Unexpected results from the re-investigation of the Beckmann rearrangement of ketoximes into amides by using TsCl

Article abstract of DOI:10.1016/j.tet.2009.07.036

TsCl (p-toluenesulfonyl chloride), a commercially available organosulfonyl chloride, has been widely used as a stoichiometric dehydrogenation reagent in the transformation of ketoximes into corresponding amides via the Beckmann rearrangement. It has been now found to catalyze the Beckmann rearrangement with high catalytic efficiency, converting a wide range of ketoximes into their corresponding amides under mild condition with good to excellent yields (up to 99% of yield with 1-5 mol % of catalyst loading).

Full text of DOI:10.1016/j.tet.2009.07.036

Tetrahedron 65 (2009) 7790–7793
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Unexpected results from the re-investigation of the Beckmann rearrangement
of ketoximes into amides by using TsCl
Hong-Jun Pi, Jin-Dong Dong, Na An, Wenting Du , Wei-Ping Deng *
*
School of Pharmacy, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, China
a r t i c l e i n f o
a b s t r a c t
Article history:
TsCl (p-toluenesulfonyl chloride), a commercially available organosulfonyl chloride, has been widely used
as a stoichiometric dehydrogenation reagent in the transformation of ketoximes into corresponding
amides via the Beckmann rearrangement. It has been now found to catalyze the Beckmann rearrange-
ment with high catalytic efficiency, converting a wide range of ketoximes into their corresponding
amides under mild condition with good to excellent yields (up to 99% of yield with 1–5 mol % of catalyst
loading).
Received 28 April 2009
Received in revised form 16 July 2009
Accepted 17 July 2009
Available online 22 July 2009
Ó 2009 Elsevier Ltd. All rights reserved.
1
. Introduction
the first highly efficient organophosphorus catalyst for the Beck-
mann rearrangement, then the second generation of organophos-
16
The Beckmann rearrangement, as a fundamental tool in organic
phorus catalyst triphosphazene (TAPC). On the other hand, a novel
mechanism was consequently proposed by Ishihara and co-workers
to account for the new generation of organocatalytic Beckmann
1
chemistry, has been successfully utilized for the transformation of
cyclohexanone oxime into caprolactam in industry. The rear-
rangement, however, requires harsh conditions such as a large
amount of a strong acid and high reaction temperature. Besides, it
also suffers from a considerable large amount of byproduct am-
monium sulfate. Hence, extensive efforts have been devoted to the
optimization of the catalytic reaction system. Many catalytic sys-
tems such as liquid phase system, vapor phase system, super-
critical water system,5 and ionic liquids system have been
developed so far. And liquid-phase catalytic Beckmann rearrange-
ment under mild conditions has become a topic of current interest
due to its advantages such as easy work-up, and industrial practi-
cability. Various catalysts were developed such as inorganic catalyst
14
rearrangement. On the basis of this hypothesis, a similar mecha-
15
16b
nism was also described for the BOP-Cl and TAPC
catalyzed
Beckmann rearrangement. Meanwhile, we also noticed that TsCl has
been widely used as a stoichiometric dehydrogenation reagent in
the transformation of ketoximes into corresponding amides via the
2
3
4
17
Beckmann rearrangement. As a consequence, we wondered if TsCl,
6
mechanistically similar to BOP-Cl, TAPC and CNC, also can catalyze
the Beckmann rearrangement efficiently. In this article, we are going
to present our results of the TsCl-catalyzed Beckmann rearrange-
ment of ketoximes to corresponding amides.
7
8
9
10
11
P
2
O
5
, HSO
3
Cl, PCl
5
, metallic Lewis acid [RhCl(cod)]
However, due to drawbacks such as toxicity, high
2
,
Yb(OTf)
3
,
2. Results and discussion
1
2
13
RuCl
3
,
HgCl
2
.
cost, and corrosiveness, these catalysts have not yet been in-
dustrially utilized.
Initially, p-toluenesulfonyl chloride TsCl was chosen for the
catalytic Beckmann rearrangement. As shown in Table 1, to our
amazement, catalytic amount (10 mol %) of TsCl was found to
smoothly convert the acetophenone oxime 1 into acetanilide 2 in
excellent yield (99% yield). Furthermore, decrease of the catalytic
amount of TsCl to 5 mol % gave acetanilide in 95% yield. It is very
clear that TsCl can catalyze the Beckmann rearrangement smoothly
and give the rearrangement product in excellent yield. Therefore, it
also indicated that a catalytic mechanism shown in Scheme 1,
similar to that of BOP-Cl and CNC system, may be accountable
for it catalytic behavior. In order to rule out the possibility of po-
tential acid (HCl) catalyzed pathway, a competitive rearrangement
experiment of acetophenone oxime 1 was carried out by using
Recently, organocatalyst for the Beckmann rearrangement has
attracted researchers’ attention for its efficiency in catalytic activity
and easy to handle during the rearrangement. Cyanuric chloride
14
(
CNC), as the first organocatalyst, was reported to be a highly ef-
ficient catalyst for the Beckmann rearrangement by Ishihara and his
co-workers. In the effort aimed at developing new catalytic system
environ-friendly for the Beckmann rearrangement, we have de-
veloped BOP-Cl15 (bis(2-oxo-3-oxazolidinyl)phosphinic chloride) as
15
14
*
Corresponding authors. Tel./fax: þ86 021 64252431.
10 mol % of HCl in anhydrous CH
3
CN at reflux temperature, and
E-mail addresses: ddwwtt@163.com (W. Du), weiping_deng@ecust.edu.cn
(
W.-P. Deng).
gave no desired amide 2.
0
040-4020/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2009.07.036

H.-J. Pi et al. / Tetrahedron 65 (2009) 7790–7793
7791
rearrangement.14 In our case, 2 mol % of TsCl solely gave much less
catalytic activity for the Beckmann rearrangement of acetophenone
oxime. Interestingly, when 0.05 equiv of water and 1 equiv (based
on oxime) were added, N-acetyl aniline was obtained in 50% (entry
Table 1
Sulfonyl chloride catalyzed Beckmann rearrangement of acetophenone oxime
OH
N
H
N
Sulfonyl Chloride
6) and 5% (entry 7), respectively, despite the fact that TsCl shows
Solvent, reflux, 1 hr
O
enough stability in aqueous solution. The same phenomenon was
reported in ionic liquid supported sulfonyl chloride catalyzed
1
2
18
O
S
O
O
Beckmann rearrangement. Anhydrous solvent somehow played
a key role on the catalytic activity of the Beckmann rearrangement.
With all above results together, 5 mol % TsCl and/or 2 mol % TsCl/
Cl
Cl
Cl
S
S
O
O
O
H CO
F3C
O
3
O
2
2 mol % ZnCl in anhydrous acetonitrile was chosen as optimal
catalytic system for Beckmann rearrangement in our study.
S
O
Cl
S-1
S-2
S-3
S-4
Entry
Sulfonyl chloride
mol %
Solvent
Yielda b (%)
,
Table 2
Effect of acids as co-catalysts on the TsCl-catalyzed Beckmann rearrangement of
acetophenone oxime
1
2
3
4
5
6
7
8
S-1
S-1
S-2
S-3
S-4
S-1
S-1
S-1
10
5
5
5
5
5
5
5
CH
CH
CH
CH
CH
Toluene
Dioxane
THF
3
3
3
3
3
CN
CN
CN
CN
CN
99
95
96
97
OH
N
H
N
TsCl, additives
50
Trace
Trace
Trace
Solvent, reflux, 1 hr
O
2
1
a
Yielda b (%)
,
Ketoximes (2 mmol) were used for the Beckmann rearrangement in anhydrous
MeCN (4 mL).
Entry
TsCl (mol %)
Additive (mol %)
None
ZnCl
None
ZnCl
InCl
1
2
3
4
5
6
7
8
5
5
2
2
2
2
5
5
95
99
35
96
67
94
50
5
b
Isolated yields.
2
(5)
OH
R2
2
(2)
(2)
N
3
4
Zn(ClO) (2)
R1
H
H
2
O (5)
O (100)
2
Cat. TsCl
HCl
a
-
R1
Ketoximes (2 mmol) were used for the Beckmann rearrangement in anhydrous
OTs
N
N
MeCN (4 mL).
b
Isolated yields.
TsO
R2
R1
R2
H+
The generality and scope of the Beckmann rearrangement of
2
ketoximes to amides catalyzed by TsCl or TsCl/ZnCl catalyst sys-
N
R1
O
R2
H
tems were then investigated (Table 3). It is obvious that excellent
yields were obtained for most of oximes, aromatic oximes in par-
ticular, in acetonitrile under reflux condition for 1 h. TsCl (2 mol %)
O
O
S
R2
Table 3
O
OH
R1
N
NH
R2
N
catalyzed Beckmann rearrangementa
Scope of TsCl/ZnCl
2
Complex A
R1
R1
R2
OH
R1
O
O
N
NH
R2
TsCl (2 mol%) / ZnCl2 (2 mol%)
Scheme 1. Proposed mechanism for TsCl-catalyzed Beckmann rearrangement.
CH CN, reflux, 1 hr
R2
3
R1
Further screen of sulfonyl chlorides showed that aryl sulfonyl
chloride bearing both electron-donating and electron-withdrawing
groups gave the very similar result. However, camphorsulfonyl
chloride S-4 shows moderate catalytic activity and gave its amide in
R1
R2
Yieldb (%)
96
Entry
1
2
(2a)
(2b)
Ph
Ph
6
Me
Ph
Ph
c
99
c
c
c
c
3 (2c)
4 (2d)
p-(Me)C H4
98
99
98
98
99
98
95
97
98
5
0% yield. The reason for lower catalytic activity of S-4 might ascribe
p-(Me)C
p-(Me)C
p-(Cl)C
p-(Me)C
p-(MeO)C
o-(MeO)C
m-(MeO)C
Ph
p-(Cl)C
(CH
C8H17
(CH
6
H
H
4
4
6 4
p-(Cl)C H
5
6
7
8
(2e)
(2f)
(2g)
(2h)
6
p-(MeO)C
p-(MeO)C
Me
Me
Me
Me
Et
Me
6
H
H
4
4
to an undetermined electronic effect on the sulfur center causing by
different substituents. In terms of availability and chemical stability,
combined with above result, TsCl was chosen for further study. Next,
we examined the effect of solvents on the Beckmann rearrangement
of acetophenone oxime. As shown in Table 1, polar solvent acetoni-
trile gave its amide in 95% yield (entry 2), while other nonpolar sol-
vents all failed to give their amides in satisfactory yields.
6
H
4
6
6
H
4
6
H
H
4
9 (2i)
10 (2j)
6
4
6
H
4
1
1
1
1 (2k)
2 (2l)
3 (2m)
d
6
H
4
40 (97 )
e
2
)
11
99 (99 )
In order to screen the optimal catalytic system, Lewis acids such
14 (2n)
15 (4)
Me
98
f
c
as ZnCl
2
, InCl
3
were also investigated as co-catalysts to further
2
)
5
<5
optimize the rearrangement.
a
Ketoximes (2 mmol) were used for the Beckmann rearrangement in anhydrous
(2 mol %).
Isolated yields (overall yields of isomeric mixtures for the cases of 2c–f).
TsCl (2 mol %) was used and the reaction mixture was refluxed for 1 h.
TsCl (10 mol %) was used and the reaction mixture was refluxed for 2 h.
The arrangement of ketoxime (100 mmol) in the MeCN (50 mL) was carried out
From Table 2, it is clear that with ZnCl
2
as co-catalyst, the
MeCN (4 mL) in the presence of TsCl (2 mol %) and ZnCl
2
b
amount of TsCl could be further decreased to 2 mol % without no-
table decrease of its catalytic activity. ZnCl (5 mol %) gave 99% yield
of the amide (entry 2). ZnCl exhibits the highest co-catalytic ac-
c
2
d
e
2
tivity among all co-catalysts (entries 4 and 5). Previous report
suggests that sole Lewis acid could hardly catalyze the Beckmann
by using TsCl (1 mol %) and ZnCl2 (1 mol %).
f
2
TsCl (5 mol %) and ZnCl (5 mol %).

7792
H.-J. Pi et al. / Tetrahedron 65 (2009) 7790–7793
without any co-catalyst could efficiently catalyze Beckmann rear-
rangement of diaryl ketoxime in almost quantitative yields pro-
viding the 5:4 ratio of corresponding isomeric mixtures caused by
the 5:4 ratio of E/Z mixture of corresponding ketoximes (entries
and 13C (100 MHz) NMR chemical shifts are reported in CDCl
3
1
13
7.27 ppm for H, 77 ppm for C as standards and coupling con-
stants (J) are reported in hertz (Hz). The following abbreviations are
used to designate signal multiplicity: s¼singlet, d¼doublet,
t¼triplet, q¼quartet, m¼multiplet, br¼broad.
2
–6). For acetophenone oxime series, excellent yields were obtained
for the monosubstituted ketoxime bearing methyl or methoxy
group on aromatic ring (entries 7–10). Unfortunately, 1-(4-chloro-
phenyl)ethanone oxime gave corresponding amide in only 40%
yield under standard reaction condition, which can be improved to
4.2. General procedure A
A solution of ketoxime (2 mmol), 1–5 mol % of TsCl and/or
–2 mol % Lewis acid in 4 mL of dry MeCN was refluxed under
97% when 10 mol % of TsCl was used (entry 12). Furthermore, the
0
Beckmann rearrangement of aliphatic oximes was also investigated.
We found that our catalytic system is applicable to aliphatic oximes
with a relatively large ring (entry 11). Scale-up test was conducted
for the Beckmann rearrangement of cyclododecanone oxime
a nitrogen atmosphere. After completion of the reaction as moni-
tored by TLC, the reaction was quenched with saturated aqueous
sodium hydrogen carbonate. The organic layer was extracted with
ethyl acetate, dried over anhydrous sodium sulfate, and concen-
trated on rotary vacuum evaporator. The resulting crude product
was purified by column chromatography on silica gel to give the
corresponding amide in high yield.
(
(
2
100 mmol). Perfect yield was obtained with TsCl (1 mol %)/ZnCl
1 mol %) (entry 13). The yield of rearrangement product of nonan-
-one oxime reached to 98%, when TsCl (5 mol %)/ZnCl (5 mol %)
was used as catalyst system (entry 14). However, the rearrangement
of cyclohexanone oxime gave only trace amount of -caprolactam by
2
2
Since all products are known, only the data of several typical
compounds were listed as following.
3
using TsCl (5 mol %), and reaction system became sluggish and
turned to brown-black after prolonging the reaction time. This re-
.2.1. Acetanilide (2a). Mp: 114–115 C (lit. mp 114–116 C); 1H
ꢀ
13
ꢀ
4
14
15
sult is very similar to that of the CNC, BOP-Cl system.
NMR:
d
2.17 (s, 3H), 7.10 (t, J¼7.4 Hz, 1H), 7.30–7.35 (m, 3H), 7.50 (d,
Interestingly, both conversion and selectivity dramatically in-
creased when the loading of TsCl increased to 20 mol % (entries 2 and
13
J¼8.0 Hz, 2H); C NMR:
d 169.38, 138.18, 128.86, 124.28, 120.35,
24.30.
3
in Table 4). Further increased to 30 mol %, better conversion and
selectivity were obtained and the decrease of reaction temperature
to 60 C gave very similar conversion and selectivity (entries 5 and
ꢀ
19
4
.2.2. N-Phenylbenzamide (2b). Mp: 164–165 C (lit. mp 164–
ꢀ
ꢀ
1
165 C); H NMR:
d
8.03 (br s, 1H), 7.87 (d, J¼7.3 Hz, 2H), 7.66 (d,
6
). When 20 mol % MsCl was employed, almost quantitative con-
J¼7.9 Hz, 2H), 7.54 (t, J¼7.3 Hz, 1H), 7.46 (t, J¼7.4 Hz, 2H), 7.37 (t,
version and 88% of selectivity were obtained (entry 7).
13
J¼7.9 Hz, 2H), 7.16 (t, J¼7.4 Hz, 1H); C NMR:
d
165.88, 137.96,
1
35.00, 131.81, 129.07, 128.76, 127.06, 124.57, 120.31.
Table 4
The catalytic Beckmann rearrangement of cyclohexanone oxime using TsCl
ꢀ
13
4
.2.3. N-p-Tolylacetamide (2g). Mp: 149–150 C (lit. mp 149–
OH
O
1
O
ꢀ
N
151 C); H NMR:
d
8.28 (br s, 1H), 7.40 (d, J¼8.2 Hz, 2H), 7.09 (d,
13
TsCl
J¼8.1 Hz, 2H), 2.30 (s, 3H), 2.12 (s, 3H); C NMR:
d
168.95, 135.53,
NH
+
CH CN
133.84, 129.36, 120.30, 24.28, 20.84.
3
3
4
5
ꢀ
13
4
.2.4. N-Phenylpropionamide (2k). Mp 106–107 C (lit. mp 103–
ꢀ
1
Sel. of 4a (%)
104 C);
H NMR:
d
8.30 (s, 1H), 7.55 (d, J¼7.8 Hz, 2H), 7.27 (t,
Entry
Catalyst (mol %)
Temp
Conv./%
J¼7.8 Hz, 2H), 7.08 (t, J¼7.4 Hz, 1H), 2.37 (q, J¼7.4 Hz, 2H), 1.21 (t,
1
2
3
4
5
6
7
TsCl (5)
Reflux
Reflux
12
24
66
74
90
35
53
83
71
94
93
88
13
J¼7.6 Hz, 3H); C NMR:
d 172.97, 138.24, 128.84, 124.12, 120.25,
TsCl (10)
TsCl (20)
TsCl (20)
TsCl (30)
TsCl (30)
MsCl (20)
Reflux
30.55, 9.81.
ꢀ
ꢀ
60
60
C
C
ꢀ
20
4
.2.5. Azacyclotridecan-2-one (2m). Mp 150–151 C (lit. mp 143–
Reflux
96
>99
ꢀ
1
145 C); H NMR:
d
6.07 (br s, 1H), 3.26 (dd, J¼5.7, 10.5 Hz, 2H),
ꢀ
60
C
2
(
2
.19–2.16 (m, 2H), 1.68–1.62 (m, 2H), 1.55–1.45 (m, 2H), 1.34–1.27
a
Ketoximes (2 mmol) were used for the Beckmann rearrangement in anhydrous
MeCN (2 mL), and the conversion and selection were determined by GC.
13
m, 14H); C NMR: d 173.57, 39.00, 36.82, 28.27, 26.73, 26.32, 26.17,
5.71, 25.20, 24.92, 24.61, 23.90.
For the case of caprolactam (4), 5–30 mol % of TsCl or MsCl was
used. The conversion and selectivity were analyzed by GC (detailed
GC analytical data please see Supplementary data).
3
. Conclusion
In summary, we have developed new catalytic system using TsCl
as highly efficient organocatalyst for Beckmann rearrangement. In
most cases, excellent yields could be obtained by using 5 mol % TsCl
Acknowledgements
or 2 mol % TsCl/2 mol % ZnCl
2
in anhydrous acetonitrile. MsCl
(
20 mol % or 30 mol % of TsCl) was necessary for cyclohexanone
This work was supported by the Natural Science Foundation of
China (No. NSFC 20602011), the Shanghai Committee of Science
and Technology (No. 06PJ14023), New Century Excellent Talents in
University, the Ministry of Education, China (No. NCET-07-0283),
and ‘111’ Project (No. B07023).
oxime case. Further studies will be focusing on exploring more
versatile and environ-benign catalytic Beckmann rearrangement
system, and clarifying the catalytic mechanism.
4
4
. Experimental
Supplementary data
.1. General methods
The 1H NMR and 13C NMR copies of most compounds and the
copies of GC data for caprolactam will be attached as Supplemen-
tary data. Supplementary data associated with this article can be
found in the online version, at doi:10.1016/j.tet.2009.07.036.
All solvents were distilled under standard procedures prior to
use under nitrogen atmosphere (For example: CH
CaH ; THF, dioxane, toluene distilled from sodium). H (400 MHz)
3
CN distilled from
1
2

H.-J. Pi et al. / Tetrahedron 65 (2009) 7790–7793
7793
References and notes
Chem. Commun. 2002, 2208; (c) Ikushima, Y.; Hatakeda, K.; Sato, O.; Yokoyama,
T.; Arai, M. Angew. Chem., Int. Ed. 1999, 38, 2910.
6
. (a) Guo, S.; Du, Z.; Zhang, S.; Li, D.; Li, Z.; Deng, Y. Green Chem. 2006, 8, 296;
1
. (a) Smith, M. B.; March, J. Advanced Organic Chemistry, 5th ed.; John Wiley &
(
b) Guo, S.; Deng, Y. Catal. Commun. 2005, 6, 225.
Sons: New York, NY, 2001; p 1415; (b) Gawley, R. E. Org. React. 1988, 35, 1.
. (a) Dalhhoff, G.; Niederer, J. P. M.; H o¨ lderich, W. F. Catal. Rev.dSci. Eng. 2001, 43,
7. Sato, H.; Yoshika, H.; Izumi, Y. J. Mol. Catal., A: Chem. 1999, 149, 25.
2
3
4
8
9
. Li, D.; Shi, F.; Guo, S.; Deng, Y. Tetrahedron Lett. 2005, 46, 671.
. Peng, J.; Deng, Y. Tetrahedron Lett. 2001, 42, 403.
381; (b) Tatsumi, T. In Fine Chemicals through Heterogeneous Catalysis; Sheldon,
R. A., van Bekkum, H., Eds.; Wiley-VCH: Weinheim, 2001; p 185; (c) Li, W. C.; Lu,
A. H.; Palkovits, R.; Schmidt, W.; Spliethoff, B.; Schuth, F. J. Am. Chem. Soc. 2005,
1
1
1
0. Arisawa, M.; Yamaguchi, M. Org. Lett. 2001, 3, 311.
1. Yadav, J. S.; Reddy, B. V. S.; Madhavi, A. V.; Ganesha, Y. S. S. J. Chem. Res., Synop.
002, 236.
2. De, S. K. Synth. Commun. 2004, 34, 3431.
3. Ramalingan, C.; Park, Y. T. J. Org. Chem. 2007, 72, 4536.
4. Furuya, Y.; Ishihara, K.; Yamamoto, H. J. Am. Chem. Soc. 2005, 127, 11240.
5. Zhu, M.; Cha, C.; Deng, W.-P.; Shi, X.-X. Tetrahedron Lett. 2006, 47, 4861.
6. (a) Zhu, M. Studies on Small Molecule Catalyzed Beckmann rearrangement.
27, 12595.
. (a) De Luca, L.; Giacomelli, G.; Porcheddu, A. J. Org. Chem. 2002, 67, 6272;
b) Kusama, H.; Yamashita, Y.; Narasaka, K. Bull. Chem. Soc. Jpn. 1995, 68,
2
1
1
1
1
(
373; (c) Wang, B.; Gu, Y.; Luo, C.; Yang, T.; Yang, L.; Suo, J. Tetrahedron Lett.
2004, 45, 3369; (d) Chandrasekhar, S.; Gopalaiah, K. Tetrahedron Lett. 2002,
43, 2455.
1
. (a) Ghiaci, M.; Abbaspur, A.; Kalbasi, R. Appl. Catal., A 2005, 287, 83; (b) Forni, L.;
A thesis for the degree of Master of Science, Jan 2007. (b) Hashimoto, M.; Obora,
Y.; Sakaguchi, S.; Ishii, Y. J. Org. Chem. 2008, 73, 2894.
Fornasari, G.; Giordano, G.; Lucarelli, C.; Katovic, A.; Trifir O` , F.; Perri, C.; Nagy, J.
B. PhysChemChemPhys 2004, 6, 1842; (c) Kim, S. J.; Jung, K. D.; Joo, O. S.; Kim, E.
J.; Kang, T. B. Appl. Catal., A: Gen. 2004, 266, 173; (d) Dongare, M. K.; Bhagwat, V.
V.; Ramana, C. V.; Gurjar, M. K. Tetrahedron Lett. 2004, 45, 4759; (e) Mao, D.;
Chen, Q.; Lu, G. Appl. Catal., A: Gen. 2003, 244, 273; (f) Maheswari, R.; Sivaku-
mar, K.; Sankarasubbier, T.; Narayanan, S. Appl. Catal., A: Gen. 2003, 248, 291;
17. Maruoka, K.; Miyazaki, T.; Ando, M.; Matsumura, Y.; Sakane, S.; Hattori, K.;
Yamamoto, H. J. Am. Chem. Soc. 1983, 105, 2831.
18. Gui, J.; Deng, Y.; Hua, Z.; Sun, Z. Tetrahedron Lett. 2004, 45, 2681.
19. Sardarian, A. R.; Shahsavari-Fard, Z.; Shahsavari, H. R.; Ebrahimi, Z. Tetrahedron
Lett. 2007, 48, 2639.
(
j) Dahlhoff, G.; Barsnick, U.; H o¨ lderich, W. F. Appl. Catal., A: Gen. 2001, 210, 83.
20. Yadav, J. S.; Subba Reddy, B. V.; Subba Reddy, U. V.; Praneeth, K. Tetrahedron Lett.
5. (a) Boero, M.; Ikeshoji, T.; Liew, C. C.; Terakura, K.; Parrinello, M. J. Am. Chem.
2008, 49, 4742.
Soc. 2004, 126, 6280; (b) Ikushima, Y.; Hatakeda, K.; Sato, M.; Sato, O.; Arai, M.