Microwave Irradiation Syntheses and Crystal Structures of Two Series of Novel Fivemembered Heterocyclic Mono-Imine Compounds.

AIM AND OBJECTIVE: The late transition metal complexes with five-membered heterocyclic mono-imine ligands have attracted much attention because of their potential application in olefin polymerization catalysis. In order to increase the coordination ability of heteroatom N and S to center metals, CH3 group was introduced into the side arm of pyrrole imine and thiophene imine respectively, to get two series of novel five-membered heterocyclic imine compounds, mono(imino)pyrroles and mono(imino)thiophenes. MATERIALS AND METHODS: Two series of novel five-membered heterocyclic compounds with the mono-imine group were synthesized from the p-toluene sulfonic acid catalyzed Schiff base condensation of aromatic amines and 2-acetylpyrrole/ 2-acetylthiophene respectively, using CH3 group to substitute the common H atom on the side arm of pyrrole imine/ thiophene imine. RESULTS: All the heterocyclic mono-imine compounds were characterized adequately by means of 1H NMR, 13C NMR, FT-IR, elementary analysis, as well as X-ray crystallographic diffraction. The reactivity differences between two precursor 2-acetylpyrrole and 2-acetylthiophene with aromatic amines were compared and discussed in detail. CONCLUSION: Compared to traditional heating methods, the solvent-free microwave irradiation seemed more efficient to prepare these series of five-membered heterocyclic mono-imine compounds, which resulted in a higher yield and cleaner product.

The Syntheses, Characterization and Crystal Structures of a Series of Heterocyclic ß-Diketones and Their Isoxazole Compounds.

BACKGROUND: In the field of coordination chemistry, the introduction of heterocyclic substituents into the structure of ß-diketone enables ligand to produce multiple coordination sites. The adoption of small steric oxime group into the structure of heterocyclic ß-diketone by Schiff-base condensation will further increase coordination sites and facilitate the generation of polynuclear structures. OBJECTIVE: A series of ß-diketones (2a-2c) containing different heterocycles such as pyridine, thiophene and furan and their corresponding isoxazole compounds (3a-3c) were synthesized. MATERIALS AND METHODS: The Claisen condensations were investigated in a solvent-free rheological phase system at room temperature to obtain heterocyclic ß-diketones 2a-2c, which further reacted with hydroxylamine hydrochloride to obtain heterocyclic isoxazoles 3a-3c. All these compounds were well characterized by EA, IR, 1H NMR and X-ray crystal diffraction to confirm the structures. Synthetic mechanisms of compounds and the effects of different heterocycles on reactivity were discussed deeply. RESULTS: 1H NMR indicated that these ß-diketones do not exist as a total diketonic form but an equilibration between diketone and enol forms in CDCl3 solvent, in which the enol form accounts for 98.0% in 2a, 94.3% in 2b, 95.5% in 2c. While the crystal structures of 2a-2c showed that the reaction allows to isolate diketones in solid state. Crystal structures of 3a-3c showed that the neutral ß-ketone oximes resonate and cyclize to form the target heterocyclic isoxazoles. CONCLUSION: SN1 nucleophilic substitution mechanism of Claisen ketoester condensation was proposed for the syntheses of 2a-2c, and SN1 single molecule nucleophilic substitution reaction mechanism was put forward for 3a-3c.

New Coordination Complexes Based on the 2,6-bis[1-(Phenylimino)ethyl] Pyridine Ligand: Effective Catalysts for the Synthesis of Propylene Carbonates from Carbon Dioxide and Epoxides.

We aimed to develop new effective catalysts for the synthesis of propylene carbonate from propylene oxide and carbon dioxide. A kind of Mx+LClx coordination complex was fabricated based on the chelating tridentate ligand 2,6-bis[1-(phenylimino)ethyl] pyridine (L). The obtained products were characterized by elemental analysis, infrared spectroscopy, ultraviolet spectroscopy, thermogravimetric analysis, and single-crystal X-ray diffraction. It was found that the catalytic activity of the complexes with different metal ions, the same ligand differed and co-catalyst, where the order of greatest to least catalytic activity was 2 > 3 > 1. The catalytic system composed of complex 2 and DMAP proved to have the better catalytic performance. The yields for complex 2 systems was 86.7% under the reaction conditions of 100 °C, 2.5 MPa, and 4 h. The TOF was 1026 h-¹ under the reaction conditions of 200 °C, 2.5 MPa, and 1 h. We also explored the influence of time, pressure, temperature, and reaction substrate concentration on the catalytic reactions. A hypothetical catalytic reaction mechanism is proposed based on density functional theory (DFT) calculations and the catalytic reaction results.

3,4-Dimethyl-N-[1-(1H-pyrrol-2-yl)ethylidene]aniline and the 1-(1-thiophen-2-yl) analogue.

The title compounds, 3,4-dimethyl-N-[1-(1H-pyrrol-2-yl)ethylidene]aniline, C14H16N2, (I), and its analogue 3,4-dimethyl-N-[1-(1-thiophen-2-yl)ethylidene]aniline, C14H15NS, (II), both have basic heterocyclic imino structures showing a planar backbone with similar features, but differing in the heteroatoms of the five-membered heterocyclic rings, i.e. N in (I) and S in (II). The dihedral angles formed by the five-membered and benzene rings are 81.78 (8) and 75.89 (7)° for (I) and (II), respectively. In (I), centrosymmetric iminopyrrole dimers are assembled by means of two inverted N-H···N hydrogen bonds and two inverted C-H···π interactions. In (II), however, molecules are linked by nonclassical C-H···N hydrogen bonds in which the molecules act as both hydrogen-bond donors and acceptors, resulting in one-dimensional supramolecular chains.

Bis{2-[(phenylimino)ethyl]-1H-pyrrol-1-ido-κ²N,N'}nickel(II): a supramolecular structure formed by C-H∙∙∙π hydrogen bonds.

In the title compound, [Ni(C12H11N2)2], the NiII cation lies on an inversion centre and has a square-planar coordination geometry. This transition metal complex is composed of two deprotonated N,N'-bidentate 2-[(phenylimino)ethyl]-1H-pyrrol-1-ide ligands around a central NiII cation, with the pyrrolide rings and imine groups lying trans to each other. The Ni-N bond lengths range from 1.894 (3) to 1.939 (2) Å and the bite angle is 83.13 (11)°. The Ni-N(pyrrolide) bond is substantially shorter than the Ni-N(imino) bond. The planes of the phenyl rings make a dihedral angle of 78.79 (9)° with respect to the central NiN4 plane. The molecules are linked into simple chains by an intermolecular C-H∙∙∙π interaction involving a phenyl ß-C atom as donor. Intramolecular C-H∙∙∙π interactions are also present.

Isomeric 3- and 4-chloro-N-[1-(1H-pyrrol-2-yl)ethylidene]aniline.

The title isomers, namely 3-chloro-N-[1-(1H-pyrrol-2-yl)ethylidene]aniline, (I), and 4-chloro-N-[1-(1H-pyrrol-2-yl)ethylidene]aniline, (II), both C12H11ClN2, differ in the position of the chlorine substitution. Both compounds have the basic iminopyrrole structure, which shows a planar backbone with similar features. The dihedral angle formed by the planes of the pyrrole and benzene rings is 75.65 (7)° for (I) and 86.56 (8)° for (II). The H atom bound to the pyrrole N atom is positionally disordered and partial protonation occurs at the imino N atom in (I), while this phenomenon is absent from the structure of (II). Packing interactions for both compounds include intermolecular N-H···N hydrogen bonds and C-H···π interactions, forming centrosymmetric dimers for both (I) and (II).

N-[1-(1H-Pyrrol-2-yl)ethyl-idene]aniline.

There are two independent mol-ecules in the asymmetric unit of the title compound, C(12)H(12)N(2), in which the pyrrole and benzene rings form dihedral angles of 72.37 (7) and 82.34 (8)°. The imino N-C bond lengths in the two mol-ecules are equal [1.286 (2) Å] and indicate C=N character. In the crystal, each mol-ecule forms a dimer with an inversion-related mol-ecule through a pair of classical N-H⋯N hydrogen bonds.

2,4,6-Trimethyl-N-[1-(1H-pyrrol-2-yl)ethyl-idene]aniline.

There are two independent mol-ecules in the asymmetric unit of the title compound, C(15)H(18)N(2), each of which features a syn disposition of the N atoms. In each mol-ecule, the pyrrole and benzene rings are essentially perpendicular, with dihedral angles of 78.90 (9) and 79.96 (9)°. In the crystal, the independent mol-ecules are connected by a pair of pyrrole-imino N-H⋯N hydrogen bonds, forming a two-mol-ecule aggregate.

2-Methyl-N-[1-(1H-pyrrol-2-yl)ethyl-idene]aniline.

There are two independent mol-ecules in the asymmetric unit of the title compound, C13H14N2, in which the dihedral angles formed by the pyrrole and benzene rings are 83.63 (8) and 87.84 (8)°. In the crystal, mol-ecules are linked via pairs of N-H⋯N hydrogen bonds, forming inversion dimers, which are further connected by C-H⋯π inter-actions.

2,5-Dimethyl-N-[1-(1H-pyrrol-2-yl)ethyl-idene]aniline.

In the title compound, C(14)H(16)N(2), the pyrrole and benzene rings form a dihedral angle of 72.37 (8)°. In the crystal, centrosymmetrically related mol-ecules are assembled into dimers by by pairs of N-H⋯N hydrogen bonds, generating rings of R(2) (2)(10) graph-set motif. C-H⋯π inter-actions also occur.