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-oxazolidinones), Rh2(4R-BNOX)4 and Rh2(4S-BNOX)4 (6), to ascertain and understand their relative effectiveness as catalysts for enantiocontrol in metal-carbene transformations. The syntheses, spectral characteristics, and X-ray structures for these dirhodium(II) compounds are reported. Each possesses two oxygen- and two nitrogen-donor atoms bound to each octahedral rhodium with a cis orientation of the nitrogen ligands. The Rh2(MEPY)4 catalysts are significantly more effective than those of Rh2(BNOX)4 in providing a high level of enantiocontrol in intermolecular and intramolecular cyclopropanation reactions, in intermolecular cyclopropenation reactions, and in intramolecular C-H insertion reactions of diazoacetates and diazoacetamides, often reaching >90% enantiomeric excesses. Molecular mechanics calculations that were able to reproduce the X-ray structures of Rh2(5S-MEPY)A and Rh2(4R-BNOX)4 have been employed to obtain the preferred conformation of the intermediate metal-carbene, but the absolute configurations of cyclopropanation products are opposite to those predicted from the preferred metal-carbene conformation. However, conformational energy minima of the styrene-carbene complex predict the observed enantiomer preference.