No abstract

Electrochemical oxidation of diphenylamines with electron‐donating and electron‐withdrawing substituents in various combinations was investigated. It was shown that the subsequent reaction channels for the radical cations are dependent on the location and electronic properties of the substituents in both phenyl rings. Guidelines for the prediction of the dominant reaction path were formulated. The conclusions developed will be useful for planning electrosynthesis. Digital simulation of the voltammograms allowed estimating the mechanism of N,N‐diaryl‐5,10‐dihydrophenazine formation (which is one of the main reaction channels); the corresponding radical cations were isolated for the first time and characterized by X‐ray, electrochemical and spectral methods. Oxidation peak potentials for diarylaminyl anions (obtained using electrochemically generated base) were measured providing information for mechanistic estimation of anti/prooxidant activity of diarylamines in radical processes.

Ni(II) complexes containing (S)-o-[N-(N-benzylprolyl)amino]benzophenone as an auxiliary chiral moiety in the form of a Schiff base with α-amino acids (α-amino acid = glycine, alanine, dehydroalanine; Gly-Ni, Ala-Ni, and Δ-Ala-Ni) were subjected to various types of electrochemical activation (oxidation, reduction, and a treatment with electrogenerated base), affording regio- and diastereoselective synthesis of novel types of binuclear Ni(II) complexes via C–C coupling. New compounds were fully characterized by HRMS, MALDI-TOF, CD, and 1H and 13C NMR (including two-dimensional techniques) spectroscopy; two complexes were characterized by X-ray diffraction analysis. The structures of the novel complexes obtained via electrosynthesis completely match the predictions (made from preliminary voltammetric investigations of the starting complexes as well as from DFT estimations of the energy and symmetry of their frontier molecular orbitals) about the nature of chemical transformations which may follow the electron transfer steps. Electrochemical oxidation of Gly-Ni and Ala-Ni allows access to new dimeric complexes linked via benzophenone moieties in the Ni(II) coordination environment. These new binuclear Ni(II) complexes are of interest as chiral redox mediators for both oxidative and reductive transformations, since they exhibit quasi-reversible electrochemical behavior (their reduced and oxidized forms are stable, at least on the time scale of cyclic voltammetry). Three other binuclear Ni(II) complexes which were obtained via reductive dimerization of the Δ-Ala-Ni complex, via nucleophilic addition of electrochemically deprotonated Gly-Ni to Δ-Ala-Ni, and via oxidative electrochemical dimerization of deprotonated Gly-Ni are of interest as convenient precursors for the stereoselective preparation of diamino dicarboxylic acids HO(O)CCH(NH2)(CH2) n (NH2)CHC(O)OH (n = 2–0), since the obtained binuclear Ni(II)–Schiff base complexes can be easily disassembled using aqueous HCl in methanol.

A Ni(II) glycine/Schiff base complex containing (S)-o-[N-(N-benzylprolyl)amino]benzophenone as an auxiliary chiral moiety was deprotonated using electrochemically generated azobenzene radical anion and used in nucleophilic addition to Michael acceptors, terminal 2,2-and 1,2-disubstituted alkenes ((2E)-1,3-diphenylprop-2-en-1-one, (E)-2-nitroethenylbenzene, 2-methylprop-2-enenitrile, Ni(II) dehydroalanine complex), creating a preparatively convenient path for asymmetric functionalization of the α-glycine carbon in the Ni(II) coordination environment, yielding new chiral Ni(II) complexes. The main advantage of the application of electrochemical techniques is the possibility of precise control of the concentration of a base and its in situ reaction with the complex. This opens up the possibility to carry out further functionalization of the anionic adduct formed in Michael addition via a successive one-pot reaction with the other electrophile. A one-pot cascade reaction of electrochemically deprotonated Ni(II) glycinate with (E)-2-nitroethenylbenzene and the successive interaction with benzyl chloride or dimethyl sulfate allowed a new oxime-containing Ni(II) complex to be obtained, which might be considered as an important synthon. All complexes were reliably characterized using HRMS and 1 H and 13 C NMR (including 2D techniques); an adduct with (2E)-1,3-diphenylprop-2en-1-one was also characterized by X-ray diffraction studies and CD spectrum. The manner of stereocontrol in the Michael addition of electrochemically deprotonated Ni(II) glycinate was shown to be different for terminal 2,2-and for 1,2-disubstituted alkenes. In the case of the 1,2-disubstituted alkene both stereocenters are already formed in the first reaction step, which is reversible and thermodynamically controlled. The second step (protonation of the anion) is fast and irreversible, and it does not influence the stereochemical result of the reaction. In contrast to the previous case, only one stereocenter is formed in the first thermodynamically controlled step for terminal alkenes, whereas the configuration of the second stereocenter is determined by a kinetically controlled protonation step.

New strategy for the molecular design of stable diarylnitroxides was elaborated based on the insertion of a bulky substituent into the ortho‐position of the phenyl ring thus disturbing its conjugation with the radical center. A series of new twisted diaryl nitroxides with tert‐butyl and trifluoromethyl substituents in different positions and combinations was obtained and fully characterized. Electron spin resonance (ESR) and density functional theory (DFT) studies confirmed that the ortho‐substituted phenyl ring is removed from the conjugation; the O–N–C–C torsion angle was shown to be dependent not only on the bulkiness of the ortho‐substituent, but it is also influenced by the electron‐donating or electron‐withdrawing ability of the substituents in both phenyl rings. These nitroxides constitute the first examples of stable diarylnitroxides with a vacant para‐position on the phenyl ring. The new approach will broaden the scope of available stable diarylnitroxyl radicals, which are practically important.