Organometallics
Article
3.88−3.82 (m, 2H), 2.32−2.21 (m, 2H), 1.68 (dt, J = 1.4, 9.8 Hz, 2H),
1.46 (dt, J = 1.4, 9.8 Hz, 2H), 1.37 (s, 6H), 1.14 (d, J = 7.3 Hz, 6H), 1.07
(d, J = 7.3 Hz, 6H), 0.62 (s, 6H) ppm. 13C {1H} NMR (176 MHz,
CDCl3): δ 167.4, 158.4, 137.4, 129.0, 128.2, 128.1, 87.1, 69.6, 63.4,
60.7, 59.7, 58.5 (d, J = 11.5 Hz), 56.1, 53.8 (d, J = 10.4 Hz), 44.1, 29.5,
27.6, 21.7, 20.9, 17.7 ppm. LRMS (ESI) calcd m/z for [M − Cl]+ 943.1,
found m/z 943.1. HRMS (ESI) calcd (found) for C44H50ClN2O6Rh2
([M − Cl]+): 943.1462 (943.1471). Anal. Calcd (%) for
C44H50Cl2N2O6Rh2(979.62 g/mol): C: 53.95; H: 5.15; N: 2.86.
Found: C: 54.02; H: 5.42; N: 2.68.
ligand 1, and higher enantioselectivity was obtained for ligand 1
for both, the cationic and the neutral complexes. The higher
yields for ligand 2 correlate well with their increased
electrochemical oxidation stability determined by their higher
onset oxidation potential in cyclic voltammetry, which increased
in the series [RhCl(1)]2 < [RhCl(2)] < [(Rh(1))2Cl]SbF6 <
[RhOH2(2)]SbF6. When comparing neutral vs charged
complexes, the neutral monomer gave better yields and
enantioselectivities, while the difference for the neutral and
charged dimers was only marginal.
The liquid confinement in microemulsions produced higher
yields than dioxane but lower enantioselectivities, which might
be related to the different orientation of the Rh complexes in/
near the amphiphilic film compared to unstructured dioxane.
A strong positive nonlinear-like effect (NLLE) regarding
diastereomeric excess of 1 and epi-1 was found in the
diastereomeric mixture of [RhCl(1)]2 and [RhCl(epi-1)]2,
while no such effect was found for varying enantiomeric purity
between [RhCl(1)]2 and [RhCl(ent-1)]2.
In the 1,4-addition to cyclopentenone 6a, complexes with
ligand 2 (both neutral and cationic) gave higher yields and
enantioselectivities than the corresponding complexes with
ligand 1. The poorer performance of ligand 1 can be rationalized
by the additional gem-dimethyl group of 1 as compared to 2,
which seems to be far away from the Rh center according to the
X-ray data (Figure 1) but is close (2.15 Å) to the substrate in the
main transition state according to DFT calculations (Figure 6).
The 1,4-addition turned out to be strongly substrate-dependent,
giving very low enantioselectivity and, in several cases, racemic
products when cyclohexenone 6b was employed as substrate.
DFT calculations explained this by showing that the negative
partial charge accumulating at the substrate O is more efficiently
stabilized in the (R)-transition states in the more rigid 6a than in
the more flexible 6b.
This combined experimental and theoretical study demon-
strated that for chiral oxazolidinone-substituted norbornadiene
ligands 1, ent-1, and 2 there is no “one size fits all” Rh catalyst.
We found and explained remarkably different reactivities and
selectivities depending on the specific reaction type, substrate,
and solvent between both monomeric and dimeric complexes
[RhCl(2)] and [RhCl(1)]2 and their cationic counterparts
[RhOH2(2)]SbF6 and [(Rh(1))2Cl]SbF6. The current results
paved the way to further exploration of monomeric Rh
complexes in asymmetric catalysis.
Synthesis of [(Rh(1))2Cl]SbF6. Under a nitrogen atmosphere,
[RhCl(1)]2 (50.0 mg, 51.0 μmol) was dissolved in CH2Cl2 (5 mL), and
AgSbF6 (17.6 mg, 51.0 μmol) was added. The reaction mixture was
stirred for 30 min at room temperature and filtered through Celite, and
the solvent was removed under reduced pressure. The crude product
was washed with hexanes (5 mL), and the cationic diene complex was
obtained as a yellow solid (60.1 mg, quant.) without further
1
purification. H NMR (700 MHz, CD2Cl2): δ 7.86−7.57 (m, 4H),
7.46−7.16 (m, 6H), 4.76−4.69 (m, 2H), 4.54−4.49 (m, 2H), 4.48−
4.43 (m, 2H), 4.01 (d, J = 3.1 Hz, 2H), 3.93−3.87 (m, 2H), 2.35−2.23
(m, 2H), 1.71 (dt, J = 1.3, 6.9 Hz, 2H), 1.44 (dt, J = 1.3, 6.9 Hz, 2H),
1.38 (s, 6H), 1.17 (d, J = 7.3 Hz, 6H,), 1.08 (d, J = 7.3 Hz, 6H), 0.64 (s,
6H) ppm. 13C {1H} NMR (176 MHz, CD2Cl2): δ 166.8, 159.8, 136.8,
129.6, 129.1, 127.9, 89.0, 70.1, 63.2 (d, J = 12.6 Hz), 61.5 (d, J = 10.2
Hz), 61.2, 60.3 (d, J = 5.7 Hz), 57.2, 55.9 (d, J = 9.5 Hz), 42.2 (d, J =
11.5 Hz), 29.8, 27.7, 21.6, 21.0, 17.4 ppm. LRMS (ESI) calcd m/z for
[M − SbF6]+ 943.2, found m/z 943.2. HRMS (ESI) calcd (found) for
C44H50ClN2O6Rh2 ([M − SbF6]+): 943.1462 (943.1466). Anal. Calcd
(%) for C44H50ClN2O6Rh2SbF6·1H2O (1179.92 g/mol): C: 44.12; H:
4.38; N: 2.34. Found: C: 44.30; H: 4.58; N: 2.34.
Synthesis of [RhOH2(2)]SbF6. Under a nitrogen atmosphere,
[RhCl(2)] (0.10 g, 0.22 mmol) was dissolved in CH2Cl2 (4 mL), and
AgSbF6 (74.4 mg, 0.22 mmol) was added. The reaction mixture was
stirred for 30 min at room temperature and filtered through Celite, and
the solvent was removed under reduced pressure. The crude product
was washed with hexanes (5 mL), and the cationic diene complex was
obtained as a yellow solid (0.15 g, quant.) without further purification.
1H NMR (700 MHz, CD2Cl2): δ 7.79−7.65 (m, 2H), 7.49−7.33 (m,
3H), 4.70−4.64 (m, 1H), 4.61−4.56 (m, 1H), 4.54−4.49 (m, 1H),
4.38−4.34 (m, 1H), 4.23 (dd, J = 2.1, 9.5 Hz, 1H), 4.13−4.07 (m, 1H),
3.87−3.83 (m, 1H), 2.66−2.58 (m, 1H), 2.42 (s, 2H), 1.64 (dt, J = 1.4,
9.8 Hz, 1H), 1.48 (dt, J = 1.4, 9.8 Hz, 1H), 0.96 (d, J = 7.3 Hz, 3H), 0.92
(d, J = 7.3 Hz, 3H) ppm. 13C {1H} NMR (176 MHz, CD2Cl2): δ 166.4,
161.3, 136.0, 129.9, 129.5, 127.6, 67.9, 66.9 (d, J = 10.7 Hz), 63.0, 61.1,
60.4 (d, J = 10.7 Hz), 59.8 (d, J = 7.7 Hz), 56.9 (d, J = 7.7 Hz), 55.5,
32.7, 29.4, 18.0, 14.7 ppm. LRMS (ESI) calcd m/z for [M − SbF6]+
444.1, found m/z 444.1. HRMS (ESI) calcd (found) for C20H23NO4Rh
([M − SbF6]+): 444.0677 (444.0679). Anal. Calcd (%) for
C44H50ClN2O6Rh2SbF6 (680.06 g/mol): C: 35.32; H: 3.41; N: 2.06.
Found: C: 35.22; H: 3.53; N: 1.84.
General Procedure for Addition Reactions with In Situ
Prepared Catalyst. Under a nitrogen atmosphere, the appropriate
ligand (10.0 μmol) and [Rh(C2H4)2Cl]2 (5.00 μmol) were dissolved in
degassed dioxane (2 mL), and the solution was stirred for 15 min at
room temperature. A degassed 3.1 M KOH solution (51.0 μL, 0.15
mmol) was added, and the reaction mixture was stirred for another 5
min at room temperature. In the case of 1,2-addition, the solution was
heated to 60 °C, triphenylboroxine 3 (74.8 mg, 0.24 mmol) and N-
tosylimide 4 (58.8 mg, 0.20 mmol) were added, and the reaction
mixture was stirred for a further 24 h at 60 °C. After dilution with
EtOAc (2 mL), the solution was filtered through a short silica column,
the solvent was removed under reduced pressure, and the crude product
was purified by column chromatography on silica. In the case of 1,4-
addition, the solution was heated to 50 °C, phenylboronic acid 7
(74.0 mg, 0.60 mmol) and the respective enone 6a,b (0.30 mmol) was
added, and the reaction mixture was stirred for 4 h at 50 °C. A saturated
NH4Cl solution (3 mL) was added, the layers were separated, and the
aqueous layer was extracted with Et2O (3 × 5 mL). The combined
organic layers were dried (Na2SO4), the solvent was removed under
EXPERIMENTAL SECTION
■
General Methods. All reactions were run under a nitrogen
atmosphere in flame-dried glassware using standard Schlenk
techniques. NMR spectra were recorded on a 500 MHz Bruker Avance
500 or a 700 MHz Bruker Avance 700 NMR spectrometer, referenced
to tetramethylsilane (δ 0.00 ppm) and calibrated on the residual solvent
peaks. Mass spectra were recorded with a Bruker Daltonics micro-TOF-
Q using electrospray ionization (ESI) with nitrogen as carrier gas.
Dioxane was degassed by bubbling nitrogen through it. All other
solvents and reagents were used as purchased. Diene ligands were
synthesized according to the literature.22
Synthesis of [RhCl(ent-1)]2. Diene ligand ent-1 (67.4 mg, 0.19
mmol) and [RhCl(C2H4)2]2 (37.3 mg, 95.9 μmol) were loaded in a
Schlenk tube under a nitrogen atmosphere, dioxane (2 mL) was added,
and the reaction mixture was stirred for 5 h at room temperature. The
remaining solids were filtered off, and the solvent was removed under
reduced pressure. After recrystallization from Et2O, the diene complex
1
was obtained as a red solid (69.0 mg, 73%). H NMR (500 MHz,
CDCl3): δ 7.71−7.64 (m, 4H), 7.33−7.27 (m, 6H), 4.70−4.64 (m,
2H), 4.51−4.45 (m, 2H), 4.42−4.38 (m, 2H), 3.91 (d, J = 3.6 Hz, 2H),
K
Organometallics XXXX, XXX, XXX−XXX
Abitaev, Karina
