Transition-Metal-Free Synthesis of Carbazoles and Indoles by an SNAr-Based "aromatic Metamorphosis" of Thiaarenes

Article abstract of DOI:10.1002/anie.201503671

Dibenzothiophene dioxides, which are readily prepared through oxidation of the parent dibenzothiophenes, undergo nucleophilic aromatic substitution with anilines intermolecularly and then intramolecularly to yield the corresponding carbazoles in a single operation. The "aromatic metamorphosis" of dibenzothiophenes into carbazoles does not require any heavy metals. This strategy is also applicable to the synthesis of indoles. Since electron-deficient thiaarene dioxides exhibit interesting reactivity, which is not observed for that the corresponding electron-rich azaarenes, a combination of a thiaarene-dioxide-specific reaction with the S_NAr-based aromatic metamorphosis allows transition-metal-free construction of difficult-to-prepare carbazoles.

Full text of DOI:10.1002/anie.201503671

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International Edition: DOI: 10.1002/anie.201503671
German Edition: DOI: 10.1002/ange.201503671
Aromatic Substitution
Transition-Metal-Free Synthesis of Carbazoles and Indoles by an SNAr-
Based “Aromatic Metamorphosis” of Thiaarenes**
M. Bhanuchandra, Kei Murakami, Dhananjayan Vasu, Hideki Yorimitsu,* and Atsuhiro Osuka
Abstract: Dibenzothiophene dioxides, which are readily
prepared through oxidation of the parent dibenzothiophenes,
undergo nucleophilic aromatic substitution with anilines
intermolecularly and then intramolecularly to yield the corre-
sponding carbazoles in a single operation. The “aromatic
metamorphosis” of dibenzothiophenes into carbazoles does
not require any heavy metals. This strategy is also applicable to
the synthesis of indoles. Since electron-deficient thiaarene
dioxides exhibit interesting reactivity, which is not observed for
that the corresponding electron-rich azaarenes, a combination
of a thiaarene-dioxide-specific reaction with the SNAr-based
aromatic metamorphosis allows transition-metal-free construc-
tion of difficult-to-prepare carbazoles.
C
arbazoles represent an important class of heteroaromatic
compounds, many of which find a wide range of applications
as biologically active agents in medicinal chemistry[1] and as
hole-transport and light-emitting materials in organic elec-
tronics.[2] For the last two decades, transition-metal catalysis
has been used for constructing a carbazole skeleton by
Scheme 1. Recent strategies for transition-metal-free synthesis of car-
bazoles.
À
catalytic C N bond formation, and includes Buchwald–
biaryls with hypervalent iodine reagents.[7] Studer and co-
workers invented an interesting approach, that is, reactions of
arynes with nitrosoarenes,[8] but the yields were moderate and
the regioselectivity was not as well controlled as is typically
observed in aryne chemistry. Tokuyama and co-workers
developed a base-mediated cyclization of 2-amino-2’-bromo-
biphenyls via arynes.[9] From similar precursors, photoinduced
cyclization occurs by a radical process to yield carbazoles.[10]
Kürti and co-workers disclosed facile reductive cyclization of
2-nitrobiaryls by means of PhMgBr.[11] Regardless of these
elegant protocols, there still remains ample room to develop
novel strategies to prepare carbazoles without recourse to
transition-metal catalysts.
Hartwig or Ullmann-type amination of halobiaryls,[3] oxida-
[4]
À
tive C H amination of aminobiaryls, or nitrene insertion
from azidobiphenyls.[5] Despite the high efficiency and
reasonably wide scope, residual transition-metal impurities
could adversely affect the biological properties or device
performance of the final products.
Transition-metal-free synthesis of carbazoles has there-
fore undergone
(Scheme 1). Liu and Knochel disclosed that halogen–magne-
sium exchange of 2-iodo-2’-triazenobiaryls resulted in forma-
tion of carbazoles.[6] The groups of Chang and Antonchick
a renaissance for the last few years
À
reported intramolecular oxidative C H amination of amino-
Recently, our group has been interested in developing
“aromatic metamorphosis”, which represents a transforma-
tion of an aromatic system into a different one through partial
disassembly of the starting aromatic ring.[12–14] Along these
lines, herein we report transition-metal-free SNAr-based
aromatic metamorphosis of dibenzothiophenes to carbazoles
via dibenzothiophene dioxides.
Our strategy begins with smooth oxidation of dibenzo-
thiophenes with aqueous hydrogen peroxide (see the Sup-
porting Information; Scheme1).[15] This conventional process
is useful as the first step for the aromatic metamorphosis.
Although aryl sulfones are known to undergo SNAr reactions,
the scope of the substitution is limited.[16–19] Another difficulty
could originate from the second intramolecular SNAr reaction
which would involve formal elimination of K2SO2, which is
a high-energy compound.[20] The realization of the second,
and the last step, sequential inter/intramolecular nucleophilic
[*] Dr. M. Bhanuchandra, Dr. K. Murakami, Dr. D. Vasu,
Prof. Dr. H. Yorimitsu, Prof. Dr. A. Osuka
Department of Chemistry, Graduate School of Science
Kyoto University, Sakyo-ku, Kyoto 606-8502 (Japan)
E-mail: yori@kuchem.kyoto-u.ac.jp
Dr. K. Murakami
The Hakubi Center for Advanced Research
Sakyo-ku, Kyoto 606-8502 (Japan)
Prof. Dr. H. Yorimitsu
ACT-C, JST, Sakyo-ku, Kyoto 606-8502 (Japan)
[**] This work was supported by Grants-in-Aid from MEXT (25107002
“Science of Atomic Layers”) and from JSPS [24685007 (Young
Scientists (A)), 26620081 (Exploratory Research)]. D.V. and K.M.
thank JSPS for financial support. H.Y. acknowledges the Japan
Association for Chemical Innovation for financial support.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201503671.
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aromatic substitution, required screening of the reaction
conditions. We selected the reaction of dibenzo[b,d]thiophene
5,5-dioxide (1a) with p-toluidine (2a) as a model reaction for
optimization [Eq. (1)]. The reaction of 1a (1.0 equiv) with 2a
(2.0 equiv) by means of lithium hexamethyldisilazide
(3.0 equiv) in 1,4-dioxane at 808C for 16 hours did not
produce the desired carbazole 3a. Interestingly, 3a was
obtained in 50% yield when NaN(SiMe3)2 was employed.
To our delight, KN(SiMe3)2 (potassium hexamethyldisilazide;
KHMDS, 0.5m toluene solution) gave the best result for
isolating 3a in 94% yield. Other potassium bases such as
K2CO3, K3PO4, KOAc, and KOtBu were totally ineffective.
1,4-Dioxane has proven to be the best cosolvent for toluene
from the KHMDS solution. The reactions in 1,2-dimethoxy-
ethane/toluene and in toluene as a sole solvent delivered 3a in
91 and 61% yield, respectively. Reducing the amount of 2a or
KHMDS led to lower yields of 3a.
the protected 3g and unprotected 3h quantitatively in 1:2.6
ratio. Unfortunately, unprotected p-aminophenol (2h)
reacted sluggishly. Despite its steric hindrance, mesitylamine
(2l) participated in the reaction to give 3l in high yield.
Moderately electron-withdrawing halo groups retarded the
reaction and also underwent dehalogenation. The reaction
with 4-chloroaniline (2m) yielded a 4:1 mixture of the desired
3m and the hydrodechlorinated byproduct 3b in 64% yield.
The fluoro analogue 2n was less reactive and afforded 3n in
30% yield. In addition, the fluoro group was substituted with
2n through an SNAr reaction to yield N-(4-(9-carbazolyl)-
phenyl)-4-fluoroaniline (3n’) as a byproduct. Alkylamines
such as p-methoxybenzylamine reacted moderately (34%
yield with p-methoxybenzylamine in 18 h), possibly because
of the less acidic NH protons. The attempted phenylation of
carbazole with diphenyl sulfone under similar reaction
conditions resulted in no conversion, probably because of
the lower nucleophilicity of potassium carbazolide.
The scope with respect to the dibenzothiophene dioxide
derivatives is broad (Table 2). Substituted or p-extended
Table 2: Scope with respect to dibenzothiophenes.
With the optimized the reaction conditions in hand, the
scope with respect to the amines was examined (Table 1). The
Table 1: Scope with respect to the anilines.
R
t [h]
3
Yield [%][a]
H
4-MeO
4-tBu
24
16
16
17
17
17
24
16
17
17
16
18
13
3b
3c
3d
3e
3 f
3g
3h
3i
3j
3k
3l
3m
3n
77
94
92
91
4-Me2N
4-CH2 =CH
4-TIPSO
4-HO
3-MeO
3,5-tBu2
3,4-OCH2O
2,4,6-Me3
4-Cl
60
28 (72[b])
26[c]
70
98
65
67
Yield is that of isolated product.
51 (13[d])
30[e] (9[f])
4-F
carbazoles were obtained efficiently. Gratifyingly, the steri-
cally demanding 4g[21] was obtained in high yield. The
reactions of substrates bearing bromo and methoxy groups
gave slightly complex mixtures, but 4h and 4i , respectively,
were isolated in moderate yield. Notably, we could synthesize
2,7-diphenyl-9-p-tolylcarbazoles (4c) in a controlled manner.
While electron-rich dibenzothiophene, as well as carbazole,
undergoes electrophilic bromination at the 3,6-positions with
exclusive regioselectivity, dibenzothiophene dioxide is elec-
tron-deficient and delivers the 2,7-dibromo product (Sche-
me 2).[15a,22] By changing the oxidation state of sulfur, we alter
[a] Yield of isolated product. [b] The yield of 3h as a byproduct. [c] 1,4-
Dioxane (6.0 mL), 1008C. [d] The yield of 3b as a byproduct. [e] 2.0 equiv
of KHMDS. [f] The yield of 3n’ as a byproduct. TIPS=triisopropylsilyl.
reactions proceeded efficiently with both electron-rich and
electron-neutral anilines to yield 3b–l in high yields. The vinyl
functionality was tolerated under the reaction conditions to
afford 3 f in moderate yield. The TIPS group of 2g partly
survived under the reaction conditions to yield a mixture of
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Scheme 2. Selective synthesis of two diphenyl isomers.[15a,22b]
the positions of the substituents in the final carbazoles 4c and
4d.[21]
Scheme 4. Sequential Diels–Alder reaction/carbazole formation.
Benzene-1,4-diamine reacted with 1a smoothly to afford
the bis(carbazolyl)benzene 5, an important structural unit
which has found diverse applications in electronic devices[23]
[Eq. (2)]. Usually, bis(carbazolyl)benzenes are obtained in
moderate yields through palladium-catalyzed or copper-
mediated reactions of 1,4-dihalobenzene with carbazole at
170–2008C.
Our SNAr-based amination was applicable to the synthesis
of indoles (Scheme 3). Although 2,3-diaryl groups are neces-
Scheme 5. Concise synthesis of p-extended dibenzoindolo[3,2-b]carba-
zoles.
We used this Diels–Alder-based approach to achieve
significantly p-extended dibenzoindolo[3,2-b]carbazoles (14;
Scheme 5). The precursor bis(sulfone) 13 was obtained from
a rather small aromatic compound (12) in 89% yield.
À
Subsequently, expected quadruple C N bond formations
occurred to give 14d and 14j in 38 and 22% yield,
respectively. In comparison with elegant yet burdensome
construction of similar dibenzoindolo[3,2-b]carbazoles for
OFET devices,[26,27] this overall transformation highlights the
synthetic potential of our carbazole synthesis.
Figure 1 shows the UV/visible and fluorescence spectra of
14j in dichloromethane.[28] The lowest energy absorption
maximum was observed at l = 496 nm, which is significantly
red-shifted compared to typical absorption of indolo[3,2-
b]carbazoles (l = 420–440 nm).[29] The compound 14j showed
bright-green fluorescence which has its intensity maximum at
l = 511 nm with a fluorescence quantum yield of 34%. Our
SNAr-based approach is thus useful for exploring p-extended
carbazoles which show interesting functions.
Scheme 3. SNAr-based synthesis of indoles.
sary to avoid undesired conjugate addition onto the vinyl
sulfoxide unit, densely arylated indoles (7a–c)[21] were formed
in excellent yields. The reaction of 6 with 4-(9-carbazolyl)ani-
line (2o) yielded an unsymmetric indole-carbazole hybrid
(7d), which will find applications in organic electrolumines-
cence.[24]
The electron-deficient benzothiophene dioxide 8 showed
uniquely high reactivity as a dienophile in the Diels–Alder
reaction with isobenzofuran (9a),[25] whereas indoles do not
react with 9a (Scheme 4). By taking advantage of the Diels–
Alder reaction, we could extend the p-conjugation to form
benzonaphthothiophene dioxide (10). Eventually, the p-
extended benzocarbazole 11 was obtained in good overall
yield from commercially available 8.
The most likely mechanism would be the sequential inter/
intramolecular SNAr reactions with highly nucleophilic potas-
sium amides (Scheme 6). The first intermolecular SNAr
reaction would be faster than the second intramolecular
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Angew. Chem. 2015, 127, 10372–10376
[1] a) H.-J. Knçlker, K. R. Reddy, Chem. Rev. 2002, 102, 4303;
b) H.-J. Knçlker, Chem. Lett. 2009, 38, 8; c) I. Bauer, H.-J.
Knçlker, Top. Curr. Chem. 2012, 309, 203; d) A. W. Schmidt,
K. R. Reddy, H.-J. Knçlker, Chem. Rev. 2012, 112, 3193.
[2] a) P. Strohriegl, J. V. Grazulevicius, Adv. Mater. 2002, 14, 1439;
b) J. V. Grazulevicius, P. Strohriegl, J. Pielichowski, K. Pieli-
chowski, Prog. Polym. Sci. 2003, 28, 1297; c) N. Blouin, M.
Leclerc, Acc. Chem. Res. 2008, 41, 1110.
Figure 1. UV/visible (solid line) and fluorescence (dotted line) spectra
of 14j in CH2Cl2.
[3] Double arylation of primary amines with 2,2’-dihalobiaryls:
a) K. Nozaki, K. Takahashi, K. Nakano, T. Hiyama, H.-Z. Tang,
M. Fujiki, S. Yamaguchi, K. Tamao, Angew. Chem. Int. Ed. 2003,
42, 2051; Angew. Chem. 2003, 115, 2097; b) T. Kitawaki, Y.
Hayashi, N. Chida, Heterocycles 2005, 65, 1561; c) A. Kuwahara,
K. Nakano, K. Nozaki, J. Org. Chem. 2005, 70, 413; d) T.
Kitawaki, Y. Hayashi, A. Ueno, N. Chida, Tetrahedron 2006, 62,
6792; e) T. Q. Hung, N. N. Thang, D. H. Hoang, T. T. Dang, A.
Villinger, P. Langer, Org. Biomol. Chem. 2014, 12, 2596; With
cyclic diaryliodonium salts: f) D. Zhu, Q. Liu, B. Luo, M. Chen,
R. Pi, P. Huang, S. Wen, Adv. Synth. Catal. 2013, 355, 2172; g) S.
Riedmüller, B. J. Nachtsheim, Beilstein J. Org. Chem. 2013, 9,
1202.
Scheme 6. Plausible mechanism.
[4] a) J. A. Jordan-Hore, C. C. C. Johansson, M. Gulias, E. M. Beck,
M. J. Gaunt, J. Am. Chem. Soc. 2008, 130, 16184; b) S. H. Cho, J.
Yoon, S. Chang, J. Am. Chem. Soc. 2011, 133, 5996; c) S. W.
Youn, J. H. Bihn, B. S. Kim, Org. Lett. 2011, 13, 3738; d) Q. Jiang,
D. Duan-Mu, W. Zhong, H. Chen, H. Yan, Chem. Eur. J. 2013,
19, 1903; e) M. Yamamoto, S. Matsubara, Chem. Lett. 2007, 36,
172.
Scheme 7. Elucidation of mechanism.
[5] a) B. J. Stokes, B. Jovanovic, H. Dong, K. J. Richert, R. D. Riell,
T. G. Driver, J. Org. Chem. 2009, 74, 3225; b) B. J. Stokes, K. J.
Richert, T. G. Driver, J. Org. Chem. 2009, 74, 6442.
reaction of 15,[30] and the carbazole formation necessitates
a high temperature. The following experiments strongly
support the mechanism (Scheme 7): 1a reacted with 2a in
the presence of KHMDS at room temperature and the
zwitterionic 16 was isolate.[21] Subsequent exposure of 16 to
KHMDS at 808C afforded 3a quantitatively. Although there
are several debatable reaction pathways for aromatic sub-
stitution of aryl sulfones,[18] the isolation of 16 strongly
indicates the intermediacy of 15 and hence the SNAr pathway
in our case.
[6] C.-Y. Liu, P. Knochel, Org. Lett. 2005, 7, 2543.
[7] a) Ref. [4b]; b) A. P. Antonchick, R. Samanta, K. Kulikov, J.
Lategahn, Angew. Chem. Int. Ed. 2011, 50, 8605; Angew. Chem.
2011, 123, 8764.
[8] S. Chakrabarty, I. Chatterjee, L. Tebben, A. Studer, Angew.
Chem. Int. Ed. 2013, 52, 2968; Angew. Chem. 2013, 125, 3041.
[9] T. Noji, H. Fujiwara, K. Okano, H. Tokuyama, Org. Lett. 2013,
15, 1946.
[10] W. D. Guerra, R. A. Rossi, A. B. Pierini, S. M. Barolo, J. Org.
Chem. 2015, 80, 928.
In conclusion, we have reported a new powerful and
practical strategy to prepare carbazoles without recourse to
transition-metal catalysts. Dibenzothiophene dioxide deriva-
tives undergo sequential inter/intramolecular nucleophilic
aromatic substitution with anilines in the presence of
KHMDS to afford a wide range of carbazoles in a single
operation. Considering that dibenzothiophene dioxides are
readily available by oxidation of the parent dibenzothio-
phenes, one can concisely achieve aromatic metamorphosis of
dibenzothiophenes to carbazoles. Since electron-deficient
thiophene dioxides exhibit reactivity different from that of
the corresponding electron-rich azaarenes, coupling a thiaar-
ene-dioxide-specific reaction with the SNAr carbazole syn-
thesis opens up a new route to difficult-to-access carbazoles.
Current efforts are directed toward exploring new aromatic
metamorphosis in our laboratory.
[11] H. Gao, Q.-L. Xu, M. Yousufuddin, D. H. Ess, L. Kürti, Angew.
Chem. Int. Ed. 2014, 53, 2701; Angew. Chem. 2014, 126, 2739.
[12] D. Vasu, H. Yorimitsu, A. Osuka, Angew. Chem. 2015, 127, 7268;
Angew. Chem. Int. Ed. 2015, 54, 7162.
[13] Diels–Alder reactions of five-membered heteroaromatics and
azines with either alkenes or alkynes result in aromatic
metamorphosis to the corresponding aromatic rings with extru-
sion of their heteroatomic units. Very recent examples: a) A.
Criado, M. Vilas-Varela, A. Cobas, D. PØrez, D. PeÇa, E.
Guitiµn, J. Org. Chem. 2013, 78, 12637; b) X. Ding, S. T. Nguyen,
J. D. Williams, N. P. Peet, Tetrahedron Lett. 2014, 55, 7002;
c) S. P. J. T. Bachollet, J. F. Vivat, D. C. Cocker, H. Adams,
J. P. A. Harrity, Chem. Eur. J. 2014, 20, 12889; d) S. Suzuki, Y.
Segawa, K. Itami, J. Yamaguchi, Nat. Chem. 2015, 7, 227.
[14] Reviews on catalytic denitrogenative transannulation of tria-
zoles: a) B. Chattopadhyay, V. Gevorgyan, Angew. Chem. Int.
Ed. 2012, 51, 862; Angew. Chem. 2012, 124, 886; b) P. Anbarasan,
D. Yadagiri, S. Rajasekar, Synthesis 2014, 46, 3004.
[15] a) Z. Duan, X. Huang, S. Fujii, H. Kataura, Y. Nishioka, Chem.
Lett. 2012, 41, 363; b) S. F. Nelsen, Y. Luo, M. N. Weaver, J. V.
Lockard, J. I. Zink, J. Org. Chem. 2006, 71, 4286; c) G.
Keywords: amination · aromatic substitution · fluorescence ·
sulfur heterocycles · synthetic methods
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.
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Communications
Barbarella, L. Favaretto, A. Zanelli, G. Gigli, M. Mazzeo, M.
Anni, A. Bongini, Adv. Funct. Mater. 2005, 15, 664.
[16] Intramolecular Smiles rearrangement is a well-known trans-
formation which includes an SNAr reaction of aryl sulfones:
W. E. Truce, E. M. Kreider, W. W. Brand, Org. React. 1970, 18,
99.
S.-M. Shuang, C. Dong, Chem. Commun. 2012, 48, 8586; c) Q.
Zhang, J. Chen, Y. Cheng, L. Wang, D. Ma, X. Jing, F. Wang, J.
Mater. Chem. 2004, 14, 895.
[24] T. Iwakuma, H. Yamamoto, Y. Hironaka, H. Ikeda, C. Hoso-
kawa, S. Tomita, T. Arakane (Idemitsu Kosan Co., Ltd.), US
8580391 B2, 2013.
[17] Some aryl sulfones undergo SNAr substitution with amines under
highly basic conditions. For instance, see: a) W. Bradley, J. Chem.
Soc. 1938, 458; b) G. Kçbrich, Chem. Ber. 1959, 92, 2981.
[18] Diphenyl sulfone reacted with fused potassium hydroxide to
afford phenol: a) S. Oae, N. Furukawa, Bull. Chem. Soc. Jpn.
1966, 39, 2260. Dibenzothiophene dioxides reacted with potas-
sium alkoxide or hydroxide at about 1608C to yield the simple
ring-opening product through one SNAr process: b) T. G.
Squires, C. G. Venier, B. A. Hodgson, L. W. Chang, J. Org.
Chem. 1981, 46, 2373; c) T. Aida, T. G. Squires, C. G. Venier,
Tetrahedron Lett. 1983, 24, 3543.
[19] Reactive azaaryl sulfones such as tetrazolyl sulfones are well
known to undergo intermolecular SNAr substitution with
a variety of nucleophiles. For instance: a) P. J. Kocienski, A.
Bell, P. R. Blakemore, Synlett 2000, 365; b) C. Aïssa, J. Org.
Chem. 2006, 71, 360; c) P. Jankowski, K. Plesniak, J. Wicha, Org.
Lett. 2003, 5, 2789.
[25] M. Nandakumar, J. Karunakaran, A. K. Mohanakrishnan, Org.
Lett. 2014, 16, 3068.
[26] M. T. Levick, I. Grace, S.-Y. Dai, N. Kasch, C. Muryn, C.
Lambert, M. L. Turner, D. J. Procter, Org. Lett. 2014, 16, 2292.
[27] Importance of benzobipyrroles in organic electronics: H. Tsuji,
Y. Yokoi, C. Mitsui, L. Ilies, Y. Sato, E. Nakamura, Chem. Asian
J. 2009, 4, 655, and references therein.
[28] The photophysical properties of 14d are very similar to those of
14j. For details, see the Supporting Information.
[29] a) J. Simokaitiene, E. Stanislovaityte, J. V. Grazulevicius, V.
Jankauskas, R. Gu, W. Dehaen, Y.-C. Hung, C.-P. Hsu, J. Org.
Chem. 2012, 77, 4924; b) K. Kawaguchi, K. Nakano, K. Nozaki, J.
Org. Chem. 2007, 72, 5119.
[30] The elimination of elusive and high-
energy K2SO2 is unlikely. After many
experiments, at this stage we speculate
that trimethylsilylation of 15 with HN-
[20] K2SO2 is an elusive species: a) P. J. Bruna, F. Grein, J. Phys.
Chem. A 2012, 116, 10229; b) D. P. Morales, A. S. Taylor, S. C.
Framer, Molecules 2010, 15, 1265.
(SiMe3)2, generated in situ, would take
place to afford 15’, which should
undergo
a cleaner SNAr reaction.
[21] X-ray crystallographic analysis unambiguously revealed the
structure. For details, see the Supporting Information.
[22] a) L. Zhang, S. Zeng, L. Yin, C. Ji, K. Li, Y. Li, Y. Wang, New J.
Chem. 2013, 37, 632; b) A. Heynderickx, A. Samat, R. Gugliel-
metti, Synthesis 2002, 213.
[23] a) B. E. Koene, D. E. Loy, M. E. Thompson, Chem. Mater. 1998,
10, 2235; b) H.-P. Shi, J.-X. Dai, L.-W. Shi, M.-H. Wang, L. Fang,
Received: April 22, 2015
Published online: July 14, 2015
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