Copper-Catalyzed Domino Reactions for the Synthesis of Phenothiazines

Article abstract of DOI:10.1002/ejoc.201500667

A method for the one-pot synthesis of phenothiazines from benzothiazoles and aryl ortho-dihalides was explored. Preliminary work on the mechanism of the reaction suggested that it follows a domino process, including the hydrolysis of benzothiazoles followed by C-S coupling and C-N coupling. The low loading of the catalyst system (5 mol-% for both copper and ligand), the mild experimental conditions (90 °C, 12 h), and the use of a green reaction medium make this synthesis very attractive to academia and industry. A copper-catalyzed domino reaction consisting of the hydrolysis of benzothiazoles followed by C-S and C-N couplings for the synthesis of phenothiazines from benzothiazoles and aryl ortho-dihalides is described. The low loading of the catalyst system, mild experimental conditions, and the use of polyethylene glycol as the solvent make this synthetic approach very attractive to academia and industry.

Full text of DOI:10.1002/ejoc.201500667

SHORT COMMUNICATION
DOI: 10.1002/ejoc.201500667
Copper-Catalyzed Domino Reactions for the Synthesis of Phenothiazines
Manna Huang,[a] Dongting Huang,[a] Xinhai Zhu,*[a] and Yiqian Wan*[a]
Keywords: Domino reactions / Copper / Green chemistry / Sulfur heterocycles / Nitrogen heterocycles
A method for the one-pot synthesis of phenothiazines from
The low loading of the catalyst system (5 mol-% for both cop-
benzothiazoles and aryl ortho-dihalides was explored. Pre- per and ligand), the mild experimental conditions (90 °C,
liminary work on the mechanism of the reaction suggested
that it follows a domino process, including the hydrolysis of
benzothiazoles followed by C–S coupling and C–N coupling.
12 h), and the use of a green reaction medium make this syn-
thesis very attractive to academia and industry.
Introduction
sphere of oxygen,[12] cyclization with benzoquinones,[13] re-
ductive cyclization of 2-nitrodiphenyl sulfides with triethyl
phosphite,[14] and thermal cyclization of aryl azides with
1,2,3-benzothiadiazole.[15] However, drawbacks and chal-
lenges of these methods still remain. The cyclization of di-
phenylamines with sulfur produces isomeric substituted
phenothiazines, which makes it difficult to isolate the de-
sired product even though iodine is used as a catalyst.[16]
The cyclization of diphenyl sulfides involving a Smiles re-
arrangement was developed to overcome this issue, but its
steric hindrance and difficult four-step route have limited
its applications.[17] The functionalization of phenothiazine
affords substituted phenothiazines, but the process often
suffers from poor regioselectivity.[7,18] Moreover, the most
clear drawback of these conventional methods is the lack
of availability of the starting materials. Several tactics, such
as using new types of starting materials and/or using a tan-
dem reaction pathway, have recently been able to circum-
vent these issues. In 2008, a palladium-catalyzed, three-
component approach to synthesize promazine was re-
ported.[19] Since then, the transition-metal-catalyzed syn-
thesis of phenothiazines has attracted significant attention.
In 2010, Ma et al.[20] reported the first copper-catalyzed
synthesis of phenothiazines in good to high yields from 2-
iodoanilines and 2-bromobenzenethiols. Using this proto-
col, the catalyst loading could reach 20 mol-%. The reaction
temperature must be sequentially held at 90 °C for 24–48 h
and then at 110 °C for 48–96 h. In 2012, Zeng et al.[21] used
aryl ortho-dihalides and ortho-aminobenzenethiols as start-
ing materials to form phenothiazines in a one-pot synthesis
but still needed a high loading of CuI (30 mol-%), a high
temperature (120 °C), and long reaction time (48 h). Inter-
estingly, Abele et al.[22] developed a new pathway toward
phenothiazines by using 2-aminobenzothiazoles as the
starting materials. However, the high reaction temperature
(170 °C), narrow substrate scope, and relatively low yields
in most cases make this method less practical.
The phenothiazine family of compounds has played a
significant role in chemistry, life sciences, and materials sci-
ence since its first member was prepared by Bernthsen in
1883.[1] Up to now, over 5000 phenothiazine derivatives
have been obtained, most of which exhibit valuable bioac-
tivities, such as neuroleptic, anticancer, antiemetic, anti-
histaminic, antipruritic, reversing multidrug resistance
(MDR), antibacterial, and analgesic properties; they have
been described in several reviews.[2] For example, over 100
phenothiazines have been used as neuroleptics in clinical
settings.
The unique features of phenothiazines, including elec-
tron-rich sulfur and nitrogen heteroatoms as well as a non-
planar ring with a butterfly conformation in the ground
state, can impede molecular aggregation and the formation
of intermolecular excimers.[3] Therefore, phenothiazines
have been introduced as scaffolds for various metal-free or-
ganic dyes for use in dye-sensitized solar cells (DSSCs),[4]
lithium-ion batteries,[5] photocatalytic hydrogen pro-
duction,[6] nonlinear optical devices,[7] organic light-emit-
ting diodes (OLEDs),[8] polymer light-emitting diodes
(PLEDs),[7] molecular wires,[9] and so on.[10]
Several methods for the synthesis of phenothiazines have
been previously reviewed.[11] The conventional methods can
be roughly classified into the following categories: (1) heat-
ing of suitable substituted diphenylamines with sulfur,
(2) Smiles rearrangement of 2-amino-2-nitrodiphenyl sulf-
ides followed by ring closure, (3) transition-metal-catalyzed
intramolecular cyclization of 2-amino-2Ј-halodiphenyl sulf-
ides, and (4) other specific methods, including cyclization of
2-aminothiophenols with cyclohexanones under an atmo-
[a] School of Chemistry and Chemical Engineering, Sun Yat-sen
University,
Guangzhou 510275, P. R. China
E-mail: ceswyq@mail.sysu.edu.cn
http://ce.sysu.edu.cn/ChemTeacher/Incumbent/207.html
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201500667.
Therefore, the development of a novel synthetic ap-
proach to phenothiazines under mild conditions, including
Eur. J. Org. Chem. 2015, 4835–4839
© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
4835

M. Huang, D. Huang, X. Zhu, Y. Wan
SHORT COMMUNICATION
a lower catalyst loading, a lower temperature, and shorter assumed that using some type of ligand might be helpful;
times, as well as a broad substrate scope is highly desired. thus, we screened several typical copper-based catalytic sys-
Modern synthesis management must seek ideal pro- tems for C–N coupling, such as copper/proline,[20] copper/
cedures in which the formation of several bonds can occur N,NЈ-dimethylethylamine,[28] and our previously reported
in one pot without the addition of any further reagents or system.[24,29] The copper/N-methoxy-1H-pyrrole-2-carbox-
catalysts and without changing the reaction conditions. amide (L1) catalyst system was found to be the most effec-
This concept has been well defined as a domino process, tive to perform the model reaction to provide phenothiazine
which has been applied in many modern syntheses of or- in 60% yield, which encouraged us to further optimize the
ganic compounds.[23]
reaction conditions.
In continuation of our interest[24] in finding more effec-
This promising result encouraged us to explore the reac-
tive synthetic approaches to phenothiazines, we herein re- tion parameters, mainly including different copper sources,
port a novel domino reaction to synthesize phenothiazines ligands, bases, and solvents, as well as their proportions.
from benzothiazoles and aryl ortho-dihalides, as shown in The optimized conditions, consisting of Cu(OAc)2 (5 mol-
Scheme 1.
%)/N-(benzyloxy)-1H-pyrrole-2-carboxamide (L2, 5 mol-
%), benzothiazole (100 mol-%), aryl ortho-dihalide
(120 mol-%), and NaOH (400 mol-%) in polyethylene glycol
(PEG)-100 (2 g) at 90 °C for 12 h were identified. Although
the loadings of both Cu(OAc)2 (5 mol-%) and L2 (5 mol-
%) for the reaction are fairly low, both are necessary for
the reaction to proceed effectively. Moreover, PEG-100 was
chosen as the solvent mainly because of its environmentally
friendly properties and its high efficiency.
After developing an effective protocol for the preparation
of phenothiazines, the selection of other benzothiazoles and
aryl ortho-dihalides was pursued to demonstrate the scope
of this novel domino reaction (Table 1).
As an initial observation, good to high yields of pheno-
thiazines were obtained from the reaction of benzothiazoles
with aryl ortho-dihalides with relatively high functional
group tolerance. Thus, benzothiazoles containing either
electron-donating (Table 1, entries 7–18) or electron-with-
drawing (Table 1, entries 19–22) groups and aryl ortho-di-
halides bearing either electron-donating (Table 1, entries 4–
6, 10–12, 16–18, 22) or electron-withdrawing (Table 1, en-
tries 2, 3, 8, 9, 14, 15, 20, 21) groups afforded the desired
products in good yields. However, benzothiazoles with very
Scheme 1. Typical starting materials for the synthesis of phenothi-
azines. (1) See ref.[15] (2) See ref.[17] (3) See ref.[14] (4) See ref.[22]
(5) See ref.[19,20] (6) The starting material in this work. (7) See
ref.[12,13,21]
Results and Discussion
Inspired by Abele’s method[22] of preparing phenothi- strong electron-withdrawing groups, for example, –NO2
azines by using 2-aminobenzothiazoles as alternative start- (Table 1, entry 23), were not successful and basic labile sub-
ing materials and considering that benzothiazoles can be strates would be problematic under the experimental condi-
reliably prepared from various sources[25] with more sta- tions.
bility than ortho-aminobenzenethiols, we speculated that
The most involved mechanisms for Ullmann couplings
developing a new synthesis protocol by replacing 2-amino- are based on either one-electron redox processes through
benzothiazoles with benzothiazoles to increase atom econ- radical intermediates, which proceed through a CuI/CuII
omy would be a challenge. Fortunately, Xu et al.[26] and catalytic cycle, or two-electron redox processes operating
Han et al.[27] reported the copper-catalyzed direct ring- through a CuI/CuIII catalytic cycle.[30] Because the addition
opening arylation of benzothiazoles with aryl iodides to af- of a radical trapping agent, for example, an equivalent of
ford 2-aminoaryl sulfides. Hence, we hypothesized that 2,2,6,6-tetramethylpiperidinooxy (TEMPO) or N-methyl-
benzothiazoles would be good starting materials in reac- imidazole, into the reaction mixture did not clearly delay
tions with aryl ortho-dihalides instead of aryl iodides to af- the reaction, the mechanism likely favors a two-electron re-
ford 2-[(2-halophenyl)thio]anilines, a key intermediate for dox process, that is, abiding by the CuI/CuIII catalytic cycle.
the further synthesis of phenothiazines through copper-cat- Overall, suggesting that the first step is a direct ring-open-
alyzed intramolecular C–N coupling. We began our study ing arylation of 1a with 2a to form 4a would be reasonable
by using benzothiazole and 1-bromo-2-iodobenzene as according to the work of Xu[26] and Han[27] (Scheme 2). We
starting materials and Xu’s[26] and Han’s[27] ligand-free, cop- successfully isolated 4a in 20% yield from the reaction mix-
per-catalyzed protocols; however, only 2-[(2-bromophenyl)- ture, which was quenched within 2 h at 90 °C. UV spec-
thio]aniline rather than the expected phenothiazine was ob- troscopy showed that neither 4a nor 3a was found in the
tained. In this case, further intramolecular C–N coupling reaction mixture if all starting materials and catalysts were
should be the key to obtaining the target phenothiazine. We simply mixed at room temperature. After heating at 90 °C
4836
www.eurjoc.org
© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2015, 4835–4839

Cu-Catalyzed Domino Reactions for the Synthesis of Phenothiazines
Table 1. Synthesis of phenothiazines.[a]
Entry
Benzothiazole
Aryl ortho-dihalide
Phenothiazine
Yield[b] [%]
1
2
3
4
5
6
7
8
1a: R = H
2a: RЈ = H
3a: R = H, RЈ = H
70
53
60
70
72
52
65
51
45
53
69
48
70
61
58
60
75
70
50
46
52
56
1a: R = H
2b: RЈ = 4-Cl
2c: RЈ = 4-F
2d: RЈ = 4-Me
2e: RЈ = 5-Me
2f: RЈ = 5-OMe
2a: RЈ = H
3b: R = H, RЈ = 8-Cl
1a: R = H
3c: R = H, RЈ = 8-F
1a: R = H
3d: R = H, RЈ = 8-Me
3e: R = H, RЈ = 7-Me
3f: R = H, RЈ = 7-OMe
3f: R = 3-OMe, RЈ = H
3g: R = 3-OMe, RЈ = 8-Cl
3h: R = 3-OMe, RЈ = 8-F
3i: R = 3-OMe, RЈ = 8-Me
3j: R = 3-OMe, RЈ = 7-Me
3k: R = 3-OMe, RЈ = 7-OMe
3e: R = 3-Me, RЈ = H
3l: R = 3-Me, RЈ = 8-Cl
3m: R = 3-Me, RЈ = 8-F
3n: R = 3-Me, RЈ = 8-Me
3o: R = 3-Me, RЈ = 7-Me
3j: R = 3-Me, RЈ = 7-OMe
3b: R = 2-Cl, RЈ = H
1a: R = H
1a: R = H
1b: R = 6-OMe
1b: R = 6-OMe
1b: R = 6-OMe
1b: R = 6-OMe
1b: R = 6-OMe
1b: R = 6-OMe
1c: R = 6-Me
1c: R = 6-Me
1c: R = 6-Me
1c: R = 6-Me
1c: R = 6-Me
1c: R = 6-Me
1d: R = 5-Cl
1d: R = 5-Cl
1d: R = 5-Cl
1d: R = 5-Cl
1e: R = 6-NO2
2b: RЈ = 4-Cl
2c: RЈ = 4-F
2d: RЈ = 4-Me
2e: RЈ = 5-Me
2f: RЈ = 5-OMe
2a: RЈ = H
2b: RЈ = 4-Cl
2c: RЈ = 4-F
2d: RЈ = 4-Me
2e: RЈ = 5-Me
2f: RЈ = 5-OMe
2a: RЈ = H
9
10
11
12
13
14
15
16
17
18
19[c]
20[c]
21[c]
22[c]
23
2b: RЈ = 4-Cl
2c: RЈ = 4-F
2e: RЈ = 5-Me
2a: RЈ = H
3p: R = 2-Cl, RЈ = 8-Cl
3q: R = 2-Cl, RЈ = 8-F
3l: R = 2-Cl, RЈ = 7-Me
3r: R = 3-NO2, RЈ = H
[d]
–
[a] Reaction conditions: benzothiazole (1 mmol), aryl ortho-dihalide (1.2 mmol), Cu(OAc)2·H2O (5 mol-%), L2 (5 mol-%), NaOH
(4 mmol), PEG-100 (2 g), N2, 90 °C, 12 h. [b] Yield of isolated product. [c] 15 h. [d] No reaction.
for 2 h, 4a was found to be the major product in the mixture phenyl)thio]-N-phenylaniline from the reaction of 4a with
with a trace amount of 3a present. The roles of 3a and 4a aniline because there is a higher probability that aniline
were gradually reversed until 3a was detected as the major would be coordinated. The fact that 3a was mainly obtained
species after 12 h. This result, along with the direct conver- in the reaction indicates that the activation of the aryl
sion of 4a into 3a in 72% yield under the experimental con- halide occurs before coordination of the nucleophile in our
ditions, confirmed that the reaction of benzothiazoles with reaction. Hence, a plausible mechanism is suggested and is
aryl ortho-dihalides underwent a successive one-pot, two- shown in Scheme 3.
step process.
Conclusions
In conclusion, we established a novel method to synthe-
Scheme 2. The domino process of the model reaction.
size phenothiazines from benzothiazoles and aryl ortho-di-
halides through one-pot, tandem copper/N-(benzyloxy)-
In the CuI/CuIII catalytic cycle, the activation of the aryl 1H-pyrrole-2-carboxamide-catalyzed reactions. The novel
halide may occur before coordination of the nucleophile to domino process, that is, hydrolysis of benzothiazoles fol-
CuI, or the coordination of the nucleophile may occur be- lowed by C–S coupling and C–N coupling, plays a substan-
fore oxidative addition.[30] To understand our reaction in tial role in the synthesis. Moreover, the use of a green sol-
more detail, 3 equivalents of aniline were added to the in- vent (polyethylene glycol-100), the low catalyst loading
tramolecular cyclization of 4a to phenothiazine. If the acti- (5 mol-% for both copper and ligand), and the mild experi-
vation of the aryl halide were to occur before coordination mental conditions (90 °C, 12 h) make this protocol useful
of the nucleophile, we would expect to obtain mainly 3a for the synthesis of phenothiazines, although the limited
because intramolecular C–N coupling is faster than inter- substrate scope has not been thoroughly investigated.
molecular C–N coupling. If the coordination of the nucleo- Moreover, our preliminary work on this mechanism may be
phile were to occur before oxidative addition, we would ex- beneficial for the further establishment of more powerful
pect to obtain a large quantity of the product 2-[(2-amino- methods for the synthesis of phenothiazines.
Eur. J. Org. Chem. 2015, 4835–4839
© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
4837

M. Huang, D. Huang, X. Zhu, Y. Wan
SHORT COMMUNICATION
Scheme 3. Proposed mechanism.
[7]
[8]
M. Sailer, A. W. Franz, T. J. Muller, Chem. Eur. J. 2008, 14,
2602–2614.
L. Yao, S. Zhang, R. Wang, W. Li, F. Shen, B. Yang, Y. Ma,
Angew. Chem. Int. Ed. 2014, 53, 2119–2123; Angew. Chem.
2014, 126, 2151–2155.
E. A. Weiss, M. J. Tauber, R. F. Kelley, M. J. Ahrens, M. A.
Ratner, M. R. Wasielewski, J. Am. Chem. Soc. 2005, 127,
11842–11850.
a) S. M. Bromfield, A. Barnard, P. Posocco, M. Fermeglia, S.
Pricl, D. K. Smith, J. Am. Chem. Soc. 2013, 135, 2911–2914;
b) T. Ito, A. Kondo, S. Terada, S.-I. Nishimoto, J. Am. Chem.
Soc. 2006, 128, 10934–10942; c) H.-W. Rhee, S. J. Choi, S. H.
Yoo, Y. O. Jang, H. H. Park, R. M. Pinto, J. C. Cameselle, F. J.
Sandoval, S. Roje, K. Han, D. S. Chung, J. Suh, J.-I. Hong, J.
Am. Chem. Soc. 2009, 131, 10107–10112.
Experimental Section
General Procedure for the Synthesis of the Phenothiazines: Cu(OAc)2·
H2O (10 mg, 0.05 mmol), L2 (10.8 mg, 0.05 mmol), benzothiazole
(1.0 mmol), aryl ortho-dihalide (1.2 mmol), NaOH (160 mg,
4.0 mmol), PEG-100 (2.0 g), and a magnetic stir bar were added to
a 10 mL sealed vial. The mixture was stirred in an oil bath pre-
heated to 90 °C for 12 or 15 h. After allowing the mixture to cool
to room temperature, the mixture was extracted with ethyl acetate
(3ϫ30 mL) and water (30 mL). The combined organic phase was
washed with brine (90 mL), dried with anhydrous Na2SO4, and
concentrated under reduced pressure. The residue was purified by
flash column chromatography on silica gel to afford the desired
product.
[9]
[10]
[11]
[12]
[13]
[14]
[15]
[16]
a) S. P. Massie, Chem. Rev. 1954, 54, 797–833; b) S. Saraf, F.
Al-Omran, B. Al-Saleh, Heterocycles 1987, 26, 239–273.
Y. Liao, P. Jiang, S. Chen, F. Xiao, G.-J. Deng, RSC Adv. 2013,
3, 18605.
a) I. Oprean, W. Schaefer, Justus Liebigs Ann. Chem. 1972, 765,
1–7; b) Y. Ueno, Pharmazie 1984, 39, 355–356.
J. I. G. Cadogan, B. S. Tait, J. Chem. Soc. Perkin Trans. 1 1975,
2396–2405.
L. Benati, P. C. Montevecchi, P. Spagnolo, J. Chem. Soc.,
Chem. Commun. 1980, 715–717.
a) N. L. Smith, J. Org. Chem. 1950, 15, 1125–1130; b) S. P.
Massie, P. K. Kadaba, J. Org. Chem. 1956, 21, 347–348; c)
G. A. Silva, L. M. Costa, F. C. Brito, A. L. Miranda, E. J. Bar-
reiro, C. A. Fraga, Bioorg. Med. Chem. 2004, 12, 3149–3158.
H. L. Yale, J. Am. Chem. Soc. 1955, 77, 2270–2272.
a) R. Y. Lai, X. Kong, S. A. Jenekhe, A. J. Bard, J. Am. Chem.
Soc. 2003, 125, 12631–12639; b) M. Hauck, J. Schoenhaber,
A. J. Zucchero, K. I. Hardcastle, T. J. J. Mueller, U. H. F. Bunz,
J. Org. Chem. 2007, 72, 6714–6725.
Acknowledgments
This work was financially supported by grants from the National
Natural Science Foundation of China (NSFC) (grant number
21272287) and Guangdong Science Foundation (grant number
2012A080201007).
[1] A. Bernthsen, Ber. Dtsch. Chem. Ges. 1883, 16, 2896–2904.
[2] a) K. Pluta, B. Morak-Mlodawska, M. Jelen, Eur. J. Med.
Chem. 2011, 46, 3179–3189; b) M. J. Ohlow, B. Moosmann,
Drug Discovery Today 2011, 16, 119–131; c) A. Jaszczyszyn, K.
Gasiorowski, P. Swiatek, W. Malinka, K. Cieslik-Boczula, J.
Petrus, B. Czarnik-Matusewicz, Pharmacol. Rep. 2012, 64, 16–
23; d) S. G. Dastidar, J. E. Kristiansen, J. Molnar, L. Amaral,
Antibiotics 2013, 2, 58–72.
[17]
[18]
[3] S. Wang, H. Wang, J. Guo, H. Tang, J. Zhao, Dyes Pigm. 2014,
[19]
T. Dahl, C. W. Tornoe, B. Bang-Andersen, P. Nielsen, M. Jor-
gensen, Angew. Chem. Int. Ed. 2008, 47, 1726–1728; Angew.
Chem. 2008, 120, 1750–1752.
109, 96–104.
[4] N. Manfredi, B. Cecconi, A. Abbotto, Eur. J. Org. Chem. 2014,
7069–7086.
[20]
[21]
D. Ma, Q. Geng, H. Zhang, Y. Jiang, Angew. Chem. Int. Ed.
2010, 49, 1291–1294; Angew. Chem. 2010, 122, 1313–1316.
C. Dai, X. Sun, X. Tu, L. Wu, D. Zhan, Q. Zeng, Chem. Com-
mun. 2012, 48, 5367–5369.
[5] S. Ergun, C. F. Elliott, A. P. Kaur, S. R. Parkin, S. A. Odom,
J. Phys. Chem. C 2014, 118, 14824–14832.
[6] H. J. Jo, J. E. Nam, D.-H. Kim, H. Kim, J.-K. Kang, Dyes
Pigm. 2014, 102, 285–292.
4838
www.eurjoc.org
© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2015, 4835–4839

Cu-Catalyzed Domino Reactions for the Synthesis of Phenothiazines
[22] a) E. Abele, R. Abele, Heterocycl. Lett. 2014, 4, 109–113; b) T.
Beresneva, E. Abele, Chem. Heterocycl. Compd. 2012, 48,
1420–1422.
[23] L. F. Tietze, G. Brasche, K. M. Gericke, Domino Reactions in
Organic Synthesis Wiley-VCH, Weinheim, Germany, 2006.
[24] M. Huang, J. Hou, R. Yang, L. Zhang, X. Zhu, Y. Wan, Syn-
thesis 2014, 46, 3356–3364.
[25] H. Ulrich, Sci. Synth. 2002, 11, 835–912.
[26] Y.-S. Feng, H.-X. Qi, W.-C. Wang, Y.-F. Liang, H.-J. Xu, Tetra-
hedron Lett. 2012, 53, 2914–2917.
[27] L. Yao, Q. Zhou, W. Han, S. Wei, Eur. J. Org. Chem. 2012,
6856–6860.
[28]
[29]
J. C. Antilla, A. Klapars, S. L. Buchwald, J. Am. Chem. Soc.
2002, 124, 11684–11688.
a) M. Huang, X. Lin, X. Zhu, W. Peng, J. Xie, Y. Wan, Eur. J.
Org. Chem. 2011, 4523–4527; b) J. Xie, X. Zhu, M. Huang, F.
Meng, W. Chen, Y. Wan, Eur. J. Org. Chem. 2010, 3219–3223;
c) X. Zhu, L. Su, L. Huang, G. Chen, J. Wang, H. Song, Y.
Wan, Eur. J. Org. Chem. 2009, 635–642.
[30] A. Casitas, X. Ribas, Chem. Sci. 2013, 4, 2301.
Received: May 24, 2015
Published Online: July 1, 2015
Eur. J. Org. Chem. 2015, 4835–4839
© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
4839