Intermolecular visible-light photoredox atom-transfer radical [3+2]-cyclization of 2-(iodomethyl)cyclopropane-1,1-dicarboxylate with alkenes and alkynes

Article abstract of DOI:10.1002/chem.201301943

Radical chemistry! A visible-light-promoted, tin/boron-free intermolecular [3+2] atom-transfer radical cyclization reaction was developed by using iridium polyphenylpridinyl complex as the sensitizer (see scheme). 2-(Iodomethyl) cyclopropane-1,1-dicarboxylate reacted with various alkenes and alkynes to form cyclopentane and cyclopentene derivatives. Copyright

Full text of DOI:10.1002/chem.201301943

COMMUNICATION
Radical chemistry! A visible-light-pro-
moted, tin/boron-free intermolecular
[3+2] atom-transfer radical cyclization
reaction was developed by using iri-
dium polyphenylpridinyl complex as
the sensitizer (see scheme). 2-(Iodome-
thyl)cyclopropane-1,1-dicarboxylate
reacted with various alkenes and
alkynes to form cyclopentane and
cyclopentene derivatives.
Organic Synthesis
X. Gu, X. Li, Y. Qu, Q. Yang, P. Li,*
Y. Yao* ......................... &&&& — &&&&
Intermolecular Visible-Light Photo-
redox Atom-Transfer Radical [3+2]-
Cyclization of 2-(Iodomethyl)cyclopro-
pane-1,1-dicarboxylate with Alkenes
and Alkynes
Chem. Eur. J. 2013, 00, 0 – 0
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DOI: 10.1002/chem.201301943
Intermolecular Visible-Light Photoredox Atom-Transfer Radical [3+2]-
Cyclization of 2-(Iodomethyl)cyclopropane-1,1-dicarboxylate with Alkenes
and Alkynes
Xiangyong Gu, Xiang Li, Yue Qu, Qi Yang, Pixu Li,* and Yingming Yao*[a]
Practical and innovative methods to construct five-mem-
bered rings are always of great interest to synthetic chemists.
Radical cyclizations are desirable processes because of their
mild conditions and high functional-group compatibility.[1]
Atom-transfer radical additions or cyclizations (ATRA/
ATRC) of halogenated alkanes to unsaturated carbon–
carbon bonds, originally reported by Kharasch and co-work-
ers,[2] have received considerable attention and have become
a powerful synthetic methodology.[3] ATRC reactions are
Many elegant visible-light-promoted intramolecular[6] and
intermolecular [2+2]-,[7] [3+2]-,[8] [4+2]-,[9] and [2+2+2]-cyc-
lization[10] reactions have recently been developed. Nonethe-
less, visible-light photocatalytic ATRC is still a challenge be-
cause the reductive products are preferentially formed in
the quenching cycle of the catalyst, leading to the loss of
halide functionality.[6c,e,11] Stephenson and co-workers re-
ported visible-light-promoted ATRA reactions of a-halocar-
bonyls and polyhalogenated alkanes to alkenes by using cat-
alysts 1 and 4.[11b,e] However, the photoredox ATRA condi-
tions were not applicable to ATRC because it was very sub-
strate dependent and significant amounts of reductive prod-
uct were observed with slight changes in alkene
substrate.[11e] Moreover, intermolecular visible-light photore-
dox ATRC mediated by Ru/Ir complexes has not been re-
ported to date.
À
particularly useful in the formation of C C bonds and the
construction of cyclic frameworks.[1e,4] However, free-radical
reactions, including ATRC, often employ stoichiometric haz-
ardous radical initiators. This drawback limits their utility
and applications in organic synthesis. The development of
environmentally benign and practical ATRC radical initia-
tors is, therefore, important.
Recent advances in visible-light photoredox chemistry
showed improved advantages over classical radical reactions
because the use of potentially hazardous radical initiators
can be avoided.[5] Typical Ir- or Ru-based visible-light sensi-
tizers used in photoredox reactions are shown in Figure 1.
In visible-light photoredox reactions, substrates contain-
À
ing activated C X bonds, such as a-halocarbonyl com-
pounds,[11a,b,12] polyhalogenated alkanes,[7b,13] benzyl bro-
mides bearing strong electron-withdrawing groups,[14] and
geminal halogenated sugars[15] have been used to generate
radical intermediates. The use
of substrates containing unacti-
vated halogen–carbon bonds
has proven very difficult be-
cause the redox potentials of
the visible-light sensitizers are
generally not high enough to
À
break such C X bonds. Break-
throughs of utilizing unactivat-
ed aryl/alkyl halide bonds in
the visible-light-promoted reac-
Figure 1. Typical Ru/Ir-based visible-light sensitizers. bpy=2,2’-bipyridine; ppy= 2-phenylpyridine; dF-
(CF3)ppy= 2-(2,4-difluorophenyl)-5-(trifluoromethyl)pyridine; dtbbpy= 4,4’-di-tert-butyl-2,2’-bipyridine.
tions have recently been repor-
ACHTUNGTRENNUNG
ted.[11c,d,16] We report herein a
photocatalytic
intermolecular
ATRC reaction of unactivated alkyl iodides that occurs
under nonreductive conditions, leading to the formation of
highly functionalized cyclic products (Scheme 1).
Two decades ago Curranꢁs group reported the [3+2]
ATRC reactions of allyl- or propargylmalonates with elec-
tron-rich olefins by using organotin reagents as the radical
initiator.[17] The reaction was further developed by Taguchi
and co-workers through a novel cyclopropane ring-opening
to generate the homoallylic radical.[18] However, stoichio-
metric alkylborane and continuous oxygen injection were re-
[a] X. Gu, X. Li, Y. Qu, Q. Yang, P. Li, Y. Yao
Key Laboratory of Organic Synthesis of Jiangsu Province
College of Chemistry, Chemical Engineering, and Materials Science
Soochow University, 199 RenAi Road, Suzhou
Jiangsu, 215123 (P. R. China)
Fax : (+86)512-65880826
E-mail: lipixu@suda.edu.cn
yaoym@suda.edu.cn
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201301943.
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Atom-Transfer Radical [3+2]-Cyclization
COMMUNICATION
and one equivalent of DIEA in a 4:1 CH2Cl2/H2O biphasic
solvent system. The desired cyclopentane 8a was obtained
in 85% GC yield (entry 9). Presumably, the addition of
water promoted the reaction by increasing the polarity of
the solvent system. In addition, the interference of IÀ (Eox =
0.54 V vs. SCE) in the radical initiation step was reduced by
dissolving IÀ into the aqueous layer.
With the optimized reaction conditions in hand, we inves-
tigated the intermolecular visible-light ATRC reaction of
compound 6 with various alkenes. The results are shown in
Table 2. Because of the electron-deficient nature of the radi-
Scheme 1. Photoredox ATRC reactions.
Table 2. Intermolecular visible-light [3+2] ATRC of 6 with alkenes.[a]
quired for the reaction to proceed to completion. We initiat-
ed the investigation of the visible-light photoredox radical
[3+2] ATRC reaction by using dimethyl-2-(iodomethyl) cy-
clopropane-1,1-dicarboxylate (6) and 2-methyl-2-(vinyloxy)-
propane (7a) as the substrates, hoping to find more practical
and greener reaction conditions. The results are summarized
in Table 1. Among the light sensitizers investigated, the re-
Entry Alkene
Product
Yield Ratio[c]
[%][b]
1
2
85
65
2.6:1
1:1
Table 1. Optimization of intermolecular visible-light [3+2] ATRC.[a]
3
88
27:1
Entry
Catalyst
Solvent
Yield [%][b]
4
5
48
47
1.8:1
7.8:1
1
2
3
4
2
3
4
5
3
–
3
3
3
3
3
3
3
3
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2/H2O (4:1)
DMF
DMF/H2O (1:1)
toluene
THF
DMSO
3[c]
69
8
39
0
5[d]
6
5[c]
0
6
22
–
7[e]
8[f]
9
55
85
74
73
1
7
8
13
18
8:1
10
11
12
13
14
n.d.
3
73
[a] Reaction conditions: 6 (1.0 mmol), alkene 7 (2.0 mmol), [IrACHTUNGTRENNUNG(ppy)3]
(1 mol%), DIEA (1.0 equiv), CH2Cl2 (3.2 mL), H2O (0.8 mL), 14 W CFL
lamp irradiation for 16 h. [b] Isolated yield. [c] Ratios were determined
by GC analysis.
[a] Reaction conditions: 6 (0.5 mmol), 7a (1.0 mmol), catalyst (1 mol%),
DIEA (1.0 equiv), solvent (4 mL), 14 W CFL lamp irradiation for 16 h.
[b] Isolated yield. [c] GC yield, dodecane was used as the internal stand-
ard. [d] No DIEA was added. [e] The reaction was carried out in the
dark. [f] 0.2 equivalents of DIEA was used.
cal generated from 6, electron-rich alkenes reacted smoothly
and gave satisfactory yields (Table 2, entries 1–3). Alkenes
with TMS groups provided lower yields, presumably due to
desilylation side reactions (entries 4 and 5). This reaction
did not proceed well with less electron-rich alkenes. The re-
actions of 2-ethyl-1-butene (7 f) and 1-hexene only gave 22
and 13% isolated yields, respectively (entries 6 and 7). Al-
though the reaction of hex-5-en-1-ol afforded 18% of the
cyclization product (entry 8), no protection of the hydroxyl
group was necessary, which showed one of the advantages
of these radical reactions.
action with [Ir
highest yield (69%; Table 1, entries 1–4). Control experi-
ments showed that [Ir(ppy)3], visible-light, and N,N-di-
ACHTUNGTRENNUNG(ppy)3] (ppy=2,2’-bipyridine) (3) afforded the
AHCTUNGTRENNUNG
isopropylethylamine (DIEA) were all essential to the reac-
tion (entries 5–7). Interestingly, substoichiometric DIEA
(0.2 equiv) afforded a 55% yield of the product, which indi-
cated that DIEA was a catalyst in this reaction (entry 8). It
was noted that the solvent has a significant effect on the re-
action (entries 9–14). The reactions in polar solvents afford-
ed higher yields (entries 10, 11, and 14). The best result was
Lewis acids have been reported to enhance the reactivity
of radicals in radical reactions.[19] Coordination of Lewis
acid to the ester carbonyl groups may enhance the electro-
obtained by using [IrACTHNUTRGNEUNG(ppy)3] (1 mol%) as the light sensitizer
Chem. Eur. J. 2013, 00, 0 – 0
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P. Li, Y. Yao et al.
ACTHNUGTRNE(UNG OAc)2-promoted [3+2] ATRC of 6 with alkenes and
Table 4. Zn
alkynes.[a]
Table 3. Screening of Lewis acids.
Entry Alkene
Product
Yield
[%][b]
Ratio[c]
7.3:1
–
Entry
Lewis acid
Equivalent
Yield [%][a]
1
2
3
4
5
6
7
AlCl3
BF3·Et2O
TiCl4
1
1
1
1
1
1
1
n.r.
1
n.r.
2
1
12
8
1
2
92
86
Ti
G
Zn
A
ZnCl2
MgSO4
8
MgBr2·6H2O
1
8
9
Mg
Yb(OTf)2·4H2O
ZnSO4·2H2O
(OAc)2·4H2O
1
1
1
1
2
2.5
3
35
48
40
58
50
71[c]
56
86[c]
3
4
5
6
78
72
75
78
7.3:1
n.d.
10
11
12
13
14
15
16
G
Zn
Zn
Zn
Zn
Zn
G
E
N
ACHTUNGTRENNUNG
E
2.5
7.0:1
6.7:1
[a] GC yield. [b] The reaction was carried out at 408C. [c] Isolated yield.
n.r. = no reaction.
philicity of the homoallylic radical intermediate toward less
electron-rich alkenes.[20] Therefore, various Lewis acids were
screened in the reaction between 6 and 7 f to achieve great-
er substrate scope (Table 3). Lewis acids, such as AlCl3, BF3,
7
89
6.3:1
8
85
68[d]
38
7.7:1
3.0:1
1.4:1
n.d.
TiCl4, TiACHTUNGTRENNUNG(OiPr)4, and ZnHCATUNGTERN(NUGN OTf)2, almost completely shut
down the radical cyclization (Table 3, entries 1–5). ZnCl2,
MgSO4, and MgBr2 did not help the reaction (entries 6–8).
9
Mg
ACHTUNGTRENNUNG(OAc)2, YbACHTUNGTRENNUNG(OTf)2, and ZnSO4 moderately improved the
reaction (entries 9–11). Among the Lewis acids screened,
ZnACHTUNGTRENNUNG(OAc)2 gave the best results (58%, entry 12). It was
10
11
postulated that Zn2+ might chelate with the bidentate radi-
cal ligand and enhance its reactivity towards less electron-
23[e]
rich alkenes. Investigation of the amount of Zn
ACHTUNGTRENNUNG
in the reaction revealed that 2.5 equivalents of ZnAHCTUNGTRENNUNG
forded the highest yield (71%, entry 14). By raising the re-
action temperature from 25 to 408C, cyclopentane 8 f was
obtained in 86% yield (entry 16).
12
13
83
81
–
With the aid of ZnACHTNUTRGNE(NUG OAc)2, the alkenes that did not per-
26.7:1
form well previously showed significantly enhanced reactivi-
ty. The yields of the reactions of compound 6 with alkenes
7e–h were raised to 92, 86, 78, and 72%, respectively
(Table 4, entries 1–4). Other terminal alkenes with various
functional groups, such as benzyl ether, silyl ether, and
ester, were all well-tolerated (Table 4, entries 5–9). Styrene
gave a moderate yield, probably due to the conjugated p-
system that lowers the electron density on the double bond
(entry 10). Allyl sulfonamide 7o was not a good substrate
for the reaction either (23%, entry 11). Substrates with 1,1-
and 1,2-disubstituted alkenes 7p–r reacted with 6 and
smoothly afforded the corresponding products 8p–r in 83,
81, and 57% yields, respectively (entries 12–14). In addition,
terminal alkynes 7s and 7t underwent cyclization with 6 and
gave cyclopentenes 8s and 8t, albeit in moderate yields (39
and 41%, entries 15 and 16).
14
57[d,e]
11.8:1
15
16
39[f]
41[f]
–
–
[a] Reaction conditions: substrate
(2.0 mmol), [Ir(ppy)3] (1 mol%), DIEA (1.0 equiv), ZnACHTUNGTRENNUNG
(2.5 equiv), CH2Cl2 (3.2 mL), H2O (0.8 mL), 14 W CFL lamp irradiation
for 16 h at 408C. [b] Isolated yield. [c] Ratios were determined by GC
analysis. [d] Reacted for 72 h. [e] 5 equivalents of alkene was used. [f] Re-
acted for 30 h. n.d = not determined.
6
(1.0 mmol), alkene or alkyne
7
G
(OAc)2·2H2O
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Atom-Transfer Radical [3+2]-Cyclization
COMMUNICATION
For ATRA or ATRC reac-
tions, a radical chain reaction
mechanism is typically pro-
posed. To study the radical
propagation, an on/off light-
switching reaction was per-
Scheme 2. Crossover ATRC.
formed. The reaction was
almost stopped when the light
source was turned off, and it
was restarted with the reintroduction of the light source
(Figure 2). This observation was confirmed by several “on/
off” cycles until the reaction was completed. This experi-
ment demonstrates that continuous radical generation under
light irradiation is necessary for the reaction to proceed.
same conditions described above, the addition of LiI to the
reaction of 9 and 7 f led to a mixture of 8 f and 10
(Scheme 2), in support of the formation of the cyclopentyl
carbinyl cationic intermediate shown in Figure 3, path a.
However, the interference of IÀ in the radical initiation step
could not be ruled out. A solid conclusion could not be
drawn at this point.
As illustrated in Figure 3, [IrACHTNUGTRENUNG(ppy)3]* is oxidized to [Ir-
AHCTUNGTRENNUNG
to compound 6 (Eox = ~0.84 V vs. SCE, see the Supporting
Information), generating homoallylic radical 11. Radical 11
then undergoes [3+2]-cyclization with an alkene to provide
radical intermediate 12. At this stage, two pathways are pos-
sible for the conversion of 12 to the final compound 8. In
pathway a, radical 12 was oxidized by [IrACTHNUTRGNEUNG
(ppy)3]+ to give pri-
mary carbon cation 13, which reacted with the iodide anion
to afford 8. In pathway b, a short-lived radical chain propa-
gation is proposed. Radical 12 abstracts an iodine atom
from the starting material 6 to afford 8, and radical 11 is re-
generated. Pathway a is supported by our experimental ob-
servations. However, more conclusive results must be ob-
tained to rule out pathway b.
Figure 2. Time profile of reaction of 6 with 7a. Light was switched off
during the “off” periods.
In conclusion, a nonreductive, intermolecular visible-light
photoredox [3+2] ATRC reaction was developed. In this re-
action, cyclopentane/cyclopentene derivatives were formed
from two structurally simpler molecules through the con-
The on/off light switching experiment indicates that the
reaction may proceed through either a radical/polar cross-
over pathway or by a short-lived radical chain propagation.
Because the halogen atom accepted an electron and leaves
as a halide anion in the visible-light photoredox radical ini-
tiation step, we postulate that the resulting halide ion may
capture the cyclopentyl carbinyl cation, as illustrated in
Figure 3, path a. To confirm this hypothesis, one equivalent
of LiI was added to the reaction of dimethyl 2-allyl-2-bro-
momalonate (9) with 3-methylenepentane (7 f). Under the
À
struction of two new C C bonds. In our study, it was found
that the homoallylic malonate radical was reactive towards
electron-rich alkenes. However, the reactions with unactivat-
ed alkenes were sluggish. ZnACHTNUGRTENUNG(OAc)2 was found to enhance
the reactivity of the radical. The use of Lewis acid expanded
the scope of the alkene substrates that undergo the reaction.
The reaction was carried out in regular lab glassware by
using a household CFL lamp. No special equipment was re-
quired. Compared to existing
methods, the procedure we
have described is operationally
simpler and does not involve
the use of stoichiometric haz-
ardous radical initiators.
In the visible-light-promoted
[3+2] ATRC reaction, unacti-
vated iodoalkanes were success-
fully used as radical precursors.
The nonreductive pathway is
different from the reported visi-
ble-light-mediated radical cycli-
Figure 3. Possible mechanism of the intermolecular visible-light [3+2] ATRC.
zation reactions. Studies dedi-
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P. Li, Y. Yao et al.
M. E. Anzovino, J. Du, T. P. Yoon, J. Am. Chem. Soc. 2008, 130,
12886; j) M. A. Ischay, Z. Lu, T. P. Yoon, J. Am. Chem. Soc. 2010,
132, 8572.
cated toward the elucidation of the nonreductive course of
these reactions are currently underway in our laboratory
and our results will be reported in due course.
[7] a) J. Du, T. P. Yoon, J. Am. Chem. Soc. 2009, 131, 14604; b) Y.-Q.
Zou, S.-W. Duan, X.-G. Meng, X.-Q. Hu, S. Gao, J.-R. Chen, W.-J.
Xiao, Tetrahedron 2012, 68, 6914; c) Z. Lu, T. P. Yoon, Angew.
Chem. Int. Ed. 2012, 51, 10329; d) M. A. Ischay, M. S. Ament, T. P.
Yoon, Chem. Sci. 2012, 3, 2807.
Acknowledgements
[8] a) Z. Lu, M. H. Shen, T. P. Yoon, J. Am. Chem. Soc. 2011, 133, 1162;
b) M. Rueping, D. Leonori, T. Poisson, Chem. Commun. 2011, 47,
9615; c) Y.-Q. Zou, L.-Q. Lu, L. Fu, N.-J. Chang, J. Rong, J.-R.
Chen, W.-J. Xiao, Angew. Chem. 2011, 123, 7309; Angew. Chem. Int.
Ed. 2011, 50, 7171; d) G. Zhao, C. Yang, L. Guo, H. Sun, R. Lin, W.
Xia, J. Org. Chem. 2012, 77, 6302; e) S. Maity, M. Zhu, R. S. Shinab-
ery, N. Zheng, Angew. Chem. Int. Ed. 2012, 51, 222.
[9] a) A. E. Hurtley, M. A. Cismesia, M. A. Ischay, T. P. Yoon, Tetrahe-
dron 2011, 67, 4442; b) S. S. Lin, M. A. Ischay, C. G. Fry, T. P. Yoon,
J. Am. Chem. Soc. 2011, 133, 19350.
Financial support is provided by NSFC (21102097), the Specialized Re-
search Fund for the Doctoral Program of Higher Education
(20103201120006), BNLMS, and the Priority Academic Program Devel-
opment of Jiangsu Higher Education Institutions (PAPD).
Keywords: cyclization · Lewis acids · photochemistry ·
radicals · visible light
[10] J. D. Parrish, M. A. Ischay, Z. Lu, S. Guo, N. R. Peters, T. P. Yoon,
Org. Lett. 2012, 14, 1640.
[1] a) K. C. Majumdar, G. V. Karunakar, B. Sinha, Synthesis 2012, 44,
2475; b) F. Dꢂnꢃs, F. Beaufils, P. Renaud, Synlett 2008, 2389; c) K. C.
Majumdar, P. K. Basu, S. K. Chattopadhyay, Tetrahedron 2006, 62,
793; d) K. C. Majumdar, P. K. Basu, P. P. Mukhopadhyay, Tetrahe-
dron 2005, 61, 10603; e) T. R. Rheault, M. P. Sibi, Synthesis 2003,
803; f) W. R. Bowman, M. O. Cloonan, S. L. Krintel, J. Chem. Soc.
[11] a) J. W. Tucker, J. M. R. Narayanam, S. W. Krabbe, C. R. J. Stephen-
son, Org. Lett. 2010, 12, 368; b) J. D. Nguyen, J. W. Tucker, M. D.
Konieczynska, C. R. J. Stephenson, J. Am. Chem. Soc. 2011, 133,
4160; c) J. D. Nguyen, E. M. D’Amato, J. M. R. Narayanam, C. R. J.
Stephenson, Nat. Chem. 2012, 4, 854; d) H. Kim, C. Lee, Angew.
Chem. Int. Ed. 2012, 51, 12303; e) C.-J. Wallentin, J. D. Nguyen, P.
Finkbeiner, C. R. J. Stephenson, J. Am. Chem. Soc. 2012, 134, 8875.
[12] a) D. A. Nicewicz, D. W. C. MacMillan, Science 2008, 322, 77;
b) J. W. Tucker, C. R. J. Stephenson, Org. Lett. 2011, 13, 5468; c) L.
Furst, B. S. Matsuura, J. M. R. Narayanam, J. W. Tucker, C. R. J. Ste-
phenson, Org. Lett. 2010, 12, 3104.
[13] a) D. B. Freeman, L. Furst, A. G. Condie, C. R. J. Stephenson, Org.
Lett. 2012, 14, 94; b) C. Dai, J. M. R. Narayanam, C. R. J. Stephen-
son, Nat. Chem. 2011, 3, 140; c) D. A. Nagib, M. E. Scott, D. W. C.
MacMillan, J. Am. Chem. Soc. 2009, 131, 10875.
[14] H. W. Shih, M. N. Vander Wal, R. L. Grange, D. W. C. MacMillan, J.
Am. Chem. Soc. 2010, 132, 13600.
Perkin Trans.
1 2001, 2885; g) W. R. Bowman, C. F. Bridge, P.
Brookes, J. Chem. Soc. Perkin Trans. 1 2000, 1; h) M. Julia, Acc.
Chem. Res. 1971, 4, 386; i) M. Julia, Pure Appl. Chem. 1967, 15, 167;
j) K. C. Majumdar, P. K. Basu, P. P. Mukhopadhyay, Tetrahedron
2004, 60, 6239.
[2] a) M. S. Kharasch, W. H. Urry, E. V. Jensen, J. Am. Chem. Soc.
1945, 67, 1626; b) M. S. Kharasch, E. V. Jensen, W. H. Urry, Science
1945, 102, 128; c) M. S. Kharasch, E. V. Jensen, W. H. Urry, J. Am.
Chem. Soc. 1945, 67, 1864.
[3] a) J. M. Munoz-Molina, T. R. Belderrain, P. J. Perez, Eur. J. Inorg.
Chem. 2011, 3155; b) T. Pintauer, Eur. J. Inorg. Chem. 2010, 2449.
[4] a) G. J. Rowlands, Tetrahedron 2010, 66, 1593; b) G. J. Rowlands,
Tetrahedron 2009, 65, 8603.
[15] R. S. Andrews, J. J. Becker, M. R. Gagne, Angew. Chem. 2010, 122,
7432; Angew. Chem. Int. Ed. 2010, 49, 7274.
[16] a) Y. Cheng, X. Gu, P. Li, Org. Lett. 2013, 15, 2664; b) M. E. Budꢂn,
J. F. Guastavino, R. A. Rossi, Org. Lett. 2013, 15, 1174.
[5] For reviews, see: a) J. Xuan, W. J. Xiao, Angew. Chem. Int. Ed. 2012,
51, 6828; b) J. W. Tucker, C. R. J. Stephenson, J. Org. Chem. 2012,
77, 1617; c) M. A. Ischay, T. P. Yoon, Eur. J. Org. Chem. 2012, 3359;
[17] D. P. Curran, M. H. Chen, E. Spletzer, C. M. Seong, C. T. Chang, J.
Am. Chem. Soc. 1989, 111, 8872.
´
d) F. Teply, Collect. Czech. Chem. Commun. 2011, 76, 859;
e) J. M. R. Narayanam, C. R. J. Stephenson, Chem. Soc. Rev. 2011,
40, 102; f) T. P. Yoon, M. A. Ischay, J. N. Du, Nat. Chem. 2010, 2,
527; g) K. Zeitler, Angew. Chem. 2009, 121, 9969; Angew. Chem. Int.
Ed. 2009, 48, 9785; h) C. K. Prier, D. A. Rankic, D. W. C. MacMil-
lan, Chem. Rev. 2013, 10.1021/cr300503r.
[18] a) O. Kitagawa, Y. Yamada, H. Fujiwara, T. Taguchi, J. Org. Chem.
2002, 67, 922; b) O. Kitagawa, Y. Yamada, A. Sugawara, T. Taguchi,
Org. Lett. 2002, 4, 1011; c) O. Kitagawa, H. Fujiwara, T. Taguchi,
Tetrahedron Lett. 2001, 42, 2165; d) O. Kitagawa, Y. Yamada, H. Fu-
jiwara, T. Taguchi, Angew. Chem. 2001, 113, 3983; Angew. Chem.
Int. Ed. 2001, 40, 3865; e) O. Kitagawa, S. Miyaji, C. Sakuma, T. Ta-
guchi, J. Org. Chem. 2004, 69, 2607.
[19] P. Renaud, M. Gerster, Angew. Chem. 1998, 110, 2704; Angew.
Chem. Int. Ed. 1998, 37, 2562.
[20] C. L. Mero, N. A. Porter, J. Am. Chem. Soc. 1999, 121, 5155.
[6] a) J. Xuan, Z.-J. Feng, S.-W. Duan, W.-J. Xiao, RSC Adv. 2012, 2,
4065; b) P. Kohls, D. Jadhav, G. Pandey, O. Reiser, Org. Lett. 2012,
14, 672; c) X. Ju, Y. Liang, P. Jia, W. Li, W. Yu, Org. Biomol. Chem.
2012, 10, 498; d) J. Xuan, Y. Cheng, J. An, L.-Q. Lu, X.-X. Zhang,
W.-J. Xiao, Chem. Commun. 2011, 47, 8337; e) J. W. Tucker, J. D.
Nguyen, J. M. R. Narayanam, S. W. Krabbe, C. R. J. Stephenson,
Chem. Commun. 2010, 46, 4985; f) J. Du, L. R. Espelt, I. A. Guzei,
T. P. Yoon, Chem. Sci. 2011, 2, 2115; g) S. Lin, C. E. Padilla, M. A.
Ischay, T. P. Yoon, Tetrahedron Lett. 2012, 53, 3073; h) E. L. Tyson,
E. P. Farney, T. P. Yoon, Org. Lett. 2012, 14, 1110; i) M. A. Ischay,
Received: May 20, 2013
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