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.
Stereoselective electrosynthesis of the first individual ( A)- and ( 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 ( A)- and ( C)-1,4-fullerene derivatives using an achiral stationary phase, a separation which has never been done before.
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