Friedländer synthesis of chiral alkyl-substituted 1,10-phenanthrolines

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.

Tautomerism and Self-Association in the Solution of New Pinene-Bipyridine and Pinene-Phenanthroline Derivatives

Two novel pinene-type ligands have been synthesized and their tautomeric and self-associating behavior studied in solution and in the solid state. The first ligand, an acetylated derivative of 5,6-pinene-bipyridine, displays keto-enol tautomerism in solution. This tautomeric equilibrium was studied by NMR and UV-Vis spectroscopy in various solvents, and the results were compared with the ones obtained through DFT calculations. The second ligand was obtained by an unusual oxidation of the phenanthroline unit of a pinene-phenanthroline derivative. This compound exists as a single tautomer in solution and aggregates both in solution (as observed by NMR) and in the solid state through H-bonding as observed by X-ray structure determination and confirmed by DFT studies.

Highly Selective and Efficient Synthesis of 7-Aminoquinolines and Their Applications as Golgi-Localized Probes

Quinoline derivatives have extensively been used for both pharmaceutical agents and bioimaging. However, typical synthesis of quinoline derivatives is generally through strong acid/base-catalyzed or metal-catalyzed methods at high temperatures. Here we report a catalyst-free synthesis of 2,4-disubstituted 7-aminoquinolines with high selectivity and good yields via the introduction of a trifluoromethyl group. It is discovered that quinolines containing both amino and trifluoromethyl groups exhibit strong intramolecular charge-transfer fluorescence with large Stokes shifts. We further applied the obtained quinolines to live-cell imaging and found that some of the derivatives can target specifically Golgi apparatus in various cell lines (HeLa, U2OS, and 4T1 cells) in vitro and the colocalization with commercial Golgi marker is retained during the mitosis in HeLa cells. Moreover, the quinoline dyes can also be used for Golgi apparatus imaging with two-photon fluorescence microscopy. These results provide new insights into developing low cost Golgi-localized probes.

Rational Design and Facile Synthesis of a Highly Tunable Quinoline-Based Fluorescent Small-Molecule Scaffold for Live Cell Imaging

Small-molecule fluorescent probes are powerful tools for chemical biology; however, despite the large number of probes available, there is still a need for a simple fluorogenic scaffold, which allows for the rational design of molecules with predictable photophysical properties and is amenable to concise synthesis for high-throughput screening. Here, we introduce a highly modular quinoline-based probe containing three strategic domains that can be easily engineered and optimized for various applications. Such domains are allotted for (1) compound polarization, (2) tuning of photophysical properties, and (3) structural diversity. We successfully synthesized our probes in two steps from commercially available starting materials in overall yields of up to 95%. Facile probe synthesis was permitted by regioselective palladium-catalyzed cross-coupling, which enables combinatorial development of structurally diverse quinoline-based fluorophores. We have further applied our probes to live-cell imaging, utilizing their unique two-stage fluorescence response to intracellular pH. These studies provide a full demonstration of our strategy in rational design and stream-lined probe discovery to reveal the diverse potential of quinoline-based fluorescent compounds.

Multifunctional Cationic Iridium(III) Complexes Bearing 2-Aryloxazolo[4,5-f][1,10]phenanthroline (N^N) Ligand: Synthesis, Crystal Structure, Photophysics, Mechanochromic/Vapochromic Effects, and Reverse Saturable Absorption

A series of 2-aryloxazolo[4,5-f][1,10]phenanthroline ligands (N^N ligands) and their cationic iridium(III) complexes (1-11, aryl = 4-NO₂-phenyl (1), 4-Br-phenyl (2), Ph (3), 4-NPh₂-phenyl (4), 4-NH₂-phenyl (5), pyridin-4-yl (6), naphthalen-1-yl (7), naphthalen-2-yl (8), phenanthren-9-yl (9), anthracen-9-yl (10), and pyren-1-yl (11)) were synthesized and characterized. By introducing different electron-donating or electron-withdrawing substituents at the 4-position of the 2-phenyl ring (1-5), or different aromatic substituents with varied degrees of π-conjugation (6-11) on oxazolo[4,5-f][1,10]phenanthroline ligand, we aim to understand the effects of terminal substituents at the N^N ligands on the photophysics of cationic Ir(III) complexes using both spectroscopic methods and quantum chemistry calculations. Complexes with the 4-R-phenyl substituents adopted an almost coplanar structure with the oxazolo[4,5-f][1,10]phenanthroline motif, while the polycyclic aryl substituents (except for naphthalen-2-yl) were twisted away from the oxazolo[4,5-f][1,10]phenanthroline motif. All complexes possessed strong absorption bands below 350 nm that emanated from the ligand-localized ¹π,π*/¹ILCT (intraligand charge transfer) transitions, mixed with ¹LLCT (ligand-to-ligand charge transfer)/¹MLCT (metal-to-ligand charge transfer) transitions. At the range of 350-570 nm, all complexes exhibited moderately strong ¹ILCT/¹LLCT/¹MLCT transitions at 350-450 nm, and broad but very weak ³LLCT/³MLCT absorption at 450-570 nm. Most of the complexes demonstrated moderate to strong room temperature phosphorescence both in solution and in the solid state. Among them, complex 7 also manifested a drastic mechanochromic and vapochromic luminescence effect. Except for complexes 1 and 4 that contain NO₂ or NPh₂ substituent at the phenyl ring, respectively, all other complexes exhibited moderate to strong triplet excited-state absorption in the spectral region of 440-750 nm. Moderate to very strong reverse saturable absorption (RSA) of these complexes appeared at 532 nm for 4.1 ns laser pulses. The RSA strength followed the trend of 7 > 11 > 9 > 3 > 2 ≈ 4 > 5 ≈ 10 ≈ 6 ≈ 8 > 1. The photophysical studies revealed that the different 2-aryl substituents on the oxazole ring impacted the singlet and triplet excited-state characteristics dramatically, which in turn notably influenced the RSA of these complexes.

Renovation of Optically Active Phenanthrolines as Powerful Chiral Ligands for Versatile Asymmetric Metal Catalysis

In the field of asymmetric synthesis, the development of new chiral ligands has been regarded as an attractive challenge for decades. Novel chiral ligands can often have a great impact on synthetic protocols. In this context, we are currently interested in the application of 1,10-phenanthroline (phen) as an entirely new class of chiral ligand. To handle this issue, we designed a chiral phen ligand that provides the N,N,O-tridentate coordination of the phen moiety and an additional phenolic hydroxyl group. As phen possesses greater coordination ability with various ions, our chiral phen ligand would be valuable as one of the "privileged" chiral ligands applied to a broad range of metal catalysts and new reactions. This account summarizes the results of the application of the chiral phen ligand to various kinds of metal catalysis.

Friedländer annulation: scope and limitations of metal salt Lewis acid catalysts in selectivity control for the synthesis of functionalised quinolines

In(OTf)₃ is the most effective catalyst among metal halides, tetrafluoroborates, perchlorates, and triflates for Friedländer quinoline synthesis in 75–92% yields.

A highly selective chemosensor for Al3+ based on 2-oxo-quinoline-3-carbaldehyde Schiff-base

A new Schiff-base ligand (1) with good fluorescence response to Al(3+), derived from 2-oxo-quinoline-3-carbaldehyde and nicotinic hydrazide, had been synthesized and investigated in this paper. Spectroscopic investigation revealed that the compound 1 exhibited a high selectivity and sensitivity toward Al(III) ions over other commonly coexisting metal ions in ethanol, and the detection limit of Al(3+) ions is at the parts per billion level. The mass spectra and Job's plot confirmed the 1:1 stoichiometry between 1 and Al(3+). Potential utilization of 1 as intracellular sensors of Al(3+) ions in human cancer (HeLa) cells was also examined by confocal fluorescence microscopy.

Design of novel chiral N,N,O-tridentate phenanthroline ligands and their application to enantioselective addition of organozinc reagents to aldehydes

The novel chiral N,N,O-tridentate ligands (BinThro) containing binaphthyl and phenanthroline units were developed. The enantioselective organozinc addition to aldehydes was performed in the presence of BinThro (S)-1 with up to 95% ee.

An efficient synthesis of quinolines derivatives promoted by a room temperature ionic liquid at ambient conditions under ultrasound irradiation via the tandem addition/annulation reaction of o-aminoaryl ketones with alpha-methylene ketones

The condensation reaction involving an o-aminoaryl ketones with alpha-methylene ketones via the tandem addition/annulation reaction by the ionic liquid of [Hbim][BF4] (IL) as a solvent with methanol as co-solvent at room temperature under ultrasound irradiation afforded the corresponding quinolines derivatives in excellent yields. The use of ionic liquid and ultrasound, which promoted this protocol under room temperature without the requirement of any added catalyst. Quinoline derivatives are important and have a potential use as special biological drug in medicine.