Palladium-catalyzed synthesis of N-arylated carbazoles using anilines and cyclic diaryliodonium salts

Article abstract of DOI:10.3762/bjoc.9.136

The direct synthesis of N-arylated carbazoles through a palladium-catalyzed amination of cyclic iodonium salts with anilines is described. In particular, electron-poor aniline derivatives reacted smoothly with only 5 mol % of Pd(OAc)₂ as catalyst to give the desired products in up to 71% yield. Furthermore, the reactivity of cyclic iodonium salts is compared with the reactivity of the corresponding cyclic bromonium analogues.

Full text of DOI:10.3762/bjoc.9.136

Palladium-catalyzed synthesis of N-arylated
carbazoles using anilines and cyclic
diaryliodonium salts
Stefan Riedmüller1,2 and Boris J. Nachtsheim*2
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2013, 9, 1202–1209.
1Merck KGaA, Performance Materials Division, Location F61/391,
Frankfurter Straße 250, 64293 Darmstadt, Germany and 2Institut für
Organische Chemie, Eberhard Karls Universität Tübingen, Auf der
Morgenstelle 18, 72076 Tübingen, Germany
doi:10.3762/bjoc.9.136
Received: 10 April 2013
Accepted: 28 May 2013
Published: 21 June 2013
Email:
Boris J. Nachtsheim* - boris.nachtsheim@uni-tuebingen.de
Associate Editor: V. M. Dong
* Corresponding author
© 2013 Riedmüller and Nachtsheim; licensee Beilstein-Institut.
License and terms: see end of document.
Keywords:
amination; carbazoles; hypervalent; iodine; iodonium salts
Abstract
The direct synthesis of N-arylated carbazoles through a palladium-catalyzed amination of cyclic iodonium salts with anilines is
described. In particular, electron-poor aniline derivatives reacted smoothly with only 5 mol % of Pd(OAc)2 as catalyst to give the
desired products in up to 71% yield. Furthermore, the reactivity of cyclic iodonium salts is compared with the reactivity of the
corresponding cyclic bromonium analogues.
Introduction
Carbazoles play an important role as core structural elements in far from the literature. The main synthetic routes are shown in
natural products (e.g., alkaloids) and pharmaceuticals [1]. In Scheme 1. The majority are transition-metal mediated. Starting
addition, the carbazole motif constitutes an immense class of from functionalized 2,2'-biphenyls (path A) [10-13] or the
materials in the rapidly growing field of molecular electronics. direct arylation [14,15] of the free NH-functionality of
In particular N-arylcarbazoles have promising electrolumines- carbazole (path B).
cent properties and have subsequently found diverse applica-
tions as hole-transport, or as host or luminescent-materials in In the past decade, hypervalent iodine chemistry has undergone
electronic devices (OLEDs) (Figure 1) [2-7]. Representative a renaissance and has developed to become a powerful area in
examples are the host molecules mCP, CBP and CBZ1-F2, the synthetic organic chemistry. Open-chained iodonium salts are
hole transporter BCz2 [8] or the recently described thermally well explored in transition-metal-mediated reactions to
activated delayed fluorescence (TADF) emitter 4CzIPN [9].
construct new C–N bonds [16-19], whereas examples dealing
with cyclic iodonium salts are underrepresented [20]. Our group
Therefore, the efficient synthesis of N-arylated carbazoles is an is interested in the development of new C–X coupling strate-
attractive goal and numerous synthetic methods are known so gies via (hypo)iodite or hypervalent iodine catalysis [21-23].
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Figure 1: Representative examples of carbazoles with hole-transport, host or luminescent properties.
Here, we wish to present an alternative Pd-catalyzed method for
the construction of N-substituted carbazoles based on a stable,
cyclic iodonium salt and electron-deficient anilines [24,25].
In the initial C–N bond-forming step of this cascade reaction, a
ring opening of the cyclic iodonium salt through the amine is
proposed to give 2'-iodobiphenyl-2-phenylamine (I). In a
second, Pd-mediated intramolecular cross-coupling, 9-phenyl-
9H-carbazole (3a) should be observed (Scheme 2).
Results and Discussion
First, we decided to prepare cyclic iodonium salt 1 as the triflate
salt, to avoid unwanted side-reactions in solution with a concur-
rent nucleophilic counterion [26]. However, 1 was synthesized
from 2-iodobiphenyl according to the established one-pot
procedure for the synthesis of diaryliodonium triflates [27,28]
by Olofsson and co-workers (Supporting Information File 1).
After promising initial experiments, we systematically opti-
mized the reaction conditions of a reaction between aniline and
cyclic iodonium salt 1 (Table 1). Various reaction parameters,
in particular the Pd catalyst, catalyst loading, the phosphine
ligand, and the temperature had a significant influence on the
outcome of this transformation. Starting with Pd2(dba)3, SPhos
(2 mol % and 4 mol %, respectively), NaOt-Bu or Cs2CO3 in
Scheme 1: Synthetic access to N-arylated carbazoles.
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Scheme 2: Proposed mechanistic motivation towards the formation of 3a.
toluene resulted only in trace amounts of 3a (Table 1, entry 1 example p-xylene, were also suitable for the reaction; in
and 2). After increasing the catalyst/ligand ratio (palladium to contrast to DME, where the yield slightly decreases (Table 1,
phosphine 5 mol % and 10 mol %, respectively) and using entry 11). We also tested t-Bu-Xantphos (Table 1, entry 8) as a
Cs2CO3 as the base, 3a could be isolated in 35% yield (Table 1, common ligand with a higher bite angle. However, this Xant-
entry 3). Next, we varied the phosphine ligands (Table 1, entries phos derivative is totally inefficient in our coupling reaction.
4–9). Xantphos was the most efficient bidentate ligand yielding Dppf and BINAP gave very similar results in isolated yield
3a in 46% yield (Table 1, entry 6). Xylenes as the solvent, for (natural bite angle of the phosphines 99° and 93°, respectively)
Table 1: Optimizing the reaction conditionsa.
entry
catalyst
5 mol %
phosphine
10 mol %
base
solvent
time [h]
temp [°C]
yield [%]b
1c
2c
3
Pd2dba3
Pd2dba3
Pd2dba3
Pd2dba3
Pd2dba3
Pd2dba3
Pd2dba3
Pd2dba3
Pd2dba3
Pd2dba3
Pd2dba3
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
SPhos
SPhos
SPhos
P(t-Bu)3
dppf
NaOt-Bu
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
toluene
toluene
toluene
toluene
toluene
toluene
p-xylene
p-xylene
p-xylene
p-xylene
DME
19
19
5
105
105
105
105
105
105
125
125
125
125
79
traced
traced
35
4
4
34
5
13
14
16
16
14
12
13
2.5
3
14
6
Xantphos
BINAP
46
7
16
8
t-Bu-Xantphos Cs2CO3
traced
34
9
DPE-Phos
Xantphos
Xantphos
Xantphos
Xantphos
Xantphos
Xantphos
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
NaOt-Bu
Cs2CO3
10
11
12
13
14
15f
42
39
p-xylene
DME
125
79
45e
12
p-xylene
p-xylene
3
125
126
traced
51
4
aAll reactions were run using iodonium salt 1 (0.35 mmol), 1.2 equiv of aniline 2a, 2.7 equiv of base, and 5 mL of solvent. bIsolated yield after column
chromatography. c2 mol % Pd2dba3 and 4 mol % SPhos were used. dProduct not isolated. e1.0 equiv of aniline 2a was used. f10 mol % Pd(OAc)2 and
20 mol % Xantphos were used. Pd2dba3 = tris(dibenzylideneacetone)dipalladium(0), SPhos = 2-dicyclohexylphosphino-2',6'-dimethoxybiphenyl, dppf
= 1,1'-bis(diphenylphosphino)ferrocene, Xantphos = 9,9-dimethyl-4,5-bis(diphenylphosphino)xanthene, BINAP = 2,2'-bis(diphenylphosphino)-1,1'-
binaphthyl, t-Bu-Xantphos = 4,5-bis(di-tert-butylphosphino)-9,9-dimethylxanthene, DPEphos = bis[(2-diphenylphosphino)phenyl] ether.
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(Table 1, entry 5 and 7). When using bidentate ligands with bite 2,2'-diiodobiphenyl, 2-(2,5-dimethylphenyl)-2'-iodobiphenyl,
angles higher than 100° (DPEphos 104°, Xantphos 108°) the and 2'-iodo-N-phenylbiphenyl-2-amine in the absence of any
reaction is more efficient and the yield increases significantly. produced 3a. Contrary to that observation, when we conducted
Next, we asked ourselves, whether other palladium salts could an analogous experiment without aniline, we observed no
be equal or better in efficiency and yield. Changing Pd2(dba)3 decomposition or byproduct formation. These results led us to
to Pd(OAc)2 had no significant impact on product yields conclude that byproduct formation is, at least partially, induced
(Table 1, entry 12). Further increase of the catalyst ratio from 5 through the nucleophilic and/or basic nature of aniline [26,32-
to 10 mol % had little effect (Table 1, entry 15).
35]. We therefore had to accept that a significant amount of
byproducts are formed during the formation of 3a, reducing our
Next, we decided to analyze the byproducts of this reaction by isolated yield.
GC–MS analysis. For this experiment the reaction was
performed according to conditions given in entry 12, Table 1. After we had gained a better understanding about byproduct
Besides the desired product 3a (56%), we could identify the formation, we decided to explore various substituted anilines
masses of 2-iodobiphenyl (22%), 2,2'-diiodobiphenyl (4%), under our optimized reaction conditions (Figure 3). Electron-
2-(2,5-dimethylphenyl)-2'-iodobiphenyl (5%), and 2'-iodo-N- rich p-toluidine (2b) gave the corresponding carbazole 3b in
phenylbiphenyl-2-amine (<2%) in significant amounts only 45% yield. Comparable results were obtained when
(Figure 2).
benzylamine (2c) was used (41% yield). Even more electron-
donating aniline derivatives, such as p-anisidine (p-methoxyani-
After a deeper literature research we came to the conclusion, line), resulted in the formation of trace amounts of the carbazole
that those byproducts should probably not only arise from side product (not shown). The aliphatic primary amines tert-butyl-
reactions within the catalytic cycle (for instance, 2-iodobiphenyl amine (2d) and propylamine (2e) were also investigated. Amine
from β-H elimination) but also from homolytic or heterolytic 2d was completely inefficient and yielded 3% of 3d compared
decomposition pathways of the diaryliodonium salt to a 79% yield of 3e, when using propylamine. However, when
(2-iodobiphenyl, 2,2'-diiodobiphenyl, and 2-(2,5-dimethyl- electron-withdrawing anilines were utilized, isolated yields of
phenyl)-2'-iodobiphenyl) [29-31]. To further verify these obser- the corresponding carbazole increased significantly. As exam-
vations, we reacted 1 in the presence of aniline (2a) for three ples, p-cyano-, p-chloro- or p-COOMe-substituted anilines
days at elevated temperature without adding a Pd catalyst. yielded 3g, 3h, and 3k in 53%, 55% and 62% yield; p-fluoro-
Again, after GC–MS analysis, we could detect 2-iodobiphenyl, aniline yielded 3f in 71%. Other fluorine-substituted anilines,
Figure 2: FID chromatogram of the reaction mixture. Only the most intense peaks were structurally assigned. x-Axis = retention time.
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Figure 3: Substrate scope. All reactions were performed using iodonium salt 1 (0.35 mmol), 1.2 equiv of primary amine 2, 2.7 equiv of Cs2CO3,
5 mol % Pd(OAc)2, 10 mol % Xantphos, and 5–8 mL p-xylene at 125 °C. Reaction times 2–4 h. Isolated yields are given in parentheses.
such as 4-(trifluoromethyl)aniline (2i) or 3,5-difluoroaniline results with the corresponding bromonium analogue. Cyclic
(2j), were also suitable with slightly decreased yields of 3i diarylbromonium salts are considerably less explored than their
(61%) and 3j (54%), respectively.
iodonium congeners as can be seen by only a handful of syn-
thetic methods described in the literature [32,36-41]. In general,
bromonium salts are more reactive but have similar reaction
behaviour [32,36]. Thus they could be helpful substrates for the
synthesis of N-arylcarbazoles from anilines. With this in mind,
we initially focussed on the synthesis of dibenzo[b,d]bromolium
chloride (5) using a procedure published by Sandin and Hay in
1952 [41] (Scheme 3).
Furthermore, meta-substituted anilines 2l and 2m were tested,
giving the N-arylated carbazoles 3l and 3m in good yields of
64% and 61%, respectively. In general, the use of fluorine-
substituted anilines showed the best results so far in this study.
However, with the perfluorinated derivative 2p, the isolated
yield of 3p was diminished to 37%. Furthermore, we used our
protocol to synthesize the N-arylcarbazole based electronic ma-
terials 3n and 3o in 40% (3n) and 23% (3o) yield.
The biphenyl derivative 4*HCl was prepared by Suzuki
coupling of 2-iodoaniline and 2-bromophenylboronic acid.
After an extensive exploration of the reaction conditions and the Diazotation of 4*HCl and cyclization gave the cyclic diaryl-
substrate scope with iodonium salt 1, we wanted to compare our bromonium chloride 5 as an off-white powder in good isolated
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Scheme 3: Synthesis of dibenzo[b,d]bromolium trifluoromethanesulfonate (6).
yield (52%) (Supporting Information File 1) [41]. Finally, 5 evident at first glance. One plausible explanation could be the
could be converted into the corresponding diarylbromonium emergence of a benzyne intermediate during synthesis, gener-
triflate 6 with silver trifluoromethanesulfonate in quantitative ated by β-elimination using aniline as a base. Subsequent
yield (Supporting Information File 1). To verify whether nucleophilic trapping of the benzyne with aniline, this time
dibenzo[b,d]bromolium trifluoromethanesulfonate (6) is reacting as a nucleophile, results in the formation of 7b. A very
indeed a more reactive surrogate for the construction of similar reactivity was described recently for a nitro-substituted
N-arylated carbazoles, we reacted 6 with p-fluoroaniline (2f) diarylbromonium salt [42]. However, the results of these experi-
according to our previously described optimized conditions ments demonstrate, that the higher reactivity of diarylbromo-
(Scheme 4).
nium salts towards nucleophilic ring opening is accompanied, to
a significant degree, by an undesired β-elimination pathway,
However, 3f was obtained in only 25% yield (Scheme 4), leading to more complex reaction mixtures and subsequently
compared to 71% when using the corresponding diaryliodo- lower yields of the desired N-arylcarbazole.
nium salt 1 (Figure 3). Apart from the desired product, we were
able to isolate two byproducts from the crude reaction mixture. Conclusion
After a systematic structure determination by one- and two- In summary, we have developed a novel synthesis of syntheti-
dimensional NMR techniques as well as mass spectrometry, we cally highly useful N-arylcarbazoles starting from cyclic
elucidated the two byproducts as the two regioisomers 2'-bro- diaryliodonium salts by a ring opening/Buchwald-amination
mo-N-(4-fluorophenyl)biphenyl-2-amine (7a, 12%) and 2'-bro- cascade using anilines and aliphatic amines as nitrogen-
mo-N-(4-fluorophenyl)biphenyl-3-amine (7b, 18%) (Scheme 4). containing substrates. With 5 mol % of Pd(OAc)2 the desired
Compound 7a was either formed during the catalytic cycle as a N-arylcarbazoles could be isolated in up to 71% yield. Finally,
reaction intermediate, which had not reacted further to the final the corresponding cyclic diarylbromonium derivatives were
product 3f, or is the result of a nucleophilic attack, caused by tested in the same reaction. Significantly lower yields were
the nucleophilic nature of aniline 2f at the electrophilic ipso- observed due to undesired side reactions involving benzyne
position in 6. The formation of the other regioisomer 7b, is not intermediates by β-elimination.
Scheme 4: Dibenzo[b,d]bromolium trifluoromethanesulfonate (6) and p-fluoroaniline (2f) to construct carbazole 3f.
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19.Beletskaya, I. P.; Davydov, D. V.; Gorovoy, M. S. Tetrahedron Lett.
2002, 43, 6221–6223. doi:10.1016/S0040-4039(02)01325-4
20.Liang, Y.; Luo, S.; Liu, C.; Wu, X.; Ma, Y. Tetrahedron 2000, 56,
2961–2965. doi:10.1016/S0040-4020(00)00166-6
21.Kloeckner, U.; Weckenmann, N. M.; Nachtsheim, B. J. Synlett 2012,
97–100. doi:10.1055/s-0031-1289902
Supporting Information
Supporting Information File 1
Experimental procedures and data of characterization of the
described compounds.
22.Hempel, C.; Weckenmann, N. M.; Maichle-Moessmer, C.;
Nachtsheim, B. J. Org. Biomol. Chem. 2012, 10, 9325–9329.
doi:10.1039/C2OB26815A
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-9-136-S1.pdf]
23.Froehr, T.; Sindlinger, C. P.; Kloeckner, U.; Finkbeiner, P.;
Nachtsheim, B. J. Org. Lett. 2011, 13, 3754–3757.
doi:10.1021/ol201439t
Acknowledgements
Financial support by the DFG and the Fonds der Chemischen
Industrie (Sachkostenzuschuss) is acknowledged. S.R. grate-
fully acknowledges Merck KGaA for financial support.
24.Letessier, J.; Detert, H. Synthesis 2012, 290–296.
doi:10.1055/s-0031-1289652
25.Sandin, R. B. J. Org. Chem. 1969, 34, 456–457.
doi:10.1021/jo01254a043
26.Beringer, F. M.; Falk, R. A. J. Chem. Soc. 1964, 4442–4451.
doi:10.1039/JR9640004442
References
1. Knölker, H.-J.; Reddy, K. R. Chem. Rev. 2002, 102, 4303–4428.
27.Bielawski, M.; Olofsson, B. Chem. Commun. 2007, 2521–2523.
doi:10.1039/B701864A
doi:10.1021/cr020059j
2. Tao, Y.; Yang, C.; Qin, J. Chem. Soc. Rev. 2011, 40, 2943–2970.
doi:10.1039/C0CS00160K
28.Bielawski, M.; Zhu, M.; Olofsson, B. Adv. Synth. Catal. 2007, 349,
2610–2618. doi:10.1002/adsc.200700373
3. Duan, L.; Hou, L.; Lee, T.-W.; Qiao, J.; Zhang, D.; Dong, G.; Wang, L.;
Qiu, Y. J. Mater. Chem. 2010, 20, 6392–6407. doi:10.1039/B926348A
4. Meng, H.; Herron, N. Organic Small Molecule Materials for Organic
Light-Emitting Diodes. In Organic Light-Emitting Materials and Devices;
Li, Z.; Meng, H., Eds.; CRC Press: Boca Raton, 2007; pp 295–412.
5. Shirota, Y. J. Mater. Chem. 2005, 15, 75–93. doi:10.1039/B413819H
6. Shirota, Y. J. Mater. Chem. 2000, 10, 1–25. doi:10.1039/A908130E
7. Shih, P.-I.; Chiang, C.-L.; Dixit, A. K.; Chen, C.-K.; Yuan, M.-C.;
Lee, R.-Y.; Chen, C.-T.; Diau, E. W.-G.; Shu, C.-F. Org. Lett. 2006, 8,
2799–2802. doi:10.1021/ol060884c
29.Beringer, F. M.; Chang, L. L. J. Org. Chem. 1971, 36, 4055–4060.
doi:10.1021/jo00825a011
30.Sato, T.; Shimizu, K.; Moriya, H. J. Chem. Soc., Perkin Trans. 1 1974,
1537–1539. doi:10.1039/P19740001537
31.Sato, T.; Shimada, S.; Shimizu, K.; Hata, K. Bull. Chem. Soc. Jpn.
1970, 43, 1918. doi:10.1246/bcsj.43.1918
32.Olah, G. A.; Sakakibara, T.; Asensio, G. J. Org. Chem. 1978, 43,
463–468. doi:10.1021/jo00397a018
33.Grushin, V. V.; Demkina, I. I.; Tolstaya, T. P.
J. Chem. Soc., Perkin Trans. 2 1992, 505–511.
8. Tsai, M.-H.; Hong, Y.-H.; Chang, C.-H.; Su, H.-C.; Wu, C.-C.;
Matoliukstyte, A.; Simokaitiene, J.; Grigalevicius, S.;
Grazulevicius, J. V.; Hsu, C.-P. Adv. Mater. 2007, 19, 862–866.
doi:10.1002/adma.200600822
doi:10.1039/P29920000505
34.Vanchikov, A. N.; Bobyleva, M. S.; Komissarova, E. E.; Kulikov, N. S.;
Tolstaya, T. P. Chem. Heterocycl. Compd. 1998, 34, 371–377.
doi:10.1007/BF02290735
9. Uoyama, H.; Goushi, K.; Shizu, K.; Nomura, H.; Adachi, C. Nature
2012, 492, 234–238. doi:10.1038/nature11687
35.Yamada, Y.; Kashima, K.; Okawara, M. Bull. Chem. Soc. Jpn. 1974,
47, 3179–3180. doi:10.1246/bcsj.47.3179
10.Zhou, Y.; Verkade, J. G. Adv. Synth. Catal. 2010, 352, 616–620.
doi:10.1002/adsc.200900846
36.Farooq, U.; Shah, A.-u.-H. A.; Wirth, T. Angew. Chem. 2009, 121,
1036–1038. doi:10.1002/ange.200805027
11.Kitawaki, T.; Hayashi, Y.; Ueno, A.; Chida, N. Tetrahedron 2006, 62,
6792–6801. doi:10.1016/j.tet.2006.04.087
Angew. Chem. Int. Ed. 2009, 48, 1018-1020.
doi:10.1002/anie.200805027
12.Kuwahara, A.; Nakano, K.; Nozaki, K. J. Org. Chem. 2005, 70,
413–419. doi:10.1021/jo048472+
37.Nesmeyanov, A. N.; Lisichkina, I. N.; Vanchikov, A. N.; Tolstaya, T. P.
Russ. Chem. Bull. 1977, 26, 1110. doi:10.1007/BF01152740
38.Nesmeyanov, A. N.; Lisichkina, I. N.; Vanchikov, A. N.; Tolstaya, T. P.
Russ. Chem. Bull. 1976, 25, 224. doi:10.1007/BF00925666
39.Nesmeyanov, A. N.; Lisichkina, I. N.; Tolstaya, T. P. Russ. Chem. Bull.
1973, 22, 2123. doi:10.1007/BF00929431
13.Nozaki, K.; Takahashi, K.; Nakano, K.; Hiyama, T.; Tang, H.-Z.;
Fujiki, M.; Yamaguchi, S.; Tamao, K. Angew. Chem. 2003, 115,
2097–2099. doi:10.1002/ange.200250648
Angew. Chem., Int. Ed. 2003, 42, 2051–2053.
doi:10.1002/anie.200250648
40.Nesmeyanov, A. N.; Tolstaya, T. P.; Lisichkina, I. N. Russ. Chem. Bull.
1968, 17, 189–190. doi:10.1007/BF00914669
14.Suzuki, K.; Hori, Y.; Kobayashi, T. Adv. Synth. Catal. 2008, 350,
652–656. doi:10.1002/adsc.200700543
41.Sandin, R. B.; Hay, A. S. J. Am. Chem. Soc. 1952, 74, 274–275.
doi:10.1021/ja01121a524
15.Xi, Z.; Liu, F.; Zhou, Y.; Chen, W. Tetrahedron 2008, 64, 4254–4259.
doi:10.1016/j.tet.2008.02.082
42.Hou, Z. J. P.; He, L. X. H. Chin. Chem. Lett. 2002, 13, 189–192.
16.Kang, S.-K.; Lee, S.-H.; Lee, D. Synlett 2000, 1022–1024.
doi:10.1055/s-2000-6673
17.Beletskaya, I. P.; Davydov, D. V.; Moreno-Mañas, M. Tetrahedron Lett.
1998, 39, 5621–5622. doi:10.1016/S0040-4039(98)01063-6
18.Davydov, D. V.; Beletskaya, I. P.; Semenov, B. B.; Smushkevich, Y. I.
Tetrahedron Lett. 2002, 43, 6217–6219.
doi:10.1016/S0040-4039(02)01326-6
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Beilstein J. Org. Chem. 2013, 9, 1202–1209.
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