Practical Synthesis of Polysubstituted Haloimidazoles from 1,1-Dibromoalkenes and Amidines

Article abstract of DOI:10.1002/ejoc.201500305

The copper-catalyzed cycloamination reaction of 1,1-dibromoalkenes with amidines affords a diverse set of polysubstituted haloimidazole derivatives. By using this strategy, high regio- and chemoselectivity has been achieved, using 4,7-diphenyl-1,10-phenanthroline as ligand without the addition of expensive catalysts to provide moderate to good yields. The copper-catalyzed cycloamination reaction of 1,1-dibromoalkenes with amidines affords a diverse set of polysubstituted haloimidazole derivatives. Moderate to good yields with high regio- and chemoselectivities were achieved by using 4,7-diphenyl-1,10-phenanthroline as ligand without the addition of expensive catalyst.

Full text of DOI:10.1002/ejoc.201500305

SHORT COMMUNICATION
DOI: 10.1002/ejoc.201500305
Practical Synthesis of Polysubstituted Haloimidazoles from
1,1-Dibromoalkenes and Amidines
Yibiao Li,*[a] Liang Cheng,[a] Yan Shao,[a] Shaohua Jiang,[a] Jialing Cai,[a] and Ning Qing[a]
Keywords: Synthetic methods / Regioselectivity / Chemoselectivity / Copper / Nitrogen heterocycles / Alkenes
The copper-catalyzed cycloamination reaction of 1,1-dibro-
moalkenes with amidines affords a diverse set of polysubsti-
tuted haloimidazole derivatives. By using this strategy, high
regio- and chemoselectivity has been achieved, using 4,7-
diphenyl-1,10-phenanthroline as ligand without the addition
of expensive catalysts to provide moderate to good yields.
Introduction
The development of general and efficient methodologies
for the synthesis of complex molecular skeletons is a central
focus of modern organic chemistry. Polysubstituted imid-
azole derivatives are the core structures of numerous natu-
ral products and pharmaceuticals that possess antimicro-
bial, anti-inflammatory, antimuscarinic, or antitubercular
activities.[1] In particular, haloimidazole derivatives are ver-
satile building blocks because the halogen atom provides an
opportunity for further functionalization through transi-
tion-metal-catalyzed reactions to form a variety of carbon–
carbon and carbon–heteroatom bonds.[2] Consequently, the
development of efficient methods for rapid construction of
halogenated and aryl-substituted imidazoles has stimulated
considerable interest. Among the various types of reactions,
functionalized haloimidazoles can be prepared through ac-
tivation of N-acylated α-aminonitrile followed by cycliza-
tion, chlorination, and tautomerization reaction.[3] Chen
and co-workers reported a simple route for the synthesis
of multisubstituted imidazole derivatives through copper-
catalyzed cycloaddition reaction from amidines and nitro-
olefins (Scheme 1, a).[4] Recently, Neuville and co-workers
reported the first example of a copper-catalyzed synthesis
of 1,2,4-trisubstituted imidazoles by using amidines and ter-
minal alkynes (Scheme 1, b). Although a variety of terminal
alkynes can participate in the reaction, internal alkynes are
unsuitable.[5]
Scheme 1. Copper-catalyzed synthesis of imidazoles from amidines.
alkenes are attractive starting materials in palladium-cata-
lyzed transformations because they are highly reactive and
readily available from inexpensive aldehydes.[7] However,
reports on the direct cycloamination of 1,1-dibromo-
alkenes with amidines to construct haloimidazoles are rare,
most likely because of the low activity the C=C–H bond of
1,1-dibromoalkenes.[8] It was shown that dehydrobromina-
tion of 1,1-dibromoalkenes can provide an efficient method
for preparing bromoalkynes as highly reactive cycload-
dition substrates. Bases that have been employed for the
synthesis of bromoalkynes from 1,1-dibromoalkenes are tet-
rabutylammonium fluoride (TBAF)[9] and tBuOK etc.[10]
We have been involved in the development of palladium-
catalyzed annulation of bromoacrylamides with isocyan-
ides, including sequences involving a direct nucleophilic ad-
dition step.[11] Subsequent to this work, we became inter-
ested in developing an annulation reaction synthesis of ha-
logenated imidazoles from 1,1-dibromoalkenes and amid-
ines (Scheme 1, c).
Although a number of methods are available for the syn-
thesis of this important scaffold,[6] straightforward and
regiodefined routes that can be used to construct haloimid-
azole derivatives from basic chemical materials, such as am-
idine and alkene derivatives, remain valuable. 1,1-Dibromo-
[a] School of Chemical & Environmental Engineering,
Wuyi University,
Jiangmen, Guangdong Province 529090, P. R. China
E-mail: leeyib268@126.com
In the development of such as sequence, several chal-
lenges must be met: 1) Although the nucleophilic addition
reaction of 1,1-dibromoalkenes (bromoalkynes) has been
http://stu.wyu.edu.cn/hhxy/
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201500305.
Eur. J. Org. Chem. 2015, 4325–4329
© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
4325

Y. Li, L. Cheng, Y. Shao, S. Jiang, J. Cai, N. Qing
SHORT COMMUNICATION
extended to numerous functionalities, it has not yet been and can also function as an effective turnover reagent
applied to unprotected amidines as a nucleophile.[12] 2) Un- (Table 1, entries 4 and 7). Similarly, lower yield of cyclo-
der the palladium or copper-catalyzed system, the reactions amination product was obtained when other bases such as
usually suffer from inevitable homocoupling of 1,1-dibro- tBuOK, KOH, and CsCO3 were used instead of
moalkenes (bromoalkynes).[7a,13] Herein, we reported the TBAF·3H2O (Table 1, entries 8–10). Compared with N,N-
realization of this cycloamination reaction for the synthesis dimethylformamide (DMF), the yields of 5-bromo-4-hept-
of 1,2,3,5-tetrasubstituted imidazoles from easily available yl-1,2-diphenyl-1H-imidazole (3) were much lower when
1,1-dibromoalkenes and amidines by using a sequence in- dimethyl sulfoxide (DMSO) and 1,4-dioxane were used as
volving dehydrobromination and annulation reaction.
the solvents (Table 1, entries 11–12). The observed yields
and regioselectivity of the cycloamination process were
strongly dependent on the nature of the bidentate nitrogen
ligand. The efficiency of the reaction was markedly lower
when 4,7-diphenyl-1,10-phenanthroline was replaced with
1,10-phenanthroline and 2,2Ј-bipyridine (Table 1, en-
tries 13–14). Under the protection of N2, the desired prod-
uct 3 was obtained in 25% yield (Table 1, entry 15). How-
ever, using copper salts as the promoter and conducting the
reaction under oxidative conditions in an O2 atmosphere
(1 atm), afforded a significant amount of alkyne homocou-
pling product and the imidazole product was obtained only
62% yield (Table 1, entry 16).[14] Finally, the best results
were obtained when 1,1-dibromonon-1-ene 1 (0.8 mmol)
and TBAF·3H2O (5.0 mmol) in DMF (3 mL) were stirred
for 6 hours at 80 °C. Subsequently, CuCl (10 mol-%), N-
phenylbenzamidine 2 (0.5 mmol) and 4,7-diphenyl-1,10-
phenanthroline (20 mol-%) were added and the mixture was
stirred for 12 h at 100 °C under air (Table 1, entry 4).
With the optimized conditions in hand, we next exam-
ined the substrate scope for the synthesis of polysubstituted
imidazoles (Scheme 2). By varying R1 substituent of 1,1-
dibromoalkenes, it was shown that a range of aryl and alkyl
1,1-dibromoalkenes could be installed. The reaction pro-
ceeded smoothly and tolerates a range of aryl- and alkyl-
1,1-dibromoalkenes to generally afford tetrasubstituted
imidazoles in moderate to good yields. Notably, bromo-sub-
stituted imidazole could be prepared, which render them
suitable substrates for further functionalization through a
range of transition-metal-catalyzed cross-coupling methods.
Notably, 2-(5,5-dibromopent-4-enyl)isoindoline-1,3-dione
also added to N-phenylbenzamidine to afford 2-[3-(5-
bromo-1,2-diphenyl-1H-imidazol-4-yl)propyl]isoindoline-
1,3-dione (3g) in moderate yield. When 1,1-dibromodec-1-
en-5-yne was examined for compatibility in this cycloamin-
ation reaction, it is worth noting that the internal C–C tri-
ple bond was tolerated under the present catalysis (3h;
Scheme 2). We then tested the catalyst system on the reac-
tions of various amidines and found that aryl- and alkyl-
amidines underwent the cycloamination reaction across 1,1-
dibromoalkenes (3i–l; Scheme 2). Moreover, these condi-
tions also provide access to imidazoles bearing fluoro sub-
stituents, which are often highly appealing to synthetic and
medicinal chemists because of the unique biological proper-
ties imparted by the fluorine atom (3j; Scheme 2).[15] No-
tably, N-naphthyl-substituted amidine also reacted with 1,1-
dibromooct-1-ene to afford functionalized halogenated
imidazoles in moderate yields (3k; Scheme 2). Unfortu-
nately, the reaction with benzamidine hydrochloride only
afforded a complex mixture, and imidazoles gave a very low
Results and Discussion
To identify an effective catalytic system for the cycloam-
ination of 1,1-dibromoalkenes, we developed conditions for
the cycloamination of 1,1-dibromoalkenes as shown in
Table 1. In optimization trials, copper salt was found to be
an efficient catalyst for this kind of transformation. When
N-phenylbenzamidine reacted with 1,1-dibromonon-1-ene
in the absence of copper catalyst, no desired product was
observed by GC-MS analysis (Table 1, entry 1). The choice
of catalyst is crucial for the success of the cycloamination
reaction. Cu(OAc)2 or CuI could provide the desired prod-
uct whereas palladium salts such as Pd(PPh3)4 and PdCl2
were totally ineffective (Table 1, entries 2–5). To our delight,
the desired halogenated imidazole was obtained in 76%
yield when 10 mol-% CuCl was employed (Table 1, entry 4).
Essential for the success of the reaction is the use of
TBAF·3H2O, which is compatible with 1,1-dibromoalkene
Table 1. Optimization of cycloamination reaction conditions.[a,b]
Entry Catalyst
Base
Solvent
Ligand Yield
[%][b]
1
2
3
4
5
6
7
8
–
TBAF·3H2O
TBAF·3H2O
TBAF·3H2O
TBAF·3H2O
TBAF·3H2O
TBAF·3H2O
–
tBuOK
KOH
CsCO3
TBAF·3H2O
TBAF·3H2O
TBAF·3H2O
TBAF·3H2O
TBAF·3H2O
TBAF·3H2O
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMSO
dioxane
DMF
DMF
DMF
DMF
L1
L1
L1
L1
L1
L1
L1
L1
L1
L1
L1
L1
L2
L3
L1
L1
Ͻ 5
55
61
Cu(OAc)2
CuI
CuCl
Pd(PPh3)4
PdCl2
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
78(76)
Ͻ 5
Ͻ 5
–
48
9
Ͻ 5
Ͻ 5
52
48
36
31
25
62
10
11
12
13
14
15[c]
16[d]
[a] Reaction conditions: (1) 1,1-dibromonon-1-ene (0.8 mmol), base
(5 mmol), solvent (3 mL), 80 °C, 6 h, then (2) catalyst (10 mol-%),
N-phenylbenzamidine (0.5 mmol), ligand (20 mol-%), 100 °C, 12 h.
[b] Isolated yield based on 2. [c] Under the protection of N2.
[d] Under O2 atmosphere (1 atm). L1 = 4,7-diphenyl-1,10-phen-
anthroline, L2 = 1,10-phen, L3 = 2,2Ј-bipyridine.
4326
www.eurjoc.org
© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2015, 4325–4329

Synthesis of Polysubstituted Haloimidazoles
Scheme 2. Synthesis and isolated yield of imidazoles from 1,1-dibromoalkenes and amidines. All reactions were performed with 1,1-
dibromoalkenes (0.8 mmol), TBAF·3H2O (5 mmol), amidine (0.5 mmol), CuCl (10 mol-%), 4,7-diphenyl-1,10-phenanthroline (20 mol-%),
DMF (3 mL), 100 °C, 10–12 h.
yield (3l; Scheme 2). However, interestingly, pyridin-2-
amine derivatives could also be used in the cycloamination
of 1,1-dibromoalkenes, giving the product in 56% yield (5a;
Scheme 3).[16]
Scheme 4. Synthesis of tetraphenylimidazole from 1,1-dibromo-1-
alkene and amidines.
Scheme 3. Synthesis of imidazo[1,2-a]pyridines from 1,1-dibro-
moalkenes and pyridin-2-amines.
To obtain further insight into the mechanism of the pres-
These haloimidazoles can be converted directly into ent catalytic process, the direct cycloamination of cholo-
1,2,3,5-tetrasubstituted imidazoles through transition- alkynes was performed, as shown in Scheme 5. Gratifyingly,
metal-catalyzed coupling reactions. During the course of chloroalkynes gave exclusively the imidazole products in
these studies, we discovered that the Pd-catalyzed coupling moderate yields. Interestingly, when bromooct-1-yne was
of 5-bromo-1,2,4-triphenyl-1H-imidazole with phenylbor- used as starting material, the reaction afforded the corre-
onic acid gave 1,2,4,5-tetraphenyl-1H-imidazole (6a) in sponding bromo-substituted imidazoles 3c in 85% yield.
82% yield, which is potentially useful for materials science Notably, the efficiency of the reaction was markedly higher
(Scheme 4).
when 1,1-dibromoalkene was replaced with bromoalkynes.
Eur. J. Org. Chem. 2015, 4325–4329
© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
4327

Y. Li, L. Cheng, Y. Shao, S. Jiang, J. Cai, N. Qing
SHORT COMMUNICATION
Scheme 5. CuCl-catalyzed cycloamination reaction of haloalkynes with amidines.
TBAF·3H2O (5 mmol) in DMF (3 mL) was stirred for 6 h at 80 °C.
CuCl (10 mol-%), N-phenylbenzamidine (0.5 mmol), and 4,7-di-
phenyl-1,10-phenanthroline (20 mol-%) were then added and the
mixture was stirred for 12 h at 100 °C under air. The progress of the
reaction was monitored by TLC, and GC–MS analysis was used to
determine when the starting materials were completely consumed.
When the reaction was complete, the reaction mixture was cooled
to room temperature, the solution was filtered though a small
amount of silica gel, and the residue was purified by silica gel pre-
parative TLC (n-hexane/EtOAc, 10:1), which furnished 3d as a
Although detailed experimental evidence is still pending,
a plausible mechanism for this reaction is outlined in
Scheme 6.[16,17] It is suggested that the reaction is initiated
by dehydrobromination of 1,1-dibromoalkenes to produce
haloalkynes under TBAF·3H2O. The alkynyl copper(III) in-
termediate A is then formed by oxidative addition of the
CuI salt to haloalkyne. Then the alkynyl copper(III) inter-
mediate A reacts with an amidine to deliver ynamide inter-
mediate B with the aid of O2.[18] Intermediate B would then
undergo an intramolecular 5-endo-dig-cyclization onto the pale-yellow oil.
activated CϵC triple bond to produce imidazolylcop-
per(III) intermediate C with the aid of copper salts,[5] which
Acknowledgments
is favored because of the steric bulk of the R3 substituent.
Reductive elimination of intermediate C would afford cy-
cloamination product D with concurrent formation of CuI.
The authors thank the National Natural Science Foundation of
China (NSFC) (grant number 21302146), Guangdong Natural Sci-
ence Foundation (grant number S2013040012354), the Foundation
for Distinguished Young Talents in Higher Education of Guang-
dong (grant number 2013LYM_0094) and the Science Foundation
for Young Teachers of Wuyi University (grant number
201210291040098) for financial support.
[1] a) G. Aridoss, S. Balasubramanian, P. Parthiban, S. Kabilan,
Eur. J. Med. Chem. 2006, 41, 268–275; b) L. Nagarapu, A.
Satyender, B. Rajashaker, K. Srinivas, P. R. Rani, K. Radhika,
G. Subhashini, Bioorg. Med. Chem. Lett. 2008, 18, 1167–1171;
c) P. Gupta, S. Hameed, R. Jain, Eur. J. Med. Chem. 2004, 39,
805–814; d) Y. Wang, G. Inguaggiato, M. Jasamai, M. Shah,
D. Hughes, M. Slater, C. Simons, Bioorg. Med. Chem. 1999, 7,
481–487; e) H. Miyachi, H. Kiyota, M. Segawa, Bioorg. Med.
Chem. Lett. 1998, 8, 2163–2168.
[2] a) E. V. Aleksandrova, A. N. Kravchenko, P. M. Kochergin,
Chem. Heterocycl. Compd. 2011, 47, 261–289; b) H. Huang, X.
Ji, W. Wu, H. Jiang, Adv. Synth. Catal. 2013, 355, 170–180.
[3] Y. L. Zhong, J. Lee, R. A. Reamer, D. Askin, Org. Lett. 2004,
6, 929–931.
Scheme 6. Tentative mechanism for cycloamination reaction.
[4] D. Tang, P. Wu, X. Liu, Y. X. Chen, S. B. Guo, W. L. Chen,
J. G. Li, B. H. Chen, J. Org. Chem. 2013, 78, 2746–2750.
[5] L. Li, L. Neuville, Org. Lett. 2013, 15, 1752–1755.
[6] a) S. W. Fox, Chem. Rev. 1943, 32, 47–71; b) T. Chandra, X.
Zou, E. J. Valente, K. Brown, J. Org. Chem. 2006, 71, 5000–
5003; c) V. G. Pawar, W. M. D. Borggraeve, K. Robeyns, L. V.
Meervelt, F. Compernolle, G. Hoornaert, Tetrahedron Lett.
2006, 47, 5451–5453; d) X. Wei, M. Zhao, Z. Du, X. Li, Org.
Lett. 2011, 13, 4636–4639.
[7] a) G. Chelucci, Chem. Rev. 2012, 112, 1344–1462; b) X. Wang,
C. Kuang, Q. Yang, Eur. J. Org. Chem. 2012, 2, 424–428; c) R.
Riveiros, L. Saya, J. P. Sestelo, L. A. Sarandeses, Eur. J. Org.
Chem. 2008, 1959–1966; d) D. H. Huh, J. S. Jeong, H. B. Lee,
H. Ryu, Y. G. Kim, Tetrahedron 2002, 58, 9925–9932; e) X.
Fan, Y. He, X. Zhang, S. Guo, Y. Wang, Tetrahedron 2011, 67,
6369–6374; f) W. Shen, T. Kohn, Z. Fu, X. Y. Jiao, S. Lai, M.
Schmitt, Tetrahedron Lett. 2008, 49, 7284–7286; g) A. Zhang,
Z. Zheng, J. Fan, W. Shen, Tetrahedron Lett. 2010, 51, 828–
831.
Conclusions
We have developed a mild and general method for the
efficient synthesis of pharmacologically important diversely
functionalized tetrasubstituted imidazoles from amidines
and 1,1-dibromoalkenes. The ready accessibility of starting
materials and the low cost of the catalytic system makes
this process a valuable alternative for the construction of
these interesting multisubstituted haloheterocycles. The
mechanism and synthetic applications of this reaction are
under investigation.
Experimental Section
Reaction of 1,1-Dibromoalkenes and Amidines. Typical Procedure
[8] a) S. G. Mewman, M. Lautens, J. Am. Chem. Soc. 2010, 132,
for 3d:
A
mixture of 1,1-dibromonon-1-ene (0.8 mmol) and
11416–11417; b) H. Xu, Y. Zhang, J. Huang, W. Chen, Org.
4328
www.eurjoc.org
© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2015, 4325–4329

Synthesis of Polysubstituted Haloimidazoles
Lett. 2010, 12, 3704–3707; c) W. Chen, Y. Zhang, L. Zhang,
M. Wang, L. Wang, Chem. Commun. 2011, 47, 10476–10478;
d) H. Xu, S. Gu, W. Chen, D. Li, J. Dou, J. Org. Chem. 2011,
76, 2448–2458; e) D. I. Chai, L. Hoffmeister, M. Lautens, Org.
Lett. 2011, 13, 106–109.
H. Zhang, J. Wang, A. Lei, J. Am. Chem. Soc. 2008, 130,
14713–14720.
[14]
[15]
a) Z. Chen, H. Jiang, A. Wang, S. Yang, J. Org. Chem. 2010,
75, 6700–6703; b) Y. Nishihara, K. Ikegashira, K. Hirabayashi,
J. Ando, A. Mori, T. Hiyama, J. Org. Chem. 2000, 65, 1780–
1787.
For selected examples, see: a) S. Purser, P. R. Moore, S. Swal-
low, V. Gouverneur, Chem. Soc. Rev. 2008, 37, 320–330; b) J.
Nie, H.-C. Guo, D. Cahard, J.-A. Ma, Chem. Rev. 2011, 111,
455–529; c) E. J. Cho, T. D. Senecal, T. Kinzel, Y. Zhang, D. A.
Watson, S. L. Buchwald, Science 2010, 328, 1679–1681; d) T.
Furuya, A. S. Kamlet, T. Ritter, Nature 2011, 473, 470–477; e)
S. Lectard, Y. Hamashima, M. Sodeoka, Adv. Synth. Catal.
2010, 352, 2708–2732.
a) J. Zeng, Y. J. Yan, M. L. Leow, X. W. Liu, Org. Lett. 2012,
14, 4386–4389; b) C. He, J. Hao, H. Xu, Y. Mo, H. Liu, J. Han,
A. Lei, Chem. Commun. 2012, 48, 11073–11075; c) K. T. J.
Loones, B. U. W. Maes, C. Meyers, J. Deruytter, J. Org. Chem.
2006, 71, 260–264; d) R. L. Yan, H. Yan, C. Ma, Z. Y. Ren,
X. A. Gao, G. S. Huang, Y. M. Liang, J. Org. Chem. 2012, 77,
2024–2028; e) Y. Gao, M. Yin, W. Wu, H. Huang, H. Jiang,
Adv. Synth. Catal. 2013, 355, 2263–2273.
[9] M. Okutani, Y. Mori, J. Org. Chem. 2009, 74, 442–444.
[10] a) W. H. Okamura, G.-D. Zhu, D. K. Hill, R. J. Thomas, K.
Ringe, D. B. Borchardt, A. W. Norman, L. J. Mueller, J. Org.
Chem. 2002, 67, 1637–1650; b) L. T. Scott, M. J. Cooney, C.
Otte, C. Puls, T. Haumann, R. Boese, A. B. Smith III, P. J. Car-
roll, A. de Meijere, J. Am. Chem. Soc. 1994, 116, 10275–10283;
c) K. C. Nicolaou, M. E. Duggan, C. K. Hwang, P. K. Somers,
J. Chem. Soc., Chem. Commun. 1985, 1359–1362.
[11] a) Y. Li, J. Zhao, H. Chen, B. Liu, H. Jiang, Chem. Commun.
2012, 48, 3545–3547; b) B. Liu, Y. Li, M. Yin, W. Wu, H. Jiang,
Chem. Commun. 2012, 48, 11446–11448; c) B. Liu, Y. Li, H.
Jiang, M. Yin, H. Huang, Adv. Synth. Catal. 2012, 354, 2288–
2300; d) H. Jiang, M. Yin, Y. Li, B. Liu, J. Zhao, W. Wu, Chem.
Commun. 2014, 50, 2037–2039; e) Y. Li, L. Cheng, X. Liu, B.
Li, N. Sun, Beilstein J. Org. Chem. 2014, 10, 2886–2891.
[12] a) M. Yamagishi, K. Nishigai, T. Hata, H. Urabe, Org. Lett.
2011, 13, 4873–4875; b) M. Yamagishi, J. Okazaki, K. NIshi-
gai, T. Hata, H. Urabe, Org. Lett. 2012, 14, 34–37; c) S. Wang,
P. Li, L. Yu, L. Wang, Org. Lett. 2011, 13, 5968–5971; d) H.
Xu, S. Gu, W. Chen, D. Li, J. Dou, J. Org. Chem. 2011, 76,
2448–2458; e) Z. Chen, J. Li, H. Jiang, S. Zhu, Y. Li, C. Qi,
Org. Lett. 2010, 12, 3262–3265; f) Z. Wu, Y. Pan, X. Zhou,
Synthesis 2011, 2255–2260.
[16]
[17] Alternatives mechanisms, such as free radical mechanism can-
not be excluded. For selected examples, see: Y. F. Wang, X.
Zhu, S. Chiba, J. Am. Chem. Soc. 2012, 134, 3679–3682.
[18] a) M. O. Frederick, J. A. Mulder, M. R. Tracey, R. P. Hsung, J.
Huang, K. C. M. Kurtz, L. Shen, C. J. Douglas, J. Am. Chem.
Soc. 2003, 125, 2368–2369; b) J. R. Dunetz, R. L. Danheiser,
Org. Lett. 2003, 5, 4011–4014; c) Y. Zhang, R. P. Hsung, M. R.
Tracey, K. C. M. Kurtz, E. L. Vera, Org. Lett. 2004, 6, 41151–
1154.
[13] a) W. Shen, S. A. Thomas, Org. Lett. 2000, 2, 2857–2860; b) A.
Coste, F. Couty, G. Evano, Synthesis 2010, 1500–1504; c) V.
Galamb, M. Gopal, H. Alper, Organometallics 1983, 2, 801–
805; d) J. P. Marino, H. N. Nguyun, J. Org. Chem. 2002, 67,
6841–6844; e) S. V. Damle, D. Seomoon, P. H. Lee, J. Org.
Chem. 2003, 68, 7085–7087; f) W. Shi, Y. Luo, X. Luo, L. Chao,
Received: March 6, 2015
Published Online: June 3, 2015
Eur. J. Org. Chem. 2015, 4325–4329
© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
4329