Electrochemically Deprotonated Chiral Nickel(II) Glycinate in Stereoselective Nucleophilic Addition to Michael Acceptors: Advantages and Limitations

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 1H and 13C NMR
(including 2D techniques); an adduct with (2E)-1,3-diphenylprop-2-en-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.