Synthesis of polyhalogenated 4,4'-bipyridines via a simple dimerization procedure

Recent Progress on the Synthesis of Bipyridine Derivatives

Bipyridine and related compounds are starting materials or precursors for a variety of valuable substances such as biologically active molecules, ligands for catalysts, photosensitizers, viologens, and supramolecular architectures. Thus, it is important to classify their synthesis methods and understand their characteristics. Representative examples include methods using homo and heterocoupling of pyridine derivatives in the presence of a catalyst. Because bipyridine compounds strongly coordinate with metal centers, a decrease in catalytic activity and yield is often observed in the reaction system. To address this issue, this review provides insights into advances over the last ~30 years in bipyridine synthesis using metal complexes under both homogeneous and heterogeneous conditions. Moreover, strategies for bipyridine synthesis involving sulfur and phosphorous compounds are examined. These alternative pathways offer promising avenues for overcoming the challenges associated with traditional catalysis methods, providing a more comprehensive understanding of the synthesis landscape.

Total Synthesis of Floyocidin B: 4,5-Regioselective Functionalization of 2-Chloropyridines

The recently discovered natural product (NP) (+)-floyocidin B with antimicrobial activity against Mycobacterium tuberculosis displays a hitherto unknown dihydroisoquinolinone scaffold in the class of the epoxyquinone NPs. The 4,5-regioselective functionalization of 2-chloropyridines was identified as a suitable strategy leading to the total syntheses of (+)-floyocidin B and analogs. In this paper, we present the long and winding evolution process to the final synthetic pathway, including model systems for route scouting and elucidation of side products, which enabled us to understand the unique reactivity of this unprecedented scaffold. A special focus was laid on method studies with different 2-chloropyridines, disclosing an unexpected effect of the 2-chloro substituent on the regioselectivity compared to 2-unsubstituted or carbon-substituted pyridines. Finally, a head-to-head comparison with the previously described synthesis of all four stereoisomers of the NP (−)-avicennone C revealed significant differences in the reactivity of these structurally closely related scaffolds.

Functionalized 4,4'-Bipyridines: Synthesis and 2D Organization on Highly Oriented Pyrolytic Graphite

Commercial 4,4'-bipyridine is a popular scaffold that is primarily employed as a linker in 3D self-assembled architectures such as metallo-organic frameworks or as a connector in 2D networks. The introduction of alkyl substituents on the bipyridine skeleton is instrumental when 4,4'-bipyridines are used as linkers to form 2D self-assembled patterns on surfaces. Here, several synthetic strategies to access 4,4'-bipyridines functionalized at various positions are described. These easily scalable reactions have been used to introduce a range of alkyl substituents at positions 2 and 2' or 3 and 3' and at positions 2,2' and 6,6' in the case of tetra-functionalization. Scanning tunneling microscopy studies of molecular monolayers physisorbed at the graphite-solution interface revealed different supramolecular patterns whose motifs are primarily dictated by the nature and position of the alkyl chains.

Enantioseparation of 5,5'-Dibromo-2,2'-Dichloro-3-Selanyl-4,4'-Bipyridines on Polysaccharide-Based Chiral Stationary Phases: Exploring Chalcogen Bonds in Liquid-Phase Chromatography

The chalcogen bond (ChB) is a noncovalent interaction based on electrophilic features of regions of electron charge density depletion (σ-holes) located on bound atoms of group VI. The σ-holes of sulfur and heavy chalcogen atoms (Se, Te) (donors) can interact through their positive electrostatic potential (V) with nucleophilic partners such as lone pairs, π-clouds, and anions (acceptors). In the last few years, promising applications of ChBs in catalysis, crystal engineering, molecular biology, and supramolecular chemistry have been reported. Recently, we explored the high-performance liquid chromatography (HPLC) enantioseparation of fluorinated 3-arylthio-4,4′-bipyridines containing sulfur atoms as ChB donors. Following this study, herein we describe the comparative enantioseparation of three 5,5′-dibromo-2,2′-dichloro-3-selanyl-4,4′-bipyridines on polysaccharide-based chiral stationary phases (CSPs) aiming to understand function and potentialities of selenium σ-holes in the enantiodiscrimination process. The impact of the chalcogen substituent on enantioseparation was explored by using sulfur and non-chalcogen derivatives as reference substances for comparison. Our investigation also focused on the function of the perfluorinated aromatic ring as a π-hole donor recognition site. Thermodynamic quantities associated with the enantioseparation were derived from van’t Hoff plots and local electron charge density of specific molecular regions of the interacting partners were inspected in terms of calculated V. On this basis, by correlating theoretical data and experimental results, the participation of ChBs and π-hole bonds in the enantiodiscrimination process was reasonably confirmed.

Factors Impacting σ- and π-Hole Regions as Revealed by the Electrostatic Potential and Its Source Function Reconstruction: The Case of 4,4'-Bipyridine Derivatives

Positive electrostatic potential (V) values are often associated with σ- and π-holes, regions of lower electron density which can interact with electron-rich sites to form noncovalent interactions. Factors impacting σ- and π-holes may thus be monitored in terms of the shape and values of the resulting V. Further precious insights into such factors are obtained through a rigorous decomposition of the V values in atomic or atomic group contributions, a task here achieved by extending the Bader-Gatti source function (SF) for the electron density to V. In this article, this general methodology is applied to a series of 4,4'-bipyridine derivatives containing atoms from Groups VI (S, Se) and VII (Cl, Br), and the pentafluorophenyl group acting as a π-hole. As these molecules are characterized by a certain degree of conformational freedom due to the possibility of rotation around the two C-Ch bonds, from two to four conformational motifs could be identified for each structure through conformational search. On this basis, the impact of chemical and conformational features on σ- and π-hole regions could be systematically evaluated by computing the V values on electron density isosurfaces (V_S) and by comparing and dissecting in atomic/atomic group contributions the V_S maxima (V_S,max) values calculated for different molecular patterns. The results of this study confirm that both chemical and conformational features may seriously impact σ- and π-hole regions and provide a clear analysis and a rationale of why and how this influence is realized. Hence, the proposed methodology might offer precious clues for designing changes in the σ- and π-hole regions, aimed at affecting their potential involvement in noncovalent interactions in a desired way.

Rational Design, Synthesis, Characterization and Evaluation of Iodinated 4,4'-Bipyridines as New Transthyretin Fibrillogenesis Inhibitors

The 3,3',5,5'-tetrachloro-2-iodo-4,4'-bipyridine structure is proposed as a novel chemical scaffold for the design of new transthyretin (TTR) fibrillogenesis inhibitors. In the frame of a proof-of-principle exploration, four chiral 3,3',5,5'-tetrachloro-2-iodo-2'-substituted-4,4'- bipyridines were rationally designed and prepared from a simple trihalopyridine in three steps, including a Cu-catalysed Finkelstein reaction to introduce iodine atoms on the heteroaromatic scaffold, and a Pd-catalysed coupling reaction to install the 2'-substituent. The corresponding racemates, along with other five chiral 4,4'-bipyridines containing halogens as substituents, were enantioseparated by high-performance liquid chromatography in order to obtain pure enantiomer pairs. All stereoisomers were tested against the amyloid fibril formation (FF) of wild type (WT)-TTR and two mutant variants, V30M and Y78F, in acid mediated aggregation experiments. Among the 4,4'-bipyridine derivatives, interesting inhibition activity was obtained for both enantiomers of the 3,3',5,5'-tetrachloro-2'-(4-hydroxyphenyl)-2-iodo-4,4'-bipyridine. In silico docking studies were carried out in order to explore possible binding modes of the 4,4'-bipyridine derivatives into the TTR. The gained results point out the importance of the right combination of H-bond sites and the presence of iodine as halogen-bond donor. Both experimental and theoretical evidences pave the way for the utilization of the iodinated 4,4'-bipyridine core as template to design new promising inhibitors of TTR amyloidogenesis.

New Hindered Amide Base for Aryne Insertion into Si-P, Si-S, Si-N, and C-C Bonds

A general method for a new, hindered lithium diadamantylamide (LDAM) base-promoted insertion of arynes into Si-P, Si-S, Si-N, and C-C bonds is described. Arynes are generated from easily available aryl triflates and halides. Subsequent reaction of the aryne with silylated phosphines, sulfides, or amines affords the insertion products. Furthermore, a one-step synthesis of anthracenes from aryl halides and aryl ketones is also demonstrated. Cyano, aryl, alkyl, trifluoromethyl, vinyl, methoxy, chloro, fluoro, and even formyl moieties are compatible with the reaction conditions. The new lithium amide affords higher yields compared with lithium tetramethylpiperidide (LiTMP)-promoted reactions. Furthermore, the bulkiness of LDAM base essentially suppresses aryne reaction with base, allowing use of aryl halides and triflates as the limiting reagents.

Polysaccharide-based chiral stationary phases as halogen bond acceptors: A novel strategy for detection of stereoselective σ-hole bonds in solution

In the last few years, halogen bonds have been exploited in a variety of research areas both in the solid state and in solution. Nevertheless, several factors make formation and detection of halogen bonds in solution challenging. Moreover, to date, few chiral molecules containing electrophilic halogens as recognition sites have been reported. Recently, we described the first series of halogen-bond-driven enantioseparations performed on cellulose tris(3,5-dimethylphenylcarbamate) by high-performance liquid chromatography. Herein the performances of amylose tris(3,5-dimethylphenylcarbamate) as halogen bond acceptor were also investigated and compared with respect to cellulose tris(3,5-dimethylphenylcarbamate). With the aim to explore the effect of polysaccharide backbone on the enantioseparations, the thermodynamic parameters governing the halogen-dependent enantioseparations on both cellulose and amylose polymers were determined by a study at variable temperature and compared. Molecular dynamics were performed to model the halogen bond in polysaccharide-analyte complexes. Chiral halogenated 4,4'-bipyridines were used as test compounds (halogen bond donors). On this basis, a practical method for detection of stereoselective halogen bonds in solution was developed, which is based on the unprecedented use of high-performance liquid chromatography as technical tool with polysaccharide polymers as molecular probes (halogen bond acceptors). The analytical strategy showed higher sensitivity for the detection of weak halogen bonds.

Silver(i) coordination polymers with 3,3′,5,5′-tetrasubstituted 4,4′-bipyridine ligands: towards new porous chiral materials

The first homochiral metal–organic framework (MOF) based on an atropisomeric 4,4′-bipyridine ligand is described.

Insights into halogen bond-driven enantioseparations

Although the halogen bond (XB) has been so far mainly studied in silico and in the solid state, its potential impact in solution is yet to be fully understood. In this study, we describe the first systematic investigation on the halogen bond in solvated environment by high-performance liquid chromatography (HPLC). Thirty three atropisomeric polyhalogenated-4,4'-bipyridines (HBipys), containing Cl, Br and I as substituents, were selected and used as potential XB donors (XBDs) on two cellulose-based chiral stationary phases (CSPs) containing potential XB acceptors (XBAs). The impact of the halogens on the enantiodiscrimination mechanism was investigated and iodine showed a pivotal role on the enantioseparation in non-polar medium. Electrostatic potentials (EPs) were computed to understand the electrostatic component of CSP-analyte interaction. Moreover, van't Hoff studies for ten HBipys were performed and the thermodynamic parameters governing the halogen-dependent enantioseparations are discussed. Finally, a molecular dynamic (MD) simulation is proposed to model halogen bond in polysaccharide-analyte complexes by inclusion of a charged extra point to represent the positive 'σ-hole' on the halogen atom. On the basis of both experimental results and theoretical data, we have profiled the halogen bond as a chemo-, regio-, site- and stereoselective interaction which can work in HPLC environment besides other known interactions based on the complementarity between selector and selectand.

Extending motifs in lithiocuprate chemistry: unexpected structural diversity in thiocyanate complexes

Lithio(thiocyanato)cuprates have been developed. These reveal planar, boat-shaped and chair-shaped Lipshutz-type dimers in the solid state, while Lipshutz-type and Gilman structures are seen in solution.

General method for functionalized polyaryl synthesis via aryne intermediates

A method for base-promoted arylation of arenes and heterocycles by aryl halides and aryl triflates is described. Additionally, in situ electrophilic trapping of ArLi intermediates generated in the reaction of benzyne with deprotonated arenes or heterocycles has been developed, providing rapid and easy access to a wide range of highly functionalized polyaryls. Base-promoted arylation methodology complements transition-metal-catalyzed direct arylation and allows access to structures that are not easily accessible via other direct arylation methods. The reactions are highly functional-group tolerant, with alkene, ether, dimethylamino, trifluoromethyl, ester, cyano, halide, hydroxyl, and silyl functionalities compatible with reaction conditions.

Halogen bonding directed supramolecular assembly in bromo-substituted trezimides and tennimides

The imide-based trezimide and tennimide macrocycle crystal structures typically aggregate as 1-D chains through C–Br⋯OC/N/π(arene) halogen bonds (withN_c≤ 0.90) that dominate the solid-state aggregation process in the absence of classical strong hydrogen bond donors.

Energetic analysis of the molecular packing of 5,5′-dibromo-2,2′-bis[4-(methylsulfanyl)phenyl]-4,4′-bipyridine: the role of π–π and halogen interactions

The crystal packing of the title compound, C24H18Br2N2S2, is rationalized using the PIXEL method, which allows a separation of the intermolecular interaction energy into Coulombic, polarization, dispersion and repulsion contributions. Infinite (\overline{1}01) molecular planes are formed through π–π stacking and other minor interactions, including a Br...S contact, with the σ hole of the Br atom pointing towards the S-atom lone pair. The title compound has crystallographically imposed twofold symmetry, with the twofold axis at the mid-point of the central C—C bond.

High-performance liquid chromatography enantioseparation of atropisomeric 4,4'-bipyridines on polysaccharide-type chiral stationary phases: Impact of substituents and electronic properties

The high performance liquid chromatography (HPLC) enantioseparation of eleven atropisomeric 4,4'-bipyridines was performed in the normal and polar organic phase mode using three cellulose-based chiral stationary phases (CSPs), namely Lux Cellulose-1, Lux Cellulose-2, Lux Cellulose-4, and two amylose-based CSPs, Chiralpak AD-H and Lux Amylose-2. n-Hexane/2-propanol mixtures and pure ethanol were employed as mobile phases. The combined use of Chiralpak AD-H and Lux Cellulose-2 allowed to enantioseparate all the considered bipyridines. Ten bipyridines were enantioseparated at the multimilligram level allowing the elution sequence determination of the enantiomers as well as their future use for the preparation of homochiral metal organic frameworks (MOFs). Moreover, the performance of the CSPs regarding the same bipyridine was different and dependent on the backbone as well as on the side chain of the polymer. The impact of substitution pattern, shape and electronic properties of the molecules on the separation behavior was investigated through the evaluation of retention factors (k), separation factors (α), resolution (R(s)) and molecular properties determined using density functional theory (DFT) calculations. In this regard, the substituents at the 3,3',5,5' positions of the 4,4'-bipyridyl rings exhibited a pivotal role on the enantioseparation.

Double N-arylation reaction of polyhalogenated 4,4'-bipyridines. Expedious synthesis of functionalized 2,7-diazacarbazoles

Unusual 2,7-diazacarbazoles were prepared in one step from readily available tetra-halogenated 4,4'-bipyridines by using a double N-arylation reaction in the presence of the Pd-XPhos catalyst system. Moderate to good yields were obtained in this site-selective Buchwald-Hartwig double amination. The functionalization of these tricyclic derivatives was performed by using Pd-catalyzed cross-coupling reactions such as the Stille and Suzuki couplings. Two compounds were analyzed by X-ray diffraction and show π-π stacking involving the diazacarbazole moieties and the phenyl rings of functionalized groups.

Base-mediated intermolecular sp2 C-H bond arylation via benzyne intermediates

A transition-metal-free method for arylation of heterocycle and arene carbon-hydrogen bonds by aryl chlorides and fluorides has been developed. The reactions proceed via aryne intermediates and are highly regioselective with respect to the C-H bond coupling component.