1,3-DIPOLAR CYCLOADDITION TO THE FE-N=C FRAGMENT .17. DIASTEREOSELECTIVITY WITH C-2-SYMMETRICAL AND NON-C-2-SYMMETRICAL CHIRALLY N-SUBSTITUTED ALPHA-DIIMINE LIGANDS, R-ASTERISK-N=C(H)-C(R')=N-R-ASTERISK AND C5H4N-2-C(R')=N-R-ASTERISK, 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 C-2-symmetric and is chiral ly N-substituted, gives the two expected diastereomers with low diaste reoselectivity, due to a small difference in the rate constants of the two reactive C=N groups in 6k exposing their re and si faces, respect ively. The 1,3-dipolar cycloaddition reaction of Fe(alpha-diimine)(CO) (3) (1, 6m) with DMAD and methyl propiolate (MP), in which a non-C-2-s ymmetric alpha-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 wi th the non-C-2-symmetric alpha-diimine ligands, which have only one re active imino group, depends on the size of the chiral group R, the di stance of the chiral group to the reacting centers, and the type of di polarophile. The two starting complexes (1 and 6m) exist in solution i n the form of two rapidly interconverting diastereomeric conformers I and II. In contrast to the C-2-symmetric case, the free energies of ac tivation for the two different conformers and their respective concent rations may be quite different here. The diastereoselectivities could not be influenced by variation of temperature, solvent, or the additio nal ligand. Kinetic resolution by a cycloreversion reaction could be e xcluded by the use of a chiral additional ligand and an achiral alpha- diimine ligand (i-Pr, H-PyCa, or i-Pr-DAB(H; H)). X-ray crystal struct ures 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 p yrrolinone ligand 8als have been determined using diastereomerically p ure products. The 5-(2-pyridyl)pyrrolin-2-one ligands 8 are decomplexe d from their precursors 5 by Me(3)NO, air oxidation, exchange by CO, a nd oxidative reactions with dihalogens or H2O2.