An effective deracemization of trans-1,2-bis (phenylsulfenyl)cyclohexane

Article abstract of DOI:10.1080/10426500902856420

A chiral S,S-donating C²-symmetric ligand containing two stereogenic centers was prepared from rac-trans-1,2-cyclohexanediol. An effective deracemization of trans-1,2-bis(phenylsulfenyl)cyclohexane was achieved by the catalyzed enantioselective

Full text of DOI:10.1080/10426500902856420

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An Effective Deracemization
of trans-1,2-Bis
(phenylsulfenyl)cyclohexane
Elżbieta Wojaczyńska a & Jacek Skarżewski a
a Department of Organic Chemistry, Faculty of
Chemistry , Wrocław University of Technology ,
Wrocław, Poland
Published online: 28 Apr 2009.
To cite this article: Elżbieta Wojaczyńska & Jacek Skarżewski (2009) An
Effective Deracemization of trans-1,2-Bis (phenylsulfenyl)cyclohexane,
Phosphorus, Sulfur, and Silicon and the Related Elements, 184:5, 1323-1331, DOI:
10.1080/10426500902856420
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Phosphorus, Sulfur, and Silicon, 184:1323–1331, 2009
Copyright © Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500902856420
An Effective Deracemization of trans-1,2-Bis
(phenylsulfenyl)cyclohexane
˙
´
˙
Elzbieta Wojaczynska and Jacek Skarzewski
Department of Organic Chemistry, Faculty of Chemistry, Wroclꢀaw
University of Technology, Wroclꢀaw, Poland
A chiral S,S-donating C2-symmetric ligand containing two stereogenic centers
was prepared from rac-trans-1,2-cyclohexanediol. An effective deracemization of
trans-1,2-bis(phenylsulfenyl)cyclohexane was achieved by the catalyzed enantios-
elective sulfoxidation, followed by the chromatographic separation of diastere-
omeric mono- and bis-sulfoxides. Subsequent recrystallization of enantiomerically
enriched bissulfoxide and its final deoxygenation gave optically pure (1R,2R)-
bis(phenylsulfenyl)cyclohexane.
Keywords Chiral resolution; chiral thioethers; sulfoxidation
INTRODUCTION
A variety of chiral compounds containing thioether functionality
have already been studied as sulfur-donating ligands in asymmetric
catalytic reactions.1 Recently, some C2-symmetric chiral S,S-donating
ligands also have been tested.2–4 However, a direct stereospecific
conversion of the available C2-symmetric chiral vic-diols to the respec-
tive chiral bis(thioethers) was often unsuccessful.4 For this reason,
only a limited number of such ligands has been examined. With our
objective to study the catalytic performance of chalcogen-containing
chiral ligands, we decided to prepare hitherto unknown non-racemic
1,2-bis(phenylsulfenyl)cyclohexane.
For this purpose we have chosen enantioselective sulfoxidation as a
means for the specific introduction of one or two additional stereogenic
Received 18 January 2008; accepted 6 February 2008.
Dedicated to Professor Marian Mikolꢀajczyk, CBMiM PAN in Lo´dz´, Poland, on the
ꢀ
occasion of his 70th birthday.
We are grateful to the Ministry of Science and Higher Education for financial support
(Grant PBZ-KBN-126/T09/2004).
Address correspondence to Jacek Skarz˙ewski, Department of Organic Chemistry, Fac-
ulty of Chemistry, Wroclꢀaw University of Technology, Wybrzez˙e Wyspian´skiego 27, 50-370
Wroclꢀaw, Poland. E-mail: jacek.skarzewski@pwr.wroc.pl
1323

1324
E. Wojaczyn´ska and J. Skarz˙ewski
centers. We hoped that the synthesis of chiral diastereomeric mono- and
bis-sulfoxides derived from cyclohexane and their separation should
allow for the resolution of racemic bis-sulfide.
An application of the enantioselective sulfoxidation and the use of
the sulfinyl group as a chiral auxiliary for the separation of enan-
tiomers has already been reported. Bortolini et al. described the opti-
cal resolution of rac-menthone by its conversion into a 1,3-dithiolane,
followed by enantioselective oxidation.5 Deoxygenation of the sepa-
rated diastereomeric S-oxides and cleavage of nonracemic dithiolanes
afforded menthone with 93% ee.5 We also used a similar procedure
for the resolution of otherwise inseparable diastereomeric alcohols.6
Moreover, due to the C2-symmetry of bis(thioether), we expected an
additional enantiomeric enrichment of product (duplication of enan-
tioselective sulfoxidation). Thus, in this article, we present a tandem
oxidation-deoxygenation reaction as an effective route to homochiral
trans-1,2-bis(phenylsulfenyl)cyclohexane.
RESULTS AND DISCUSSION
The attempted nucleophilic substitution of bis-sulfonyl esters of
enantiomeric (1R,2R)-cyclohexanediol (1) gave
a complex mix-
ture. Also the corresponding reaction of (1R,2R)-1 with phenyl
disulfide–tributylphosphine (the Hata reaction) yielded trans-1,2-
bis(phenylsulfenyl)cyclohexane (2) as a diastereomeric mixture (72%
yield; trans:cis 81:19; 1S, 2S, dextrorotatory, 50% ee) (Scheme 1).4b
SCHEME 1
In order to circumvent these obstacles, we developed an alter-
native procedure to obtain this product in the enantiomerically
pure form. Starting from commercially available racemic trans-1,2-
cyclohexanediol (1), we applied the same Hata reaction conditions. Nu-
cleophilic substitution with this substrate furnished again 2 as the in-
separable mixture of trans and cis isomers (67% yield; trans:cis 82:18).
The obtained mixture was subjected to the catalyzed enantiose-
lective sulfoxidation procedure. The oxidation of sulfides was carried
out using the optimized Bolm catalytic system, based on vanadyl
acetylacetonate, chiral Schiff base 3, and 30% aqueous hydrogen

Deracemisation of trans-1,2-Bis(phenylsulfenyl)cyclohexane
1325
SCHEME 2
peroxide as a stoichiometric oxidant.7,8 We have already used this
catalytic system successfully oxidizing various mono- and bis-sulfides,
applying (S)-(-)-N-(3-phenyl-5-nitrosalicylidene)valinol 3a as a chiral
ligand. The stereochemical outcome of this sulfoxidation for alkyl aryl
sulfides was in agreement with the absolute configuration of the ligand,
i.e., the (S)-ligand gave mainly (S)-arylsulfinyl product.3,6,9 Here we also
examined newly prepared (S)-(-)-N-(3,5-diiodosalicylidene)valinol 3b as
well as the commercially available original Bolm’s ligand 3c (Scheme 2).
As we started with the racemic trans-bis-sulfide, which also con-
tained ca. 18% of cis isomer, one could expect a rather complicated mix-
ture of oxidation products. Fortunately, with all the catalysts used, the
reaction furnished mainly mono- and bis-trans-sulfoxides (67%–80%
total isolated yield) and no overoxidation products (sulphones) were ob-
served. Moreover, the catalytic system limited the number of stereoiso-
mers. Regardless of which of the catalytic ligands 3 was applied, we
were able to isolate and characterize the same two diastereomeric
products of monooxidation 4 and two diastereomeric bis(sulfoxides)

1326
E. Wojaczyn´ska and J. Skarz˙ewski
FIGURE 1 CD spectra of 5a (solid line), 5b (dotted line), and 4a (dashed line)
–4
in acetonitrile solution in 10 M concentration range.
5. A preliminary chromatographic separation (silica gel, hexane:ethyl
acetate, 1:1) led to the fractions containing 4 and 5, respectively. Both
fractions were subjected to the next chromatographic separation. This
way we managed to isolate two diastereomers of 1-phenylsulfinyl-2-
phenylsulfenylcyclohexane (4a and b) and two diastereomers of 1,2-
bis(phenylsulfinyl)cyclohexane (5a and b).
We expected the preference for (SS)-monosulfoxides and (SS, SS)-bis-
sulfoxides, because in all experiments, catalytic (S)-(−)-3 ligands were
used. The configuration assignment of products was verified by the
respective CD spectra. A correlation of the absolute configuration of
phenylsulfinyl chromophore and the sign of the Cotton effects is well
documented.10 The representative spectra are shown in Figure 1. For
all compounds (monosulfoxides 4 and bis-sulfoxides 5) we observed
a positive CE at ca. 220 nm and a negative one at ca. 250–255 nm.
The sign of Cotton effects indicates (S) absolute configuration of sulfur
stereogenic center, and the value of ꢀε is significantly enhanced if two
phenylsulfinyl groups with identical configurations are present.
Diastereomeric monosulfoxides 4a and 4b were then deoxygenated11
using TiCl4 and NaI to give the respective 1,2-bis(phenylsulfe-
nyl)cyclohexanes 2. The results are collected in Table I. Thus, the
deoxygenation of two diastereomeric monosulfoxides yielded two op-
tically active antipodes of trans-1,2-bis(phenylsulfenyl)cyclohexane 2.
Enantiomeric excess was determined using chiral HPLC. Because the

Deracemisation of trans-1,2-Bis(phenylsulfenyl)cyclohexane
1327
TABLE I Deoxygenation of 1-Phenylsulfinyl-2-phenyl
sulfenylcyclohexane 4
[α]D
Yield of
[α]D
ee of
Substrate
(c, CH2Cl2)
2 [%]
(c, CH2Cl2)
2 [%]
4a
4b
–10.4 (0.96)
–114.8 (1.68)
61 (S,S)
65 (R,R)
+43.0 (0.60)
35
40
–49.0 (1.00)
sign of specific rotation of the optically active 2 resulted from deoxy-
genation of 4a was in agreement with that for (+)-(1S, 2S)-2 obtained
from (1R,2R)-1, we assigned 4a as (1S,2S,SS:1R,2R,RS)-4 and 4b as
(1R,2R,SS:1S,2S,RS)-4, respectively (the dominating enantiomer indi-
cated). Enantiomeric excesses were rather moderate, 35% and 40% ee,
respectively.
However, more effective enantioenrichment could be expected for
bis-sulfoxides. The double sulfoxidation should lead to some amplifi-
cation of their enantiomeric excesses (“duplication,” Horeau’s rule).12
Moreover, their crystallization opens the possibility for an addi-
tional increase of optical purity of the final product. Indeed, for
one of the diastereomers (5b), a significant enantiomeric enrich-
ment was observed. After recrystallization from methylene chloride-
hexane mixture, (1R,2R,SS,SS)-1,2-bis(phenylsulfinyl)cyclohexane (5b)
was obtained with enantiomeric purity exceeding 95%. Deoxygenation
of (1R,2R,SS,SS)-1,2-bis(phenylsulfinyl)cyclohexane 5b yielded 97% of
(1R,2R)-2 in over 95% ee (by chiral HPLC).
The signs and values of optical rotation of the deoxygenation prod-
ucts 2 further confirm the postulated configurations of starting mono-
and bis-sulfoxides and are in agreement with those reported for (1S,2S)-
2.4b
CONCLUSION
We described a protocol for the optical resolution of racemic 1,2-
bis(phenylsulfenyl)cyclohexane, comprising the enantioselective sul-
foxidation, separation of diastereomeric bissulfoxides, and the subse-
quent deoxygenation of pure diastereomers.
The above procedure offers an efficient way to enantiomerically
pure (1R,2R)-1,2-bis(phenylsulfenyl)cyclohexane from racemic 1,2-
cyclohexanediol.

1328
E. Wojaczyn´ska and J. Skarz˙ewski
EXPERIMENTAL
General
Melting points were determined using a Boetius hot-stage apparatus
and are uncorrected. IR spectra were recorded on a Perkin Elmer 1600
FTIR spectrophotometer. 1H and 13C NMR spectra were obtained with
a Bruker CPX (1H, 300 MHz) or a Bruker Avance (1H, 500 MHz) spec-
trometer using TMS as an internal standard. UV and CD spectra were
recorded for CH3CN solutions using a Hewlett-Packard 8452 diode ar-
ray spectrophotometer and a JASCO J 600 spectropolarimeter, respec-
tively. Optical rotations were measured using an Optical Activity Ltd.
Model AA-5 automatic polarimeter. GC/MS analyses were determined
on a Hewlett-Packard 5890 II gas chromatograph (25 m capillary col-
umn) with a Hewlett Packard mass spectrometer 5971 A operating on
the electron impact mode (70 eV). High resolution mass spectra were
recorded using a microTOF-Q instrument utilizing electrospray ioniza-
tion mode. Separations of products by chromatography were performed
on silica gel 60 (230–400 mesh) purchased from Merck. Thin layer
chromatography analyses were performed using silica gel 60 precoated
plates (Merck). HPLC measurements were performed on KNAUER
HPLC PUMP 64 using KNAUER Variable Wavelength Monitor and
CHIRACEL OD-H column.
1,2-Bis(phenylsulfenyl)cyclohexane (2) was obtained form racemic
trans-1,2-cyclohexanediol (1) using six equivalents of diphenyl disulfide
and eight equivalents of tributylphosphine as described in our previous
papers.3,4 Yield = 67%, trans:cis 82:18.
N-salicylidenevalinol derivatives 3a and 3b were prepared according
to the procedure in the literature.8
(S)-(-)-N-(3,5-diiodosalicylidene)valinol (3b)
Yield 88%. Mp = 129–130◦C (hexane/diethyl ether). [α]D = –68.8 (c =
1
1.104, CH2Cl2). H NMR (300 MHz, CDCl3): δ = 0.88 (d, J = 4.0 Hz,
3H, Me), 0.90 (d, J = 4.1 Hz, 3H, Me), 1.82–1.91 (m, 1H,∗CH), 3.13–
3.16 (m, 1H, CH(CH3)2), 3.37–3.95 (m, 3H, CH2OH), 7.34 (d, J = 2.1
Hz, 1H, ArH), 7.84 (d, J = 2.1 Hz, 1H, ArH), 7.96 (s, 1H, CH N), 14.61
(s, 1H, OH). 13C NMR (CDCl3): δ = 18.9 and 20.1 (CH(CH3)2), 30.0
(CH(CH3)2), 64.4 (CH2OH), 74.8 (∗CH), 76.5 (C-1ꢀ), 92.9 (C-5ꢀ), 117.7
(C-3ꢀ), 141.3 (C-6ꢀ), 150.2 (C-4ꢀ), 164.9 (CH N), 166.7 (C-2ꢀ). IR (KBr):
3296, 2960, 1638, 1538, 1508, 1438, 1217, 1067, 859, 664 cm−1. Anal.
Calcd. for C12H15NO2I2 (M = 459.062) C, 31.40; H, 3.29; N, 3.05. Found:
C, 31.44; H, 3.26; N, 2.96. HRMS (ESI): Mcalcd for [M+H]+: 459.9270;
Found 459.9286.

Deracemisation of trans-1,2-Bis(phenylsulfenyl)cyclohexane
1329
Oxidation of Sulfides with VO(acac)2 and Chiral Schiff Base
Vanadyl acetylacetonate (5.2 mg, 0.02 mmol) and the ligand 3
(0.03 mmol) were dissolved in a test tube in dichloromethane (4 mL),
and the solution was stirred for 5 min at 25◦C. After the addition of the
bis-sulfide 2 (2 mmol), the solution was cooled to 0◦C, and 30% H2O2
(0.26 mL, 2.3 mmol) was added dropwise during 10 min. The mixture
was stirred for 20 h at 0◦C and extracted with CH2Cl2 (2 × 5 mL). The
combined organic extracts were washed with H2O and brine and dried
over Na2SO4. The solvent was removed in vacuo, the crude product
was submitted to the chromatography on silica gel (hexane:ethyl ac-
etate, 1:1), and the solid products were recrystallized from methylene
chloride/hexane.
(1S,2S,SS:1 R,2R,RS)-1-Phenylsulfinyl-2-
phenylsulfenylcyclohexane (4a)
Yield 15% (for ligand 3a). [α]D = −10.4 (c = 0.96, CH2Cl2). 1H NMR
(300 MHz, CDCl3): δ = 1.00–1.06 (m, 1H, cyclohexane ring), 1.24–1.42
(m, 2H, cyclohexane ring), 1.60–1.76 (m, 3H, cyclohexane ring), 2.12–
2.18 (m, 1H, cyclohexane ring), 2.27–2.36 (m, 1H, cyclohexane ring),
2.96–3.00 (m, 1H, ∗CHS), 3.18–3.25 (m, 1H, ∗CHSO), 7.24–7.35 (m, 3H,
ArH), 7.39–7.44 (m, 2H, ArH), 7.46–7.54 (m, 3H, ArH), 7.67–7.72 (m,
2H, ArH).13C NMR (CDCl3): δ = 22.2 (C-3), 24.0 (C-5), 24.2 (C-4), 32.0
∗
∗
(C-6), 45.0 (C-2, CHS), 65.7 (C-1, CHSO), 126.2, 127.9, 129.3, 129.5,
131.8, 132.7 (CAr), 133.4 (C-1ꢀꢀ, SCAr), 141.0 (C-1’, SOCAr). IR(KBr):
3056, 2934, 2857, 1582, 1475, 1441, 1082, 1041, 750, 692, 530 cm−1
.
Rf = 0.64 (hexane:ethyl acetate 1:1). Anal. Calcd. for C18H20OS2 (M
= 316.483): C, 68.31; H, 6.37; S, 20.26. Found: C, 66.78; H, 6.30; S,
19.70%.13
(1R,2R,SS:1S,2S,RS)-1-Phenylsulfinyl-2-
phenylsulfenylocyclohexane (4b)
1
Yield 23% (for ligand 3a). [α]D = −114.8 (1.68, CH2Cl2). H NMR
(500 MHz, CDCl3): δ = 1.01–1.08 (m, 1H, cyclohexane ring), 1.20–1.28
(m, 2H, cyclohexane ring), 1.44–1.52 (m, 1H, cyclohexane ring), 1.56–
1.68 (m, 3H, cyclohexane ring), 2.13–2.17 (m, 1H, cyclohexane ring),
2.30–2.35 (m, 1H, ∗CHS), 3.17–3.22 (m, 1H, ∗CHSO), 7.15–7.23 (m, 3H,
ArH), 7.34–7.42 (m, 7H, ArH).13C NMR (CDCl3): δ = 21.0 (C-3), 24.5
(C-5), 25.5 (C-4), 34.7 (C-6), 47.2 (C-2, ∗CHS), 67.0 (C-1, ∗CHSO), 124.9,
128.2, 129.4, 130.9, 133.9 (CAr), 133.7 (C-1ꢀꢀ, SCAr), 142.6 (C-1’, SOCAr).
IR(film): 3055, 2935, 1582, 1476, 1442, 1085, 1042, 752, 692, 533 cm−1
Rf = 0.65 (hexane:ethyl acetate 1:1). Anal. Calcd. for C18H20OS2 (M =
.

1330
E. Wojaczyn´ska and J. Skarz˙ewski
316.483): C, 68.31; H, 6.37; S, 20.26. Found: C, 66.59; H, 6.34; S,
19.46%.13
(1S,2S,SS,SS:1R,2R,RS,RS)-1,2-Bis(phenylsulfinyl)cyclohexane
(5a)
Yield 14% (for ligand 3b). Mp = 132–134◦C. [α]D = −208.3 (c =
0.36, CH2Cl2, 40% ee). 1H NMR (300 MHz, CDCl3): δ = 1.45–1.58
(m, 4H, cyclohexane ring), 1.72–1.82 (m, 2H, cyclohexane ring), 2.15–
∗
2.23 (m, 2H, cyclohexane ring), 3.19 (bs, 2H, CH), 7.51–7.54 (m, 6H,
ArH), 7.61–7.65 (m, 4H, ArH). 13C NMR (CDCl3): δ = 22.8, 23.3, 60.9
(∗CHSO), 125.9, 129.7, 132.1, 142.2 (SOCAr). IR(KBr): 3050, 2958,
2828, 1732, 1470, 1442, 1082, 1038, 996, 755, 701, 693, 502 cm−1
.
Rf = 0.21 (hexane:ethyl acetate 1:1). Anal. Calcd. for C18H20O2S2
(M = 332.482): C, 65.02; H, 6.06; S, 19.29. Found: C, 64.64; H, 6.11;
S, 19.09%. HPLC (Chiracel OD-H column, hexane:i-PrOH, 97:3, flow
rate 1.0 mL/min, λ = 205 nm) tR(5a, major) = 17.7 min., tR(5a, minor)
= 15.8 min.
(1R,2R,SS,SS)-1,2-Bis(phenylsulfinyl)cyclohexane (5b)
Yield 15% (for ligand 3b); 7% (after recrystallization from methylene
chloride/hexane). Mp = 123–125◦C. [α]D = −304.6 (c = 0.302, CH2Cl2,
1
>95% ee). H NMR (300 MHz, CDCl3): δ = 1.13–1.20 (m, 2H, cyclo-
hexane ring), 1.37–1.41 (m, 2H, cyclohexane ring), 1.70–1.81 (m, 4H,
∗
cyclohexane ring), 2.94–2.97 (m, 2H, CH), 7.52–7.62 (m, 10H, ArH).
13C NMR (CDCl3): δ = 20.6, 24.3, 61.8 (∗CHSO), 124.9, 129.6, 131.2,
141.1 (SOCAr). IR (KBr): 3439, 3052, 2934, 2860, 1579, 1475, 1441,
1085, 1044, 747, 697, 689, 535 cm−1. Rf = 0.46 (hexane:ethyl acetate
1:1). Anal. Calcd. for C18H20O2S2 (M = 332.482): C, 65.02; H, 6.06; S,
19.29. Found: C, 64.80; H, 6.13; S, 19.27%. HPLC (Chiracel OD-H col-
umn, hexane : i-PrOH, 95:5, flow rate 1.0 mL/min, λ = 205m) tR(5b,
major) = 12.3 min., tR(5b, minor) = 15.8 min.
Sulfoxide Deoxygenation
To a magnetically stirred solution of 1-phenylsulfinyl-2-phenyl-
sulfenylcyclohexane 4 (0.052 g, 0.16 mmol) in CH3CN (5 mL), TiCl4
(0.32 mL of 1 M solution in CH2Cl2, 0.32 mmol) and NaI (0.071 g, 0.48
mmol) were added at room temperature. The mixture turned brown
almost immediately. After 20 h stirring, the mixture was quenched
with diluted aqueous KOH (3 mL) and extracted with Et2O (3 ×
10 mL). The combined organic extracts were washed with 10% aq.
Na2S2O3 (10 mL) and H2O (15 mL), and dried with MgSO4. The sol-

Deracemisation of trans-1,2-Bis(phenylsulfenyl)cyclohexane
1331
vent was removed, and the residue was chromatographed on silica
gel column using hexane:ethyl acetate (4:1 v/v) mixture to give 1,2-
bisphenylsulfenylcyclohexane 2.
For the deoxygenation of bissulfoxides 5 the amounts of TiCl4 and
NaI were doubled.
(1R,2R)-2 from (1R,2R,SS,SS)-5b: Yield 91%. [α]D = −122.0 (c = 0.80,
CH2Cl2, >95% ee). For (1S,2S)-2 [α]D = +120.0 (c2.20, CH2Cl2, > 95%
ee).4bHPLC (Chiracel OD-H column, hexane : i-PrOH, 99.75 : 0.25, flow
rate 1.0 mL/min, λ = 205 nm): tR((1R,2R)-2) = 8.5 min; tR((1S,2S)-2) =
9.3 min.
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[13] The oily samples of 4a and 4b contained ca 2.5% of incombustible impurity (SiO2).