Enhanced enantiocontrol in catalytic metal carbene transformations with dirhodium (II) tetrakis[methyl 2-oxooxazolidin-4(S)-carboxylate], Rh2(4S-MEOX)4

The synthesis, spectral characteristics, and X-ray structures for dirhodium(II) tetrakis[methyl 2-oxooxazolidin-4(S)-carboxylate], Rh2(4S-MEOX)4, and the related dirhodium(II) tetrakis[methyl 5(R)-methyl-2-oxooxazolidin-4(S)-carboxylate], Rh2(4S-THREOX)4, are reported. Comparison is made between these 2-oxooxazolidin-ligated dirhodium(II) catalysts for metal carbene transformations and those with comparable 2-oxopyrrolidine ligands, especially dirhodium(II) tetrakis[methyl 2-oxopyrrolidine-5(S)-carboxylate], Rh2(5S-MEPY)4. Structure-selectivity comparisons reveal that Rh2(4S-MEOX)4 provides higher enantiocontrol and, in some cases, higher diastereocontrol than Rh2(5S-MEPY)4 in intramolecular carbon-hydrogen insertion reactions of sterically demanding diazoacetates and diazoacetamides and is the catalyst of choice for highly enantioselective diazodecomposition of adamantyl diazoacetates and N-(tert)butyldiazoacetamides. The structurally analogous Rh2(4S-THREOX)4 is nearly identical with Rh2(4S- MEOX)4 in its overall effect on selectivity. The net advantage of Rh2(4S-MEOX)4 lies in its wider openness for metal carbene transformations.