Pd-carbene migratory insertion: Application to the synthesis of trifluoromethylated alkenes and dienes

Article abstract of DOI:10.1002/chem.201304143

Pd-catalyzed cross-coupling of halides with CF₃-substituted diazo compounds or N-tosylhydrazones has been explored for the synthesis of CF₃-substituted alkenes and 1,3-butadienes. Pd-carbene migratory insertion plays the key role in these transformations. Copyright

Full text of DOI:10.1002/chem.201304143

DOI: 10.1002/chem.201304143
Communication
&
Trifluoromethylation
Pd–Carbene Migratory Insertion: Application to the Synthesis of
Trifluoromethylated Alkenes and Dienes
Xi Wang,[a] Yan Xu,[a] Yifan Deng,[a] Yujing Zhou,[a] Jiajie Feng,[a] Guojing Ji,[a] Yan Zhang,[a]
and Jianbo Wang*[a, b]
Abstract: Pd-catalyzed cross-coupling of halides with CF3-
substituted diazo compounds or N-tosylhydrazones has
been explored for the synthesis of CF3-substituted alkenes
and 1,3-butadienes. Pd–carbene migratory insertion plays
the key role in these transformations.
Scheme 1. General strategies for preparing trifluoromethylated alkenes.
olefination of aromatic aldehydes with CF3CH2PPh3+OTfÀ,[11b]
Introduction of the trifluoromethyl group can lead to profound
changes in both physical and biological properties of organic
molecules, and thus the trifluoromethyl substituent is a signifi-
cant structural element of many pharmaceuticals, agrochemi-
cals, and functional materials.[1] However, CF3-bearing com-
pounds are absent in the natural world. Therefore, it is highly
important to develop general and practical methods to intro-
duce the trifluoromethyl group into organic molecules.[2]
In the past decades, the scientific community has witnessed
remarkable advances in the synthesis of trifluoromethylated
arenes,[3–6] alkynes,[7] and alkanes.[8] Along with these develop-
ments, the synthesis of trifluoromethylated alkenes has also re-
ceived considerable attention because compounds containing
vinyl-CF3 moieties can be used as versatile synthons in the syn-
thesis of fluorinated organic compounds.[9]
and reverse Wittig coupling of benzylphosphonium ylides with
CF3C(OEt)OH[11c] (Scheme 1, strategy B). The third approach in-
volves modification of the molecules containing a vinyl-CF3
moiety (such as CF3-substituted vinyl halides, CF3-substituted
vinyl boron reagents, and CF3- substituted olefins; Scheme 1,
strategy C).[12]
Although significant progress has been made in the synthe-
sis of multisubstituted trifluoromethyl alkenes, there are still
a number of problems: some of the methods for the construc-
tion of Cvinyl–CF3 bonds requires the use of relatively expensive
trifluoromethylation reagents and functionalized alkenes that
are not readily accessible (such as alkenyl halides, sulfonates,
and boronic acids). The construction of C=CCF3 bonds by
using the Horner reaction or Wittig reaction generally suffer
from limited substrates scope, low E/Z selectivity, and cumber-
some preparation of CF3-containing ylides.[11a,b]
The previously established methods for the synthesis of tri-
fluoromethylated alkenes can be classified into three different
strategies (Scheme 1). The first approach involves transition-
metal-catalyzed or -mediated conversion of a halide,[4c,10a] sulfo-
nate,[10b] boron group,[3a,d,f,10c] or carboxyl group[10d] into a CF3
group (Scheme 1, strategy A). The second approach involves
construction of the carbon–carbon double bond by the Horner
reaction of aromatic aldehydes with CF3CH2P(O)Ph2,[11a] Wittig
We and others have recently demonstrated that Pd-cata-
lyzed cross-coupling reactions of halides with diazo com-
pounds are powerful for the synthesis of alkenes.[13] Moreover,
in recent years, trifluoromethyldiazomethane, generated in situ
from 2,2,2-trifluoroethylamine hydrochloride[14] and 1-aryl-2,2,2-
trifluorodiazoethanes,[15] has been explored as a CF3-bearing
precursor of complex trifluoromethylated molecules.
Thus, we have conceived that by using CF3-bearing diazo
compounds as substrates, it should be possible to develop
straightforward methods for the synthesis of CF3-containing al-
kenes by using Pd-catalyzed coupling reactions (Scheme 2a).
The reaction is supposed to follow the general pathway as pre-
viously proposed (Scheme 2b).[13] Oxidative addition of Pd0 to
halides, followed by decomposition of CF3-containing diazo
compounds, forms the Pd carbene as the key intermediate.
Subsequent migratory insertion leads to a PdII species, from
which b-hydride elimination occurs to afford trifluoromethylat-
ed alkenes. The possible b-fluoride elimination is supposedly
not competitive with the corresponding b-hydride elimina-
tion.[16]
[a] X. Wang, Y. Xu, Y. Deng, Y. Zhou, J. Feng, G. Ji, Prof. Dr. Y. Zhang,
Prof. Dr. J. Wang
Beijing National Laboratory of Molecular Sciences (BNLMS)
and Key Laboratory of Bioorganic Chemistry and
Molecular Engineering of Ministry of Education
College of Chemistry, Peking University
Beijing 100871 (China)
Fax: (+8610)6275-1708
E-mail: wangjb@pku.edu.cn
[b] Prof. Dr. J. Wang
The State Key Laboratory of Organometallic Chemistry
Shanghai Institute of Organic Chemistry
Chinese Academy of Sciences
354 Fenglin Lu, Shanghai 200032 (China)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201304143.
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Communication
Figure 1. X-Ray structure of 3a. Thermal ellipsoids shown at 30% probabili-
ty.
Scheme 2. Synthesis of trifluoromethylated alkenes by using Pd-catalyzed
coupling.
entry 10) and 3a was obtained in 93% yield and with excellent
E/Z selectivity under the optimized conditions with suitable
amount of LiOtBu and Et3N (Table 1, entry 11).
To this end, we examined the cross-coupling of benzyl bro-
mide (1a) with 1-phenyl-2,2,2-trifluorodiazoethane (2) in the
presence of [Pd2dba3]/(2-furyl)3P, which was proved to be effi-
cient in our previous work.[13c] After preliminary screening of
solvents, toluene turned out to be the suitable solvent
(Table 1, entries 1–4). The yield could be increased at elevated
temperature (Table 1, entry 5). Further optimizations of various
base at 808C revealed that LiOtBu was the suitable base for
the reaction (Table 1, entries 6–8). Finally, slightly increasing
the amount of 2 to 1.05 equivalents resulted in better yield,
albeit with trace amounts of 1a (3%) being recovered (Table 1,
entry 9). Adding Et3N led to full conversion of 1a (Table 1,
The E configuration of product 3a was confirmed by X-ray
crystallographic analysis, as shown in Figure 1.[17]
The high stereoselectivity for (E)-3a formation can be ration-
alized as follows Scheme 3: two phenyl groups prefer an
Scheme 3. Rationalization of stereoselectivity.
Table 1. Optimization of reaction conditions.[a]
eclipsed relationship in the transition state of the syn b-H elim-
ination from the palladium intermediate, presumably owing to
the attractive p–p stacking interaction,[18] thus leading to (E)-
3a as the major product.
With the optimized reaction conditions established, we then
evaluated the scope of the reaction using various benzyl bro-
Entry
Base
[equiv]
Solvent
T
[8C]
2
Yield
[%][b]
[equiv]
mides
1
and
1-phenyl-2,2,2-trifluorodiazoethane
(2)
1
2
3
4
5
6
7
8[c]
9[d]
10
11[e]
Et3N (3.0)
Et3N (3.0)
Et3N (3.0)
Et3N (3.0)
DCE
MeCN
50
50
50
50
80
80
80
80
80
80
80
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
trace
trace
51
76
84
40
90
93
96
(Scheme 4). The data show that benzyl bromides with elec-
tron-donating substituents on the aryl ring are amenable to
this process, thus affording trifluoromethyl alkenes in good
yields and with excellent E/Z ratio (3b, 3d). The use of a sub-
strate with a methyl substituent at an ortho position led to
a decline in E/Z ratio (3c). High yields of product were also ob-
tained with benzyl bromides bearing electron-withdrawing
groups (3e–g), whereas a moderate yield of product was ob-
tained by using 4-nitrobenzylbromide (1h). 2-(Bromomethyl)-
naphthalene (1i) also worked well, providing 3i in moderate
yield and high E/Z selectivity.
dioxane
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
Et3N (3.0)
Cs2CO3 (3.0)
(iPr)2NH (3.0)
LiOtBu (3.0)
LiOtBu (3.0)
LiOtBu (1.0)/Et3N (2.0)
LiOtBu (2.0)/Et3N (0.5)
1.05
1.05
1.05
73
93
[a] Reaction conditions: 1a (0.30 mmol), 2, solvent (2.0 mL), 5 h. [b] Yield
of isolated product. [c] 1a (4%) was isolated. [d] 1a (3%) was isolated.
[e] 1a was completely converted. The product 3a E/Z> 20:1 was con-
firmed by 19F NMR analysis.
To further expand the scope of this transformation, we used
the corresponding CF3-bearing N-tosylhydrazones, 4a–f, as
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Scheme 4. Pd-catalyzed coupling of benzyl bromides with 1-phenyl-2,2,2-tri-
fluorodiazoethane. Reaction conditions: benzyl bromide 1a–i (0.3 mmol), 2
(0.315 mmol, 1.05 equiv), toluene (2 mL), 808C, 5 h. [a] Yield of isolated prod-
uct. [b] E/Z ratio determined by 19F NMR analysis.
diazo substrate precursors. Compared to diazo compounds, N-
tosylhydrazones are stable, easy to handle, and readily avail-
able from the corresponding ketones. To avoid a side reaction
involving CF3-bearing N-tosylhydrazones and toluene, dioxane
was employed as the solvent. To our delight, the coupling re-
action proceeded equally well with the CF3-bearing N-tosylhy-
drazones and exhibited good reactivity and stereoselectivity
with benzyl bromides containing either electron-rich or -defi-
cient substituents (Scheme 5). The coupling reaction proceed-
ed smoothly with a series of trifluoromethyl-containing N-tosyl-
hydrazones, 4a–f, thus providing the corresponding trisubsti-
tuted olefins, 3a, 3c–e and 3g–o, in moderate to good yields.
Notably, we also observed a decrease in E/Z selectivity when
benzyl bromides bearing an ortho substituent were used as
the substrate (3c).
Scheme 5. Pd-catalyzed coupling of benzyl bromides with N-tosylhydra-
zones. Reaction conditions: benzyl bromide 1 (0.3 mmol), N-tosylhydrazones
4a–f (0.48 mmol, 1.6 equiv), dioxane (5 mL), 808C, 5 h. [a] Yield of isolated
product. [b] E/Z ratio was determined by 19F NMR analysis.
Next, the Pd-catalyzed cross-coupling of allyl bromides 5a–
e with 1-phenyl-2,2,2-trifluorodiazoethane (2) was investigated.
To identify appropriate reaction conditions, several experi-
ments were conducted with allyl bromide (5a) and diazo sub-
strate 2. The main results are as following: 1) the screening of
catalyst/ligand/base at room temperature led to tri(2-furyl)-
phosphine/Et3N being identified as the best conditions,
Pd(OAc)2 showing considerable catalytic efficiency; 2) among
the solvents commonly used in Pd-catalyzed reactions of diazo
compounds (toluene, dioxane, DCE, CH3CN), CH3CN was the
best solvent for the transformation; 3) the best results were
obtained when the ratio 5a/2 was maintained at 1:1. Because
the Pd carbene derived from 1-phenyl-2,2,2-trifluorodiazo-
ethane is more active than nonfluorinated diazo compounds,
an excess amount of 2 lead to further reaction of product 6a
with 2, as proved by GC-MS analysis; 4) the use of N-tosylhy-
drazone 4a gave product in low yield (27% 19F NMR yield)
owing to direct nucleophilic attack at allyl bromide by the ni-
trogen-based anion of N-tosylhydrazones after deprotonation.
Thus, under the optimized reaction conditions, the treatment
Scheme 6. Pd-catalyzed coupling of allyl bromides with 1-phenyl-2,2,2-tri-
fluorodiazoethane. Reaction conditions: allyl bromide 5a–e (0.3 mmol), 1-
phenyl-2,2,2-trifluorodiazoethane (2) (0.3 mmol, 1.0 equiv), CH3CN (0.5 mL),
258C, 24 h. [a] Yield of isolated product. [b] E/Z ratio determined by 19F NMR
analysis. [c] Allyl chloride (5a’) was used in this reaction.
of 5a (0.3 mmol) with 2 in 0.5 mL CH3CN afforded 6a in 64%
yield upon isolation with 2:1 E/Z ratio (Scheme 6).
With the optimized reaction conditions established, we sur-
veyed the scope of the substrates with a series of allyl halides,
5a–e (Scheme 6). Allyl chloride (5a’) also worked well, giving
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6a in a slightly improved yield. With substituted allyl bromides
as substrates, the reaction proceeded smoothly and the ex-
pected 1,3-butadiene products (6b–e) were obtained in good
yields with increased selectivity. Notably, the reaction with 2-
phenyl allyl bromide (5d) also proceeded smoothly, providing
6d in 75% yield and high E/Z ratio.
a wide range of functional groups are tolerated. The products
can serve as valuable intermediates for the construction of tri-
fluoromethyl-containing complex molecules. Future efforts will
be aimed at exploring more carbene-based transformations in
the synthesis CF3-containing molecules.
Finally, we investigated the Pd-catalyzed coupling of aryl
bromides 7a–h with N-tosylhydrazones 8a–e derived from tri-
fluoroacetylalkanes (Scheme 7). The coupling reaction proceed-
Keywords: alkene synthesis · cross-coupling reactions · diazo
compounds · palladium · trifluoromethylation
[1] a) P. Kirsch, Modern Fluoroorganic Chemistry: Synthesis, Reactivity, Appli-
cations, Wiley-VCH, Weinheim, 2004; b) K. Mꢁller, C. Faeh, F. Diederich,
Science 2007, 317, 1881; c) T. Furuya, A. S. Kamlet, T. Ritter, Nature 2011,
473, 470.
[2] For reviews, see: a) G. K. S. Prakash, A. K. Yudin, Chem. Rev. 1997, 97,
757; b) M. Shimizu, T. Hiyama, Angew. Chem. 2005, 117, 218; Angew.
Chem. Int. Ed. 2005, 44, 214; c) M. Schlosser, Angew. Chem. 2006, 118,
5558; Angew. Chem. Int. Ed. 2006, 45, 5432; d) J. A. Ma, D. Cahard, J. Flu-
orine Chem. 2007, 128, 975; e) J. A. Ma, D. Cahard, Chem. Rev. 2008, 108,
PR1; f) O. A. Tomashenko, V. V. Grushin, Chem. Rev. 2011, 111, 4475; g) T.
Besset, C. Schneider, D. Cahard, Angew. Chem. 2012, 124, 5134; Angew.
Chem. Int. Ed. 2012, 51, 5048; h) Y. Macꢂ, E. Magnier, Eur. J. Org. Chem.
2012, 2479; i) N. Shibata, A. Matsnev, D. Cahard, Beilstein J. Org. Chem.
2010, 6, 65; j) X. F. Wu, H. Neumann, M. Beller, Chem. Asian J. 2012, 7,
1744; k) A. Studer, Angew. Chem. 2012, 124, 9082; Angew. Chem. Int. Ed.
2012, 51, 8950; l) T. Liang, C. N. Neumann, T. Ritter, Angew. Chem. 2013,
125, 8372; Angew. Chem. Int. Ed. 2013, 52, 8214.
[3] For selected examples of Cu-catalyzed or -mediated C(aryl)–CF3 bond
formation, see: a) H. Morimoto, T. Tsubogo, N. D. Litvinas, J. F. Hartwig,
Angew. Chem. 2011, 123, 3877; Angew. Chem. Int. Ed. 2011, 50, 3793;
b) J. Xu, D. F. Luo, B. Xiao, Z. J. Liu, T. J. Gong, Y. Fu, L. Liu, Chem.
Commun. 2011, 47, 4300; c) O. A. Tomashenko, E. C. Escudero-Adꢃn,
M. M. Belmonte, V. V. Grushin, Angew. Chem. 2011, 123, 7797; Angew.
Chem. Int. Ed. 2011, 50, 7655; d) L. L. Chu, F. L. Qing, J. Am. Chem. Soc.
2012, 134, 1298; e) P. Novꢃk, A. Lishchynskyi, V. V. Grushin, Angew.
Chem. 2012, 124, 7887; Angew. Chem. Int. Ed. 2012, 51, 7767.
[4] For selected examples of Pd-catalyzed or -mediated C(aryl)–CF3 bond
formation, see: a) V. V. Grushin, W. J. Marshall, J. Am. Chem. Soc. 2006,
128, 12644; b) X. S. Wang, L. Truesdale, J. Q. Yu, J. Am. Chem. Soc. 2010,
132, 3648; c) E. J. Cho, T. D. Senecal, T. Kinzel, Y. Zhang, D. A. Watson,
S. L. Buchwald, Science 2010, 328, 1679.
[5] For selected examples of Ag-mediated C(aryl)–CF3 bond formation, see:
a) Y. Ye, S. H. Lee, M. S. Sanford, Org. Lett. 2011, 13, 5464; b) A. Hafner, S.
Brꢄse, Angew. Chem. 2012, 124, 3773; Angew. Chem. Int. Ed. 2012, 51,
3713; c) Y. W. Zeng, L. J. Zhang, Y. C. Zhao, C. F. Ni, J. W. Zhao, J. B. Hu, J.
Am. Chem. Soc. 2013, 135, 2955; d) X. Wang, Y. Xu, F. Y. Mo, G. J. Ji, D.
Qiu, J. J. Feng, Y. X. Ye, S. N. Zhang, Y. Zhang, J. Wang, J. Am. Chem. Soc.
2013, 135, 10330.
Scheme 7. Pd-catalyzed coupling of aryl bromides with N-tosylhydrazones.
Reaction conditions: aryl bromide 7a–h (0.3 mmol), N-tosylhydrazones 8a–
e (0.39 mmol, 1.3 equiv), dioxane (2 mL), 1108C, 12 h. [a] Yield of isolated
product unless otherwise noted. [b] 1.05 equiv of N-tosylhydrazone was
used. [c] E/Z ratio determined by 19F NMR analysis. [d] These yields are based
on 19F NMR with PhCF3 as internal standard.
[6] For selected examples of radical trifluoromethylation of arenes, see:
a) B. R. Langlois, E. Laurent, N. Roidot, Tetrahedron Lett. 1991, 32, 7525;
b) Y. Ji, T. Brueckl, R. D. Baxter, Y. Fujiwara, I. B. Seiple, S. Su, D. G. Black-
mond, P. S. Baran, Proc. Natl. Acad. Sci. USA 2011, 108, 14411; c) D. A.
Nagib, D. W. C. MacMillan, Nature 2011, 480, 224.
[7] For selected examples of C(sp)–CF3 bond formation, see: a) L. L. Chu,
F. L. Qing, J. Am. Chem. Soc. 2010, 132, 7262; b) D. F. Luo, J. Xu, Y. Fu,
Q. X. Guo, Tetrahedron Lett. 2012, 53, 2769.
[8] For selected examples of C(sp3)–CF3 bond formation, see: a) X. Mu, T.
Wu, H. Y. Wang, Y. L. Guo, G. S. Liu, J. Am. Chem. Soc. 2012, 134, 878;
b) T. S. N. Zhao, K. J. Szabꢅ, Org. Lett. 2012, 14, 3966; c) R. Shimizu, H.
Egami, Y. Hamashima, M. Sodeoka, Angew. Chem. 2012, 124, 4655;
Angew. Chem. Int. Ed. 2012, 51, 4577; d) Y. Li, A. Studer, Angew. Chem.
2012, 124, 8345; Angew. Chem. Int. Ed. 2012, 51, 8221; e) Y. Yasu, T.
Koike, M. Akita, Angew. Chem. 2012, 124, 9705; Angew. Chem. Int. Ed.
2012, 51, 9567; f) R. Zhu, S. L. Buchwald, J. Am. Chem. Soc. 2012, 134,
12462; g) J. Xu, B. Xiao, C. Q. Xie, D. F. Luo, L. Liu, Y. Fu, Angew. Chem.
2012, 124, 12719; Angew. Chem. Int. Ed. 2012, 51, 12551; h) M. Y. Hu,
C. F. Ni, J. B. Hu, J. Am. Chem. Soc. 2012, 134, 15257; i) D. F. Lu, C. L. Zhu,
H. Xu, Chem. Sci. 2013, 4, 2478; j) S. Mizuta, S. Verhoog, K. M. Engle, T.
ed at elevated temperature (1108C) and with relatively high
catalyst loading ([Pd2dba3], 6 mol%), because CF3-containing
N-tosylhydrazones are less active than nonfluorinated N-tosyl-
hydrazones analogues in this transformation. The aryl bro-
mides bearing either electron-donating or -withdrawing
groups react smoothly to produce the a-trifluoromethylsty-
renes (9a–h) in moderate to good yields. CF3-containing trisub-
stituted alkenes (9k,l) and tetrasubstituted alkenes (9i,j) were
also obtained by using the corresponding 2,2,2-trifluoroacetyl-
alkane N-tosylhydrazones (8b–e).
In summary, we have developed a set of Pd-catalyzed cou-
pling reactions of benzyl halides/allylic halides/aryl halides with
CF3-bearing N-tosylhydrazones/diazo compounds for the syn-
thesis of trifluoromethylated alkenes and conjugated dienes.
The transformations proceed under mild conditions and
Chem. Eur. J. 2014, 20, 961 – 965
www.chemeurj.org
964
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication
Khotavivattana, M. O’Duill, K. Wheelhouse, G. Rassias, M. Mꢂdebielle, V.
Gouverneur, J. Am. Chem. Soc. 2013, 135, 2505.
G. A. Olah, Org. Lett. 2012, 14, 1146; e) M. Shimizu, T. Fujimoto, H. Mine-
zaki, T. Hata, T. Hiyama, J. Am. Chem. Soc. 2001, 123, 6947.
[13] For reviews, see: a) Y. Zhang, J. Wang, Eur. J. Org. Chem. 2011, 1015;
b) J. Barluenga, C. Valdꢂs, Angew. Chem. 2011, 123, 7626; Angew. Chem.
Int. Ed. 2011, 50, 7486; c) Q. Xiao, Y. Zhang, J. Wang, Acc. Chem. Res.
2013, 46, 236.
[14] a) B. Morandi, E. M. Carreira, Angew. Chem. 2010, 122, 950; Angew.
Chem. Int. Ed. 2010, 49, 938; b) B. Morandi, B. Mariampillai, E. M. Car-
reira, Angew. Chem. 2011, 123, 1133; Angew. Chem. Int. Ed. 2011, 50,
1101; c) S. A. Kꢁnzi, B. Morandi, E. M. Carreira, Org. Lett. 2012, 14, 1900;
d) C. B. Liu, W. Meng, F. Li, S. Wang, J. Nie, J. A. Ma, Angew. Chem. 2012,
124, 6331; Angew. Chem. Int. Ed. 2012, 51, 6227; e) G. A. Molander, L. N.
Cavalcanti, Org. Lett. 2013, 15, 3166.
[15] J. R. Denton, D. Sukumaran, H. M. L. Davies, Org. Lett. 2007, 9, 2625.
[16] For b-fluoride elimination on rhodium, see: a) T. Miura, Y. Ito, M. Muraka-
mi, Chem. Lett. 2008, 37, 1006; for b-fluoride elimination on iridium,
see: b) J. Choi, D. Y. Wang, S. Kundu, Y. Choliy, T. J. Emge, K. Krogh-Jes-
persen, A. S. Goldman, Science 2011, 332, 1545.
[17] CCDC-968366 contains the supplementary crystallographic data for this
paper. These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
[18] E. Krenske, K. N. Houk, Acc. Chem. Res. 2013, 46, 979.
[9] a) D. Bonnet-Delpon, J. P. Begue, T. Lequeux, M. Ourevitch, Tetrahedron
1996, 52, 59; b) J.-P. Bꢂguꢂ, D. Bonnet-Delpon, M. H. Rock, J. Chem. Soc.
Perkin Trans. 1 1996, 1409; c) J. Ichikawa, H. Fukui, Y. Ishibashi, J. Org.
Chem. 2003, 68, 7800; d) T. Mori, J. Ichikawa, Chem. Lett. 2004, 33, 1206;
e) J. Ichikawa, T. Mori, Y. Iwai, Chem. Lett. 2004, 33, 1354; f) T. Mori, Y.
Iwai, J. Ichikawa, Chem. Lett. 2005, 34, 778; g) J. Ichikawa, Y. Iwai, R.
Nadano, T. Mori, M. Ikeda, Chem. Asian J. 2008, 3, 393; h) J. Ichikawa, M.
Ikeda, M. Hattori, Heterocycles 2009, 77, 1285; i) K. Fuchibe, M. Takaha-
shi, J. Ichikawa, Angew. Chem. 2012, 124, 12225; Angew. Chem. Int. Ed.
2012, 51, 12059.
[10] a) A. Hafnera, S. Brꢄse, Adv. Synth. Catal. 2011, 353, 3044; b) E. J. Cho,
S. L. Buchwald, Org. Lett. 2011, 13, 6552; c) A. T. Parsons, T. D. Senecal,
S. L. Buchwald, Angew. Chem. 2012, 124, 3001; Angew. Chem. Int. Ed.
2012, 51, 2947; d) Z. B. He, T. Luo, M. Y. Hu, Y. J. Cao, J. B. Hu, Angew.
Chem. 2012, 124, 4010; Angew. Chem. Int. Ed. 2012, 51, 3944.
[11] a) T. Kobayashi, T. Eda, O. Tamura, H. Ishibashi, J. Org. Chem. 2002, 67,
3156; b) T. Hanamoto, N. Morita, K. Shindo, Eur. J. Org. Chem. 2003,
4279; c) S. M. Landge, D. A. Borkin, B. Tçrçk, Lett. Org. Chem. 2009, 6,
439.
[12] a) B. Jiang, Q. F. Wang, C. G. Yang, M. Xu, Tetrahedron Lett. 2001, 42,
4083; b) X. Y. Liu, M. Shimizu, T. Hiyama, Angew. Chem. 2004, 116, 897;
Angew. Chem. Int. Ed. 2004, 43, 879; c) T. Fuchikami, M. Yatabe, I. Ojima,
Synthesis 1981, 365; d) G. K. S. Prakash, H. S. Krishnan, P. V. Jog, A. P. Iyer,
Received: October 23, 2013
Published online on December 20, 2013
Chem. Eur. J. 2014, 20, 961 – 965
www.chemeurj.org
965
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