Sunlight-Driven Decarboxylative Alkynylation of α-Keto Acids with Bromoacetylenes by Hypervalent Iodine Reagent Catalysis: A Facile Approach to Ynones

Article abstract of DOI:10.1002/anie.201503479

A novel and practical decarboxylative alkynylation of α-keto acids with bromoacetylenes is catalyzed by hypervalent iodine(III) reagents when irradiation by sunlight at room temperature. The product ynones are generated in good yields. Experiments show that results obtained with blue light (λ=450-455 nm) are comparable to those obtained when using sunlight. Mechanistic studies demonstrate that the sunlight-driven decarboxylation undergoes a radical process.

Full text of DOI:10.1002/anie.201503479

.
Angewandte
Communications
International Edition: DOI: 10.1002/anie.201503479
German Edition: DOI: 10.1002/ange.201503479
Photochemistry
Sunlight-Driven Decarboxylative Alkynylation of a-Keto Acids with
Bromoacetylenes by Hypervalent Iodine Reagent Catalysis: A Facile
Approach to Ynones**
Hui Tan, Hongji Li,* Wangqin Ji, and Lei Wang*
Abstract: A novel and practical decarboxylative alkynylation
of a-keto acids with bromoacetylenes is catalyzed by hyper-
valent iodine(III) reagents when irradiation by sunlight at
room temperature. The product ynones are generated in good
yields. Experiments show that results obtained with blue light
(l = 450–455 nm) are comparable to those obtained when
using sunlight. Mechanistic studies demonstrate that the sun-
light-driven decarboxylation undergoes a radical process.
aerobic oxidation (Scheme 1, Path B).[3g] Very recently, Li
and co-workers reported an iridium- and rhodium-catalyzed
À
chelation-assisted formyl C H alkynylation (Scheme 1,
Path B).[3h] Despite the successful synthesis of ynones, current
methods always suffer from significant limitations, including
the requirement of high temperature, excessive additives, and
toxic metals.[3] Therefore, developing a mild and green
method for the synthesis of ynones is highly desirable.
A photocatalysis strategy may provide a better alternative
in organic synthesis because it avoids additional ligands,
bases, and elevated temperatures.[4] Recent studies in photo-
T
he prevalence of carbonyl compounds as structural motifs[1]
in natural products[1c–e] and pharmaceutical compounds[1f] has
been recognized in organic synthesis. Especially ynones,
which are often used as building blocks for the construction
of target molecules with unique properties.[2] In the literature,
ynones are used as attractive precursors to a number of
heterocycles, such as furans,[2e] pyrazoles,[2f,g] flavones,[2h] and
others.[2i–l] Because of these facts, a variety of strategies have
been developed for the synthesis of ynones.[3] For examples,
Muller and co-workers employed a Pd/Cu bimetallic catalyst
to realize the one-pot coupling of acyl chlorides with terminal
alkynes, and this method was also extended to ynediones
(Scheme 1, Path A).[3f] More recently, Huang and co-workers
developed an alternative approach to ynones from aldehydes
and hypervalent alkynyl iodides through gold-catalyzed
À
chemistry have reported the formation of C C and C–
heteroatom bonds by using visible-light and either ruthenium
or iridium complexes, or organic dyes as photoredox cata-
lysts.[5] It is well known that among renewable energy sources,
sunlight is the largest energy source.[6] Using sunlight would
solve the energy crisis and the drawbacks of traditional
photochemical reactions which require UV-light.[7] Recently,
a-keto acids were used as acylating reagents, through
a decarboxylative process to form an acyl free radical along
with extrusion of CO2, in ketone synthesis owing to their high
reactivity.[8] Meanwhile, the application of hypervalent iodine
reagents (HIRs) has been been used in organic synthesis,[9]
and HIRs were effective reagents for organic transformations
through visible-light photoredox catalysis in the presence of
[Ru(bpy)3](PF6)2.[9c,d] Based on this understanding and our
work,[8d,e, 10] we herein report a novel sunlight-driven decarb-
oxylative alkynylation of a-keto acids with bromoacety-
lenes[11] using an HIR catalyst under photolysis but in the
absence of a visible-light photoredox catalyst, thus represent-
ing an energy-efficient approach to ynones (Scheme 1,
Path C).
Our initial investigation focused on the model reaction of
2-oxo-2-phenylacetic acid (1a) with (bromoethynyl)benzene
(2a) at room temperature (Table 1). Inspired by the common
photoredox catalysts,[4,5] the model reaction was performed
with [Ru(bpy)3]Cl2·6H2O (1.0 mol%) as photocatalyst and
a catalytic amount of an HIR as an additive under irradiation
from a fluorescent bulb (18 W) for 8 hours in toluene.
Fortunately, addition of PhI(OAc)2 (30 mol%) gave the
desired product 3a in 12% yield (entry 1). Subsequently,
a series of HIRs, such as BI-OAc, BI-OTf, BI-OH, and BI-
alkyne, were examined, and an improved yield of 3a was
observed in the presence of BI-OH (entries 2–5). Eosin Yand
Na2-Eosin Y gave lower yields of 3a (entries 6 and 7).
Furthermore, 40% yield of 3a was achieved when this
transformation was performed without the photoredox cata-
lyst (entry 8). To our delight, the reaction exposed to sunlight
irradiation underwent the decarboxylative coupling to give 3a
Scheme 1. Synthetic strategies for ynones.
[*] H. Tan, Dr. H. Li, W. Ji, Prof. Dr. L. Wang
Department of Chemistry, Huaibei Normal University
Huaibei, Anhui 235000 (PR China)
E-mail: leiwang@chnu.edu.cn
Prof. Dr. L. Wang
State Key Laboratory of Organometallic Chemistry, Shanghai
Institute of Organic Chemistry, Chinese Academy of Sciences
Shanghai 200032 (PR China)
[**] Financial support was provided by National Natural Science
Foundation of China (21372095, 21402060).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201503479.
8374
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2015, 54, 8374 –8377

Angewandte
Chemie
Table 1: Optimization of the reaction conditions.[a]
Entry Light source
Photocatalyst
HIR
Yield [%][b]
1
2
3
4
5
6
7
8
light[c]
[Ru(bpy)3]Cl2·6H2O PhI(OAc)2 12
light[c]
[Ru(bpy)3]Cl2·6H2O BI-OAc
[Ru(bpy)3]Cl2·6H2O BI-OTf
[Ru(bpy)3]Cl2·6H2O BI-OH
[Ru(bpy)3]Cl2·6H2O BI-Alkyne
17
8
30
16
light[c]
light[c]
light[c]
light[c]
Eosin Y
Na2-Eosin Y
BI-OH
BI-OH
BI-OH
BI-OH
BI-OH
BI-OH
BI-OH
BI-OH
BI-OH
BI-OH
BI-OH
–
23
25
40
66
trace
21
63
38
15
light[c]
light[c]
–
–
–
–
–
–
–
–
–
–
–
–
–
9
sunlight
red LED
green LED
blue LED
purple LED
UV (226 nm)
sunlight
sunlight
sunlight
–
10
11
12
13
14
15
16
17
18
19
20
49[d]
67[e]
n.r.
n.r.
n.r.[f]
19[g]
BI-OH
BI-OH
BI-OH
–
–
[a] Reaction conditions: 2-oxo-2-phenylacetic acid (1a, 0.60 mmol),
(bromoethynyl)benzene (2a, 0.20 mmol), photocatalyst (1.0 mol%),
HIR (30 mol%), PhMe (1.0 mL) at room temperature in air for 8 h.
[b] Yield of isolated product. [c] Fluorescent bulb (18 W). [d] BI-OH
(20 mol%). [e] BI-OH (40 mol%). [f] 608C. [g] 908C. bpy=2,2’-bipyr-
idine, n.r.=no reaction.
Scheme 2. The scope of a-keto acids and bromoacetylenes. Reaction
conditions: 1 (0.60 mmol), 2 (0.20 mmol), BI-OH (30 mol%), PhMe
(1.0 mL), sunlight irradiation at room temperature for 8 h. [a] Yield of
isolated product. [b] 2 equiv of PhI(OAc)2.
in 66% yield (entry 9). Further screening of light sources
demonstrated that blue LED (450–455 nm) plays an impor-
tant role in this alkynylation, thus providing 3a in a yield
comparable to that obtained under sunlight irradiation
(entries 10–14). Changing the amount of BI-OH up to
40 mol% did not enhance the transformation (entries 15
and 16). The addition of BI-OH is essential in the reaction
(entry 17). Specifically, no 3a was formed when the reaction
was performed at either room temperature or 608C, and 19%
yield of 3a was obtained at 908C in the dark (entries 18–20). It
was found that decomposition of BI-OH produces 2-iodo-
benzoic acid and accounted for 30 mol% of BI-OH (a higher
catalytic amount) used in this reaction.[12] In addition, solvent
screening indicated that toluene is the best reaction medium
(see Table S1 in the Supporting Information).
To our knowledge, this sunlight-driven reaction is novel
and represents a successful example of photochemistry. The
optimized reaction conditions were applied to the decarbox-
ylative alkynylation of a variety of a-keto acids (1) with
bromoacetylenes (2), as described in Scheme 2. Firstly,
bromoacetylenes with F, Cl, Br, MeCO, Me, nPr, nBu, and
MeO groups, at para-positions of the benzene rings in 2-aryl-
1-bromoethynes, reacted with 1a to afford the products 3b–
i in 62–80% yields. The results indicated that bromoacety-
lenes with attached electron-withdrawing groups on the
benzene rings seemed to accelerate the transformation, and
bromoacetylenes with electron-donating groups gave the
inferior yields (3b–e veresus 3 f–i). In contrast, 2-aryl-1-
bromoethynes bearing Br and Me at the ortho-positions, and
F and Me at the meta-positions of the benzene rings
demonstrated comparable reactivity in the reaction, with
the formation of the corresponding ynones in good yields (3j–
m). 4-Methyl-2-oxopentanoic acid (1b) reacted with a series
of ortho-, meta-, and para-substituted 2-aryl-1-bromoethynes,
and the desired products 3n–u were obtained in 60–74%
yields. In contrast, a variety of 2-oxo-2-arylacetic acids were
used to extend the substrate scope. When 2-oxo-2-arylacetic
acids, with ortho-substitutes, such as F, Cl, Br, Me, and MeO
on the benzene rings, reacted with (bromoethynyl)benzene
(2a), the corresponding ynones (3v–z) were isolated in
acceptable yields (of the the ortho-position effect is ignored).
When the reactions of 2-oxo-2-(meta- or para-substituted)-
phenylacetic acids with 2a were performed under the
Angew. Chem. Int. Ed. 2015, 54, 8374 –8377
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
8375

.
Angewandte
Communications
optimized reaction conditions, there was no noticeable
influence on the formation of ynones (3aa–ah). It should be
noted that dimethyl-substituted 2-oxo-2-arylacetic acids in
the reaction with 2a generated the anticipated ynones (3ai
and 3aj) in 57 and 61% yields. The introduction of two strong
electron-donating groups (RO) on the aromatic ring of 2-oxo-
2-arylacetic acid obviously hindered the decarboxylative
process. For example, 2-(3,4-dimethoxyphenyl)-2-oxoacetic
acid and 2-(benzo[d][1,3]dioxol-5-yl)-2-oxoacetic acid reacted
with 2a to provide 3ak (15%) and 3al (13%), respectively.
Fortunately, replacing BI-OH with a stoichiometric amount
of PhI(OAc)2 gave moderate yields of 3ak and 3al. It is
important to note that the reaction of aliphatic bromoacety-
lene (1-bromohept-1-yne) with 2-(4-methoxyphenyl)-2-oxo-
acetic acid produced 3an in 44% yield.
We next attempted to explore the more challenging
alkynylation using a terminal alkyne, instead of bromoacety-
lene, with an a-keto acid. When the reaction of phenyl-
acetylene with 1a was performed in PhMe with BI-OH as
additive and irradiated by sunlight at room temperature for
8 hours, 3a was isolated in 23% yield. To improve the yield of
3a, the formation of (bromoethynyl)benzene in situ was
considered to accelerate this process. When NBS, AgNO3,
and phenylacetylene were added into the reaction, 3a was
obtained in 36% yield (see the Supporting Information for
full details).
To understand this decarboxylative process, we conducted
intermediate-trapping experiments as shown in Scheme 3. In
the presence of BI-OH and sunlight irradiation, the benzoyl
radical derived from 1a was in situ trapped by TEMPO
(2,2,6,6-tetramethyl-1-piperidinyloxy), thus affording 4 in
58% yield (Scheme 3a).[13] Next, an equivalent of BI-
Alkyne was directly used for the alkynylation of 1a, and
only trace amounts of 3a were observed. Interestingly, when
employing 30 mol% BI-OH as the catalyst 3a was formed in
66% yield (Scheme 3b). The prepared hypervalent iodine
reagent 5 was used to react with 2a, thus affording 3a in 63%
yield (Scheme 3c). These results imply that BI-Alkyne and 5
may be important intermediates involved in the reaction.
Moreover, BHT (2,6-di-tert-butyl-4-methylphenol)[14] could
trap both the iodanyl radical and benzoyl radical to generate 6
and 7 (Scheme 3d).
Scheme 3. Preliminary mechanistic study and proposed mechanism.
example of sunlight-driven radical alkynylation has an
advantage over the reported methods. Our ongoing studies
are focused on gaining detailed insights into the reaction
mechanism and applying this strategy to other organic
transformations.
Based on the preliminary study, a possible mechanism for
the reaction is proposed in Scheme 3. Firstly, BI-OH reacts
with 2-oxo-2-phenylacetic acid (1a) to form the intermediate
5,[15] and is initiated by sunlight irradiation to generate the Experimental Section
iodanyl radical A[9e, 16] and acyl radical B. Next, A reacts with
2a to give BI-Alkyne and a Br radical. Then an addition[9b–d]
of the benzoyl radical, from decarboxylation of B (FT-IR
analysis of the resulting CO2 gas; see the Supporting
Information),[10] to BI-Alkyne occurs to form C, followed by
releasing 3a and A. Additionally, a radical coupling reaction
between Br radical and A produces D, which finally under-
goes hydrolysis to generate BI-OH.
General procedure for sunlight-driven decarboxylative alkynylation:
A 5 mL oven-dried reaction vessel equipped with a magnetic stirrer
bar was charged with a-oxocarboxylic acid (1; 0.60 mmol), alkynyl
bromide (2; 0.20 mmol), BI-OH (0.06 mmol), and toluene (1.0 mL) in
air. The reaction vessel was exposed to sunlight at room temperature
for 8 h. After completion of the reaction, the reaction mixture was
concentrated to yield the crude mixture, which was further purified by
flash chromatography (silica gel, petroleum ether/ethyl acetate = 15:1
to 30:1) to give the desired product 3.
In conclusion, a sunlight-driven radical alkynylation of a-
keto acids with bromoacetylenes catalyzed by BI-OH has
been developed. This alkynylation tolerates a series of
substituted groups and affords ynones in good yields.
Considering the precedent in ynone synthesis, the rare
Keywords: acylation · alkynes · hypervalent compounds ·
photochemistry · radicals
8376
www.angewandte.org
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2015, 54, 8374 –8377

Angewandte
Chemie
How to cite: Angew. Chem. Int. Ed. 2015, 54, 8374–8377
Angew. Chem. 2015, 127, 8494–8497
Am. Chem. Soc. 2014, 136, 5257 – 5260; c) R. Tomita, Y. Yasu, T.
Koike, M. Akita, Angew. Chem. Int. Ed. 2014, 53, 7144 – 7148;
Angew. Chem. 2014, 126, 7272 – 7276; d) D. C. Fabry, J. Zoller, S.
Raja, M. Rueping, Angew. Chem. Int. Ed. 2014, 53, 10228 –
10231; Angew. Chem. 2014, 126, 10392 – 10396; e) A. Noble,
S. J. McCarver, D. W. C. MacMillan, J. Am. Chem. Soc. 2015,
137, 624 – 627.
[1] a) J.-C. Wang, S.-S. Ng, M. J. Krische, J. Am. Chem. Soc. 2003,
125, 3682 – 3683; b) B. M. Trost, Z. T. Ball, J. Am. Chem. Soc.
2004, 126, 13942 – 13944; c) K. C. Nicolaou, D. Sarlah, D. M.
Shaw, Angew. Chem. Int. Ed. 2007, 46, 4708 – 4711; Angew.
Chem. 2007, 119, 4792 – 4795; d) B. G. Vong, S. H. Kim, S.
Abraham, E. A. Theodorakis, Angew. Chem. Int. Ed. 2004, 43,
3947 – 3951; Angew. Chem. 2004, 116, 4037 – 4041; e) C. J. For-
syth, J. Xu, S. T. Nguyen, I. A. Samdal, L. R. Briggs, T.
Rundberget, M. Sandvik, C. O. Miles, J. Am. Chem. Soc. 2006,
128, 15114 – 15116; f) M. Hadjeri, E. L. Peiller, C. Beney, N.
Deka, M. A. Lawson, C. Dumontet, A. Boumendjel, J. Med.
Chem. 2004, 47, 4964 – 4970.
[2] a) D. M. DꢀSouza, T. J. J. Muller, Nat. Protoc. 2008, 3, 1661 –
1665; b) D. A. Rooke, E. M. Ferreira, J. Am. Chem. Soc. 2010,
132, 11926 – 11928; c) B. H. Xu, G. Kehr, R. Frçhlich, B.
Wibbeling, B. Schirmer, S. Grimme, G. Erker, Angew. Chem.
Int. Ed. 2011, 50, 7183 – 7186; Angew. Chem. 2011, 123, 7321 –
7324; d) V. S. Aulakh, M. A. Ciufolini, J. Am. Chem. Soc. 2011,
133, 5900 – 5903; e) C. G. Lee, K. Y. Lee, S. Lee, J. N. Kim,
Tetrahedron 2005, 61, 8705 – 8710; f) J. D. Kirkham, S. J. Edeson,
S. Stokes, J. P. Harrity, Org. Lett. 2012, 14, 5354 – 5357; g) M. S.
Mohamed Ahmed, K. Kobayashi, A. Mori, Org. Lett. 2005, 7,
4487 – 4489; h) E. Awuah, A. Capretta, Org. Lett. 2009, 11,
3210 – 3213; i) A. S. Karpov, E. Merkul, F. Rominger, T. J. J.
Muller, Angew. Chem. Int. Ed. 2005, 44, 6951 – 6956; Angew.
Chem. 2005, 117, 7112 – 7117; j) P. Bannwarth, A. Valleix, D.
GrØe, R. GrØe, J. Org. Chem. 2009, 74, 4646 – 4649; k) Z. She, D.
Niu, L. Chen, M. A. Gunawan, X. Shanja, W. H. Hersh, Y. Chen,
J. Org. Chem. 2012, 77, 3627 – 3633; l) K. Matsumura, S.
Hashigushi, T. Ikarya, R. Noyori, J. Am. Chem. Soc. 1997, 119,
8738 – 8739.
[6] N. S. Lewis, D. G. Nocera, Proc. Natl. Acad. Sci. USA 2006, 103,
15729 – 15735.
[7] a) P. Esser, B. Pohlmann, H.-D. Scharf, Angew. Chem. Int. Ed.
Engl. 1994, 33, 2009 – 2023; Angew. Chem. 1994, 106, 2093 –
2108; b) M. A. Ischay, M. E. Anzovino, J. Du, T. P. Yoon, J.
Am. Chem. Soc. 2008, 130, 12886 – 12887.
[8] a) P. Fang, M.-Z. Li, H.-B. Ge, J. Am. Chem. Soc. 2010, 132,
11898 – 11899; b) M. Kim, J. Park, S. Sharma, A. Kim, E. Park,
J. H. Kwak, Y. H. Jung, I. S. Kim, Chem. Commun. 2013, 49,
925 – 927; c) Z.-Y. Li, D.-D. Li, G.-W. Wang, J. Org. Chem. 2013,
78, 10414 – 10420; d) H. Li, P. Li, L. Wang, Chem. Commun.
2013, 49, 9170 – 9172; e) H. Li, P. Li, Hui, Tan, L. Wang, Chem.
Eur. J. 2013, 19, 14432 – 14436.
[9] For a selected review, see: a) V. V. Zhdankin, P. J. Stang, Chem.
Rev. 2008, 108, 5299 – 5358; For selected examples, see: b) X.
Liu, Z. Wang, X. Cheng, C. Li, J. Am. Chem. Soc. 2012, 134,
14330 – 14333; c) H. Huang, G. Zhang, L. Gong, S. Zhang, Y.
Chen, J. Am. Chem. Soc. 2014, 136, 2280 – 2283; d) H. Huang, K.
Jia, Y. Chen, Angew. Chem. Int. Ed. 2015, 54, 1881 – 1884;
Angew. Chem. 2015, 127, 1901 – 1904; One example with an HIR
catalyst under visible-light photolysis: e) S. A. Moteki, A. Usui,
S. Selvakumar, T. Zhang, K. Maruoka, Angew. Chem. Int. Ed.
2014, 53, 11060 – 11064; Angew. Chem. 2014, 126, 11240 – 11244.
[10] L. Chen, H. Li, F. Yu, L. Wang, Chem. Commun. 2014, 50,
14866 – 14869.
[11] For some selected examples, see: a) W. Wu, H. Jiang, Acc. Chem.
Res. 2014, 47, 2483 – 2504; b) W. Shi, Y. Luo, X. Luo, L. Chao, H.
Zhang, J. Wang, A. Lei, J. Am. Chem. Soc. 2008, 130, 14713 –
14720.
[12] a) M. Ochiai, T. Ito, H. Takahashi, A. Nakanishi, M. Toyonari, T.
Sueda, S. Goto, M. Shiro, J. Am. Chem. Soc. 1996, 118, 7716 –
7730; b) M. Ochiai, A. Nakanishi, T. Ito, J. Org. Chem. 1997, 62,
4253 – 4259.
[3] a) M. S. M. Ahmed, A. Mori, Org. Lett. 2003, 5, 3057 – 3060;
b) H. C. Brown, C. P. Garg, J. Am. Chem. Soc. 1961, 83, 2952 –
2953; c) Y. Maeda, N. Kakiuchi, S. Matsumura, T. Nishimura, T.
Kawamura, S. Uemura, J. Org. Chem. 2002, 67, 6718 – 6724;
d) D. A. Alonso, C. Nµjera, M. C. Pacheco, J. Org. Chem. 2004,
69, 1615 – 1619; e) K. Nozaki, N. Sato, H. Takaya, J. Org. Chem.
1994, 59, 2679 – 2681; f) C. Boersch, E. Merkul, T. J. J. Muller,
Angew. Chem. Int. Ed. 2011, 50, 10448 – 10452; Angew. Chem.
2011, 123, 10632 – 10636; g) Z. Wang, L. Li, Y. Huang, J. Am.
Chem. Soc. 2014, 136, 12233 – 12236; h) H. Wang, F. Xie, Z. Qi,
X. Li, Org. Lett. 2015, 17, 920 – 923.
[13] J. Liu, Q. Liu, H. Yi, C. Qin, R. Bai, X. Qi, Y. Lan, A. Lei, Angew.
Chem. Int. Ed. 2014, 53, 502 – 506; Angew. Chem. 2014, 126, 512 –
516.
[14] K. Sun, X. Wang, G. Li, Z. Zhu, Y. Jiang, B. Xiao, Chem.
Commun. 2014, 50, 12880 – 12883.
[15] O. A. El Seoud, M. Ferreira, W. A. Rodrigues, M. F. Ruasse, J.
Phys. Org. Chem. 2005, 18, 173 – 182.
[16] a) K. Matcha, R. Narayan, A. P. Antonchick, Angew. Chem. Int.
Ed. 2013, 52, 7985 – 7989; Angew. Chem. 2013, 125, 8143 – 8147;
b) H.-Q. Do, R. M. Kashif Khan, O. Daugulis, J. Am. Chem. Soc.
2008, 130, 15185 – 15192.
[4] a) J. M. R. Narayanam, C. R. J. Stephenson, Chem. Soc. Rev.
2011, 40, 102 – 113; b) J. Xuan, W.-J. Xiao, Angew. Chem. Int. Ed.
2012, 51, 6828 – 6838; Angew. Chem. 2012, 124, 6934 – 6944;
c) C. K. Prier, D. A. Rankic, D. W. C. MacMillan, Chem. Rev.
2013, 113, 5322 – 5363; d) D. M. Schultz, T. P. Yoon, Science 2014,
343, 1239176; e) D. Prasad Hari, T. Hering, B. Kçnig, Angew.
Chem. Int. Ed. 2014, 53, 725 – 728; Angew. Chem. 2014, 126, 743 –
747; f) K. Zeitler, Angew. Chem. Int. Ed. 2009, 48, 9785 – 9789;
Angew. Chem. 2009, 121, 9969 – 9974.
[5] a) L. J. Allen, P. J. Cabrera, M. Lee, M. S. Sanford, J. Am. Chem.
Soc. 2014, 136, 5607 – 5610; b) Z. Zuo, D. W. C. MacMillan, J.
Received: April 16, 2015
Published online: June 1, 2015
Angew. Chem. Int. Ed. 2015, 54, 8374 –8377
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
8377