Rh(III)-Catalyzed C7-Alkylation of Isatogens with Malonic Acid Diazoesters

Rh(III)-catalyzed C7-alkylation of isatogens (indolin-3-one N-oxides) with malonic acid diazoesters has been developed. This strategy utilizes oxygen anion on the N-oxide group of isatogens as a directing group and successfully achieves the synthesis of a series of C7-alkylated isatogens with moderate to good yields (48-86% yields). Moreover, the N-oxides of isatogens can not only serve as the simple directing group for C7-H bond cleavage but also be deoxidized for easy removal.

C-H functionalization of quinoline N-oxides catalyzed by Pd(II) complexes: a computational study

Pd(II) catalysts, particularly the acetate salt in acetic acid, tended to favor regioselective C-H activation of quinoline N-oxides (QOs) at the C2 position. However, Pd(II)Cl2 was shown to catalyze their C-H activation at C8 and, in the presence of water, C8-H activation was accompanied by the formation of 2-quinolinones. The aim of the DFT study described in this work was to shed light on the complete mechanism of these competing catalytic reactions, when PdCl2 reacts with QO and benzaldehyde in dichloroethane. C-H activation of QO was the first step of the reaction and involved either a metallacycle, with a CQO-Pd(II) σ-bond and a C(8)-H-Pd(II) agostic bond, or an η3-QO complex, with three carbon atoms of the heteroring of QO binding PdCl2. The first situation led to the unusual C8 activation and the second to C2 activation. The σ-metallacycle undergoes C8-H activation and the energy of the TOF determining the transition state to form the product is ∼17 kcal mol-1, while for the reaction through the π-metallacycle (C2-H activation) the corresponding energy is higher (∼29 kcal mol-1) and thus is not competitive under the same conditions. The reaction proceeding through the σ-complex, activating the C8 position, is preferred, in agreement with experimental results. Both reactions involve oxidation of Pd(II) to Pd(IV) and the catalyst is regenerated. When small amounts of water are added to the reaction mixture, C8-H activation (acylation) results from the same σ-metallacycle with the same barrier, but the simultaneous formation of 2-quinolinones is more complicated. It starts with OH- attack at the C2 position, and is followed by the migration of two hydrogen atoms, and the final reductive elimination step ends with Pd(0). The higher barriers for the migration and reoxidation of Pd(0) are associated with the more demanding reaction conditions. The different reactivity of Pd(II)(OAc)2 under analogous conditions is clarified, as it is only capable of forming the above mentioned π-complex and thus of activating the C2 position of QO. This catalyst can preferentially activate the C8-H bond under rather different conditions, including in particular acetic acid medium, as shown by other authors.

Direct Electrochemical Synthesis of 2,3-Disubstituted Quinoline N-oxides by Cathodic Reduction of Nitro Arenes

The use of electric current in synthetic organic chemistry offers a sustainable tool for the selective reductive synthesis of quinoline N-oxides starting from easily accessible nitro compounds. The reported method employs mild and reagent-free conditions, a simple undivided cell, and constant current electrolysis set-up which provides conversion with a high atom economy. The synthesis of 30 differently substituted quinoline N-oxides was successfully performed in up to 90 % yield. Using CV studies, the mechanism of the selective formation of the quinoline N-oxides was elucidated. The technical relevance of the described reaction could be shown in a 50-fold scale-up reaction.

Heterogeneous Photocatalysis as a Potent Tool for Organic Synthesis: Cross-Dehydrogenative C-C Coupling of N-Heterocycles with Ethers Employing TiO2/N-Hydroxyphthalimide System under Visible Light

Despite the obvious advantages of heterogeneous photocatalysts (availability, stability, recyclability, the ease of separation from products and safety) their application in organic synthesis faces serious challenges: generally low efficiency and selectivity compared to homogeneous photocatalytic systems. The development of strategies for improving the catalytic properties of semiconductor materials is the key to their introduction into organic synthesis. In the present work, a hybrid photocatalytic system involving both heterogeneous catalyst (TiO₂) and homogeneous organocatalyst (N-hydroxyphthalimide, NHPI) was proposed for the cross-dehydrogenative C-C coupling of electron-deficient N-heterocycles with ethers employing t-BuOOH as the terminal oxidant. It should be noted that each of the catalysts is completely ineffective when used separately under visible light in this transformation. The occurrence of visible light absorption upon the interaction of NHPI with the TiO₂ surface and the generation of reactive phthalimide-N-oxyl (PINO) radicals upon irradiation with visible light are considered to be the main factors determining the high catalytic efficiency. The proposed method is suitable for the coupling of π-deficient pyridine, quinoline, pyrazine, and quinoxaline heteroarenes with various non-activated ethers.

Photoinduced carbamoylation reactions: unlocking new reactivities towards amide synthesis

The preparation of amide-containing compounds is among the most interesting and challenging topics for the synthetic community. Such relevance is given by their reactive aspects explored in the context of organic synthesis and by the direct application of these compounds as pharmaceuticals and useful materials, and their key roles in biological structures. A simple and straightforward strategy for the amide moiety installation is the use of carbamoyl radicals - this nucleophilic one-electron intermediate is prone to undergo a series of transformations, providing a range of structurally relevant derivatives. In this review, we summarize the latest advances in the field from the perspective of photoinduced protocols. To this end, their synthetic applications are organized accordingly to the nature of the radical precursor (formamides through HAT, 4-substituted-1,4-dihydropyridines, oxamic acids, and N-hydroxyphthalimido esters), the mechanistic aspects also being highlighted. The discussion also includes a recent approach proceeding via photolytic C-S cleavage of dithiocarbamate-carbamoyl intermediates. By exploring fundamental concepts, this material aims to offer an understanding of the topic, which will encourage and facilitate the design of new synthetic strategies applying the carbamoyl radical.

Oxamic acids: useful precursors of carbamoyl radicals

This review article describes the recent development in the chemistry of carbamoyl radicals generated from oxamic acids. This mild and efficient method compares well with previous methods of generation of these nucleophilic radicals. The oxidative decarboxylation of oxamic acids can be mediated through thermal, photochemical, electrochemical or photoelectrochemical means, generating carbamoyl radicals, which may further add to unsaturated systems to provide a broad range of important amides. Oxidative decarboxylation of oxamic acids also offers a straightforward entry for the preparation of urethanes, ureas, and thioureas.

Direct C-H aminocarbonylation of N-heteroarenes with isocyanides under transition metal-free conditions

A C-C bond forming amide synthesis through direct C-H aminocarbonylation of N-heteroarenes with isocyanides was developed. The reaction was mediated by an inorganic persulfate salt under transition metal-free conditions. Mechanistic studies suggested a radical pathway for this reaction without the participation of H₂O and O₂. This method also showed merits of substrate availability, easy operation and atom economy. It provided an efficient route for straightforward synthesis of N-heteroaryl amides.

Cross-dehydrogenative Coupling Reactions Between Formamidic C(sp2)-H and X-H (X = C, O, N) Bonds

In the last decade, the scientific community has witnessed explosive growth in research on the direct carbamoylation of C-H and X-H (X = N, O) bonds with formamides via cross-dehydrogenative coupling reactions. This novel approach is an effective means of preparing a variety of carboxamide, carbamate as well as urea derivatives, which are prevalent in medicinal chemistry and natural product synthesis. This review elaborates the most important advances and developments in the field, with an emphasis on the reaction patterns and mechanisms.

Co(III)-Catalyzed C-H Amidation of Nitrogen-Containing Heterocycles with Dioxazolones under Mild Conditions

A cobalt(III)-catalyzed C-8 selective C-H amidation of quinoline N-oxide using dioxazolone as an amidating reagent under mild conditions is disclosed. The reaction proceeds efficiently with excellent functional group compatibility. The utility of the current method is demonstrated by gram scale synthesis of C-8 amide quinoline N-oxide and by converting this amidated product into functionalized quinolines. Furthermore, the developed catalytic method is also applicable for C-7 amidation of N-pyrimidylindolines and ortho-amidation of benzamides.

Silver-catalyzed direct C-H oxidative carbamoylation of quinolines with oxamic acids

A silver-catalyzed efficient and direct C-H carbamoylation of quinolines with oxamic acids to access carbamoylated quinolines has been developed through oxidative decarboxylation reaction. The reaction proceeds smoothly over a broad range of substrates with excellent functional group tolerance and excellent yields under mild conditions.

Deoxygenative Amination of Azine-N-oxides with Acyl Azides via [3 + 2] Cycloaddition

A transition-metal-free deoxygenative C-H amination reaction of azine-N-oxides with acyl azides is described. The initial formation of an isocyanate from the starting acyl azide via a Curtius rearrangement can trigger a [3 + 2] dipolar cycloaddition of polar N-oxide fragments to generate the aminated azine derivative. The applicability of this method is highlighted by the late-stage and sequential amination reactions of complex bioactive compounds, including quinidine and fasudil. Moreover, the direct transformation of aminated azines into various bioactive N-heterocycles illustrates the significance of this newly developed protocol.

A mild and metal-free synthesis of 2- and 1-alkyl/aryl/dialkyl-aminoquinolines and isoquinolines

A simple synthetic strategy has been developed for the synthesis of 2- and 1-alkyl/aryl/dialkylaminoquinolines and isoquinolines from the easily available quinoline and isoquinoline-N-oxides, different amines, triflic anhydride as activating agent and acetonitrile as solvent in a one-pot reaction under metal-free conditions at 0 °C to room temperature.

Regioselective Arylation of Quinoline N-Oxides (C8), Indolines (C7) and N-tert-Butylbenzamide with Arylboronic Acids

Herein, we disclose Ru(II)-catalyzed regioselective distal C(sp2)-H arylation of quinoline N-oxide with arylboronic acids to 8-arylquinolines. In the developed method, the Ru(II)-catalyst shows dual activity, that is, distal C-H activation of quinoline N-oxides followed by in situ deoxygenation of arylated quinoline N-oxide in the same pot. The current catalytic method features use of Ru metal as the catalyst and arylboronic acids as the arylating source under mild reaction conditions. Use of the Rh(III)-catalyst in place of Ru(II) under the same conditions afforded 8-arylquinoline N-oxides with excellent regioselectivity. Furthermore, the developed Ru(II) catalytic system is also extended for the C(sp2)-H arylation of indolines, N-tert-butylbenzamide, and 6-(5H)-phenanthridinone. Formation of the quinoline N-oxide coordinated ruthenium adduct is found to be the key reaction intermediate, which has been characterized by single crystal X-ray diffraction and NMR spectroscopy.

Transition-metal free direct C–H functionalization of quinoxalin-2(1H)-ones with oxamic acids leading to 3-carbamoyl quinoxalin-2(1H)-ones

A practical transition-metal free decarboxylative coupling reaction of oxamic acids with quinoxalin-2(1H)-ones has been developed under mild conditions.

Transition-metal- and oxidant-free three-component reaction of quinoline N-oxides, sodium metabisulfite and aryldiazonium tetrafluoroborates via a dual radical coupling process

A convenient and straightforward three-component transformation of quinoline N-oxides, sodium metabisulfite and aryldiazonium tetrafluoroborates has been developed, providing the target products in moderate to good yields. Compared with previous studies, the present methodology avoids the use of transition-metal catalysts and excess oxidants, providing a simple and practical alternative approach for the construction of 2-sulfonylquinolines. Control experiments indicate that a dual radical coupling process is responsible for this reaction.