DIRHODIUM(II) TETRAKIS(CARBOXAMIDATES) WITH CHIRAL LIGANDS - STRUCTURE AND SELECTIVITY IN CATALYTIC METAL CARBENE TRANSFORMATIONS

Structure-selectivity comparisons are made between chiral dirhodium(II ) tetrakis(methyl 2-oxopyrrolidine-5-carboxylates), Rh2(5S-MEPY)4 and Rh2(5R-MEPY)4 (5), and dirhodium(II) tetrakis(4-benzyl-2-oxazolidinone s), Rh2(4R-BNOX)4 and Rh2(4S-BNOX)4 (6), to ascertain and understand t heir relative effectiveness as catalysts for enantiocontrol in metal-c arbene transformations. The syntheses, spectral characteristics, and X -ray structures for these dirhodium(II) compounds are reported. Each p ossesses two oxygen- and two nitrogen-donor atoms bound to each octahe dral rhodium with a cis orientation of the nitrogen ligands. The Rh2(M EPY)4 catalysts are significantly more effective than those of Rh2(BNO X)4 in providing a high level of enantiocontrol in intermolecular and intramolecular cyclopropanation reactions, in intermolecular cycloprop enation reactions, and in intramolecular C-H insertion reactions of di azoacetates and diazoacetamides, often reaching >90% enantiomeric exce sses. Molecular mechanics calculations that were able to reproduce the X-ray structures of Rh2(5S-MEPY)A and Rh2(4R-BNOX)4 have been employe d to obtain the preferred conformation of the intermediate metal-carbe ne, but the absolute configurations of cyclopropanation products are o pposite to those predicted from the preferred metal-carbene conformati on. However, conformational energy minima of the styrene-carbene compl ex predict the observed enantiomer preference.