Synthesis of Phenanthrene Derivatives by Intramolecular Cyclization Utilizing the [1,2]-Phospha-Brook Rearrangement Catalyzed by a Bronsted Base

Article abstract of DOI:10.1002/chem.201501377

The synthesis of functionalized phenanthrene derivatives was achieved by intramolecular cyclization utilizing the [1,2]-phospha-Brook rearrangement under Bronsted base catalysis. Treatment of biaryl compounds having an α-ketoester moiety and an alkyne moiety at the 2 and 2′ positions, respectively, with diisopropyl phosphite in the presence of a catalytic amount of phosphazene base P2-tBu provides 9,10-disubstituted phenanthrene derivatives in high yields. This reaction involves the generation of an ester enolate through an umpolung process, that is, addition of diisopropyl phosphite to a keto moiety followed by the [1,2]-phospha-Brook rearrangement, the intramolecular addition to an alkyne, and the [3,3] rearrangement of the allylic phosphate moiety in a consecutive fashion.

Full text of DOI:10.1002/chem.201501377

DOI: 10.1002/chem.201501377
Communication
&
Synthetic Methods
Synthesis of Phenanthrene Derivatives by Intramolecular
Cyclization Utilizing the [1,2]-Phospha-Brook Rearrangement
Catalyzed by a Brønsted Base
Azusa Kondoh,[a] Takuma Aoki,[b] and Masahiro Terada*[a, b]
progress in biaryl synthesis by transition metal-catalyzed cross-
coupling reactions,[5] this approach has become a straightfor-
Abstract: The synthesis of functionalized phenanthrene
derivatives was achieved by intramolecular cyclization uti-
lizing the [1,2]-phospha-Brook rearrangement under
Brønsted base catalysis. Treatment of biaryl compounds
having an a-ketoester moiety and an alkyne moiety at the
2 and 2’ positions, respectively, with diisopropyl phosphite
in the presence of a catalytic amount of phosphazene
base P2-tBu provides 9,10-disubstituted phenanthrene de-
rivatives in high yields. This reaction involves the genera-
tion of an ester enolate through an umpolung process,
that is, addition of diisopropyl phosphite to a keto moiety
followed by the [1,2]-phospha-Brook rearrangement, the
intramolecular addition to an alkyne, and the [3,3] rear-
rangement of the allylic phosphate moiety in a consecutive
fashion.
ward one. In addition, a variety of functionalities can be easily
introduced into the phenanthrene skeleton by using function-
alized biaryl compounds. To achieve this, several reactions
have been employed, including the [4+2] cycloaddition of
biaryl compounds with alkynes,[6] the intramolecular cyclization
of alkynylated biaryl compounds,[7] and intramolecular C9=C10
double bond-forming reactions, including ring-closing alkene
metathesis, McMurry coupling, and aldol condensation, and
others.[8,9] Despite such significant advances, however, there
still remain some issues on the introduction of functionalities
into the resulting phenanthrene skeleton in a regioselective
manner. Therefore, a new and efficient methodology that can
expand the scope of accessible phenanthrene derivatives
would be highly valuable. We have been focusing on the [1,2]-
phospha-Brook rearrangement,[10] which involves the migration
of a dialkoxyphosphoryl moiety of an a-hydroxy phosphonate
from carbon to oxygen under the influence of a Brønsted base
to generate an a-oxygenated carbanion, as a useful tool for
the development of novel synthetic reactions.[11] In the course
of our studies on new carbon–carbon bond-forming reactions
that are characterized by the direct generation of enolates
from a-keto carbonyl compounds through an umpolung pro-
cess, that is, the addition of a dialkyl phosphite to a keto
moiety followed by the [1,2]-phospha-Brook rearrangement
under the influence of a Brønsted base catalyst,[11b] we envis-
aged the application of the methodology to the formation of
a C9=C10 double bond of the phenanthrene skeleton from
biaryl compounds. Our reaction design is shown in Scheme 1,
in which biaryl compounds 1, having an a-ketoester moiety
and an alkyne moiety at the 2 and 2’ positions, respectively,
are used as a substrate. At first, deprotonation of dialkyl phos-
phite 2 followed by the addition of the resulting anion to
a keto moiety of 1 provides alkoxide A. Subsequently, the
[1,2]-phospha-Brook rearrangement proceeds to generate ester
enolate B. The addition of enolate B to an alkyne followed by
protonation affords cyclic compound 3 along with the regener-
ation of the Brønsted base or the anion of 2. Finally, the rear-
rangement of the allylic phosphate moiety of 3, where aroma-
tization should serve as the driving force, takes place to pro-
vide phenanthrene derivatives 4. The products of the designed
reaction have an ester moiety and an oxygenated alkyl group
at the C9 and C10 positions, respectively, which potentially
function as handles for further manipulation. Herein we report
a new method for the synthesis of highly functionalized phen-
Phenanthrene derivatives have attracted a great deal of atten-
tion because of their presence in numerous natural products,[1]
as well as their interesting biological activities.[2] They are also
important structural motifs in material science because of their
photoconducting, photochemical, and electroluminescent
properties.[3] Therefore, the development of general and effi-
cient methods for the synthesis of highly functionalized phen-
anthrene derivatives is imperative in organic synthesis, and
massive efforts have been undertaken over the years. The con-
ventional approach involves the intramolecular aryl–aryl bond
formation from stilbene derivatives. In that approach, the
Pschorr reaction, photocyclization, radical cyclization, and
other oxidative cyclization reactions are used for the aryl–aryl
bond formation.[4] Another approach that has drawn much at-
tention in recent years is the construction of a phenanthrene
skeleton from biaryl compounds. Owing to recent substantial
[a] Dr. A. Kondoh, Prof. Dr. M. Terada
Research and Analytical Center for Giant Molecules
Graduate School of Science, Tohoku University
Aramaki, Aoba-ku, Sendai 980-8578 (Japan)
E-mail: mterada@m.tohoku.ac.jp
[b] T. Aoki, Prof. Dr. M. Terada
Department of Chemistry, Graduate School of Science
Tohoku University, Aramaki, Aoba-ku, Sendai 980-8578 (Japan)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201501377.
Chem. Eur. J. 2015, 21, 12577 – 12580
12577
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication
Table 1. Initial screening for reaction conditions.[a]
Yield [%][b]
Entry
1
2
Solvent
Base
T
4
4
3
5
6
1
2
3
4
5
6
7
8
1a 2a
1b 2a
1b 2a
1b 2a
1b 2a
1b 2a
1b 2b
1b 2b
DMSO
DMSO
MeCN
THF
EtOAc
toluene
MeCN
THF
P2-tBu
P2-tBu
P2-tBu
P2-tBu
P2-tBu
P2-tBu
P2-tBu
P2-tBu
RT 4aa <1
RT 4ba 17
RT 4ba 57
RT 4ba 37
RT 4ba 25
RT 4ba 11
RT 4bb 44
0
3
0
30
52
62
0
0
0
0
8
30
17
19
13
57 <1 25
Scheme 1. Reaction design.
37
87
75
88
0
0
0
0
0
0
0
0
0
0
0
28
4
3
16
0
0
0
0
0
0
0
0
1
0
0
6
12
6
RT 4bb
RT 4bb
8
9
9
1b 2b 1,4-dioxane P2-tBu
anthrene derivatives utilizing the [1,2]-phospha-Brook rear-
rangement under Brønsted base catalysis.
10
11
12
13
14
15
16
17
18
19
20
1b 2b
1b 2b
1b 2b
EtOAc
MeCN
THF
P2-tBu
P2-tBu reflux 4bb 87
P2-tBu reflux 4bb 65
RT 4bb 11
Our investigation commenced with the treatment of 1a,
which has a phenyl group at the terminus of an alkyne moiety,
with diethyl phosphite (2a) in the presence of a catalytic
amount of phosphazene base P2-tBu in DMSO at room tem-
perature. Only a trace amount of 4aa was detected along with
other products, including ester 5aa, which was formed by the
protonation of the ester enolate generated via the [1,2]-phos-
pha-Brook rearrangement, and tetracyclic compound 6aa,
which was formed by the nucleophilic attack of a vinyl anion
on the phosphorus center and the concomitant elimination of
one of the ethoxides on the phosphorus after cyclization
(Table 1, entry 1).[12] In order to facilitate the addition of the
ester enolate to the alkyne in preference to protonation, in
other words, to prevent the formation of 5, a nitro group was
introduced to the para position of the phenyl group at the
alkyne terminus (Table 1, entry 2). As expected, the formation
of 5 was completely suppressed by using 1b as the substrate.
In addition, desired phenanthrene 4ba was formed, albeit in
low yield.[13] Screening of the solvent improved the combined
yield of 3ba and 4ba. However, a substantial amount of by-
product 6ba was formed in all cases (Table 1, entries 3–6). Fur-
ther improvement of the combined yield of 3 and 4 was ac-
complished by employing diisopropyl phosphite (2b) instead
of diethyl phosphite (Table 1, entries 7–10). By using 2b, the
nucleophilic attack of a vinyl anion on the phosphorus center,
which resulted in the formation of byproduct 6bb, was pre-
vented probably because of the bulkiness around the phos-
phorus center of the phosphate moiety. Finally, the elevated
temperature was found to facilitate the [3,3] rearrangement of
the allylic phosphonate moiety of 3bb (Table 1, entries 11–14)
and the desired phenanthrene 4bb was obtained in a quantita-
tive yield in ethyl acetate at reflux (entry 14). Further investiga-
tion of Brønsted base catalyst revealed that the use of strong
organic bases was essential for promoting the reaction
(Table 1, entries 15–20). P4-tBu, which is a stronger organic
base than P2-tBu, resulted in the formation of phenanthrene
4bb in good yield. Weaker organic bases, such as P1-tBu,
29
0
1b 2b 1,4-dioxane P2-tBu reflux 4bb 97
1b 2b
1b 2b
1b 2b
1b 2b
1b 2b
1b 2b
1b 2b
EtOAc
EtOAc
EtOAc
EtOAc
EtOAc
EtOAc
EtOAc
P2-tBu reflux 4bb 99 (92) <1
P4-tBu reflux 4bb 83
P1-tBu reflux 4bb
TBD reflux 4bb
KOtBu reflux 4bb 20
KHMDS reflux 4bb 26
0
0
0
0
<1
0
2
2
NaH reflux 4bb
9
[a] Reaction conditions: 1 (0.10 mmol), 2 (0.10 mmol), base (0.010 mmol),
solvent (3.0 mL), 7 h. [b] Yields are based on 31P NMR measurement of the
crude mixture. Trimethyl phosphate was used as the internal standard.
Yield of isolated product is given in parentheses.
1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD), and 1,8-diazabicy-
clo[5.4.0]undec-7-ene (DBU), as well as inorganic bases, such as
KOtBu, potassium hexamethyldisilazide (KHMDS) and NaH
were much less effective than P2-tBu.
To clarify whether ester 5 was the intermediate for the for-
mation of 3 and 4, 5bb was treated with P2-tBu in ethyl ace-
tate at reflux (Scheme 2); only a trace amount of 4bb was de-
tected in the crude reaction mixture and 5bb was recovered,
clearly indicating that the cyclization occurred directly from
the enolate generated by the [1,2]-phospha-Brook rearrange-
ment. Thus, this experiment demonstrates the usability of
[1,2]-phospha-Brook rearrangement for the generation of the
enolate of less acidic nucleophiles, such as esters.
With the optimized conditions in hand, the scope of the re-
action was investigated with various types of biaryl com-
Chem. Eur. J. 2015, 21, 12577 – 12580
www.chemeurj.org
12578
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication
tetraphene, and chrysene derivatives. It is particularly notable
that regioisomers, such as 4ib and 4jb, were easily accessible
by changing the position of the substituents on the starting
biaryl compounds. To confirm the necessity of the nitro sub-
stituent on the phenyl group at the alkyne terminus, sub-
strates having different electron-withdrawing groups were ex-
amined (Scheme 4); the substrate with an ethoxycarbonyl
group instead of a nitro group also underwent the reaction.
However, the yield was moderate because of the formation of
a substantial amount of byproducts such as 6. A trifluorometh-
yl substituent did not facilitate cyclization.
Scheme 2. Control experiment.
pounds (Scheme 3). The introduction of an electron-donating
group, as well as an electron-withdrawing group at the para
position of the keto ester moiety did not affect the reaction,
and the corresponding products were obtained in high yields.
In contrast, substrates with an electron-withdrawing group at
the para position of an alkyne moiety required a longer reac-
tion time for completion of the reaction. Although those sub-
strates underwent cyclization as smoothly as others, the [3,3]
rearrangement proceeded very slowly. Owing to the mildness
of the reaction conditions, a wide range of functional groups,
such as chloro, trifluoromethyl, acetal, ester, and nitro groups,
were compatible. Furthermore, the reaction was applicable to
the construction of a tetracyclic structure. The reaction of
biaryl compounds having a naphthyl moiety provided a variety
of tetracyclic compounds, including benzo[c]phenanthrene,
Scheme 4. Reaction with substrates having an electron-withdrawing group
at the para position of the phenyl group at the alkyne terminus.[a] [a] Yields
are based on 31P NMR measurement of the crude mixture. Trimethyl phos-
phate was used as the internal standard.
The product of this reaction possesses a benzylic phosphate
moiety, which could function as a handle for further manipula-
tion, particularly as a leaving group (Scheme 5). For example,
the reaction of 4bb with sodium borohydride resulted in the
cleavage of the phosphate moiety to afford diarylmethane 7 in
high yield. In addition, the treatment of 4bb with TfOH provid-
ed lactone 8 in good yield. The nitro group of 8 could be re-
duced to amino by using zinc as a reducing agent to provide
9 in high yield.
In conclusion, synthesis of functionalized phenanthrene de-
rivatives was achieved by intramolecular cyclization utilizing
Scheme 3. Substrate scope.[a] [a] Yields refer to isolated products unless oth-
erwise noted. [b] The reaction was performed for 12 h. [c] The reaction was
performed for 24 h. [d] The reaction was performed in 1,4-dioxane at reflux
for 16 h. [e] Yield is based on 31P NMR measurement of the crude mixture.
Trimethyl phosphate was used as the internal standard.
Scheme 5. Transformation of products.
Chem. Eur. J. 2015, 21, 12577 – 12580
www.chemeurj.org
12579
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication
132, 14006–14008; c) K. Kanno, Y. Liu, A. Iesato, K. Nakajima, T. Takaha-
shi, Org. Lett. 2005, 7, 5453–5456; d) A. B. Mandal, G.-H. Lee, Y.-H. Liu,
S.-M. Peng, M. Leung, J. Org. Chem. 2000, 65, 332–336; e) R. C. Larock,
M. J. Doty, Q. Tian, J. M. Zenner, J. Org. Chem. 1997, 62, 7536–7537.
[7] a) Y. Kwon, I. Kim, S. Kim, Org. Lett. 2014, 16, 4936–4939; b) C. Shu, L.
Li, C.-B. Chen, H.-C. Shen, L.-W. Ye, Chem. Asian J. 2014, 9, 1525–1529;
c) Y. Kwon, H. Cho, S. Kim, Org. Lett. 2013, 15, 920–923; d) H.-C. Shen,
J.-M. Tang, H.-K. Chang, C.-W. Yang, R.-S. Liu, J. Org. Chem. 2005, 70,
10113–10116; e) V. Mamane, P. Hannen, A. Fürstner, Chem. Eur. J. 2004,
10, 4556–4575; f) T. Yao, M. A. Campo, R. C. Larock, Org. Lett. 2004, 6,
2677–2680; g) A. Fürstner, V. Mamane, Chem. Commun. 2003, 2112–
2113; h) A. Fürstner, V. Mamane, J. Org. Chem. 2002, 67, 6264–6267;
i) J. D. Tovar, T. M. Swager, J. Organomet. Chem. 2002, 653, 215–222.
[8] a) A. Iuliano, P. Piccioli, D. Fabbri, Org. Lett. 2004, 6, 3711–3714; b) T. J.
Katz, T. M. Sivavec, J. Am. Chem. Soc. 1985, 107, 737–738; c) A.-E. Gies,
M. Pfeffer, J. Org. Chem. 1999, 64, 3650–3654; d) Y. H. Kim, H. Lee, Y. J.
Kim, B. T. Kim, J.-N. Heo, J. Org. Chem. 2008, 73, 495–501; e) Y. Xia, Z.
Liu, Q. Xiao, P. Qu, R. Ge, Y. Zhang, J. Wang, Angew. Chem. Int. Ed. 2012,
51, 5714–5717; Angew. Chem. 2012, 124, 5812–5815; f) K. Bera, S.
Sarkar, S. Jalal, U. Jana, J. Org. Chem. 2012, 77, 8780–8786; g) K. Mon-
sieurs, G. Rombouts, P. Tapolcsµnyi, P. Mµtyus, B. V. W. Maes, Synlett
2006, 3225–3230; h) G. A. Kraus, K. Hoover, N. Zhang, Tetrahedron Lett.
2002, 43, 5319–5321; i) C. B. de Koning, J. P. Michael, A. L. Rousseau, Tet-
rahedron Lett. 1998, 39, 8725–8728; j) F. Dubois, M. Gingras, Tetrahe-
dron Lett. 1998, 39, 5039–5040; k) M. E. Jung, A. Hagiwara, Tetrahedron
Lett. 1991, 32, 3025–3028.
the [1,2]-phospha-Brook rearrangement under Brønsted base
catalysis. This operationally simple reaction involves the gener-
ation of an ester enolate through addition of diisopropyl phos-
phite to a keto moiety followed by the [1,2]-phospha-Brook re-
arrangement, the intramolecular addition to an alkyne, and the
[3,3] rearrangement of the allylic phosphate moiety in a consec-
utive fashion to provide phenanthrene derivatives having
transformable functionalities at both C9 and C10 positions.
Further exploration of new reactions utilizing the [1,2]-phos-
pha-Brook rearrangement is now in progress.
Experimental Section
To a solution of 1b (40 mg, 0.10 mmol) and diisopropyl phosphite
2b (17 mL, 0.10 mmol) in EtOAc (3.0 mL) was added a solution of
P2-tBu in THF (2.0m, 5.0 mL, 0.010 mmol). The resulting mixture
was then stirred at reflux for 7 h. After the mixture had cooled to
room temperature, the reaction was quenched by addition of satu-
rated aqueous NH4Cl, and the product was extracted with EtOAc
(30 mL”3). The combined organic layer was dried over Na2SO4 and
the solvent was removed by evaporation. The residue was purified
by column chromatography (hexane/EtOAc=2:1) to provide 4bb
(52 mg, 0.092 mmol, 92%) as a white solid.
[9] For selected recent other syntheses of phenanthrene derivatives, see:
a) M. L. Hossain, F. Ye, Z. Liu, Y. Xia, Y. Shi, L. Zhou, Y. Zhang, J. Wang, J.
Org. Chem. 2014, 79, 8689–8699; b) M. Murai, N. Hosokawa, D. Roy, K.
Takai, Org. Lett. 2014, 16, 4134–4137; c) R. Wakabayashi, T. Kurahashi, S.
Matsubara, Synlett 2013, 24, 2297–2301; d) M. Saifuddin, P. K. Agarwal,
B. Kundu, J. Org. Chem. 2011, 76, 10122–10128; e) Y. Kuninobu, T. Tatsu-
zaki, T. Matsuki, K. Takai, J. Org. Chem. 2011, 76, 7005–7009; f) Y. Chen,
G. Li, Y. Liu, Adv. Synth. Catal. 2011, 353, 392–400; g) Z. Liu, R. C. Larock,
Angew. Chem. Int. Ed. 2007, 46, 2535–2538; Angew. Chem. 2007, 119,
2587–2590, and references therein.
Acknowledgements
This research was partially supported by a Grant-in-Aid for Sci-
entific Research on Innovative Areas “Advanced Molecular
Transformations by Organocatalysts” from MEXT (Japan) and
a Grant-in-Aid for Scientific Research from the JSPS.
[10] For selected examples, see: a) C. C. Bausch, J. S. Johnson, Adv. Synth.
˙
˙
˘
Catal. 2005, 347, 1207–1211; b) A. S. Demir, Ö. Reis, A. C¸. Igdir, I. Esirin-
gü, S. Eymur, J. Org. Chem. 2005, 70, 10584–10587; c) A. S. Demir, B.
Reis, Ö. Reis, S. Eymür, M. Gçllü, S. Tural, G. Saglam, J. Org. Chem. 2007,
72, 7439–7442; d) A. S. Demir, I. Esiringü, M. Gçllü, Ö. Reis, J. Org. Chem.
2009, 74, 2197–2199; e) L. El Ka¨ım, L. Gaultier, L. Grimaud, A. Dos San-
tos, Synlett 2005, 2335–2336; f) M. Hayashi, S. Nakamura, Angew. Chem.
Int. Ed. 2011, 50, 2249–2252; Angew. Chem. 2011, 123, 2297–2300;
g) M. T. Corbett, D. Uraguchi, T. Ooi, J. S. Johnson, Angew. Chem. Int. Ed.
2012, 51, 4685–4689; Angew. Chem. 2012, 124, 4763–4767.
Keywords: rearrangement · phosphazenes · organocatalysis ·
cyclization · phenanthrenes
[1] a) A. J. Floyd, S. F. Dyke, S. E. Ward, Chem. Rev. 1976, 76, 509–562; b) Z.
Li, Z. Jin, R. Huang, Synthesis 2001, 2365–2378.
[2] a) W. T. Colwell, V. Brown, P. Christie, J. Lange, C. Reece, K. Yamamoto,
D. W. Henry, J. Med. Chem. 1972, 15, 771–775; b) J. G. Cannon, P. R.
Khonje, J. P. Long, J. Med. Chem. 1975, 18, 110–112; c) M. G. Banwell, A.
Bezos, C. Burns, I. Kruszelnicki, C. R. Parish, S. Su, M. O. Sydnes, Bioorg.
Med. Chem. Lett. 2006, 16, 181–185; d) L. Wei, A. Brossi, R. Kendall, K. F.
Bastow, S. L. Morris-Natschke, Q. Shi, K.-H. Lee, Bioorg. Med. Chem. 2006,
14, 6560–6569.
[3] a) F. D. Lewis, E. L. Burch, J. Phys. Chem. 1996, 100, 4055–4063; b) F. D.
Lewis, S. V. Barancyk, E. L. Burch, J. Am. Chem. Soc. 1992, 114, 3866–
3870; c) A. M. Machado, M. Munaro, T. D. Martins, L. Y. A. Dµvila, R. Giro,
M. J. Caldas, T. D. Z. Atvars, L. C. Akcelrud, Macromolecules 2006, 39,
3398–3407; d) R. Liu, J. P. S. Farinha, M. A. Winnik, Macromolecules
1999, 32, 3957–3963.
[11] a) A. Kondoh, M. Terada, Org. Lett. 2013, 15, 4568–4571; b) A. Kondoh,
T. Aoki, M. Terada, Org. Lett. 2014, 16, 3528–3531.
[12] The structures of byproducts 5 and 6 were determined by NMR spec-
troscopy and HRMS. Compounds related to 5 and 6 were reported in
our previous work (ref [11b]). In addition, less than 10% yield of 4aa’
was isolated only in the reaction of 1a with 2a.
[4] a) F. W. Wassmundt, W. F. Kiesman, J. Org. Chem. 1995, 60, 196–201;
b) J. F. Almeida, L. Castedo, D. Fernµndez, A. G. Neo, V. Romero, G. Tojo,
Org. Lett. 2003, 5, 4939–4941; c) D. C. Harrowven, M. I. T. Nunn, D. R.
Fenwick, Tetrahedron Lett. 2002, 43, 3185–3187; d) K. Wang, M. Lü, A.
Yu, X. Zhu, Q. Wang, J. Org. Chem. 2009, 74, 935–938, and references
therein.
[13] The structure of 4ba was confirmed by single-crystal X-ray diffraction
analysis. CCDC 1054798 contains the supplementary crystallographic
data for this paper. These data are provided free of charge by The Cam-
bridge Crystallographic Data Centre.
[5] J. Hassan, M. SØvignon, C. Gozzi, E. Schulz, M. Lemaire, Chem. Rev. 2002,
102, 1359–1469.
Received: April 8, 2015
Published online on July 21, 2015
[6] a) A. Matsumoto, L. Ilies, E. Nakamura, J. Am. Chem. Soc. 2011, 133,
6557–6559; b) C. Wang, S. Rakshit, F. Glorius, J. Am. Chem. Soc. 2010,
Chem. Eur. J. 2015, 21, 12577 – 12580
www.chemeurj.org
12580
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim