoxylation of phenols has been developed by using PIDA (phenyliodine diacetate) as a sole acetoxy source to synthesize pyrocatechol derivatives for the first time. The key feature of this method is the use of earth-abundant metal cobalt as the green and inexpensive catalyst for the acetoxylation of C(sp²)-H bonds under neutral reaction conditions. Furthermore, the gram-scale reaction and late-stage functionalization demonstrated the usefulness of this method.

Site-Selective C-H Oxygenation via Aryl Sulfonium Salts

Herein, we report a two-step process forming arene C-O bonds in excellent site-selectivity at a late-stage. The C-O bond formation is achieved by selective introduction of a thianthrenium group, which is then converted into C-O bonds using photoredox chemistry. Electron-rich, -poor and -neutral arenes as well as complex drug-like small molecules are successfully transformed into both phenols and various ethers. The sequence differs conceptually from all previous arene oxygenation reactions in that oxygen functionality can be incorporated into complex small molecules at a late stage site-selectively, which has not been shown via aryl halides.

Transition Metal-Catalyzed Directing-Group-Assisted C-H Activation of Phenols

Strategies that exploit directing groups to control the site selectivity in the C-H activation of arenes have received much attention during the past two decades. In light of the importance of phenol derivatives in the areas of pharmaceuticals, agrochemicals, and materials science, transition-metal-catalyzed C-H activation of phenols has proven to be an extremely useful tool in organic synthesis. This Minireview summarizes the current state-of-the-art direct C-H activation of phenol derivatives under transition-metal catalysis.

Late-Stage Aromatic C-H Oxygenation

Synthetic methods for oxidative aromatic C-O bond formation are sparse, despite their demand in metabolite synthesis for drug discovery and development. We report a novel methodology for late-stage C-O bond formation of arenes. The reaction proceeds with excellent functional group tolerance even for highly functionalized substrates. The resulting aryl mesylates provide access to potential human metabolites of pharmaceuticals, and may be used directly to install a C-F bond to block metabolic hotspots. A charge-transfer interaction between the reagent bis(methanesulfonyl) peroxide and the substrate arenes may be relevant for the chemoselective functionalization of arenes over other functional groups.

A comprehensive overview of directing groups applied in metal-catalysed C-H functionalisation chemistry

The present review is devoted to summarizing the recent advances (2015-2017) in the field of metal-catalysed group-directed C-H functionalisation. In order to clearly showcase the molecular diversity that can now be accessed by means of directed C-H functionalisation, the whole is organized following the directing groups installed on a substrate. Its aim is to be a comprehensive reference work, where a specific directing group can be easily found, together with the transformations which have been carried out with it. Hence, the primary format of this review is schemes accompanied with a concise explanatory text, in which the directing groups are ordered in sections according to their chemical structure. The schemes feature typical substrates used, the products obtained as well as the required reaction conditions. Importantly, each example is commented on with respect to the most important positive features and drawbacks, on aspects such as selectivity, substrate scope, reaction conditions, directing group removal, and greenness. The targeted readership are both experts in the field of C-H functionalisation chemistry (to provide a comprehensive overview of the progress made in the last years) and, even more so, all organic chemists who want to introduce the C-H functionalisation way of thinking for a design of straightforward, efficient and step-economic synthetic routes towards molecules of interest to them. Accordingly, this review should be of particular interest also for scientists from industrial R&D sector. Hence, the overall goal of this review is to promote the application of C-H functionalisation reactions outside the research groups dedicated to method development and establishing it as a valuable reaction archetype in contemporary R&D, comparable to the role cross-coupling reactions play to date.

Highly Diastereoselective Palladium-Catalyzed Oxidative Carbocyclization of Enallenes Assisted by a Weakly Coordinating Hydroxyl Group

A highly diastereoselective palladium-catalyzed oxidative carbocyclization-borylation of enallenes assisted by a weakly coordinating hydroxyl group was developed. The reaction afforded functionalized cyclohexenol derivatives, in which the 1,3-relative stereochemistry is controlled (d.r. > 50:1). Other weakly coordinating oxygen-containing groups (ketone, alkoxide, acetate) also assisted the carbocyclization toward cyclohexenes. The reaction proceeds via a ligand exchange on Pd of the weakly coordinating group with a distant olefin group. The high diastereoselectivity of the hydroxyl-directed reaction could be rationalized by a face-selective coordination of the distant olefin. It was demonstrated that the primary coordination of the close-by oxygen-containing functionality was necessary for the reaction to occur and removal of this functionality shut down the reaction.

Heterologous pathway assembly reveals molecular steps of fungal terreic acid biosynthesis

Terreic acid is a potential anticancer drug as it inhibits Bruton's tyrosine kinase; however, its biosynthetic molecular steps remain unclear. In this work, the individual reactions of terreic acid biosynthesis were determined by stepwise pathway assembly in a heterologous host, Pichia pastoris, on the basis of previous knockout studies in a native host, Aspergillus terreus. Polyketide synthase AtX was found to catalyze the formation of partially reduced polyketide 6-methylsalicylic acid, followed by 3-methylcatechol synthesis by salicylate 1-monooxygenase AtA-mediated decarboxylative hydroxylation of 6-methylsalicylic acid. Our results show that cytochrome P450 monooxygenase AtE hydroxylates 3-methylcatechol, thus producing the next product, 3-methyl-1,2,4-benzenetriol. A smaller putative cytochrome P450 monooxygenase, AtG, assists with this step. Then, AtD causes epoxidation and hydroxyl oxidation of 3-methyl-1,2,4-benzenetriol and produces a compound terremutin, via which the previously unknown function of AtD was identified as cyclooxygenation. The final step involves an oxidation reaction of a hydroxyl group by a glucose-methanol-choline oxidoreductase, AtC, which leads to the final product: terreic acid. Functions of AtD and AtG were determined for the first time. All the genes were reanalyzed and all intermediates and final products were isolated and identified. Our model fully defines the molecular steps and corrects previous results from the literature.

Silicon-Tethered Strategies for C-H Functionalization Reactions

Selective and efficient functionalization of ubiquitous C-H bonds is the Holy Grail of organic synthesis. Most advances in this area rely on employment of strongly or weakly coordinating directing groups (DGs) which have proven effective for transition-metal-catalyzed functionalization of C(sp²)-H and C(sp³)-H bonds. Although most directing groups are important functionalities in their own right, in certain cases, the DGs become static entities that possess very little synthetic leverage. Moreover, some of the DGs employed are cumbersome or unpractical to remove, which precludes the use of this approach in synthesis. It is believed, that development of a set of easily installable and removable/modifiable DGs for C-H functionalization would add tremendous value to the growing area of directed functionalization, and hence would promote its use in synthesis and late-stage functionalization of complex molecules. In particular, silicon tethers have long provided leverage in organic synthesis as easily installable and removable/modifiable auxiliaries for a variety of processes, including radical transformations, cycloaddition reactions, and a number of TM-catalyzed methods, including ring-closing metathesis (RCM) and cross-coupling reactions. Employment of Si-tethers is highly attractive for several reasons: (1) they are easy to handle/synthesize and are relatively stable; (2) they utilize cheap and abundant silicon precursors; and (3) Si-tethers are easily installable and removable/modifiable. Hence, development of Si-tethers for C-H functionalization reactions is appealing not only from a practical but also from a synthetic standpoint, since the Si-tether can provide an additional handle for diversification of organic molecules post-C-H functionalization. Over the past few years, we developed a set of Si-tether approaches for C-H functionalization reactions. The developed Si-tethers can be categorized into four types: (Type-1) Si-tethers possessing a reacting group, where the reacting group is delivered to the site of functionalization; (Type-2) Si-tethers possessing a DG, designed for selective C(sp²)-H functionalization of arenes; (Type-3) reactive Si-tethers for C-H silylation of organic molecules; and finally, (Type-4) reactive Si-tethers containing a DG, developed for selective C-H silylation/hydroxylation of challenging C(sp³)-H bonds. In this Account, we outline our advances on the employment of silicon auxiliaries for directed C-H functionalization reactions. The discussion of the strategies for employment of different Si-tethers, functionalization/modification of silicon tethers, and the methodological developments on C-C, C-X, C-O, and C-Si bond forming reactions via silicon tethers will also be presented. While the work described herein presents a substantial advance for the area of C-H functionalization, challenges still remain. The use of noble metals are required for the C-H functionalization methods presented herein. Also, the need for stoichiometric use of high molecular weight silicon auxiliaries is a shortcoming of the presented concept.

Cu-Catalyzed Aerobic Oxidative C-H/C-O Cyclization of 2,2'-Binaphthols: Practical Synthesis of PXX Derivatives

Cu-catalyzed C-H/C-O cyclization of 2,2'-binaphthol, using air as an oxidant, was developed. C-H functionalization of binaphthols occurred at the 8,8'-position to form peri-xanthenoxanthenes that exhibit high charge-carrier mobility. The reaction can tolerate a wide variety of functional groups and afforded the corresponding peri-xanthenoxanthene derivatives via an efficient procedure.