Ligand-Enabled Catalytic C-H Arylation of Aliphatic Amines by a Four-Membered-Ring Cyclopalladation Pathway

Article abstract of DOI:10.1002/anie.201508912

A palladium-catalyzed C-H arylation of aliphatic amines with arylboronic esters is described, proceeding through a four-membered-ring cyclopalladation pathway. Crucial to the successful outcome of this reaction is the action of an amino-acid-derived ligand. A range of hindered secondary amines and arylboronic esters are compatible with this process and the products of the arylation can be advanced to complex polycyclic molecules by sequential C-H activation reactions. Four corners: A palladium-catalyzed C-H arylation of aliphatic amines with arylboronic esters is described to proceed by a four-membered-ring cyclopalladation pathway. Crucial to the successful outcome of this reaction is the action of an amino-acid-derived ligand. A range of hindered secondary amines and arylboronic esters are compatible with this process and the products of the arylation can be advanced to complex polycyclic molecules by sequential C-H activation reactions.

Full text of DOI:10.1002/anie.201508912

.
Angewandte
Communications
International Edition: DOI: 10.1002/anie.201508912
German Edition: DOI: 10.1002/ange.201508912
Homogeneous Catalysis
À
Ligand-Enabled Catalytic C H Arylation of Aliphatic Amines by
a Four-Membered-Ring Cyclopalladation Pathway
Chuan He and Matthew J. Gaunt*
Abstract: A palladium-catalyzed CÀH arylation of aliphatic
amines with arylboronic esters is described, proceeding
through a four-membered-ring cyclopalladation pathway.
Crucial to the successful outcome of this reaction is the
action of an amino-acid-derived ligand. A range of hindered
secondary amines and arylboronic esters are compatible with
this process and the products of the arylation can be advanced
to complex polycyclic molecules by sequential CÀH activation
reactions.
T
he development of new catalytic methods which enable the
functionalization of aliphatic CÀH bonds is an important
[
1,2]
challenge to the continued advance of chemical synthesis.
Central to many of the developments in this area is the
facilitating role of polar functional groups which steer the CÀ
[3]
H bond cleavage by a process called cyclometallation.
Among many elegant developments, palladium-catalyzed
aliphatic CÀH activation directed by synthetically versatile
[
4]
functionalities such as carboxylic acid derivatives, hydroxy
[5]
[6]
motifs, and oximes have been the focus of most attention,
and have delivered numerous new transformations through
a variety of activation modes. In contrast, the use of the amine
functionality as a directing group for cyclopalladation is less
[3b, 7]
common,
and is surprising given their importance to the
function of biologically relevant molecules, and most suc-
cessful cases require protecting groups or auxiliaries to
modulate the metal-binding properties of the nucleophilic
[
8]
nitrogen motif. As a result, catalytic strategies for the CÀH
activation of aliphatic amines remain underdeveloped.
Recently, we reported that secondary aliphatic amines can
undergo CÀH activation through a distinct four-membered-
Scheme 1. Palladium-catalyzed CÀH activation of aliphatic amines.
[9]
ring cyclopalladation pathway (Scheme 1a). Key to the
could expand the toolbox of direct functionalization reactions
[4a,10]
success of this unusual CÀH activation is the steric hindrance
to an arylation process.
In addition, the novel products
around the secondary amine motif. We believe these inter-
actions promote the formation of the putative monoamine/
would represent highly substituted variants of the biologically
[11]
relevant phenthylamine scaffold.
II
Pd complex as a result of destabilizing the usually more
Herein, we report the development of a palladium-
favorable bis(amine) palladium complex (Scheme 1b). This
novel activation process enabled the development of catalytic
CÀH carbonylation and CÀH amination processes to strained
nitrogen heterocycles and CÀH acetoxylation to amino
catalyzed CÀH arylation of secondary aliphatic amines with
arylboronic esters (Scheme 1c). Crucial to the successful
outcome of this reaction is an amino-acid ligand for the
[12]
palladium catalyst,
and it also underpins preliminary
alcohol derivatives (Scheme 1a). As part of the evolution of
studies towards enantioselective reaction. Furthermore, the
this distinct CÀH activation mode we questioned whether we
arylated products provide a platform for further catalytic CÀ
H activation reactions to readily generate previously unex-
plored complex amines, which could be attractive to practi-
tioners of medicinal chemistry.
[
*] Dr. C. He, Prof. M. J. Gaunt
Department of Chemistry, University of Cambridge
Lensfield Road, Cambridge, CB2 1EW (UK)
E-mail: mjg32@cam.ac.uk
At the outset of our studies, stoichiometric reactions
between a preformed hindered amine cyclopalladation com-
plex (1) and various organoboron reagents were investigated
using t-amyl alcohol as a solvent (Scheme 2). We found that
the desired reaction, to give 3a, was observed with both
Homepage: http://www-gaunt.ch.cam.ac.uk
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201508912.
1
5840
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2015, 54, 15840 –15844

Angewandte
Chemie
[a]
Table 2: Scope of amine phenylation.
Scheme 2. Initial screening for arylation. PhBPin=phenylboronic acid
pinacol ester.
3
a, 61% (11%)
3b, 63% (8%)
3c, 68%
3d, 50%
gram scale
PhB(OH)2 (2a) and PhBPin (2b) in the presence of
benzoquinone and sodium hydrogen carbonate. Even
though the yields were low, we were encouraged to find that
the arylation process was viable.
Initial catalytic conditions were examined with 10 mol%
of Pd(OAc) and Ag CO as the oxidant and were based on
[b]
3
e, trace
3 f, 42%
3g, 54%
3h, 45% (8%)
2
2
3
reaction conditions originally reported by Yu et al.
[a] Yields are of isolated products. Yields in brackets are those of the
diarylation products. [b] 1.36:1 mixture of regioisomers observed. The
other isomer results from arylation on the methyl group closest to the
[13]
(
Table 1). Despite initial failure using 2a, we did find that
CH OBn. See the Supporting Information for details.
2
Table 1: Selected optimization for CÀH arylation.
noteworthy that the gram-scale reaction starting from
1
0 mmol 2b could produce 1.73 grams of the monoarylation
product 3b. Surprisingly, almost no reaction was seen with the
morpholinone derivative (to form 3e), despite our previous
success with this type of amine, and acyclic and less-hindered
[14]
[a]
Entry
1a
2
5 (20 mol%)
Yield [%]
amines were unreactive under these reaction conditions.
(
equiv)
(equiv)
3a
4a
However, we were pleased to find that a functionalized
piperidine derivative (3 f), a seven-membered-ring azepine
1
2
3
4
5
6
7
8
1.0
1.0
1.0
1.0
1.0
2.0
2.0
2.0
2.0
2a (1.5)
2b (1.5)
2b (1.5)
2b (1.5)
2b (1.5)
2b (1.0)
2b (1.0)
2b (1.0)
2b (1.0)
none
none
0
3
0
0
(3g), and a morpholine scaffold (3h), all productively form
Ac-Gly-OH (5a)
Ac-Val-OH (5b)
Ac-Leu-OH (5c)
Ac-Gly-OH (5a)
Ac-Val-OH (5b)
Ac-Leu-OH (5c)
Ac-Phe-OH (5d)
33
34
35
50
56
53
61
19
21
19
7
10
10
11
the corresponding arylated products, and constitute hindered
and previously unexplored variants of the pharmaceutically
relevant phenethyl amine motif.
The reaction was readily extended to a variety of
arylboronic acid pinacol esters (ArBPins) in good yields
(Table 3). ArBPins with substituents at the meta (3k,p) and
para (3i,j, 3l–o, 3t–w) positions of the aromatic ring afforded
the desired products in good yields, while the ortho substitu-
ents lowered the reactivity (3q), presumably because of
a deleterious steric effect. Both electron-withdrawing and
electron-donating groups on the aromatic ring of ArBPins
were well tolerated, and ArBPins displaying halogen sub-
stituents could also be introduced to give the desired arylation
products (3l–n). Extended aromatic groups (3r–t) as well as
more functionally complex ArBPins could also be trans-
formed into the amine scaffold (3u,v). Unfortunately, heter-
oarylboronates that contained pyridine- or thiophene-type
motifs were unreactive under these reaction conditions.
To increase the complexity of the arylamine products
generated from this reaction we sought to exploit the pathway
through which the diarylation side product (4) was formed.
We questioned whether the previously deleterious second
cyclopalladation pathway could be harnessed to enable
a subsequent CÀH activation event with a different coupling
partner. Towards this end, we found that CÀH carbon-
[
b]
9
1
[
a] Yield determined by GC or H NMR analysis using triphenylmethane
as an internal standard. [b] Reaction at 808C. BQ=benzoquinone.
the reaction of 2b in combination with amino-acid deriva-
tives, recently introduced by Yu and co-workers as enabling
[12]
ligands for CÀH activation with palladium complexes,
showed a dramatic improvement in conversion into the
desired product 3a (entries 3–5). In addition, we also
observed significant quantities of the corresponding diary-
lation product 4a, where the second arylation takes place on
2
the ortho sp -carbon atom of the new phenyl group. However,
by changing the stoichiometry of the reaction and an
extensive screen of reaction parameters (see the Supporting
Information for details), optimal reaction conditions were
found to involve the use of N-acetyl phenylalanine as a ligand
to produce arylated amine 3a in 61% yield (entries 6–9).
In assessing the scope of the catalytic reaction, we found
that a modest range of readily available amine derivatives
were suitable substrates for the CÀH arylation (Table 2).
[
15]
[16]
[17]
ylation,
CÀH alkenylation,
and CÀH alkynylation
Besides the standard piperidine derivatives (3a,b), methyl-
enepiperidine (3c), and fluorinated piperidine (3d) deriva-
tives could also be readily tolerated to yield 50% to 68%. It is
reactions of 3b successfully led to functionalization on the
ortho-position of the newly installed phenyl group, thus
forming
a
range of architecturally complex scaffolds
Angew. Chem. Int. Ed. 2015, 54, 15840 –15844
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 15841

.
Angewandte
Communications
[
a]
Table 3: Scope of aryl transfer.
3
i, 58%
l, 61%
o, 60%
r, 72%
3j, 52%
3m, 55%
3p, 67%
3s, 52%
3k, 50%
3n, 57%
3q, 28%
3t, 63%
3
Scheme 3. Sequential CÀH activation. Reagents and conditions:
a) 10 mol% Pd(OAc) , CO (1 atm), AgOAc, PhMe, 1108C, 48 h;
2
b) 10 mol% Pd(OAc) , ethyl acrylate, Ag CO , PivOH, DCE, 1208C, air,
2
2
3
4
8 h; c) 10 mol% Pd(OAc) , (bromoethynyl)triisopropulsilane,
2
3
biphenyl-2-carboxylic acid, KHCO , DCE, 1008C, N , 24 h. DCE=1,2-
3
2
dichloroethane.
[a]
Table 4: Towards enantioselective CÀH arylation.
3
3
u, 52%
3v, 68%
3w, 65%
[
a] Yields are those of the isolated products. Boc=tert-butoxycarbonyl,
TIPS=triisopropylsilyl.
5b, 70%
4% ee
5c, 63%
20% ee
5e, 64%
20% ee
5 f, 64%
35% ee
2
(
Scheme 3; 6–8). Moreover, we could extend this sequential
strategy further by performing either CÀH carbonylation or
CÀH alkenylation on the amine 8 to form the polycyclic
amine products 9 and 10. Taken together, these sequential CÀ
H activation processes provide direct access to a structural
and functionally diverse series of complex amine products
prepared in only three steps from a readily available amine
5d, 70%
40% ee
5g, 70%
48% ee
5h, 40%
60% ee
[
a] See the Supporting Information for details. Yield determined by
H NMR analysis using triphenylmethane as an internal standard.
1
[
18]
starting material.
group resulted in a modest improvement to 48% ee (5g).
Further improvement was gained by changing the amino acid
to a b-phenyl-phenylalanine combined with the bulky acyl
side chain, and thus a 60% ee was obtained (5h). At this
stage, we need further mechanistic evidence to help us
rationally design new ligands for this enantioselective trans-
formation, but the results presented herein represent a rare
example of catalytic enantioselective arylation and provide an
Finally, we investigated the potential for an asymmetric
CÀH arylation on the basis that Yu et al. have provided
numerous examples wherein the amino-acid-derived ligands
[19]
can provide asymmetric induction. We screened a range of
amino-acid ligands under similar reaction conditions (at 60
rather than 808C) and a selection of the ligands assessed is
shown in Table 4 (see the Supporting Information for more
details). We found that the l-phenylalanine-derived ligand 5d
gave the best yield and a maximum enantiomeric excess of
[
20]
exciting starting point for further development.
In summary, we have developed a new palladium-
4
0%. Interestingly, only N-acyl substituents displayed any
catalyzed CÀH arylation of aliphatic amines. Central to the
reactivity, and we found that increasing the bulk of the N-acyl
success of this reaction is the presence of amino-acid-derived
1
5842 www.angewandte.org
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2015, 54, 15840 –15844

Angewandte
Chemie
ligands. The process works for a variety of hindered amines
and arylboronic esters to generate novel cyclic arylated
amines. We believe that this palladium-catalyzed CÀH
K. M. Engle, J.-Q. Yu, Isr. J. Chem. 2010, 50, 605 – 616; d) O.
Baudoin, Chem. Soc. Rev. 2011, 40, 4902 – 4911.
[
[
3] a) A. E. Shilov, G. B. Shulꢀpin, Chem. Rev. 1997, 97, 2879 – 2932;
b) A. D. Ryabov, Chem. Rev. 1990, 90, 403 – 424; c) J. Dupont,
C. S. Consorti, J. Spencer, Chem. Rev. 2005, 105, 2527 – 2571.
4] a) R. Giri, N. Maugel, J.-J. Li, D.-H. Wang, S. P. Breazzano, L. B.
Saunders, J.-Q. Yu, J. Am. Chem. Soc. 2007, 129, 3510 – 3511;
b) H. A. Chiong, Q.-N. Pham, O. Daugulis, J. Am. Chem. Soc.
2007, 129, 9879 – 9884.
[5] a) Y. Lu, D.-H. Wang, K. M. Engle, J.-Q. Yu, J. Am. Chem. Soc.
2010, 132, 5916 – 5921; b) X. Wang, Y. Lu, H.-X. Dai, J.-Q. Yu, J.
Am. Chem. Soc. 2010, 132, 12203 – 12205; c) E. M. Simmons, J. F.
Hartwig, Nature 2012, 483, 70 – 73.
functionalization proceeds through a four-membered-ring
cyclometallation pathway and we have demonstrated the
basis of a catalytic enantioselective reaction. Furthermore,
3
linking the C(sp )ÀH arylation and iterative divergent C-
2
(
sp )ÀH functionalization reactions provides a facile and
straightforward tactic for the rapid construction of complex
molecules.
[
6] a) L. V. Desai, K. L. Hull, M. S. Sanford, J. Am. Chem. Soc. 2004,
Experimental Section
1
2
26, 9542 – 9543; b) Z. Ren, F. Mo, G. Dong, J. Am. Chem. Soc.
012, 134, 16991 – 16994.
General procedure for the catalytic CÀH arylation: In a 10 mL vial
equipped with stir bar, arylboronic acid pinacol ester (0.2 mmol),
[
7] a) A. D. Ryabov, I. K. Sakodinskaya, A. K. Yatsimirsky, J.
Chem. Soc. Dalton Trans. 1985, 2629 – 2638; b) K. Orito, A.
Horibata, T. Nakamura, H. Ushito, H. Nagasaki, M. Yuguchi, S.
Yamashita, M. Tokuda, J. Am. Chem. Soc. 2004, 126, 14342 –
Pd(OAc) (4.5 mg, 0.02 mmol), Ac-(L)-Phe-OH (8.3 mg, 0.04 mmol),
2
NaHCO (67.3 mg, 0.8 mmol), Ag CO (110.3 mg, 0.4 mmol), and 1,4-
3
2
3
benzoquinone (10.8 mg, 0.1 mmol) were combined, followed by the
addition of t-amyl-OH (1 mL) and amine substrates (0.4 mmol). Then
the vial was sealed under air with a screw cap and a Teflon septum,
placed in a preheated oil bath at the described temperature, and
stirred for the stated time. The reaction mixture was cooled to room
temperature and filtered through celite by eluting with ethyl acetate.
The filtrates were further filtered through SCX-2 by eluting with
methanol and then an ammonia methanol solution (1m). The
ammonia methanol solution filtrate was concentrated in vacuo to
recover the amine compounds from the crude reaction mixture. The
mixture of amines was then purified by flash column chromatography
1
4343; c) J. Albert, X. Ariza, T. Calvet, M. Font-Bardia, J.
Garcia, J. Granell, A. Lamela, B. Lopez, M. Martinez, L. Ortega,
A. Rodriguez, D. Santos, Organometallics 2013, 32, 649 – 659.
8] a) O. Daugulis, H.-Q. Do, D. Shabashov, Acc. Chem. Res. 2009,
[
4
2, 1074 – 1086; b) G. Rouquet, N. Chatani, Angew. Chem. Int.
Ed. 2013, 52, 11726 – 11743; Angew. Chem. 2013, 125, 11942 –
1959; c) G. He, G. Chen, Angew. Chem. Int. Ed. 2011, 50, 5192 –
1
5196; Angew. Chem. 2011, 123, 5298 – 5302; d) J.-J. Li, T.-S. Mei,
J.-Q. Yu, Angew. Chem. Int. Ed. 2008, 47, 6452 – 6455; Angew.
Chem. 2008, 120, 6552 – 6555; e) X. Ye, Z. He, T. Ahmed, K.
Weise, N. G. Akhmedov, J. L. Petersen, X. Shi, Chem. Sci. 2013,
(
Et N washed silica gel column) under the stated conditions to
3
provide the pure arylation product.
4
, 3712 – 3716; f) C. Wang, C. Chen, J. Zhang, J. Han, Q. Wang,
K. Guo, P. Liu, M. Guan, Y. Yao, Y. Zhao, Angew. Chem. Int. Ed.
014, 53, 9884 – 9888; Angew. Chem. 2014, 126, 10042 – 10046;
2
Acknowledgements
g) N. Rodríguez, J. A. Romero-Revilla, M. . Fernµndez-
IbµÇez, J. C. Carretero, Chem. Sci. 2013, 4, 175 – 179.
9] a) A. McNally, B. Haffemayer, B. S. L. Collins, M. J. Gaunt,
Nature 2014, 510, 129 – 133; b) A. P. Smalley, M. J. Gaunt, J. Am.
Chem. Soc. 2015, 137, 10632 – 10641.
We are grateful to the EPSRC (C.H. & M.J.G.), ERC (M.J.G.)
and the Marie Curie Foundation (C.H.) for funding. Mass
spectrometry data were acquired at the EPSRC UK National
Mass Spectrometry Facility at Swansea University.
[
[10] a) A. Lazareva, O. Daugulis, Org. Lett. 2006, 8, 5211 – 5213;
b) V. G. Zaitsev, D. Shabashov, O. Daugulis, J. Am. Chem. Soc.
Keywords: amines · amino acids · CÀH activation ·
2005, 127, 13154 – 13155; c) D.-H. Wang, M. Wasa, R. Giri, J.-Q.
Yu, J. Am. Chem. Soc. 2008, 130, 7190 – 7191.
11] “Phenethylamines (2-Phenylethylamines)”: F. Zaragoza Dçr-
wald in Lead Optimization for Medicinal Chemists: Pharmaco-
kinetic Properties of Functional Groups and Organic Com-
pounds, Wiley-VCH, Weinheim, 2012.
homogeneous catalysis · palladium
[
[
How to cite: Angew. Chem. Int. Ed. 2015, 54, 15840–15844
Angew. Chem. 2015, 127, 16066–16070
12] a) K. M. Engle, D.-H. Wang, J.-Q. Yu, J. Am. Chem. Soc. 2010,
[
1] a) C. Jia, T. Kitamura, Y. Fujiwara, Acc. Chem. Res. 2001, 34,
33 – 639; b) K. Godula, D. Sames, Science 2006, 312, 67 – 72;
1
32, 14137 – 14151; b) K. M. Engle, P. S. Thuy-Boun, M. Dang, J.-
6
Q. Yu, J. Am. Chem. Soc. 2011, 133, 18183 – 18193.
13] K. S. L. Chan, M. Wasa, L. Chu, B. N. Laforteza, M. Miura, J.-Q.
Yu, Nat. Chem. 2014, 6, 146 – 150.
c) H. M. L. Davies, J. R. Manning, Nature 2008, 451, 417 – 424;
d) H. M. L. Davies, J. Du Bois, J.-Q. Yu, Chem. Soc. Rev. 2011,
[
[
4
0, 1855 – 1856; e) J. Yamaguchi, A. D. Yamaguchi, K. Itami,
Angew. Chem. Int. Ed. 2012, 51, 8960 – 9009; Angew. Chem.
012, 124, 9092 – 9142; f) I. A. I. Mkhalid, J. H. Barnard, T. B.
Marder, J. M. Murphy, J. F. Hartwig, Chem. Rev. 2010, 110, 890 –
14] Acyclic and less-hindered amines did not undergo arylation.
2
9
3
31; g) J. Wencel-Delord, F. Glorius, Nat. Chem. 2013, 5, 369 –
75; h) X. Chen, K. M. Engle, D.-H. Wang, J.-Q. Yu, Angew.
Chem. Int. Ed. 2009, 48, 5094 – 5115; Angew. Chem. 2009, 121,
[
[
[
[
15] B. Haffemayer, M. Gulias, M. J. Gaunt, Chem. Sci. 2011, 2, 312 –
5
1
196 – 5217; i) T. W. Lyons, M. S. Sanford, Chem. Rev. 2010, 110,
147 – 1169; j) L. Ackermann, Chem. Rev. 2011, 111, 1315 – 1345;
3
15.
16] Q. Wang, J. Han, C. Wang, J. Zhang, Z. Huang, D. Shi, Y. Zhao,
Chem. Sci. 2014, 5, 4962 – 4967.
17] Y. Zhao, G. He, W. A. Nack, G. Chen, Org. Lett. 2012, 14, 2948 –
k) D. A. Colby, R. G. Bergman, J. A. Ellman, Chem. Rev. 2010,
10, 624 – 655; l) L. McMurray, F. O’Hara, M. J. Gaunt, Chem.
1
Soc. Rev. 2011, 40, 1885 – 1898.
2
951.
[
2] a) R. Jazzar, J. Hitce, A. Renaudat, J. Sofack-Kreutzer, O.
Baudoin, Chem. Eur. J. 2010, 16, 2654 – 2672; b) H. Li, B.-J. Li,
Z.-J. Shi, Catal. Sci. Technol. 2011, 1, 191 – 206; c) M. Wasa,
18] A. Nadin, C. Hattotuwagama, I. Churcher, Angew. Chem. Int.
Ed. 2012, 51, 1114 – 1122; Angew. Chem. 2012, 124, 1140 – 1149.
Angew. Chem. Int. Ed. 2015, 54, 15840 –15844
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 15843

.
Angewandte
Communications
[
19] a) R. Giri, B.-F. Shi, K. M. Engle, N. Maugel, J.-Q. Yu, Chem.
Soc. Rev. 2009, 38, 3242 – 3272; b) K. M. Engle, J.-Q. Yu, J. Org.
Chem. 2013, 78, 8927 – 8955.
20] a) M. Wasa, K. M. Engle, D. W. Lin, E. J. Yoo, J.-Q. Yu, J. Am.
Chem. Soc. 2011, 133, 19598 – 19601; b) K. S. L. Chan, H.-Y. Fu,
J.-Q. Yu, J. Am. Chem. Soc. 2015, 137, 2042 – 2046; c) D. Katayev,
M. Nakanishi, T. Buergi, E. P. Kuendig, Chem. Sci. 2012, 3,
Chem. Soc. 2014, 136, 4841 – 4844; g) L. Liu, A.-A. Zhang, R.-J.
Zhao, F. Li, T.-J. Meng, N. Ishida, M. Murakami, W.-X. Zhao,
Org. Lett. 2014, 16, 5336 – 5338; h) J. Kim, M. Sim, N. Kim, S.
Hong, Chem. Sci. 2015, 6, 3611 – 3616; i) Z.-Q. Lin, W.-Z. Wang,
S.-B. Yan, W.-L. Duan, Angew. Chem. Int. Ed. 2015, 54, 6265 –
6269; Angew. Chem. 2015, 127, 6363 – 6367.
[
1
5
422 – 1425; d) T. Saget, N. Cramer, Angew. Chem. Int. Ed. 2012,
1, 12842 – 12845; Angew. Chem. 2012, 124, 13014 – 13017; e) R.
Shintani, H. Otomo, K. Ota, T. Hayashi, J. Am. Chem. Soc. 2012,
34, 7305 – 7308; f) D.-W. Gao, Q. Yin, Q. Gu, S.-L. You, J. Am.
Received: September 23, 2015
Published online: November 5, 2015
1
1
5844 www.angewandte.org
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2015, 54, 15840 –15844