A new oxidatively stable ligand for the chiral functionalization of amino acids in Ni(II)-Schiff base complexes.

A new oxidatively stable (S)-N-benzylproline-derived ligand ((S)-N-(2-benzoyl-5-tert-butylphenyl)-1-benzylpyrrolidine-2-carboxamide) and its Ni(II)-Schiff base complexes formed of glycine, serine, and dehydroalanine are reported. A bulky tert-butyl substituent in the phenylene fragment precludes unwanted oxidative dimerization of the Schiff base complex, making it suitable for targeted electrochemically induced oxidative modification of the amino acid side chain. Experimental and DFT studies showed that the additional tert-butyl group increases the dispersion interactions in the Ni coordination environment making the complexes more conformationally rigid and provides a higher level of thermodynamically controlled stereoselectivity as compared to the parent Belokon complex. Additionally, functionalization with the tert-butyl group significantly enhances the reactivity of the deprotonated glycine complex towards electrophiles as compared to the anionic species formed from the original Belokon complex. Solubility of the t-Bu-containing ligand and its Schiff base complexes is increased, facilitating scaling-up the reaction procedure and isolation of the functionalized amino acid.

Stable Spiro-Fused Diarylaminyl Radicals: A New Type of a Neutral Mixed-Valence System.

A new type of neutral mixed-valence system was synthesized using a facile one-pot procedure. The spiro-conjugated framework is additionally "fastened" with a biphenyl bridge, which does not directly participate in spin delocalization but makes the molecule stable and influences the reorganization energy and the energy barrier of the intramolecular electron transfer. The in-depth experimental and quantum-chemical study allowed determining the radicals as the Class II Robin-Day-mixed-valence systems. The structure of the radicals was confirmed by the X-ray data, which are relatively rare for Class II MV molecules. Advanced properties of the radicals, such as an ambipolar redox behavior and panchromatic absorption in the visible and NIR regions, along with their stability, make them of interest for materials science. All radicals demonstrate the SOMO-HOMO inversion phenomenon, which was supported by the DFT and the experimental study.

Reductive opening of a cyclopropane ring in the Ni(II) coordination environment: a route to functionalized dehydroalanine and cysteine derivatives.

The involvement of an α,α-cyclopropanated amino acid in the chiral Ni(II) coordination environment in the form of a Schiff base is considered as a route to electrochemical broadening of the donor-acceptor cyclopropane concept in combination with chirality induction in the targeted products. A tendency to the reductive ring-opening and the follow-up reaction paths of thus formed radical anions influenced by substituents in the cyclopropane ring are discussed. Optimization of the reaction conditions opens a route to the non-proteinogenic amino acid derivatives containing an α-ß or ß-γ double C=C bond in the side chain; the regioselectivity can be tuned by the addition of Lewis acids. One-pot combination of the reductive ring opening and subsequent addition of thiols allows obtaining the cysteine derivatives in practical yields and with high stereoselectivity at the removed ß-stereocenter.

Pyridine-Containing Donor-Acceptor Diarylnitroxides: Noncovalent Stabilization of the Redox States.

A series of new pyridyl- or 2-pyridyloxide-containing donor-acceptor diarylnitroxides was obtained and characterized; high stability of the ortho-2-pyridyl-containing diarylnitroxides was determined by kinetic measurements (τ1/2 =1733 h in benzene). Comparative voltammetric study of new nitroxides and their analog in which the Py replaces the Ph group revealed both through-bond and through-space stabilization of the NO redox states with the pyridyl/ 2-pyridyloxide moiety, providing reversibility of both oxidation and reduction processes. Adaptive conformational behavior of new pyridyl/pyridyloxide containing nitroxides upon one-electron oxidation and reduction was confirmed by DFT calculations. Stimuli-responsive conformational changes allow switching on/off dispersion and electrostatic interactions within the molecule and increase stability of the redox states. Spectroelectrochemical measurements provided experimental evidence for reversibility of the through-space stabilization of the oxidized state of the nitroxides with the neighboring pyridine lone pair.

Solvent-triggered stereoselectivity of α,α-cyclopropanation of amino acids in the Ni(ii) chiral coordination environment.

Experimental and DFT investigation of the α,α-cyclopropanation of amino acids via nucleophilic addition of the deprotonated glycine Ni(ii)-Schiff-base complex, containing the (S)-proline derivatives as an auxiliary chiral moiety, to alkyl α-bromoacrylates was performed. It was demonstrated that the predominant configuration of the newly formed α-stereocenter is (S), regardless of the solvent used but the smart choice of solvent allows high diastereoselectivity at the removed ß-stereocenter to be obtained, which commonly is rather rare. DFT analysis of the reaction path provides a rationale for the stereochemical outcome observed. The cyclopropanated complexes exhibit stereodependent redox activity, thus supporting that this is a general phenomenon inherent to this class of Ni Schiff-base derivatives, accounting for the influence of the peripheral groups in the metal coordination environment on the relative impact of different parts of the molecule in the frontier orbitals via conformational changes.

Complexes of Cobalt(II) Iodide with Pyridine and Redox Active 1,2-Bis(arylimino)acenaphthene: Synthesis, Structure, Electrochemical, and Single Ion Magnet Properties.

Complexes [(dpp-BIAN)0CoIII2]·MeCN (I) and [(Py)2CoI2] (II) were synthesized by the reaction between cobalt(II) iodide and 1,2-bis(2,6-diisopropylphenylimino)acenaphthene (dpp-BIAN) or pyridine (Py), respectively. The molecular structures of the complexes were determined by X-ray diffraction. The Co(II) ions in both compounds are in a distorted tetrahedral environment (CoN2I2). The electrochemical behavior of complex I was studied by cyclic voltammetry. Magnetochemical measurements revealed that when an external magnetic field is applied, both compounds exhibit the properties of field-induced single ion magnets.

Which Stereoinductor Is Better for Asymmetric Functionalization of α-Amino Acids in a Nickel(II) Coordination Environment? Experimental and DFT Considerations.

The results of extended comparative investigation of nickel(II) Schiff base complexes (containing various auxiliary chiral moieties) commonly used as a methodological platform for the asymmetric synthesis of tailor-made α-amino acids are provided. The following issues are addressed: 1) redox activity (determining the possibility for electrochemically induced reactions); 2) quantitative estimation of the reactivity of deprotonated complexes towards electrophiles; and 3) quantum-chemical estimation of noncovalent interactions in the metal coordination environment (which shed light on the origin of the stereochemical outcome observed for different stereoinductors). Possible mechanisms that determine the relationship between the stereochemical configuration of a molecule and its electronic structure are discussed. The DFT-calculated HOMO-LUMO energies and localization, as well as relative energies for the (S)- and (R)-alanine derivatives, that determine the stereoinduction efficiency in thermodynamically controlled reactions in nickel(II) coordination are provided. The computational data are supported by experimental results on the monobenzylation of glycine derivatives.

Chameleonic Behavior of the α-Methylcyclopropyl Group and Its Through-Space Interactions: A Route to Stabilized Three Redox States in Diarylnitroxides.

The α-methylcyclopropyl (MCP) group with conformationally dependent electronic properties is suggested as an additional structural "instrument" for stabilization of both open-shell and ionic states of diarylnitroxides, to be used as "ambipolar" redox active materials. New MCP-substituted diphenylnitroxides (fully characterized by electrochemical, spectral, and X-ray data) are the most stable nitroxides of the diaryl type known to date [τ1/2 in benzene exceeds three months (2310 h)]. The radicals are capable to reversible oxidation and reduction, yielding stable oxoammonium cations and aminoxyl anions. DFT investigation of the electronic structure and geometry of the compounds confirmed the conformational switching of the cyclopropyl orientation relative to the adjacent aromatic π system is dependent on the nitroxide's redox state. Additional through-space stabilizing interaction between the π-acceptor orbital of the NO+ moiety and the cyclopropyl "banana" bond orbital was also detected, highlighting its good hyperconjugative ability. The estimated σ(para)MCP value (-0.32) confirms its strong electron-donating properties.

Amination of Aryl Boronic Acids with Alkylnitrites: A Convenient Complement to Cu-Promoted Reductive Amination.

Copper-catalyzed amination of aryl boronic acids with alkylnitrites leading to symmetrical diarylamines with a practical 50-80% yield was elaborated. Two C(sp2)-N bonds are formed in the one-pot process under mild conditions. This new approach to diarylamines is a complement to the Cu-assisted reductive amination of aryl boronic acids avoiding preliminary synthesis of nitrosoarenes. The possible reaction scheme based on quantum chemical calculations was suggested, clarifying key intermediates.

Diarylamine/diarylnitroxide cycle: quantum chemical and electrochemical estimation.

Possibility for diarylamine/diarylnitroxide cycling via electrochemical N-O bond formation/bond breaking processes was considered using quantum-chemical and electrochemical methods. It was shown that electrochemically reduced diarylnitroxides undergo fast N-O bond cleavage in the presence of oxophilic Li ions. The possible reaction scheme was suggested. In contrast, in the presence of Na+ salts, aminoxyl anions are stable and can be considered as possible anodic redox active material in energy storage systems utilizing Na+ ions migration. Direct oxygenation of diarylaminyl radicals with dioxygen yielding diarylnitroxide is not feasible; multiple competing routes involving the aromatic moiety are observed instead. The results obtained shed light on possibilities and limitations for functioning of nitroxide-based redox active electrode materials.

Copper-Assisted Amination of Boronic Acids for Synthesis of Bulky Diarylamines: Experimental and DFT Study.

Comparative investigation of copper-assisted oxidative and reductive amination showed that the latter was preferable for the synthesis of bulky diarylamines. DFT estimation of the mechanism of copper(I)-assisted reductive amination of boronic acids with aryl nitroso compounds was performed and possible active species were identified. DFT estimation of the steric penalty revealed that the barrier for the transmetalation step for the hindered nitroso compound was almost the same as that for the unsubstituted one, whereas a bulky group in the boronic acid increased the activation energy. A DFT study of the influence of the electronic properties of the substituents in both reactants on the activation energy revealed that the optimal combination for the synthesis of unsymmetrical diarylamines to provide better yields was an electron-rich aryl boronic acid and an electron-deficient nitroso compound. By using these helpful guidelines, a series of new bulky diarylamines were obtained and fully characterized.

Individual (f,t A)- and (f,t C)-Fullerene-Based Nickel(II) Glycinates: Protected Chiral Amino Acids Directly Linked to a Chiral π-Electron System.

Stereoselective electrosynthesis of the first individual (f,t A)- and (f,t C)-1,4-fullerene derivatives with a non-inherently chiral functionalization pattern is described, as well as the first example of an optically pure protected primary amino acid directly linked to the fullerene through only the chiral α-amino-acid carbon atom. An application of an auxiliary chiral nickel-Schiff base moiety as derivatizing agent allowed separation of (f,t A)- and (f,t C)-1,4-fullerene derivatives using an achiral stationary phase, a separation which has never been done before.