Chalcogen Atom Competition in the Coordination of Bicyclic β-Lactam Derivatives to a Dirhodium Tetracarboxylate Complex

Abstract

NMR spectra of five enantiopure β-lactam derivatives with varying oxygen and sulfur atoms (1 to 5) have been recorded in the presence of an equimolar amount of a 2:1-mixture of (R)-Rh* and (S)-Rh* [(R/S)-Rh*]. No carboxylate ligand exchange between the two enantiomeric dirhodium complexes was observed. In general, sulfur is a much better donor than oxygen in axial coordination to rhodium. If both chalcogen atoms are equal (S in 1 and O in 4) the doubly bonded atom is the prefered binding site (C 9 =X). In the compounds with mixed chalcogen atoms, sulfur prevails as donor regardless of its bond order. This behaviour can be rationalized by comparing calculated molecular orbital energies and electronic charges. Olefinic groups do not contribute to binding significantly. All β-lactam derivatives were enantiopure so that it was possible to monitor significant chemical shift differences of the 1 H, 13 C and 19 F NMR resonances for the two Rh* enantiomers. An experiment with racemic 4-vinyloxyazetidin-2-one (6) and (R)-Rh* suggests that the dirhodium method is effective for enantiodifferentiation of β-lactams.