Evaluation of Structural Effects of Acyclic and Macrocyclic Ligands Derived from (R,R)-Cyclohexane-1,2-diamine in the Enantioselective Cu(II)-Catalyzed Henry Reaction
Two new ligands with acyclic or macrocyclic structure were prepared from (R,R)-cyclohexane-1,2-diamine, the
former by condensation with 5-[1-methyl-1-(1H-pyrrol-2-yl)ethyl]-1H-pyrrole-2-carboxaldehyde, and the latter by double
condensation with 5,5'-(1H-pyrrole-2,5-diyl)bis(furan-2-carboxaldehyde). Subsequent reduction of the intermediate diimine
and tetraimine with sodium borohydride afforded the corresponding amines, which were tested as ligands in the
Henry reaction of benzaldehyde with nitromethane. The results were compared with those obtained by analogous acyclic
and macrocyclic ligands and enabled to assess that the macrocyclic structure of pyrrole-containing ligand was beneficial
for achieving high enantioselectivity. Moreover, it was confirmed that the asymmetric induction of ligands containing the
pyrrole ring, even if a furan ring is also present, is opposite to that displayed by analogous furan- and thiophenecontaining
ligands with the same chirality of the cyclohexane-1,2-diamine moiety.
Keywords: trans-cyclohexane-1, 2-diamine, enantioselective catalysis, furan, henry reaction, macrocyclic ligand, pyrrole.
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