Enantioselective syntheses of sulfoxides in octahedral ruthenium(II) complexes via a chiral-at-metal strategy

Article abstract of DOI:10.1021/ic502898e

The preparation of chiral 2-(alkylsulfinyl)phenol compounds by enantioselective coordination-oxidation of the thioether ruthenium complexes with a chiral-at-metal strategy has been developed. The enantiomerically pure sulfoxide complexes δ-[Ru(bpy)₂{(R)-LO-R}](PF₆) (bpy is 2,2′-bipyridine, HLO-R is 2-(alkylsulfinyl)phenol, R = Me (δ-1a), Et (δ-2a), iPr (δ-3a), Bn (δ-4a), and Nap (δ-5a)) and δ-[Ru(bpy)₂{(S)-LO-R}](PF₆) (R = Me (δ-1a), Et (δ-2a), iPr (δ-3a), Bn (δ-4a), and Nap (δ-5a)) have been synthesized by the reaction of δ-[Ru(bpy)₂(py)₂]²⁺ or δ-[Ru(bpy)₂(py)₂]²⁺ with the prochiral thioether ligands 2-(alkylthio)phenol (HL-R), followed by enantioselective oxidation with m-CPBA as oxidant. The X-ray crystallography was used to verify the stereochemistry of ruthenium complexes and sulfur atoms. The configurations of the ruthenium complexes are stable during the coordination and oxidation reactions. Moreover, the chiral sulfoxide ligands are enantioselectively generated by controlling of the configuration of ruthenium centers in the course of oxidation reaction. That is, the δ configuration at the ruthenium center generates the S sulfoxide ligand; on the contrary, the δ configuration of the ruthenium complex originates the R sulfoxide ligand. Acidolysis of δ-[Ru(bpy)₂{(R)-LO-R}](PF₆) and δ-[Ru(bpy)₂{(S)-LO-R}](PF₆) complexes in the presence of TFA-MeCN afforded the chiral ligands (R)-HLO-R and (S)-HLO-R in 96-99% ee values, respectively. Importantly, the chiral ruthenium complexes can be recycled as δ/δ-[Ru(bpy)₂(MeCN)₂](PF₆)₂ and reused in a next reaction cycle with complete retention of the configurations at ruthenium centers.

Full text of DOI:10.1021/ic502898e

Article
pubs.acs.org/IC
Enantioselective Syntheses of Sulfoxides in Octahedral Ruthenium(II)
Complexes via a Chiral-at-Metal Strategy
Zheng-Zheng Li, A-Hao Wen, Su-Yang Yao, and Bao-Hui Ye*
MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, School of Chemistry and Chemical Engineering, Sun Yat-Sen
University, Guangzhou 510275, China
S
* Supporting Information
ABSTRACT: The preparation of chiral 2-(alkylsulfinyl)-
phenol compounds by enantioselective coordination−oxida-
tion of the thioether ruthenium complexes with a chiral-at-
metal strategy has been developed. The enantiomerically pure
sulfoxide complexes Δ-[Ru(bpy)2{(R)-LO-R}](PF6) (bpy is
2,2′-bipyridine, HLO-R is 2-(alkylsulfinyl)phenol, R = Me (Δ-
1a), Et (Δ-2a), iPr (Δ-3a), Bn (Δ-4a), and Nap (Δ-5a)) and
Λ-[Ru(bpy)2{(S)-LO-R}](PF6) (R = Me (Λ-1a), Et (Λ-2a),
iPr (Λ-3a), Bn (Λ-4a), and Nap (Λ-5a)) have been
synthesized by the reaction of Δ-[Ru(bpy)2(py)2]2+ or Λ-
[Ru(bpy)2(py)2]2+ with the prochiral thioether ligands 2-
(alkylthio)phenol (HL-R), followed by enantioselective
oxidation with m-CPBA as oxidant. The X-ray crystallography was used to verify the stereochemistry of ruthenium complexes
and sulfur atoms. The configurations of the ruthenium complexes are stable during the coordination and oxidation reactions.
Moreover, the chiral sulfoxide ligands are enantioselectively generated by controlling of the configuration of ruthenium centers in
the course of oxidation reaction. That is, the Λ configuration at the ruthenium center generates the S sulfoxide ligand; on the
contrary, the Δ configuration of the ruthenium complex originates the R sulfoxide ligand. Acidolysis of Λ-[Ru(bpy)2{(R)-LO-
R}](PF6) and Δ-[Ru(bpy)2{(S)-LO-R}](PF6) complexes in the presence of TFA−MeCN afforded the chiral ligands (R)-HLO-
R and (S)-HLO-R in 96−99% ee values, respectively. Importantly, the chiral ruthenium complexes can be recycled as Δ/Λ-
[Ru(bpy)2(MeCN)2](PF6)2 and reused in a next reaction cycle with complete retention of the configurations at ruthenium
centers.
the Δ and Λ enantiomers in the presence of chiral sulfoxides.
Therefore, the chiral-at-metal framework may be used to
recognize and originate the enantiomeric sulfoxides. Indeed, a
literature survey revealed that the first chiral sulfoxide
compound oxidated enantioselectively from thioether in the
presence of Δ-[Ru(dmp)2(MeCN)2](Λ-Trisphat)2 complex
(where dmp is 2,9-dimethyl-1,10-phenanthroline) has been
reported by Fontecave and co-workers.8 However, the
enantiomeric excess (ee) values are low in 7−18%. In the
previous work, we have developed an approach for the
preparation of chiral 2-alkylsulfinylbenzonate compounds up
to 93% ee value by means of oxidation of the thioether
complexes to sulfoxide complexes in a chiral-at-metal
asymmetric envionment, followed by release of the oxidated
ligand.9 The configurations of the ruthenium centers are stable
during the coordinated reaction of the thioether ligand to the
ruthenium center, followed by oxidation of the thioether
complexes to the corresponding sulfoxide complexes. More-
over, the chirality of the sulfoxide ligand is controlled by the
chirality of the ruthenium complexes in the oxidation reaction;
INTRODUCTION
■
Enantiomerically pure sulfoxides are of great importance in
synthetic and medicinal chemistry.1,2 Two classical methods
have been developed for the synthesis of chiral sulfoxide. One is
the Andersen method.1a,b,3 However, this synthetic approach
was experimentally tedious and an organometallic reagent
needs to be used. The second approach is the enantioselective
oxidation of thioethers to sulfoxides using either chiral oxidant
or achiral oxidant under an asymmetric environment such as
the well-known Sharpless reagent.4 The oxidation of thioethers,
considered as the most direct and practical approach for the
preparation of sulfoxides, has attracted much attention over the
years.5,6 Nevertheless, these methods suffered from the low
turnover numbers and enantiomeric excess (ee) value, and
overoxidation to sulfone. In particular, the enantioselective
oxidation of dialkyl sulfides is still not well resolved. Therefore,
it is still highly desirable to develop new approaches to
supplement the existing methodologies for synthesis of chiral
sulfoxides.
Chiral sulfoxide ligands have been found to be efficiently
enantioselective recognition with an octahedral [Ru(bpy)2Cl2]
(bpy is 2,2′-bipyridine) complex.7 We believe that the
octahedral stereocenters allow the straightforward generation
Received: December 4, 2014
© XXXX American Chemical Society
A
DOI: 10.1021/ic502898e
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Inorganic Chemistry
Article
(d, 1H), 8.84 (d, 1H), 8.41(m, 3H), 8.31 (d, 3H), 8.12 (t, 1H), 7.94
(m, 3H), 7.81 (t, 1H), 7.64 (m, 2H), 7.52 (t, 1H), 7.29 (m, 2H), 7.16
(t, 1H), 6.92 (t, 1H), 6.51 (d, 1H), 6.44 (t, 1H), 1.41 (s, 3H). Λ-
[Ru(bpy)2(L-Me)](PF6) (Λ-1). Yield 90% based on Λ-Ru; ee, 98%;
CD (Δε/M−1 cm−1, MeCN): 279 nm (−66), 297 nm (+122), 362 nm
(+16). Δ-[Ru(bpy)2(L-Me)](PF6) (Δ-1). Yield 91% based on Δ-Ru;
ee, 98%; CD (Δε/M−1 cm−1, MeCN): 279 nm (+70), 297 nm
(−128), 362 nm (−17).
therefore, the predetermined chirality of sulfoxide is originated.
To the best of our knowledge, though the asymmetric catalysis
based on chiral ligands have been well documented,10 the
report for the asymmetric synthesis using a chiral-at-metal
complex as the unique source is still rare.11,12 On the other
hand, the oxidation of thiolato and thioether complexes has
been observed;13,14 however, the enantioselective oxidation in
situ of the thioether ligands generation of the predetermined
chirallty of the sulfoxide ligands is unexplored.9 As part of an
ongoing study, we extend the approach of “coordination
oxidation in situ” to synthesize the chiral ortho hydroxyl phenyl
sulfoxides which have been used as chiral auxiliaries for
asymmetric synthesis.12g In this paper, the synthesis of 15
Ru(II)-bpy-thioether complexes, rac-[Ru(bpy)2(L-R)](PF6)
(HL-R is 2-(alkylthio)phenol, R = Me (rac-1), Et (rac-2), iPr
(rac-3), Bn (rac-4), and Nap (rac-5)), Δ/Λ-[Ru(bpy)2(L-
R)](PF6) (R = Me (Δ-1 and Λ-1), Et (Δ-2 and Λ-2), iPr (Δ-3
and Λ-3), Bn (Δ-4 and Λ-4), and Nap (Δ-5 and Λ-5)), and the
oxidated products rac-[Ru(bpy)2(LO-R)](PF6) (HLO-R is
2(alkylsulfinyl)phenol, R = Me (rac-1a), Et (rac-2a), iPr (rac-
3a), Bn (rac-4a), and Nap (rac-5a)), Δ-[Ru(bpy)2{(R)-LO-
R}](PF6) (R = Me (Δ-1a), Et (Δ-2a), iPr (Δ-3a), Bn (Δ-4a),
and Nap (Δ-5a)), and Λ-[Ru(bpy)2{(S)-LO-R}](PF6) (R =
Me (Λ-1a), Et (Λ-2a), iPr (Λ-3a), Bn (Λ-4a) and Nap (Λ-
5a)), have been reported. Moreover, the chiral ortho hydroxyl
phenyl sulfoxides ((R/S)-HLO-R) afford by acidolysis of the
chiral sulfoxide complexes in the presence of TFA−MeCN in
95−97% yields with an ee value up to 98.8%. In the meantime,
the chirality of the ruthenium complex completely retains; it
can be recovered and reused in a new synthetic cycle. The
method reported here may offer a new choice for the synthesis
of chiral sulfoxide compounds.
[Ru(bpy)2(L-Et)](PF6) (rac-2). Yield 94% (reaction conditions are
similar to rac-1). Anal. Calcd for C28H25F6N4OPRuS: C 47.26, H 3.54,
N 7.87, S 4.51. Found: C 47.20, H 3.57, N 7.81, S 4.49; ESI-MS: m/z
1
= 567 [M − PF6]+; H NMR (300.1 MHz, CD3CN): δ 9.22 (d, 1H),
8.89 (d, 1H), 8.48(d, 1H), 8.36 (m, 3H), 8.13 (t, 1H), 7.95 (m, 3H),
7.80 (t, 1H), 7.68 (t, 1H), 7.50 (m, 2H), 7.34 (d, 1H), 7.26 (t, 1H),
7.17 (t, 1H), 6.91 (t, 1H), 6.51 (d, 1H), 6.43 (t, 1H), 2.21 (m, 1H),
1.64 (m, 1H), 0.65 (t, 3H). Λ-[Ru(bpy)2(L-Et)](PF6) (Λ-2). Yield,
85% based on Λ-Ru; ee, 98%; CD (Δε/M−1 cm−1, MeCN): 279 nm
(−83), 298 nm (+152), 362 nm (+23). Δ-[Ru(bpy)2(L-Et)](PF6) (Δ-
2). Yield 85% based on Δ-Ru; ee, 98%; CD (Δε/M−1 cm−1, MeCN):
279 nm (+79), 297 nm (−142), 362 nm (−20).
[Ru(bpy)2(L-iPr)](PF6) (rac-3). Yield 93% (reaction conditions are
similar to rac-1). Anal. Calcd for C29H27F6N4OPRuS: C 48.00, H 3.75,
N 7.72, S 4.42. Found: C 47.95, H 3.75, N 7.70, S 4.38; ESI-MS: m/z
1
= 581 [M − PF6]+; H NMR (300.1 MHz, CD3CN): δ 9.24 (d, 1H),
9.01 (d, 1H), 8.50(d, 1H), 8.34 (m, 3H), 8.13 (t, 1H), 8.05 (d, 1H),
7.94 (t, 1H), 7.86 (t, 1H), 7.80 (t, 1H), 7.70 (t, 1H), 7.49 (t, 1H), 7.28
(m, 3H), 7.16 (t, 1H), 6.90 (t, 1H), 6.48 (d, 1H), 6.40 (t, 1H), 2.58
(m, 1H), 0.87 (d, 3H), 0.32 (d, 3H). Λ-[Ru(bpy)2(L-iPr)](PF6) (Λ-
3). Yield, 87% based on Λ-Ru; ee, 98%; CD (Δε/M−1 cm−1, MeCN):
280 nm (−77), 298 nm (+140), 360 nm (+24). Δ-[Ru(bpy)2(L-
iPr)](PF6) (Δ-3). Yield 88% based on Δ-Ru; ee, 98%; CD (Δε/M−1
cm−1, MeCN): 280 nm (+76), 298 nm (−135), 360 nm (−23).
[Ru(bpy)2(L-Bn)](PF6) (rac-4). Yield 91% (reaction conditions are
similar to rac-1). Anal. Calcd for C33H27F6N4OPRuS: C 51.23, H 3.52,
N 7.24, S 4.14. Found: C 51.02, H 3.60, N 7.18, S 4.10; ESI-MS: m/z
1
= 629 [M − PF6]+; H NMR (300.1 MHz, CD3CN): δ 9.26 (d, 1H),
8.92 (d, 1H), 8.34(m, 2H), 8.24 (d, 1H), 8.14 (t, 1H), 7.95 (t, 1H),
7.87 (m, 3H), 7.75 (t, 1H), 7.54 (m, 2H), 7.34 (d, 1H), 7.21 (m, 2H),
6.98 (m, 5H), 6.63 (d, 2H), 6.53 (d, 1H), 6.39 (t, 1H), 3.83 (d, 1H),
3.06 (d, 1H). Λ-[Ru(bpy)2(L-Bn)](PF6) (Λ-4). Yield, 83% based on
Λ-Ru; ee, 98%; CD (Δε/M−1 cm−1, MeCN): 280 nm (−61), 299 nm
(+121), 364 nm (+20). Δ-[Ru(bpy)2(L-Bn)](PF6) (Δ-4). Yield 83%
based on Δ-Ru; ee, 98%; CD (Δε/M−1 cm−1, MeCN): 280 nm (+65),
299 nm (−128), 362 nm (−21).
EXPERIMENTAL SECTION
■
Materials. RuCl3·3H2O, 2,2′-bipyridine, 2-hydroxythiophenol, 2-
(methylthio)phenol, bromoethane, 2-bromopropane, benzyl bromide,
2-(bromomethyl)naphthalene, 3-chloroperoxybenzoic acid (m-CPBA),
and other chemicals were purchased from commercial sources.
[Ru(bpy)2Cl2]·2H2O,15 Λ-[Ru(bpy)2(py)2][O,O′-dibenzoyl-D-tar-
trate]·12H2O (Λ-Ru), and Δ-[Ru(bpy)2(py)2][O,O′-dibenzoyl-L-
tartrate]·12H2O (Δ-Ru) were prepared by the literature procedures.16
Syntheses of Thioether Ligands. The ligands were synthesized
according to the reported procedure.17
2-(Ethylthio)phenol (HL-Et). Yield 85% (reaction in 0.5 h). ESI-
MS: m/z = 153 [M − H]−; 1H NMR (300.1 MHz, CDCl3): δ 7.46 (d,
1H), 7.26 (t, 1H), 6.99 (d, 1H), 6.87 (t, 1H), 6.77 (s, 1H), 2.43 (q,
2H), 1.24 (d, 3H).
2-(Isopropylthio)phenol (HL-iPr). Yield 86% (reaction in 3 h). ESI-
MS: m/z = 167 [M − H]−; 1H NMR (300.1 MHz, CDCl3): δ 7.47 (d,
1H), 7.30 (t, 1H), 7.02 (d, 1H), 6.90 (m, 2H), 3.12 (m, 1H), 1.28 (d,
6H).
2-(Benzylthio)phenol (HL-Bn). Yield 83% (reaction overnight).
ESI-MS: m/z = 215 [M − H]−; 1H NMR (300.1 MHz, CDCl3): δ 7.29
(m, 5H), 7.11 (m, 2H), 6.95 (d, 1H), 6.83 (t, 1H), 6.57 (s, 1H), 3.87
(s, 2H).
2-(Naphthalthio)phenol (HL-Nap). Yield 82% (reaction over-
night). ESI-MS: m/z = 265 [M − H]−; 1H NMR (300.1 MHz,
CDCl3): δ 7.79 (m, 2H), 7.67 (m, 1H), 7.45 (m, 2H), 7.38 (s, 1H),
7.33 (d, 1H), 7.24 (m, 2H), 6.92 (d, 1H), 6.76 (t, 2H), 4.01 (s, 2H).
Syntheses of Thioether Complexes. The thioether complexes
were synthesized by the similar procedure of our previous report.9
[Ru(bpy)2(L-Me)](PF6) (rac-1). Yield 98% based on cis-Ru(bpy)2Cl2.
The reactants were added to 18 mL of EtOH and 2 mL of H2O, and
stirred at 90 °C for 6 h. Anal. Calcd for C27H23F6N4OPRuS: C 46.49,
H 3.32, N 8.03, S 4.60. Found: C 46.41, H 3.33, N 8.00, S 4.55; ESI-
MS: m/z = 553 [M − PF6]+; 1H NMR (300.1 MHz, CD3CN): δ 9.20
[Ru(bpy)2(L-Nap)](PF6) (rac-5). Yield 91% (reaction conditions are
similar to rac-1). Anal. Calcd for C37H29F6N4OPRuS: C 53.95, H 3.55,
N 6.80, S 3.89. Found: C 53.75, H 3.65, N 6.75, S 3.78; ESI-MS: m/z
1
= 679 [M − PF6]+; H NMR (300.1 MHz, CD3CN): δ 9.34 (d, 1H),
8.92 (d, 1H), 8.34(d, 1H), 8.28 (d, 1H), 8.17 (t, 1H), 8.04 (d, 1H),
7.95 (t, 1H), 7.81 (m, 3H), 7.70 (m, 1H), 7.48 (m, 5H), 7.34 (d, 1H),
7.19 (m, 3H), 6.90 (m, 5H), 6.56 (d, 1H), 6.43 (t, 1H), 4.24 (d, 1H),
3.30 (d, 1H). Λ-[Ru(bpy)2(L-Nap)](PF6) (Λ-5). Yield 88% based on
Λ-Ru; ee, 98%; CD (Δε/M−1 cm−1, MeCN): 279 nm (−55), 299 nm
(+107), 368 nm (+21). Δ-[Ru(bpy)2(L-Nap)](PF6) (Δ-5). Yield 88%
based on Δ-Ru; ee, 98%; CD (Δε/M−1 cm−1, MeCN): 279 nm (+59),
299 nm (−109), 368 nm (−22).
Synthesis of Sulfoxide Complexes. Caution! m-CPBA is
potentially explosive beyond 85% purity and irritates the mucous
membranes, respiratory tract, eyes, and skin. Moreover, skin contact with
m-CPBA causes burns and blisters. Therefore, it is recommended that m-
CPBA should only be used in a chemical fume hood.
Method A. The sulfoxide complexes were sprepared by the similar
procedure of our previous report.9
[Ru(bpy)2(LO-Me)](PF6) (rac-1a). Yield 95% based on rac-1. Anal.
Calcd for C27H23F6N4O2PRuS: C 45.44, H 3.25, N 7.85, S 4.49.
Found: C 45.31, H 3.29, N 7.80, S 4.45; ESI-MS: m/z = 569 [M −
1
PF6]+; H NMR (300.1 MHz, CD3CN): δ 10.05 (d, 1H), 8.51 (d,
1H), 8.43(m, 3H), 8.38 (d, 1H), 8.20 (t, 1H), 8.06 (m, 2H), 7.94 (m,
2H), 7.67 (t, 1H), 7.58 (m, 2H), 7.38 (m, 2H), 7.28 (t, 1H), 7.10 (t,
1H), 6.60 (m, 2H), 2.30 (s, 3H). Λ-[Ru(bpy)2{(S)-LO-Me}](PF6)
B
DOI: 10.1021/ic502898e
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Inorganic Chemistry
Article
Table 1. Crystallographic Data for rac-2a·CH2Cl2, Λ-2a·CH2Cl2, Δ-2a·CH2Cl2, and (R)-HLO-Bn
complex
rac-2a·CH2Cl2
Λ-2a·CH2Cl2
Δ-2a·CH2Cl2
(R)-HLO-Bn
molecular formula
C29H27Cl2F6N4O2PRuS
812.55
C29H27Cl2F6N4O2PRuS
812.55
C29H27Cl2F6N4O2PRuS
812.55
C13H12O2S
232.29
Mr
crystal system
monoclinic
P21/c
monoclinic
C2
monoclinic
C2
orthorhombic
P212121
6.7450(2)
7.3199(2)
23.9210(9)
90
space group
a/Å
10.9404(2)
14.2532(2)
20.8005(4)
101.441(2)
3179.09(10)
4
21.5650(2)
8.79600(10)
17.9140(2)
105.1050(10)
3280.63(6)
4
21.6534(3)
8.84210(10)
17.9476(2)
105.063(2)
3318.21(8)
4
b/Å
c/Å
β/deg
V/Å3
Z
1181.04(7)
4
Dc (g cm−3
μ (mm−1
)
1.698
1.645
1.626
1.306
)
7.265
7.040
6.961
2.288
data/restraints/para
6265/0/416
0.0480
6215/1/418
0.0416
6458/1/418
0.0301
1806/0/146
0.0323
a
R1[I > 2σ(I)]
b
wR2 (all data)
0.1367
0.1155
0.0772
0.0760
Flack parameter
GOF on F2
−0.015(9)
1.103
−0.030(7)
1.035
0.00(2)
1.087
1.027
Δρmax/Δρmin (e Å−3
)
1.109/−1.043
0.886/−0.881
0.521/−0.426
0.115/−0.205
a
b
R1 = ∑∥Fo| − |Fc∥/∑|Fo|. wR2 = [∑w(Fo − Fc2)2/∑w(Fo )2]1/2
.
2
2
(Λ-1a). Yield 95% based on Λ-1; ee, 98%; CD (Δε/M−1 cm−1,
MeCN): 274 nm (−112), 290 nm (+142). Δ-[Ru(bpy)2{(R)-LO-
Me}](PF6) (Δ-1a). Yield 95% based on Δ-1; ee, 98%; CD (Δε/ M−1
cm−1, MeCN): 274 nm (+120), 290 nm (−145).
1H), 7.13 (m, 2H), 6.97 (m, 4H), 6.85 (d, 1H), 6.63 (m, 2H), 5.06 (d,
1H), 3.86 (d, 1H). Λ-[Ru(bpy)2{(S)-LO-Nap}](PF6) (Λ-5a). Yield
92% based on Λ-5; ee, 98%; CD (Δε/M−1 cm−1, MeCN): 279 nm
(−100), 295 nm (+75). Δ-[Ru(bpy)2{(R)-LO-Nap}](PF6) (Δ-5a).
Yield 92% based on Δ-5; ee, 98%; CD (Δε/ M−1 cm−1, MeCN): 279
nm (+99), 295 nm (−66).
[Ru(bpy)2(LO-Et)](PF6) (rac-2a). Yield 94% based on rac-2. Anal.
Calcd for C28H25F6N4O2PRuS: C 46.22, H 3.46, N 7.70, S 4.41.
Found: C 46.21, H 3.45, N 7.64, S 4.35; ESI-MS: m/z = 583 [M −
Mothod B (One-Pot Method). The one-pot synthesis of sulfoxide
complexes was carried out with the similar procedure of ours.9 The
1
PF6]+; H NMR (300.1 MHz, CD3CN): δ 10.15 (d, 1H), 8.45 (m,
1
yield of Λ-3a is 81%. Its enantiomeric excess is determined by H
5H), 8.23(t, 1H), 7.98 (m, 4H), 7.70 (t, 1H), 7.59 (t, 1H), 7.52 (d,
1H), 7.34 (t, 1H), 7.28 (t, 1H), 7.23 (d, 1H), 7.09 (t, 1H), 6.58 (m,
2H), 3.07 (m, 1H), 2.10 (m, 1H), 0.64 (t, 3H). Λ-[Ru(bpy)2{(S)-LO-
Et}](PF6) (Λ-2a). Yield 95% based on Λ-2; ee, 98%; CD (Δε/M−1
cm−1, MeCN): 277 nm (−117), 291 nm (+154) Δ-[Ru(bpy)2{(R)-
LO-Et}](PF6) (Δ-2a): Yield 94% based on Δ-2; ee, 98%; CD (Δε/
M−1 cm−1, MeCN): 277 nm (+109), 291 nm (−136).
NMR and found to be larger than 98%.
Syntheses of Sulfoxide Compounds and Recovery of the
Chiral Complex. A sulfoxide complex (0.1 mmol) was added to the
solution of CH3CN (3 mL) containing 1.0 mmol of TFA. The
solution was stirred at 80 °C for 2 h. After cooling to room
temperature, the solvent was removed under reduced pressure and
subjected to silica gel chromatography with EtOAc, and then MeCN−
H2O (10:1) as eluents. After removal of the solvent and drying in a
desiccator with silica gel, chiral sulfoxide compounds and Λ/Δ-
[Ru(bpy)2(MeCN)2](CF3COO)(PF6) were obtained. To convert into
PF6 salt, 20 mL of water was used to dissolve [Ru(bpy)2(MeCN)2]-
(CF3COO)(PF6), and then solid KPF6 was added. Furthermore, 15
mL and 2 × 10 mL of CH2Cl2 were used to extract the product from
the aqueous phase, and the CH2Cl2 phase was combined and dried
over MgSO4. After filtration and concentration, Λ/Δ-[Ru(bpy)2-
(MeCN)2](PF6)2 was obtained in the yield of 94%.
[Ru(bpy)2(LO-iPr)](PF6) (rac-3a). Yield 95% based on rac-3. Anal.
Calcd for C29H27F6N4O2PRuS: C 46.96, H 3.67, N 7.55, S 4.32.
Found: C 46.91, H 3.65, N 7.54, S 4.35; ESI-MS: m/z = 597 [M −
1
PF6]+; H NMR (300.1 MHz, CD3CN): δ 10.15 (d, 1H), 8.72 (d,
1H), 8.55(d, 1H), 8.39 (d, 3H), 8.24 (t, 1H), 7.96 (m, 4H), 7.74 (t,
1H), 7.54 (t, 1H), 7.48 (d, 1H), 7.30 (m, 2H), 7.04 (m, 2H), 6.54 (m,
2H), 2.87 (m, 1H), 1.02 (d, 3H), 0.34 (d, 3H). Λ-[Ru(bpy)2{(S)-LO-
iPr}](PF6) (Λ-3a). Yield 94% based on Λ-3; ee, 98%; CD (Δε/ M−1
cm−1, MeCN): 278 nm (−105), 293 nm (+148). Δ-[Ru(bpy)2{(R)-
LO-iPr}](PF6) (Δ-3a). Yield 95% based on Δ-3; ee, 98%; CD (Δε/
M−1 cm−1, MeCN): 278 nm (+106), 293 nm (−143).
2-(Methylsulfinyl)phenol (HLO-Me). Yield, 95%. ESI-MS: m/z =
155 [M − H]−; 1H NMR (300.1 MHz, CDCl3): δ 10.25 (s, 1H), 7.37
(t, 1H), 7.07 (d, 1H), 6.93 (m, 2H), 2.97 (s, 3H). For (R)-HLO-Me,
yield 95% based on Λ-1a; ee, 97.0% (mobile phase: hexane/
[Ru(bpy)2(LO-Bn)](PF6) (rac-4a). Yield 93% based on rac-4. Anal.
Calcd for C33H27F6N4O2PRuS: C 50.19, H 3.45, N 7.09, S 4.06.
Found: C 50.21, H 3.35, N 7.14, S 4.15; ESI-MS: m/z = 645 [M −
(EtOH:MeOH)/TFA = 90/(3:1)10/0.1); CD (Δε/M−1 cm−1
,
1
PF6]+; H NMR (300.1 MHz, CD3CN): δ 10.14 (d, 1H), 8.55 (d,
MeCN): 219 nm (−23), 243 nm (+52). For (S)-HLO-Me, yield
95% based on Δ-1a; ee, 97.6%; CD (Δε/M−1 cm−1, MeCN): 219 nm
(+23), 243 nm (−52).
1H), 8.39(d, 1H), 8.36 (d, 1H), 8.20 (m, 2H), 8.00 (m, 2H), 7.83 (m,
2H), 7.77 (m, 1H), 7.67 (t, 1H), 7.59 (t, 1H), 7.40 (d, 1H), 7.30 (t,
1H), 7.11 (m, 3H), 6.90 (d, 1H), 6.88 (t, 2H), 6.67 (d, 2H), 6.61 (d,
1H), 6.56 (t, 1H), 4.68 (d, 3H), 3.66 (d, 1H). Λ-[Ru(bpy)2{(S)-LO-
Bn}](PF6) (Λ-4a). Yield 93% based on Λ-4; ee, 98%; CD (Δε/M−1
cm−1, MeCN): 278 nm (−116), 294 nm (+118). Δ-[Ru(bpy)2{(R)-
LO-Bn}](PF6) (Δ-4a). Yield 93% based on Δ-4; ee, 98%; CD (Δε/
M−1 cm−1, MeCN): 278 nm (+126), 294 nm (−123).
2-(Ethylsulfinyl)phenol (HLO-Et). Yield 95%. ESI-MS: m/z = 169
1
[M − H]−; H NMR (300.1 MHz, CDCl3): δ 10.46 (s, 1H), 7.36 (t,
1H), 7.01 (d, 1H), 6.76 (m, 1H), 3.13 (m, 2H), 1.33 (t, 3H). For (R)-
HLO-Et, yield 93% based on Λ-2a; ee, 96.7% (mobile phase: hexane/
(EtOH:MeOH)/TFA = 95.5/(3:1)4.5/0.1); CD (Δε/M−1 cm−1,
MeCN): 221 nm (−26), 245 nm (+47). For (S)-HLO-Et, yield 94%
based on Δ-2a; ee, 95.5%; CD (Δε/M−1 cm−1, MeCN): 221 nm
(+27), 245 nm (−49)
2-(Isopropylsulfinyl)phenol (HLO-iPr). Yield 96%. ESI-MS: m/z =
183 [M − H]−; 1H NMR (300.1 MHz, CDCl3): δ 10.63 (s, 1H), 7.36
(t, 1H), 6.98 (d, 1H), 6.89 (m, 1H), 3.21 (m, 1H), 1.37 (d, 3H), 1.27
(d, 3H). For (R)-HLO-iPr, yield 96% based on Λ-3a; ee, 98.8%
[Ru(bpy)2(LO-Nap)](PF6) (rac-5a). Yield 92% based on rac-5. Anal.
Calcd for C37H29F6N4O2PRuS: C 52.92, H 3.48, N 6.67, S 3.82.
Found: C 53.01, H 3.25, N 6.64, S 3.89; ESI-MS: m/z = 695 [M −
1
PF6]+; H NMR (300.1 MHz, CD3CN): δ 10.21 (d, 1H), 8.55 (d,
1H), 8.35(t, 2H), 8.23 (t, 1H), 8.02 (t, 1H), 7.94 (m, 2H), 7.80 (m,
2H), 7.71 (d, 1H), 7.60 (m, 2H), 7.48 (m, 3H), 7.34 (d, 1H), 7.26 (t,
C
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Scheme 1. Synthetic Pathway for Complexes and Chiral Sulfoxides
(mobile phase: hexane/(EtOH:MeOH)/TFA = 92/(3:1)8/0.1); CD
(Δε/M−1 cm−1, MeCN): 214 nm (−36), 222 nm (−39), 248 nm
(+55). For (S)-HLO-iPr, yield 97% based on Δ-3a; ee, 98.6%; CD
(Δε/M−1 cm−1, MeCN): 213 nm (+39), 222 nm (+40), 248 nm
(−53).
alkylation product was observed in the experiment conditions,
indicating that the alkylation reaction was high chemical
selectivity. The products were confirmed by MS and NMR
spectra. For the complexes, the racemic thioether complexes
rac-1 to rac-5 were obtained when cis-Ru(bpy)2Cl2 reacted with
the corresponding thioether ligands in EtOH−H2O at 90 °C
for 6 h. When the chiral precursors Δ/Λ-[Ru(bpy)2(py)2]2+
were used to react with the prochiral thioethers at 120 °C in
ethylene glycol, the corresponding chiral complexes Δ/Λ-
[Ru(bpy)2(L-R)](PF6) (Δ/Λ-1, Δ/Λ-2, Δ/Λ-3, Δ/Λ-4, and
Δ/Λ-5) were obtained in yields of 83−91% after chromatog-
raphy column separation. They were characterized by 1H NMR,
and positive ESI-MS spectroscopy. CD spectra were employed
to observe the optical activity of the thioether complexes. As
expected, the racemic complexes are all optically silent, whereas
Δ/Λ-1, Δ/Λ-2, Δ/Λ-3, Δ/Λ-4, and Δ/Λ-5 are optically active.
As shown in Figure 1, Λ-1 and Δ-1 have an intensive Cotton
effect at 279 and 297 nm. They are almost mirror images. The
analogous cases were also observed for Δ/Λ-2, Δ/Λ-3, Δ/Λ-4,
2-(Benzylsulfinyl)phenol (HLO-Bn). Yield 97%. ESI-MS: m/z = 231
[M − H]−; 1H NMR (300.1 MHz, CDCl3): δ 10.20 (s, 1H), 7.30 (m,
4H), 7.02 (d, 2H), 6.89 (d, 1H), 6.76 (t, 1H), 6.66 (d, 1H), 4.37 (d,
1H), 4.30 (d, 1H). For (R)-HLO-Bn, yield 97% based on Λ-4a; ee,
96.5% (mobile phase: hexane/(EtOH:MeOH)/TFA = 92/(3:1)8/
0.1). CD (Δε/M−1 cm−1, MeCN): 224 nm (−27), 251 nm (+51). For
(S)-HLO-Bn, yield 98% based on Δ-4a; ee, 97.4%; CD (Δε/M−1
cm−1, MeCN): 225 nm (+27), 251 nm (−48) .
2-(Naphthalsulfinyl)phenol (HLO-Nap). Yield 97%. ESI-MS: m/z
1
= 281 [M − H]−; H NMR (300.1 MHz, CDCl3): δ 10.25 (s, 1H),
7.76 (m, 3H), 7.48 (m, 3H), 7.34 (t, 1H), 7.13 (d, 1H), 6.90 (d, 1H),
6.71 (t, 1H), 6.62 (d, 1H), 4.56 (d, 1H), 4.45 (d, 1H). For (R)-HLO-
Nap, yield 97% based on Λ-5a; ee, 96.7% (mobile phase: hexane/
(EtOH:MeOH)/TFA = 80/(3:1)20/0.1); CD (Δε/M−1 cm−1
,
MeCN): 230 nm (−16), 254 nm (+48). For (S)-HLO-Nap, yield
97% based on Δ-5a; ee, 97.4%; CD (Δε/M−1 cm−1, MeCN): 230 nm
(+17), 254 nm (−51).
Single-Crystal X-ray Crystallography. The data collection and
the structure resolution for rac-2a·CH2Cl2, Δ-2a·CH2Cl2, Λ-2a·
CH2Cl2, and (R)-HLO-Bn were carried out as the previous report.9
The crystal data and parameters for the complexes are presented in
Table 1. CCDC reference numbers 1036023 ((R)-HLO-Bn), 1036024
(for rac-2a), 1036025 (for Δ-2a), and 1036026 (for Λ-2a) contain the
supplementary crystallographic data for the compounds. These data
can be obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
RESULTS AND DISCUSSION
■
Syntheses and Characterization of the Ligands and
Thioether Complexes. Scheme 1 presents the pathway for
the synthesis of the ligands and complexes. The 2-(alkylthio)-
phenol (HL-R) ligands were smoothly synthesized by the
reaction of 2-hydroxythiophenol and alkyl bromide in the
presence of KHCO3 as a base at room temperature.17 The
bulky substituents such as Bn and Nap need to extend the
reaction time to afford the satisfactory results. No hydro-
Figure 1. CD spectra of Δ-1 and Λ-1 in MeCN (50 μM).
D
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and Δ/Λ-5 (see Figures S1−S4 in the Supporting Information
and the Experimental Section).
Cotton effect at 274 and 290 nm. The similar cases were also
observed for Δ/Λ-2a, Δ/Λ-3a, Δ/Λ-4a, and Δ/Λ-5a (see
Figures S8−S12 in the Supporting Information and the
Experimental Section).
The enantiopurities of the thioether complexes were
1
measured by H NMR spectra, because the Δ and Λ isomers
have different chemical shifts when S-binol was added.18 As
shown in Figure 2, the resonance peak at 8.84 ppm in rac-1,
The chemical shifts of the H6 of bpy at 10.05 ppm in 1a,
10.15 ppm in 2a, 10.15 ppm in 3a, 10.14 ppm in 4a, and 10.21
ppm in 5a are markedly low-field shifted in comparison with
those of the corresponding thioether complexes (9.20, 9.22,
9.24, 9.26, and 9.34 ppm). These may be attributed to the
hydrogen bonding between the α-H and sulfoxide oxygen atom
(vide infra). The 1H chemical shifts at 1.41 ppm in 1, 2.21 and
1.64 ppm in 2, 2.58 ppm in 3, 3.83 and 3.06 ppm in 4, and 4.24
and 3.30 ppm in 5, assigned to the methyl or methylene (−S-
CH−) in the thioether ligands, are low-field shifted to the
corresponding 2.30 ppm in 1a, 3.07 and 2.10 ppm in 2a, 2.87
ppm in 3a, 4.68 and 3.66 ppm in 4a, and 5.06 and 3.86 ppm in
5a, demonstrating that the oxidation reaction indeed occurs
1
because sulfoxide is an electron-withdrawing group. The H
NMR spectra of rac-3a, Λ-3a, and Δ-3a in the absence or
presence of S-binol in CD3CN are presented in Figure 4. The
Figure 2. 1H NMR spectra of rac-1 (a), rac-1 and 40 equiv of S-binol
(b), Λ-1 and 40 equiv of S-binol (c), Δ-1 and 40 equiv of S-binol (d)
in CD3CN.
assigned to the H6 of bpy, is split into 8.81 and 8.73 ppm when
40 equiv of S-binol was added. They have the same chemical
shifts with the corresponding Λ and Δ enantiomers. Thereby,
the enantiopurities of Δ and Λ enantiomers can be calculated
from the integrals of these peaks and were found to be larger
than 98%. The same approach is used to examine the
enantiopurities of the other thioether complexes (see Figures
S5−S8 in the Supporting Information). Their enantiopurities
were found to be larger than 98%. The above results indicate
that the configurations of ruthenium complexes are stable in the
coordination reaction.
Syntheses and Structural Characterization of Sulf-
oxide Complexes. Sulfoxide complexes were obtained by the
directed oxidation of the thioether complexes in the presence of
m-CPBA in methanol at room temperature. The yields are
excellent in 93−95%. It was noteworthy that no sulfone
product was detected. The sulfoxide complexes were
characterized by NMR and MS spectroscopy. CD spectra
were used to examine the optical activity of the corresponding
complexes. As shown in Figure 3, Λ-1a and Δ-1a have a strong
1
Figure 4. H NMR spectra of rac-3a (a), rac-3a with 40 equiv of S-
binol (b), Λ-3a with 40 equiv of S-binol (c), Δ-3a with 40 equiv of S-
binol (d) in CD3CN.
resonance peak at 10.15 ppm is split into two peaks at 10.06
and 10.01 ppm, which are in accord with those of Δ and Λ
enantiomers. Moreover, the peak at 8.72 ppm is also split and
shifted to 8.67 and 8.61 ppm, which are consistent with those of
Λ and Δ enantiomers. The similar cases are also observed in
the other complexes (see Figures S13−S16 in the Supporting
Information): the peak at 10.05 ppm for rac-1a is split into
10.00 and 9.94 ppm, that at 10.15 ppm for rac-2a is split into
10.08 and 10.03 ppm, those at 10.14 and 8.55 ppm for rac-4a
are split into 10.09 and 10.04 ppm, and 8.51 and 8.44 ppm, and
those at 10.21 and 8.53 ppm for rac-5a are split into 10.15 and
10.11 ppm, and 8.51 and 8.45 ppm. The enantiopurities of Δ
and Λ enantiomers are calculated from the integrals of these
peaks and are found to be larger than 98%. The experiments
also demonstrated that the ruthenium complexes are stable in
the oxidation reaction under the experimental conditions.
The racemic complex rac-2a crystallizes in the P21/c space
group. As shown in Figure 5, the sulfoxide ligand is indeed
generated in situ. Each Ru(II) ion is coordinated by two bpy
and one LO-Et ligand in a distorted octahedral geometry. The
sulfur atom is bound to the Ru(II) ion with a distance of
2.250(1) Å; the Ru1−S1 bond length is comparable with those
of the ruthenium sulfoxide complexes.7c,9,14 The bond distance
Figure 3. CD spectra of complexes Δ-1a and Λ-1a in MeCN (50 μM).
E
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reaction under the experiment conditions and mediates the
chirality of sulfoxide in situ oxidation.
Synthesis and Characterization of Sulfoxide Com-
pounds. The sulfoxide ligands can be released from the
corresponding sulfoxide complexes in the presence of TFA and
displaced by the coordinated solvent CH3CN.9,20 Indeed, when
the sulfoxide complexes were added to the TFA−CH3CN
solution, and then stirred at 80 °C for 2 h, the sulfoxide
compounds HLO-R were obtained in 95−97% after a column
separation. The sulfoxide compounds were characterized by MS
and NMR spectroscopy. CD spectra was also used to examine
their optical activity (see Figures S19−S23 in the Supporting
Information). Furthermore, the enantiopurities of (R/S)-HLO-
R were measured by means of chiral HPLC analyses (see
Figure 6 and Figures S24−27, Supporting Information). The ee
Figure 5. View of the structures of a pair of cations in rac-2a (left, Δ-
R; and right, Λ-S). Selected bond angles (deg) and distances (Å):
Ru1−O1 = 2.076(3), Ru1−S1 = 2.250(1), Ru1−N1 = 2.060(4), Ru1−
N2 = 2.058(4), Ru1−N3 = 2.070(4), Ru1−N4 = 2.105(3), S1−O2 =
1.487(4), N4−Ru1−S1 = 168.5(1). ORTEP drawing with 50%
probability thermal elipsoids.19
of Ru1−N4 (2.105(3) Å) is slightly longer than those of Ru−
N(1−3) (2.058−2.070 Å), indicating the stronger trans effect
of the coordination sulfur atom.7c The S−O bond length
(1.487(4) Å) is in the range of those reported.9,14 The bulk
ethyl group of the LO-Et ligand is away from the H6 of bpy to
relieve the repulsion. Moreover, the sulfoxide oxygen atom
generated in situ indeed anchors on the other side and further
forms a hydrogen bond with the H6 of bpy (C11···O2 = 3.080
Å, H···O2 = 2.227 Å, ∠C11−H−O2 = 150.5°). Only a pair of
enantiomeric cations Δ-R and Λ-S are found in each asymmtric
unit, though two pairs of diasteromeric cations (Δ-S and Δ-R,
Λ-S and Λ-R) would be produced theoretically during the
oxidation process. Upon careful examination of the crystal
structure, the fact that enantioselective origination of the
absolute configuration at the sulfur atom mediated by the chiral
ruthenium center is found, namely, the R configuration at the
sulfur atom is generated by the Δ metal-centered parent,
whereas the S configuration at the sulfur atom is produced by
the Λ metal-centered environment. This also inspires us to
observe the oxidation reaction in an asymmetric environment.
Indeed, the cases were also found in the chiral sulfoxide
complexes Δ-2a and Λ-2a, which were generated by the
oxidation of the chiral thioether complexes Δ-2 and Λ-2,
respectively. The absolute configurations of sulfur atoms and
ruthenium centers in complexes Δ-2a and Λ-2a were measured
by single-crystal X-ray diffraction (see Figures S17 and S18 in
the Supporting Information). The structures verify that the Δ
and Λ configurations are at the ruthenium centers of Δ-2a and
Λ-2a, respectively, which are consistent with the configurations
of their parents, indicating that the configurations of ruthenium
complexes are retained during the oxidation reaction. The Flack
parameters of Δ-2a (−0.030(7)) and Λ-2a (−0.015(9))
demonstrate that the chiral assignments at the complexes are
correct. Most important, only the R configuration of the LO-Et
ligand is found in Δ-2a; on the contrary, only the S
configuration of the LO-Et ligand is observed in Λ-2a. These
further demonstrate that the configuration at the ruthenium
center mediates the sulfoxide chirality during the oxidation
process. Therefore, this strategy can be employed to originate
the predetermined chirality of sulfoxide compounds.
Figure 6. HPLC traces of racemic HLO-iPr (a), (R)-HLO-iPr with
98.8% ee value (b), and (S)-HLO-iPr with 98.6% ee value (c).
values are 97.0% for (R)-HLO-Me, 96.7% for (R)-HLO-Et,
98.8% for (R)-HLO-iPr, 96.5% for (R)-HLO-Bn, and 96.7%
for (R)-HLO-Nap, and 97.6% for (S)-HLO-Me, 95.5% for (S)-
HLO-Et, 98.6% for (S)-HLO-iPr, 97.4% for (S)-HLO-Bn, and
97.4% for (S)-HLO-Nap. The experiment results show that no
significant racemization occurred during the acidolysis reaction.
It is noteworthy that the sulfur atom configuration converts
from R to S upon release from the coordination. (S)-HLO-iPr
has been prepared by the Andersen method,7c involving
nucleophilic substitution of 1,2:5,6-di-isopropylidene-α-D-
glucofuranosyl-(+)-(R)-2-propanesulfinate21 by the freshly
prepared Grignard reagent and then deprotection of the
methoxyl group. Here, we developed a new approach to
synthesize the chiral sulfoxide compounds with high yields and
enantiopurities. Importantly, the use of the organometallic
reagent is avoided, and the procedures are easy to be
controlled.
The absolute configuration of (R)-HLO-Bn was further
identified by single-crystal X-ray diffraction. It crystallizes in the
P212121 space group. The molecular structure of (R)-HLO-Bn
is presented in Figure 7. The sulfur atom is a R configuration.
The Flack parameter 0.00(7) demonstrates that the chiral
assignment at the sulfur atom is correct. The S1−O2 distance
of 1.504(2) Å is compared to that reported.9
Recovery and Reuse of the Chiral Ruthenium
Complex. The affording of chiral sulfoxide compounds with
high yields and high enantiomeric excesses indicates that the
configuration of the sulfur atom is retained during the acidolysis
reaction. This prompts us to observe the configuration of the
metal center upon removal of the chiral sulfoxide ligand, and to
recycle and reuse the chiral ruthenium complex as a new
precursor. When 10 equiv of TFA was added to the acetonitrile
solution of Λ-3a at 80 °C in the dark, the new complex Λ-
One-pot method for the synthetic sulfoxide complex Λ-3a
was also developed. The total yield of the sulfoxide complex
was 81% (see the Experimental Section). The enantiopurity of
the one-pot product was measured by NMR spectra, and the ee
value is larger than 98%. The result also proves that the
ruthenium-centered configuration is stable in the course of
F
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thioether ruthenium complexes with a chiral-at-metal strategy
has been developed. The configurations of ruthenium
complexes are stable during the coordination and oxidation
processes. Moreover, the ruthenium centered configurations
mediate the origination of sulfoxide chirality in the course of
oxidation. The chiral sulfoxide can be released by the acidolysis
of the corresponding sulfoxide complex with TFA in the
presence of CH3CN. Importantly, the chiral ruthenium
complex can be recycled and reused in a next reaction cycle
with complete retention of their configurations.
ASSOCIATED CONTENT
* Supporting Information
The detailed experiments. the CD, H NMR, crystal structure,
and HPLC figures for the compounds. This material is available
free of charge via the Internet at http://pubs.acs.org.
■
Figure 7. View of the structure of (R)-HLO-Bn. Selected bond
distances (Å) and angles (deg): S1−O2 = 1.504(2), C6−S1 =
1.787(2), C7−S1 = 1.820(2), O2−S1−C6 = 104.9(1), C7−S1−C6 =
99.3(1), O2−S1−C7 = 104.4(1). ORTEP drawing with 50%
probability thermal elipsoids.19
S
1
AUTHOR INFORMATION
Corresponding Author
*E-mail: cesybh@mail.sysu.edu.cn (B.-H.Y).
Notes
■
7d
[Ru(bpy)2(CH3CN)2](PF6)2 was afforded in 94% yield after
a facile silica gel chromatographic column separation. Its
enantiopurity was examined by conversion into the correspond-
ing thioether complex Λ-3 (see the Experimental Section) and
The authors declare no competing financial interest.
1
measured by H NMR spectra. The ee value was found to be
larger than 98% calculated from the integrals of H6 peaks (see
Figure S28, Supporting Information), indicating that the chiral
ruthenium complex would be recyclable under the reaction
conditions.
ACKNOWLEDGMENTS
This work was supported by the NSF of China (21272284) and
the DPF of MOE of China (No. 0171110004).
■
Moreover, the reusability of the recycled chiral ruthenium
complex was also examined. Indeed, Δ-3a was obtained in a
yield of 83% by the reaction of the recycled Δ-[Ru(bpy)2-
(MeCN)2](PF6)2 and HL-iPr in EtOH at 80 °C for 0.5 h
under an argon atmosphere, followed by oxidation using m-
CPBA as an oxidant (see the Experimental Section, one-pot
method). Its enantiopurity was determined by 1H NMR
spectra. The ee value was found to be larger than 98% (see
Figure S29, Supporting Information). The results demonstrate
that the chiral ruthenium complex is reusable under the
reaction conditions, as shown in Scheme 2.
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■
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CONCLUSIONS
■
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Scheme 2. Reaction Cycle for the Synthesis of Chiral
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