Selective reduction of carbonyl groups in the presence of low-valent titanium reagents

Article abstract of DOI:10.1016/j.tetlet.2014.02.072

The chemoselective reduction of several structurally diverse compounds containing carbonyl groups was achieved in the presence of low-valent titanium reagents. This novel synthetic method provides easy access to highly selective reduction of carbonyl groups, and possesses several advantages including one-step procedure, convenient manipulation, good to excellent yields, and short reaction times.

Full text of DOI:10.1016/j.tetlet.2014.02.072

Tetrahedron Letters 55 (2014) 2238–2242
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Selective reduction of carbonyl groups in the presence of low-valent
titanium reagents
⇑
⇑
Wei Lin, Ming-Hua Hu, Xian Feng, Lei Fu, Cheng-Pao Cao, Zhi-Bin Huang , Da-Qing Shi
Key Laboratory of Organic Synthesis of Jiangsu Province, College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
The chemoselective reduction of several structurally diverse compounds containing carbonyl groups was
achieved in the presence of low-valent titanium reagents. This novel synthetic method provides easy
access to highly selective reduction of carbonyl groups, and possesses several advantages including
one-step procedure, convenient manipulation, good to excellent yields, and short reaction times.
Ó 2014 Elsevier Ltd. All rights reserved.
Received 23 October 2013
Revised 18 February 2014
Accepted 20 February 2014
Available online 28 February 2014
Keywords:
Chemoselective reduction
Carbonyl groups
Low-valent titanium reagent
Introduction
important molecules.11 Therefore, several methods for the synthe-
sis of substituted indolin-2-ones have been described;12 including
The selective reduction of functional groups is one of the most
important and basic chemical transformations in organic
chemistry.1 The catalytic reduction of carbonyl groups is used to
synthesize biologically active compounds, for instance, cyclic
amide derivatives, which have good antifungal, antidepressant,
and antitubercular activities.2 The selective reduction of carbonyl
groups is an important transformation in chemistry and represents
a challenge in both chemical and pharmaceutical industries.
The use of metal hydrides or H2 with metal catalysts has been
widely used in the reduction of carbonyl groups.3 Numerous meth-
ods have been described in recent reports dealing with this type of
reduction, using various metal catalysts such as Au,4 Fe,5 Ir,6 Pt7 or
Rh.8 However, the high costs and limited availabilities of these
metals have led researchers to look for inexpensive, easily
available, and environmental friendly alternatives. Sodium borohy-
dride (NaBH4) has been used for carbonyl reduction under metal-
and solvent-free conditions.9,10 However, this method is suffering
from long reaction times and lack of chemoselectivity.9 Solid acids
in combination with NaBH4 could improve both the reaction rate
and chemoselectivity.10 It can be seen that the selective reduction
of carbonyl groups in the presence of other reducible functional-
ities in a molecule remains a challenge.
Wolff–Kishner–Huang method.13 Recently, some improved meth-
ods have been reported,14 although they remain unsatisfactory
with respect to ease of operation, high yield, reasonable reaction
time, and general applicability. Thus, there is a need for the devel-
opment of concise and efficient method for the synthesis of this
heterocyclic skeleton.
Low-valent titanium reagents could efficiently promote reduc-
tive coupling of carbonyl compounds.15 There are increased inter-
ests of this reagents, and many other functional groups can also be
coupled.16 Recently, we have reported the cyclodimerization of
a,b-unsaturated ketones and a,b-unsaturatednitriles promoted by
this reagent and yielded functionalized cyclopentanes17 and cyclo-
pentenes,18 respectively. It is well known that the carbonyl group
can easily be reduced by low-valent titanium reagent to afford only
reductive coupling products. However, few literatures report the
selective reduction of carbonyl groups promoted by low-valent
titanium reagents. As a part of our ongoing research on the
synthesis of heterocyclic compounds using low-valent titanium
reagents,19 we report our preliminary results on the chemoselec-
tive reduction of the carbonyl group of isatin and several structur-
ally diverse compounds with low-valent titanium reagents.
The indolin-2-one ring is one of the fundamental heterocycles,
Results and discussion
widely distributed in
a variety of natural and biologically
In our preliminary evaluation of the transformation, the
reductive reaction of 1a was evaluated under various conditions
and summarized in Table 1. We examined the effects of different
low-valent titanium systems. The target compound 2a was not
⇑
Corresponding authors. Tel.: +86 512 65880049; fax: +86 512 65880095.
E-mail addresses: zbhuang@suda.edu.cn (Z.-B. Huang), dqshi@suda.edu.cn
(D.-Q. Shi).
http://dx.doi.org/10.1016/j.tetlet.2014.02.072
0040-4039/Ó 2014 Elsevier Ltd. All rights reserved.

W. Lin et al. / Tetrahedron Letters 55 (2014) 2238–2242
2239
observed with TiCl4/Fe (Table 1, entry 1), while it was obtained in
55% yield in the presence of TiCl4/Al (Table 1, entry 2). We then
found that TiCl4/Zn system gave the best results for the synthesis
of 2a (89% yield, Table 1, entry 3). To further optimize the reaction
conditions, similar experiments were carried out using different
ratios of 1a to TiCl4 from 1:2 to 1:5. The yield of product 2a in-
creased as the ratio was increased from 1:2 to 1:3 (Table 1, entries
6 and 3). However, further increases in the ratio to 1:4 and 1:5
failed to improve the yield of product 2a (Table 1, entries 7 and
8). We also examined the effect of the temperature (Table 1, entries
obtained in good yields (Table 3), as a result of hydrogen bonds
in 1-(2-hydroxy-1H-indol-1-yl) ethanon.
Table 3
Reduction of 1-acetylisatins20
O
R
TiCl4/Zn
R
O
O
N
N
H
THF, r.t. 5min
O
O
3
and 9–11), and the highest yield was obtained at room
4
3
temperature.
Entry
Products
R
Isolated yield (%)
Under the optimized conditions, we found that the method was
applicable to a broad substrate scope bearing 4-, 5-, 6-, 7-, 4,7-di,
5,6-di and 5,7-disubstitution (Table 2). This method could be
applied to isatins with electron deficient groups or electron-rich
group. Leading the final products in up to 94% yield. N-substituted
isatins were examined under the same conditions. The correspond-
1
2
3
4a
4b
4c
H
5-Cl
5-F
94
91
86
ing 1-(2-hydroxy-1H-indol-1-yl)ethanone derivatives
4 were
Table 4
Optimization of conditions for selective reduction of 2-carbonylthioacetamides
O
O
O
OH
O
TiCl4/M
THF
N
N
N
+
H3C
Table 1
S
S
S
H3C
H3C
Optimization of reductive reaction
7a
5a
6a
O
Entry
TiCl4/M
Ratioa
Temp. (°C)
Time (min)
Yieldb (%)
TiCl4/M
O
O
6a
7a
N
H
THF
N
H
1
2
3
4
5
6
7
8
9
Fe
Al
1:3
1:3
1:3
1:3
1:3
1:4
1:2
1:3
1:3
1:3
rt
rt
rt
rt
rt
rt
rt
40
60
Reflux
20
15
5
25
30
5
46
55
91
65
42
87
75
55
15
0
12
9
4
11
8
5
2a
1a
Zn
Mg
Sm
Zn
Zn
Zn
Zn
Zn
Entry
TiCl4/M
Ratioa
Temp (°C)
Time (min)
Isolated yield (%)
1
2
Fe
Al
1:3
1:3
1:3
1:3
1:3
1:2
1:4
1:5
1:3
1:3
1:3
rt
rt
rt
rt
rt
rt
rt
rt
60
65
30
60
60
30
30
30
30
30
30
Trace
55
89
Trace
42
79
87
60
62
55
3
4
5
6
7
8
9
10
11
Zn
Mg
Sm
Zn
Zn
Zn
Zn
Zn
Zn
5
4
30
30
30
24
62
88
10
a
Ratio of 5a and low-valent Ti reagent.
The yields were determined by HPLC–MS.
b
40
60
Reflux
42
a
Ratio of 1a and low-valent titanium reagent.
Table 5
Synthesis of 2-hydroxythioacetamide derivatives 621
OH
S
C
O
C
S
C
R1
R2
R1
R2
TiCl4/Zn
THF, r.t.
Ar
C
H
N
Ar
N
Table 2
Reduction of isatins20
6
5
O
Entry Product Ar
R1
R2
Time
(min)
Isolated
yield (%)
TiCl4/Zn
THF, r.t.
R
O
R
O
N
H
N
1
2
3
4
5
6a
6b
6c
6d
6e
4-CH3C6H4
4-BrC6H4
4-ClC6H4
2-ClC6H4
4-
CH2CH2OCH2CH2
CH2CH2OCH2CH2
CH2CH2OCH2CH2
CH2CH2OCH2CH2
CH2CH2OCH2CH2
5
5
5
5
8
91
89
85
82
90
H
2
1
Entry
Product
R
Time (min)
Isolated yield (%)
1
2
3
4
5
2a
2b
2c
2d
2e
2f
H
4-Cl
4-Br
5-Br
5-Cl
5
5
5
6
5
94
91
93
88
90
87
85
90
88
89
90
92
89
86
CO2CH3C6H4
6
7
8
9
6f
6g
6h
6i
Naphth-1-yl CH2CH2OCH2CH2
Naphth-2-yl CH2CH2OCH2CH2 10
Furan-2-yl
Thiophene-
2-yl
8
89
87
85
86
CH2CH2OCH2CH2 10
CH2CH2OCH2CH2 10
6
5-F
6
7
8
9
2g
2h
2i
5-CH3
6-Br
6-Cl
5,6-F2
7-CF3
7-Br
4,7-Cl2
10
5
10
11
12
13
6j
6k
6l
4-ClC6H4
2-ClC6H4
Naphth-1-yl CH3
C6H5
C6H5
CH3
CH3
CH3
CH3
CH3
H
6
8
8
8
79
78
84
87
5
5
5
5
5
5
10
11
12
13
14
2j
2k
2l
2m
2n
6m
4-
CH3C6H5
4-
CH3OC6H5
14
6n
H
8
90
5-Cl-7-CH3

2240
W. Lin et al. / Tetrahedron Letters 55 (2014) 2238–2242
Table 6
Synthesis of thioacetamide derivatives 721
S
C
O
C
S
C
R1
R2
R1
R2
H2
C
TiCl4/Zn
Ar
N
Ar
N
THF, reflux
7
5
Entry
Product
Ar
R1
R2
Time (min)
Isolated yield (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
7a
7b
7c
7d
7e
7f
7g
7h
7i
7j
7k
7l
7m
7n
4-CH3C6H4
4-BrC6H4
4-ClC6H4
2-ClC6H4
4-CO2CH3C6H4
Naphth-1-yl
Naphth-2-yl
Furan-2-yl
Thiophene-2-yl
4-ClC6H4
2-ClC6H4
Naphth-1-yl
C6H5
CH2CH2OCH2CH2
CH2CH2OCH2CH2
CH2CH2OCH2CH2
CH2CH2OCH2CH2
CH2CH2OCH2CH2
CH2CH2OCH2CH2
CH2CH2OCH2CH2
CH2CH2OCH2CH2
CH2CH2OCH2CH2
CH3
30
30
30
30
30
35
35
40
40
30
30
35
40
40
88
85
85
85
75
87
82
78
76
80
74
79
85
89
CH3
CH3
CH3
H
CH3
CH3
4-CH3C6H5
4-CH3OC6H5
C6H5
H
To further explore this method, we examined 2-carbonylthioac-
etamide derivatives under different conditions (Table 4). Partially
reduced alcohol 6a was obtained in 91% yield under the above
optimized conditions (Table 4, entry 3), however, elevating the
reaction temperature to reflux was able to exclusively furnish the
desired thioamide 7a in 88% yield only in 30 min. (Table 4, entry
10).
With these optimal conditions in hand, several thioacetamide
derivatives were examined at both rt and refluxed conditions.
When the reaction was conducted using TiCl4/Zn at room temper-
ature for about 5–10 min, we obtained 2-hydroxythioacetamide
derivatives 6 (Table 5). Thioacetamide derivatives 7 were produced
at reflux temperature after about 30 min (Table 6). Phenyl, naph-
thalen-1-yl, thiophene, and furan groups, were well tolerated un-
der these reaction conditions with satisfactory yields (up to 91%).
The structures of the products were identified from their IR, 1H
NMR, 13C NMR, and HRMS spectra. The structures of compounds 6b
and 7b were further confirmed by X-ray diffraction analysis22
(Figs. 1 and 2).
reduced to intermediate B. Then product 2 was obtained by hydro-
lysis and tautomerization.
In conclusion, a facile and efficient method to selectively reduce
several structurally diverse carbonyl-group-containing compounds
was realized using low-valent Ti reagents. This method possesses
high selectivity, convenient manipulation, good to excellent yields,
and short reaction time. This novel method is feasible in selective
reduction of carbonyl groups. Further exploitation of this method
and applications of these products are underway.
Diphenylethanedione derivatives 8 were examined as well with
the above two conditions. The reaction proceeded under both con-
ditions to give 2-hydroxy-1,2-diphenylethanone derivatives 9 and
1,2-diphenylethanone derivative 10 selectively (Table 7).
According to literature,15 we propose the following mechanism
of this reaction (Scheme 1). TiCl4 is first reduced by Zn dust to give
low-valent titanium species (Ti(0)). In the initial step, 1 was re-
duced by low-valent titanium to intermediate A, which was further
Figure 2. Molecular structure of compounds 7b.
Table 7
Syntheses of compounds 9 and 10
OH O
O
C
O
C
O
C
TiCl4/Zn
TiCl4/Zn
H2
C
Ar
C
H
C
Ar
Ar
Ar
Ar
Ar
THF, reflux 30-40 min
THF, r,t. 5-10 min
10
9
8
Entry
Ar
Product
Isolated yield (%)
1
2
3
4
5
6
7
8
C6H5
9a
9b
9c
88
85
86
85
83
83
81
84
p-BrC6H4
p-FC6H4
p-CH3C6H4
C6H5
p-BrC6H4
p-FC6H4
p-CH3C6H4
9d
10a
10b
10c
10d
Figure 1. Molecular structure of compounds 6b.

W. Lin et al. / Tetrahedron Letters 55 (2014) 2238–2242
2241
O
O
Ti (II)
O
deoxygenation
Ti (0)
R
OTi(III)O
R
O
R
N
H
N
N
H
H
B
1
A
HCl
H2O
R
O
R
OH
N
H
N
H
C
2
Scheme 1. The proposed mechanism for the reduction of isatins.
16. (a) McMurry, J. E. Acc. Chem. Res. 1983, 16, 405; (b) Fürstner, A.; Bogdanovi, B.
Angew. Chem., Int. Ed. 1996, 35, 2443; (c) Shi, D. Q.; Rong, L. C.; Wang, J. X.;
Zhuang, Q. Y.; Wang, X. S.; Hu, H. W. Tetrahedron Lett. 2003, 44, 3199.
17. Zhou, L. H.; Shi, D. Q.; Gao, Y.; Shen, W. B.; Dai, G. Y.; Chen, W. X. Tetrahedron
Lett. 1997, 38, 2729.
18. Zhou, L. H.; Tu, S. J.; Shi, D. Q.; Dai, G. Y.; Chen, W. X. Synthesis 1998, 851.
19. (a) Lin, W.; Dou, G. L.; Hu, M. H.; Cao, C. P.; Huang, Z. B.; Shi, D. Q. Org. Lett.
2013, 15, 1238; (b) Lin, W.; Hu, M. H.; Feng, X.; Cao, C. P.; Huang, Z. B.; Shi, D. Q.
Tetrahedron 2013, 69, 6721; (c) Sun, F.; Feng, X.; Zhao, X.; Huang, Z. B.; Shi, D. Q.
Tetrahedron 2012, 68, 3851; (d) Shi, D. Q.; Dou, G. L.; Shi, C. L.; Li, Z. Y.; Ji, S. J.
Synthesis 2007, 3117; (e) Dou, G. L.; Shi, D. Q. J. Comb. Chem. 2008, 10, 810; (f)
Shi, D. Q.; Dou, G. L.; Li, Z. Y.; Ni, S. N.; Li, X. Y.; Wang, X. S.; Wu, H.; Ji, S. J.
Tetrahedron 2007, 63, 9764; (g) Shi, D. Q.; Rong, S. F.; Dou, G. L.; Wang, M. M. J.
Comb. Chem. 2010, 12, 25; (h) Dou, G. L.; Sun, F.; Zhao, X.; Shi, D. Q. Chin. J.
Chem. 2011, 29, 2465.
Acknowledgements
We are grateful for financial support from the Major Basic
Research Project of the Natural Science Foundation of the Jiangsu
Higher Education Institutions (No. 10KJA150049), the Natural
Science Foundation of the Jiangsu Higher Education Institutions
(No. 11KJB150014), a project fund from the Priority Academic
Project Development of the Jiangsu Higher Education Institutions
and a Project Funded by the Jiangsu College Graduate Research
and Innovation Project of the Jiangsu Province Department of
Education (CXZZ12_0809).
20. General procedure for the synthesis of compounds 2 and 4: TiCl4 (0.7 mL, 6 mmol)
was added to a stirred suspension of Zn powder (0.78 g, 12 mmol) in freshly
distilled anhydrous THF (15 mL) at room temperature (rt) under a dry N2
atmosphere. After completion of the addition, the mixture was refluxed for 2 h.
The suspension of the low-valent titanium reagent thus-formed was cooled to
rt. A solution of isatin or its derivatives 1 or 3 (2 mmol) in THF (10 mL) was
added dropwise. The mixture was stirred at room temperature for about 5 min
under N2. After this period, the thin layer chromatography (TLC) analysis of the
mixture showed the reaction completed. The reaction mixture was quenched
with 3% HCl (15 mL) and extracted with CHCl3 (3 Â 50 mL). The combined
extracts were washed with water (3 Â 50 mL) and dried over anhydrous
Na2SO4. After evaporation of the solvent under reduced pressure, the crude
product was purified by column chromatography (petroleum ether/ethyl
acetate = 5:1) to give the pure products 2 or 4. Indolin-2-one (2a): Yellow solid,
References and notes
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yield 94%; mp 124–125 °C; IR (KBr)
m: 3213, 3066, 2828, 1700, 1606, 1468,
1321, 1227, 742, 662 cmÀ1 1H NMR (400 MHz, CDCl3) d: 3.55 (s, 2H, CH2), 6.90
;
(d, J = 7.6 Hz, 1H, ArH), 7.02 (t, J = 7.6 Hz, 1H, ArH), 7.22 (t, J = 7.6 Hz, 2H, ArH),
8.97 (s, 1H, NH). N-Acetylindolin-2-one (4a): White solid, yield 94%; mp 116–
118 °C; IR (KBr) m ;
: 3400, 3155, 1713, 1618, 1463, 1299, 1155, 797 cmÀ1 1H
NMR (300 MHz, CDCl3) d: 2.63 (s, 3H, CH3), 3.39 (s, 1H, OH), 5.18 (s, 1H, CH),
7.22 (d, J = 8.1 Hz, 1H, ArH), 7.37 (t, J = 8.1 Hz, 1H, ArH), 7.47 (d, J = 7.2 Hz, 1H,
ArH), 8.18 (d, J = 8.4 Hz, 1H, ArH); HRMS Calcd for m/z C10H9NO2 (M+):
175.0633. Found: 175.0634.
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21. General procedure for the synthesis of compounds 6 and 7: TiCl4 (0.7 mL, 6 mmol)
was added to a stirred suspension of Zn powder (0.78 g, 12 mmol) in freshly
distilled anhydrous THF (20 mL) at room temperature (rt) under a dry N2
atmosphere. After completion of the addition, the mixture was refluxed for 2 h.
The suspension of the low-valent titanium reagent thus-formed was cooled to
rt. A solution of 2-carbonylthioacetamide 5 (2 mmol) in THF (10 mL) was
added dropwise. The mixture was stirred at room temperature for about 5–
10 min (for 6) or refluxed for about 30–40 min (for 7) under N2. After this
period, the thin layer chromatography (TLC) analysis of the mixture showed
the reaction completed. The reaction mixture was quenched with 3% HCl
(15 mL) and extracted with CHCl3 (3 Â 50 mL). The combined extracts were
washed with water (3 Â 50 mL) and dried over anhydrous Na2SO4. After
evaporation of the solvent under reduced pressure, the crude product was
purified by recrystallization from 95% ethanol or column chromatography
(petroleum ether/ethyl acetate = 5:1) to give the pure products 6 or 7. 2-
Hydroxy-1-morpholino-2-(p-tolyl)ethanethione (6a): White solid, yield 91%; mp
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Y. R.; Suk, J. Y.; Kim, B. S. Tetrahedron Lett. 1999, 40, 8219; (c) Blasko, G.;
Lukacs, G.; Reiter, J.; Florian, E.; Porcs-Makkay, M.; Mezei, T.; Simig, G. WO
9736895A1, 1997.; (d) Mahaney, P. E.; Heffernan, G. D.; Coghlan, R. D. US
2007072897, 2007.; (e) Deng, B.; Bie, P.; Liu, W.; Yang, S.; Zhang, L. WO
2007085205, 2007.
101–102 °C; IR (KBr)
m: 3425, 2914, 1635, 1501, 1406, 1268, 1108, 999, 829,
586 cmÀ1 1H NMR (400 MHz, CDCl3) d: 2.34 (s, 3H, CH3), 3.03–3.06 (m, 1H,
.
CHÀH), 3.51–3.67 (m, 4H, 2Â CH2), 3.81(s, 1H, CHÀH), 4.12–4.17 (m, 1H, CH-
H), 4.55–4.58 (m, 1H, CHÀH), 5.27–5.35 (m, 2H, OH+CH), 7.15–7.30 (m, 4H,
ArH); 13C NMR (75 MHz, CDCl3) d: 21.3, 50.4, 52.2, 65.8, 66.4, 73.8, 127.1,
129.9, 137.8, 138.6, 203.4. HRMS Calcd for m/z C13H17NO2S (M+): 251.0980.
Found: 251.0980. 1-Morpholino-2-(p-tolyl)ethanethione (7a): Yellow solid, yield
12. Masanori, S. Heterocycles 2007, 73, 537.
13. Mologni, L.; Rostagno, R.; Brussolo, S.; Knowles, P. P.; Murray-Rust, J.; Rosso, E.;
Zambon, A.; Scapozza, L.; McDonald, N. Q.; Lucchini, V.; Gambacorti-Passerini,
C. Bioorg. Med. Chem. 2010, 18, 1482.
88%; mp 102 °C; IR (KBr)
m
: 2971, 2846, 1896, 1630, 1500, 1432, 1269, 1182,
1107, 1027, 955, 845, 786 cmÀ1
;
1H NMR (400 MHz, CDCl3) d: 2.32 (s, 3H, CH3),
14. (a) Soriano, D. S. J. Chem. Educ. 1993, 70, 332; (b) Crestini, C.; Saladino, R. Synth.
Commun. 1994, 24, 2835; (c) Kuo, L. H.; Hsu, J. P.; Chen, C. T. U.S. 5973165,
1999.
15. (a) McMurry, J. E. Acc. Chem. Res. 1974, 7, 281; (b) McMurry, J. E. Chem. Rev.
1989, 89, 1513; (c) Shohji, N.; Kawaji, T.; Okamoto, S. Org. Lett. 2011, 13, 2626;
(d) Kumar, A.; Samuelson, A. G. Eur. J. Org. Chem. 2011, 951.
3.39 (t, J = 5.2 Hz, 2H, CH2), 3.62 (t, J = 4.8 Hz, 2H, CH2), 3.73 (t, J = 5.2 Hz, 2H,
CH2), 4.30 (s, 2H, CH2), 4.34 (t, J = 4.8 Hz, 2H, CH2), 7.12 (d, J = 8.4 Hz, 2H, ArH),
7.19 (d, J = 8.0 Hz, 2H, ArH); HRMS Calcd for m/z C13H17NOS (M+): 235.1031.
Found: 235.1031.
22. Crystallographic data for the structures of compounds 6b and 7b have been
deposited at the Cambridge Crystallographic Data Center, and the deposit

2242
W. Lin et al. / Tetrahedron Letters 55 (2014) 2238–2242
numbers are CCDC-952316 and CCDC-956340, respectively. Copy of available
b = 93.318(8)°,
(I > 2 ), wR2 = 0.1884. Crystal data of 7b: molecular formula: C12H14BrNOS, a
c
= 96.359(11)°, V = 1341.4(7) Å3, T = 293(2) K, R1 = 0.0596
material can be obtained free of charge on application to CCDC, 12 Union
Road, Cambridge CB2 1EZ, UK (fax: +44 1223 336 033; e-mail: deposit@
ccdc.cam.ac.uk). Crystal data of 6b: molecular formula: C12H14BrNO2S, a yellow
r
yellow crystal (0.50 Â 0.28 Â 0.10 mm), k (Mo-K ) = 0.71073 Å, crystal system:
a
orthorhombic, space group: Pbca, a = 14.2009(15) Å, b = 6.3853(7) Å,
crystal (0.60 Â 0.30 Â 0.05 mm), k(Mo-K ) = 0.71070 Å, crystal system: triclinic,
c = 28.382(2) Å, = 90°, b = 90°,
a
c
= 90°, V = 2573.7(7) Å3, T = 298(2) K,
a
space group: P-1, a = 8.565(3) Å, b = 11.021(3) Å, c = 14.356(5) Å,
a
= 93.499(9)°,
R1 = 0.0612 (I > 2r), wR2 = 0.1820.