Iron-catalyzed C-H fuctionalization of indoles

Article abstract of DOI:10.1002/adsc.201100334

An easily available iron catalyst was developed to accomplish the C-H functionalization of indoles with α-aryl-α-diazoesters in high yields under mild conditions. The asymmetric C-H functionalization of indoles was also realized by using iron complexes of

Full text of DOI:10.1002/adsc.201100334

COMMUNICATIONS
DOI: 10.1002/adsc.201100334
À
Iron-Catalyzed C H Fuctionalization of Indoles
Yan Cai,a Shou-Fei Zhu,a Guo-Peng Wang,a and Qi-Lin Zhoua,*
a
State Key Laboratory and Institute of Elemento-organic Chemistry, Nankai University, Tianjin 300071, Peopleꢀs Republic
of China
Fax : (+86)-22-2350-0011; e-mail: qlzhou@nankai.edu.cn
Received: April 28, 2011; Revised: July 9, 2011; Published online: November 7, 2011
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adcs.201100334.
Abstract: An easily available iron catalyst was de-
veloped to accomplish the C H functionalization of
including rhodium, copper, and ruthenium have been
À
À
developed for the C H functionalization of indoles in
indoles with a-aryl-a-diazoesters in high yields
the past decades,[7] only quite recently was a highly
À
À
under mild conditions. The asymmetric C H func-
enantioselective C H functionalization of indoles dis-
tionalization of indoles was also realized by using
iron complexes of chiral spiro bisoxazolines with up
to 78% ee.
closed by Fox and co-workers.[8] With catalytic
amounts of [Rh2ACHTNUTRGNE(NUG S-NTTL)4] {dirhodium(II) tetra-
kis[N-(1,8-naphthaloyl)-(S)-tert-leucinate]}, the a-
alkyl-a-diazoesters reacted with indoles at C-3 to pro-
duce a-alkyl-a-indolylacetates in high yields and high
enantioselectivities (79–99% ee). From a synthetic
point of view, the indole functionalization with a-aryl-
a-diazoesters is a very useful reaction for the con-
struction of a-aryl-a-indolylacetates, which are key in-
termediates for the synthesis of a wide range of bioac-
À
Keywords: C H functionalization; chiral spiro li-
gands; indoles; iron
À
Transition metal-catalyzed C H functionalization by tive compounds. For instance, Pfizerꢀs chiral endothe-
diazo compounds represents one of the most efficient lin antagonists UK-350,926 and UK-350,862
[1]
À
C H bond activation approaches. Thanks to the de- (Figure 1) can be easily prepared via indole function-
velopment of chiral dirhodium(II) catalysts, asymmet- alization with the corresponding a-aryl-a-diazoest-
ric C H functionalization of a variety of sp hybrid ers.[9] However, when Davies et al.[10] tried the C H
3
À
À
À
C H bonds by metal carbenes derived from diazo functionalization of 1,2-dimethylindole with methyl a-
compounds has become a reliable and widely applied phenyl-a-diazoacetate by using the catalyst [Rh
C
method in organic synthesis nowadays.[2] Although DOSP)4] [rhodium(II) (S)-N-(dodecyl-benzenesul-
2
À
the sp hybrid C H functionalization has been exten-
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
sively investigated,[1a] only a few asymmetric sp C H
2
À
À
functionalization reactions were documented. In 1995, catalysts for C H functionalization of indoles with a-
Hashimoto et al.[3] reported an intramolecular asym- aryl-a-diazoesters are still highly desired.
À
metric phenyl ring C H functionalization via desym-
metrization by using the catalyst [Rh
N
[rhodium(II) N-phthaloyl-(S)-tert-leucinate] with 88–
98% ee. Schmaltz[4] and Merlic[5] utilized copper- and
2
À
rhodium-catalyzed asymmetric sp C H functionaliza-
tion reactions to prepare planar-chiral ferrocenes and
(h6-arene)tricarbonylchromium complexes in good
enantioselectivities (78% and 90% ee, respectively).
Since the indole ring is probably the most ubiqui-
tous heterocycle in nature, the development of ap-
proaches for the functionalization of indoles has at-
tracted extensive attention recently.[6] Transition
À
metal-catalyzed C H functionalization of indoles pro-
vides one of the most efficient methods for the syn-
Figure 1. Chiral endothelin antagonists UK-350,926 and
thesis of indole derivatives. Although several catalysts UK-350,862.
Adv. Synth. Catal. 2011, 353, 2939 – 2944
ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2939

Yan Cai et al.
COMMUNICATIONS
À
Table 1. Iron-catalyzed C H functionalization of N-benzyl-
(TMEDA) was introduced as a ligand, the yield of
the reaction significantly increased to 96% (entry 9).
Besides DCE, CHCl3 and toluene were also suitable
solvents for the reaction in terms of reaction rate and
yield (entries 10 and 11). On the contrary, the coordi-
nating solvents THF and CH3CN dramatically slowed
down the reaction and gave very low yields (entries 12
and 13). The reaction can also be performed at room
temperature albeit a slightly longer reaction time was
indole with a-diazo-a-phenylacetate.[a]
Entry [Fe]
Ligand
Solvent Time [h] Yield [%][b]
needed (entry 14). The catalyst FeACHTNURTGNE(UNG ClO4)2/TMEDA
showed high activity, with 1 mol% being sufficient for
achieving high yield (entry 15). This result demon-
strates that the iron catalyst is comparable with rhodi-
1
2
3
4
5
6
FeCl2
FeCl3
Fe
Fe(OAc)2 none
Fe(ClO4)3 none
Fe(BF4)2 none
Fe(ClO4)2 none
(ClO4)2 none
G
U
E
R
none
none
(acac)2 none
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
30
30
30
30
6
4
4
3
3
10
5
G
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
R
N
T
NR[c]
45
50
54
60
72
96
92
90
15
10
96
93
À
um and copper catalysts for the C H functionaliza-
tion of indoles.[7]
Under the optimal reaction conditions, various a-
7
À
aryl-a-diazoacetates were examined in the C H func-
8[d]
9[d]
Fe
Fe
tionalization of 1-benzylindole (2a). All tested a-aryl-
a-diazoacetates with different substituents on the
phenyl ring gave the desired products in high to excel-
lent yields (85–95%) (Table 2, entries 2–10). Diazo
compounds 1h and 1i with a 2-substituted phenyl
group required a higher catalyst loading for obtaining
satisfactory yields (entries 8 and 9). Additionally,
methyl a-naphthyl-a-diazoacetate (1k) exhibited a
similar reactivity to that of 1a, affording the corre-
sponding product in 92% yield (entry 11). The sub-
stituent effect on the indole ring was then investigat-
ed. An electron-withdrawing group at the 5-position
of indole had a negligible effect on the reaction and
the corresponding products were isolated in high
yields (entries 12–14). On the contrary, an electron-
10[d] Fe
11[d] Fe
12[d] Fe
13[d] Fe
14[d,e] Fe
15[d,f] Fe
4
4
48
ACHTUNGTRENNUNG(ClO4)2 TMEDA CH3CN 48
A
ACHTUNGTRENNUNG
10
5
[a]
Reaction conditions: [Fe]/ligand/1a/2a=0.015/0.018/0.36/
0.3 (mmol), in 5 mL of solvent at 608C.
Isolated yield.
[b]
[c]
[d]
[e]
[f]
No reaction.
6 mol% NaBArF was added.
The reaction was performed at 258C.
Using 1 mol% of catalyst.
As part of our continuing efforts on transition donating group at C-2, C-3, C-4, and C-5 of the indole
metal-catalyzed carbene transformations,[11] we herein significantly reduced the reaction rate and yield, and
À
report an iron-catalyzed C H functionalization of in- a higher reaction temperature was required for full
doles with a-aryl-a-diazoesters. An easily available conversion (entries 15–18). For 3-methyl-substituted
À
À
iron catalyst was developed to accomplish the C H indole 2g, the C H functionalization took place at
functionalization of indoles in high yields. The asym- C-2 of the indole ring to give the corresponding prod-
À
metric C H functionalization of indoles was also real- uct in 72% yield (entry 17). Besides the a-aryl-a-diaz-
ized by using iron complexes of chiral spiro bisoxazo- oacetates, a-diazopropionate and a-diazoacetate,
lines with up to 78% ee.
which contain a-methyl and a-H moieties, respective-
À
In the initial studies, the reaction of 1-benzylindole ly, were also studied. Both of them underwent the C
(2a) with methyl a-diazophenylacetate (1a) was per- H functionalization under standard reaction condi-
formed in 1,2-dichloroethane (DCE) at 608C in the tions, however, the competitive carbene dimerization
presence of 5 mol% iron catalyst. Firstly, various iron reaction became significant and the yields for desired
catalysts were evaluated (Table 1). Except for products were only moderate (42% and 36%, respec-
Fe
ACHTUNGTRENNUNG(acac)2, all the tested Fe(II) and FeACHTUNGTRENNUNG
À
moted the reaction to give the desired product,
methyl 2-(1-benzylindol-3-yl)-2-phenylacetate (3aa), functionalization of indoles was then tried. We first
in low to moderate yields (Table 1, entries 1–7). The investigated chiral spirobisoxazoline ligands (Table 3,
ionic catalysts Fe
forded higher yields (entries 5–7). With Fe
catalyst precursor, the addition of NaBArF {sodium sertion reactions.[11] We were delighted to find that
tetrakis[3,5-bis(trifluoromethyl)phenyl]borate}, which the ligand (Ra,S,S)-4a containing phenyl groups on the
ACHTUGNTRNE(NUGN OAc)2, FeACHTNUTRGEG(NNUN BF4)2 and FeACTHUNGTRENNUNG
AHCTUNGTRENNUNG
ACHTUNGTRENNUNG
À
provided a bulky and non-coordinating anion, slightly oxazoline rings afforded the C H functionalization
improved the reaction rate and yield (entry 8). To our product in high yield (90%) with moderate enantiose-
delight, when N,N,N’,N’-tetramethylethylenediamine lectivity (61% ee) in the reaction of 1a with indole 2a
2940
asc.wiley-vch.de
ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2011, 353, 2939 – 2944

À
Iron-Catalyzed C H Fuctionalization of Indoles
[a]
À
Table 2. Iron-catalyzed C H functionalization of indoles with a-aryl-a-diazoesters.
Entry
Ar (1)
R (2)
3
Time [h]
Yield [%]
1
2
3
4
5
6
7
C6H5 (1a)
H (2a)
H (2a)
H (2a)
H (2a)
H (2a)
H (2a)
H (2a)
H (2a)
H (2a)
H (2a)
H (2a)
5-F (2b)
5-Cl (2c)
5-Br (2d)
5-Me (2e)
4-BnO (2f)
3-Me (2g)
2-Me (2h)
3aa
3ba
3ca
3da
3ea
3fa
3ga
3ha
3ia
5
5
4
2
7
6
5
5
6
1
6
4
4
5
15
18
12
14
93
95
95
93
90
93
93
92
85
88
92
91
93
91
85
74
72
82
4-Br-C6H4 (1b)
4-Me-C6H4 (1c)
4-MeO-C6H4 (1d)
3-Br-C6H4 (1e)
3-Me-C6H4 (1f)
3-MeO-C6H4 (1g)
2-Cl-C6H4 (1h)
2-Me-C6H4 (1i)
3,4-(MeO)2C6H3 (1j)
2-naphthyl (1k)
C6H5 (1a)
C6H5 (1a)
C6H5 (1a)
C6H5 (1a)
C6H5 (1a)
C6H5 (1a)
C6H5 (1a)
8[b]
9[b,c]
10
11
12
13
14
15[d]
16[d]
17[d,e]
18[d]
3ja
3ka
3ab
3ac
3ad
3ae
3af
3ag
3ah
[a]
Reaction conditions and analysis: the same as those of Table 1, entry 15.
5 mol% of catalyst was used.
The reaction was performed at 408C.
The reaction was performed at 808C.
The reaction took place at C-2 of indole.
[b]
[c]
[d]
[e]
[a]
À
Table 3. Iron-catalyzed asymmetric C H functionalization of indoles: optimization of reaction conditions.
Entry
Ligand
[M]
R1
Time [h]
Yield [%][b]
ee [%][c]
1
2
3
4
5
6
7
8
N
Fe
Fe
Fe
Fe
Fe
CuCl
AgOTf
AuCl
G
Bn (2a)
Bn (2a)
Bn (2a)
Bn (2a)
Bn (2a)
Bn (2a)
Bn (2a)
Bn (2a)
Bn (2a)
Me (2i)
TBS (2j)[e]
15
18
35
48
48
1
20
48
20
30
8
50
90
70
À26
61
16
12
–
41
29
49
27
60
73
24
NR[d]
70
74
24
49
90
9
10
11
CoCl2
Fe
Fe
92
[a]
Reaction conditions: [M]/4/NaBArF/1a/2=0.015/0.018/0.018/0.3/0.3 (mmol), in 5 mL CHCl3 at 408C.
Isolated yield.
Determined by SFC using a chiral column.
No reaction.
TBS=tert-butyldimethylsilyl.
[b]
[c]
[d]
[e]
Adv. Synth. Catal. 2011, 353, 2939 – 2944
ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
2941

Yan Cai et al.
COMMUNICATIONS
[a]
À
Table 4. Iron-catalyzed asymmetric C H functionalization of indoles with a-aryl-a-diazoesters.
Entry
Ar (1)
R (2)
3
Time [h]
Yield [%]
ee [%]
1
2
3
4
5
6
7
8
C6H5 (1a)
H (2j)
H (2j)
H (2j)
H (2j)
H (2j)
H (2j)
H (2j)
H (2j)
H (2j)
H (2j)
H (2j)
H (2j)
Cl (2k)
Me (2l)
3aj
3lj
8
4
4
8
8
4
3
4
24
30
10
24
6
92
89
86
86
86
88
92
90
76
60
86
70
80
94
73
71
65
63
67
71
78
76
60
39
72
65
73
60
4-Cl-C6H4 (1l)
4-Br-C6H4 (1b)
4-Me-C6H4 (1c)
3-F-C6H4 (1m)
3-Cl-C6H4 (1n)
3-Br-C6H4 (1e)
3-CF3-C6H4 (1o)
3-MeO-C6H4 (1g)
2-Me-C6H4 (1i)
3,4-Cl2C6H3 (1p)
2-naphthyl (1k)
C6H5 (1a)
3bj
3cj
3mj
3nj
3ej
3oj
3gj
3ij
3pj
3kj
3ak
3al
9
10
11
12
13
14
C6H5 (1a)
8
[a]
Reaction conditions and analysis: the same as those of Table 3, entry 11.
(entry 2). When the reaction was performed on a However, the reaction of methyl a-(2-chlorophenyl)-
double scale (0.6 mmol), the same levels of reactivity a-diazoacetate (1h) yielded a racemic product in 76%
and enantioselectivity (18 h, 91%, 61% ee) were ob- yield. The indoles with 5-Cl (2k) and 5-Me (2l) sub-
served. No epimerization of the product 3aa was ob- stituents also underwent the reaction smoothly with
served after stirring for 12 hour under the reaction high yields and good enantioselectivities (entries 13
conditions, indicating that the stereochemistry of the and 14). It is worthy of mention that most of the
À
C H functionalization products of indoles is stable products are solid and can be easily purified by re-
under the reaction conditions. Other chiral bisoxazo- crystallization. For instance, the ee value of product
line ligands such as BOX and Pybox were less effi- 3lj was enhanced to 99% after once recrystallization
[12]
À
cient in this C H functionalization reaction. Other from petroleum ether. The absolute configuration of
transition metals including copper, silver, gold, and 3lj was determined to be S by means of single crystal
cobalt were also evaluated and lower enantioselecti- X-ray diffraction analysis (see Supporting Information
vies were obtained (entries 6–9). The N-protecting for details).[13]
group of indole strongly affected the enantioselectivi-
ty of the reaction, with N-TBS (tert-butyldimethylsil- C H functionalization of indoles is proposed, which is
yl)-indole (2j) giving the best result (entry 11). analogous with that of rhodium- and copper-catalyzed
A plausible mechanism for present iron-catalyzed
À
The substrate scope of the reaction was studied. reaction (Scheme 1).[7] The diazoester was decom-
For all tested a-aryl-a-diazoacetates with para- or posed by iron catalyst to generate an iron carbene
meta-substituted phenyl ring, high yields (76–92%) (step a), which reacted with indole to form a metal-
and moderate to good enantioselectivities (60–78% associated zwitterionic intermediate I (step b). A
ee) were obtained (Table 4, entries 2–9). The diazoest- proton migration from C-3 of the indole to the a-posi-
ers with an electron-withdrawing group on the phenyl tion of the ester took place to give the product and
ring gave higher enantioselectivity, with methyl a-(3- regenerate the iron catalyst (step c). The chiral induc-
bromophenyl)-a-diazoacetate (1e) showing the high- tion is realized via an enantioselective proton migra-
est enantioselectivity (78% ee) (entry 7). The reaction tion. A study on the kinetic isotope effect (KIE=kH/
was quite sensitive to the steric hindrance of the sub- kD) of the C-3 deuterium-labeled indole by competi-
strate. The substitution at the ortho-position of the tive experiments was performed (see Supporting In-
phenyl ring of the substrate dramatically slowed down formation for details) and a KIE value of 5.06 was ob-
the reaction and provided a low yield and low enan- served, which was comparable with that in the rhodi-
À
tioselectivity. For example, the reaction of methyl a- um-catalyzed functionalization of aromatic C H
(2-methylphenyl)-a-diazoacetate (1i) afforded the bonds.[14] The significant first order KIE value means
product 3ij in only 60% yield with 39% ee (entry 10). that the proton transfer is most likely the rate-deter-
2942
asc.wiley-vch.de
ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2011, 353, 2939 – 2944

À
Iron-Catalyzed C H Fuctionalization of Indoles
Scheme 1. Proposed mechanism.
mining step. Alternative mechanisms via a cyclopro- Experimental Section
À
panation-ring opening sequence or a concerted C H
À
insertion cannot be excluded at this stage. Additional- General Procedure for the Fe-Catalyzed C H
ly, the possibility that iron(II) activates the a-aryl-a- Functionalization of Indoles
diazoacetate by association with its terminal nitrogen,
FeACHTUNGTRNE(UNG ClO4)2·4H2O (2.5 mg, 0.0075 mmol), NaBArF (8.5 mg,
which does not form an iron carbene intermediate, is
also plausible. Further studies to identify the accurate
mechanism are undergoing in this laboratory.
In summary, we have developed the first iron-cata-
lyzed C H functionalization of indoles with a-aryl-a-
diazoacetates, which provided an efficient method for
the construction of useful a-aryl-a-indolylacetate de-
rivatives. The asymmetric version of this reaction was
also accomplished with up to good enantioselectivities
by using iron complexes of chiral spiro bisoxazoline li-
gands. This study further indicates a high potential for
application of sustainable and environmentally benign
iron catalysts in carbene transformations.
0.009 mmol, 1.2 mol%), and 2 mL of 1,2-dichloroethane
(DCE) were introduced into a Schlenk tube under argon.
1 mL of solution of TMEDA (0.009M, 0.009 mmol,
1.2 mol%) in DCE was added. The resulting suspension was
stirred under an argon atmosphere at room temperature for
À
4 h.
A solution of methyl a-diazophenylacetate (1a)
(158.4 mg, 0.9 mmol) and 1-benzylindole (2a) (155.3 mg,
0.75 mmol) in DCE (2 mL) was injected into the reaction
mixture at 608C. The resulting mixture was stirred at 608C
for the specified time, and the solvent was evaporated under
reduced pressure. The crude product was purified by flash
chromatography on silica gel with petroleum ether/ethyl
acetate (10:1, v/v).
General Procedure for the Fe-Catalyzed Asymmetric
À
C H Functionalization of Indoles
FeACHTUNGTRNE(NUG ClO4)2·4H2O (5.0 mg, 0.015 mmol), (Ra,S,S)-4a (9.2 mg,
0.018 mmol, 6 mol%), NaBArF (16.9 mg, 0.018 mmol,
6 mol%), and 3 mL of CHCl3 were introduced into a
Adv. Synth. Catal. 2011, 353, 2939 – 2944
ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
2943

Yan Cai et al.
COMMUNICATIONS
Schlenk tube under argon. The resulting suspension was
stirred under an argon atmosphere at room temperature for
Melloni, S. Tommasi, A. Umani-Ronchi, Synlett 2005,
1199.
4 h.
A solution of methyl a-diazophenylacetate (1a)
[7] For representative examples, see: Rh a) J. L. Wood,
B. M. Stoltz, H. J. Dietrich, D. A. Pflum, J. Am. Chem.
Soc. 1997, 119, 9641; b) M. Salim, A. Capretta, Tetrahe-
dron 2000, 56, 8063; c) R. Gibe, M. A. Kerr, J. Org.
Chem. 2002, 67, 6247; d) S. Muthusamy, C. Gunana-
than, S. A. Babu, Chem. Commun. 2002, 824; e) Y. J.
Lian, H. M. L. Davies, Org. Lett. 2010, 12, 924; Cu
f) M. B. Johansen, M. A. Kerr, Org. Lett. 2010, 12,
4956; Ru g) W. W. Chan, S. H. Yeung, W. Y. Yu, Org.
Lett. 2010, 12, 604.
(52.8 mg, 0.3 mmol) and 1-(tert-butyldimethylsilyl)indole
(2j) (69.3 mg, 0.3 mmol) in 2 mL CHCl3 was injected into
the reaction mixture at 408C. The resulting mixture was
stirred at 408C for the specified time, and the solvent was
evaporated under reduced pressure. The crude product was
purified by flash chromatography on silica gel with petrole-
um ether/ethyl acetate (20:1, v/v).
[8] A. DeAngelis, V. W. Shurtleff, O. Dmitrenko, J. M. Fox,
J. Am. Chem. Soc. 2011, 133, 1650.
Acknowledgements
[9] a) D. J. Rawson, K. N. Dack, R. P. Dickinson, K. James,
Bioorg. Med. Chem. Lett. 2002, 12, 125; b) C. P. Ash-
croft, S. Challenger, D. Clifford, A. M. Derrick, Y. Ha-
jikarimian, K. Slucock, T. V. Silk, N. M. Thomson, J. R.
Williams, Org. Process Res. Dev. 2005, 9, 663.
[10] Y. J. Lian, H. M. L. Davies, J. Am. Chem. Soc. 2010,
132, 440.
We thank the National Natural Science Foundation of China,
and the National Basic Research Program of China (973
Program, No.2010CB833300, 2011CB808600), and “111”
Project of the Ministry of Education of China (Grant No.
B06005) for financial support.
À
[11] N H insertion: a) B. Liu, S.-F. Zhu, W. Zhang, C.
Chen, Q.-L. Zhou, J. Am. Chem. Soc. 2007, 129, 5834;
References
À
O H insertions: b) C. Chen, S.-F. Zhu, B. Liu, L.-X.
Wang, Q.-L. Zhou, J. Am. Chem. Soc. 2007, 129, 12616;
c) S.-F. Zhu, C. Chen, Y. Cai, O.-L. Zhou, Angew.
Chem. 2008, 120, 946; Angew. Chem. Int. Ed. 2008, 47,
932; d) S.-F. Zhu, Y. Cai, H.-X. Mao, J.-H. Xie, Q.-L.
Zhou, Nature Chem. 2010, 2, 546; e) S.-F. Zhu, X.-G.
Song, Y. Li, Y. Cai, Q.-L. Zhou, J. Am. Chem. Soc.
[1] For reviews, see: a) M. P. Doyle, M. A. McKervey, T.
Ye, Modern Catalytic Methods for Organic Synthesis
with Diazo Compounds, Chaps. 3 and 6.3, Wiley, New
York, 1998; b) R. Giri, B.-F. Shi, K. M. Engle, N.
Maugel, J.-Q. Yu, Chem. Soc. Rev. 2009, 38, 3242; c) K.
Godula, D. Sames, Science 2006, 312, 67.
[2] For reviews, see: a) M. P. Doyle, R. Duffy, M. Ratnikov,
L. Zhou, Chem. Rev. 2010, 110, 704; b) H. M. L.
Davies, J. R. Manning, Nature 2008, 451, 417;
c) H. M. L. Davies, R. E. J. Beckwith, Chem. Rev. 2003,
103, 2861.
[3] N. Watanabe, Y. Ohtake, S.-i. Hashimoto, M. Shiro, S.
Ikegami, Tetrahedron Lett. 1995, 36, 1491.
[4] S. Siegel, H.-G. Schmalz, Angew. Chem. 1997, 109,
2569; Angew. Chem. Int. Ed. Engl. 1997, 36, 2456.
[5] C. A. Merlic, A. L. Zechman, M. M. Miller, J. Am.
Chem. Soc. 2001, 123, 11101.
À
2010, 132, 16374; S H insertion: f) Y.-Z. Zhang, S.-F.
Zhu, Y. Cai, H.-X. Mao, Q.-L. Zhou, Chem. Commun.
2009, 5362; Si H insertion: g) Y.-Z. Zhang, S.-F. Zhu,
L.-X. Wang, Q.-L. Zhou, Angew. Chem. 2008, 120,
8624; Angew. Chem. Int. Ed. 2008, 47, 8496.
À
[12]
(S,S)-Box
[2,2’-(1-methylethyliene)bis(4,5-dihydro-4-
phenyloxazole)]: 12 h, 72%, 10% ee; (S,S)-Pybox [2,6-
bis(4,5-dihydro-4-phenyloxazole)pyridine]: 48 h, 64%,
racemic.
[13] CCDC 821999 contains the supplementary crystallo-
graphic data for 3lj. These data can be obtained free of
charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
[14] S. Babu, M. D. Hrytsak, T. Durst, Can. J. Chem. 1989,
67, 1071.
[6] For reviews, see: a) R. J. Sundberg, Indoles, Academic
Press, London, UK, 1996; b) G. R. Humphrey, J. T.
Kuethe, Chem. Rev. 2006, 106, 2875; c) S. Cacchi, G.
Fabrizi, Chem. Rev. 2005, 105, 2873; d) M. Bandini, A.
2944
asc.wiley-vch.de
ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2011, 353, 2939 – 2944