1,5-Dihydropyrrol-2-ones from (1,4-diaza-1,3-diene)tricarbonyliron, (dad)Fe(CO)3, and alkyne: IV. Electronic and steric effects along the reaction coordinate. Regioselectivities with unsymmetrical dad (methylglyoxal-bis-isopropylimine) or alkyne (methyl propynoate)

Abstract The [2.2.2]bicyclic intermediate observed in the title reaction in the case of dad ligands containing at least one unsubstituted imine carbon atom (aldimino group) were previously shown to be stabilized by introduction of a phosphite ligand, and on the basis of the bond distances revealed by X-ray analysis of the stabilized complex the stabilizing effect has been attributed to a strengthening of the FeC bond trans - to the P-donor atom, i.e., to one of the bonds broken in the final rearrangement reaction. By utilization of this stabilization, the heretofore unobservable [2.2.2] intermediate with a biacetyl derived dad (bis-ketimine) was observed, and identified by IR spectroscopy. Its rearrangement product, the first example of this type of species containing a phosphite ligand, was isolated. Its NMR ( 1 H, 13 C, 31 P) spectra reveal that only the isomer with the phosphorus in the thermodynamically most favorable position is formed. With sterically demanding dad-substituents, products from the final rearrangement are obtained only in poor yields. A mechanistic sequence which accounts for the findings is proposed. The initial step in the reaction sequence, in which the dad and alkyne become CC connected, has been found to be very sensitive to electronic influences. In reactions with unsymmetrical dad (methylglyoxal-bis-isopropylimine) and alkyne (methyl propynoate), respectively, the formation of exclusively one of the possible regioisomers is observed. The products have been characterized, and their structures determined by MS, IR, 1 H, and 13 C NMR spectroscopy.