Synthesis of Fluorinated Homoallylic Compounds by Fluoroalkyl Radical Mediated Ring Opening of Methylenecyclopropanes

Article abstract of DOI:10.1002/ejoc.201600380

The fluoroalkyl (Rf) radical mediated ring opening of methylenecyclopropanes (MCPs) 1 and subsequent atom transfer were investigated. In the first part, a copper-initiated radical reaction of Rf-X (X = I, Br) with MCP 1 gave homoallylic halides 2 in excellent yields. Similarly, radical reaction of the RfTMS/CsF/PhI(OCOR)₂ system with MCP 1 led to homoallylic alcohol esters 3 in moderate to good yields. An efficient synthesis of fluoroalkyl (Rf)-substituted homoallylic halides 2 and homoallylic alcohol esters 3 by fluoroalkyl radical mediated ring opening of methylenecyclopropanes (MCPs) 1 followed by an atom-transfer process is developed.

Full text of DOI:10.1002/ejoc.201600380

DOI: 10.1002/ejoc.201600380
Communication
Radical Rearrangements
Synthesis of Fluorinated Homoallylic Compounds by Fluoroalkyl
Radical Mediated Ring Opening of Methylenecyclopropanes
[
a]
[a]
Huajun Xie and Bo Xu*
Abstract: The fluoroalkyl (Rf) radical mediated ring opening of 2 in excellent yields. Similarly, radical reaction of the RfTMS/
methylenecyclopropanes (MCPs) 1 and subsequent atom trans- CsF/PhI(OCOR) system with MCP 1 led to homoallylic alcohol
2
fer were investigated. In the first part, a copper-initiated radical esters 3 in moderate to good yields.
reaction of Rf–X (X = I, Br) with MCP 1 gave homoallylic halides
Introduction
fluorinated homoallylic compound C. According to a recent re-
[
6]
view by Studer and Curran, if all the propagation steps in a
Organic compounds containing perfluoroalkyl (Rf) groups, es-
pecially the trifluoromethyl group (CF ), have been used in nu-
merous applications in many fields such as medicinal chemistry,
agrichemicals, and materials science. Many newly developed
methods including transition-metal-catalyzed or transition-
metal-mediated perfluoroalkylations or trifluoromethylations
have facilitated the synthesis of Rf-containing compounds.
However, efficient and convenient introduction of Rf groups
into organic molecules continues to be a long-standing chal-
lenge for synthetic chemists.
Methylenecyclopropanes (MCPs) are highly strained but
readily accessible molecules. Being highly functional com-
pounds with high strain, they have been used as versatile build-
ing blocks in synthesis. Typically, transition metals or Brønsted
acids/Lewis acids catalyze the ring opening of MCPs to generate
structurally complex molecules. On the other hand, radical-
4 –1
radical-chain mechanism are relatively fast (e.g., k > ca. 10 s )
Scheme 1), the reaction meets the criteria for a good chain
3
(
reaction. Because termination (radical–radical coupling) and in-
hibition (radical–solvent or radical–inhibitor) will always happen
in a radical reaction, if the rate of one of the propagation steps
[
1]
3
–1
is much below 10 s , then this chain may not propagate well
because of the fact that the slow step cannot compete with
the termination and inhibition processes. In the proposed chain
reaction (Scheme 1), the addition of the radical to the alkene
[
2]
[
1,2]
4
8
–1 [5,6]
(
k ≈ 10 to 10 s ),
the rearrangement of the cyclopropyl
1
7
–1
1
2 [5]
methyl radical (k ≈ 10 s , depending on R and R ), and
atom transfer (k = ≈ 10 –10 s , Y = I) are all relatively fast,
and the above process (innate cycle ) will be inherently effi-
2
[
3]
5
9
–1
[6]
3
[6]
[
3]
cient provided that there is a suitable mode of initiation and
absent inhibition.
[
3]
[
4]
mediated ring opening of MCPs has received less attention.
Herein, we report an efficient synthesis of fluoroalkyl-substi-
tuted homoallylic halides 2 and homoallylic alcohol esters 3 by
fluoroalkyl radical mediated ring opening of 1 followed by an
atom-transfer process.
Results and Discussion
Scheme 1. Reaction of MCPs with a fluoroalkyl radical.
In general, for MCP 1 (R , R2 ≠ H, Scheme 1), a radical will
1
add to the less sterically crowded α-carbon atom to generate
The generation of fluoroalkyl radicals, especially trifluoro-
[
4]
[1f,2b–2e,7]
cyclopropyl methyl radical A, which undergoes fast ring- methyl radicals, has been extensively studied.
We fo-
[
5]
opening rearrangement to give radical B. As a highly reactive cused our research on using readily available and relatively in-
alkyl radical, B can go through an atom-transfer process to give expensive fluoroalkyl radical precursors (Scheme 2, methods A
and B). In general, fluoroalkyl radicals can be generated from
Rf–X (X = I, Br; e.g., CF I) (method A) or Rf–M (M = Si; e.g.,
3
[
a] College of Chemistry, Chemical Engineering and Biotechnology, Donghua
CF -TMS) (method B). Although fluoroalkyl radicals can also be
generated from electrophilic trifluoromethylation reagents (e.g.,
3
University
2
999 North Renmin Lu, Shanghai 201620, P. R. China
[2d]
Togni reagent, Umemoto reagent), CF SO X, and many other
3
2
E-mail: bo.xu@dhu.edu.cn
[
7]
sources, these methods usually need expensive electrophilic
trifluoromethylation reagents and/or complex reaction condi-
tions, so we will focus on methods A and B.
http://www.dhu.edu.cn/ef/24/c5958a126756/page.htm
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/ejoc.201600380.
Eur. J. Org. Chem. 2016, 2594–2598
2594
© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication
(
Table 1, entries 15 and 16). Because it is well established that
[
6]
radicals can be generated from R–I by low-valent metals, we
thought that the real initiator might not have been CuI itself
but could have been Cu generated in situ through dispropor-
0
0
tionation or reduction of CuI. We found that Cu (copper pow-
der) itself indeed mediated this reaction, albeit less efficiently
Scheme 2. Generations of fluoroalkyl radicals.
(
Table 1, entries 17–20). The lower efficiency can be explained
by the large particle size of copper powder (around 0.5–2 μm
in diameter). Alkyl iodides tend to decompose to generate I ,
2
We used the reaction of MCP 1a with readily available n-
[
6]
which is a potent radical reaction inhibitor, so low-valent met-
als here are not only an initiator, but they could also function
as an I2 scavenger. We also investigated a photoinitiation
method by using Ru(bpy) Cl (bpy = 2,2′-bipyridyl) and visible
C F I as our first model system (Table 1). The common radical
4
9
initiator BEt did not give a clean transformation (Table 1, en-
3
[
8]
try 1). A combination of a low-valent metal, FeBr , and Cs CO
2
2
3
3
2
with 1,2-dichloroethane (DCE) as the solvent gave desired prod-
uct 2a in 24 % yield (Table 1, entry 2). The yield was dramati-
cally improved upon switching the solvent to 1,4-dioxane
light, and it also worked well (Table 1, entry 21), but photoinitia-
(
Table 1, entry 3), and the reaction did not work well in solvents
such as DMF and CH CN (Table 1, entries 4 and 5). We also
Table 2. Scope for synthesis of 2.
3
screened other metal sources such as FeSO ·7H O, FeCl ·4H O,
4
2
2
2
PdCl (PPh ) , and CuI (Table 1, entries 6–9). They also gave
2
3 2
good conversions, and among them, CuI gave the best result
Table 1, entry 9). We then investigated the effect of the num-
ber of equivalents of Cs CO on the reaction (Table 1, entries 9–
(
2
3
1
1); a higher amount of Cs CO (2 equiv.) led to a quantitative
2 3
yield of 2a (Table 1, entry 11). Reducing the amount of CuI
or shortening the reaction time led to incomplete conversion
(
Table 1, entries 12–14). We found CuI and Cs CO could also
2 3
mediate the reaction, but in a much less efficient manner
Table 1. Screening conditions.
Entry Initiator (equiv.-%)
Et B (300)
Additive (equiv.) Solvent
Yield[a] [%]
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
3
–
THF
complex
FeBr
FeBr
FeBr
FeBr
2
2
2
2
(10)
(10)
(10)
(10)
Cs
Cs
Cs
Cs
Cs
Cs
Cs
Cs
Cs
Cs
Cs
Cs
Cs
Cs
–
2
2
2
2
2
2
2
2
2
2
2
2
2
2
CO
CO
CO
CO
CO
CO
CO
CO
CO
CO
CO
CO
CO
CO
3
3
3
3
3
3
3
3
3
3
3
3
3
3
(0.8)
(0.8)
(0.8)
(0.8)
(0.8)
(0.8)
(2)
(0.8)
(1.5)
(2)
DCE
dioxane
DMF
24
80
complex
CH
3
CN
18
76
48
78
83
95
99 (94[b])
88
32
76
15
40
7
FeSO
FeCl
PdCl
4
·7H
·4H
(PPh
2
O (10)
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
DMSO
2
2
O (10)
2
3 2
) (10)
CuI (10)
CuI (10)
CuI (10)
CuI (5)
CuI (10)
CuI (10)
–
0
1
2
3
4
5
6
7
8
9
0
1
(2)
(2)
(2)
(2)
[
[
c]
d]
CuI (10)
[
[
[
[
[
e]
0
Cu (10)
Cs
Cs
–
Cs
DBU (2)
2
2
CO
CO
3
(2)
(2)
e,f]
e]
0
Cu (640)
3
88
22
5
0
Cu (640)
e]
0
Cu (640)
2
CO
3
(2)
DMSO
g]
[Ru] (0.5)/hv
CH
3
CN
99
[
4
[
a] Yield determined by GC. [b] Yield of isolated product. [c] Reaction time:
h. [d] Reaction time: 6 h. [e] Copper powder (ca. 0.5–2 μm in diameter).
f] Reaction time: 72 h. [g] Ru(bpy) Cl and visible light; DBU = 1,8-diazabi-
[a] Reaction conditions: 1 (0.3 mmol), CuI (0.03 mmol), Rf–X (0.6 mmol),
3
2
Cs
2 3
CO (0.6 mmol), dioxane (1 mL), 60 °C, 18–24 h. [b] Yield of isolated prod-
0
cyclo[5.4.0]undec-7-ene.
uct. [c] Cu was used.
Eur. J. Org. Chem. 2016, 2594–2598
www.eurjoc.org
2595
© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication
tion requires an expensive Ru-based photosensitizer, and we chromatography. Similar to the reaction of MCPs with Rf–X, un-
will use a CuI-based initiator.
symmetrical MCPs gave a mixture of Z/E isomers (Table 4, en-
With the optimized conditions in hand, we explored the tries 4, 9, and 10).
scope of this method for the synthesis of 2 (Table 2). Most
substrates gave almost quantitative yields. Some MCPs (e.g., 1b)
Table 4. Scope for the synthesis of 3.[a]
gave slightly lower yields. Because GC analysis of this reaction
mixture indicated a clean transformation, we think the lower
yield may be caused by the instability of iodide products 2
during column separation (formation of I was detected). Un-
2
symmetrical MCPs give a mixture of Z/E isomers (Table 2, en-
tries 6 and 7; E/Z ≈ 1:1 to 1:2). The standard method with the
0
use of CuI did not work for Br–CF COOEt, but the Cu /DMSO
2
system (Table 1, entry 19) worked well (Table 2, entries 8 and
9).
Because many fluoroalkyl iodides such as CF I are gases at
3
room temperature, we tried to seek alternative precursors to
generate the CF radical. Me SiCF is readily available and is a
3
3
3
liquid at room temperature. Hypervalent iodine reagents are
[
2d]
common oxidants in radical chemistry.
Although PhI(OAc)2-
mediated radical trifluoromethylation by using Me SiCF has
3
3
[
9]
been reported, there are few examples of atom transfer with
the use of PhI(OAc) . We used the reaction of MCP 1a with
2
Me SiCF and PhI(OAc) as our model system (Table 3). First, we
3
3
2
investigated the amount of the fluoride source on the reaction
Table 3, entries 1–4), and 3 equivalents of CsF gave the best
(
result (Table 3, entry 3). Additives such as NaOAc gave a cleaner
reaction (Table 3, entry 5), but other additives such as 1,4-cyclo-
hexadiene and AgNO had deleterious effect (Table 3, entries 6
3
and 7). We also screened the effect of solvents, but none
worked as well as CH CN (Table 3, entries 8–10).
3
Table 3. Screening conditions for the synthesis of 3.
Entry
CsF [equiv.] Solvent
Additive
Temp. [°C]
Yield[a] [%]
1
2
3
4
5
6
7
8
9
1
1.5
3
4
3
3
3
3
3
CH
CH
CH
CH
CH
CH
CH
3
3
3
3
3
3
3
CN
CN
CN
CN
CN
CN
CN
–
–
–
–
60
25
25
25
25
25
25
25
25
25
17
37
52
40
62 (50)[
b]
NaOAc
CHD
AgNO
NaOAc
NaOAc
NaOAc
[
c]
3
0
2
16
2
[
d]
3
[
(
a] Reaction conditions: 1 (0.3 mmol), PhI(RCOO)2 (0.45 mmol), Cs
2 3
CO
NMP
CH Cl
THF
0.6 mmol), CsF (0.9 mmol), NaOAc (0.3 mmol), CH CN(1 mL), r.t., 3 h. [b] Yield
3
2
2
of isolated product.
1
0
3
[
a] Yield determined by GC. [b] Yield of isolated product. [c] CHD = 1,4-
cyclohexadiene. [d] AgNO was used to replace CsF.
3
The proposed mechanisms are shown in Scheme 3. In the
reaction of 1 with Rf–X (Scheme 3, a), first, a low-valent metal,
0
potentially Cu , reacts with Rf–I to generate the Rf radical. This
With the optimized conditions for the synthesis of 3 in hand, radical adds to the less sterically crowded α-carbon atom of
[
4]
we explored the scope of the reaction (Table 4). Most substrates MCP 1 to generate cyclopropyl methyl radical A, which under-
gave moderate yields. These low yields are due to the formation goes fast ring-opening rearrangement to give radical B. Radical
of unidentified non-fluorinated products, but these non-fluorin- B is a highly reactive alkyl radical that can go through an atom-
ated products could be easily removed by silica gel column transfer process with Rf–X to give homoallylic halide 2. Similarly,
Eur. J. Org. Chem. 2016, 2594–2598
www.eurjoc.org
2596
© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication
Typical Procedure for the Synthesis of 3: Under an argon atmos-
phere, MCP 1a (59.4 mg, 0.3 mmol), Me SiCF (128 mg, 0.9 mmol,
3
3
3
equiv.), NaOAc (24.6 mg, 0.3 mmol, 1 equiv.), PhI(OAc) (145 mg,
2
0.45 mmol, 1.5 equiv.), and CsF (137 mg, 0.9 mmol, 3 equiv.) were
dissolved in dry CH CN (1 mL). The mixture was stirred at 25 °C for
3
18 h, and the reaction was quenched with water. The aqueous solu-
tion was extracted with EtOAc, and the combined organic phase
was washed with brine and dried with MgSO . After filtration, the
4
filtrate was concentrated under reduced pressure, and the residue
was purified by column chromatography (silica gel, hexane/EtOAc)
1
to afford product 3a (49 mg, 50 %) as a colorless oil. H NMR
(
400 MHz, CDCl , Me Si): δ = 1.57–1.65 (m, 4 H), 2.08–2.16 (m, 5 H),
3 4
2
4
.63 (t, J = 8 Hz, 2 H), 2.79–2.86 (m, 2 H), 3.01–3.05 (m, 1 H), 4.07–
.1 5 (m, 2 H), 7.22–7.24 (m, 3 H), 7.31–7.35 (m, 2 H) ppm. C NMR
1
3
Scheme 3. Proposed mechanisms.
(100 MHz, CDCl , Me Si): δ = 20.95, 26.82, 31.65, 31.76, 35.10,
3 4
3
1
5.45, 44.15, 63.13, 118.32 (q, J = 28 Hz), 124.72 (q, J = 274 Hz),
1
9
26.31, 126.77, 128.48, 145.67, 150.72 (q, J = 3 Hz), 170.98 ppm.
F
in the reaction of 1 with the Rf-TMS/CsF/PhI(OAc)2 system
NMR (376 MHz, CDCl , Me Si): δ = –56.16 (s) ppm. IR (neat):
3
4
(Scheme 3, b), first, Rf-TMS reacts with fluoride to generate the
ν˜ = 701, 754, 1043, 1104, 1236, 1322, 1453, 1655, 1743, 2861,
–
1
+
CF anion, which is then oxidized by PhI(OAc) to give the CF
2930 cm . HRMS (ESI+): calcd. for C H F O [M + H] 327.1572;
3
2
3
18 22 3 2
radical. Similar to Scheme 3a, newly generated alkyl radical B found 327.1566.
goes through an atom-transfer process with PhI(OAc) to give
2
[
10,11]
fluorinated homoallylic acetate 3.
The low yields for the
synthesis of 3 can be explained by the fact that PhI(OAc) is
not as good an atom-transfer reagent as Rf–I.
2
Acknowledgments
The authors are grateful to the National Natural Science Foun-
dation of China (NSFC) for financial support (grant number
NSFC-21472018).
Conclusion
Keywords: Radicals · Rearrangement · Fluorine · Ring-
opening · Small ring systems
In summary, we developed an efficient synthesis of fluoroalkyl
(
Rf)-substituted homoallylic halides 2 and homoallylic alcohol
esters 3 by fluoroalkyl radical mediated ring opening of MCPs
and a subsequent atom-transfer process. Further applications
[
1] a) L. Chu, F.-L. Qing, Acc. Chem. Res. 2014, 47, 1513–1522; b) X.-H. Xu, K.
Matsuzaki, N. Shibata, Chem. Rev. 2015, 115, 731–764; c) J. Charpentier,
N. Fruh, A. Togni, Chem. Rev. 2015, 115, 650–682; d) O. A. Tomashenko,
V. V. Grushin, Chem. Rev. 2011, 111, 4475–4521; e) J. Wang, M. Sanchez-
Rosello, J. L. Acena, C. del Pozo, A. E. Sorochinsky, S. Fustero, V. A. Solo-
shonok, H. Liu, Chem. Rev. 2014, 114, 2432–2506; f) E. Merino, C. Nevado,
Chem. Soc. Rev. 2014, 43, 6598–6608; g) S. Purser, P. R. Moore, S. Swallow,
V. Gouverneur, Chem. Soc. Rev. 2008, 37, 320–330.
1
of the products as well as expansion of our method to other
radical reactions are under investigation in our laboratory.
Experimental Section
[2] a) Z. Jin, G. B. Hammond, B. Xu, Aldrichim. Acta 2012, 45, 67–83; b) C.
Alonso, E. Martínez de Marigorta, G. Rubiales, F. Palacios, Chem. Rev.
Typical Procedure for the Synthesis of 2: Under an argon atmos-
phere, MCP 1a (39.6 mg, 0.2 mmol), CuI (3.8 mg, 0.02 mmol, 10 mol-
2015, 115, 1847–1935; c) P. Gao, X.-R. Song, X.-Y. Liu, Y.-M. Liang, Chem.
Eur. J. 2015, 21, 7648–7661; d) T. Koike, M. Akita, J. Fluorine Chem. 2014,
167, 30–36; e) C. Zhang, Org. Biomol. Chem. 2014, 12, 6580–6589.
%
), Cs CO (130.3 mg, 0.4 mmol, 2 equiv.), and n-C F I (138.4 mg,
2
3
4
9
0
.4 mmol, 2 equiv.) were dissolved in dry dioxane (1 mL). The mix-
[3] a) M. Shi, L.-X. Shao, J.-M. Lu, Y. Wei, K. Mizuno, H. Maeda, Chem. Rev.
2010, 110, 5883–5913; b) A. Brandi, S. Cicchi, F. M. Cordero, A. Goti,
ture was stirred at 60 °C for 18 h, and the reaction was quenched
Chem. Rev. 2014, 114, 7317–7420.
by the addition of aq. HCl. The mixture was then extracted with
hexanes,[
12]
[4] a) Z.-Z. Zhu, K. Chen, L.-Z. Yu, X.-Y. Tang, M. Shi, Org. Lett. 2015, 17, 5994–
and the organic layers were combined and dried with
5
997; b) L. Yu, Y. Wu, T. Chen, Y. Pan, Q. Xu, Org. Lett. 2013, 15, 144–147;
MgSO . After filtration, the filtrate was concentrated under reduced
4
c) T. Kippo, K. Hamaoka, I. Ryu, J. Am. Chem. Soc. 2013, 135, 632–635; d)
M. Miao, X. Huang, J. Org. Chem. 2009, 74, 5636–5639; e) N. Hayashi, Y.
Hirokawa, I. Shibata, M. Yasuda, A. Baba, J. Am. Chem. Soc. 2008, 130,
2912–2913; f) W.-J. Fu, X. Huang, J. Organomet. Chem. 2007, 692, 740–
745; g) L. Yu, X. Huang, Synlett 2007, 1371–1374; h) J.-W. Huang, M. Shi,
J. Org. Chem. 2005, 70, 3859–3863; i) K. Tsuchii, M. Imura, N. Kamada, T.
Hirao, A. Ogawa, J. Org. Chem. 2004, 69, 6658–6665; j) H. Zhou, X. Huang,
W. Chen, J. Org. Chem. 2004, 69, 5471–5472; k) L.-P. Liu, M. Shi, Chem.
Commun. 2004, 2878–2879; l) B. Xu, Y. Chen, M. Shi, Tetrahedron Lett.
pressure, and the residue was purified by column chromatography
(
silica gel, hexanes/EtOAc) to afford product 2a (102.3 mg, 94 %) as
1
a colorless oil. H NMR (400 MHz, CDCl , Me Si): δ = 1.59–1.71 (m,
3
4
2
3
H), 2.05–2.24 (m, 4 H), 2.80–2.90 (m, 4 H), 3.00–3.04 (m, 1 H), 3.11–
.22 (m, 2 H), 7.25–7.29 (m, 3 H), 7.34–7.38 (m, 2 H) ppm. 13C NMR
(
100 MHz, CDCl , Me Si): δ = 1.73, 32.06, 32.94, 33.91, 35.22, 35.67,
3 4
4
1
5
–
3.98, 100–120 (m, multiple carbon atoms of –CF CF CF CF ),
20.56 (t, J = 20 Hz), 126.42, 126.74, 128.52, 145.32, 154.02 (t, J =
Hz) ppm. 19F NMR (376 MHz, CDCl , Me Si): δ = –80.9 (m, 3 F),
2 2 2 3
2002, 43, 2781–2784; m) S. I. Kozhushkov, M. Brandl, A. de Meijere, Eur.
3
4
J. Org. Chem. 1998, 1535–1542.
104.2 (m, 2 F), –122.6 (m, 2 F), –125.9 (m, 2 F) ppm. IR (neat):
[
5] V. W. Bowry, J. Lusztyk, K. U. Ingold, J. Am. Chem. Soc. 1991, 113, 5687–
ν˜ = 533, 746, 860, 1025, 1133, 1238, 1347, 1452, 1639, 2861,
5
698.
–1
+
2
930 cm . HRMS (ESI+): calcd. for C H F I [M+ H] 545.0388;
[6] A. Studer, D. P. Curran, Angew. Chem. Int. Ed. 2016, 55, 58–102; Angew.
Chem. 2016, 128, 58–106.
1
9 19 9
found 545.0384.
Eur. J. Org. Chem. 2016, 2594–2598
www.eurjoc.org
2597
© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication
[
[
7] X. Wang, Y. Zhang, J. Wang, Zhongguo Kexue: Huaxue 2012, 42, 1–11.
8] T. Xu, C. W. Cheung, X. Hu, Angew. Chem. Int. Ed. 2014, 53, 4910–4914;
Angew. Chem. 2014, 126, 5010–5014.
[11] S. Wang, L. N. Guo, H. Wang, X.-H. Duan, Org. Lett. 2015, 17, 4798–
4
801.
[
12] T. Xu, C. W. Cheung, X. Hu, Angew. Chem. Int. Ed. 2014, 53, 4910–4914;
Angew. Chem. 2014, 126, 5010–5014.
[
9] Y.-F. Wang, G. H. Lonca, S. Chiba, Angew. Chem. Int. Ed. 2014, 53, 1067–
1
071; Angew. Chem. 2014, 126, 1085–1089.
Received: March 27, 2016
Published Online: May 2, 2016
[
10] U. Kloeckner, B. J. Nachtsheim, Chem. Commun. 2014, 50, 10485–10487.
Eur. J. Org. Chem. 2016, 2594–2598
www.eurjoc.org
2598
© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim