The first earth-abundant
cobalt-catalyzed highly branched- and
enantioselective allylic amination of racemic branched allylic carbonates
bearing alkyl groups with both aromatic and aliphatic amines has been
developed. The process allows rapid access of allylic amines in high
yields with exclusively branched selectivity and excellent enantioselectivities
(normally 99% ee) under mild reaction conditions.
Two organic moieties, known as ligands, having -OMe and -SePh as the ortho substituent attached to the aniline moiety of the parent 2-anilino-4,6-di-tert-butylphenol ligand, were synthesized. The ligands reacted with CuCl2·2H2O in a 2:1 ratio in CH3CN in the presence of Et3N and provided the corresponding mononuclear Cu(II)-diradical complexes 1 (-OMe) and 2 (-SePh). Complex 1 was square planar, while complex 2 was in distorted square planar geometry due to the secondary coordination between the Se atom and the central Cu(II) center. Both complexes were comprised of multi-paramagnetic centers and exhibited an St = 1/2 ground state as established by variable-temperature magnetic susceptibility measurements. X-band electron paramagnetic resonance measurements indicated the presence of an unpaired electron at the Cu(II) center in complex 1 and at the ligand center (π-radical) in complex 2. The extent of the secondary interaction was found to be dependent on the "softness" of the donor atom belonging to the ortho substituent.
Cobalt-catalyzed highly branched-
and enantioselective allylic
alkylation of malononitriles has been developed. Chiral γ,δ-unsaturated
malononitriles could be synthesized with >20:1 branched/linear
regioselectivity
and up to 99% enantiomeric excess from easily accessible racemic allylic
carbonates under mild reaction conditions. The electron-rich and sterically
less hindered bisoxazolinephosphine ligand is essential to realize
the high reactivity in the carbon–carbon bond formation process.
Ligand H3Sami(Mixed(tBu)) was composed of two different compartments, a redox-active 2-aminophenol and a salen salicylidene. Both compartments were linked via a benzyl linker. The ligand reacted with CuCl2·2H2O under air in the presence of Et3N and provided the corresponding monoradical-coordinated mononuclear Cu(II) complex (1). Complex 1, in solution, reacted with air and provided complex 2 via ligand-centered oxygenation at the benzyl-CH2 position. Both complexes were characterized via IR, mass spectrometry, X-ray single-crystal diffraction, variable-temperature magnetic susceptibility, cyclic voltammograms (CVs), and UV-vis/NIR spectroscopic techniques. X-ray crystallographic analyses clearly showed almost equally distorted square planar geometry around the Cu(II) atom in both complexes. However, the bending of the radical-containing C6 ring compared to the N1-Cu1-O1 plane was different in both complexes. While complex 1 was paramagnetic and showed a ferromagnetic coupling between the d(x(2)-y(2)) magnetic orbital of Cu(II) ion and the p(z) orbital of coordinated π-radical, complex 2 was diamagnetic by experiencing a strong antiferromagnetic coupling between the two magnetic orbitals. UV-vis/NIR spectra of the complexes were dominated by charge-transfer transitions. CVs of the complexes showed two reversible one-electron oxidations and one reversible one-electron reduction. E(1/2)(ox2) and E(1/2)(red1) potentials were different in both complexes, while E(1/2)(ox1) values were almost the same and the process corresponded to the formation of phenoxyl radical. Theoretical studies were also performed to understand the magnetic coupling phenomena, and TD-DFT calculations were employed for the assignment of charge-transfer absorption bands.
Rhodium(I)/bisoxazolinephosphine
combination has been developed
as a general catalyst to achieve the dynamic kinetic asymmetric allylation
of a variety of nitrogen, carbon, oxygen, and sulfur pronucleophiles
from branched racemic allylic carbonates. Exclusive branch-selectivity
and up to 99% enantiomeric excess could be obtained under neutral
conditions. Linear allylic substrates (both Z and E) could be converted to the same chiral branched products
with excellent regio- and enantioselectivities as well. Chiral π-allyl-Rh(III)/NPN
intermediate was isolated and characterized to understand the origin
of the high selectivities.
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