Au(iii)-catalyzed intermolecular amidation of benzylic C-H bonds

Article abstract of DOI:10.1039/c2ob26857d

Au(iii)-catalyzed intermolecular amidations of benzylic C-H bonds with sulfonamides and carboxamides are described. The protocol with the Au-bipy complex/N-bromosuccinimide system provides practical applications for synthesis of various amides via C-H activations. The reaction proceeds with high efficiency to give the corresponding amines, which are extremely useful synthetic intermediates.

Full text of DOI:10.1039/c2ob26857d

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Au(III)-catalyzed intermolecular amidation of benzylic C–H bonds†
Yan Zhang,a,b Bainian Feng*a and Chengjian Zhub
Received 15th August 2012, Accepted 16th October 2012
DOI: 10.1039/c2ob26857d
Au(III)-catalyzed intermolecular amidations of benzylic C–H
bonds with sulfonamides and carboxamides are described.
The protocol with the Au–bipy complex/N-bromosuccinimide
system provides practical applications for synthesis of
various amides via C–H activations. The reaction proceeds
with high efficiency to give the corresponding amines, which
are extremely useful synthetic intermediates.
Scheme 1 Gold-complex catalyzed amidation of benzylic C–H bonds.
Introduction
Transition metal catalyzed activations of C–H bonds, especially
sp3 C–H bonds, and subsequent C–C and C–N bond formations
which avoid the use of prefunctionalized starting materials are
valuable and straightforward synthetic strategies.1 The direct C–
N bond conversions of C–H bonds are of fundamental impor-
tance in organic synthesis, owing to the high prevalence of nitro-
gen-containing molecules in natural and pharmaceutical
compounds.2 The selective amidation among many different
C–H bonds remains a challenge.3 Considerable achievements have
been made within the past several years in order to realize the
intramolecular and intermolecular amidations via the activations
of C–H bonds, particularly with regard to allylic and benzylic
C–H bonds. Three factors must be considered in the amidations
of C–H bonds: metal catalysts, oxidants and nitrogen sources.
The reported metal catalysts include rhodium, ruthenium, silver,
iron, copper and other metal complexes.4–9 PhI(OAc)2,
t-BuOOAc, t-BuOOH and N-bromosuccinimide can be used in
amidations of C–H bonds.9 The C–H amidation methodologies
reported usually proceed through transition metal–nitrene
(imido) intermediates4–7 and alternative nitrene derivatives such
as chloramines-T,10 bromamine-T,11 and tosyloxycarbamates12
have been used as the primary nitrogen sources.
of C–H bonds has highlighted their remarkable reactivity and
leads to a significant increase in their utilisation.16 He et al.
achieved nitrene insertion into aromatic C–H bonds in a Au(III)
catalyzed process for the formation of carbon–nitrogen bonds.17
Insertion of PhI = NTs into a number of arenes was achieved at
room temperature in the presence of 2 mol% AuCl3. The authors
commented that this procedure was only applicable to benzene
rings possessing three or more substituents, which may be
related to the higher nucleophilicities of these substrates. We
have reported some Au–bipydine complex catalyzed formations
of C–C, C–O and C–N bonds.18 Oxidative α-cyanation of ter-
tiary amines was catalyzed by Au complexes with trimethylsilyl
cyanide to afford the corresponding α-aminonitriles in the pres-
ence of tert-butyl hydroperoxide in good to excellent yields
under acid-free conditions at room temperature. Inspired by the
previous good results, we are searching for a convenient,
efficient, and general Au complex/oxidant system to achieve the
selective amidation of C–H bonds. Fortunately, an excellent
Au–bipy/N-bromosuccinimide system was developed for the
amidation of sp3 C–H bonds (Scheme 1).
Au catalysis has rapidly become a hot topic in chemistry in
the past decade.13 Au species are equally effective as hetero-
geneous or homogeneous catalysts,14 which show excellent
results in diversified reactions.15 The use of powerful and envir-
onmentally benign Au(I) and Au(III) catalysts in functionalization
Results and discussion
Initially, ethylbenzene (5b) and p-toluenesulfonamide (PTSA,
6a) were chosen as the model substrates to optimize the catalysis
conditions, including catalysts, oxidants and solvents. The
results are listed in Table 1. Some Au catalysts, Au–bipy
complex (1), Au–pyridine complex (2), Au–picolinic acid
complex (3), Bu4NAuCl4, AuCl3 and AuClPPh3, were tested in
CH3CN (Fig. 1). Among the Au catalysts tested, Au–bipy
complex (1) was found to be the most effective catalyst for the
reaction (entry 5). No reaction was observed without using any
catalyst (entry 1), AuClPPh3 also showed no catalytic activity
aSchool of Medicine and Pharmaceutics, Jiangnan University, Wuxi
214122, China. E-mail: zhangyan@jiangnan.edu.cn;
Fax: +86-0519-85197052
bSchool of Chemistry and Chemical Engineering, State Key Laboratory
of Coordination Chemistry, Nanjing University, Nanjing 210093, China
†Electronic supplementary information (ESI) available. See DOI:
10.1039/c2ob26857d
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Table 1 Optimization of the gold-catalyzed amidation of ethylbenzene
with p-toluenesulfonamidea
Table 2 Gold–bipy/NBS catalyzed amidation of benzylic sp3C–H
bonds with sulphonamidesa
Entry
1
5
6
Product 4
Yieldb [%]
78
2
3
4
5a
73
70
72
Time
(h)
Yieldb
(%)
Entry
Catalyst (mol%)
Oxidant
Solvent
1
2
3
4
5
6
7
8
None
NBS
NBS
NBS
NBS
NBS
NBS
NBS
NBS
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
MeOH
10
10
10
8
Trace
Trace
21
55
68
51
42
60
6a
6b
AuClPPh3 (10)
AuCl3 (10)
Bu4NAuCl4 (10)
1 (10)
2 (10)
3 (10)
8
5b
12
13
8
1 (10)
9
1 (10)
1 (10)
1 (10)
1 (10)
1 (10)
1 (10)
1 (3)
1 (3)
NBS
NBS
NCS
PhI(OAc)2
t-BuOOH
t-BuOOAc
NBS
EtOH
8
8
8
8
8
8
8
3
62
75
70
Trace
Trace
Trace
75
5
6
6b
6a
81
71
10
11
12
13
14
15c
16d
Toluene
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
NBS
78
7
5d
6b
74
a Reaction conditions: ethylbenzene (0.5 mmol), p-toluenesulfonamide
(0.5 mmol), solvent (3 ml), oxidant (0.5 mmol) and gold catalyst
(10 mol%). b Isolated yield. c 3 mol% gold complex 1. d 1 mmol of
ethylbenzene was used.
8
9
6a
6a
70
71
10
5f
6b
73
Fig. 1 Gold complexes 1, 2 and 3.
11
12
13
6a
6b
6a
72
69
73
(entry 2). Although other gold complexes showed moderate cata-
lytic activities, the reactions required relatively longer times to
reach the reasonable yields (entries 4–7). AuCl3, which was
reported as the effective catalyst for the insertion of PhI = NTs
into arenes,17 showed a little catalytic activity for the reaction
(entry 3). The effect of solvents (without any previous procedure
for the commercially available solvents) was also investigated.
Acetonitrile was the most effective solvent, although ethanol,
methanol, and toluene could also be used (entries 7–9). A
slightly lower yield was achieved when NCS replaced NBS as
the oxidant (entry 11). No product was detected when other oxi-
dants such as PhI(OAc)2, t-BuOOAc, t-BuOOH and NCS were
used (entries 12–14). The reaction also provided an excellent
yield when the amount of Au–bipy complex (1) was reduced to
3 mol% (entry 15). The use of excess ethylbenzene (2 equiv.)
shortened the reaction time to 2–3 h with a similar yield (entry
16). After the optimization of catalysts, oxidants and solvents,
the following amidations were carried out under the standard
conditions: 3 mol% Au–bipy as the catalyst, NBS as the oxidant,
and acetonitrile as the solvent .
5g
14
6b
71
15
16
6a
6b
68
69
5j
a Reaction conditions: benzylic reagent (1 mmol), amide (0.5 mmol),
acetonitrile (3 ml), N-bromosuccinide (NBS) (0.5 mmol) and gold-
complex 1 (3 mol%). b Isolated yield.
In order to explore the scopes of substrates for the amidations
of benzylic sp3 C–H bonds in the Au–bipy complex/NBS
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Table 3 Gold–bipy/NBS catalyzed amidation of benzylic sp3C–H
bonds with carboxamidesa
Entry
1
5
6
Product 4
Yieldb (%)
71
5a
2
5a
73
Scheme 2 (a) Conversion of sulfonamide to N-bromosulfonamide; (b)
gold catalyzed reaction of ethylbenzene with N-bromosulfonamide.
3
4
5
5b
5b
5b
6c
70
70
69
6d
6
7
5c
5c
6c
85
81
6d
8
5d
6c
73
9
5d
5e
6d
6c
71
70
10
Scheme 3 (a) Effect of a radical inhibitor on the gold catalyzed amida-
tion; (b) effect of a radical inhibitor on the bromination of TsNH2; (c)
11
12
5f
6c
6c
69
75
effect of
bromosulfonamide.
a radical inhibitor on the reaction starting from the
5g
withdrawing property of the substituents on benzene rings has
no obvious effect on the reaction time and yield. For the carbox-
amides and sulfonamides, the electronic effects of the substrates
are prime, the substrates containing the stronger electron-with-
drawing groups show higher reactivity, and their order of activity
is sulfonamides > benzamides.
13
14
5g
5h
6e
6c
70
72
To gain insight into the gold-catalyzed amidation of benzylic
C–H bonds, several control experiments were carried out to elu-
cidate the mechanism. As shown in Scheme 2, reaction of sulfo-
nylamide (6a) with NBS in acetonitrile produced N-bromo
sulfonylamide, a similar product was obtained and identified by
Sudalai.19 Ethylbenzene (5b) and Au-complex (1) were added to
the resulting solution, and the reaction provided 70% yield.
Adding the radical inhibitor TEMPO (2,2,6,6-tetramethylpiperi-
dinooxy) to the reaction of 5b and 6a dramatically decreased the
reaction rate and yield (Scheme 3a). Two control experiments
were carried out to elucidate which step the radical inhibitor had
an effect on. As shown in Schemes 3b and 3c, the separate bro-
mination step was stopped by the radical inhibitor, while the
reaction starting from the bromo-amide still worked in the pres-
ence of TEMPO.
15
5i
6e
71
a Reaction conditions: benzylic reagent (1 mmol), amide (0.5 mmol),
acetonitrile (3 ml), N-bromosuccinide (NBS) (0.5 mmol) and gold-
complex 1 (3 mol%). b Isolated yield.
system, a series of benzylic substrates were investigated. As
shown in Tables 2 and 3, all the substrates examined were selec-
tively and efficiently converted into the corresponding amidation
products in moderate yields. The activity order of benzylic
reagents is triphenylmethane > diphenylmethane > ethylbenzene
= tetrahydronaphthalene. The electron-donating or electron-
In light of our previous work on the Au-catalyzed oxidative
C–C coupling, a plausible mechanism for the Au-catalyzed
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and Technological Special Project (2009ZX09103-081) is also
acknowledged.
Notes and references
1 (a) C.-J. Li, Acc. Chem. Res., 2009, 42, 335; (b) J. J. Li, T. S. Mei and
J. Q. Yu, Angew. Chem., Int. Ed., 2008, 47, 6452; (c) D. H. Wang,
M. Was, R. Giri and J. Q. Yu, J. Am. Chem. Soc., 2008, 130, 7190;
(d) O. Basle and C.-J. Li, Org. Lett., 2008, 10, 3661.
2 Modern Amination Reactions, ed. A. Ricci, Wiley-VCH, Weinheim,
Germany, 2000.
3 (a) K. Godula and D. Sames, Science, 2006, 312, 67; (b) R. G. Bergman,
Nature, 2007, 446, 391; (c) H. Chen, S. Schlecht, T. C. Semple and
J. F. Hartwig, Science, 2000, 287, 1995; (d) J.-Y. Cho, M. K. Tse,
D. Holmes, R. E. Maleczka Jr. and M. R. Smith III, Science, 2002, 295,
305; (e) J. A. Labinger and J. E. Bercaw, Nature, 2002, 417, 507;
(f) Z. Li, D. S. Bohle and C.-J. Li, Proc. Natl. Acad. Sci. U. S. A., 2006,
103, 8928; (g) Z. Li and C.-J. Li, Org. Lett., 2004, 6, 4997; (h) Z. Li and
C.-J. Li, J. Am. Chem. Soc., 2004, 126, 11810; (i) Z. Li and C.-J. Li,
J. Am. Chem. Soc., 2005, 127, 3672.
4 For some recent reviews of C–H bond amidations, see: (a) F. Collet,
R. H. Dodd and P. Dauban, Chem. Commun., 2009, 5061;
(b) H. M. L. Davies and M. S. Long, Angew. Chem., Int. Ed., 2005, 44,
3518; (c) J. Du Bois, Chemtracts, 2005, 18, 1; (d) C. G. Espino and
J. Du Bois, in Modern Rhodium-Catalyzed Organic Reactions, ed.
P. A. Evans, Wiley-VCH, Weinheim, 2005, p. 379; (e) J. A. Halfen, Curr.
Org. Chem., 2005, 9, 657.
Scheme 4 Plausible mechanism for the gold catalyzed amidation of
benzylic bonds (L = 2,2′-bipyridine).
amidation of benzylic sp3C–H bonds is shown in Scheme 4.
Initially, AuI species is generated in situ from the reduction of
AuIII by the solvent or substrates.18b,20 Reaction of carboxamide
or sulfonylamide with NBS yields N-bromocarboxamide or
N-bromosulfonamide 7. Exchange of AuI species with a proton
in 7 gives intermediate complex 8. Isomerization of 8 gives Au–
nitrene complex 9. 9 combines with the sp3 C–H bond of the
benzylic substrate to form the transition state 10, and release of
gold species in 10 gives the target product 4.
5 Representative
intramolecular
metal–nitrene-type
amidations:
(a) S. M. Paradine and M. C. White, J. Am. Chem. Soc., 2012, 134,
2036; (b) M. Ichinose, H. Suematsu, Y. Yasutomi, Y. Nishioka, T. Uchida
and T. Katsuki, Angew. Chem., Int. Ed., 2011, 50, 9884; (c) H. J. Lu,
H. L. Jiang, L. Wojtas and X. P. Zhang, Angew. Chem., Int. Ed., 2010,
49, 10192; (d) E. Milczek, N. Boudet and S. Blakey, Angew. Chem.,
2008, 120, 6931; E. Milczek, N. Boudet and S. Blakey, Angew. Chem.,
Int. Ed., 2008, 47, 6825; (e) M. Kim, J. V. Mulcahy, C. G. Espino and
J. Du Bois, Org. Lett., 2006, 8, 1073; (f) H. Lebel, K. Huard and
S. Lectard, J. Am. Chem. Soc., 2005, 127, 14198; (g) J. Zhang,
P. W. H. Chan and C.-M. Che, Tetrahedron Lett., 2005, 46, 5403;
(h) Y. Cui and C. He, Angew. Chem., Int. Ed., 2004, 43, 4210;
(i) C. G. Espino, K. W. Fiori, M. Kim and J. Du Bois, J. Am. Chem. Soc.,
2004, 126, 15378; ( j) J.-L. Liang, S.-X. Yuan, J.-S. Huang and
C.-M. Che, J. Org. Chem., 2004, 69, 3610; (k) C. Fruit and P. Muller,
Helv. Chim. Acta, 2004, 87, 1607; (l) J.-L. Liang, S.-X. Yuan,
J.-S. Huang, W.-Y. Yu and C.-M. Che, Angew. Chem., Int. Ed., 2002, 41,
3465; (m) C. G. Espino, P. M. Wehn, J. Chow and J. Du Bois, J. Am.
Chem. Soc., 2001, 123, 6935; (n) C. G. Espino and J. Du Bois, Angew.
Chem., Int. Ed., 2001, 40, 598; (o) R. Breslow and S. H. Gellman, J. Am.
Chem. Soc., 1983, 105, 6728.
Conclusions
In conclusion, we have demonstrated the novel Au-catalyzed
amidation of benzylic C–H bonds via C–H activation. The reac-
tion proceeds with high efficiency to give the corresponding
amines which are extremely useful synthetic intermediates in the
construction of biologically important compounds. Research is
currently underway to elucidate the mechanism and to apply the
principle to other catalytic systems.
Experimental section
General procedure for amidation of benzylic sp3C–H bonds with
amides
6 Representative
intermolecular
metal–nitrene-type
amidations:
(a) A. Iglesias, R. Alvarez, A. R. de Lera and K. Muniz, Angew. Chem.,
Int. Ed., 2012, 51, 2225; (b) Z. K. Ni, Q. Zhang, T. Xiao, Y. Y. Zheng,
Y. Li, H. W. Zhang, J. P. Zhang and Q. Liu, Angew. Chem., Int. Ed.,
2012, 51, 1244; (c) Y. H. Ye, J. Zhang, G. Wang, S. Y. Chen and
X. Q. Yi, Tetrahedron, 2011, 67, 4649; (d) D. A. Powell and H. Fan,
J. Org. Chem., 2010, 75, 2726; (e) R. H. Fan, W. X. Li, D. M. Pu and
L. Zhang, Org. Lett., 2009, 11, 1425; (f) C. G. Liang, F. Collet,
F. Robert-Peillard, P. Muller, R. H. Dodd and P. Dauban, J. Am. Chem.
Soc., 2008, 130, 343; (g) Z. Wang, Y. M. Zhang, H. Fu, Y. Y. Jiang and
Y. F. Zhao, Org. Lett., 2008, 10, 1863; (h) Z. G. Li, D. A. Capretto,
R. Rahaman and C. He, Angew. Chem., Int. Ed., 2007, 46, 5184;
(i) M. R. Fructos, S. Trofimenko, M. M. Díaz-Requejo and P. J. Pérez,
J. Am. Chem. Soc., 2006, 128, 11784; ( j) H.-Y. Thu, W.-Y. Yu and
C.-M. Che, J. Am. Chem. Soc., 2006, 128, 9048; (k) S. K.-Y. Leung,
W.-M. Tsui, J.-S. Huang, C.-M. Che, J.-L. Liang and N. Zhu, J. Am.
Chem. Soc., 2005, 127, 16629; (l) M. Yamawaki, H. Tsutsui, S. Kitagaki,
M. Anada and S. Hashimoto, Tetrahedron Lett., 2002, 43, 9561;
(m) Y. Kohmura and T. Katsuki, Tetrahedron Lett., 2001, 42, 3339;
(n) J. Yang, R. Weinberg and R. Breslow, Chem. Commun., 2000, 531;
(o) S.-M. Au, J.-S. Huang, C.-M. Che and W.-Y. Yu, J. Org. Chem.,
2000, 65, 7858; (p) X.-Q. Yu, J.-S. Huang, X.-G. Zhou and C.-M. Che,
Org. Lett., 2000, 2, 2233; (q) I. Nageli, C. Baud, G. Bernardinelli,
Y. Jacquier, M. Moran and P. Muller, Helv. Chim. Acta, 1997, 80, 1087;
(r) J. P. Mahy, G. Bedi, P. Battioni and D. Mansuy, Tetrahedron Lett.,
A 25 mL round-bottom flask equipped with a magnetic stirrer
was charged with benzylic reagent (1 mmol), amide (0.5 mmol),
acetonitrile (3 ml), N-bromosuccinide (NBS) (0.5 mmol) and
gold-complex 1 (3 mol%). The resulting mixture was continu-
ously stirred at 70 °C for 2–4 h. At the end of the reaction, the
mixture was filtered and the filtrate was extracted with ethyl
acetate (3–10 mL). The combined organic layer was washed
with brine, dried over anhydrous sodium sulfate and concen-
trated under reduced pressure. The residue was purified by
column chromatography on silica gel and the fraction was col-
lected and concentrated to give the desired product.
Acknowledgements
We gratefully acknowledge the National Natural Science Foun-
dation of China (20832001, 20972065, 21074054) and the
National Basic Research Program of China ((2007CB925103,
2010CB92330) for their financial support. The Major Scientific
9140 | Org. Biomol. Chem., 2012, 10, 9137–9141
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1988, 29, 1927; (s) R. Breslow and S. H. Gellman, J. Chem. Soc., Chem.
Commun., 1982, 1400.
J. Org. Chem., 2001, 66, 30; (d) R. Vyas, G.-Y. Gao, J. D. Harden and
X. P. Zhang, Org. Lett., 2004, 6, 1907.
7 (a) Y. Kohmura, K.-I. Kawasaki and T. Katsuki, Synlett, 1997, 1456;
(b) K. Smith, C. D. Hupp, K. L. Allen and G. A. Slough, Organometal-
lics, 2005, 24, 1747; (c) J. S. Clark and C. Roche, Chem. Commun.,
2005, 5175.
12 (a) H. Lebel, K. Huard and S. Lectard, J. Am. Chem. Soc., 2005, 127,
14198; (b) H. Lebel and K. Huard, Org. Lett., 2007, 9, 639; (c) H. Lebel,
O. Leogane, K. Huard and S. Lectard, Pure Appl. Chem., 2006, 78, 363.
13 (a) A. S. K. Hashmi and G. J. Hutchings, Angew. Chem., Int. Ed., 2006,
45, 7896; (b) A. S. K. Hashmi, Chem. Rev., 2007, 107, 3180;
(c) T. C. Boorman and I. Larrosa, Chem. Soc. Rev., 2011, 40, 1910;
(d) M. N. Hopkinson, A. D. Gee and V. Gouverneur, Chem.–Eur. J.,
2011, 17, 8248; (e) T. de Haro and C. Nevado, Synthesis, 2011, 2530;
(f) L. P. Liu and G. B. Hammond, Chem. Soc. Rev., 2012, 41, 3129.
14 W. B. Wang, G. B. Hammond and B. Xu, J. Am. Chem. Soc., 2012, 134,
5697.
15 L. Guczi, A. Beck and Z. Paszti, Catal. Today, 2012, 181, 26.
16 A. Hashmi, K. Stephen, A. M. Schuster, S. Litters, F. Rominger and
M. Pernpointner, Chem.–Eur. J., 2011, 17, 5661.
17 Z. G. Li, D. A. Carpretto, R. O. Rahaman and C. He, J. Am. Chem. Soc.,
2007, 129, 12058.
8 For other strategies for the amination of allylic and benzylic C–H bonds,
see: (a) W. Adam and O. Krebs, Chem. Rev., 2003, 103, 4131;
(b) M. Johannsen and K. A. Jørgensen, Chem. Rev., 1998, 98, 1689.
9 (a) M. R. Fructos, S. Trofimenko, M. M. Díaz-Requejo and P. J. Pérez,
J. Am. Chem. Soc., 2006, 128, 11784; (b) R. Bhuyan and
K. M. Nicholas, Org. Lett., 2007, 9, 3957; (c) D. P. Albone,
S. Challenger, A. M. Derrick, S. M. Fillery, J. L. Irwin, C. M. Parsons,
H. Takada, P. C. Taylor and D. J. Wilson, Org. Biomol. Chem., 2005, 3,
107; (d) G. Pelletier and D. A. Powell, Org. Lett., 2006, 8, 6031;
(e) Y. Zhang, H. Fu, Y. Jiang and Y. Zhao, Org. Lett., 2007, 9, 3813.
10 (a) D. P. Albone, S. Challenger, A. M. Derrick, S. M. Fillery, J. L. Irwin,
C. M. Parsons, H. Takada, P. C. Taylor and D. J. Wilson, Org. Biomol.
Chem., 2005, 3, 107; (b) L. Simkhovich and Z. Gross, Tetrahedron Lett.,
2001, 42, 8089; (c) D. P. Albone, P. S. Aujla, P. C. Taylor, S. Challenger
and A. M. Derrick, J. Org. Chem., 1998, 63, 9569.
18 (a) Y. Zhang, H. Peng, M. Zhang, Y. X. Cheng and C. J. Zhu, Chem.
Commun., 2011, 47, 2354; (b) J. Xie, H. M. Li, J. C. Zhou, Y. X. Cheng
and C. J. Zhu, Angew. Chem., 2012, 124, 1278.
11 (a) G.-Y. Gao, J. D. Harden and X. P. Zhang, Org. Lett., 2005, 7, 3191;
(b) J. D. Harden, J. V. Ruppel, G.-Y. Gao and X. P. Zhang, Chem.
Commun., 2007, 4644; (c) B. M. Chanda, R. Vyas and A. V. Bedekar,
19 (a) V. V. Thakur, S. K. Talluri and A. Sudalai, Org. Lett., 2003, 5, 861;
(b) S. K. Talluri and A. Sudalai, Org. Lett., 2005, 7, 855.
20 X. Q. Yao and C. J. Li, J. Am. Chem. Soc., 2004, 126, 6884.
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