Synthesis and Single-Crystal X-ray Characterization of 4,4‘ ‘-Functionalized 4‘-(4-Bromophenyl)-2,2‘:6‘,2‘ ‘-terpyridines

An In-Depth Exploration of Six Decades of the Kröhnke Pyridine Synthesis

The Kröhnke Pyridine Synthesis has been discovered about six decades ago (1961), by Fritz Kröhnke and Wilfried Zecher at the University of Giessen. The original method involved the reaction of α-pyridinium methyl ketone salts with α,β-unsaturated carbonyl compounds in the presence of a nitrogen source, frequently ammonium acetate. Since its discovery, the Kröhnke methodology has been demonstrated to be suitable for the preparation of mono-, di-, tri- and tetra-pyridines, with important applications in several research fields. Over the years, a number of modifications to the original approach have been developed and reported, enabling for the broad applicability of these methods even in modern days, also for the synthesis of non-pyridine compounds. In this critical and tutorial review, we will thoroughly explore and discuss the potential of the original method, the refinements that have been made over the years, as well as some applications arising from each type of pyridine and/or non-pyridine compounds produced by Kröhnke's approach.

Direct C-H Sulfonylimination of Pyridinium Salts

A direct pyridinium C-H sulfonylimination has been developed for the synthesis of sulfonyl iminopyridine derivatives with high efficiency. This transformation features the direct and efficient formation of a C═N bond with a high functional group tolerance under metal-free conditions. The spectroscopic properties potentially enable these sulfonyl iminopyridine compounds to be useful new emitting materials.

Synthesis of N-Formyl-2-benzoyl Benzothiazolines, 2-Substituted Benzothiazoles, and Symmetrical Disulfides from N-Phenacylbenzothiazolium Bromides

An unusual aerobic hydrolysis-cascade reaction has been developed with N-phenacylbenzothiazolium bromides by treatment with organic and inorganic base. The corresponding N-formyl-2-benzoyl benzothiazoline and 2-substituted benzothiazole products were obtained in moderate to good yields under mild reaction conditions. Also, symmetrical disulfide was formed when keto group was replaced with ester. The scopes of the reactions are fairly broad tolerating aryl, heteroaryl, and alkyl groups.

Formation of diverse polycyclic spirooxindoles via three-component reaction of isoquinolinium salts, isatins and malononitrile

The triethylamine promoted three-component reaction of N-(4-nitrobenzyl), N-ethoxycarbonylmethylisoquinolinium bromide, isatins and malononitrile in ethanol afforded spiro[indoline-3,2′-pyrrolo[2,1-a]isoquinolines] in good yields and with high diastereoselectivity. The similar reaction of N-cyanomethylisoquinolinium chloride mainly gave complex indolo[2″,3″:2′,3′]pyrrolo[3′,4′:4,5]pyrrolo[2,1-a]isoquinoline derivatives. However, the three-component reaction of N-cyanomethylisoquinolinium chloride, isatins and ethyl cyanoacetate mainly resulted in functionalized spiro[indoline-3,8′-pyrido[2′,3′:4,5]pyrrolo[2,1-a]isoquinolines].

Synthesis and properties of fluorescent 4'-azulenyl-functionalized 2,2':6',2″-terpyridines

4′-Azulenyl-substituted terpyridines were efficiently synthesized following the Kröhnke methodology via azulenylchalcone intermediates. These azulenyl-containing terpyridines showed fluorescent emission with a fluorescence quantum yield varying from 0.14, in the case of parent terpyridine, to 0.64 when methyl groups are grafted on the azulenyl seven-membered ring. According to the crystal structures and TDDFT calculations, different twisting of the aromatic constituents is responsible for the observed fluorescent behavior. The electrochemical profile contains one-electron oxidation/reduction steps, which can only be explained on the basis of the redox behavior of the azulene unit. The ability of the 4′-azulenyl 2,2′:6′,2″-terpyridine to bind poisoning metal cations was studied by UV–vis titrations using aqueous solutions of Hg(II) and Cd(II) chlorides as illustrative examples.

Crystal structure of 4'-{[4-(2,2':6',2''-terpyrid-yl-4'-yl)phen-yl]ethyn-yl}biphenyl-4-yl (2,2,5,5-tetra-methyl-1-oxyl-3-pyrrolin-3-yl)formate benzene 2.5-solvate

The title compound, C44H35N4O3·2.5C6H6 (1), consists of a terpyridine and a N-oxylpyrroline-3-formate group separated by an aromatic spacer, viz. 4-(phenyl-ethyn-yl)-1,1'-biphenyl. It crystallized in the triclinic space group P-1 with two and a half benzene solvate mol-ecules (one benzene mol-ecule is located about an inversion center), while the di-chloro-methane solvate (2) of the same mol-ecule [Ackermann et al. (2015 ▸). Chem. Commun. 51, 5257-5260] crystallized in the tetra-gonal space group P42/n, with considerable disorder in the mol-ecule. In (1), the terpyridine (terpy) group assumes an all-trans conformation typical for terpyridines. It is essentially planar with the two outer pyridine rings (B and C) inclined to the central pyridine ring (A) by 8.70 (15) and 14.55 (14)°, respectively. The planes of the aromatic spacer (D, E and F) are nearly coplanar with dihedral angles D/E, D/F and E/F being 3.42 (15), 5.80 (15) and 4.00 (16)°, respectively. It is twisted with respect to the terpy group with, for example, dihedral angle A/D being 24.48 (14)°. The mean plane of the N-oxylpyrroline is almost normal to the biphenyl ring F, making a dihedral angle of 86.57 (16)°, and it is inclined to pyridine ring A by 72.61 (15)°. The intra-molecular separation between the O atom of the nitroxyl group and the N atom of the central pyridine ring of the terpyridine group is 25.044 (3) Å. In the crystal, mol-ecules are linked by pairs of C-H⋯O hydrogen bonds, forming inversion dimers. The dimers stack along the c axis forming columns. Within and between the columns, the spaces are occupied by benzene mol-ecules. The shortest oxygen-oxygen separation between nitroxyl groups is 4.004 (4) Å. The details of the title compound are compared with those of the di-chloro-methane solvate (2) and with the structure of a related mol-ecule, 4'-{4-[(2,2,5,5-tetra-methyl-N-oxyl-3-pyrrolin-3-yl)ethyn-yl]phen-yl}-2,2':6',2''-terpyridine (3), which has an ethynylphenyl spacer [Meyer et al. (2015). Acta Cryst. E71, 870-874].

Diastereoselective synthesis of spiro[benzo[d]pyrrolo[2,1-b]thiazole-3,3'-indolines] via cycloaddition reaction of N-phenacylbenzothiazolium bromides and 3-methyleneoxindoles

The domino cycloaddition reactions of N-phenacylbenzothiazolium bromides with 3-phenacylideneoxindoles or ethyl 2-(2-oxoindolin-3-ylidene)acetates in ethanol at room temperature in the presence of triethylamine as a base afforded functionalized spiro[benzo[d]pyrrolo[2,1-b]thiazole-3,3'-indolines] in good yields and with high diastereoselectivity. The similar reactions of N-phenacylthiazolium bromides with 3-phenacylideneoxindoles resulted in the corresponding functionalized spiro[indoline-3,7'-pyrrolo[2,1-b]thiazoles] in satisfactory yields and also with high diastereoselectivity.

The crystal structure of 4'-{4-[(2,2,5,5-tetra-methyl-N-oxyl-3-pyrrolin-3-yl)ethyn-yl]phen-yl}-2,2':6',2''-terpyridine

The terpyridine group of the title compound, C31H27N4O, assumes an all-transoid conformation and is essentially planar with the dihedral angles between the mean planes of the central pyridine and the two outer rings amounting to 3.87 (5) and 1.98 (5)°. The pyrroline-N-oxyl group commonly seen in such nitroxyls is found in the title structure and the mean plane of the pyrroline ring subtends a dihedral angle of 88.44 (7)° to the mean plane of the central pyridine ring. The intra-molecular separation between the nitrogen atom of the central pyridine unit of the terpyridine group and the nitroxyl group is 14.120 (2) Å. In the crystal, the mol-ecules are arranged in layers stacked along [001]. Slipped face-to-face π-π inter-actions between the pyridine rings are observed along this direction with the shortest centroid-centroid distances amounting to 3.700 (1) and 3.781 (1) Å. Furthermore, edge-on C-H⋯π inter-actions between the phenyl-ene rings of neighbouring mol-ecules are observed along this direction. A two-dimensional C-H⋯O hydrogen-bonded network is formed within the (010) plane. The shortest O⋯O separation between neighbouring mol-ecules is 5.412 (3) Å.

Phosphonate-functionalized heteroleptic ruthenium(II) bis(2,2′:6′,2″-terpyridine) complexes

The heteroleptic complexes [Ru(1)(4)][PF6]2, [Ru(2)(4)][PF6]2, [Ru(Phtpy)(4)][PF6]2, and [Ru(pytpy)(4)][PF6]2 (Phtpy = 4′-phenyl-2,2′:6′,2″-terpyridine, pytpy = 4′-(4-pyridyl)-2,2′:6′,2″-terpyridine, 1 and 2 = 4-methyl ester substituted derivatives of Phtpy and pytpy, 4 = ethyl 2,2′:6′,2″-terpyridine-4′-phosphonate) have been prepared. The single crystal structure of ligand 1 (1 = methyl 4-carboxy-4′-phenyl-2,2′:6′,2″-terpyridine) is reported. The introduction of the 4-methyl ester group causes a small red shift in the MLCT band of the ruthenium(II) complexes and a small shift to a more positive potential for the Ru2+/Ru3+ couple. The new complexes should serve as a useful starting point for development of ruthenium(II) dyes suited for sensitization of p-type semiconductors.

Synthesis and photophysical properties of some 6,6″-functionalized terpyridine derivatives

The synthesis and photophysical properties of several 6,6″ symmetrically substituted 4′-aryl-2,2′:6′,2″-terpyridine derivatives are reported herein. The UV-Vis spectra in acetonitrile as well as in dichloromethane show two intense bands in the UV areas 252–262 nm and 275–290 nm while the fluorescence emission spectra are only slightly influenced by chemical derivatization.

8-(4-Chlorobenzylidene)-4-(4-chlorophenyl)-2-phenyl-5,6,7,8-tetrahydroquinoline

In the crystal structure of the title compound, C₂₈H₂₁Cl₂N, π–π inter­actions link pairs of mol­ecules into centrosymmetric dimers with a distance of 3.756 (3) Å between the centroids of the pyridine rings. Weak inter­molecular C—H⋯Cl hydrogen bonds further link these dimers into chains propagating along [01]. The pyridine ring forms dihedral angles of 21.52 (1) and 55.87 (2)°, respectively, with the phenyl ring and the 4-chlorophenyl ring.

Anthracene functionalized terpyridines – synthesis and properties

The synthesis of several symmetrically 4,4″-functionalized 2,2′:6′,2″-terpyridines is reported. In addition to the biscarboxylic acid 4,4″-tpy(CO₂H)₂ (3), the anthryl esters 4,4″-tpy(CO₂CH₂Anth)₂ (5a) and 4,4″-tpy(CO₂CH₂CH₂OAnth)₂ (5b) were synthesized. Furthermore, both anthryl esters were used to synthesize symmetric iron(II)-bis(terpyridine)complexes 6a–b. Irradiation experiments were carried out with both the free ligands and an iron(II)-complex in order to investigate the photochemistry of the compounds.

Efficient synthesis of diarylidene octahydroacridines by one-pot multi-component tandem reactions

A one-pot multi-component reaction is developed for the efficient synthesis of 4,5-dibenzylidene octahydroacridines in high yields. The reaction is performed by the tandem reaction of three molar equivalent aromatic aldehydes with two molar equivalent 4-alkylcyclohexanone in the system of NH4OAc/HOAc under microwave irradiation.

Metallosupramolecular polyelectrolytes self-assembled from various pyridine ring-substituted bisterpyridines and metal ions: photophysical, electrochemical, and electrochromic properties

This work presents several metallosupramolecular coordination polyelectrolytes (MEPEs) self-assembled from rigid, pi-conjugated, pyridine ring functionalized bisterpyridines and metal ions. The MEPEs are water-soluble and display different colors spanning the entire visible regions. Optical, electrochemical, and electrochromic properties of the obtained MEPEs are presented. The results show that the properties are profoundly affected by the nature of the substituents at the peripheral pyridine rings. Namely, MEPEs assembled from the electron-rich OMe group modified ligands exhibit high switching reversibility and stability and show a lower switching potential than the unsubstituted and electron-deficient Br-substituted analogues. The response times can be tuned either by the design of the ligands or by the choice of the metal ions to cover a broad time scale from under 1 s to several minutes. The optical memory is enhanced from 30 s to longer than 15 min as a comparison of unsubstituted and substituted MEPEs shows. Thus, the significantly enhanced stability and the ease of tuning the properties render this type of supramolecular assembly attractive as electrochromic materials for applications in a large variety of areas. Most importantly, we presented the structure-property relationships of MEPEs, which lays the groundwork for further design of new bisterpyridine-based metallosupramolecular functional materials.