Organonickel(II) complexes containing aryl ligands with chiral pyrrolidinyl ring systems; syntheses and use as homogeneous catalysts for the Kharasch addition reaction

Article abstract of DOI:10.1002/recl.19941130502

New chiral organonickel(II) complexes 3a-d have been synthesized containing monoanionic terdentate ligands of the type 2,6-bis<(1-pyrrolidinyl)methyl>phenyl. 3a R1=Me(racemic), R2=H, 3b R1=Me(S), R2=H, 3c R1=Me(R), R2=Me(R), 3d R1=C(O)OPr-i(S), R2=H.Chirality is introduced at the nitrogen donor atoms, which are part of the pyrrolidinyl ring systems, by placing substituents on the 2 or 5 positions of the rings.The complexes have been characterized variously by 1H- and 13C-NMR spectroscopy, UV/Vis spectroscopy and X-ray crystallography.Crystals of 3b are monoclinic, space group P2₁, with a = 11.572(2), b = 11.804(2), c=13.249(3) Angstroem, β=107.00(2) deg, Z=4, final R=o.o43 for 3043 observed reflections.Crystals of 3c are tetragonal, space group P4₁2₁2₁, with a = b = 12.2957(14) Angstroem, c = 26.418(3) Angstroem, Z = 8, final R = 0.052 for 1479 observed reflections.Crystals of 3d are tetragonal, space group P4₁2₁2₁ with a = b = 11.0869(5), c = 21.718(2) Angstroem, Z = 4, final R = 0.046 for 1469 observed reflections.The catalytic ability of the nickel complexes in the addition reaction of polyhalogenated compounds with alkenes (Kharasch addition) is markedly influenced by the geometry around the nickel atom and only 3a and 3b prove to be effective catalysts.The highest enantiomeric excesses of 16-17percent obtained with 3b and L-menthyl methacrylate in this catalytic reaction indicate that the C₂ chiral fence produced by the Me-substituted pyrrolidinyl rings is not very effective for the transfer of chiral information.