Friedel-Crafts-Type Intermolecular C-H Silylation of Electron-Rich Arenes Initiated by Base-Metal Salts

Article abstract of DOI:10.1002/anie.201510469

An electrophilic aromatic substitution (S_EAr) with a catalytically generated silicon electrophile is reported. Essentially any commercially available base-metal salt acts as an initiator/catalyst when activated with NaBAr^F₄. The thus-generated Lewis acid then promotes the S_EAr of electron-rich arenes with hydrosilanes but not halosilanes. This new C-H silylation was optimized for FeCl₂ /NaBAr^F₄, affording good yields at catalyst loadings as low as 0.5 mol %. The procedure is exceedingly straightforward and comes close to typical Friedel-Crafts methods, where no added base is needed to absorb the released protons.

Full text of DOI:10.1002/anie.201510469

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201510469
German Edition: DOI: 10.1002/ange.201510469
Lewis Acid Catalysis
À
Friedel–Crafts-Type Intermolecular C H Silylation of Electron-Rich
Arenes Initiated by Base-Metal Salts
Qin Yin, Hendrik F. T. Klare, and Martin Oestreich*
Abstract: An electrophilic aromatic substitution (SEAr) with
a catalytically generated silicon electrophile is reported.
Essentially any commercially available base-metal salt acts as
an initiator/catalyst when activated with NaBArF4 . The thus-
generated Lewis acid then promotes the SEAr of electron-rich
À
arenes with hydrosilanes but not halosilanes. This new C H
silylation was optimized for FeCl2 /NaBArF4 , affording good
yields at catalyst loadings as low as 0.5 mol%. The procedure
is exceedingly straightforward and comes close to typical
Friedel–Crafts methods, where no added base is needed to
absorb the released protons.
T
he installation of silicon groups at arenes by electrophilic
aromatic substitution (SEAr) was identified as an important
synthetic goal nearly four decades ago.[1] However, this
seemingly straightforward challenge has not lost its relevance
because, then as now, there have been no practical solutions
reported.[2,3] The key issue is that the Brønsted acid released
in SEAr rapidly triggers the reverse reaction, known as
protodesilylation. Hence, protons must be absorbed irrever-
sibly from this equilibrium. This was achieved by using neutral
Me3SiOTf and Et3N as solvent in the C3-selective silylation of
indoles and pyrroles (e.g., 1!2; Scheme 1, top).[4] Recent
variations employing Ph3SiH activated by stoichiometric
[Ph3C]+[B(C6F5)4]À or catalytic B(C6F5)3 (not shown) also
require the addition of a base.[5] An alternative approach
involves catalytic generation of the silicon electrophile by
À
Scheme 1. C H silylation of electron-rich heterocycles by SEAr.
ArF =3,5-bis(trifluoromethyl)phenyl, Dmp=2,6-dimesitylphenyl,
rs=regioselectivity, Tf=trifluoromethanesulfonyl.
We selected the electrophilic silylation of electron-rich
N,N-dimethylaniline as a model reaction (4!5; Table 1). The
Lewis acid FeCl2 was used as the catalyst, and the halosilanes
Me2PhSiCl and Me2PhSiI, as well as the hydrosilane
Me2PhSiH were chosen as the silylating agents. As expected,
treatment of 4 in the presence of FeCl2 (5 mol%) with excess
(5 equiv) of either of these reagents at 1008C for prolonged
reaction times did not show any conversion (Table 1,
entries 1–3). To boost the Lewis acidity of FeCl2, we decided
À
À
cooperative Si H bond activation of hydrosilanes at the Ru
S bond of tethered cationic complexes.[6] After transfer of the
to add equimolar amounts of NaBArF , based on the catalyst
4
+
À
silicon cation from the [S Si] site to the nucleophilic
employed. This usually brings in the weakly coordinating
heterocycle, the sulfur atom acts as an internal base,
eventually forming dihydrogen (e.g., 1!3; Scheme 1,
bottom).[7] These transformations have essentially remained
tetrakis[3,5-bis(trifluoromethyl)phenyl]borate
anion
as
a replacement for the chloride ligand. This technique is
often used to transform well-defined neutral metal complexes
into cationic ones, but we are not aware of examples of its
application to conventional metal salts. We hoped to form
À
the only examples of intermolecular electrophilic C H
silylation.[8–10] We demonstrate in this work that activated
base-metal salts are also capable of initiating or catalyzing the
“[FeCl]+[BArF ]À”, although the actual structure, in other
4
À
silylation of arene C H bonds with hydrosilanes but not
words, its degree of aggregation or composition, is likely to be
more complicated. The additive had no or little effect on the
reactions with the halosilanes but changed the outcome with
the hydrosilane dramatically (entries 4–6). The FeCl2/
chlorosilanes. The new procedure almost resembles the
Friedel–Crafts reaction in its classical sense, with no addition
of base required.[11]
NaBArF catalyst system afforded 5 regioselectively with
4
87% conversion in 70% yield. NaBArF alone did not
4
[12]
À
promote the C H silylation (entries 7–9).
We repeated
[*] Dr. Q. Yin, Dr. H. F. T. Klare, Prof. Dr. M. Oestreich
Institut für Chemie, Technische Universität Berlin
Strasse des 17. Juni 115, 10623 Berlin (Germany)
E-mail: martin.oestreich@tu-berlin.de
the reactions of the halosilanes with 2,6-lutidine as a proton
scavenger (entries 10 and 11). While the reaction with the
chlorosilane was again unsuccessful, the iodosilane indeed
reacted, converting 4 into 5 with 65% conversion. Control
experiments with FeCl2/2,6-lutidine or 2,6-lutidine alone
Homepage: http://www.organometallics.tu-berlin.de
Supporting information and ORCID(s) from the author(s) for this
article are available on the WWW under http://dx.doi.org/10.1002/
anie.201510469.
confirmed that NaBArF as an additive is necessary
4
(entries 12–14). This outcome corroborates the fact that
3204
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2016, 55, 3204 –3207

Angewandte
Communications
Chemie
À
Table 1: Development of the Friedel–Crafts-type intermolecular C H
0.5 mol% and with less hydrosilane (3 equiv); the yield was
72% in both cases.
silylation catalyzed by iron Lewis acids.[a]
The new method is moderately general with regard to the
hydrosilane (Table 2). As verified for the FeCl2/NaBArF
4
catalyst system, an aryl substituent at the silicon atom of the
hydrosilane is required, and equally high conversions were
achieved with mono-, di-, and trihydrosilanes (Table 2,
entries 1, 2, 8, and 9); Ph3SiH seems to be too sterically
hindered (entry 3). Representative results with yields are
summarized in Figure 1 (4!5, 6, or 12). Hydrosilanes solely
decorated with alkyl, alkoxy, and/or silyloxy substituents did
not react at all (entries 4–7).
Entry Iron salt
Additive
X
T [8C] rs[b]
Conv. [%][c]
1
2
3
4
5
6
7
8
9
10
FeCl2
FeCl2
FeCl2
FeCl2
FeCl2
FeCl2
–
–
–
–
Cl
I
H
Cl
I
H
Cl
I
H
Cl
100
100
100
100
100
100
100
100
100
100
–
–
–
–
–
no conv.
trace
no conv.
no conv.
trace
NaBArF
4
NaBArF
4
[d]
NaBArF
97:3 87 (70)[e]
4
Table 2: Screening of suitable hydrosilanes.[a]
NaBArF
–
–
–
–
no conv.[f]
no conv.[f]
no conv.
no conv.
4
–
–
NaBArF
4
NaBArF
4
FeCl2
NaBArF /
4
2,6-lutidine[g]
11
FeCl2
NaBArF /
I
100 >99:1 65
4
2,6-lutidine[g]
Entry
Hydrosilane (equiv)
Aniline
rs[b]
Conv. [%][c]
12
13
14
15
16
17
18
19
20
21
22[i]
FeCl2
FeCl2
–
2,6-lutidine[g] Cl
100
100
100
100
–
–
–
no conv.
trace
trace
2,6-lutidine[g]
2,6-lutidine[g]
I
I
1
2
3
4
5
6
7
8
9
Me2PhSiH (5)
MePh2SiH (5)
Ph3SiH (5)
5
6
7
8
9
10
11
12
13
97:3
99:1
>99:1
87
83
30
no conv.
no conv.
no conv.
no conv.
85
FeBr2
NaBArF
H
H
H
H
H
H
H
H
98:2 90
4
Fe(OAc)2 NaBArF
100 >99:1 35
Et3SiH (5)
–
–
–
4
4
FeCl3
FeCl2
FeCl2
FeCl2
FeCl2
FeCl2
NaBArF
AgSbF6
NaBArF
NaBArF
NaBArF
NaBArF
100
100
97:3 83
no conv.
EtMe2SiH (5)
(EtO)3SiH (5)
(Me3SiO)2MeSiH (5)
Ph2SiH2 (2)
–
60 >99:1 70
–
4
4
4
4
140
96:4 88
98:2
MW[h]
97:3 95 (78)[e]
95:5 90 (58)[e]
[d]
PhSiH3 (1)
–
80
100
[a] Reactions were performed on a 0.1 mmol scale in a sealed tube.
[b] para:ortho ratio determined by GLC analysis. [c] Determined by GLC
analysis with tetracosane as internal standard. [d] Mixture of mono- and
disubstituted silanes obtained.
[a] Reactions were performed on a 0.1 mmol scale in a sealed tube.
[b] para:ortho ratio determined by GLC analysis. [c] Determined by GLC
analysis with tetracosane as internal standard. [d] Exactly the same result
was obtained with anhydrous FeCl2 (99.99%): 83% conv., rs=98:2.
[e] Yield after purification by flash chromatography on silica gel.
[f] Addition of 2,6-lutidine (1.0 equiv) did not make any difference.
[g] 1.0 equiv. [h] Reaction was performed with microwave (MW) heating
at 1408C for 2 h. [i] Reaction was performed in an open flask.
À
Figure 1. C H silylation of N,N-dimethylaniline with phenyl-substituted
hydrosilanes (see Table 2).
protons and the reverse reaction caused by them cannot be
ignored (see Scheme 1, top).[4,5] Other iron(II) and iron(III)
salts were also effective, and those with halide counteranions
were superior (entries 15–17). This is likely due to the ability
of NaBArF4 to preferentially abstract halide anions. The use of
The optimized conditions (see Table 1, entries 6 and 21)
were then applied to the N,N-dialkyl-substituted anilines 14–
23, as well as to indoline 24 and the 1,2,3,4-tetrahydroquino-
lines 25 and 26 (Scheme 2). We routinely ran the reactions at
1008C for 60 h but microwave heating at 1408C for 12 h or
less was also employed in selected cases. Yields were
generally good for ortho- and meta-substituted substrates,
and regioselectivity in favor of para substitution was excellent
throughout. However, neither a methyl nor a fluoro group in
the aniline para position, as in 19 and 23, steered the reaction
toward the ortho positions. In contrast to the successful use of
fluorinated anilines, the related chlorinated and brominated
AgSbF6 instead of NaBArF was not successful (entry 18).[13]
4
Further optimization of the catalyst system revealed that
the use of 10 mol% of FeCl2/NaBArF4 and 10 mol% NaBArF
4
at 5.0 mol% FeCl2 had little effect. A more pronounced
influence was seen when the reaction was run in a solvent;
conversion dropped to 60% in toluene (1m). The conversion
remained nearly unchanged in a temperature window from 60
to 1408C (Table 1, entry 6 vs. entries 19 and 20). However,
microwave heating to 1408C resulted in substantial rate
acceleration, reaching 95% conversion within 2 h (entry 21).
If desired, it is possible to perform the SEAr in an open flask
without exclusion of air and moisture (entry 22). To demon-
strate the preparative utility on larger scale (2 mmol), we
repeated the reaction at 1008C for 60 h (entry 6) and 1408C
(MW) for 2 h (entry 21) at a catalyst loading as low as
À
anilines did participate in the C H silylation but significant
dehalogenation occurred (see the Supporting Information for
details). As expected from the result with ortho-methyl-
substituted 17, cyclic compounds 24–26 also furnished the C5-
and C6-silylated heterocycles in useful yields. The lower yield
Angew. Chem. Int. Ed. 2016, 55, 3204 –3207
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
3205

Angewandte
Communications
Chemie
Scheme 4. Reaction scope III: Pyrroles and indoles. Si=SiMe2Ph.
reacted with moderate efficiency to afford regioisomeric 47
and 48 in an 80:20 ratio (Scheme 4, top). Indole 1 was
converted into the expected C3-silylated indole 3 but also
À
formed 49, with the C H bond at C5 additionally silylated
(Scheme 4, bottom).[16] This stands in contrast to existing
literature precedence,[4,5b, 7,9] and is likely the result of
reversible indole-to-indoline reduction, where the transient
À
indoline undergoes para-selective C H silylation (see 24!37;
Scheme 2, 808C instead of 1008C to suppress competing
indoline-to-indole oxidation).
We were intrigued by the fact that the rather simple
combination of an iron(II) or iron(III) salt with NaBArF
4
brings about an otherwise difficult SEAr with an in situ
generated silicon electrophile. We then tested various com-
mercially available di- and trivalent metal chlorides and
Scheme 2. Reaction scope I: Acyclic and cyclic N,N-disubstituted ani-
line derivatives.
triflates and were surprised to find that all of them catalyzed
F
À
obtained for indoline 24 is due to indoline-to-indole oxida-
tion, which is why this reaction demanded milder temper-
atures.
the C H silylation of N,N-dimethylaniline with NaBAr as
an additive (4!5; Figure 2).
4
The finding that such a broad range of metal salts together
Free NH groups as in the anilines 40 and 41 underwent
with NaBArF is catalytically active raises the question of
4
[14,15]
À
(two-fold) dehydrogenative Si N couplings
prior to the
whether the same catalyst is released from these combina-
planned SEAr reaction (40!42 but not 43 and 41!44 and 45;
Scheme 3). However, the resulting N,N-bissilylated aniline 42
is not sufficiently nucleophilic anymore, and 43 was not
detected. Conversely, aniline 44, which has one silyl group at
tions. Our initial assumption was that NaBArF generates
4
a cationic Lewis acid catalyst by chloride or triflate abstrac-
tion from the metal salt. It is, however, known that the BArF
À
4
anion is not necessarily stable toward certain noble-metal
À
À
the nitrogen atom, is not fully deactivated, and the C H
silylation did furnish 45 with good conversion.
cations, and C B bond cleavage concomitant with attachment
of an ArF group to the metal center is well documented.[17]
Such transmetalation could also come into play here and
liberate tris[3,5-bis(trifluoromethyl)phenyl]borane,[18] yet
Activated heterocycles such as N-substituted pyrroles and
À
indoles are the preferred substrates for electrophilic C H
silylation (see Scheme 1).[4,5b, 7,9] And indeed, pyrrole 46
another potential catalyst for the present SEAr.[19] We
analyzed the action of FeCl2 on NaBArF by H, 11B, and
1
4
19F NMR spectroscopy at room and elevated temperatures
but could not detect any free BArF in the resulting complex
3
mixtures. At this stage, the true nature of the catalyst, either
À
Figure 2. Testing other metal salts in the C H silylation of N,N-
dimethylaniline [4!5: metal chloride or triflate (5 mol%), NaBArF
À
Scheme 3. Reaction scope II: C H silylation in the presence of free
NH group(s). Si=SiMe2Ph.
4
(5 mol%), neat, 1008C, 24 h].
3206 www.angewandte.org
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2016, 55, 3204 –3207

Angewandte
Communications
Chemie
an in situ formed ArF-substituted base metal[17] or a boron
Lewis acid, cannot be formulated.[20] The mechanism of the
generation of the silicon electrophile as well as an overall
analysis are the subject of ongoing investigations.
Synthesis 2015, 47, 2347 – 2366; d) Z. Xu, W.-S. Huang, J. Zhang,
L.-W. Xu, Synthesis 2015, 47, 3645 – 3668.
[9] An iron-catalyzed C3 indole silylation that potentially proceeds
through an SEAr mechanism: Y. Sunada, H. Soejima, H.
Nagashima, Organometallics 2014, 33, 5936 – 5939.
À
The present intermolecular C H silylation by SEAr
À
[10] Intramolecular Friedel–Crafts-type C H silylations: a) S. Fur-
complements the existing state of the art defined by noble-
ukawa, J. Kobayashi, T. Kawashima, J. Am. Chem. Soc. 2009,
131, 14192 – 14193; b) Ref. [5a] (stoichiometric, using [Ph3C]+-
[B(C6F5)4]À); c) L. D. Curless, M. J. Ingleson, Organometallics
2014, 33, 7241 – 7246 (catalytic, using B(C6F5)3); d) L. Omann,
M. Oestreich, Angew. Chem. Int. Ed. 2015, 54, 10276 – 10279;
Angew. Chem. 2015, 127, 10414 – 10418 (catalytic, involving
metal-[21] and KOtBu-catalyzed[22] procedures.[8] The use of
a base-metal salt together with NaBArF and a hydrosilane
4
rather than the more obvious chlorosilane is key to success.
No base is required to absorb the proton released from the
Wheland intermediate, which is why the new method is close
to classic Friedel–Crafts reactions.
À
cooperative Si H bond activation, see Ref. [6]).
[11] a) C. Friedel, J. M. Crafts, J. Chem. Soc. 1877, 32, 725; b) C. C.
Price, Org. React. 1946, 3, 1 – 82.
[12] These reagents are similar to the Me3SiCl/Ag[B(C6F5)4] combi-
nation introduced by Mukaiyama and co-workers for aldol
reactions: M. Yanagisawa, T. Shimamura, D. Iida, J.-i. Matsuo, T.
Mukaiyama, Chem. Pharm. Bull. 2000, 48, 1838 – 1840.
[13] The FeCl3/AgSbF6 catalyst system applied to Friedel – Crafts
alkylation starting from secondary alcohols: L. R. Jefferies, S. P.
Cook, Org. Lett. 2014, 16, 2026 – 2029.
Acknowledgements
Q.Y. gratefully acknowledges the Alexander von Humboldt
Foundation for a postdoctoral fellowship (2015–2016). M.O. is
indebted to the Einstein Foundation (Berlin) for an endowed
professorship. We thank Dr. Elisabeth Irran (TU Berlin) for
X-ray analysis.
À
[14] Examples of relevant dehydrogenative Si N coupling reactions:
a) S. Itagaki, K. Kamata, K. Yamaguchi, N. Mizuno, Chem.
Commun. 2012, 48, 9269 – 9271; b) T. Tsuchimoto, Y. Iketani, M.
Sekine, Chem. Eur. J. 2012, 18, 9500 – 9504; c) Ref. [7b]; d) L.
Greb, S. Tamke, J. Paradies, Chem. Commun. 2014, 50, 2318 –
2320.
À
Keywords: C H activation · electrophilic substitution ·
À
hydrosilanes · Lewis acids · Si H activation
À
[15] An example of a two-fold dehydrogenative Si N coupling of
How to cite: Angew. Chem. Int. Ed. 2016, 55, 3204–3207
Angew. Chem. 2016, 128, 3256–3260
a free aniline and PhSiH3: W. Xie, H. Hu, C. Cui, Angew. Chem.
Int. Ed. 2012, 51, 11141 – 11144; Angew. Chem. 2012, 124, 11303 –
11306.
[1] D. Häbich, F. Effenberger, Synthesis 1979, 841 – 876.
[16] The indole C5 position is difficult to access and there are no
[2] BCl3-catalyzed preparation of PhSiCl3 from benzene and Cl3SiH
at 300 – 3508C under autogenous pressure: A. Wright, J.
Organomet. Chem. 1978, 145, 307 – 314, and references therein.
À
previous reports of its direct C H silylation. C3-Protodesilyla-
tion of 49 on silica gel affords C5-silylated indole 50 quantita-
tively (see the Supporting Information).
À
[3] Other attempted or unusual intermolecular electrophilic C H
[17] a) W. V. Konze, B. L. Scott, G. J. Kubas, Chem. Commun. 1999,
1807 – 1808 [platinum(II)]; b) H. Salem, L. J. W. Shimon, G.
Leitus, L. Weiner, D. Milstein, Organometallics 2008, 27, 2293 –
2299 [rhodium(III)]; c) S. G. Weber, D. Zahner, F. Rominger,
B. F. Straub, Chem. Commun. 2012, 48, 11325 – 11327 [gold(I)].
silylations: a) G. P. Sollott, W. R. Peterson, Jr., J. Am. Chem.
Soc. 1967, 89, 5054 – 5056 [AlCl3-mediated SEAr of ferrocene
with R3ÀnSiCln+1, R2Si(Cl)NR’2, or R3SiNR’2 (R = aryl or alkyl,
R’ = Me or Et, and n = 0 or 1); yields not exceeding 53%];
b) G. A. Olah, T. Bach, G. K. S. Prakash, J. Org. Chem. 1989, 54,
3770 – 3771 [AlCl3-catalyzed SEAr of benzene and toluene with
R3SiCl (R = alkyl) in the presence of Hünigꢀs base; approx-
imately 1% of desired silylated arene formed]; c) C. Ungur-
enas¸u, Rev. Roum. Chim. 1998, 43, 1087 – 1089 [SEAr of
anthracene with PcSi(AlCl4)2 (Pc = phthalocyanine) at 2008C;
85% yield of isolated product].
À
[18] To our knowledge, there are no examples of Si H bond
activation promoted by this underused electron-deficient
borane. An application in dihydrogen activation: T. J. Herring-
ton, A. J. W. Thom, A. J. P. White, A. E. Ashley, Dalton Trans.
2012, 41, 9019 – 9022.
[19] It must be noted here that 5 mol% of B(C6F5)3 and even BArF
3
promoted our model reaction 4!5 with 91% and 89%
conversion, respectively. Added base (1.0 equiv of 2,6-dichlor-
opyridine) was in fact detrimental, furnishing 46% conversion
with B(C6F5)3 as catalyst.[5b]
[4] U. Frick, G. Simchen, Synthesis 1984, 929 – 930.
[5] a) S. Furukawa, J. Kobayashi, T. Kawashima, Dalton Trans. 2010,
39, 9329 – 9336 (2,6-lutidine was stoichiometrically added; 31%
yield of Ph4Si for benzene as the arene); b) L. D. Curless, E. R.
Clark, J. J. Dunsford, M. J. Ingleson, Chem. Commun. 2014, 50,
5270 – 5272 (in addition 2,6-dichloropyridine was stoichiometri-
cally or catalytically added; yield ꢀ 70% for heteroarenes).
[6] T. Stahl, P. Hrobµrik, C. D. F. Kçnigs, Y. Ohki, K. Tatsumi, S.
Kemper, M. Kaupp, H. F. T. Klare, M. Oestreich, Chem. Sci.
2015, 6, 4324 – 4334.
[7] a) H. F. T. Klare, M. Oestreich, J.-i. Ito, H. Nishiyama, Y. Ohki,
K. Tatsumi, J. Am. Chem. Soc. 2011, 133, 3312 – 3315; b) C. D. F.
Kçnigs, M. F. Müller, N. Aiguabella, H. F. T. Klare, M. Oes-
treich, Chem. Commun. 2013, 49, 1506 – 1508.
[20] [Ph3C]+[B(C6F5)4]À (5 mol%) with and without 2,6-lutidine
(1.0 equiv) yielded less than 10% conversion in our model
reaction 4!5.[5a,10a] We therefore exclude hidden catalysis by
silicon cations or hydronium ions.
[21] a) C. Cheng, J. F. Hartwig, Science 2014, 343, 853 – 857; b) C.
Cheng, J. F. Hartwig, J. Am. Chem. Soc. 2014, 136, 12064 – 12072;
c) C. Cheng, J. F. Hartwig, J. Am. Chem. Soc. 2015, 137, 592 –
595.
[22] A. A. Toutov, W.-B. Liu, K. N. Betz, A. Fedorov, B. M. Stoltz,
R. H. Grubbs, Nature 2015, 518, 80 – 84.
À
[8] Up-to-date reviews of catalytic silylation of unactivated C H
bonds: a) C. Cheng, J. F. Hartwig, Chem. Rev. 2015, 115, 8946 –
8975; b) Y. Yang, C. Wang, Sci. China Chem. 2015, 58, 1266 –
1279; c) R. Sharma, R. Kumar, I. Kumar, B. Singh, U. Sharma,
Received: November 11, 2015
Revised: December 11, 2015
Published online: January 28, 2016
Angew. Chem. Int. Ed. 2016, 55, 3204 –3207
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
3207