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

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.

Mechanistic Approach Toward the C4-Selective Amination of Pyridines via Nucleophilic Substitution of Hydrogen

The development of site-selective functionalization of N-heteroarenes is highly desirable in streamlined synthesis. In this context, direct amination of pyridines stands as an important synthetic methodology, with particular emphasis on accessing 4-aminopyridines, a versatile pharmacophore in medicinal chemistry. Herein, we report a reaction manifold for the C4-selective amination of pyridines by employing nucleophilic substitution of hydrogen (SNH). Through 4-pyridyl pyridinium salt intermediates, 4-aminopyridine products are obtained in reaction with aqueous ammonia without intermediate isolation. The notable regioselectivity was achieved by the electronic tuning of the external pyridine reagents along with the maximization of polarizability in the proton elimination stage. Further mechanistic investigations provided a guiding principle for the selective C-H pyridination of additional N-heteroarenes, presenting a strategic avenue for installation of diverse functional groups.

TsCl promoted deoxygenative phosphorothiolation of quinoline N-oxides towards S-quinolyl phosphorothioates

A convenient, efficient and practical approach for the synthesis of S-quinolyl phosphorothioates via cheap TsCl promoted deoxygenative C2-H phosphorothiolation of quinoline N-oxides with readily available triethylammonium O,O-dialkylphosphorothioates was developed. The reaction performed well under transition-metal-free conditions at room temperature with a very short reaction time (10-20 min). Preliminary studies showed that the current transformation underwent a nucleophilic substitution process.

Phosphonium Salt-Promoted C2-H Functionalization of Heterocyclic N-Oxides

We report the development of a phosphonium salt as a remarkable activating agent that enables the direct conversion of C2-H bonds of a variety of heterocyclic N-oxides into C2-N, C2-O, or C2-S bonds with high efficiency. The phosphonium salt was prepared on a >150 g scale in a single step and is tolerant of multiple functionalities.

Synthesis and Photophysical Properties of α-(N-Biphenyl)-Substituted 2,2'-Bipyridine-Based Push-Pull Fluorophores

A series of new α-(N-biphenyl)-substituted 2,2'-bipyridines were obtained through the combination of the ipso-nucleophilic aromatic substitution of the C5-cyano group, aza-Diels-Alder and Suzuki cross-coupling reactions, starting from 5-cyano-1,2,4-triazines. For the obtained compounds, photophysical and fluorosolvatochromic properties were studied. Fluorophores 3l and 3b demonstrated unexpected AIEE activity, while 3a and 3h showed promising nitroexplosive detection abilities.

N-Oxide-Induced Ugi Reaction: A Rapid Access to Quinoline-C2-amino Amides via Deoxygenative C(sp2)-H Functionalization

A logic-based replacement of the carboxylic acid component of the Ugi reaction by quinoline N-oxides has been developed. In this approach, the carboxylic isostere, quinoline N-oxide, plays a vital role by shifting the equilibria toward the product side with irreversible addition onto the C2-position of the N-oxide. Thus, aldehydes react with amines, isocyanides, and quinoline N-oxides to furnish quinoline four-component Ugi adducts. The unique reactivity of N-oxides with Ugi components opens an efficient synthetic route for the preparation of biologically active compounds.

C2-Selective, Functional-Group-Divergent Amination of Pyrimidines by Enthalpy-Controlled Nucleophilic Functionalization

Synthesis of heteroaryl amines has been an important topic in organic chemistry because of their importance in small-molecule discovery. In particular, 2-aminopyrimidines represent a highly privileged structural motif that is prevalent in bioactive molecules, but a general strategy to introduce the pyrimidine C2-N bonds via direct functionalization is elusive. Here we describe a synthetic platform for site-selective C-H functionalization that affords pyrimidinyl iminium salt intermediates, which then can be transformed into various amine products in situ. Mechanism-based reagent design allowed for the C2-selective amination of pyrimidines, opening the new scope of site-selective heteroaryl C-H functionalization. Our method is compatible with a broad range of pyrimidines with sensitive functional groups and can access complex aminopyrimidines with high selectivity.

Site-Selective Deoxygenative Amination of Azine N-Oxides with Carbodiimides under Catalyst-, Activator-, Base-, and Solvent-Free Conditions

An operationally simple method for synthesizing 2-amino azines via [3+2] dipolar cycloaddition of azine N-oxide with carbodiimide has been demonstrated. The reaction can proceed smoothly under simple heating conditions without any transition metal catalyst, activator, base, and solvent. This transformation demonstrates a broad substrate scope and produces CO₂ as the only co-product. The applicability of this method is highlighted by the late-stage modification of bioactive molecules, including quinine, (±)-α-tocopherol, and tryptamine modified quinoline.

Site-Selective C8-Alkylation of Quinoline N-Oxides with Maleimides under Rh(III) Catalysis

The site-selective modification of quinolines and their analogs has emerged as a pivotal topic in medicinal chemistry and drug discovery. Herein, we describe the rhodium(III)-catalyzed C8-alkylation of quinoline N-oxides with maleimides as alkylating agents, resulting in the formation of bioactive succinimide-containing quinoline derivatives. The reaction proceeds under mild conditions with complete functional group tolerance.

Recent advances in the synthesis and reactivity of quinoxaline

Quinoxalines are observed in several bioactive molecules and have been widely employed in designing molecules for DSSC's, optoelectronics, and sensing applications. Therefore, developing newer synthetic routes as well as novel ways for their functionalization is apparent.