Copper catalysed direct amidation of methyl groups with N-H bonds

Fifty Shades of Phenanthroline: Synthesis Strategies to Functionalize 1,10-Phenanthroline in All Positions

1,10-Phenanthroline (phen) is one of the most popular ligands ever used in coordination chemistry due to its strong affinity for a wide range of metals with various oxidation states. Its polyaromatic structure provides robustness and rigidity, leading to intriguing features in numerous fields (luminescent coordination scaffolds, catalysis, supramolecular chemistry, sensors, theranostics, etc.). Importantly, phen offers eight distinct positions for functional groups to be attached, showcasing remarkable versatility for such a simple ligand. As a result, phen has become a landmark molecule for coordination chemists, serving as a must-use ligand and a versatile platform for designing polyfunctional arrays. The extensive use of substituted phenanthroline ligands with different metal ions has resulted in a diverse array of complexes tailored for numerous applications. For instance, these complexes have been utilized as sensitizers in dye-sensitized solar cells, as luminescent probes modified with antibodies for biomaterials, and in the creation of elegant supramolecular architectures like rotaxanes and catenanes, exemplified by Sauvage's Nobel Prize-winning work in 2016. In summary, phen has found applications in almost every facet of chemistry. An intriguing aspect of phen is the specific reactivity of each pair of carbon atoms ([2,9], [3,8], [4,7], and [5,6]), enabling the functionalization of each pair with different groups and leading to polyfunctional arrays. Furthermore, it is possible to differentiate each position in these pairs, resulting in non-symmetrical systems with tremendous versatility. In this Review, the authors aim to compile and categorize existing synthetic strategies for the stepwise polyfunctionalization of phen in various positions. This comprehensive toolbox will aid coordination chemists in designing virtually any polyfunctional ligand. The survey will encompass seminal work from the 1950s to the present day. The scope of the Review will be limited to 1,10-phenanthroline, excluding ligands with more intracyclic heteroatoms or fused aromatic cycles. Overall, the primary goal of this Review is to highlight both old and recent synthetic strategies that find applicability in the mentioned applications. By doing so, the authors hope to establish a first reference for phenanthroline synthesis, covering all possible positions on the backbone, and hope to inspire all concerned chemists to devise new strategies that have not yet been explored.

Metal-Free Chemoselective Oxidation of 4-Methylquinolines into Quinoline-4-Carbaldehydes

A convenient protocol for the synthesis of quinoline-4-carbaldehydes via chemoselective oxidation of 4-methylquinolines using hypervalent iodine(III) reagents as oxidant is described. This method highlights metal-free and mild reaction conditions, nice yield, good functional group tolerance, and high chemoselectivity.

Urea hydrogen peroxide-initiated synthesis of pyranopyrazoles through oxidative coupling under base- and metal-free conditions by physical grinding method

A novel multicomponent one-pot expeditious synthesis of highly functionalized and pharmaceutically fascinated pyranopyrazoles has been developed. This reaction occurs via tandem Knoevenagel condensation reaction of methyl aryl derivatives, 3-methyl pyrazolone and malononitrile in the presence of urea hydrogen peroxide under the physical grinding method. The present methodology offers several benefits such as available green and cheap starting materials, solvent-free, mild reaction conditions, high atom economy, eco-friendly standards, excellent yields and easy isolation of the products without column chromatographic separation.

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.

One-pot imine synthesis from methylarenes and anilines under air over heterogeneous Cu oxide-modified CeO2 catalyst

Selective one-pot synthesis of imines from methylarenes and anilines with air as an oxidant was substantiated by heterogeneous Cu oxide-modified CeO₂ (CuO_x–CeO₂) catalyst without additives.

Transition-Metal-Free Oxidation of Benzylic C-H Bonds of Six-Membered N-Heteroaromatic Compounds

A novel oxidation of benzylic C-H bonds for the synthesis of diverse six-membered N-heteroaromatic aldehydes and ketones has been developed. The obvious advantages of this approach are the simple operation, mild reaction conditions, and without use of toxic reagent and transition metal. The present method should provide a useful access for the synthesis and modification of N-heterocycles.

Iodine mediated oxidative cross-coupling of unprotected anilines and heteroarylation of benzothiazoles with 2-methylquinoline

Iodine-promoted oxidative C-H/C-H cross-coupling of unprotected anilines and 2-methylquinoline to furnish C4-carbonylated aniline (4-aminophenyl)(quinoline-2-yl) methanones in moderate to good yields has been demonstrated. This work provides the first site-selective approach for the synthesis of free amino groups containing methanones including unprecedented C-H functionalization rather than the N-H functionalization of unprotected anilines via the Kornblum oxidation of 2-methylquinoline. Furthermore, we noticed that the incorporation of KOH under standard conditions provides 2-heteroarylbenzothiazoles from benzothiazoles and 2-methylquinoline in good to excellent yields. These transformations do not require any transition metals or peroxides and tolerate various functional groups such as methoxy, hydroxy, bromo, chloro and nitro groups. Moreover, a plausible mechanistic pathway is proposed.

Platinum(ii)-catalyzed selective para C-H alkoxylation of arylamines through a coordinating activation strategy

A highly efficient method to selectively install alkoxy onto the para position of arylamines via a coordinating activation strategy has been reported. Various substrates are compatible, providing the corresponding products in good to excellent yields. This strategy gives an efficient and practical solution for the synthesis of unsymmetrical aryl ethers. A free radical pathway mechanism is advised for transformation.

Copper Complexes of Multidentate Carboxamide Ligands

The copper coordination chemistry of two multidentate carboxamido ligands derived from HL¹ (offering two quinolyl and one carboxamide donor) and H₄L² (with two pyridine(dicarboxamido) units linked by naphthalene spacers) was explored. The former was chosen because upon deprotonation it would provide a monoanionic mer-coordinating N-donor set that would model the putative deprotonated form of the His-brace in copper monooxygenases, while the latter was designed to bind two copper ions and enable comparisons to other systems with different ligand spacers. Upon reaction with Cu(I)-mesityl, HL¹ yielded a symmetric dimer (L¹Cu)₂ in which each bis(quinolyl)amide ligand binds via two N-donors to one Cu(I) ion and via the third to the other Cu(I) center. Monomeric Cu(II) complexes [L ¹ Cu(H ₂ O) ₂ ](OTf) and L ¹ ₂ Cu were also characterized. Treatment of H₄L² with Cu(OTf)₂ and excess Me₄NOH (in CH₃CN, pyridine/H₂O, or MeOH) yielded complexes with anions of general formula [L ² Cu ₂ (X)]^n-, where X = CH₃CONH^- (n = 1), CO₃ ^2- (n = 2), or MeO^- (n = 1). X-ray structures of these complexes revealed the (L²)^4- ligand binding to two Cu(II) ions in an open paddle-wheel geometry, with an additional bridging ligand (X) completing the square planar coordination sphere of each metal ion. The open paddlewheel motif differs from the more 'open' puckered geometry seen with related ligands with different spacer units.

Oxidative C(sp3)–H functionalization of methyl-azaheteroarenes: a facile route to 1,2,4-triazolo[4,3-a]pyridines

An oxidative [4 + 1] annulation for triazolo[4,3-a]pyridine–quinoline linked diheterocycle synthesis via functionalization of the sp³ C–H bonds of 2-methyl-azaheteroarenes has been developed.

Copper-catalyzed α-C–H acyloxylation of carbonyl compounds with terminal alkynes

Herein, a copper/TBHP catalyst system for the α-C–H acyloxylation of carbonyl compounds is developed using terminal alkynes as the acyloxy source.

Co/NHPI-mediated aerobic oxygenation of benzylic C-H bonds in pharmaceutically relevant molecules

A simple cobalt(ii)/N-hydroxyphthalimide catalyst system has been identified for selective conversion of benzylic methylene groups in pharmaceutically relevant (hetero)arenes to the corresponding (hetero)aryl ketones. The radical reaction pathway tolerates electronically diverse benzylic C-H bonds, contrasting recent oxygenation reactions that are initiated by deprotonation of a benzylic C-H bond. The reactions proceed under practical reaction conditions (1 M substrate in BuOAc or EtOAc solvent, 12 h, 90-100 °C), and they tolerate common heterocycles, such as pyridines and imidazoles. A cobalt-free, electrochemical, NHPI-catalyzed oxygenation method overcomes challenges encountered with chelating substrates that inhibit the chemical reaction. The utility of the aerobic oxidation method is showcased in the multigram synthesis of a key intermediate towards a drug candidate (AMG 579) under process-relevant reaction conditions.

Cu-catalyzed aerobic oxidative C–CN bond cleavage of benzyl cyanide for the synthesis of primary amides

Cu-catalyzed oxidative amidation of benzyl cyanide for primary amides is successfully developed. Using readily available NH₄Cl and Cu/O₂ catalytic oxidation system offers new opportunities for C–CN bond cleavage and primary amides bond formation.

Synthesis of pyrrolo[1,2-a]quinoxalines via copper or iron-catalyzed aerobic oxidative carboamination of sp3C–H bonds

We developed an aerobic oxidative carboamination of sp³C–H bonds with 2-(1H-pyrrol-1-yl)anilines for synthesis of pyrrolo[1,2-a]quinoxalines.

TBHP-promoted direct oxidation reaction of benzylic Csp3–H bonds to ketones

A metal-free oxidation system employingtert-butyl hydroperoxide (TBHP) has been developed for selective oxidation of structurally diverse benzylic sp³C–H bonds.

Nonclassical Routes for Amide Bond Formation

The present review offers an overview of nonclassical (e.g., with no pre- or in situ activation of a carboxylic acid partner) approaches for the construction of amide bonds. The review aims to comprehensively discuss relevant work, which was mainly done in the field in the last 20 years. Organization of the data follows a subdivision according to substrate classes: catalytic direct formation of amides from carboxylic and amines ( section 2 ); the use of carboxylic acid surrogates ( section 3 ); and the use of amine surrogates ( section 4 ). The ligation strategies (NCL, Staudinger, KAHA, KATs, etc.) that could involve both carboxylic acid and amine surrogates are treated separately in section 5 .

Base metal-catalyzed benzylic oxidation of (aryl)(heteroaryl)methanes with molecular oxygen

The methylene group of various substituted 2- and 4-benzylpyridines, benzyldiazines and benzyl(iso)quinolines was successfully oxidized to the corresponding benzylic ketones using a copper or iron catalyst and molecular oxygen as the stoichiometric oxidant. Application of the protocol in API synthesis is exemplified by the alternative synthesis of a precursor to the antimalarial drug Mefloquine. The oxidation method can also be used to prepare metabolites of APIs which is illustrated for the natural product papaverine. ICP-MS analysis of the purified reaction products revealed that the base metal impurity was well below the regulatory limit.

Green synthesis of primary, secondary, and tertiary amides through oxidative amidation of methyl groups with amine hydrochlorides over recyclable CoFe2O4 NPs

This protocol provides high yield, short reaction time, and operational simplicity, green and low cast procedure for the synthesis of all types of amides.

Transition metal-free oxidative ortho-acylation of phenols with N-heteroarylmethanes via double C–H activation

The direct oxidative acylation of phenols with N-heteroarylmethanes via sp³C–H and sp²C–H double activation was achieved under metal-free conditions. The reaction proceeds in a facile I₂/DMSO/O₂ system regio-selectively to produce valuable (2-hydroxyphenyl)arylmethanones. A plausible mechanism was proposed.

Copper-catalysed aerobic oxidative esterification of N-heteroaryl methanes with alcohols

Efficient copper-catalysed aerobic oxidative esterification of N-heteroaryl methanes with alcohols has been developed. A variety of N-heteroaryl esters including those with functional groups are produced in good to excellent yields under the present reaction conditions.