Sulfur-containing chiral bis(oxazolines) tested in copper-catalyzed asymmetric cyclopropanation

Article abstract of DOI:10.1080/00397910500213948

The novel C₂-symmetric sulfur-containing chiral bis(oxazoline) compound has been synthesized and characterized by X-ray crystallography. Highly enantioselective and diastereoselective cyclopropanation reactions have been performed using the cop

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Sulfur‐Containing Chiral
Bis(oxazolines) Tested in
Copper‐Catalyzed Asymmetric
Cyclopropanation
Ming Zhang Gao a , Bo Wang b , Deyuan Kong a
,
Ralph A. Zingaro a , Abraham Clearfield a & Zun Le
Xu b
a Department of Chemistry, Texas A&M University,
College Station, Texas, USA
b School of Chemistry and Chemical Engineering,
Zhongshan University, Guangzhou, China
Available online: 22 Aug 2006
To cite this article: Ming Zhang Gao, Bo Wang, Deyuan Kong, Ralph A. Zingaro,
Abraham Clearfield & Zun Le Xu (2005): Sulfur‐Containing Chiral Bis(oxazolines)
Tested in Copper‐Catalyzed Asymmetric Cyclopropanation, Synthetic Communications:
An International Journal for Rapid Communication of Synthetic Organic Chemistry,
35:20, 2665-2673
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Synthetic Communicationsw, 35: 2665–2673, 2005
Copyright # Taylor & Francis, Inc.
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910500213948
Sulfur-Containing Chiral Bis(oxazolines)
Tested in Copper-Catalyzed Asymmetric
Cyclopropanation
Ming Zhang Gao
Department of Chemistry, Texas A&M University, College Station,
Texas, USA
Bo Wang
School of Chemistry and Chemical Engineering, Zhongshan University,
Guangzhou, China
Deyuan Kong, Ralph A. Zingaro, and Abraham Clearfield
Department of Chemistry, Texas A&M University, College Station,
Texas, USA
Zun Le Xu
School of Chemistry and Chemical Engineering, Zhongshan University,
Guangzhou, China
Abstract: The novel C2-symmetric sulfur-containing chiral bis(oxazoline) compound
has been synthesized and characterized by X-ray crystallography. Highly enantioselec-
tive and diastereoselective cyclopropanation reactions have been performed using the
copper-bis(oxazoline) catalyst.
Keywords: Asymmetric catalysis, bis(oxazolinyl)thiophene, bis(oxazolone), copper,
cyclopropanation
Received in the USA May 18, 2005
Address correspondence to Abraham Clearfield, Department of Chemistry, Texas
A&M University, College Station, TX 77843-3255, USA. E-mail: clearfield@mail.
chem.tamu.edu.
2665

2666
M. Z. Gao et al.
During the past decade, important advances in the field of asymmetric
catalysis have been made.[1] The cyclopropanation of alkenes with diazoace-
tate has been one of the highlights of metal-catalyzed asymmetric synthesis
since the report of Nozaki and coworkers.[2] Catalysts containing various
metals and chiral ligands, such as Schiff Base–Cu,[3] salen–Co,[4] Rh2(5S-
MEPY),[5] porphyrin–Ru,[6] and especially the C2-symmetric chiral bis(oxa-
zolines) metal complexes, have proved to be efficient stereoselective catalysts
for a wide range of asymmetric cyclopropanations.[7] Typical examples are the
metal complexes of chiral bis(oxazolinyl)propane 1 and bis(oxazolinyl)pyri-
dine 2, which gave desirable enantioselectivity.[8]
Recently, we have reported some new sulfur-containing chiral bis(oxazo-
linyl)thiophenes 3.[7b,9] Relatively little work has been reported on the sulfur-
containing bis(oxazoline) for asymmetric catalysis.[10] This prompted us to
report our preliminary results on the structure of the new ligand together
with its application in catalytic asymmetric cyclopropanation reactions
between alkenes and ethyl diazoacetate (EDA) using the copper complex.
The chiral bis(oxazolines) 3 (thiobox) have been prepared using two
methods (Scheme 1). The reaction of the diacid chloride 4 with chiral
b-amino alcohols forms dihydroxy diamide compounds 5 (a), which are sub-
sequently treated with thionyl chloride to afford dichloride diamide
compounds (b). These compounds are transformed into 3 by treatment with
.
a base (c). The improved route includes one step (d, using BF3 Et2O or
DAST) instead of the two steps (b) and (c). This results not only in a simpli-
fication of procedures but also in an improved yield of the bis(oxazoline).
Scheme 1.

Sulfur-Containing Chiral Bis(oxazolines)
2667
These new ligands were characterized by IR, NMR, MS and elemental
analysis.
To better understand the structure of the new chiral bis(oxazoline) ligands
3, the structure of the ligand 3a was determined by X-ray crystallography.
Compound 3a was obtained in the form of air-stable, colorless crystals by
slow evaporation from a solution of 3a in acetone–hexane (v/v 1 : 3). The per-
spective view of compound 3a is shown in Figure 1.
Examination of the crystal structure of 3a shows that the thiophene unit
with two oxazoline moieties possesses a twist angle and that they do not
lie in the same plane. The N-S-N angle is quite large; this differs from the
structure of the previously reported pybox.[8c,11] In 3, the thiophene
functions as a backbone, and the sulfur atom is part of a strong p-donor
system. The chiralty is introduced by the two-oxazoline moieties near the
sulfur. These C2-symmetric bis(oxazolines) present a new class of N-S-N
chelating tridentate ligands.
These ligands were successfully utilized for the Cu(I)-catalyzed cyclo-
propanation of alkenes with ethyl diazoacetate. This is the commonly
investigated model reaction with chiral ligands.[8,12] The asymmetric cyclo-
propanation was carried out in CH2Cl2 in the presence of 4% mol of Cu(I)
catalyst by mixing Cu(SO2CF3)2 and the bis(oxazoline) ligands 3 to give
cyclopropanecarboxylate as a mixture of trans and cis isomers (Scheme 2).
1
The ratio of trans to cis isomers was determined by H NMR, and the
enantiomeric excess of these mixtures was determined by HPLC using a
chiral column. The results are summarized in Table 1. Catalytic cyclopropana-
tion experiments of 1,1-diphenylethene with ethyl diazoacetate afforded only
low yields of cyclopropane products because of the presence of traces of
water. Therefore, several desiccants such as CaSO4 and MgSO4 were tested
˚
and high yields were obtained using 4 A molecular sieve.
Using ligand 3a and substrate 1,1-diphenylethene, studies were carried
out to determine the optimum reaction conditions. The choice of solvents
has a great effect on both the enantioselectivity and the yield. Dichloromethane
Figure 1. Molecular structure of 3a.

2668
M. Z. Gao et al.
Scheme 2.
was found to give the best results. Slightly lower enantioselectivities were
obtained in chloroform and dichloroethane, and the enantioselectivity and
yield decreased in THF (entry 1). Reduction of the reaction temperature
from 40 8C to 0 8C did not noticeably affect the reaction, but increased enan-
tioselectivity from 85% (entry 4) to 93% (entry 6) lowered the yield from
88% to 73%.
Changing ligand (3b, 3c, 3d) on the cyclopropanation of alkenes has a great
effect on the trans/cis diastereoselectivity and enantioselectivity. As can be
seen from entries 10–15 in Table 1, when the R groups of the ligand are
changed from ethyl to bulkier groups (iso-Pr; Ph; tert-butyl), higher trans:cis
ratios and ee values for the cyclopropanation products of styrene, p-chlorostyr-
ene, and 4-vinylanisole were observed in all cases. The best results were
obtained with the tert-butyl group, where the enantioselectivity of 97% ee
(entry 9) and a trans:cis ratio of 91 : 9 and 90% ee (entry 15) of trans product
were obtained. This is consistent with the results previously reported.[13]
Compared with the results reported for catalyst Cu–pybox,[8d] the results
of enantioselectivity and diastereoselectivity of cyclopropanation product are
improved by the use of Cu–thiobox. Compared with result reported for the
catalyst Ru–pybox,[8b,8c] with respect to ee values and yield of cycloprpana-
tion of 1,1-diphenylethene, better results are reported herein. In the case of
styrene and substituent styrenes, the ee values and trans/cis stereoselectivity
of the cyclopropanation product are similar, but a little lower. These differ-
ences may result from the differences in catalyst structure. With respect to
the structure of the catalyst, it is expected that the N-S-N bonds coordinated
to Cu have a larger angle than that of N-N-N bonds coordinated to Cu. This
may result in easier access to the reactive substrate and is favorable for a
large sterically bulky substrate. In addition, Cu-S coordination is much
stronger than Cu-N coordination, because sulfur and copper are both soft
centers. The Cu-N coordination represents a soft–hard interaction, and a
soft–soft interaction is expected to be much stronger.
In conclusion, a series of new chiral sulfur-containing bis(oxazolines)
with thiophene as a backbone has been synthesized. Preliminary results in
the asymmetric cyclopropanation of alkenes with diazoacetate have been
obtained. A high enantioselectivity has been obtained in the cyclopropanation
of diphenylethene using the Cu(I)-bis(oxazoline) catalyst. The trans/cis
stereoselectivity and enantioselectivity are moderate to good. The preliminary
results suggest that these novel bis(oxazolines) may function as new catalysts
for asymmetric reactions. Further studies on the asymmetric reactions are
currently being investigated.

Table 1. Enantioselective cyclopropanation catalyzed by the Cu-bis(oxazolines)
Entry
Ligand
R1
R2
T (8C)
Solvent
Trans:cisa
%ee (trans)b
%ee (cis)
Yield %c
1
2
3a
3a
3a
3a
3a
3a
3b
3c
3d
3a
3b
3c
3d
3d
3d
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
H
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
rt
rt
rt
40
rt
0
THF
ClCH2CH2Cl
CHCl3
—
—
77
88
86
85
91
93
89
86
97
60
81
73
88
85
90
77
88
86
85
91
93
89
86
97
64
83
75
84
82
87
61
79
80
88
84
73
76
56
71
89
80
72
78
80
76
3
—
—
4
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
5
—
—
6
7
rt
rt
rt
rt
rt
rt
rt
rt
rt
—
—
8
9
—
10
11
12
13
14
15
63:37
84:16
76:24
89:11
88:12
91:9
H
H
H
H
4-Cl-Ph
4-MeO-Ph
H
1
aThe ratio of trans and cis was determined by H NMR.
bEe values were determined by chiral HPLC analysis (Chiralcel OD-H or OJ column).
cIsolated yield of a mixture of trans and cis based on EDA.

2670
M. Z. Gao et al.
EXPERIMENTAL
Melting points were on determined on a Thiele apparatus and are uncorrected.
1H NMR spectra were recorded with a Varian 300 spectrometer. Optical
rotations were measured using a Perkin-Elmer 241 polarimeter at 589 nm in
a cell of path length 10 cm. Chiral HPLC analyses were performed on a
Beckman 110B chromatographic system with a Beckman 168 UV detector
(254 nm). The chiral columns were purchased from Daicel Chemical
Industries, Ltd. The crystallographic measurements were carried out on a
Siemens P4 diffractometer with graphite-monochromated Mo-Ka radiation
˚
(l ¼ 0.71073 A) and 12-kW rotating generator. The data were collected at
110 K. The structure was solved and refined using the programs SHELXS-97
(Sheldrick, 1997) and SHELXL (Sheldrick, 1997). The program X-Seed
(Barbour, 1999) was used as an interface to the SHELX programs.
General Procedure for Cyclopropopanation Reaction
The ligand 3 (0.21 mmol) and Cu(SO2CF3)2 (72 mg, 0.2 mmol) were taken
under nitrogen in anhydrous CH2Cl2 (4 mL). The mixture was stirred at
reflux temperature for 30 min. After cooling to rt, phenylhydrazine (three
˚
drops) and 4A molecular sieves (MS, 0.6 g) were added. Then, the mixture
was stirred for 5 min and the 1,1-diphenylethene or other alkene (10.0 mmol,
excess) was added, followed by slow addition of diazoester (5.0 mmol) in
CH2Cl2 (3mL) for 4 h. The final reaction was stirred for another 16h. The
solvent was removed under reduced pressure and the crude product was
separated by flash column chromatography to afford the cyclopropyl ester
products in 56–89% isolated yield.
Ethyl 2,2-diphenylcyclopropane-1-carboxylate. Colorless oil. Analytical
HPLC Chiralcel OD-H, isopropanol/hexane (0.4%, 0.5 mL/min21),
tR ¼ 20.1 min [(2)-R], tR ¼ 21.8 min [(þ)-S]. IR: 3061, 3039, 2998, 2935,
2908, 1746, 1601, 1506, 1449, 1421, 1372, 1344, 1264, 1182, 1086, 1026,
1
970, 918, 751 cm21. H NMR (300 MHz, CDCl3), dH: 1.02 (t, J ¼ 7.2 Hz,
3H, CH2CH3), 1.61 (dd, J ¼ 4.9, 8.1 Hz, 1H, CHH), 2.18 (dd, J ¼ 5.9,
4.9 Hz, 1H, CHH), 2.55 (dd, J ¼ 8.1, 5.9 Hz, 1H, CH), 4.03–4.85 (m, 2H,
CH2Me), 7.16–7.40 (m, 10H, PhH). 13C NMR (75 MHz, CDCl3), dC: 20.9,
28.8, 41.2, 126.9, 127.3, 127.9, 128.7, 128.8, 129.9, 144.8, 176.8. MS
(FAB): 266 (M, 50%), 192 (100%), 164,120, 106, 91, 77, 59. Anal. calcd.
for C18H18O2: C, 81.17; H, 6.81. Found: C, 81.37; H, 6.69.
Ethyl trans-2-phenylcyclopropane-1-carboxylate. Colorless oil. GC-column
CB1, helium 1.2 mL/min21. tR ¼ 34.9 min (1S, 2R), tR ¼ 38.0 min
(1R, 2S), tR ¼ 39.1 min (1R, 2R), tR ¼ 40.2 min (1S, 2S). IR: 3031, 2984,
2935, 2910, 1899, 1726, 1498, 1452, 1407, 1395, 1328, 1270, 1220, 1188,
1
1095, 1046, 1015, 950, 857, 819, 789, 738, 468 cm21. H NMR (300 MHz,

Sulfur-Containing Chiral Bis(oxazolines)
2671
CDCl3), dH: 1.30 (t, J ¼ 7.5 Hz, 3H, CH3), 1.33 (ddd, J ¼ 4.5, 6.5, 8.5 Hz, 1H,
CHH), 1.60 (ddd, J ¼ 4.5, 5.4, 9.1 Hz, 1H, CHH), 1.91 (ddd, J ¼ 4.2, 5.4,
9.1 Hz, 1H, CH), 2.53 (ddd, J ¼ 4.2, 6.5, 9.1 Hz, 1H), 4.17 (q, J ¼ 7.5 Hz,
CH2Me), 7.11–7.32 (m, 5H, PhH). 13C NMR (75 MHz, CDCl3), dC: 11.5,
14.5, 22.2, 25.3, 60.5, 127.1, 128.3, 129.8, 136.9, 172.3. HRMS (EI), calcd.
for C12H15O2 (M þ H): 191.1072; found: 191.1075.
Selected data for 3a, (2)-2,5-bis[40(R)-ethyloxazolin-20-yl]thiophene.
Mp ¼ 89–908C. [a]2D2 ¼ 294.88 (c 1.00, CH2Cl2). IR: 2966, 2931, 2896,
1
1644, 1461, 1361, 1293, 1039, 1011, 934, 842 cm21. H NMR (CDCl3), dH:
0.98 (t, J ¼ 7.4 Hz, 6H, 2 ꢀ CH3), 1.61–1.71 (m, 4H, 2 ꢀ CH2), 4.05 (dd,
J ¼ 7.7, 8.0 Hz, 2H, 2 ꢀ OCHH), 4.48 (dd, J ¼ 7.7, 8.0 Hz, 2H,
2 ꢀ OCHH), 4.18–4.30 (m, 2H, 2 ꢀ NCH), 7.54 (s, 2H, thiophene-H).
13C NMR (CDCl3), dc: 9.83, 26.4, 66.3, 72.6, 130.1, 133.6, 158.6; MS
(FAB): 279 (M þ 1, 100%), 249, 208, 195, 111. Anal. calcd. for
C14H18N2O2S: C, 59.68; H, 6.44; N, 9.94. Found: C, 59.80; H, 6.56; N, 10.02.
3b, (2)-2,5-bis[40(S)-isopropylaxazolin-20-yl]thiophene. Mp ¼ 66–688C.
[a]2D0 ¼ 229.58 (c 0.36, acetone). IR: 2958, 2902, 2871, 1645, 1524, 1362,
1310, 1250, 1043, 1010, 948, 828 cm21 1H NMR (CDCl3),dH: 0.92
;
(d, J ¼ 7.0 Hz, 6H, 2 ꢀ CH3), 1.13 (d, J ¼ 7.0 Hz, 6H, 2 ꢀ CH3), 1.82–
1.89 (m, 2H, CHMe2), 4.10 (dd, J ¼ 9.0, 15.0 Hz, 2H, 2 ꢀ OCHH), 4.14
(dd, J ¼ 8.0, 15.5 Hz, 2H, 2 ꢀ OHH), 4.41 (dd, J ¼ 8.0, 9.0 Hz, 2H,
2 ꢀ NCH), 7.56 (s, 2H, thiophene-H). 13C NMR,dC: 18.09, 18.83, 32.76,
70.74, 72.80, 130.24, 133.79, 158.59; MS (FAB): 307 (M þ 1, 100%), 263,
221, 151, 111. Anal. calcd. for C16H22N2O2S: C, 62.72, H, 7.26; N, 9.14; S,
10.47; Found: C, 62.85; H, 7.52; N, 8.99; S, 10.20.
3c, (2)-2,5-bis[40-(R)-phenyloxozolin-20-yl]thiophene. Mp: 128–1318C;
[a]2D6 ¼ 257.38(c 0.20, CH2Cl2); IR: 3089, 3058, 3032, 2965, 2926, 2900,
1642, 1553, 1476, 1452, 1359, 1303, 1274, 1240, 1048, 1028, 934, 817, 698,
1
538 cm21. H NMR (CDCl3), dH: 4.32 (dd, J ¼ 8.0, 8.5 Hz, 2H, 2 ꢀ NCH),
4.81 (dd, J ¼ 8.5, 10.0 Hz, 2H, OCHH), 5.40 (dd, J ¼ 8.0, 9.5 Hz, 2H,
OCHH), 7.28–7.38 (m, 10H, PhH), 7.67 (s, 2H, thiophene-H). 13C NMR,
dC: 70.33, 75.36, 126.72, 127.77, 128.80, 130.83, 133.80, 141.75, 159.87;
MS (FAB): 375 (M þ 1, 58%), 256, 229, 165, 149, 52 (100%). Anal. calcd.
for C22H18N2O2S: C, 70.57; H, 4.84; N, 7.48. Found: C, 70.21; H, 5.03; N, 7.24.
3d, (þ)-2,5-bis[40-(S)-terebutyloxazolin-20-yl]thiophene. Mp: 119–1228C,
[a]2D0 ¼ þ5.448(c 0.55, acetone); IR: 3053, 3026, 2970, 2901, 2853, 1650,
1602, 1477, 1451, 1032, 1008, 949, 819, 696 cm21 1H NMR (CDCl3),
.
dH: 0.97 (s, 18H, 2 ꢀ C(CH3)3), 4.03 (dd, J ¼ 8.0, 10.0 Hz, 2H, 2 ꢀ OHH),
4.23 (dd, J ¼ 8.0, 8.5 Hz, 2H, 2 ꢀ NCH), 4.34 (dd, J ¼ 8.5, 10.0 Hz, 2H,
2 ꢀ OCHH), 7.52 (2H, s, thiophene-H). 13C NMR, dC: 25.79, 34.05, 53.79,
69.25, 129.91, 133.80, 158.39; MS (FAB): 335 (M þ 1, 100%), 277, 223,
221, 179, 165, 95, 69. Anal. calcd. for C18H24N2O2S: C, 64.64; H, 7.83;
N, 8.37; S, 9.58. Found: C, 64.97; H, 8.08; N, 8.14; S, 9.13.

2672
M. Z. Gao et al.
X-Ray Crystallography
X-ray crystallographic data for 3a was deposited with Cambridge Crystallo-
graphic Data Center, No. CCDC-232494. These data can be obtained free of
charge via web, www.ccdc.cam.ac.uk/conts/retrieving.html; e-mail, deposit@
ccdc.cam.ac.uk; or mail, the Cambridge Crystallographic Data Center, 12
Union Road, Cambridge CB21EZ, UK. Empirical formula: C7H9NOS0.50.
Formula weight: 139.18. Temperature: 110(2) K. Wavelength: 0.71073 A.
˚
Crystal system: Monoclinic. Space group: C2. Unit cell dimensions: a ¼
˚
˚
˚
3.
15.010 (2) A, a ¼ 908; b ¼ 6.3193 (9) A, b ¼ 107.126 (2)8; c ¼ 7.6881 (11) A,
3
˚
g ¼ 908. Volume: 696.93 (17) A . Z: 4. Density (calculated): 1.326 mg/m .
Absorption coefficient: 0.232 mm21. F(000): 296 Crystal size: 0.93 ꢀ
0.60 ꢀ 0.14 mm3. Theta range for data collection: 2.84 to 28.238. Index
ranges: 218 ꢁ h ꢁ 19, 27 ꢁ k ꢁ 8, 29 ꢁ l ꢁ 10. Reflections collected:
2165. Independent reflections: 1367 [R(int) ¼ 0.0120]. Completeness to
theta ¼ 28.238: 92.1%. Absorption correction: SADABS. Refinement method:
Full-matrix least-squares on F2. Data/restraints/parameters: 1367/1/89.
Goodness of fit on F2: 1.049. Final R indices [I . 2sigma(I)]: R1 ¼ 0.0290,
wR2 ¼ 0.0734. R indices (all data): R1 ¼ 0.0292, wR2 ¼ 0.0737. Absolute
structure parameter: 0.00 (8). Extinction coefficient: 0.0029 (16). Largest
˚
diff. peak and hole: 0.226 and 20.196 e.A.
ACKNOWLEDGMENTS
The authors acknowledge the assistance of the Robert A. Welch Foundation
(A-084) and the Department of Energy, Basic Sciences Division (DE-FG03-
00ER 15806), for financial support.
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