Palladium-catalyzed difluorocarbene transfer enables access to enantioenriched chiral spirooxindoles

We disclose herein an unprecedented Pd-catalyzed difluorocarbene transfer reaction, which assembles a series of structurally interesting chiral spiro ketones with generally over 90% ee. Commercially available BrCF2CO2K serves as the difluorocarbene precursor, which is harnessed as a user-friendly and safe carbonyl source in this transformation. Preliminary mechanistic studies exclude the formation of free CO in the reaction process, and importantly, we also find that BrCF2CO2K outcompete gaseous CO and several common CO surrogates in this asymmetric process. The reaction mechanism, including the in-situ progressive release of the difluorocarbene, the rapid migratory insertion of ArPd(II) = CF2 species, and subsequent defluorination hydrolysis by water to introduce the carbonyl group, accounts for the overall high efficiency and uniqueness. This work clearly showcases the advantage and potential of the difluorocarbene in synthesis and supplies a mechanistically distinct route for asymmetric carbonylative cyclization reactions.

Sustainable preparation of 2-acylbenzothiazoles under the cooperation of ionic liquids and microwave irradiation

Microwave irradiation (MW) and ionic liquids (ILs) are two of the most promising relatively greener synthetic approaches to preparing value-added chemicals. Herein, a series of 2-acylbenzothiazole derivatives were synthesized for the first time from commercially available α-bromoacetophenones and disulfane-diyl-dianilines through the cooperation of ionic liquids and microwave irradiation under metal- and extra-additives-free conditions. A plausible mechanism involving the successive IL-induced enolation has been proposed.

Palladium-Catalyzed Modular Assembly of P-Stereogenic and Axially Chiral Phosphinooxazoles (PHOX) Ligands by C-P Bond Cleavage/Intermolecular C(sp2)-H Bond Functionalization

Chiral P,N-ligands are of great interest and importance in the fields of metal-catalyzed enantioselective transformations and have found numerous applications spanning drug and polymer synthesis. Here, modular assembly of diverse P-stereogenic and axially chiral phosphinooxazoles ligands is achieved through palladium-catalyzed asymmetric cleavage of C-P bond/intermolecular C-H bond functionalization in high atroposelectivities and diastereoselectivities of up to >99% ee and >25:1 dr. This protocol features broad substrate scope and provides an avenue for facile construction of new P-stereogenic and axially chiral phosphinooxazoles ligands directly from the phosphonium salts and benzoxazoles/benzothiazoles. Evaluation of the synthesized P-stereogenic and axially chiral phosphinooxazoles ligands in two model catalytic asymmetric reactions illustrates the potential of our strategy to access valuable chiral molecules.

Plausible PEPPSI catalysts for direct C-H functionalization of five-membered heterocyclic bioactive motifs: synthesis, spectral, X-ray crystallographic characterizations and catalytic activity

In this study, a series of benzimidazolium salts were synthesized as asymmetric N-heterocyclic carbene (NHC) precursors. Nine novel palladium complexes with the general formula [PdX2(NHC)(pyridine)] were synthesized using benzimidazolium salts in the PEPPSI (Pyridine Enhanced Precatalyst Preparation, Stabilization and Initiation) theme. All synthesized Pd(ii) complexes are stable. The synthesized compounds were thoroughly characterized by respective spectroscopic techniques, such as 1HNMR, 13C NMR, FTIR spectroscopy, X-ray crystallography and elemental analysis. The geometric structure of the palladium N-heterocyclic carbene has been optimized in the framework of density functional theory (DFT) using the B3LYP-D3 dispersion functional with LANL2DZ as a basis set. The on/off mechanism of pyridine assisted Pd-NHC complexes made them the best C-H functionalized catalysts for regioselective C-5 arylated products. Five membered heterocyclic compounds such as 2-acetyl furan, furfuryl acetate 2-acetylthiophene and N-methylpyrrole-2-carboxaldehyde were treated with numerous aryl bromides and arylchlorides under optimal catalytic reaction conditions. Interestingly, all the prepared catalysts possessed essential structural features that facilitated the formation of desired coupling products in quantitative yield with excellent selectivity. The arylation reaction of bromoacetophenone was highly catalytically active with only 1 mol% catalyst loading at 150 °C for 2 hours. To check the efficiency of the synthesized complexes, three different five member heterocyclic substrates (2-acetylfuran, 2-acetylthiophen, 2-propylthaizole) were tested with a number of aryl bromides bearing both electron-donating and electron-withdrawing groups on para position. The data in Tables 2-4. Indicated that electron-donating groups on the para position of aryl halide decreased the catalytic conversion while electron-withdrawing groups increased the catalytic conversion this was due to the high nucleophilicity of the electron-donating substituents.

Direct Dehydrogenative Access to Unsymmetrical Phenones

The first non-directed dehydrogenative phenone coupling method of methylarenes with aromatic C-H bonds, displaying a large substrate scope, is herein reported. This reaction represents a far more direct atom- and step-efficient alternative to the classical Friedel-Crafts or Suzuki-Miyaura derived acylation reactions. The method can be carried out on a gram scale and was successfully applied to the synthesis of several Ketoprofen drug analogues.

Fe/S-Catalyzed synthesis of 2-benzoylbenzoxazoles and 2-quinolylbenzoxazoles via redox condensation of o-nitrophenols with acetophenones and methylquinolines

An Fe/S catalyst generated in situ from FeCl₂·4H₂O and elemental sulfur S₈ in the presence of a tertiary amine as a base was found to catalyze efficiently a 6e^- redox condensation of o-nitrophenols with acetophenones and methylquinolines. The condensed products 2-benzoylbenzoxazoles and 2-quinolylbenzoxazoles were obtained in reasonable yields with water as the only byproduct at a temperature as low as 80 °C.

Accessing C2-Functionalized 1,3-(Benz)azoles through Transition Metal-Catalyzed C-H Activation

The skeletal presence of 1,3-azoles in a variety of bioactive natural products, pharmacophores, and organic materials demands the derivatization of such heteroarenes regioselectively. Plenty of cross-coupling as well as cyclocondensation reactions have been performed to build up these skeletons but remained commercially unrealizable. A couple of severe drawbacks are faced by these traditional protocols that require a more straightforward strategy to obviate them. Transition metal-catalyzed C-H functionalization has emerged as a superior alternative in that context. 1,3-Azoles and their benzo counterparts have been extensively functionalized exploiting both noble and earth-abundant transition metals. Lately, C-2 functionalization have gained much traction due to the ease of attaining high regioselectivity and installation of synthetically manipulative functionalities. This critical review presents a bird's eye view of all major C-2 functionalization of (benz)azoles catalyzed by a diverse set of metals performed over the past 15 years.

Enhancing stability by trapping palladium inside N-heterocyclic carbene-functionalized hypercrosslinked polymers for heterogeneous C-C bond formations

Catalyst deactivation caused by the aggregation of active metal species in the reaction process poses great challenges for practical applications of supported metal catalysts in solid-liquid catalysis. Herein, we develop a hypercrosslinked polymer integrated with N-heterocyclic carbene (NHC) as bifunctional support to stabilize palladium in heterogeneous C-C bond formations. This polymer supported palladium catalyst exhibits excellent stability in the one-pot fluorocarbonylation of indoles to four kinds of valuable indole-derived carbonyl compounds in cascade or sequential manner, as well as the representative Suzuki-Miyaura coupling reaction. Investigations on stabilizing effect disclose that this catalyst displays a molecular fence effect in which the coordination of NHC sites and confinement of polymer skeleton contribute together to stabilize the active palladium species in the reaction process. This work provides new insight into the development of supported metal catalysts with high stability and will also boost their efficient applications in advanced synthesis.

Catalyst deactivation caused by the aggregation of active metal species poses great challenges for supported metal catalyzed solid-liquid reactions. Here, the authors develop a hypercrosslinked polymer integrated with N-heterocyclic carbene (NHC) as bifunctional support to stabilize palladium in heterogeneous C-C bond formations.

Tunable C–H arylation and acylation of azoles with carboxylic acids by Pd/Cu cooperative catalysis

Direct C–H arylation and acylation of azoles with carboxylic acids are achieved selectively through Pd/Cu cooperative catalysis: biaryls are generated selectively with dppp as ligand, while biaryl ketones are obtained with high selectivity using dpph or Ph₂PCy as ligand.

A Synthetic Approach to Dimetalated Arenes Using Flow Microreactors and the Switchable Application to Chemoselective Cross-Coupling Reactions

In spite of their potential utility, the chemistry of dimetalated arenes is still in its infancy because they are extremely difficult to synthesize. We report a novel method of synthesizing arenes bearing a boryl group and a metallic substituent, such as boryl, silyl, stannyl, or zincyl groups, in an integrated flow microreactor based on the generation and reactions of aryllithiums bearing a trialkyl borate moiety. The bimetallic arenes showed a remarkable chemoselectivity in palladium-catalyzed cross-coupling reactions. The selectivity was switched by the selection of the metal species that constitutes the dimetalated arenes as well as appropriate catalysts.

Palladium catalyzed carbonylative generation of potent, pyridine-based acylating electrophiles for the functionalization of arenes to ketones

We describe here the design of a palladium catalyzed route to generate aryl ketones via the carbonylative coupling of (hetero)arenes and aryl- or vinyl-triflates. In this, the use of the large bite angle Xantphos ligand on palladium provides a unique avenue to balance the activation of the relatively strong C(sp²)–OTf bond with the ultimate elimination of a new class of potent Friedel–Crafts acylating agent: N-acyl pyridinium salts. The latter can be exploited to modulate reactivity and selectivity in carbonylative arene functionalization chemistry, and allow the efficient synthesis of ketones with a diverse array of (hetero)arenes.

A palladium catalyzed approach to the overall carbonylative functionalization of arenes to form ketones with aryl- and vinyl-triflates is described.

One-Pot Multicomponent Reaction of Catechols, Ammonium Acetate, and Aldehydes for the Synthesis of Benzoxazole Derivatives Using the Fe(III)-Salen Complex

The Fe(III)-salen complex has been applied successfully as a catalyst for the novel, simple, efficient, and one-pot multicomponent synthesis of benzoxazole derivatives from catechols, ammonium acetate as the nitrogen source, and aldehydes (nontoxic and cheap alternatives of amines) for the first time. Using this procedure, a wide range of benzoxazoles was successfully synthesized in the presence of a catalyst in EtOH under mild conditions, and all products were obtained in excellent yields. To the best of our knowledge, this method is the first example of the multicomponent synthesis of benzoxazole derivatives using these starting materials. The notable features such as the use of air that is considered as a benign oxidant and EtOH as a green solvent, ease of product separation, readily available and inexpensive aldehydes, and mild conditions make our procedure more efficient and practical for organic synthesis. Moreover, the current protocol is successfully applied to synthesize desirable products on a large scale.

A palladium-catalyzed C-H functionalization route to ketones via the oxidative coupling of arenes with carbon monoxide

We describe the development of a new palladium-catalyzed method to generate ketones via the oxidative coupling of two arenes and CO. This transformation is catalyzed by simple palladium salts, and is postulated to proceed via the conversion of arenes into high energy aroyl triflate electrophiles. Exploiting the latter can also allow the synthesis of unsymmetrical ketones from two different arenes.

A palladium catalyzed route to prepare aryl ketones from their two fundamental building blocks, two arenes and carbon monoxide, is described.

i-Pr2NMgCl·LiCl Enables the Synthesis of Ketones by Direct Addition of Grignard Reagents to Carboxylate Anions

The direct preparation of ketones from carboxylate anions is greatly limited by the required use of organolithium reagents or activated acyl sources that need to be independently prepared. Herein, a specific magnesium amide additive is used to activate and control the addition of more tolerant Grignard reagents to carboxylate anions. This strategy enables the modular synthesis of ketones from CO₂ and the preparation of isotopically labeled pharmaceutical building blocks in a single operation.

Catalytic C-H/C-F Coupling of Azoles and Acyl Fluorides

A method for the palladium/copper-catalyzed direct acylation of azoles with acyl fluorides is described. This study reports the first examples of acyl fluorides being used as acylation reagents in transition-metal-catalyzed aromatic C-H bond functionalization reactions. Depending on the reaction temperature, decarbonylative coupling may also occur. Mechanistic studies suggest that the cleavage of the aromatic C-H bond, promoted by a copper-phosphine species, is not the rate-limiting step of this acylation.

Pd/Cu Cocatalyzed Oxidative Tandem C-H Aminocarbonylation and Dehydrogenation of Tryptamines: Synthesis of Carbolinones

The Pd/Cu cocatalyzed oxidative tandem C-H aminocarbonylation and dehydrogenation was developed, affording carbolinones with molecular oxygen as the terminal oxidant. Natural product strychnocarpine and its derivatives were prepared conveniently using this strategy.

Natural product syntheses via carbonylative cyclizations

This review summarizes the application of various transition metal-catalyzed/mediated carbonylative cyclization reactions in natural product total synthesis.

Pd-Catalyzed C(sp2)–H aminocarbonylation using the Langlois reagent as a carbonyl source

An efficient Pd-catalyzed C(sp²)–H aminocarbonylation of aryl carboxamides assisted by an N,S-bidentate directing group was developed.

Palladium(II)/N-Heterocyclic Carbene-Catalyzed Direct C-H Acylation of Heteroarenes with N-Acylsaccharins

N-Acylsaccharin represents a facile acyl group transfer agent to heteroarenes in the presence of Pd(II)/NHC complexes appended with a pyrene unit. Catalytic acylation of heteroarenes was enhanced by the noncovalent interaction between the pyrene unit and substrates. High functional group tolerance, broad substrate scope, and moderate to good yields of 2-acylated azoles are added features of this method.

Rh-catalyzed aerobic oxidative cyclization of anilines, alkynes, and CO

We describe a novel Rh-catalyzed C–H cyclization of a wide range of anilines with alkynes and CO. Particularly, simple primary anilines and readily prepared tertiary anilines can be easily converted to quinolin-2(1H)-ones via C–N bond cleavage.

Transition-metal-catalyzed oxidative C–H cyclization of anilines has been an attractive and powerful strategy for the efficient construction of N-heterocycles. However, primary and tertiary anilines are rarely employed in this strategy due to the relative instability with strong oxidants or the presence of three C–N bonds. We describe here a novel Rh-catalyzed C–H cyclization of a wide range of anilines with alkynes and CO, using an aerobic oxidative protocol. Particularly, the simple primary anilines and readily prepared tertiary anilines could be easily converted to quinolin-2(1H)-ones, which are high value-added, biologically significant N-heterocycles, via C–N bond cleavage.

OMS-2-Supported Cu Hydroxide-Catalyzed Benzoxazoles Synthesis from Catechols and Amines via Domino Oxidation Process at Room Temperature

In the presence of manganese oxide octahedral molecular sieve (OMS-2) supported copper hydroxide Cu(OH)_x/OMS-2, aerobic synthesis of benzoxazoles from catechols and amines via domino oxidation/cyclization at room temperature is achieved. This heterogeneous benzoxazoles synthesis initiated by the efficient oxidation of catechols over Cu(OH)_x/OMS-2 tolerates a variety of substrates, especially amines containing sensitive groups (hydroxyl, cyano, amino, vinyl, ethynyl, ester, and even acetyl groups) and heterocycles, which affords functionalized benzoxazoles in good to excellent yields by employing low catalyst loading (2 mol % Cu). The characterization and plausible catalytic mechanism of Cu(OH)_x/OMS-2 are described. The notable features of our catalytic protocol such as the use of air as the benign oxidant and EtOH as the solvent, mild conditions, ease of product separation, being scalable up to the gram level, and superior reusability of catalyst (up to 10 cycles) make it more practical and environmentally friendly for organic synthesis.

Metal-catalyzed C–H functionalization involving isocyanides

This review demonstrates the metal-catalyzed incorporation of isocyanides into C–H functionalization to establish complicated reactions utilizing synergistic effects.

Covalent triazine framework-supported palladium as a ligand-free catalyst for the selective double carbonylation of aryl iodides under ambient pressure of CO

Carbonylation of aryl iodides with amines under atmospheric pressure of CO, catalyzed by Pd/CTFs (covalent triazine frameworks) without any specific additives, leads to the highly selective synthesis of α-ketoamides.

Sulfoximinocarbonylation of aryl halides using heterogeneous Pd/C catalyst

A heterogeneous Pd/C catalyzed three component protocol is developed for the sulfoximinocarbonylation of aryl halides.

Base-Promoted Domino Reaction of 5-Substituted 2-Nitrosophenols with Bromomethyl Aryl Ketones: A Transition-Metal-Free Approach to 2-Aroylbenzoxazoles

The reaction of 5-substituted 2-nitrosophenols with bromomethyl aryl ketones and related compounds employing K2CO3 as a base in refluxing THF and DMF at 80 °C, respectively, delivers 2-aroylbenzoxazoles in a single step with yields up to 85%. The new method involves an intermolecular nucleophilic substitution followed by intramolecular 1,2-addition and elimination. It allows an efficient and practical access to 2-aroylbenzoxazoles under transition-metal-free conditions.