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.
