Chiral Bis(oxazoline) Ligands as C2-Symmetric Chiral Auxiliaries for the Synthesis of Enantiomerically Pure Bis-Cyclometalated Rhodium(III) Complexes

Article abstract of DOI:10.1021/acs.organomet.9b00533

The synthesis of enantiomerically pure bis-cyclometalated rhodium(III) complexes using chiral bis(oxazoline) ligands as C₂-symmetric chiral auxiliaries is described. Bis(oxazolines) are versatile chiral ligands for asymmetric catalysis but have

Full text of DOI:10.1021/acs.organomet.9b00533

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Cite This: Organometallics XXXX, XXX, XXX−XXX
Chiral Bis(oxazoline) Ligands as C2‑Symmetric Chiral Auxiliaries for
the Synthesis of Enantiomerically Pure Bis-Cyclometalated
Rhodium(III) Complexes
Yvonne Grell, Nemrud Demirel, Klaus Harms, and Eric Meggers*
̈
Fachbereich Chemie, Philipps-Universitat Marburg, Hans-Meerwein-Strasse 4, 35043 Marburg, Germany
S
* Supporting Information
ABSTRACT: The synthesis of enantiomerically pure bis-cyclometalated
rhodium(III) complexes using chiral bis(oxazoline) ligands as C2-symmetric
chiral auxiliaries is described. Bis(oxazolines) are versatile chiral ligands for
asymmetric catalysis but have not been applied to the resolution of racemic
mixtures of transition-metal complexes. Due to their C2 symmetry, chiral
bis(oxazolines) are particularly useful for the synthesis of nonracemic transition-
metal complexes with lower symmetry, and this is demonstrated with the synthesis
of an enantiomerically pure rhodium(III) complex containing two different cyclometalated ligands.
(III) complexes.9,10 Recently, we realized a limitation of its
INTRODUCTION
■
practicability when we introduced a synthetic method for the
preparation of bis-cyclometalated rhodium(III) catalysts with
two different cyclometalating ligands.11 Due to its lower
symmetry and the non-C2 symmetry of the chiral salicyloxazo-
line, four diastereomers were obtained upon coordination of
the chiral salicyloxazoline auxiliary to the bis-cyclometalated
rhodium complex, two featuring metal-centered Λ config-
uration and two Δ configuration. This posed an additional
challenge for the separation of the Λ- and Δ-configured
stereoisomers. A C2-symmetric chiral auxiliary bidentate ligand
would solve this problem.
Here, we report the use of chiral bis(oxazoline) (BOX)
ligands as C2-symmetric chiral auxiliaries for the synthesis of
bis-cyclometalated rhodium(III) complexes. C2-symmetric
chiral bis(oxazolines) belong to one of the most popular
classes of chiral ligands for the synthesis of chiral transition-
metal complexes used in asymmetric catalysis due to their C2
symmetry, which reduces the number of competing transition
states, and due to their easy and flexible synthesis.12,13
However, to the best of our knowledge, the use of BOX
ligands as chiral auxiliaries for the synthesis of nonracemic
metal complexes has not been reported before.
Nonracemic transition-metal complexes play an important role
as chiral catalysts for the asymmetric synthesis of chiral
compounds used in academia and industry.1 They typically
contain chiral ligands to control the overall chirality of the
metal complexes. Following a different strategy, we and others
have recently demonstrated that chiral transition-metal
complexes composed from entirely achiral ligands can be
exquisite transition-metal catalysts for a large variety of
asymmetric conversions, including asymmetric photocataly-
sis.2,3 Such chiral-at-metal complexes feature a configuration-
ally stable stereogenic metal center for generating metal-
centered chirality which at the same time must be a reactive
metal center for performing the asymmetric catalysis.
Controlling the metal-centered configuration with tailored
chiral ligands is well established.4 However, chiral-at-metal
complexes consist of entirely achiral ligands so that modified or
different synthetic strategies need to be employed. Chiral-
auxiliary-mediated methods for the synthesis of enantiomeri-
cally pure metal complexes have been reported, including using
chiral counterions, cleavable chiral linkers, and temporary
chiral ligands in which the coordinative strength can be
varied.4,5 Our group6,7 and others8 have had great success with
the development and application of such temporarily
coordinating chiral bidentate ligands as chiral auxiliaries for
the synthesis of enantiomerically pure octahedral transition-
metal complexes. Upon binding of the bidentate auxiliary
ligand to the central metal, it either implements the metal-
centered configuration or leads to a mixture of two
diastereomers which can be resolved. Finally, the auxiliary is
removed in a traceless fashion, typically upon labilization with
a Brønsted acid, to provide the nonracemic metal complexes
(Figure 1).
RESULTS AND DISCUSSION
■
We first explored the use of simple C2-symmetric BOX ligands
as chiral auxiliaries for the synthesis of our standard bis-
cyclometalated rhodium(III) catalyst Λ- and Δ-RhS,9,14 in
which rhodium is cyclometalated by two 5-tert-butyl-2-
phenylbenzothiazoles and the octahedral coordination sphere
is supplemented by two acetonitriles. The monocationic
complexes are typically synthesized as their hexafluorophos-
A fluorinated salicyloxazoline, first reported by Monari,
Bandini, and Ceroni,8b has been our auxiliary of choice for the
synthesis of enantiomerically pure bis-cyclometalated rhodium-
Received: August 6, 2019
© XXXX American Chemical Society
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Scheme 1. Chiral Bis(oxazoline) Mediated Synthesis of Λ-
and Δ-RhS
substituted BOX ligand (S,S)-2 as a promising next alternative,
expecting stabilizing π−π-stacking interactions of the phenyl
moieties with the cyclometalated ligands of the rhodium
complex. Following the above conditions, we obtained Λ- and
Δ-(S,S)-Rh2 by reaction of rac-RhS with the ligand (S,S)-2,
with conversion being complete after only 1.5 h. Indeed, the
diastereomeric mixture of Λ- and Δ-(S,S)-Rh2 could be
chromatographed on deactivated silica gel without showing
any traces of degradation of the complexes, providing the
single diastereomers Λ- and Δ-(S,S)-Rh2 in high yields of 45%
and 47%, respectively. Absolute configurations were assigned
on the basis of the crystal structure of Λ-(S,S)-Rh2 (Figure 2).
The BOX ligand coordinates as a six-membered chelate in its
monodeprotonated form to generate a neutral rhodium
complex.15 The structure also reveals the anticipated π−π-
stacking interactions between the two phenyl moieties of the
BOX ligand (S,S)-2 and the benzothiazole moieties of the
rhodium complex.
With the individual diastereomers in hand, we next engaged
in the synthesis of single enantiomers using an acid-induced
replacement of the coordinated BOX ligand by two acetonitrile
ligands, followed by counterion exchange with NH4PF6.
Accordingly, the addition of 10.0 equiv of trifluoroacetic acid
(TFA) to a suspension of either Λ- or Δ-(S,S)-Rh2 in
acetonitrile was first executed at room temperature (Table 1,
entry 1). Although TLC indicated completion of the reaction
after 1 h, 1H NMR of the isolated compound after performance
Figure 1. Chiral auxiliaries for the resolution of stereoisomers of bis-
cyclometalated rhodium(III) complexes used as chiral catalysts. AN =
acetonitrile, CN and C′N′ = cyclometalated ligands.
phate salts. Despite all ligands being achiral, octahedral metal-
centered chirality leads to Λ (left-handed helical twist) and Δ
enantiomers (right-handed helical twist). Chiral bis(oxazoline)
ligands were easily prepared via a single-step procedure starting
from readily available β-amino alcohols and symmetrically
disubstituted diethyl malonimidate dihydrochloride. We
started our investigations with the isopropyl-substituted BOX
ligand (S,S)-1 (Scheme 1). Gratifyingly, the addition of 1.10
equiv of this ligand to a suspension of rac-RhS in ethanol in the
presence of 3 equiv of K2CO3 led to the formation of the
corresponding Λ- and Δ-configured auxiliary complexes after
only 1 h at room temperature. Analysis of the 1H NMR of the
crude material confirmed the conversion to a 1:1 mixture of
diastereomeric complexes by showing a combination of two
different sets of signals as well as the full consumption of the
racemic starting material (see the Supporting Information for
more details). However, when we attempted to separate the
two diastereomers via column chromatography on deactivated
silica gel (1% of Et3N), we found that they largely decomposed
during purification, resulting in only low yields of about 20%
for each diastereomer Λ- and Δ-(S,S)-Rh1 after chromato-
graphic resolution. Therefore, we envisioned the phenyl-
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order to improve the reaction outcome with regard to the
target complex. Table 1 shows a number of different
conditions.
Notably, increasing the amount of TFA to 20.0 equiv in
addition to prolonging the reaction time and executing the
reaction at elevated temperature could significantly enhance
the obtained ratio in favor of the desired complex, with RhS
now being formed as the major species (Table 1, entry 2).
Conducting the reaction at 50 °C for 22 h finally resulted in
complete dissociation of the BOX ligand from the metal
center, providing RhS as the sole product of the reaction in
81% yield (entry 3). Alternatively, the application of 5.00 equiv
of methanesulfonic acid (MsOH) instead of TFA already
allowed the transformation to proceed at room temperature in
a significantly reduced reaction time and with reasonable
product to side product ratios (entries 4 and 5). Readjustment
to 10.0 equiv of MsOH afforded the single enantiomers Λ-RhS
(entry 6) and Δ-RhS after 6 h with retention of the absolute
configuration in excellent yields of 98% and 99%, respectively.
HPLC performed on a chiral stationary phase exhibited the
high enantiomeric purity (>99% ee) of the individual
enantiomers (Figure 3).
Having established the conditions for the chiral bis-
(oxazoline)-mediated synthesis of nonracemic rhodium(III)
catalysts on the basis of the bis-cyclometalated complex RhS,
we then sought out to transfer the strategy to the synthesis of
an enantiomerically pure rhodium complex with two different
cyclometalating ligands. To this end, rac-RhNS was prepared
according to our previously reported procedure11 and reacted
with BOX ligand (S,S)-2 (Scheme 2).
Figure 2. Crystal structure of the auxiliary complex Λ-(S,S)-Rh2 as an
ORTEP drawing with 50% probability thermal ellipsoids. Solvent
molecules are omitted for clarity.
of the subsequent anion exchange and purification of the crude
material by regular silica gel chromatography showed that
correspondingly Λ- or Δ-(S,S)-Rh2-H was obtained as the
major product, while the desired complex Λ- or Δ-RhS was
only formed in traces (see the Supporting Information for
more details). Since both monocationic complexes had the
same Rf value, they were isolated as a mixture. The relative
amount of RhS in comparison to (S,S)-Rh2-H was determined
by 1H NMR and revealed that the initial conditions led to the
formation of RhS in a ratio of only 1:5.5 (Table 1, entry 1).
Thus, different reaction conditions were next attempted in
As expected, this resulted in the formation of only two
diastereomers due to the C2-symmetric nature of the chiral
auxiliary. Λ-(S,S)-Rh3 (44%) and Δ-(S,S)-Rh3 (45%) could
be obtained conveniently after separation of the diastereomeric
complexes by silica gel chromatography. Configurations were
assigned with reference to the obtained crystal structure of Λ-
(S,S)-Rh3 shown in Figure 4. The subsequent stereospecific
a
Table 1. Optimization of the Acid-Induced Cleavage of the Auxiliary Ligand
conditions (1st step)
b
entry
acid (equiv)
T (°C)
t (h)
RhS:(S,S)-Rh2-H
c
1
TFA (10)
TFA (20)
TFA (20)
MsOH (5)
MsOH (5)
MsOH (10)
room temp
40
50
0 to room temp
room temp
room temp
1
15
22
5
6
6
1:5.5
1.5:1
>20:1 (81%)
5.2:1
14:1
>20:1 (98%)
2
3
4
d
e
5
e
d
6
a
Reaction conditions first step unless specified otherwise: Λ- or Δ-(S,S)-Rh2 was dissolved in MeCN (0.04 M), the indicated acid was added in
b
one portion, and the resulting solution was stirred at the indicated temperature for the indicated time under an atmosphere of nitrogen. Ratios
were determined by 1H NMR after column chromatographic purification of the second step. Isolated yields are only provided, if conversion to Λ-/
c
d
e
Δ-RhS was complete. Λ-(S,S)-Rh3 was employed. Isolated yields of RhS are provided in parentheses. MsOH was added at 0 °C, and the
resulting solution was stirred for a further 15 min at 0 °C before the ice bath was removed.
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Scheme 2. Chiral Bis(oxazoline) Mediated Synthesis of Λ-
and Δ-RhNS
Figure 3. HPLC traces on a chiral stationary phase: (a) racemic
complex RhS as a reference; (b) Δ enantiomer of RhS synthesized
with (S,S)-2 as the chiral auxiliary; (c) Λ enantiomer of RhS
synthesized with (S,S)-2 as the chiral auxiliary. Conditions: Daicel
Chiralpak IB-N5 column (250 × 4.6 mm), column temperature 25
°C, λabs = 254 nm, flow rate 0.6 mL/min, solvent A: 0.1% aqueous
TFA, solvent B: MeCN, gradient 40−50% B in 180 min, 50% B
maintained for a further 60 min.
substitution of the coordinated auxiliary ligand gave the
enantiopure complexes Λ-RhNS and Δ-RhNS (95% yield
each) with retention of configuration by following the above
optimized conditions. CD spectra confirmed their mirror-
image structures (see the Supporting Information). Since a
satisfactory separation of the two enantiomers Λ- and Δ-RhNS
could not be achieved by chiral HPLC, the single enantiomers
were reconverted into Λ- and Δ-(S,S)-Rh3, respectively, in
order to validate the enantiopurity via determination of the
diastereomeric ratio from the 1H NMR of the crude materials.
Hereby, Λ- as well as Δ-(S,S)-Rh3 were obtained with a dr
value of more than 20:1 (see the Supporting Information for
more details).
CONCLUSIONS
■
In conclusion, we developed the first auxiliary-mediated
strategy for the synthesis of nonracemic bis-cyclometalated
rhodium(III) catalysts using a simple chiral bis(oxazoline)
ligand as the chiral auxiliary and demonstrated the feasibility of
the approach with the preparation of the established
benzothiazole complex Λ- and Δ-RhS. In the first step, a
pair of two chromatographically stable diastereomers was
obtained by reaction of a racemic mixture of the complex with
a phenyl-substituted BOX ligand. The crucial acid-induced
dissociation of the coordinated BOX ligand in the subsequent
second step was accomplished despite the ability of the ligand
to coordinate in its neutral form, providing the final catalyst
conveniently with excellent enantiomeric purities (>99% ee for
Figure 4. Crystal structure of auxiliary complex Λ-(S,S)-Rh3 as an
ORTEP drawing with 50% probability thermal ellipsoids. Solvent
molecules are omitted for clarity.
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added diethyl malonimidate dihydrochloride (1.00 g, 4.33 mmol, 1.00
equiv) and the resulting solution was stirred for 45 h at room
temperature. Bulb-to-bulb distillation gave bis((S)-4-isopropyl-4,5-
dihydrooxazol-2-yl)methane ((S,S)-1) (797 mg, 3.34 mmol, 77%) as
a white waxy solid. Analytical data were in agreement with the
literature.19 1H NMR (300 MHz, CDCl3): δ 4.25 (dd, J = 8.9, 7.6 Hz,
2H), 4.01−3.88 (m, 4H), 3.33 (s, 2H), 1.80−1.69 (m, 2H), 0.94 (d, J
= 6.8 Hz, 6H), 0.86 (d, J = 6.8 Hz, 6H) ppm.
Synthesis of (S,S)-2. Following general procedure I, to a solution of
(S)-2-phenylglycinol (1.19 g, 8.65 mmol, 2.00 equiv) in CH2Cl2 (43.3
mL) was added diethyl malonimidate dihydrochloride (1.00 g, 4.33
mmol, 1.00 equiv) and the resulting solution was stirred for 68 h at
room temperature. Purification by bulb-to-bulb distillation yielded
bis((S)-4-phenyl-4,5-dihydrooxazol-2-yl)methane ((S,S)-2) (1.01 g,
3.30 mmol, 76%) as a yellow oil. Analytical data were in agreement
with the literature.20 1H NMR (300 MHz, CD2Cl2): δ 7.37−7.25 (m,
10H), 5.26−5.20 (m, 2H), 4.68 (dd, J = 10.2, 8.4 Hz, 2H), 4.14 (t, J =
8.2 Hz, 2H), 3.53 (s, 2H) ppm.
General Procedure II: Synthesis of Chiral Bis(oxazolinato)
Rhodium(III) Complexes. The racemic rhodium complex (1.00
equiv), K2CO3 (3.00 equiv), and BOX ligand (S,S)-2 (1.10 equiv)
were suspended in EtOH (25 mM, absolute) and stirred at room
temperature until TLC indicated completion of the reaction. The
reaction mixture was diluted with EtOAc and filtered over a short plug
of Celite. After removal of the solvent under reduced pressure, the
resulting mixture of two diastereomers was transferred to a silica gel
column with EtOAc and a few drops of CH2Cl2 (HPLC grade) for
dissolution and purified by column chromatography (n-pentane/
EtOAc with a gradient, plus 1% Et3N). The silica gel employed was
deactivated prior to use by stirring in the initial solvent mixture
together with 1% of Et3N for about 10 min.
Synthesis of Λ- and Δ-(S,S)-Rh2. Following general procedure II,
rac-RhS (80.0 mg, 92.7 μmol, 1.00 equiv), K2CO3 (38.4 mg, 0.28
mmol, 3.00 equiv), and BOX ligand (S,S)-2 (31.2 mg, 0.10 mmol,
1.10 equiv) were suspended in EtOH (3.78 mL, absolute) and stirred
for 1.5 h at room temperature. Purification by column chromatog-
raphy (n-pentane/EtOAc, gradient 100/1 → 80/1 → 60/1, plus 1%
Et3N) afforded Λ-(S,S)-Rh2 (39.4 mg, 41.9 μmol, 45%) and Δ-(S,S)-
Rh2 (40.9 mg, 43.5 μmol, 47%) as yellow solids.
each enantiomer) and in high yields. Additionally, we
complemented our studies with the synthesis of the bis-
cyclometalated complex Λ- and Δ-RhNS, which has a lower
symmetry due to two different cyclometalating ligands, thereby
disclosing the particular benefit of the C2-symmetric auxiliary.
This process reduces the amount of possible stereoisomers to
two, which represents a significant enhancement of our
previously reported synthetic protocol since it greatly facilitates
the separation of the diastereomers and renders the
preparation of this class of catalysts less time consuming.
Thus, chiral BOX ligands are an economical tool for the
synthesis of nonracemic bis-cyclometalated rhodium com-
plexes, which have been used extensively over the past few
years as chiral transition-metal catalysts.7,14,16,17 Further
investigations addressing the generality of the introduced
auxiliary-mediated approach for the asymmetric synthesis of
chiral transition-metal catalysts different from rhodium are
currently being pursued.
EXPERIMENTAL SECTION
■
General Methods and Materials. See the Supporting
Information for experimental details on the synthesis of cyclo-
metalating ligands and the racemic rhodium(III) complexes. All
reactions were carried out under a nitrogen atmosphere in oven-dried
glassware unless noted otherwise. Solvents were distilled under
nitrogen from calcium hydride (MeCN, CH2Cl2), sodium/benzo-
phenone (THF), or sodium (toluene) prior to use. Reagents that
were purchased from commercial suppliers were used without further
purification. Flash column chromatography was performed with silica
gel 60 M from Macherey-Nagel (irregularly shaped, 230−400 mesh,
pH 6.8, pore volume 0.81 mL g−1, mean pore size 66 Å, specific
surface 492 m2 g−1, particle size distribution 0.5% < 25 μm and 1.7%
1
> 71 μm, water content 1.6%). H NMR and 13C{1H} NMR spectra
were recorded on a Bruker AV II 300 MHz, AV III HD 250 MHz, AV
III 500 MHz, AV III HD 500 MHz, or AV II 600 MHz spectrometer
at ambient temperature. Chemical shift values δ are reported in ppm
with the solvent resonance as internal standard. All 13C NMR signals
are singlets unless noted otherwise. Samples for NMR measurements
were about 15−30 mg of the substance dissolved in 0.60 mL of
deuterated solvent. 1D-1H spectra were aquired with 65536 data
points, 16 transients, a sweep width of 11−20 ppm, and a relaxation
delay of 6−7 s. 1D-13C spectra were aquired with 65536 data points,
2000−8000 transients, a sweep width of 262 ppm, and a relaxation
delay of 2−3 s. IR spectra were recorded on a Bruker Alpha FT-IR
spectrometer. CD spectra were acquired with a JASCO J-810 CD
spectropolarimeter (600−200 nm, data pitch 0.5 nm, bandwidth 1
nm, response 1 s, sensitivity standard, scanning speed 50 nm/min,
accumulation of three scans). High-resolution mass spectrometry was
performed on a Finnigan LTQ-FT Ultra mass spectrometer (Thermo
Fischer Scientific) using ESI or APCI as the ionization source. EI
mass spectra were recorded on an AccuTOF GCv instrument
(JEOL). Melting points were determined on a Mettler Toledo MP70
apparatus using capillary tubes closed on one end. Chiral HPLC was
performed on an Agilent 1260 instrument.
General Procedure I: Synthesis of Chiral Bis(oxazoline)
Ligands. According to a slightly modified procedure,18 to a solution
of the β-amino alcohol (2.00 equiv) in CH2Cl2 (0.10 M) was added
diethyl malonimidate dihydrochloride (1.00 equiv). The resulting
cloudy solution was stirred at room temperature until TLC confirmed
full consumption of the starting materials. The reaction mixture was
diluted with H2O and extracted three times with CH2Cl2. The
combined organic layers were washed once with brine and dried over
Na2SO4, and the solvent was removed under reduced pressure.
Purification of the obtained oily residue by bulb-to-bulb distillation
(Kugelrohr distillation, 150 °C at 0.2 mbar) afforded the respective
bis(oxazoline).
Λ-(S,S)-Rh2. TLC (n-pentane/EtOAc 30/1 + 1% Et3N): Rf = 0.30.
1H NMR (300 MHz, CD2Cl2): δ 8.44 (d, J = 1.2 Hz, 2H), 7.76 (d, J =
8.5 Hz, 2H), 7.59 (dd, J = 8.5, 1.8 Hz, 2H), 6.90−6.87 (m, 2H),
6.77−6.68 (m, 6H), 6.36 (br s, 3H), 6.18−6.15 (m, 3H), 5.95 (br s,
4H), 4.59 (dd, J = 8.6, 2.9 Hz, 2H), 4.50 (t, J = 8.3 Hz, 2H), 4.40 (s,
1H), 3.69 (dd, J = 7.9, 3.0 Hz, 2H), 1.55 (s, 18H) ppm. 13C NMR
(126 MHz, CD2Cl2): δ 176.9 (d, JC,Rh = 2.9 Hz, 2C), 172.9 (d, JC,Rh
=
29.2 Hz, 2C), 169.9 (2C), 151.9 (2C), 151.6 (2C), 145.3 (2C), 142.3
(2C), 133.0 (2C), 129.7 (4C), 127.3 (4C), 126.4 (2C), 125.7 (2C),
125.5 (2C), 123.3 (2C), 121.9 (2C), 121.7 (2C), 118.9 (2C), 74.9
(2C), 70.1 (2C), 53.7, 35.6 (2C), 32.0 (6C) ppm. IR (neat): ν 3043
̃
(w), 2958 (w), 2924 (w), 2855 (w), 1737 (w), 1607 (w), 1578 (w),
1529 (m), 1471 (w), 1439 (w), 1412 (w), 1360 (w), 1344 (w), 1319
(w), 1286 (w), 1261 (w), 1239 (w), 1201 (w), 1146 (w), 1100 (w),
1063 (w), 1032 (w), 990 (w), 959 (w), 931 (w), 892 (w), 875 (w),
846 (w), 813 (w), 755 (w), 723 (m), 697 (w), 669 (w), 647 (w), 581
(w), 543 (w), 462 (w) cm−1. HRMS (APCI): m/z calcd for
C53H50N4O2RhS2 [M + H]+ 941.2425, found 941.2416. Mp: 273 °C
dec (EtOAc). CD (CH2Cl2): γ, nm (Δε, M−1 cm−1) 411 (−48), 369
(+110), 352 (+91), 339 (+54), 329 (+87), 304 (−41), 268 (+73),
250 (+110), 220 (−131), 207 (−24).
Δ-(S,S)-Rh2. TLC (n-pentane/EtOAc 30/1 + 1% Et3N): Rf = 0.23.
1H NMR (500 MHz, CD2Cl2): δ 9.00 (d, J = 1.7 Hz, 2H), 7.91 (d, J =
8.6 Hz, 2H), 7.64 (dd, J = 8.6, 1.8 Hz, 2H), 7.27 (dd, J = 7.1, 0.4 Hz,
2H), 6.77 (m, 6H), 6.61 (m, 4H), 6.45 (m, 2H), 5.90 (dd, J = 7.5, 0.8
Hz, 2H), 5.47 (d, J = 7.8 Hz, 2H), 4.39 (s, 1H), 4.11−4.07 (m, 2H),
3.72 (dd, J = 10.7, 9.1 Hz, 2H), 3.52 (dd, J = 10.9, 8.3 Hz, 2H), 1.54
(s, 18 H) ppm. 13C NMR (126 MHz, CD2Cl2): δ 176.8 (d, JC,Rh = 3.2
Hz, 2C), 173.5 (d, JC,Rh = 30.2 Hz, 2C), 171.6 (2C), 151.6 (2C),
151.1 (2C), 143.9 (2C), 139.5 (2C), 134.3 (2C), 128.9 (2C), 127.7
(4C), 127.2 (4C), 126.0 (2C), 124.6 (2C), 124.1 (2C), 121.9 (2C),
Synthesis of (S,S)-1. Following general procedure I, to a solution of
L-valinol (893 mg, 8.65 mmol, 2.00 equiv) in CH2Cl2 (43.3 mL) was
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121.5 (2C), 120.0 (2C), 74.7 (2C), 70.1 (2C), 57.3, 35.8 (2C), 31.7
̃
(6C) ppm. IR (neat): ν 3055 (w), 2959 (w), 2885 (w), 1730 (w),
357 (−54), 323 (+20), 310 (+5), 295 (+27), 245 (−46), 215 (+35),
207 (−6), 203 (+7).
1602 (w), 1580 (w), 1538 (m), 1461 (w), 1438 (w), 1413 (w), 1356
(w), 1312 (w), 1291 (w), 1279 (w), 1252 (w), 1233 (w), 1203 (w),
1154 (w), 1123 (w), 1102 (w), 1059 (w), 1039 (m), 989 (w), 959
(w), 932 (w), 888 (w), 845 (w), 809 (w), 778 (w), 750 (w), 721 (w),
695 (w), 670 (w), 647 (w), 607 (w), 585 (w), 536 (w), 459 (w)
cm−1. HRMS (APCI): m/z calcd for C53H50N4O2RhS2 [M + H]+
941.2425, found 941.2439. Mp: 167 °C (EtOAc). CD (CH2Cl2): γ,
nm (Δε, M−1 cm−1) 417 (+43), 377 (−47), 361 (−46), 332 (+80),
312 (−99), 283 (+21), 270 (+33), 248 (−33), 228 (+27), 221 (−9),
217 (+115), 207 (+30).
Synthesis of Λ- and Δ-(S,S)-Rh3. Following general procedure II,
rac-RhNS (36.9 mg, 39.3 μmol, 1.00 equiv), K2CO3 (16.3 mg, 0.12
mmol, 3.00 equiv), and BOX ligand (S,S)-2 (13.3 mg, 0.04 mmol,
1.10 equiv) were suspended in EtOH (1.60 mL, absolute) and stirred
for 2.5 h at room temperature. Purification by column chromatog-
raphy (n-pentane/EtOAc, gradient 20/1 → 15/1 → 10/1 → 7/1, plus
1% Et3N) afforded Λ-(S,S)-Rh3 (17.7 mg, 17.4 μmol, 44%) and Δ-
(S,S)-Rh3 (18.0 mg, 17.7 μmol, 45%) as yellow solids.
Λ-(S,S)-Rh3. TLC (n-pentane/EtOAc 5/1 + 1% Et3N): Rf = 0.40.
1H NMR (500 MHz, CD2Cl2): δ 8.46 (d, J = 1.6 Hz, 1H), 7.97 (d, J =
1.6 Hz, 1H), 7.76 (d, J = 8.6 Hz, 1H), 7.54 (dd, J = 8.5, 1.8 Hz, 1H),
7.51 (dd, J = 8.7, 1.8 Hz, 1H), 7.23 (d, J = 8.7 Hz, 1H), 6.97 (dd, J =
7.1, 0.6 Hz, 1H), 6.93 (dd, J = 7.4, 1.3 Hz, 1H), 6.80−6.77 (m, 1H),
6.74−6.67 (m, 6H), 6.33 (br s, 2H), 6.26 (d, J = 7.4 Hz, 1H), 6.16 (d,
J = 7.5 Hz, 1H), 5.96 (br s, 4H), 5.75 (br s, 1H), 4.63 (dd, J = 8.7, 3.2
Hz, 1H), 4.55−4.48 (m, 3H), 4.33 (s, 1H), 3.70 (dd, J = 8.1, 3.2 Hz,
1H), 3.63 (s, 3H), 3.61 (dd, J = 7.4, 2.3 Hz, 1H), 2.19−2.13 (m, 9H),
1.90−1.84 (m, 6H), 1.52 (s, 9H) ppm. 13C NMR (126 MHz,
CD2Cl2): δ 176.3 (d, JC,Rh = 3.5 Hz, 1C), 174.3 (d, JC,Rh = 30 Hz, 1C),
173.8 (d, JC,Rh = 29 Hz, 1C), 170.0, 169.8, 158.9 (d, JC,Rh = 3.1 Hz,
1C), 152.3, 151.7, 147.3, 146.2, 145.4, 142.3, 141.0, 136.6, 134.2,
133.6, 133.1, 129.9, 129.4, 128.5, 127.2 (2C), 126.8 (2C), 126.5
(2C), 126.1, 125.6 (2C), 125.3, 125.1, 123.9, 122.5, 121.6, 121.5,
121.3, 120.9, 118.8, 114.4, 108.7, 74.8, 74.7, 70.2, 70.1, 53.0, 43.8
(3C), 37.1 (3C), 37.1, 35.4, 32.2 (3C), 31.9, 29.6 (3C) ppm. IR
General Procedure III: Acid-Induced Removal of Coordi-
nated Auxiliary Ligand. To a solution of Λ- or Δ-(S,S)-2/3 (1.00
equiv) in MeCN (0.04 M) was added methanesulfonic acid (10.0
equiv) in one portion at 0 °C. After 15 min, the ice bath was removed
and the solution was stirred for a further 6 h at room temperature.
Alternatively, the reaction mixture may be stirred for 16 h without
affecting the ee value of the final catalyst. The solvent was removed
under reduced pressure (250−50 mbar for a maximum of 10 min),
NH4PF6 (15.0 equiv) was added to the residual yellow oil, and MeCN
(0.02 M) was added. The resulting suspension was stirred for 30 min
at room temperature before the solvent was removed under reduced
pressure. Purification by column chromatography (CH2Cl2/MeCN
20/1) provided the enantiomerically pure rhodium(III) complexes.
Synthesis of Λ-RhS. Following general procedure III with Λ-(S,S)-
Rh2 (20.0 mg, 21.3 μmol, 1.00 equiv), Λ-RhS (18.0 mg, 20.9 μmol,
98%) was obtained as a pale yellow solid after column chromato-
graphic purification. The enantiomeric excess was established by
HPLC analysis using a Daicel Chiralpak IB-N5 column, ee >99.9%
(HPLC: 254 nm, H2O + 0.1% TFA/MeCN 40−50% MeCN in 180
min, 50% MeCN maintained for a further 60 min, flow rate 0.6 mL/
min, 25 °C, tR(Δ-RhS) = 210.0 min, tR(Λ-RhS) = 217.8 min).
Analytical data were in agreement with the literature.9 1H NMR (300
MHz, CD2Cl2): δ 8.50 (br s, 2H), 8.02 (d, J = 8.6 Hz, 2H), 7.72 (dd,
J = 8.6, 1.7 Hz, 2H), 7.67 (dd, J = 7.6, 1.2 Hz, 2H), 7.03 (dt, J = 7.5,
0.8 Hz, 2H), 6.83 (dt, J = 7.6, 1.4 Hz, 2H), 6.21 (d, J = 7.8 Hz, 2H),
2.18 (br s, 6H), 1.46 (s, 18H) ppm.
Synthesis of Δ-RhS. Following general procedure III with Δ-(S,S)-
Rh2 (22.0 mg, 23.4 μmol, 1.00 equiv), Δ-RhS (19.9 mg, 23.1 μmol,
99%) was obtained as a pale yellow solid after purification by column
chromatography. The enantiomeric excess was established by HPLC
analysis using a Daicel Chiralpak IB-N5 column, ee >99% (HPLC:
254 nm, H2O + 0.1% TFA/MeCN 40−50% MeCN in 180 min, 50%
MeCN maintained for a further 60 min, flow rate 0.6 mL/min, 25 °C,
tR(Δ-RhS) = 210.0 min, tR(Λ-RhS) = 217.8 min). Analytical data
were in agreement with the literature.9 1H NMR (300 MHz, CD2Cl2):
δ 8.48 (d, J = 1.5 Hz, 2H), 8.03 (d, J = 8.6 Hz, 2H), 7.72 (dd, J = 8.6,
1.8 Hz, 2H), 7.67 (dd, J = 7.6, 1.2 Hz, 2H), 7.03 (dt, J = 7.5, 0.9 Hz,
2H), 6.83 (dt, J = 7.6, 1.3 Hz, 2H), 6.20 (d, J = 7.8 Hz, 2H), 2.17 (br
s, 6H), 1.46 (s, 18H) ppm.
Synthesis of Λ-RhNS. Following general procedure III with Λ-
(S,S)-Rh3 (20.0 mg, 19.7 μmol, 1.00 equiv), Λ-RhNS (17.5 mg, 18.7
μmol, 95%) was obtained as a pale yellow solid after column
chromatographic purification. Since the enantiomeric excess could not
be established by chiral HPLC, Λ-RhNS was reconverted into Λ-
(S,S)-Rh3 and a dr value of >20:1 was determined by 1H NMR of the
crude material (see the Supporting Information for details). CD
(CH3OH): λ, nm (Δε, M−1 cm−1) 389 (−29), 355 (+88), 298 (−95),
244 (+65), 228 (+14), 220 (+26), 214 (−17), 206 (+66). All other
analytical data were in agreement with rac-RhNS (see the Supporting
Information).
Synthesis of Δ-RhNS. Following general procedure III with Δ-
(S,S)-Rh3 (30.0 mg, 29.5 μmol, 1.00 equiv), Δ-RhNS (26.2 mg, 27.9
mmol, 95%) was obtained as a pale yellow solid after purification by
column chromatography. Since the enantiomeric excess could not be
established by chiral HPLC, Δ-RhNS was reconverted into Δ-(S,S)-
Rh3 and a dr value of >20:1 was determined by 1H NMR of the crude
material (see the Supporting Information). CD (CH3OH): λ, nm
(Δε, M−1 cm−1) 389 (+30), 355 (−91), 298 (+101), 244 (−65), 228
(−18), 220 (−29), 214 (+10), 206 (−67). All other analytical data
were in agreement with rac-RhNS (see the Supporting Information).
(neat): ν 2900 (m), 2848 (w), 1612 (w), 1580 (w), 1531 (w), 1476
̃
(w), 1451 (w), 1414 (w), 1347 (w), 1322 (w), 1288 (w), 1260 (w),
1207 (w), 1111 (w), 1064 (w), 1034 (w), 992 (w), 962 (w), 933 (w),
867 (w), 797 (w), 758 (w), 724 (w), 696 (w), 668 (w), 637 (w), 615
(w), 542 (w), 460 (w) cm−1. HRMS (APCI): m/z calcd for
C60H59N5O2RhS [M + H]+ 1016.3439, found 1016.3464. Mp: 141 °C
dec (EtOAc). CD (CH3OH): γ, nm (Δε, M−1 cm−1) 388 (−13), 353
(+60), 299 (−42), 269 (+29), 260 (+25), 245 (+57), 216 (−69), 202
(−18).
Δ-(S,S)-Rh3. TLC (n-pentane/EtOAc 5/1 + 1% Et3N): Rf = 0.27.
1H NMR (500 MHz, CD2Cl2): δ 9.05 (d, J = 1.5 Hz, 1H), 8.54 (d, J =
1.4 Hz, 1H), 7.92 (d, J = 8.6 Hz, 1H), 7.59 (dd, J = 8.6, 1.7 Hz, 1H),
7.56 (dd, J = 8.7, 1.7 Hz, 1H), 7.47 (d, J = 8.7 Hz, 1H), 7.42 (d, J =
7.7 Hz, 1H), 7.31 (d, J = 7.6 Hz, 1H), 6.76−6.72 (m, 4H), 6.69 (d, J
= 7.3 Hz, 2H), 6.62−6.57 (m, 4H), 6.47−6.42 (m, 2H), 5.90−5.83
(m, 2H), 5.57 (d, J = 7.8 Hz, 1H), 5.41 (d, J = 7.8 Hz, 1H), 4.37 (s,
1H), 4.17 (s, 3H), 4.07−4.03 (m, 2H), 3.71 (m, 2H), 3.57 (dd, J =
9.9, 8.3 Hz, 1H), 3.43 (dd, J = 11.3, 8.3 Hz, 1H), 2.22−2.16 (m, 6H),
2.12 (m, 3H), 1.82 (m, 6H), 1.53 (s, 9H) ppm. 13C NMR (126 MHz,
CD2Cl2): δ 176.2 (d, JC,Rh = 3.1 Hz, 1C), 175.1 (d, JC,Rh = 31.0 Hz,
1C), 173.8 (d, JC,Rh = 30.3 Hz, 1C), 171.7, 171.2, 160.0 (d, JC,Rh = 3.7
Hz, 1C), 152.0, 151.1, 146.6, 144.4, 143.8, 140.9, 139.8, 135.2, 134.3,
134.1, 133.8, 129.1, 128.5, 127.6 (4C), 127.5, 127.3 (2C), 127.3
(2C), 125.9, 125.8, 124.6, 123.1, 122.9, 121.8, 121.3, 121.0, 120.8,
120.4, 116.1, 108.9, 74.9, 74.5, 70.1, 69.9, 56.5, 43.6 (3C), 37.4, 37.2
(3C), 35.5, 32.4, 32.0 (3C), 29.6 (3C) ppm. IR (neat): ν 2899 (m),
̃
2847 (w), 1603 (w), 1581 (w), 1534 (w), 1504 (w), 1475 (w), 1458
(w), 1437 (w), 1415 (w), 1353 (w), 1310 (w), 1289 (w), 1256 (w),
1232 (w), 1209 (w), 1149 (w), 1103 (w), 1062 (w), 1034 (w), 989
(w), 960 (w), 865 (w), 799 (w), 751 (w), 723 (w), 695 (w), 668 (w),
650 (w), 609 (w), 536 (w), 459 (w) cm−1. HRMS (APCI): m/z calcd
for C60H59N5O2RhS [M + H]+ 1016.3439, found 1016.3468. Mp: 181
°C dec (EtOAc). CD (CH3OH): γ, nm (Δε, M−1 cm−1) 393 (+19),
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.9b00533.
F
DOI: 10.1021/acs.organomet.9b00533
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Synthesis of ligands and metal complexes, experimental
Article
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details, CD spectra, NMR spectra, HPLC traces, and
crystallographic data (PDF)
2017, 50, 320−330.
(8) (a) Chepelin, O.; Ujma, J.; Wu, X.; Slawin, A. M. Z.; Pitak, M.
Accession Codes
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Inorg. Chem. 2016, 55, 6018−6026. (e) Li, L.-P.; Yao, S.-Y.; Ou, Y.-L.;
Wei, L.-Q.; Ye, B.-H. Diastereoselective Synthesis and Photophysical
Properties of Bis-Cyclometalated Ir(III) Stereoisomers with Dual
CCDC 1939489−1939490 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Author
*E-mail for E.M.: meggers@chemie.uni-marburg.de.
■
ORCID
Eric Meggers: 0000-0002-8851-7623
Notes
The authors declare no competing financial interest.
Stereocenters. Organometallics 2017, 36, 3257−3265.
ACKNOWLEDGMENTS
We gratefully acknowledge funding from the Deutsche
Forschungsgemeinschaft (ME 1805/13-1).
■
(9) Ma, J.; Shen, X.; Harms, K.; Meggers, E. Expanding the family of
bis-cyclometalated chiral-at-metal rhodium(III) catalysts with a
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chemistry. Nat. Protoc. 2018, 13, 605−632.
(11) Grell, Y.; Hong, Y.; Huang, X.; Mochizuki, T.; Xie, X.; Harms,
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