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1.
Dalton Trans ; 49(25): 8636-8644, 2020 Jul 07.
Artigo em Inglês | MEDLINE | ID: mdl-32598408

RESUMO

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.

2.
Molecules ; 25(9)2020 Apr 28.
Artigo em Inglês | MEDLINE | ID: mdl-32354044

RESUMO

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.

3.
Chemistry ; 26(31): 7074-7082, 2020 Jun 02.
Artigo em Inglês | MEDLINE | ID: mdl-32187746

RESUMO

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.

4.
Chemistry ; 26(30): 6793-6804, 2020 May 26.
Artigo em Inglês | MEDLINE | ID: mdl-32065686

RESUMO

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.

5.
Org Lett ; 21(24): 10028-10032, 2019 Dec 20.
Artigo em Inglês | MEDLINE | ID: mdl-31800249

RESUMO

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.

6.
Heliyon ; 5(10): e02735, 2019 Oct.
Artigo em Inglês | MEDLINE | ID: mdl-31720473

RESUMO

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.

7.
Chemistry ; 23(51): 12575-12584, 2017 Sep 12.
Artigo em Inglês | MEDLINE | ID: mdl-28700103

RESUMO

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.

8.
Chem Asian J ; 12(10): 1075-1086, 2017 May 18.
Artigo em Inglês | MEDLINE | ID: mdl-28281332

RESUMO

A series of novel highly soluble double-caged [60]fullerene derivatives were prepared by means of lithium-salt-assisted [2+3] cycloaddition. The bispheric molecules feature rigid linking of the fullerene spheres through a four-membered cycle and a pyrrolizidine bridge with an ester function CO2 R (R=n-decyl, n-octadecyl, benzyl, and n-butyl; compounds 1 a-d, respectively), as demonstrated by NMR spectroscopy and X-ray diffraction. Cyclic voltammetry studies revealed three closely overlapping pairs of reversible peaks owing to consecutive one-electron reductions of fullerene cages, as well as an irreversible oxidation peak attributed to abstraction of an electron from the nitrogen lone-electron pair. Owing to charge delocalization over both carbon cages, compounds 1 a-d are characterized by upshifted energies of frontier molecular orbitals, a narrowed bandgap, and reduced electron-transfer reorganization energy relative to pristine C60 . Neat thin films of the n-decyl compound 1 a demonstrated electron mobility of (1.3±0.4)×10-3  cm2 V-1 s-1 , which was comparable to phenyl-C61 -butyric acid methyl ester (PCBM) and thus potentially advantageous for organic solar cells (OSC). Application of 1 in OSC allowed a twofold increase in the power conversion efficiencies of as-cast poly(3-hexylthiophene-2,5-diyl) (P3HT)/1 devices relative to the as-cast P3HT/PCBM ones. This is attributed to the good solubility of 1 and their enhanced charge-transport properties - both intramolecular, owing to tightly linked fullerene cages, and intermolecular, owing to the large number of close contacts between the neighboring double-caged molecules. Test P3HT/1 OSCs demonstrated power-conversion efficiencies up to 2.6 % (1 a). Surprisingly low optimal content of double-caged fullerene acceptor 1 in the photoactive layer (≈30 wt %) favored better light harvesting and carrier transport owing to the greater content of P3HT and its higher degree of crystallinity.

9.
Angew Chem Int Ed Engl ; 56(10): 2704-2708, 2017 03 01.
Artigo em Inglês | MEDLINE | ID: mdl-28140502

RESUMO

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.

10.
Chemistry ; 19(52): 17969-79, 2013 Dec 23.
Artigo em Inglês | MEDLINE | ID: mdl-24248883

RESUMO

Novel difluoromethylenated [70]fullerene derivatives, C70(CF2 )n (n=1-3), were obtained by the reaction of C70 with sodium difluorochloroacetate. Two major products, isomeric C70(CF2 ) mono-adducts with [6,6]-open and [6,6]-closed configurations, were isolated and their homofullerene and methanofullerene structures were reliably determined by a variety of methods that included X-ray analysis and high-level spectroscopic techniques. The [6,6]-open isomer of C70(CF2 ) constitutes the first homofullerene example of a non-hetero [70]fullerene derivative in which functionalisation involves the most reactive bond in the polar region of the cage. Voltammetric estimation of the electron affinity of the C70(CF2 ) isomers showed that it is substantially higher for the [6,6]-open isomer (the 70-electron π-conjugated system is retained) than the [6,6]-closed form, the latter being similar to the electron affinity of pristine C70. In situ ESR spectroelectrochemical investigation of the C70(CF2 ) radical anions and DFT calculations of the hyperfine coupling constants provide evidence for the first example of an inter-conversion between the [6,6]-closed and [6,6]-open forms of a cage-modified fullerene driven by an electrochemical one-electron transfer. Thus, [6,6]-closed C70(CF2 ) constitutes an interesting example of a redox-switchable fullerene derivative.


Assuntos
Fulerenos/química , Isomerismo , Modelos Moleculares , Estereoisomerismo
11.
Inorg Chem ; 49(7): 3370-86, 2010 Apr 05.
Artigo em Inglês | MEDLINE | ID: mdl-20184372

RESUMO

The mechanism of benzyl alcohol oxidation with dioxygen activated by Cu complexes with biquinolyl (biQ)-containing ligands was investigated using theoretical DFT calculations to identify the active species and to reveal similarities with naturally occurring oxidations performed by Cu-containing enzymes. The detailed potential energy profiles for two possible reaction paths with the alpha-H atom abstraction key step (via direct benzaldehyde formation and via hydroxylation of benzyl alcohol followed by the expulsion of a water molecule) were calculated for the both triplet and singlet states of a Cu(II)-superoxo complex and a Cu(III)-oxo complex. The DFT estimated activation barrier for the alpha-H-atom abstraction (which is involved in the both pathways) from a molecule of benzyl alcohol by the triplet biquinolyl-containing Cu(II)-superoxo complex is 16.4 kcal/mol. This value is reasonable for an enzymatic C-H bond activation and is very close to that calculated for benzylic C-H bond activation in dopamine and formylglycine. Thus, both reaction pathways are reasonable, and the existence of the parallel pathways cannot be ruled out. However, the first path (via direct benzaldehyde formation) is less complicated and the entropy factor should facilitate this route. (biQ)Cu(III)-oxo species formed in the catalytic cycle can also be involved in the further catalytic oxidation of the benzyl alcohol. DFT exploration performed for the triplet and singlet (biQ)Cu(III)-oxo complexes revealed that this process is barrierless and much more exothermic and, consequently, might be rather fast. It also includes the alpha-H-atom abstraction step and results in the formation of benzaldehyde. Thus, the primary catalytic cycle in which the Cu-superoxo complex performs the process can be coupled with the second cycle which is governed by the Cu-oxo complex formed in the first cycle.


Assuntos
Álcool Benzílico/química , Cobre/química , Oxigênio/química , Quinolinas/química , Modelos Moleculares , Oxirredução , Teoria Quântica
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