Enantioselective oxidation of thioethers to sulfoxides by means of a structural template with chiral-at-metal ruthenium complexes

Article abstract of DOI:10.1002/cplu.201402243

Treatment of cis-[Ru(bpy)₂Cl₂]·2H₂O or Δ/Λ-[Ru(bpy)₂(py)₂]²⁺ (bpy=2,2′-bipyridine, py=pyridine) with the prochiral thioether ligands 2-alkylthiobenzoic acid (HOS-R) produces the corresponding thioether complexes rac-[Ru(bpy)₂(OS-R)](PF₆) (R=Me (rac-1), iPr (rac-2), 2-benzylthiobenzonate (Bn) (rac-3)) and Δ/Λ-[Ru(bpy)₂(OS-R)](PF₆) (R=Me (Δ-1/Λ-1), iPr (Δ-2/Λ-2), Bn (Δ-3/Λ-3)) with retention of the configurations at chiral metal centers. In situ oxidation of the thioether complexes by metachloroperoxybenzoic acid provides the corresponding sulfoxide complexes rac-[Ru(bpy)₂(OSO-R)](PF₆) (OSO-R is 2-alkylsulfinylbenzonate, R=Me (rac-1a), iPr (rac-2a), Bn (rac-3a)), Δ-[Ru(bpy)₂{(R)-OSO-R}](PF₆) (R=Me (Δ-1a), iPr (Δ-2a), Bn (Δ-3a)), and Λ-[Ru(bpy)₂{(S)-OSO-R}](PF₆) (R=Me (Λ-1a), iPr (Λ-2a), Bn (Λ-3a)) in yields of 95% with 98% ee values. The absolute configurations at the metal centers and sulfur atoms were determined by means of X-ray crystallography. The results indicate that the configurations of the metal centers are retained and have the function of controlling sulfoxide chirality during the oxidation process. The Δ metal-centered configuration enantioselectively generates an R-configuration sulfoxide, and the Λ configuration enantioselectively forms an S-configuration sulfoxide in the course of the in situ oxidation reaction, thereby resulting in a predetermined chirality of the sulfoxide ligands. The predetermined chirality of sulfoxides (S)-HOSO-R and (R)-HOSO-R were obtained by the treatments of the corresponding sulfoxide complexes Δ-[Ru(bpy)₂{(R )-OSO-R}](PF₆) and Λ-[Ru(bpy)₂{(S)-OSO-R}](PF₆) with trifluoroacetic acid in yields of 90% with 83.5-92.9% ee values.

Full text of DOI:10.1002/cplu.201402243

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DOI: 10.1002/cplu.201402243
Enantioselective Oxidation of Thioethers to Sulfoxides by
Means of a Structural Template with Chiral-at-Metal
Ruthenium Complexes
Zheng-Zheng Li, Su-Yang Yao, and Bao-Hui Ye*[a]
Treatment of cis-[Ru(bpy)2Cl2]·2H2O or D/L-[Ru(bpy)2(py)2]2+
(bpy=2,2’-bipyridine, py=pyridine) with the prochiral thioeth-
er ligands 2-alkylthiobenzoic acid (HOSÀR) produces the corre-
sponding thioether complexes rac-[Ru(bpy)2(OSÀR)](PF6) (R=
Me (rac-1), iPr (rac-2), 2-benzylthiobenzonate (Bn) (rac-3)) and
D/L-[Ru(bpy)2(OSÀR)](PF6) (R=Me (D-1/L-1), iPr (D-2/L-2), Bn
(D-3/L-3)) with retention of the configurations at chiral metal
centers. In situ oxidation of the thioether complexes by meta-
chloroperoxybenzoic acid provides the corresponding sulfox-
ide complexes rac-[Ru(bpy)2(OSOÀR)](PF6) (OSOÀR is 2-alkylsul-
finylbenzonate, R=Me (rac-1a), iPr (rac-2a), Bn (rac-3a)), D-
[Ru(bpy)2{(R)-OSOÀR}](PF6) (R=Me (D-1a), iPr (D-2a), Bn (D-
3a)), and L-[Ru(bpy)2{(S)-OSOÀR}](PF6) (R=Me (L-1a), iPr (L-
2a), Bn (L-3a)) in yields of 95% with 98% ee values. The abso-
lute configurations at the metal centers and sulfur atoms were
determined by means of X-ray crystallography. The results indi-
cate that the configurations of the metal centers are retained
and have the function of controlling sulfoxide chirality during
the oxidation process. The D metal-centered configuration
enantioselectively generates an R-configuration sulfoxide, and
the L configuration enantioselectively forms an S-configura-
tion sulfoxide in the course of the in situ oxidation reaction,
thereby resulting in a predetermined chirality of the sulfoxide
ligands. The predetermined chirality of sulfoxides (S)-HOSOÀR
and (R)-HOSOÀR were obtained by the treatments of the corre-
sponding sulfoxide complexes D-[Ru(bpy)2{(R)-OSOÀR}](PF6)
and L-[Ru(bpy)2{(S)-OSOÀR}](PF6) with trifluoroacetic acid in
yields of 90% with 83.5–92.9% ee values.
Introduction
Chiral sulfoxides are of great interest for organic chemistry and
biochemistry since they serve as intermediates in the synthesis
of numerous sulfur-containing drugs and can also be widely
used as auxiliary ligands in asymmetric synthesis.[1] Several ap-
proaches, including separation of a racemic mixture, transfor-
mation of a reagent from the chiral pool, and the use of
a chiral catalyst for enantioselective synthesis, are used for the
synthesis of chiral sulfoxides. Among them, the enantioselec-
tive transformations using a chiral catalyst are particularly at-
tractive.[2–4] Many excellent asymmetric sulfoxidation reactions
have been developed since the first enantioselective sulfoxida-
tion was reported by the groups of Kagan[5] and Modena[6] in
1984. However, they still have certain disadvantages, which in-
clude low turnover numbers and enantiomeric excess (ee)
value, the need to precisely control the reaction conditions
and the water content, and overoxidation to sulfone, which
has stimulated the search for new efficient catalysts and cata-
lytic reactions.[1]
the use of a chiral-at-metal complex with an achiral ligand as
a source for the asymmetric environment for catalysis has
been still unexploited.[8] This might be limited to the lack of
the available approach for the preparation of the chiral-at-
metal complex in an enantiopure form and depends on its
configurational stability in a catalysis reaction.[9] Octahedral
ruthenium(II) complexes with a d6 electron structure are very
stable and good candidates for insight into the chiral-at-metal
catalysis reaction. About ten years ago, Fontecave and co-
workers reported the first instance of the enantioselective oxi-
dation of thioethers to sulfoxides mediated by the chiral-at-
metal complex D/L-[Ru(dmp)2(MeCN)2]2+ (dmp=2,9-dimethyl-
1,10-phenanthroline).[10] Although its stereoselectivity (ee is 7–
18%) is modest, the chirality mediated by the “chiral-at-metal”
opens a new approach for asymmetric synthesis of sulfoxide
and encourages us to search for an efficient approach for the
synthesis of chiral sulfoxides mediated by means of a chiral-at-
metal ruthenium complex.
Relative to metal-dependent asymmetric catalysis based on
a chiral ligand responsible for the asymmetric environment,[7]
Previous studies have demonstrated that diastereoselectivity
has occurred in the reaction of [Ru(bpy)2Cl2] (bpy=2,2’-bipyri-
dine) and chiral sulfoxides,[9b,c,11] which has been successfully
exploited for the auxiliary-mediated and catalytic synthesis of
chiral ruthenium complexes by Meggers and co-workers.[9g,i]
We perceive that octahedral stereocenters permit the straight-
forward generation of D and L enantiomers, which might
serve as a rigid scaffold for the generation of the enantiomeric
sulfoxides. Crystal structural analysis of the racemic thioether
complex [Ru(bpy)2(OSÀMe)]+ (OSÀMe=2-methylthiobenzo-
[a] Z.-Z. Li, S.-Y. Yao, Prof. Dr. B.-H. Ye
MOE Key Laboratory of Bioinorganic and Synthetic Chemistry
School of Chemistry and Chemical Engineering
Sun Yat-Sen University, Guangzhou 510275 (P. R. China)
Fax: (+86)20 84112245
E-mail: cesybh@mail.sysu.edu.cn
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cplu.201402243.
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dation of metal–thioether in situ to generate enantiomeric sulf-
oxide is unprecedented. The preliminary results for the suc-
cessful synthesis of chiral sulfoxides (R)/(S)-2-isopropylsulfinyl-
benzoic acid (HOSOÀiPr) in situ in good ee values[15] led us to
extend this approach to synthesize other chiral sulfoxides and
systematically investigate the effect of the substituents of thio-
ether on the enantioselectivity. In this study, we present the
synthesis and characterization of a series of RuII complexes
with the prochirally chelate thioether ligands rac-[Ru(bpy)2-
(OSÀR)](PF6) (R=Me (rac-1), iPr (rac-2), 2-benzylthiobenzonate
(Bn) (rac-3)), D/L-[Ru(bpy)2(OSÀR)](PF6) (R=Me (D-1 and L-1),
iPr (D-2 and L-2), Bn (D-3 and L-3)), and their corresponding
oxidation products rac-[Ru(bpy)2(OSOÀR)](PF6) (R=Me (rac-1a),
iPr (rac-2a), Bn (rac-3a)), D-[Ru(bpy)2{(R)-OSOÀR}](PF6) (R=Me
(D-1a), iPr (D-2a), Bn (D-3a)) and L-[Ru(bpy)2{(S)-OSOÀR}](PF6)
(R=Me (L-1a), iPr (L-2a), Bn (L-3a)). Moreover, the predeter-
mined chirality of sulfoxides was also obtained by the treat-
ments of the sulfoxide complexes with trifluoroacetic acid
(TFA) in MeCN in good yields and ee values.
Scheme 1. In situ oxidation of prochiral thioether to chiral sulfoxide.
nate) indicates that the methyl group of the thioether always
keeps away from the a-pyridyl proton to avoid steric conges-
tion and the lone electron pair of the coordinated sulfur atom
sits near the a-pyridyl proton (see Scheme 1).[12] That is to say,
the position of the lone electron pair is predetermined in this
kind of complex. When the thioether is oxidated in situ to the
corresponding sulfoxide OSOÀMe (OSOÀMe=2-methylsulfinyl-
benzonate), the adducted oxygen atom prefers to settle on
the site near the a-pyridyl proton to form a hydrogen bond
between them, thereby leading to the predetermined chirality
of sulfoxide. Although the direct oxidation of metal-bound thi-
olato[13] and thioether[12,14] ligands has been reported, the oxi-
Results and Discussion
Synthesis and structural characterization of thioether com-
plexes
The synthetic procedures for ruthenium complexes and sulfox-
ides are briefly summarized in Scheme 2. The racemic cis-[Ru-
(bpy)2Cl2][16] and chiral D/L-[Ru(bpy)2(py)2]2+[17] (py=pyridine)
were used as the precursors and treated with the prochiral thi-
oether ligands HOSÀR (R=Me, iPr, and Bn) in ethylene glycol
at 1208C under argon protection. The corresponding racemic
products [Ru(bpy)2(OSÀR)](PF6) (rac-1, rac-2, and rac-3) were
obtained in yields of 80–85% after column chromatography
Scheme 2. A summary of the syntheses of sulfoxide compounds.
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separation, and the corresponding chiral products D/L-[Ru-
(bpy)2(OSÀR)](PF6) (D-1, D-2, D-3 and L-1, L-2, L-3) were af-
forded in yields of 71–81% after column chromatography sep-
aration. Although rac-1 and rac-3 have been synthesized using
different methods,[14b,c] they were also synthesized using the
present procedure. As expected, circular dichroism (CD) spectra
of the racemic complexes rac-1, rac-2, and rac-3 are silent,
whereas D/L-1, D/L-2, and D/L-3 are optically active. The CD
spectra of the two enantiomers are almost mirror images with
a positive Cotton effect at 282 nm and a negative Cotton
effect at 295 nm for D-1, and a negative Cotton effect at
281 nm and a positive Cotton effect at 295 nm for L-1 (see
Figure S1 in the Supporting Information), a positive Cotton
effect at 282 nm and a negative Cotton effect at 297 nm for D-
2, and a negative Cotton effect at 281 nm and a positive
Cotton effect at 297 nm for L-2 (see Figure S2), a positive
Cotton effect at 282 nm and a negative Cotton effect at
297 nm for D-3, and a negative Cotton effect at 282 nm and
a positive Cotton effect at 297 nm for L-3 (see Figure S3),
thereby suggesting that the configuration at the metal is the
dominant factor in the appearance of the spectra.
Figure 1. Crystal structures of rac-2 (left, Ru1ÀS1=2.331(1) ꢁ, Ru1À
O1=2.087(2) ꢁ, O1ÀRu1ÀS1=90.27(6)8) and D-3 (right, Ru1ÀS1=2.312(1) ꢁ,
Ru1ÀO1=2.081(3) ꢁ, O1ÀRu1ÀS1=89.63(8)8). ORTEP picture with thermal
ellipsoids drawn at 50% probability. The anion and solvent molecules are
omitted for clarity.
signment of chirality at the metal center is correct and the ab-
solute configuration at the metal center can be retained
during the reaction. The Ru1ÀS1 distance (2.312(1) ꢁ) is com-
parable to the reported one.[12,15] The benzyl group is away
from the a-pyridyl proton to avoid steric congestion and p–p
stacking with the bpy at a distance of 3.358 ꢁ. Moreover,
metal-centered configurations of all thioether complexes pre-
sented here were assigned relative to the two crystal struc-
tures by means of CD spectra.
The NMR spectra of the racemates and enantiomers were
almost identical under the experimental conditions; however,
the two enantiomers become distinguishable in the presence
of (S)-1,1’-binaphthol ((S)-binol) as a chiral NMR spectroscopic
shift reagent.[18] The resonance peak at d=9.09 ppm for rac-
1 was split into a pair of peaks and shifted to d=8.97 and
8.91 ppm (see Figure S4), at d=9.47 ppm for rac-2 was split
and shifted to d=9.30 and 9.22 ppm (see Figure S5), and at
d=9.42 ppm for rac-3 was split and shifted to d=9.29 and
9.23 ppm (see Figure S6). These peaks are assigned to the a-H
of the pyridyl ring in bpy. Moreover, the peaks at d=8.97 ppm
for rac-1, d=9.30 ppm for rac-2, and d=9.29 ppm for rac-3
are consistent with the corresponding L enantiomers, and the
others are assigned to the corresponding D enantiomers. The
spectra show very high enantiopurity in both D and L enan-
tiomers, and these peaks can be used to determine ee values.
The enantiopurities were found to be greater than 98% ee
from the ratio of the integrals of the a-H peaks of the two
enantiomers. The above results demonstrate that the metal-
centered configurations are completely retained in the course
of the reaction.
In situ generation and structural characterization of sulfox-
ide complexes
The oxidation of the thioether complexes to the corresponding
sulfoxide complexes was performed by the modified proce-
dure of Rack[14b,c] with meta-chloroperoxybenzoic acid (m-
CPBA) as an oxidant in methanol (see Scheme 2). It should be
pointed out that the reaction time in OSÀBn complexes was
prolonged to 24 h to afford a satisfactory yield because of an
electron-withdrawing effect of the benzyl group. The excess
amount of m-CPBA and its reduced product, 3-chlorobenzoic
acid, were removed by ultrasonic extraction from the solid
ruthenium product with diethyl ether. The yields were almost
quantitative (95–97%) and no overoxidation product sulfone
complex was found in the reaction. The other oxidant such as
H2O2 was also tried to be used to oxidate the thioether com-
plexes; however, the yield was very low. The identity and for-
mulation of the corresponding complexes was confirmed by
NMR spectroscopy, MS, and IR spectroscopy as well as by ele-
mental analysis (see the Experimental Section).
A new signal at 1105 cmÀ1 for 1a and 1093 cmÀ1 for 2a and
3a, which can be assigned to n(S=O) in the S-bonded com-
plexes and is in good agreement with the literature values for
sulfoxide complexes,[19] indicates that the corresponding sulf-
oxide complexes are indeed generated in situ. The CD spectra
of the enantiomers are almost mirror images, as shown in Fig-
ures 2 and Figure S7, thus indicating that the corresponding
complexes are optically active.
Single crystals of rac-2·H2O were grown from a solution in
methanol. It crystallizes in space group P21/c. As shown in
Figure 1, the chelating thioether is bound to the [Ru(bpy)2]2+
moiety and features a slight twist of the phenyl ring relative to
the carboxylate group. The Ru1ÀS1 distance is 2.331(1) ꢁ,
which is in accord with the reported value.[12,15] As expected,
the isopropyl group of OSÀiPr is away from the a-H of the pyr-
idyl ring to avoid steric congestion. The absolute metal-cen-
tered configuration of the D enantiomer was determined by X-
ray crystallography for two complexes: one preliminary report-
ed instance for D-2[15] in addition to D-3 shown in Figure 1. It
crystallizes in a chiral space group P21. The structure confirms
a D configuration at the ruthenium center, which is consistent
with the configuration of its parent, and the Flack parameter
(À0.016(7)) is close to zero, thus demonstrating that the as-
The 1H and 13C chemical shifts of the alkyl groups (-SÀCH-) at
d=1.83 and 15.61 ppm in 1; d=2.85 and 42.69 ppm in 2; and
d=3.81, 3.21, and 44.23 ppm in 3 are lowfield-shifted relative
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1
Figure 3. The excerpts of the aromatic region H NMR spectra in CD3CN:
(a) rac-3a, (b) rac-3a in the presence of (S)-binol (40 equiv), (c) D-3a in the
presence of (S)-binol (40 equiv), and (d) L-3a in the presence of (S)-binol
(40 equiv).
excesses. The ee values were found to be greater than 98%
from the ratio of the integrals of the a-H peaks of the two
enantiomers, thereby demonstrating that the metal-centered
configurations are stable in the oxidation reaction.
Single-crystal structures of rac-2a·H2O and rac-3a·0.5CH3OH
were measured by X-ray diffraction. As shown in Figure 4 and
Figure S10, the sulfoxide ligands are indeed generated in situ.
The sulfur atoms are bound to RuII ions with Ru1ÀS1 distances
of 2.236(1) ꢁ for rac-2a and 2.269(1) ꢁ for rac-3a; these values
are in accord with the reported distance for rac-1a
(2.213(1) ꢁ)[12] but shorter than those of thioether complexes
(2.331(1) ꢁ for rac-2 and 2.312(1) ꢁ for D-3). The SÀO bond
lengths of 1.478(2) ꢁ for rac-2a and 1.505(5) ꢁ for rac-3a are
consistent with the reported bond lengths.[12,15] As expected,
the methyl and benzyl groups of the sulfoxide ligands are far
from the a-H of the pyridyl ring to avoid steric congestion and
the added oxygen atoms indeed sit on the site near the a-pyr-
idyl proton with a hydrogen bond to each other (C20···O3=
3.430 ꢁ for rac-2a and C10···O3=3.312 ꢁ for rac-3a). The sulf-
oxide complexes are racemic; therefore, four isomers (D-R, D-S,
Figure 2. CD spectra of L-1a and D-1a (above), and L-3a and D-3a
(below) in CH3CN (40 mm).
to the corresponding d=2.64 and 47.44 ppm in 1a; d=3.14
and 59.66 ppm in 2a; and d=4.14, 3.82, and 64.10 ppm in 3a,
also indicating that the oxygen atom is definitely added to the
sulfur atom because of the strong electron-withdrawing effect
of the sulfoxide group. The excerpts of the aromatic region
1H NMR spectra in CD3CN are shown in Figure 3 and Figures S8
and S9. The peak at d=9.13 ppm for rac-1a, d=9.22 ppm for
rac-2a, and d=9.27 ppm for rac-
3a, which are assigned to the a-
H of the pyridine ring, is split
into two peaks and highfield-
shifted to d=9.01 and 8.95 ppm,
d=9.10 and 9.05 ppm, and d=
9.19 and 9.12 ppm, respectively,
in the presence of (S)-binol
(40 equiv) as a chiral NMR spec-
troscopic shift reagent. These
values are in accord with those
of L and D enantiomers. The
spectra show very high enantio-
purity in both D and L enantio-
Figure 4. Crystal structures of rac-2a (Ru1ÀS1=2.236(1) ꢁ, Ru1ÀO1=2.090(2) ꢁ, S1ÀO3=1.478(2) ꢁ; left, D-R
isomer; right, L-S isomer). ORTEP picture with thermal ellipsoids drawn at 50% probability. The anion and solvent
mers, and these peaks can be
used to determine enantiomeric molecules are omitted for clarity.
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L-R, and L-S) would be generated in theory during the oxida-
tion reaction. However, structural analyses show that only two
configurations D-R and L-S were found in the crystal struc-
tures. The absolute stereochemistry at the sulfur atom is as-
signed to the R configuration in the D metal-centered configu-
ration according to the Cahn–Ingold–Prelog priority rules, and
the S configuration of sulfoxide is named in the L metal-cen-
tered configuration complex. That is, the D metal-centered
configuration enantioselectively generates the R-configuration
sulfoxide, and the L configuration enantioselectively forms the
S-configuration sulfoxide in the course of the oxidation reac-
tion, thereby resulting in predetermined chirality for the sulfox-
ides. Moreover, the retention of the absolute metal-centered
configuration and enantioselective generation of chiral sulfox-
ide complexes D-2a and L-2a in the course of oxidation reac-
tion were also observed by means of X-ray crystallography,
which have been reported by us recently.[15]
no free ligand was obtained under similar conditions, thus in-
dicating the acidolysis reaction was suppressed. By increasing
the concentration of acetic acid the acidolysis reaction was
promoted; however, the racemization occurred simultaneously,
thereby demonstrating that an acid of appropriate strength
plays a crucial role in the acidolysis reaction. Furthermore,
CH3OH and CH2Cl2 solvents were used in place of CH3CN under
identical conditions. However, the fact that only partial acidoly-
sis reaction occurred to afford (R)-HOSOÀBn in yields of 43 and
57%, respectively, indicates that a coordinated solvent such as
CH3CN is advantageous for the removal of the ligand. No sig-
nificant racemization of sulfoxide was observed in CH2Cl2 and
CH3OH solvents, and the enantiopurity came close to that in
CH3CN.
The identity and formulation of the corresponding com-
pounds were confirmed by NMR spectroscopy, MS, and IR
spectroscopy as well as elemental analysis (see the Experimen-
tal Section). As shown in Figure 5 and Figure S11, the sulfox-
ides are optically active. Their CD spectra are mirror images
with a Cotton effect at 293 nm for (R)/(S)-HOSOÀMe, 296 nm
for (R)/(S)-HOSOÀiPr, and 300 nm for (R)/(S)-HOSOÀBn in
CH2Cl2.
To optimize the synthetic procedures, a one-pot method
was also developed. After reaction of the chiral precursors D/
L-[Ru(bpy)2(py)2]2+ with the prochiral HOSÀR (R=Me, iPr, and
Bn) ligands in ethylene glycol for four hours, oxidant m-CPBA
(2 equiv) in methanol was directly added to the above reaction
mixtures. The corresponding sulfoxide complexes were ob-
tained in yields of 73–83% after separation over several steps
(see the Experimental Section). The CD spectra show that they
are optically active. Their enantiopurities were determined by
NMR spectra in the presence of (S)-binol as a chiral shift re-
agent. The ee values were found to be greater than 98% from
the ratio of the integrals of the a-H peaks of the two enantio-
mers, thus demonstrating that the absolute configurations at
the metal centers are retained over the course of reactions and
the configurations of the metal centers have the function of
controlling sulfoxide chirality in situ oxidation, thus leading to
enantioselective oxidation of prochiral thioether to chiral sulf-
oxide.
Acidolysis of sulfoxide complexes to afford chiral sulfoxides
under retention of configuration
Following the method of Meggers et al.,[20] the sulfoxide li-
gands can be removed in the presence of acid since the bind-
ing strength of the sulfoxide ligand would be decreased
through protonation of the carboxylate. Indeed, following
treatments of the sulfoxide complexes with TFA in CH3CN at
808C for 2 h in the dark, the pure sulfoxides HOSOÀR were iso-
lated in yields of 90–92%. The chirality at metal centers and
sulfoxides are configuration-stable under the experimental
conditions.[15] To examine the influence of acidolysis conditions
on the configuration of sulfoxide,[21] the acidolysis of L-3a was
observed in various acids and solvents. When HCl (5 equiv)
was used in place of TFA in CH3CN, the acidolysis reaction
went smoothly to afford the (R)-HOSOÀBn ligand in a yield of
96%. However, the ee value decreased to 7.9% as determined
1
by H NMR spectra using (S)-binol as a chiral shift reagent. Fur-
ther experiments showed that (R)-HOSOÀBn is configuration-
unstable under the experimental conditions.[22] When acetic
acid (20 equiv) was added to the solution of L-3a in CH3CN,
Figure 5. CD spectra of (R)/(S)-HOSOÀMe (above) and (R)/(S)-HOSOÀBn
(below) in CH2Cl (40 mm).
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Although the NMR spectra of the two enantiomers are iden-
tical, the 1H NMR spectra of (R)/(S)-HOSOÀMe and (R)/(S)-
HOSOÀBn become distinguishable in the presence of (S)-binol
(10 equiv) as a chiral NMR spectroscopic shift reagent. The
methyl resonance peak at d=2.96 ppm for HOSOÀMe was
split into a pair of peaks and shifted to d=2.82 and 2.80 ppm
in the presence of (S)-binol, which are in accord with the R and
S enantiomers (see Figure S12), respectively. The enantiopuri-
ties were found to be 85.0% ee for the R configuration and
83.5% ee for the S configuration on the basis of the ratio of
the integrals of the methyl peaks of the two enantiomers. For
HOSOÀBn, the double peaks for methylene at d=4.48 and
4.44 ppm were split into triple peaks at d=4.34, 4.30, and
4.26 ppm in the presence of (S)-binol (see Figure S13). The ee
values were found to be 92.9% for the R configuration and
92.5% for the S configuration from the ratio of the integrals of
the methylene peaks of the two enantiomers. Furthermore, the
enantiopurities of (R)-HOSOÀBn and (S)-HOSOÀBn were also
determined by chiral HPLC analysis (see Figure 6) and found to
Figure 7. Crystal structures of (R)-HOSOÀMe (left, S1ÀO3=1.521(2) ꢁ, S1À
C7=1.810(2) ꢁ, S1ÀC8=1.793(2) ꢁ, O3-S1-C8=103.9(1)8, O3-S1-
C7=103.8(1)8, C7-S1-C8=98.6(1)8) and (R)-HOSOÀBn (right, S1À
O3=1.508(2) ꢁ, S1ÀC7=1.808(3) ꢁ, S1ÀC8=1.831(2) ꢁ, O3-S1-C8=101.8(1)8,
O3-S1-C7=104.8(1)8, C7-S1-C8=99.6(1)8). ORTEP picture with thermal ellip-
soids drawn at 50% probability.
ing the assignment of chirality at the sulfur centers is correct.
It should be pointed out that the absolute stereochemistry at
the sulfur atoms changes from S to R upon removal of coordi-
nation according to the Cahn–Ingold–Prelog priority rules. The
S1ÀO3 distances of 1.521(2) ꢁ for (R)-HOSOÀMe and 1.508(2) ꢁ
for (R)-HOSOÀBn are comparable to the reported ones
(1.515(2) ꢁ).[14b] Moreover, the sulfur atom configurations of all
sulfoxides presented here were assigned relative to the two
crystal structures by means of CD spectra.
Conclusion
We have demonstrated here that the absolute configurations
at the ruthenium centers of these complexes are completely
retained during the formation of thioether complexes and oxi-
dation in situ to generate sulfoxide complexes. Moreover, the
configurations of the metal centers have the function of con-
trolling sulfoxide chirality during the oxidation process, thus
generating the predictable chirality of the sulfoxides. The chiral
sulfoxides can be obtained by treatments of the corresponding
sulfoxide complexes with TFA in CH3CN in high yields with re-
tention of their configurations. The oxidation that generates
enantiomeric sulfoxide in situ is unprecedented, which could
provide a new approach for the enantioselective synthesis of
sulfoxides.
Figure 6. HPLC traces demonstrating the enantiopurity of synthetic HOSOÀ
Bn: (a) rac-HOSOÀBn, (b) (R)-HOSOÀBn, and (c) (S)-HOSOÀBn.
be 92.6 and 92.9% ee, respectively. The enantiopurities calcu-
lated from the two techniques are comparable. We also at-
tempted to measure the enantiopurities of (R)/(S)-HOSOÀiPr by
1
means of their H NMR spectra using (S)-binol as a chiral shift
reagent; however, the two enantiomers were indistinguishable
under the experimental conditions. Therefore, their enantio-
purities were estimated by chiral HPLC analysis (see Figure S14)
and found to be 88.2% ee for (R)-HOSOÀiPr and 91.6% ee for
(S)-HOSOÀiPr. The increase in ee values from the methyl (83.5
and 85.0%) to the isopropyl (88.2 and 91.6%) to the benzyl
group (92.6 and 92.9%) indicates that the bulky substituent on
thioether is advantageous for the enantioselectivity of sulfox-
ide. Apparently, the bulky substituent on sulfoxide should in-
crease its stability during the acidolysis process.
Experimental Section
General procedures
All chemicals were commercially available and used as purchased
unless otherwise noted. All manipulations were carried out under
an Ar or N2 atmosphere unless otherwise noted. The reactions that
involved the formation of chiral ruthenium complexes were carried
out in the dark as a precaution against light-induced decomposi-
tion and isomerization. Column chromatography was performed
with silica gel (200–300 mesh) under reduced light. The precursors
The absolute configurations of (R)-HOSOÀMe and (R)-HOSOÀ
Bn were determined by means of X-ray crystallography. As
shown in Figure 7, they all crystallize in the chiral space group
P212121. The structures verify an R configuration at the sulfur
center, and the Flack parameters (0.01(2) for (R)-HOSOÀMe and
À0.01(2) for (R)-HOSOÀBn) are close to zero, thus demonstrat-
[Ru(bpy)2Cl2]·2H2O,[16]
D-[Ru(bpy)2(py)2][O,O’-dibenzoyl-d-tartra-
te]·12H2O,[17] and L-[Ru(bpy)2(py)2][O,O’-dibenzoyl-l-tartrate]·12
H2O,[17] and the prochiral sulfide ligands OSÀiPr[23] and OSÀBn[24]
were synthesized according to methods described in the literature.
Elemental (C, H, N, and S) analyses were performed with an Ele-
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mentar Vario EL analyzer. Electrospray ionization mass spectra (ESI-
MS) were obtained with a Thermo LCQ DECA XP mass spectrome-
159.81, 159.05, 158.88, 158.27, 155.33, 154.17, 152.42, 151.16,
142.93, 138.61, 137.97, 137.56, 136.65, 136.52, 133.62, 131.73,
130.38, 128.54, 127.96, 127.14, 126.93, 126.49, 124.77, 124.70,
124.22, 123.99, 42.69, 22.24, 21.59 ppm; IR (KBr): n˜ =3428, 1593,
1552, 1464, 1444, 1361, 842, 763, 731, 557 cmÀ1; ESI-MS: m/z: 608
[MÀPF6]+; elemental analysis calcd (%) for C30H27F6N4O2PRuS: C
47.81, H 3.61, N 7.43, S 4.25; found: C 47.65, H 3.80, N 7.32, S 3.98.
Compound D-2: Yield: 81% (D-[Ru(bpy)2(py)2][O,O’-dibenzoyl-d-tar-
trate]·12H2O was used as the precursor); ee 98% (determined by
1H NMR spectroscopy using (S)-binol as a chiral shift reagent); CD
(MeCN): l (De)=282 (+42), 297 nm (À106mÀ1 cmÀ1). Compound
L-2: Yield: 81% (L-[Ru(bpy)2(py)2][O,O’-dibenzoyl-l-tartrate]·12H2O
1
ter. H and 13C NMR spectra were obtained with a Varian Mercury-
Plus 300 spectrometer by using the chemical shift of the solvent as
an internal standard. IR spectra were obtained with a Nicolet 330
FTIR spectrometer. CD spectra were recorded with a JASCO J-810
CD spectropolarimeter (1 s response, 3.41 nm bandwidth, scanning
speed of 200 nmminÀ1, accumulation of 3 scans). Chiral HPLC chro-
matogram analyses were carried out with a Shimadzu LC 20 with
an SPD-20A UV detector (Daicel Chiralpak AY-H column, 250 mmꢂ
4.6 mm, hexane/(EtOH/MeOH)/TFA=85:(3:1)15:0.3, flow rate:
1 mLminÀ1, column temperature 358C, 254 nm).
1
was used as the precursor); ee 98% (determined by H NMR spec-
troscopy using (S)-binol as a chiral shift reagent); CD (MeCN):
l (De)=281 (À40), 297 nm (+101mÀ1 cmÀ1).
General procedures for the preparation of the thioether
complexes
A
ruthenium precursor (0.5 mmol), thioether ligand HOSÀR
Compound rac-3
(0.6 mmol), K2CO3 (0.25 mmol), and ethylene glycol (4 mL) were
added into a three-necked flask. The mixture was magnetically
stirred and heated at 1208C for 4 h under argon protection. Then
a saturated aqueous KPF6 solution (15 mL) was added to the
cooled reaction mixture. CH2Cl2 (3ꢂ15 mL) was used to extract the
product. The organic extract was subjected to silica gel column
chromatography with CH3CN and later CH3CN/H2O/KNO3 (saturat-
ed)=100:1:0.4 solution as eluents. After removal of the solvent,
water (20 mL) was used to dissolve the resulting product, and an
excess amount of solid KPF6 was added to the solution. Then
CH2Cl2 (15 mL) was added to the solution and the layers were sep-
arated. The aqueous phase was extracted with CH2Cl2 (2ꢂ10 mL),
and the combined organic phase were dried over MgSO4, filtered,
concentrated, and dried under high vacuum.
Yield: 80% (cis-[Ru(bpy)2Cl2] was used as the precursor, reaction
conditions: 1208C, 6 h). 1H NMR (300.1 MHz, CD3CN): d=9.42 (d,
1H), 8.59 (d, 1H), 8.37 (d, 1H), 8.29 (t, 2H), 8.20 (m, 2H), 7.97 (d,
1H), 7.84 (m, 4H), 7.69 (d, 1H), 7.26 (m, 6H), 7.10 (t, 2H), 6.98 (t,
2H), 6.62 (d, 2H), 3.81 (d, 1H), 3.21 ppm (d, 1H); 13C NMR
(75.5 MHz, CD3CN): d=170.41, 160.03, 159.10, 158.70, 158.12,
155.38, 154.05, 152.23, 151.20, 141.79, 138.51, 138.12, 137.63,
136.56, 134.88, 134.62, 133.99, 131.47, 130.30, 129.60, 129.57,
129.37, 129.22, 129.19, 128.52, 128.41, 127.07, 126.98, 126.65,
124.74, 124.65, 124.22, 124.14, 44.23 ppm; IR (KBr): n˜ =3428, 1585,
1552, 1467, 1444, 1421, 1345, 841, 766, 728, 696, 557, 465 cmÀ1
;
ESI-MS: m/z: 657 [MÀPF6]+; elemental analysis calcd (%) for
C34H27F6N4O2PRuS: C 50.94, H 3.39, N 6.99, S 4.00; found: C 50.81, H
3.42, N 6.80, S 3.91. Compound D-3: Yield: 76% (D-[Ru(bpy)2(py)2]
[O,O’-dibenzoyl-d-tartrate]·12H2O was used as the precursor); ee
98% (determined by 1H NMR spectroscopy using (S)-binol as
a chiral shift reagent); CD (MeCN): l (De)=234 (À15), 282 (+62),
297 nm (À169mÀ1 cmÀ1). Compound L-3: Yield: 76% (L-[Ru-
(bpy)2(py)2][O,O’-dibenzoyl-l-tartrate]·12H2O was used as the pre-
cursor); ee 98% (determined by 1H NMR spectroscopy using (S)-
binol as a chiral shift reagent); CD (MeCN): l (De)=235 (+15), 282
(À60), 297 nm (+165mÀ1 cmÀ1).
Compound rac-1
Yield: 85% (cis-[Ru(bpy)2Cl2] was used as the precursor; EtOH
(18 mL) and H2O (2 mL) were used as solvent at 908C for 6 h).
Compound D-1: Yield: 71% (D-[Ru(bpy)2(py)2][O,O’-dibenzoyl-d-tar-
trate]·12H2O was used as the precursor); ee 98% (determined by
1H NMR spectroscopy by using (S)-binol as a chiral shift reagent).
1H NMR (300.1 MHz, CD3CN): d=9.09 (d, 1H), 8.79 (d, 1H), 8.47 (d,
1H), 8.34 (t, 3H), 8.17 (t, 1H), 7.92 (m, 3H), 7.77 (m, 3H), 7.57 (d,
1H), 7.48 (d, 1H), 7.40 (m, 2H), 7.27 (dd, 2H), 7.17 (t, 1H), 1.83 ppm
(s, 3H); 13C NMR (75.5 MHz, CD3CN): d=168.21, 160.45, 160.14,
159.02, 158.64, 155.09, 153.97, 153.50, 153.32, 143.92, 137.78,
137.11, 137.03, 136.50, 133.34, 132.17, 127.42, 126.84, 126.78,
126.22, 125.41, 124.76, 124.33, 124.04, 123.91, 123.52, 123.29,
15.61 ppm; IR (KBr): n˜ =3433, 1601, 1550, 1467, 1443, 1426, 1365,
841, 763, 557 cmÀ1; CD (MeCN): l (De)=282 (+42), 295 nm
(À110 mÀ1 cmÀ1); ESI-MS: m/z: 581 [MÀPF6]+; elemental analysis
calcd (%) for C28H23F6N4O2PRuS: C 46.35, H 3.19, N 7.72, S 4.42;
found: C 46.21, H 3.25, N 7.64, S 4.35. Compound L-1: Yield: 71%
(L-[Ru(bpy)2(py)2][O,O’-dibenzoyl-l-tartrate]·12H2O was used as the
General procedures for the preparation of the sulfoxide
complexes
Method A: A ruthenium thioether complex (2.5 mmol) and m-CPBA
(5 mmol) were dissolved in methanol (100 mL). The solution was
stirred at room temperature for 4 h. The solvent was removed
under reduced pressure to yield a yellow-orange solid. Using Et2O
(3ꢂ20 mL) to ultrasonically extract the solid for 10 min, the result-
ing solid was filtered, washed with Et2O, and air-dried.
precursor); CD (MeCN):
108mÀ1 cmÀ1).
l
(De)=281 (À39), 295 nm (+
Compound rac-1a
Yield: 95% (rac-1 was used as the starting material). Compound D-
1a: Yield: 95% (D-1 was used as the starting material); ee 98%
(determined by 1H NMR spectroscopy using (S)-binol as a chiral
shift reagent). 1H NMR (300.1 MHz, CD3CN): d=9.13 (d, 1H), 8.84 (d,
1H), 8.61 (d, 1H), 8.37 (m, 4H), 8.20 (d, 1H), 8.05 (t, 1H), 7.93 (m,
4H), 7.79 (m, 2H), 7.57 (t, 1H), 7.49 (d, 1H), 7.40 (t, 1H), 7.26 (t,
2H), 2.64 ppm (s, 3H); 13C NMR (75.5 MHz, CD3CN): d=166.61,
158.83, 158.74, 158.46, 156.84, 156.10, 154.27, 152.66, 150.79,
143.99, 140.24, 140.02, 138.84, 138.35, 133.95, 133.78, 133.21,
Compound rac-2
Yield: 80% (cis-[Ru(bpy)2Cl2] was used as the precursor). 1H NMR
(300.1 MHz, CD3CN): d=9.46 (d, 1H), 8.66 (d, 1H), 8.54 (d, 1H), 8.36
(d, 1H), 8.27 (m, 3H), 8.06 (d, 1H), 7.87 (m, 4H), 7.73 (d, 1H), 7.63
(d, 1H), 7.41 (m, 2H), 7.24 (m, 3H), 7.12 (t, 1H), 2.85 (m, 1H), 0.78
(d, 3H), 0.51 ppm (d, 3H); 13C NMR (75.5 MHz, CD3CN): d=171.05,
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132.35, 131.19, 128.91, 127.78, 127.64, 127.55, 125.43, 124.67,
124.62, 124.47, 47.44 ppm; IR (KBr): n˜ =3431, 1604, 1557, 1466,
1446, 1425, 1367, 1105, 843, 764, 558 cmÀ1; CD (MeCN): l (De)=
275 (+16), 292 nm (À39mÀ1 cmÀ1); ESI-MS: m/z: 596 [MÀPF6]+; ele-
mental analysis calcd (%) for C28H23F6N4O3PRuS: C 45.29, H 3.26, N
7.54, S 4.32; found: C 45.11, H 3.45, N 7.34, S 4.21. Compound L-
1a: Yield: 95% (L-1 was used as the starting material); ee 98%
(determined by 1H NMR spectroscopy using (S)-binol as a chiral
cooling, methanol (50 mL) and m-CPBA (35 mg, 0.20 mmol) were
added to the reaction mixture. The resulting solution was stirred at
room temperature for 4 h (24 h for 3). After that, the methanol sol-
vent was removed under reduced pressure and a saturated aque-
ous KPF6 solution (5 mL) was added. CH2Cl2 (3ꢂ20 mL) was used to
extract the product, and the organic extract was subjected to
silica-gel column chromatography with acetonitrile and later
CH3CN/H2O/KNO3 (saturated)=100:1:0.4 as eluents. After removal
of the solvent, water (10 mL) was used to dissolve the resulting
product, and an excess amount of solid KPF6 was added into the
solution. Then CH2Cl2 (15 mL) was added to the solution and the
layers were separated. The aqueous phase was further extracted
with CH2Cl2 (2ꢂ10 mL), and the combined organic phase was
dried over MgSO4, filtered, concentrated, and dried under high
vacuum. Yield: 73% for D-1a and L-1a, 83% for D-2a and L-2a,
and 81% for D-3a and L-3a. The ee values are >98% (determined
shift reagent); CD (MeCN):
38mÀ1 cmÀ1).
l
(De)=275 (À11), 293 nm (+
Compound rac-2a
Yield: 96% (rac-2 was used as the starting material). 1H NMR
(300.1 MHz, CD3CN): d=9.22 (d, 1H), 8.99 (d, 1H), 8.60 (d, 1H), 8.33
(m, 3H), 8.28 (d, 1H), 8.05 (t, 2H), 7.93 (m, 4H), 7.74 (dd, 2H), 7.53
(t, 1H), 7.37 (t, 1H), 7.27 (m, 3H), 3.14 (m, 1H), 0.68 (d, 3H),
0.58 ppm (d, 3H); 13C NMR (75.5 MHz, CD3CN) d=169.07, 158.90,
158.87, 158.69, 157.14, 156.26, 154.41, 152.72, 150.36, 140.08,
139.99, 139.22, 138.57, 138.24, 136.20, 133.53, 133.06, 131.20,
128.94, 127.61, 127.47, 127.17, 126.42, 125.35, 124.55, 124.43,
124.38, 59.66, 16.98, 14.97 ppm; IR (KBr): n˜ =3429, 1603, 1556,
1455, 1445, 1359, 1093, 843, 765, 730, 555 cmÀ1; ESI-MS: m/z: 624
[MÀPF6]+; elemental analysis calcd (%) for C30H27F6N4O3PRuS: C
46.82, H 3.54, N 7.28, S 4.17; found: C 46.51, H 3.68, N 7.46, S 4.01.
Compound D-2a: Yield: 96% (D-2 was used as the starting materi-
1
by H NMR spectroscopy using (S)-binol as a chiral shift reagent).
General procedures for the preparation of the sulfoxide
compounds
A ruthenium sulfoxide complex (0.1 mmol), trifluoroacetic acid
(0.5 equiv), and CH3CN (3 mL) were added into a three-necked
flask. The mixture was magnetically stirred and heated at 808C for
2 h under argon protection. The reaction mixture was cooled to
room temperature and concentrated to give an orange solid. After
addition of H2O (10 mL) to the orange solid, the aqueous phase
was extracted with Et2O (3ꢂ15 mL). The Et2O solutions were com-
bined and dried over MgSO4 and then filtered. The solvent was re-
moved under reduced pressure and dried under high vacuum to
give a white powder.
1
al); ee 98% (determined by H NMR spectroscopy using (S)-binol as
a chiral shift reagent); CD (MeCN): l (De)=276 (+13), 292 nm
(À52mÀ1 cmÀ1). Compound L-2a: Yield: 96% (L-2 was used as the
starting material); ee 98% (determined by 1H NMR spectroscopy
using (S)-binol as a chiral shift reagent); CD (MeCN): l (De)=277
(À21), 293 nm (+51mÀ1 cmÀ1).
rac-HOSOÀMe
Compound rac-3a
Compound rac-1a was used as the starting material. Yield: 90%.
(S)-HOSOÀMe: Compound D-1a was used as the starting material.
Yield: 97% (rac-3 was used as the starting material, reaction time
was 24 h). Compound D-3a: Yield: 95% (D-3 was used as the start-
ing material, reaction time was 24 h); ee 98% (determined by
1H NMR spectroscopy using (S)-binol as a chiral shift reagent).
1H NMR (300.1 MHz, CD3CN): d=9.26 (d, 1H), 8.81 (d, 1H), 8.55 (d,
1H), 8.36 (m, 4H), 8.06 (t, 1H), 7.93 (m, 5H), 7.38 (m, 5H), 7.19 (m,
3H), 7.02 (t, 2H), 6.54 (d, 2H), 4.14 (d, 1H), 3.82 ppm (d, 1H);
13C NMR (75.5 MHz, CD3CN): d=169.28, 158.83, 158.76, 158.55,
156.92, 156.22, 154.26, 152.63, 150.55, 140.38, 140.22, 140.17,
138.91, 138.40, 133.85, 133.10, 131.78, 131.75, 131.27, 131.26,
129.14, 129.13, 128.88, 128.72, 128.70, 127.91, 127.56, 127.54,
126.65, 125.61, 124.69, 124.57, 124.49, 64.10 ppm; IR (KBr): n˜ =
3440, 1595, 1557, 1468, 1446, 1424, 1349, 1093, 842, 766, 730, 698,
1
Yield: 90%; ee 83.5% (determined by H NMR spectroscopy using
1
(S)-binol as a chiral shift reagent). H NMR (300.1 MHz, CD3Cl): d=
8.32 (d, 1H), 8.16 (d, 1H), 7.87 (t, 1H), 7.61 (t, 1H), 2.96 ppm (t,
3H); 13C NMR (75.5 MHz, CD3Cl): d=167.98, 149.00, 134.40, 131.65,
130.68, 127.08, 124.35, 43.77 ppm; IR (KBr): n˜ =2920, 2852, 2458,
1812, 1699, 1588, 1441, 1306, 1113, 1051, 972, 948, 802, 752, 693,
517 cmÀ1; CD (CH2Cl2): l (De)=295 nm (À41mÀ1 cmÀ1); ESI-MS: m/
z: 183 [MÀH]À; elemental analysis calcd (%) for C8H8O3S: C 52.16, H
4.38, S 17.41; found: C 52.00, H 4.40, S 17.35. (R)-HOSOÀMe: Com-
pound L-1a was used as the starting material. Yield: 90%; ee
85.0% (determined by 1H NMR spectroscopy using (S)-binol as
a chiral shift reagent); CD (CH2Cl2): l (De)=293 nm (+43mÀ1 cmÀ1).
558, 505 cmÀ1
;
CD (MeCN):
l
(De)=275 (+20), 292 nm
(À92mÀ1 cmÀ1); ESI-MS: m/z: 673 [MÀPF6]+; elemental analysis
calcd (%) for C34H27F6N4O3PRuS: C 49.94, H 3.33, N 6.85, S 3.92;
found: C 49.77, H 3.47, N 6.68, S 3.85. Compound L-3a: Yield:
95% (L-3 was used as the starting material, reaction time was
rac-HOSOÀiPr
Compound rac-2a was used as the starting material. Yield: 91%.
1H NMR (300.1 MHz, CDCl3) d=8.18 (m, 2H), 7.81 (t, 1H), 7.59 (t,
1H), 3.27 (m, 2H), 1.54 (d, 1H), 0.98 ppm (t, 1H); 13C NMR
(75.5 MHz, CD3Cl): d=167.92, 145.41, 133.38, 131.89, 130.39,
127.66, 126.08, 52.97, 18.80, 12.05 ppm; IR (KBr): n˜ =2966, 2928,
2473, 1896, 1689, 1586, 1467, 1281, 1143, 1108, 1055, 1033, 963,
801, 759, 694, 536 cmÀ1; elemental analysis calcd (%) for C10H12O3S:
C 56.58, H 5.70, S 15.11; found: C 56.23, H 5.95, S 14.89; ESI-MS: m/
z: 211 [MÀH]À. (S)-HOSO-iPr: Compound D-2a was used as the
starting material. Yield: approximately 90%; ee 91.6% (determined
1
24 h); ee 98% (determined by H NMR spectroscopy using (S)-binol
as a chiral shift reagent); CD (MeCN): l (De)=275 (À20), 293 nm
(+94mÀ1 cmÀ1).
Method B (one-pot method): D-[Ru(bpy)2(py)2][O,O’-dibenzoyl-d-
tartrate]·12H2O/L-[Ru(bpy)2(py)2][O,O’-dibenzoyl-l-tartrate]·12H2O
(112 mg, 0.1 mmol), HOSÀR (0.12 mmol), K2CO3 (6.9 mg,
0.05 mmol), and ethylene glycol (0.8 mL) were added into a 10 mL
three-necked flask. The mixture was magnetically stirred and
heated at 1208C for 4 h (6 h for 3) under argon protection. After
by chiral HPLC analysis); CD (MeCN):
l
(De)=296 nm
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(À110 mÀ1 cmÀ1). (R)-HOSO-iPr: Compound L-2a was used as the
starting material. Yield: approximately 90%; ee 88.2% (determined
[1] a) I. Fernꢃndez, N. Khiar, Chem. Rev. 2003, 103, 3651; b) J. Legros, J. R.
Dehli, C. Bolm, Adv. Synth. Catal. 2005, 347, 19; c) K. Kaczorowska, Z. Ko-
larska, K. Mitka, P. Kowalski, Tetrahedron 2005, 61, 8315; d) B. Jiang, H.
Huang, J. Luo, Z.-Y. Li, Chin. J. Org. Chem. 2005, 25, 1542; e) Q. Y. Huang,
J. Zhu, J. G. Deng, Chin. J. Org. Chem. 2005, 25, 496; f) M. Mellah, A. Voi-
turiez, E. Schulz, Chem. Rev. 2007, 107, 5133; g) Q. Zeng, Prog. Chem.
by chiral HPLC analysis); CD (MeCN):
l (De)=296 nm (+
108mÀ1 cmÀ1).
´
´
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rac-HOSOÀBn
Compound rac-3a was used as the starting material. Yield: 92%.
(S)-HOSOÀBn: Compound D-3a was used as the starting material.
1
Yield: 90%; ee 92.9% (determined by chiral HPLC analysis). H NMR
(300.1 MHz, CD3CN): d=8.12 (d, 1H), 7.81 (d, 1H), 7.72 (t, 1H), 7.60
(t, 1H), 7.29 (m, 3H), 7.15 (m, 2H), 4.41 (d, 1H), 3.83 ppm (d, 1H);
13C NMR (75.5 MHz, CD3Cl): d=168.37, 145.72, 133.79, 131.54,
131.21, 130.77, 130.65, 130.41, 128.65, 128.39, 128.36, 127.44,
125.75, 61.74 ppm; IR (KBr): n˜ =2870, 2614, 2482, 1700, 1587, 1404,
1303, 1250, 1144, 1109, 1060, 1006, 793, 746, 695, 641, 496 cmÀ1
;
CD (MeCN): l (De)=300 nm (À128mÀ1 cmÀ1); ESI-MS: m/z: 259
[MÀH]À; elemental analysis calcd (%) for C14H12O3S: C 64.60, H 4.65,
S 12.32; found: C 64.55, H 4.67, S 12.9. (R)-HOSOÀBn: Compound
L-3a was used as the starting material. Yield: 90%; ee 92.6% (de-
termined by chiral HPLC analysis); CD (MeCN): l (De)=300 nm (+
126mÀ1 cmÀ1).
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Crystallographic analysis
Crystals suitable for X-ray diffraction were grown from CHCl3 solu-
tions layered with hexane for (R)-HOSOÀMe and (R)-HOSOÀBn,
from CH2Cl2 solutions layered with toluene for D-2, D-2a, and L-
2a, and from CH2Cl2 solutions layered with CH3CH2OH for D-3.
Single crystals of rac-2, rac-2a, and rac-3a were grown from
CH3OH solutions. Single-crystal X-ray diffraction data were collect-
ed with a Rigaku R-AXIS Spider IP diffractometer with graphite-
monochromated MoKa radiation (l=0.71073 ꢁ, for rac-2, rac-2a,
and rac-3a) at 150 K or an Oxford Gemini S Ultra CCD area detec-
tor diffractometer with graphite-monochromated CuKa radiation
(l=1.54178 ꢁ, for D-2, D-2a, L-2a, D-3, (R)-HOSOÀMe, and (R)-
HOSOÀBn) at 160 K. All of the data were corrected for absorption
effects using the multiscan technique. The structures were solved
by direct methods using SHELXS-97 programs[25] and refined by
full-matrix least-squares technique on F2 using SHELXL-97 pro-
grams.[26] Anisotropic thermal parameters were applied to all non-
hydrogen atoms. The organic hydrogen atoms of the ligands were
generated geometrically.
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CCDC 1001979 (rac-2·H2O), 990972 (D-2·0.25CH2Cl2·0.25H2O),
1001980 (rac-2a·H2O), 990805 (D-2a·0.5H2O), 990806 (L-
2a·0.5H2O), 1001981 (D-3·CH3CH2OH), 1001978 (rac-3a·0.5CH3OH),
1001983 ((R)-HOSOÀMe), and 1001982 ((R)-HOSOÀMe) contain the
supplementary crystallographic data for this paper. These data can
be obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Acknowledgements
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This study was supported by the NSF of China (21071154 and
21272284), the DPF of the Ministry of Education of China
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Keywords: chirality
ruthenium · synthetic methods
·
enantioselectivity
·
oxidation
·
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Published online on && &&, 0000
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FULL PAPERS
Delta force: Treatment of D-[Ru-
Z.-Z. Li, S.-Y. Yao, B.-H. Ye*
(bpy)2(py)2]2+ (bpy=2,2’-bipyridine,
py=pyridine) or L-[Ru(bpy)2(py)2]2+
with thioether ligands, then oxidation in
situ, provides the corresponding D-[Ru-
(bpy)2{(R)-OSOÀR}]+ or L-[Ru(bpy)2{(S)-
OSOÀR}]+ with 98% ee values, which
can be converted into the correspond-
ing chiral sulfoxides in yields of 90%
with 83–93% ee values (see scheme).
&& – &&
Enantioselective Oxidation of
Thioethers to Sulfoxides by Means of
a Structural Template with Chiral-at-
Metal Ruthenium Complexes
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These are not the final page numbers!