Binuclear and polynuclear transition metal complexes with macrocyclic ligands. 4. New polydentate azomethine ligands based on 2,5-diformylpyrrole and 2,6-diformylpyridine

The reactions of 2,5-diformylpyrrole (1) and 2,6-diformylpyridine (2) with propane-1,3-diamine afforded new macrocyclic Schiff"s bases 5 and 6, respectively. Their structures were established by NMR spectroscopy and mass spectrometry. Binuclear copper(ii) and nickel(ii) complexes with ligand 5 were synthesized. Pentadentate Schiff"s base, viz., 2,6-bis[(2-aminophenylimino)methyl]pyridine, was prepared by demetallation of its complex with Cd(ClO₄)₂ using Na₂S. In solutions, the latter Schiff"s base is quantitatively transformed into 2,6-bis(benzoimidazolyl)pyridine under the action of atmospheric oxygen or other mild oxidizing agents.     

Synthetic Diversity from a Versatile and Radical Nitrating Reagent

We leverage the slow liberation of nitrogen dioxide from a newly discovered, inexpensive succinimide-derived reagent to allow for the C−H diversification of alkenes and alkynes. Beyond furnishing a library of aryl β-nitroalkenes, this reagent provides unparalleled access to β-nitrohydrins and β-nitroethers. Detailed mechanistic studies strongly suggest that a mesolytic N−N bond fragmentation liberates a nitryl radical. Using in situ photo-sensitized, electron paramagnetic resonance spectroscopy, we observed direct evidence of a nitryl radical in solution by nitrone spin-trapping. To further exhibit versatility of N-nitrosuccinimide under photoredox conditions, the late-stage diversification of an extensive number of C−H partners to prepare isoxazolines and isoxazoles is presented. This approach allows for the formation of an in situ nitrile oxide from a ketone partner, the presence of which is detected by the formation of the corresponding furoxan when conducted in the absence of a dipolarophile. This 1,3-dipolar cycloaddition with nitrile oxides and alkenes or alkynes proceeds in a single-operational step using a mild, regioselective, and general protocol with broad chemoselectivity.     

Catalytic ipso-Nitration of Organosilanes Enabled by Electrophilic N-Nitrosaccharin Reagent

Nitroaromatic compounds represent one of the essential classes of molecules that are widely used as feedstock for the synthesis of intermediates, the preparation of nitro-derived pharmaceuticals, agrochemicals, and materials on both laboratory and industrial scales. We herein disclose the efficient, mild, and catalytic ipso-nitration of organotrimethylsilanes, enabled by an electrophilic N-nitrosaccharin reagent and allows chemoselective nitration under mild reaction conditions, while exhibiting remarkable substrate generality and functional group compatibility. Additionally, the reaction conditions proved to be orthogonal to other common functionalities, allowing programming of molecular complexity via successive transformations or late-stage nitration. Detailed mechanistic investigation by experimental and computational approaches strongly supported a classical electrophilic aromatic substitution (S_EAr) mechanism, which was found to proceed through a highly ordered transition state.     

New strategy and methods for constructing artificial macrocyclic anion receptors. Selective binding of tetrahedral oxoanions

A new rational strategy for assembling highly selective neutral macrocyclic anionic receptors proposed by the authors is considered. The strategy includes preliminary theoretical modeling of supramolecular complexes, analysis of synthetic paths for receptor preparation, selection and synthesis of building blocks followed by their cyclocondensation under thermo-dynamic control in the presence of acid corresponding to the target template anion, which provides anion-induced combinatorial selection of the macrocyclic ligand in the emerging dynamic combinatorial library. Analysis of the properties of the obtained anion receptors showed that the number of hydrogen bonds formed between the receptor and the guest anion mainly determines the binding energy. The receptor selectivity for a particular anion is determined by the nature of coordination sites, their geometry in the macrocyclic cavity, and the overall conformation rigidity of the macrocycle. Using the developed strategy and novel synthetic approaches, a large series of highly selective anion receptors with record binding constants (up to 10^p7 L mol^p-1 in both highly and weakly solvating media) was constructed and the structures of the host—guest complexes were studied in detail by both experimental and theoretical methods     

Recognition of perrhenate and pertechnetate by a neutral macrocyclic receptor

The synthesis and anion binding properties of a neutral macrocyclic receptor bearing H-bond donor coordination sites are described. The anion binding studies by use of UV-vis and ⁹⁹Tc NMR methods revealed that the receptor can coordinate perrhenate and pertechnetate in dimethylsulfoxide and chloroform solutions with the relatively high binding constants, viz. log K _a > 4. The coordination mode of the perrhenate to the receptor was determined by a single-crystal X-ray diffraction analysis.