Copper-mediated synthesis of 1,2,3-triazoles from N-tosylhydrazones and anilines

Article abstract of DOI:10.1002/anie.201306416

NNNifty targets: In a straightforward copper-mediated synthesis of 1,4-disubstituted and 1,4,5-trisubstituted 1,2,3-triazoles, readily available aniline and N-tosylhydrazone substrates underwent cyclization through Ci￡N and Ni￡N bond formation (see scheme

Full text of DOI:10.1002/anie.201306416

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DOI: 10.1002/anie.201306416
Nitrogen Heterocycles
Copper-Mediated Synthesis of 1,2,3-Triazoles from N-Tosylhydrazones
and Anilines**
Zhengkai Chen, Qiangqiang Yan, Zhanxiang Liu, Yiming Xu, and Yuhong Zhang*
1,2,3-Triazoles have found wide application in chemistry,
biology, and materials science.[1] Therefore, general methods
for their synthesis are of considerable importance.[2] Conven-
tionally, triazoles are prepared by the uncatalyzed thermally
induced dipolar cycloaddition of alkynes with organic azides,
as explored by Huisgen and co-workers.[3] A significantly
improved version of the Huisgen-type reaction is the copper-
catalyzed azide–alkyne cycloaddition (CuAAC) discovered
by the research groups of Sharpless[4] and Meldal[2a]
(Scheme 1). Copper catalysis leads to a remarkable increase
lyzed azide–alkyne cycloaddition reaction.[9] However, all of
these transformations involve the use of sodium azides or
organic azides, which are toxic and potentially shock-sensitive
(explosive). Furthermore, the preparation of 1,4,5-trisubsti-
tuted 1,2,3-triazoles by the CuAAC reaction is very limited
owing to the reduced reactivity of internal alkynes and
uncontrolled regioselectivity.[10]
À
The transition-metal-promoted formation of C N bonds
has become one of the most important strategies for the
synthesis of heterocycles.[11] In particular, synthesis through
À
C H cleavage has attracted much attention owing to its high
atom- and step-economical characteristics.[12] As stable and
reactive reagents, hydrazones have been widely studied in
both coupling reactions[13] and C H activation reactions[14] as
À
good precursors. Herein, we report a synthesis of 1,4-
disubstituted
and
1,4,5-trisubstituted
1,2,3-triazoles
(Scheme 1) from readily available N-tosylhydrazones and
anilines. The corresponding triazoles were obtained with
unprecedented regioselectivity under mild reaction condi-
tions. The reaction involves the spontaneous formation of
À
À
À
a C N bond through C H cleavage and N N bond formation.
The N-tosylhydrazone substrates were obtained quite
conveniently by the simple stirring of N-tosylhydrazines with
aryl ethanone derivatives in methanol. We initially examined
the reaction between p-toluidine (1b) and N-tosylhydrazone
2a in the presence of Cu(OAc)2 (1 equiv) and LiOtBu
(2 equiv) in toluene at 1008C. The reaction proceeded in
a regioselective manner to give exclusively the 1,4-disubsti-
tuted isomer 4-phenyl-1-p-tolyl-1H-1,2,3-triazole (3b) in
55% yield (Table 1, entry 1). Further studies revealed that
the addition of PivOH (2 equiv) significantly improved the
reaction to give the triazole 3b in 70% yield (Table 1,
entry 2). Surprisingly to us, the use of PivOH (2 equiv) in the
absence of LiOtBu resulted in the formation of triazole 3b in
88% yield (Table 1, entry 3). The yield decreased to 36%
when the reaction was carried out without LiOtBu and PivOH
(Table 1, entry 4). Other copper salts tested, including CuCl2,
Cu(OTf)2, Cu(OTFA)2, CuI, and CuBr, showed poor reac-
tivity (see the Supporting Information). In the absence of
a copper salt, no triazole product was observed. The reaction
could also be performed in 1,4-dioxane, N,N-dimethylforma-
mide (DMF), and CH3CN (see the Supporting Information).
When the reaction was performed under a N2 atmosphere, the
yield decreased to 42% (Table 1, entries 7). However, the
reaction was almost halted by the side reaction of the
dimerization of N-tosylhydrazone when the reaction was
performed under an atmosphere of pure oxygen (Table 1,
entry 8).
Scheme 1. Two distinct approaches to 1,2,3-triazoles. Ts=p-toluenesul-
fonyl.
in the reaction rate and the regioselectivity for the 1,4-
disubstituted triazole isomer. The CuAAC reaction has had
a huge impact on organic synthesis as the premier example of
a “click reaction”.[5] Other routes to 1,2,3-triazoles include the
regioselective 1,3-dipolar cycloaddition of an azide with an
enamine or ketone by organocatalysis,[6] the palladium-
catalyzed reaction of alkenyl halides with sodium azide,[7]
the copper-catalyzed cycloaddition of organic azides with 1-
iodoalkynes or 1-bromoalkynes,[8] and the ruthenium-cata-
[*] Z. Chen, Q. Yan, Z. Liu, Y. Xu, Prof. Dr. Y. Zhang
ZJU-NHU United R&D Center
Department of Chemistry, Zhejiang University
Hangzhou 310027 (China)
E-mail: yhzhang@zju.edu.cn
Prof. Dr. Y. Zhang
State Key Laboratory of Applied Organic Chemistry
Lanzhou University
Lanzhou 730000 (China)
[**] We gratefully acknowledge the National Basic Research Program of
China (no. 2011CB936003), the Natural Science Foundation of
China (no. 2107216 and no. 21272205), and the Program for the
Zhejiang Leading Team of Science and Technology Innovation for
their financial support.
Various anilines could be used in this reaction (represen-
tative results are summarized in Scheme 2). Generally, the
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201306416.
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Table 1: Optimization of the reaction conditions.[a]
reaction proceeded smoothly with a variety of anilines to
afford a wide range of 1,4-diaryl-substituted 1,2,3-triazoles.
Electron-rich anilines showed high reactivity to afford the
corresponding triazoles 3b–o in good yields. However, aniline
substrates with electron-withdrawing substituents at the para
position, including F, Cl, Br, and COOMe groups, showed
lower reactivity, and the corresponding triazoles 3p–r and 3t
were obtained in lower yields. When the electron-withdraw-
ing group was at the meta position (products 3s and 3u),
higher reactivity was observed owing to a decreased con-
jugation effect. Significantly, both ortho- and meta-substituted
anilines were converted into the triazole products in good
yields (products 3c,d and 3j,k). These results suggest that the
steric bulk of the anilines had little effect on the reaction.
Disubstituted anilines underwent the cyclization quite well
(products 3e–h, 3m). 2-Phenylaniline and 2-(thiophen-2-
yl)aniline were successfully converted into the corresponding
products 3v,w under the standard reaction conditions. Nota-
bly, 1-naphthylamine and quinolin-8-amine participated in
the reaction efficiently to give the corresponding triazoles
3x,y. The reaction was highly regioselective, with the exclu-
sive formation of 1,4-disubstituted 1,2,3-triazoles. The treat-
ment of aliphatic amines under these reaction conditions gave
a complex mixture, and the desired product was not isolated.
The structure of the 1,4-disubstituted triazole 3v was further
confirmed by single-crystal X-ray diffraction analysis (see the
Supporting Information).
Entry
Additive
Atmosphere
Yield [%][b]
1
2
3
4
5
6
7
8
LiOtBu
PivOH/LiOtBu
PivOH
–
PivOH
PivOH
PivOH
PivOH
air
air
air
air
air
air
N2
O2
55
70
88
36
85[c]
86[d]
42
17[e]
[a] Reaction conditions: p-toluidine (0.4 mmol), 2a (0.2 mmol), Cu-
(OAc)2 (1.0 equiv), additive (2.0 equiv), toluene (2 mL), 1008C, 12 h.
[b] Yield of the isolated product. [c] The reaction was carried out with
1.5 equivalents of Cu(OAc)2. [d] The reaction was carried out with
2.0 equivalents of Cu(OAc)2. [e] The reaction was carried out with
a catalytic amount of Cu(OAc)2 (20 mol%). Piv=pivaloyl.
To further evaluate the scope of the reaction, we next
investigated the transformation of a range of N-tosylhydra-
zones with different substitution patterns (Scheme 3). The
reaction system displayed good tolerance toward a range of
functional groups. Aromatic groups bearing electron-donat-
ing (products 4e–q) or electron-withdrawing substituents
(products 4a–d) were all tolerated, but the higher reactivity of
electron-rich substrates led to the formation of the products in
higher yields. Steric hindrance had little effect on this
transformation, and both ortho- and meta-substituted N-
tosylhydrazones presented good reactivity (products 4 f,g and
4k,l). Disubstituted N-tosylhydrazones participated in the
reaction smoothly to afford the desired products 4h,i and 4m
in good yields. 1-Naphthyl, 2-naphthyl, and 2-fluorenyl N-
tosylhydrazones were good substrates for this transformation,
which gave the desired products 4r–t smoothly. Interestingly,
the corresponding ferrocenyl N-tosylhydrazone was compat-
ible with the reaction conditions and was converted into the
ferrocenyl-substituted triazole 4x in 84% yield. Furthermore,
triazoles with heterocyclic substituents, such as furyl, thio-
phenyl, and benzofuryl groups, were obtained in high yields
from the corresponding N-tosylhydrazones (products 4u–w).
It is not easy to prepare the 1,4,5-trisubstituted triazoles
by the CuAAC reaction owing to the reduced reactivity of
internal alkynes and regioselectivity issues. We therefore
turned our attention to the possibility of extending the scope
of the reaction to the use of hydrazone substrates derived
from a-substituted aryl ketones to enable the formation of
1,4,5-trisubstituted 1,2,3-triazoles. To our delight, hydrazones
derived from various 3,4-dihydronaphthalen-1(2H)-ones
reacted with aniline to give the corresponding 1,4,5-trisub-
stituted triazoles under the reaction conditions (Scheme 4,
Scheme 2. Synthesis of 1,2,3-triazoles from a variety of aniline deriva-
tives. Reaction conditions: 1 (0.6 mmol), 2a (0.3 mmol), Cu(OAc)2
(0.3 mmol), PivOH (0.6 mmol), toluene (3 mL), 1008C, 12 h. The
yields given are for the isolated product.
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Scheme 4. Formation of 1,4,5-trisubstituted triazoles. Reaction condi-
tions: aniline (0.6 mmol), 2 (0.3 mmol), Cu(OAc)2 (0.3 mmol), PivOH
(0.6 mmol), toluene (3 mL), 1008C, 12 h. The yields given are for the
isolated product. [a] The reaction was carried out at 1158C.
N-tosylhydrazine in toluene was stirred for 1 h and then
treated with aniline, PivOH, and Cu(OAc)2 under the
standard reaction conditions (Scheme 5).
Scheme 3. Synthesis of 1,2,3-triazoles from a variety of N-tosylhydra-
zones. Reaction conditions: aniline (0.6 mmol), 2 (0.3 mmol), Cu-
(OAc)2 (0.3 mmol), PivOH (0.6 mmol), toluene (3 mL), 1008C, 12 h.
The yields given are for the isolated product. [a] The reaction was
carried out under a N2 atmosphere.
5a–d). More importantly, the hydrazone formed from 1-
benzosuberone also reacted well to afford the corresponding
triazole 5e in 63% yield. A higher reaction temperature was
required when the a substituent was a methyl or a phenyl
group (Scheme 4, 5 f–l). Thus, this new transformation
provides an efficient approach to 1,4,5-trisubstituted 1,2,3-
triazoles.
Following the success of the formation of triazoles from
aniline and N-tosylhydrazones, we explored the possibility of
performing a one-pot transformation of the aniline, the aryl
ethanone, and N-tosylhydrazine. Significantly, the triazole
could be obtained in almost the same yield from the one-pot
tandem reaction, in which a mixture of the aryl ethanone and
Scheme 5. One-pot synthesis of triazole 3b.
Preliminary mechanistic investigations excluded the pos-
sibility that the reaction occurs by a radical process.[15] The
reaction mechanism of this new transformation for the
synthesis of 1,2,3-triazoles is not clear at the moment. On
the basis of the imine–enamine shift observed in the presence
of transition metals,[16] we speculate that the N-tosylhydra-
zone may isomerize to A as shown in Scheme 6 in the
presence of Cu(OAc)2 and PivOH. Subsequent oxidation
gives a 1-tosyl-2-vinyldiazene B,[17] which undergoes an aza-
Michael addition with aniline to generate the intermediate C.
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Keywords: aniline derivatives · copper · hydrazones ·
.
synthetic methods · triazoles
Scheme 6. Plausible intermediates in the transformation.
[1] a) H. Wamhoff in Comprehensive Heterocyclic Chemistry, Vol. 5
(Eds.: A. R. Katritzky, C. W. Rees), Pergamon, Oxford, 1984,
pp. 669; b) S. T. Abu-Orabi, M. A. Atfah, I. Jibril, F. M. Mariꢀi,
A. A. Ali, J. Heterocycl. Chem. 1989, 26, 1461 – 1468.
[2] a) C. W. Tornøe, C. Christensen, M. Meldal, J. Org. Chem. 2002,
67, 3057 – 3064; b) A. K. Feldman, B. Colasson, V. V. Fokin, Org.
Lett. 2004, 6, 3897 – 3899; c) F. Himo, T. Lovell, R. Hilgraf, V. V.
Rostovtsev, L. Noodleman, K. B. Sharpless, V. V. Fokin, J. Am.
Chem. Soc. 2005, 127, 210 – 216; d) B. Chattopadhyay, C. I. R.
Vera, S. Chuprakov, V. Gevorgyan, Org. Lett. 2010, 12, 2166 –
2169; e) S. W. Kwok, J. R. Fotsing, R. J. Fraser, V. O. Rodionov,
V. V. Fokin, Org. Lett. 2010, 12, 4217 – 4219.
The copper-catalyzed cyclization of intermediate C with the
[18]
À
formation of a N N bond and subsequent aromatization
produce the triazole.
We prepared compound C, treated it with Cu(OAc)2
(20 mol%) and PivOH (2.0 equiv), and indeed obtained the
1,4-disubstituted 1,2,3-triazole 3a in 82% yield (Scheme 7).
Powder X-ray diffraction analysis of the residue formed under
[3] a) R. Huisgen, Angew. Chem. 1963, 75, 604 – 637; Angew. Chem.
Int. Ed. Engl. 1963, 2, 565 – 598; b) R. Huisgen, Angew. Chem.
1963, 75, 742 – 754; Angew. Chem. Int. Ed. Engl. 1963, 2, 633 –
645; c) R. Huisgen, R. Knorr, L. Mçbius, G. Szeimies, Chem. Ber.
1965, 98, 4014 – 4021.
[4] V. V. Rostovtsev, L. G. Green, V. V. Fokin, K. B. Sharpless,
Angew. Chem. 2002, 114, 2708 – 2711; Angew. Chem. Int. Ed.
2002, 41, 2596 – 2599.
[5] a) Click Chemistry for Biotechnology and Materials Science
(Ed.: J. Lahann), Wiley, Weinheim, 2009; b) A. E. Speers, G. C.
Adam, B. F. Cravatt, J. Am. Chem. Soc. 2003, 125, 4686 – 4687;
c) L. V. Lee, M. L. Mitchell, S.-J. Huang, V. V. Fokin, K. B.
Sharpless, C.-H. Wong, J. Am. Chem. Soc. 2003, 125, 9588 – 9589;
d) J. E. Moses, A. D. Moorhouse, Chem. Soc. Rev. 2007, 36,
1249 – 1262; e) M. Meldal, C. W. Tornøe, Chem. Rev. 2008, 108,
2952 – 3015; f) P. Thirumurugan, D. Matosiuk, K. Jozwiak,
Chem. Rev. 2013, 113, 4905 – 4979.
[6] a) D. B. Ramachary, K. Ramakumar, V. V. Narayana, Chem.
Eur. J. 2008, 14, 9143 – 9147; b) L. J. T. Danence, Y. Gao, M. Li,
Y. Huang, J. Wang, Chem. Eur. J. 2011, 17, 3584 – 3587; c) M.
Belkheira, D. E. Abed, J.-M. Pons, C. Bressy, Chem. Eur. J. 2011,
17, 12917 – 12921.
[7] J. Barluenga, C. Valdꢁs, G. Beltrꢂn, M. Escribano, F. Aznar,
Angew. Chem. 2006, 118, 7047 – 7050; Angew. Chem. Int. Ed.
2006, 45, 6893 – 6896.
[8] a) B. H. M. Kuijpers, G. C. T. Dijkmans, S. Groothuys, P. J. L. M.
Quaedflieg, R. H. Blaauw, F. L. van Delft, F. P. J. T. Rutjes,
Synlett 2005, 3059 – 3062; b) J. E. Hein, J. C. Tripp, L. B. Kras-
nova, K. B. Sharpless, V. V. Fokin, Angew. Chem. 2009, 121,
8162 – 8165; Angew. Chem. Int. Ed. 2009, 48, 8018 – 8021; c) C.
Spiteri, J. E. Moses, Angew. Chem. 2010, 122, 33 – 36; Angew.
Chem. Int. Ed. 2010, 49, 31 – 33.
[9] a) L. Zhang, X. Chen, P. Xue, H. H. Y. Sun, I. D. Williams, K. B.
Sharpless, V. V. Fokin, G. Jia, J. Am. Chem. Soc. 2005, 127,
15998 – 15999; b) B. C. Boren, S. Narayan, L. K. Rasmussen, L.
Zhang, H. Zhao, Z. Lin, G. Jia, V. V. Fokin, J. Am. Chem. Soc.
2008, 130, 8923 – 8930.
[10] a) J. Wei, J. Chen, J. Xu, L. Cao, H. Deng, W. Sheng, H. Zhang,
W. Cao, J. Fluorine Chem. 2012, 133, 146 – 154; b) J. Li, Y. Zhang,
D. Wang, W. Wang, T. Gao, L. Wang, J. Li, G. Huang, B. Chen,
Synlett 2010, 1617 – 1622.
[11] a) A. V. Gulevich, A. S. Dudnik, N. Chernyak, V. Gevorgyan,
Chem. Rev. 2013, 113, 3084 – 3213; b) T. R. M. Rauws, B. U. W.
Maes, Chem. Soc. Rev. 2012, 41, 2463 – 2497; c) R. Martꢃn, M. R.
Rivero, S. L. Buchwald, Angew. Chem. 2006, 118, 7237 – 7240;
Angew. Chem. Int. Ed. 2006, 45, 7079 – 7082; d) B. Zou, Q. Yuan,
D. Ma, Angew. Chem. 2007, 119, 2652 – 2655; Angew. Chem. Int.
Ed. 2007, 46, 2598 – 2601; e) M. Nagamochi, Y.-Q. Fang, M.
Lautens, Org. Lett. 2007, 9, 2955 – 2958; f) P. Sang, M. Yu, H. Tu,
J. Zou, Y. Zhang, Chem. Commun. 2013, 49, 701 – 703.
Scheme 7. Preparation of 3a from compound C.
the standard reaction conditions revealed the formation of
copper metal (see the Supporting Information). The use of
more than 1 equivalent of Cu(OAc)2 was shown to be
unnecessary (Table 1, entries 5 and 6). In fact, the formation
of a red precipitate forebodes the success of the reaction.
More elaborate research is required for the elucidation of the
reaction mechanism.
In conclusion, we have developed a new general method
for the synthesis of 1,4-disubstituted and 1,4,5-trisubstituted
triazoles from anilines and N-tosylhydrazones through
a copper-mediated reaction. Notable features of this trans-
formation include the exclusive formation of 1,4- or 1,4,5-
substituted regioisomers, its broad scope with respect to both
the N-tosylhydrazone and the aniline substrate, the low cost
of reagents, and the convenient operating conditions. This
discovery could have important consequences for the syn-
thesis of triazole structures. Further investigations to extend
the reaction scope and elucidate the reaction mechanism are
in progress.
Experimental Section
Typical procedure: p-Toluidine (1b; 0.6 mmol) was added to a mixture
of Cu(OAc)2 (0.3 mmol), PivOH (0.6 mmol), and the N-tosylhydra-
zone 2a (0.3 mmol) in toluene (3 mL), and the resulting mixture was
stirred at 1008C in air for 12 h. The reaction mixture was then cooled
to ambient temperature, and the solvent was removed in vacuo.
Purification of the crude product by column chromatography on silica
gel with petroleum ether/EtOAc (10:1) afforded 3b (62 mg, 88%) as
a white solid.
Received: July 23, 2013
Revised: September 24, 2013
Published online: October 15, 2013
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[12] a) P. Thansandote, M. Lautens, Chem. Eur. J. 2009, 15, 5874 –
5883; b) G. Brasche, S. L. Buchwald, Angew. Chem. 2008, 120,
1958 – 1960; Angew. Chem. Int. Ed. 2008, 47, 1932 – 1934;
c) D. A. Colby, R. G. Bergman, J. A. Ellman, J. Am. Chem.
Soc. 2008, 130, 3645 – 3651; d) Q. Xiao, W.-H. Wang, G. Liu, F.-K.
Meng, J.-H. Chen, Z. Yang, Z.-J. Shi, Chem. Eur. J. 2009, 15,
7292 – 7296; e) A. Armstrong, J. C. Collins, Angew. Chem. 2010,
122, 2332 – 2335; Angew. Chem. Int. Ed. 2010, 49, 2282 – 2285;
f) A. E. Wendlandt, A. M. Suess, S. S. Stahl, Angew. Chem. 2011,
123, 11256 – 11283; Angew. Chem. Int. Ed. 2011, 50, 11062 –
11087; g) Y. Yang, C. Xie, Y. Xie, Y. Zhang, Org. Lett. 2012,
14, 957 – 959.
[13] a) J. Barluenga, C. Valdꢁs, Angew. Chem. 2011, 123, 7626 – 7640;
Angew. Chem. Int. Ed. 2011, 50, 7486 – 7500; b) Z. Shao, H.
Zhang, Chem. Soc. Rev. 2012, 41, 560 – 572; c) Q. Xiao, Y. Zhang,
J. Wang, Acc. Chem. Res. 2013, 46, 236 – 247; d) J. Barluenga, N.
QuiÇones, M.-P. Cabal, F. Aznar, C. Valdꢁs, Angew. Chem. 2011,
123, 2398 – 2401; Angew. Chem. Int. Ed. 2011, 50, 2350 – 2353;
e) E. Brachet, A. Hamze, J.-F. Peyrat, J.-D. Brion, M. Alami,
Org. Lett. 2010, 12, 4042 – 4045; f) L. Zhou, F. Ye, J. Ma, Y.
Zhang, J. Wang, Angew. Chem. 2011, 123, 3572 – 3576; Angew.
Chem. Int. Ed. 2011, 50, 3510 – 3514; g) J. Barluenga, M. Tomꢂs-
Gamasa, F. Aznar, C. Valdꢁs, Nat. Chem. 2009, 1, 494 – 499; h) J.
Barluenga, M. Tomꢂs-Gamasa, C. Valdꢁs, Angew. Chem. 2012,
124, 6052 – 6054; Angew. Chem. Int. Ed. 2012, 51, 5950 – 5952.
[14] a) X. Zhao, G. Wu, Y. Zhang, J. Wang, J. Am. Chem. Soc. 2011,
133, 3296 – 3299; b) T. Yao, K. Hirano, T. Satoh, M. Miura,
Angew. Chem. 2012, 124, 799 – 803; Angew. Chem. Int. Ed. 2012,
51, 775 – 779; c) Q. Xiao, L. Ling, F. Ye, R. Tan, L. Tian, Y.
Zhang, J. Wang, J. Org. Chem. 2013, 78, 3879 – 3885.
[15] The addition of radical-scavenging reagents, such as 2,2,6,6-
tetramethyl-1-piperidinyloxy (TEMPO) or 2,6-di-tert-butyl-4-
methylphenol (BHT), did not influence the reaction (see the
Supporting Information).
[16] a) B. Qian, S. Guo, J. Shao, Q. Zhu, L. Yang, C. Xia, H. Huang, J.
Am. Chem. Soc. 2010, 132, 3650 – 3651; b) M. Rueping, N.
Tolstoluzhsky, Org. Lett. 2011, 13, 1095 – 1097; c) B. Qian, P. Xie,
Y. Xie, H. Huang, Org. Lett. 2011, 13, 2580 – 2583; d) H. Komai,
T. Yoshino, S. Matsunaga, M. Kanai, Org. Lett. 2011, 13, 1706 –
1709.
[17] a) J. M. Hatcher, D. M. Coltart, J. Am. Chem. Soc. 2010, 132,
4546 – 4547; b) C. E. Sacks, P. L. Fuchs, J. Am. Chem. Soc. 1975,
97, 7372 – 7374.
[18] a) J. J. Neumann, M. Suri, F. Glorius, Angew. Chem. 2010, 122,
7957 – 7961; Angew. Chem. Int. Ed. 2010, 49, 7790 – 7794; b) S.
Ueda, H. Nagasawa, J. Am. Chem. Soc. 2009, 131, 15080 – 15081;
c) D.-G. Yu, M. Suri, F. Glorius, J. Am. Chem. Soc. 2013, 135,
8802 – 8805; d) M. Suri, T. Jousseaume, J. J. Neumann, F. Glorius,
Green Chem. 2012, 14, 2193 – 2196; e) H. Mori, K. Sakamoto, S.
Mashito, Y. Matsuoka, M. Matsubayashi, K. Sakai, Chem.
Pharm. Bull. 1993, 41, 1944 – 1947.
13328 www.angewandte.org
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