1,3-Dipolar Cycloaddition to the FeNC Fragment. 17. Diastereoselectivity with C₂- and non-C₂-Symmetric Chirally N-Substituted α-Diimine Ligands, R*NC(H)C(R‘)NR* and C₅H₄N-2-C(R‘)NR*, with Dimethyl Acetylenedicarboxylate (DMAD) and Methyl Propiolate (MP). Decomplexation of the Pyrrolinone Ligand of Fe(CO)₃(pyrrolinone) Complexes

The 1,3-dipolar cycloaddition reaction of Fe[R*-DAB(H;H)](CO)₃ (6k; R*-DAB(H;H) = R*NC(H)C(H)NR*) with dimethyl acetylenedicarboxylate (DMAD), in which the R*-DAB(H;H) ligand is C₂-symmetric and is chirally N-substituted, gives the two expected diastereomers with low diastereoselectivity, due to a small difference in the rate constants of the two reactive CN groups in 6k exposing their re and si faces, respectively. The 1,3-dipolar cycloaddition reaction of Fe(α-diimine)(CO)₃ (1, 6m) with DMAD and methyl propiolate (MP), in which a non-C₂-symmetric α-diimine ligand (R*-DAB(H;Me) and Py-2-C(R‘)NR* (R*,R‘-PyCa) is chirally N-substituted, gives the two expected diastereomers with medium to high diastereoselectivity. The diastereoselectivity with the non-C₂-symmetric α-diimine ligands, which have only one reactive imino group, depends on the size of the chiral group R*, the distance of the chiral group to the reacting centers, and the type of dipolarophile. The two starting complexes (1 and 6m) exist in solution in the form of two rapidly interconverting diastereomeric conformers I and II. In contrast to the C₂-symmetric case, the free energies of activation for the two different conformers and their respective concentrations may be quite different here. The diastereoselectivities could not be influenced by variation of temperature, solvent, or the additional ligand. Kinetic resolution by a cycloreversion reaction could be excluded by the use of a chiral additional ligand and an achiral α-diimine ligand (i-Pr, H-PyCa, or i-Pr-DAB(H; H)). X-ray crystal structures have been determined from the respective diastereomeric mixtures of the pyrrolinone complex 5cks and the [2.2.2] bicyclic product 7aks. The structures of the [2.2.2] bicyclic product 4aks and of the free pyrrolinone ligand 8als have been determined using diastereomerically pure products. The 5-(2-pyridyl)pyrrolin-2-one ligands 8 are decomplexed from their precursors 5 by Me₃NO, air oxidation, exchange by CO, and oxidative reactions with dihalogens or H₂O₂.