Synthesis and Structures of (2,2-cis)-Dirhodium(II) Tetrakis[methyl 1-acyl-2-oxoimidazolidine-4(S)-carboxylates]. Chiral Catalysts for Highly Stereoselective Metal Carbene Transformations

Abstract

Dirhodium(II) tetracarboxamidates derived from chiral methyl 1-acyl-2-oxoimidazolidine-4(S)-carboxylates are highly enantioselective and diastereoselective catalysts for metal carbene transformations of diazoacetates. Four of these catalysts have been prepared by ligand substitution with dirhodium(II) acetate and characterized spectroscopically and by X-ray structural analysis. The simplest member of the series, (2,2-cis)-dirhodium(II) tetrakis[methyl 1-acetyl-2-oxoimidazolidine-4(S)-carboxylate], (2,2-cis)-Rh2(4S-MACIM)4, forms monoclinic crystals in space group P21 with cell constants of a = 9.959(1) Å, b = 20.100(1) Å, c = 11.678(1) Å, β = 107.61(1)°, V = 2228.1(3) Å3, and Z = 2. (2,2-cis)-Rh2(4S-MBOIM)4, whose 1-acyl group is phenyl, forms tetragonal crystals in space group P41212 with cell constants of a = 18.352(1) Å, c = 17.067(1) Å, V = 5748.1(6) Å3, and Z = 4. The most enantioselective catalyst in many metal carbene transformations is the one with 3-phenylpropanoyl at the 1-position, (2,2-cis)-Rh2(4S-MPPIM)4; crystals are monoclinic in space group P21 with cell constants of a = 11.589(1) Å, b = 12.225(1) Å, c = 21.802(2) Å, β = 94.429(8)°, V = 3079.6(5) Å3, and Z = 2. The highest level of diastereocontrol has been achieved with the dirhodium(II) 1-acyl-2-oxoimidazolidine having the 1-acyl group equal to cyclohexylacetyl, (2,2-cis)-Rh2(4S-MCHIM)4, whose crystals are triclinic in space group P1 with cell constants of a = 11.720(1) Å, b = 12.067(1) Å, c = 12.773(1) Å, α = 71.794(3)°, β = 75.765(4)°, γ = 72.789(3)°, V = 1615.5(2) Å3, and Z = 1. The (3,1) isomer in which ligand orientation places three nitrogens and one oxygen on one rhodium and three oxygens and one nitrogen bound to the second rhodium is formed as a byproduct of the (2,2-cis) isomer in three of the four catalyst preparations; for (3,1)-Rh2(4S-MACIM)4, crystals are orthorhombic in space group P212121 with cell constants of a = 11.568(1) Å, b = 19.344(1) Å, c = 19.551(2) Å, V = 4375.0(6) Å3, and Z = 4. A third tetrasubstituted isomer, proposed to be (4,0)-Rh2(4S-MACIM)4 based on spectral information and the absence of enantiocontrol in selected metal carbene transformations, is also formed, but in amounts that do not exceed 5% of the total, and this compound isomerizes during the time course of the substitution reaction. Under the reaction conditions employed for their synthesis, the (2,2-cis) and (3,1) isomers reach equilibrium at 85:15. The effectiveness of these chiral dirhodium(II) oxazolidinones as catalysts for enantioselective and diastereoselective intramolecular reactions of diazoacetates has been evaluated.