Structure-selectivity comparisons are made between chiral dirhodium(I1) tetrakis( methyl 2-oxopyrrolidine-5-carboxylates), R~z (~S -M E P Y )~ and Rhz(5R-MEPY)d (5), and dirhodium(I1) 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(I1) 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 Rhz(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 R~z (~S -M E P Y )~ 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. Dirhodium(I1) compounds possessing four chiral pyrrolidone or oxazolidinone ligands are effective catalysts for highly enantioselective metal-carbene transformations.14 Optical yields of greater than 90% have been achieved in olefin cyclopropanation with diazoacetate esters,* alkyne cyclopropenation,3 and even carbon-hydrogen insertion reactions.4 The basis for this high level of enantiocontrol has been attributed to the organization of the four chiral amide ligands around the dirhodium nucleus and the electronic orientation and the stabilization of the intermediate metal-carbene.5 A limited number of dirhodium(I1) tetrakis(carb0xamidate) compounds have been synthesized and characterized. Those formed from acetamide6 and trifl~oroacetamide~ are constructed so that in the major isomer two oxygen-and two nitrogen-donor atoms (2,2) are bonded to each octadehedral rhodium in a cis configuration (eq 1). When prepared in an acetamide melt by tH, 1 successive substitutions of acetate from dirhodium(I1) tetraacetate, only one tetrakis(acetamidate) 1 has been obtained: but Doyle, M. P.; Brandes, B. D.; Kazala, A. P.; Pieters, R. J.; Jarstfer, M. B.; Watkins, L. M.; Eagle, C. T. Tetrahedron Lett. 1990, 31, 6613. (2) Doyle, M. P.; Pieters, R. J.; Martin, S. F.; Austin, R. E.; Oalmann, C. J.; Miiller, P.

No 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.

No abstract