PdCl2 and N-hydroxyphthalimide co-catalyzed C(sp2)-H hydroxylation by dioxygen activation

NH4OAc-Promoted Domino Route to Hydroxyarylated Unsymmetrical Pyridines under Neat Conditions

A new metal-, oxidant-, and solvent-free ecofriendly domino method has been established for modular synthesis of a diverse range of medicinally promising hydroxyarylated unsymmetrical pyridines in good to high chemical yields with an excellent regioselectivity. This domino process involves a range of N-sulfonyl ketimines as C,N-binucleophiles, enolizable ketones, and aromatic/heteroaromatic aldehydes using ammonium acetate as an ideal promoter under neat conditions, which creates two new C-C bonds and one C-N bond. Notably, the neutral reaction conditions are mild enough to tolerate a range of functionalities and cover a variety of substrates, thus bestowing a powerful avenue to access tri- and tetrasubstituted pyridines including carbo- and heterocyclic fused ones. Interestingly, a practical, scalable, and high-yielding synthesis of pyridylphenol derivatives was successfully accomplished by our unique method.

Palladium-catalyzed intramolecular aerobic alkenylhydroxylation of allenamides with alkenyl iodides

An efficient palladium-catalyzed aerobic alkenylhydroxylation cyclization of allenamide derivatives was developed. Mechanistic studies indicated that the reaction might undergo a radical process.

Palladium-catalyzed ortho-selective C–H hydroxylation of carboxybenzyl-protected benzylamines

A convenient approach to synthesize o-hydroxybenzylamine via Pd-catalyzed hydroxylation of benzylamine with a removable Cbz-amide as the directing group was developed.

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.

Rhodium(III)-Catalyzed Regioselective C(sp2)-H Functionalization of 7-Arylpyrazolo[1,5-a]pyrimidines with Dioxazolones as Amidating Agents

Rh(III)-catalyzed C-H functionalization of 7-arylpyrazolo[1,5-a]pyrimidines was developed wherein the pyrazolo[1,5-a]pyrimidine moiety is reported for the first time to direct the C-H bond activation. Various 7-arylpyrazolo[1,5-a]pyrimidines underwent smooth C-H amidation with alkyl-, aryl-, and heteroaryl-substituted dioxazolones to afford the products in moderate to good yields. Mechanistic studies suggest that a six-membered rhodacycle intermediate involving N1 might play a key role in the regioselective catalytic cycle.

Palladium-Catalyzed C-H Bond Functionalization Reactions Using Phosphate/Sulfonate Hypervalent Iodine Reagents

A new and operationally simple approach for palladium-catalyzed C-H functionalization reactions utilizing an organophosphorus/sulfonate hypervalent iodine reagent as both an oxidant and the source of a functional group has been developed. Through this method, the oxidative phosphorylation-, sulfonation-, and hydroxylation of unactivated benzyl C(sp³)-H bonds, along with the hydroxylation and arylation of aryl C(sp²)-H bonds, are successfully realized under mild conditions and with excellent site-selectivity. The versatile C-OSO₂R bond provides a platform for a wide array of subsequent diversification reactions.

NH3 -Driven Benzene C-H Activation with O2 that Opens a New Way for Selective Phenol Synthesis

Catalytic benzene C-H activation toward selective phenol synthesis with O₂ remains a stimulating challenge to be tackled. Phenol is currently produced industrially by the three-steps cumene process in liquid phase, which is energy-intensive and not environmentally friendly. Hence, there is a strong demand for an alternative gas-phase single-path reaction process. This account documents the pivotal confined single metal ion site platform with a sufficiently large coordination sphere in β zeolite pores, which promotes the unprecedented catalysis for the selective benzene hydroxylation with O₂ under coexisting NH₃ by the new inter-ligand concerted mechanism. Among alkali and alkaline-earth metal ions and transition and precious metal ions, single Cs⁺ and Rb⁺ sites with ion diameters >0.300 nm in the β pores exhibited good performances for the direct phenol synthesis in a gas-phase single-path reaction process. The single Cs⁺ and Rb⁺ sites that possess neither significant Lewis acidic-basic property nor redox property, cannot activate benzene, O₂ , and NH₃ , respectively, whereas when they coadsorbed together, the reaction of the inter-coadsorbates on the single alkali-metal ion site proceeds concertedly (the inter-ligand concerted mechanism), bringing about the benzene C-H activation toward phenol synthesis. The NH₃ -driven benzene C-H activation with O₂ was compared to the switchover of the reaction pathways from the deep oxidation to selective oxidation of benzene by coexisting NH₃ on Pt₆ metallic cluster/β and Ni₄ O₄ oxide cluster/β. The NH₃ -driven selective oxidation mechanism observed with the Cs⁺ /β and Rb⁺ /β differs from the traditional redox catalysis (Mars-van Krevelen) mechanism, simple Langmuir-Hinshelwood mechanism, and acid-base catalysis mechanism involving clearly defined interaction modes. The present catalysis concept opens a new way for catalytic selective oxidation processes involving direct phenol synthesis.

Palladium catalyzed chemo- and site-selective C-H acetoxylation and hydroxylation of oxobenzoxazine derivatives

An efficient protocol for the introduction of acetoxy and hydroxy functionalities on unactivated aryl sp2 carbons of oxobenzoxazine derivatives via an ortho-C-H activation reaction using a palladium catalyst has been developed for the first time. Interestingly, this intermolecular C-H functionalization reaction takes place in a facile and simple manner with high chemo- and site selectivity.

Nickel-catalyzed regioselective C–H oxygenation: new routes for versatile C–O bond formation

Nickel-catalyzed regioselective C–H oxygenation reactions of chelating arenes using iodobenzene diacetate, alcohols, and benzoic acids respectively as attacking reagents have been developed for the first time.

Recent advances in intramolecular C–O/C–N/C–S bond formationviaC–H functionalization

This review presents the construction of C–X bonds (X = O/N/S) by using intramolecular C–H functionalization for the synthesis of heterocyclic compounds.

Ruthenium(II)-Catalyzed Regioselective C-8 Hydroxylation of 1,2,3,4-Tetrahydroquinolines

Ru(II)-catalyzed chelation-assisted highly regioselective C8-hydroxylation of 1,2,3,4-tretrahydroquinolines has been developed. Various 1,2,3,4-tetrahydroquinolines underwent smooth C8-H hydroxylation with cheap and safe K₂S₂O₈ as the oxidant and oxygen source to furnish the corresponding products in good to excellent yields with high tolerance of the functional groups. The choice of a readily installable and removable N-pyrimidyl directing group is the key to catalysis. Mechanistic studies suggest the involvement of a six-membered ruthenacycle intermediate in the catalytic cycle. The method can also be extended to the direct hydroxylation of other (hetero)arene C-H bonds.

DABCO- and DBU-promoted one-pot reaction of N-sulfonyl ketimines with Morita-Baylis-Hillman carbonates: a sequential approach to (2-hydroxyaryl)nicotinate derivatives

An intriguing DABCO-catalyzed and DBU-promoted one-pot synthesis of an important class of (2-hydroxyaryl)pyridine derivatives bearing a carboxylate or a nitrile group suitably placed at C3 position of the aza-ring has been achieved in acceptable chemical yields with a broad functional group tolerance. This sequential C-C/C-N bond making process proceeds through a regioselective allylic alkylation/aza-Michael reaction between MBH carbonates derived from an acrylate/acrylonitrile and N-sulfonyl ketimines as C,N-binucleophiles catalyzed by DABCO, followed by elimination of SO2 under the influence of base and subsequent aromatization in an open atmosphere.

Cobalt-Catalyzed Direct C-H Thiolation of Aromatic Amides with Disulfides: Application to the Synthesis of Quetiapine

A direct C(sp²)-H thiolation of aromatic amides with disulfides was developed. The coupling reaction proceeds between the thioether radical and cobaltacycle intermediate. This method exhibits a relatively broad substrate scope and high functional group compatibility. A mechanistic study indicates that the cobalt(IV) intermediate is probably formed during the course of the reaction. The thiolation product can be transformed to Quetiapine, which is an atypical antipsychotic agent approved for the treatment of schizophrenia and bipolar disorder.

Cobalt-catalyzed peri-selective alkoxylation of 1-naphthylamine derivatives

A cobalt-catalyzed C(sp²)-H alkoxylation of 1-naphthylamine derivatives has been disclosed, which represents an efficient approach to synthesize aryl ethers with broad functional group tolerance. It is noteworthy that secondary alcohols, such as hexafluoroisopropanol, isopropanol, isobutanol, and isopentanol, were well tolerated under the current catalytic system. Moreover, a series of biologically relevant fluorine-aryl ethers were easily obtained under mild reaction conditions after the removal of the directing group.

Catalyzed and Electrocatalyzed Oxidation of l-Tyrosine and l-Phenylalanine to Dopachrome by Nanozymes

Catalyzed oxygen insertion into C-H bonds represents a continuous challenge in chemistry. Particularly, driving this process at ambient temperature and aqueous media represents a "holy grail" in catalysis. We report on the catalyzed cascade transformations of l-tyrosine or l-phenylalanine to dopachrome in the presence of l-ascorbic acid/H₂O₂ as oxidizing mixture and CuFe-Prussian Blue-like nanoparticles, Fe₃O₄ nanoparticles or Au nanoparticles as catalysts. The process involves the primary transformation of l-tyrosine to l-DOPA that is further oxidized to dopachrome. The transformation of l-phenylalanine to dopachrome in the presence of CuFe-Prussian Blue-like nanoparticles and l-ascorbic acid/H₂O₂ involves in the first step the formation of l-tyrosine and, subsequently, the operation of the catalytic oxidation cascade of l-tyrosine to l-DOPA and dopachrome. Electron spin resonance experiments demonstrate that ascorbate radicals and hydroxyl radicals play cooperative functions in driving the different oxygen-insertion processes. In addition, the aerobic elecrocatalyzed oxidation of l-tyrosine to dopachrome in the presence of naphthoquinone-modified Fe₃O₄ nanoparticles and l-ascorbic acid is demonstrated. In this system, magnetic-field attraction of the naphthoquinone-modified Fe₃O₄ nanoparticles onto the electrode allows the quinone-mediated electrocatalyzed reduction of O₂ to H₂O₂ (bias potential -0.5 V vs SCE). The electrogenerated H₂O₂ is then utilized to promote the transformation of l-tyrosine to dopachrome in the presence of l-ascorbic acid and Fe₃O₄ catalyst.

GO–Cu7S4 catalyzed ortho-aminomethylation of phenol derivatives with N,N-dimethylbenzylamines: site-selective oxidative CDC

An efficient ortho-selective C–H aminomethylation of phenols and naphthols with N,N-dimethylbenzylamines using a GO–Cu₇S₄ nanocatalyst is reported.

Pd-Catalyzed Ortho C-H Hydroxylation of Benzaldehydes Using a Transient Directing Group

The direct Pd-catalyzed ortho C-H hydroxylation of benzaldehydes was achieved using 4-chloroanthranilic acid as the transient directing group, 1-fluoro-2,4,6-trimethylpyridnium triflate as the bystanding oxidant, and p-toluenesulfonic acid as the putative oxygen nucleophile. The unusual C-H chlorination and polyfluoroalkoxylation reactions signaled the importance of external nucleophiles to the outcome of Pd(IV) reductive eliminations.

Oxygenation via C-H/C-C Bond Activation with Molecular Oxygen

The selective oxidation of organic molecules is a fundamentally important component of modern synthetic chemistry. In the past decades, direct oxidative C-H and C-C bond functionalization has proved to be one of the most efficient and straightforward methods to synthesize complex products from simple and readily available starting materials. Among these oxidative processes, the use of molecular oxygen as a green and sustainable oxidant has attracted considerable attention because of its highly atom-economical, abundant, and environmentally friendly characteristics. The development of new protocols using molecular oxygen as an ideal oxidant is highly desirable in oxidation chemistry. More importantly, the oxygenation reaction of simple molecules using molecular oxygen as the oxygen source offers one of the most ideal processes for the construction of O-containing compounds. Aerobic oxidation and oxygenation by enzymes, such as monooxygenase, tyrosinase, and dopamine β-monooxygenase, have been observed in some biological C-H bond hydroxylation processes. Encouraged by these biological transformations, transition-metal- or organocatalyst-catalyzed oxygenation through dioxygen activation has attracted academic and industrial prospects. In this Account, we describe some advances from our group in oxygenation via C-H/C-C bond activation with molecular oxygen as the oxidant and oxygen source for the synthesis of O-containing compounds. Under an atmosphere of O₂ (1 atm) or air (1 atm), we have successfully incorporated one or two O atoms from O₂ into simple and readily available substrates through C-H, C-C, C═C, and C≡C bond cleavage by transition-metal catalysis, organocatalysis, and photocatalysis. Moreover, we have devised cyclization reactions with molecular oxygen to construct O-heterocycles. Most of these transformations can tolerate a broad range of functional groups. Furthermore, on the basis of isotope labeling experiments, electron paramagnetic resonance spectral analysis, and other mechanistic studies, we have demonstrated that a single electron transfer process via a carbon radical, peroxide radical, or hydroxyl radical is involved in these aerobic oxidation and oxygenation reactions. These protocols provide new approaches for the green synthesis of various α-keto amides, α-keto esters, esters, ketones, aldehydes, formamides, 2-oxoacetamidines, 2-(1H)-pyridones, phenols, tertiary α-hydroxy carbonyls, p-quinols, β-azido alcohols, benzyl alcohols, tryptophols, and oxazoles, which have potential applications in the preparation of natural products, bioactive compounds, and functional materials. In most cases, inexpensive and low-toxicity Cu, Fe, Mn, or NHPI was found to be an efficient catalyst for the transformation. The high efficiency, low cost, high oxygen atom economy, broad substrate scope, and practical operation make the developed oxygenation system very attractive and practical. Moreover, the design of new types of molecular-oxygen- or air-based oxidation and oxygenation reactions can be anticipated.

Rhodium(III)-Catalyzed Selective C-H Acetoxylation and Hydroxylation Reactions

An efficient Cp*Rh(III)-catalyzed, chelation-assisted C(sp²)-H acetoxylation and hydroxylation reaction has been developed for the first time. The reaction proceeds under mild conditions and allows for selective preparation of C-H acetoxylation and hydroxylation products, thus providing a good complement to previous C-H oxygenation reactions and expanding the field of Cp*Rh(III)-catalyzed C-H functionalizations.

A bioinspired and biocompatible ortho-sulfiliminyl phenol synthesis

Synthetic methods inspired by Nature often offer unique advantages including mild conditions and biocompatibility with aqueous media. Inspired by an ergothioneine biosynthesis protein EgtB, a mononuclear non-haem iron enzyme capable of catalysing the C-S bond formation and sulfoxidation, herein, we discovered a mild and metal-free C-H sulfenylation/intramolecular rearrangement cascade reaction employing an internally oxidizing O-N bond as a directing group. Our strategy accommodates a variety of oxyamines with good site selectivity and intrinsic oxidative properties. Combining an O-N bond with an X-S bond generates a C-S bond and an S=N bond rapidly. The newly discovered cascade reaction showed excellent chemoselectivity and a wide substrate scope for both oxyamines and sulfenylation reagents. We demonstrated the biocompatibility of the C-S bond coupling reaction by applying a coumarin-based fluorogenic probe in bacterial lysates. Finally, the C-S bond coupling reaction enabled the first fluorogenic formation of phospholipids, which self-assembled to fluorescent vesicles in situ.

Synthesis of (NHC)Pd(salicylaldimine)Cl complexes through template-directed ortho-aromatic metaloxylation of NHC-palladacycles derived from arylimines

Reactions of imidazoliums with palladacycles afforded NHC-palladacycle complexes. The reactivity of the NHC-palladacycles was explored and the results demonstrated that they could be template-directed ortho-aromatic metaloxylated in the presence of tert-butyl hydroperoxide (TBHP), affording (NHC)Pd(salicylaldimine)Cl complexes. Furthermore, the NHC-palladacycles showed efficient catalytic activities toward the ortho-aromatic hydroxylation of arylimines.