Yb(OTf)3-catalyzed construction of indole derivatives through formal [3+3] cycloaddition of 1, 1-vinylidenecyclopropanediesters with nitrones

Article abstract of DOI:10.1002/chem.200902510

"Camical equation presented" Indulging in indoles: Indole derivatives are efficiently afforded through Yb(OTf)₃-catalyzed formal [3 + 3] cycloaddition of 1, 1-vinylidenecyclopro-panediesters with nitrones in moderate to good yields under mild r

Full text of DOI:10.1002/chem.200902510

COMMUNICATION
DOI: 10.1002/chem.200902510
Yb
ACHTUNGTRENNUNG(OTf)3-Catalyzed Construction of Indole Derivatives through Formal
ACHTUNGTRENNUNG[3+3] Cycloaddition of 1,1-Vinylidenecyclopropanediesters with Nitrones
Lei Wu and Dr Min Shi*[a]
The 1,3-dipolar cycloaddition reaction is a highly efficient
opening reaction, providing novel cycloadducts.[6] Thus far,
the chemistry of vinylidenecyclopropanes (VDCPs) has
been extensively explored in our laboratory and in other
groups during the last several years. Many novel intramolec-
ular rearrangements or cycloaddition reactions with carbon–
carbon or carbon–heteroatom multiple bonds, such as
imines, aldehydes, nitriles, and a,b-unsaturated ketones,
took place smoothly either upon heating and photoirradia-
tion[7] or catalyzed/mediated by Lewis and Brønsted acids,
giving the corresponding [2+2], [3+2], or [3+3] cycload-
ducts in moderate to good yields under mild conditions.[8–10]
and powerful method for the assembly of a variety of bio-
logically important heterocycles in a convergent fashion
from simple precursors.[1] Of particular interest to us is the
Lewis acid catalyzed cycloaddition reaction of highly strain-
ed small rings with nitrones 2 for the construction of N-con-
taining heterocycles. Thus far, the groups of Brandi and de
Meijere have systematically studied the 1,3-dipolar cycload-
dition of methylenecyclopropanes (MCPs) with nitrones,[2]
affording the exocyclic type of [3+2] addition products in
good yields. Later on, Young and Kerr[3] reported a [3+3]
cycloaddition of donor–acceptor (D-A) type of cyclopro-
To our delight, we found that in the YbACTHNUTRGENUN(G OTf)3-catalyzed re-
pane diesters with nitrones, catalyzed by Yb
mild conditions. Recently, the group of Wang[4] has reported
the first example of Yb(OTf)3-promoted distal [3+3] cyclo-
(OTf)3 under
actions of vinylidenecyclopropanes containing two ester
groups (VDCP diesters) 1[11] with nitrones 2, the proximal
[3+3] cycloaddition products 3 were exclusively obtained
with a high regioselectivity. To the best of our knowledge,
this is the first example of Lewis acid catalyzed proximal
[3+3] cycloaddition of VDCPs with 1,3-dipoles.
AHCTUNGTRENNUNG
addition of MCP diesters with nitrone 1,3-dipoles to give
the N,O-heterocyclic products in good yields. On the basis
of above examples, it is quite clear that the release of highly
strained energy associated with the ring opening of a cyclo-
propane moiety in an organic molecule might lead to multi-
ple transformations, the selectivity of which depends on the
nature and pattern of the substituents on the cyclopropane
ring as well as the adjacent positions.[5] Inspired by these re-
sults, we anticipated that the geminal installation of two
electron-withdrawing groups (EWGs) at the cyclopropane
ring connected with an allene moiety, for example, vinylid-
Initial experiments were carried out by using VDCP di-
AHCTUNGTREGeNNNU ster 1a and nitrone 2a as the substrates in the presence of
a series of Lewis acids to determine the optimal reaction
conditions (please see the Supporting Information for the
details). The typical results are summarized in Table 1. We
found that the reaction could be finished within 10 min at
room temperature and the corresponding product 3aa was
obtained in the yield of 32% using 1a (1.0 equiv) and ni-
A
trone 2a (3.0 equiv) catalyzed by YbACTHNGURETNNU(G OTf)3 (20 mol%) in
bond cleavage and lead to high regioselectivity of the ring-
THF (Table 1, entry 1). Solvent screening showed that the
highest yield was achieved in toluene (Table 1, entries 1–4)
and decreasing the amount of nitrone 2a resulted in a little
higher yield of 53% (Table 1, entry 5). Several commonly
[a] L. Wu, Prof. D. M. Shi
used Lewis acids were also tested and, of these, YbACHTUNGTRENNUNG(OTf)3
State Key Laboratory of Organometallic Chemistry
Shanghai Institute of Organic Chemistry
Chinese Academy of Sciences, 354 Fenglin Lu
Shanghai 200032 (China)
Fax : (+86)21-64166128
E-mail: Mshi@mail.sioc.ac.cn
.
afforded better results than Zn
or Sn(OTf)2 etc. (Table 1, entries 6–9, see the Supporting In-
formation for the details). Decreasing the amount of Yb-
(OTf)3 to 10 mol%, the similar result was obtained without
ACHTUTGNRNENUG(OTf)2, ScCAHTUNGTERN(NUGN OTf)3, BF3 Et2O,
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
the distinct diminishment of the yield (Table 1, entry 10).
The highest yield of 72% was achieved when the reaction
was conducted in toluene at room temperature using a
excess amount of VDCP 1a (1.5 equiv) and nitrone 2a
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200902510. It contains 13C and
1H NMR spectroscopic and analytic data for 1, 3 and 4 and X-ray
crystal data of 3ae and 4aa.
Chem. Eur. J. 2010, 16, 1149 – 1152
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1149

Table 1. Optimization of Lewis acid catalyzed reaction conditions of
1d with an aliphatic nPr group connected to the carbon
atom of the allene moiety led to low yields under the same
reaction conditions. To improve yields, we optimized the re-
action conditions again with respect to substrates 1b and
1d. After variation of reaction temperature and time, the
moderate yields of the cycloadducts were obtained (Table 2,
entries 5, 6, and 9).
The structure of the compound 3ae was established by
single-crystal X-ray diffraction (Figure 1; CIF data are pre-
sented in the Supporting Information).[12]
VDCP-diester 1a with nitrone 2a.[a]
1a:2a
Cat. ([mol%])
Additive
Solvent
Yield 3aa [%][b]
1
2
3
4
5
6
7
8
9
1.0:3.0
1.0:3.0
1.0:3.0
1.0:3.0
1.0:1.0
1.0:1.0
1.0:1.0
1.0:1.0
1.0:1.0
1.0:1.0
1.5:1.0
Yb
Yb
Yb
Yb
Yb
N
–
–
–
–
–
–
–
–
THF
32
29
50
CH2Cl2
toluene
MeOH
toluene
toluene
toluene
toluene
toluene
toluene
toluene
decomposed
53
Zn
Sc
N
30
25
U
.
BF3 Et2O (20)
Sn(OTf)2 (20)
NR[c]
NR[c]
51
G
–
–
10
11
Yb
Yb
4 ꢁ MS
72
[a] All the reactions were carried out with VDCP diester 1a and nitrone
2a in solvent catalyzed by Lewis acid at RT. [b] Isolated yield. [c] NR=
no reaction.
(1.0 equiv) catalyzed by YbACHTNUTRGENNG(U OTf)3 (10 mol%) with the addi-
tion of 4 ꢁ molecular sieves (50 mg for 0.2 mmol of nitrone)
(Table 1, entry 11).
With these optimal reaction conditions identified, the
scope of the reaction was investigated further with respect
to various VDCP diesters 1 and nitrones 2. The results are
summarized in Table 2. As for VDCP diesters 1a and 1c
with an electron-deficient substituent (Cl) on the aryl ring,
the reactions proceeded smoothly to give 3 in moderate to
good yields under optimized conditions, irrespective of the
electronic nature of the aryl rings on the C,N-diarylnitrones
(Table 2, entries 1–4, 7, and 8). However, VDCPs 1b with a
moderately electron-rich methyl group on the aryl ring and
Figure 1. X-ray crystal structure of 3ae.
Table 2. Yb
G
2.[a]
Interestingly, all the products 3 from the formal [3+3] cy-
cloaddition of VDCP diesters and nitrones could undergo
ring closure to afford the indole derivatives 4 in good yields
under mild reaction conditions in the presence of trifluoro-
methanesulfonic acid CF3SO3H (HOTf) (1.5 equiv)
(Table 3), thus opening up a new route to synthesize indole
derivatives. The structure of the compound 4aa was charac-
terized by spectroscopic analysis and confirmed by single-
crystal X-ray diffraction (Figure 2; CIF data are given in the
Supporting Information).[12]
Although the mechanism of this reaction is still under in-
vestigation, a plausible mechanism involving a formal [3+3]
cycloaddition pathway is proposed in Scheme 1 to rational-
ize the reaction. The strained bonds in the cyclopropane
ring have long been considered to have a significant p char-
acter[13] and in 1,1-cyclopropane diesters, this bond can be
polarized and further weakened by coordination of a Lewis
acid to one or both of the ester moieties.[3] Thus, initially
Ar1
Ar2
Ar3
Product,
Yield [%][b]
1
2
3
4
1a Ph
1a Ph
1a Ph
1a Ph
2b Ph
p-ClC6H4
p-MeOC6H4 3ac, 62
Ph
3ab, 66
2c
2d p-BrC6H4
2e p-MeOC6H4 Ph
Ph
6[d] 1b p-MeC6H4 2e p-MeOC6H4 Ph
Ph
3ad, 68
3ae, 60
3ba, 61
3be, 60
3ca, 66
3cd, 65
3da, 42
5[c] 1b p-MeC6H4 2a Ph
7
8
1c
1c
p-ClC6H4
p-ClC6H4
2a Ph
2d p-BrC6H4
2a Ph
Ph
Ph
Ph
9[e] 1d nPr
[a] All reactions were conducted by adding VDCP diesters 1 (0.3 mmol,
1.5 equiv) in toluene (6.0 mL) to the solution of nitrones 2 (0.2 mmol,
1.0 equiv), YbACHTUNGTRENNUNG(OTf)3 (10 mol%) and 4 ꢁ MS (50 mg) in toluene (2.0 mL)
at RT unless otherwise specified. [b] Isolated yield. [c] The reaction was
conducted at 08C. [d] The reaction was conducted at À208C. [e] The re-
action mixtures were heated at 808C for 20 min.
VDCP diester 1a is activated by YbACTHNUTRGNENUG(OTf)3, forming a com-
1150
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Chem. Eur. J. 2010, 16, 1149 – 1152

Indole Chemistry
COMMUNICATION
Table 3. Translation of compounds 3 mediated by HOTf to indole deriva-
tives 4.[a]
Ar1
R2
Ar3
Product,
Yield [%][b]
1
2
3
4
5
6
7
8
9
3aa
3ab
3ac
3ad
3ae
3ba
3be
3ca
3 cd
Ph
Ph
Ph
Ph
H
H
H
Br
MeO
H
MeO
H
Ph
p-ClC6H4
p-MeOC6H4
Ph
Ph
Ph
Ph
Ph
Ph
4aa, 83
4ab, 82
4ac, 77
4ad, 80
4ae, 85
4ba, 81
4be, 82
4ca, 80
4cd, 79
Ph
p-MeC6H4
p-MeC6H4
p-ClC6H4
p-ClC6H4
Br
[a] All reactions were conducted with the substrates 3 (0.2 mmol) in
CH2Cl2 (5.0 mL) cooled in ice water for a while before adding HOTf
(1.5 equiv) and then warmed to RT to continue the reaction. [b] Isolated
yield.
Scheme 1. A plausible reaction mechanism.
In conclusion, we have established a novel method for the
construction of indole derivatives through Yb(OTf)3-cata-
AHCTUNGTRENNUNG
lyzed formal [3+3] cycloaddition of vinylidenecyclopro-
panes bearing two EWGs at the cyclopropane ring with ni-
trones. The reaction is applicable to a variety of VDCPs and
nitrones with a moderate yield under mild reaction condi-
tions. Further efforts are in progress regarding the scope
and mechanistic details.
Experimental Section
Figure 2. X-ray crystal structure of 4aa.
General remarks: 1H and 13C NMR spectra were recorded at 400 and
100 MHz, respectively. Mass and HRMS spectra were recorded by EI
method. Organic solvents used were dried by standard methods when
necessary. Satisfactory CHN microanalyses were obtained with an ana-
lyzer. Commercially obtained reagents were used without further purifi-
cation. All these reactions were monitored by TLC with silica gel coated
plates. Flash column chromatography was carried out using silica gel at
increased pressure. VDCP diesters 1 were prepared according to the lit-
erature.[11]
plex between one or two of the carbonyl groups.[3,14] Subse-
quently, a stepwise annulation mechanism is proposed in-
volving an initial nucleophilic attack of the negatively
charged oxygen atom of nitrone dipole 2a onto the cyclo-
propane, leading to the opening of the cyclopropyl ring and
the formation of zwitterionic intermediate A, which under-
goes a cyclization to afford the formal [3+3] cycloaddition
intermediate B. Presumably, the nitrogen–oxygen bond of
intermediate B can be easily cleaved and subsequently rear-
ranges to afford the final product 3aa either in a concerted
way (path a)[15] or in a stepwise way through a zwitterionic
intermediate C (path b). The product 3aa mediated by
HOTf may undergo an intramolecular Friedel–Crafts aryl-
General procedure for Yb
of 1,1-vinylidenecyclopropanediesters with nitrones: Under an argon at-
mosphere, nitrone 2 (0.2 mmol, 1.0 equiv), Yb(OTf)3 (10 mol%) and 4 ꢁ
ACHTUNGRTENUN(NG OTf)3-catalyzed formal [3+3] cycloaddition
AHCTUNGTRENNUNG
molecular sieves (50 mg) were placed in a Schlenk tube, and then toluene
(2.0 mL) was added. The mixture was stirred at room temperature
(208C) for 5 min and then a solution of VDCP diester 1 (0.3 mmol,
1.5 equiv) in toluene (6.0 mL) was added into the reaction mixture. The
reaction completed within 10 min. Then the solvent was quickly removed
under reduced pressure and the residue was purified by a short path
silica gel flash column chromatography immediately.
ACHTUNGTRENNUNG
ation (a typical Scholl reaction),[16] affording the indole de-
General procedure for HOTf-mediated translations of compounds 3 to
rivative 4aa.
indole derivatives: Compound 3 (0.2 mmol) was dissolved in dichlorome-
Chem. Eur. J. 2010, 16, 1149 – 1152
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
1151

M. Shi and L. Wu
thane (5.0 mL) in a Schlenk tube and cooled to 08C, and then at this
temperature HOTf (0.3 mmol, 1.5 equiv) was added in. The mixture was
stirred at room temperature (208C) for about additional 50 h. After the
reaction was completed the solvent was removed under reduced pressure
and the residue was purified by silica gel flash column chromatography.
Vinylidenecyclopropanediester 1a: A light yellow oil. 1H NMR (CDCl3,
300 MHz, TMS): d=2.63 (d, J=4.5 Hz, 2H; CH2), 3.80 (s, 3H; CH3),
3.81 (s, 3H; CH3), 6.59 (t, J=4.5 Hz, 1H; CH), 6.02 (s, 1H; CH), 7.21–
7.26 (m, 2H; Ar), 7.29–7.32 ppm (m, 3H; Ar); 13C NMR (CDCl3,
75 MHz, TMS): d=19.9, 35.2, 52.57, 52.61, 86.8, 102.2, 126.9, 127.4, 128.4,
133.5, 166.9, 167.3, 188.8 ppm; IR (CH2Cl2): n˜ =2925, 2927, 2854, 2370,
2342, 23013, 1738, 1436, 1276, 1234, 1108, 693 cmÀ1; MS (%): m/z (%):
258 (2) [M]+, 242 (9), 155 (13), 113 (28), 99 (31), 85 (64), 71 (81), 57
(100); HRMS (EI): m/z calcd for C15H14O4: 258.0892; found: 258.0891.
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Molchanov, L. V. Stepakova, G. L. Starova, R. R. Kostikov, Russ. J.
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Acknowledgements
Shanghai Municipal Committee of Science and Technology (06XD14005,
08dj1400100-2), National Basic Research Program of China
(ACTHNGUTER(NNUG 973)-
[9] A. V. Stepakov, A. G. Larina, A. P. Molchanov, L. V. Stepakova,
G. L. Starova, R. R. Kostikov, Russ. J. Org. Chem. 2007, 43, 40–45.
[10] a) J.-M. Lu, M. Shi, J. Org. Chem. 2008, 73, 2206–2210; b) J.-M. Lu,
M. Shi, Chem. Eur. J. 2009, 15, 6065–6073; c) M. Shi, L.-F. Yao,
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2008, 14, 10219–10222; f) M. Shi, W. Li, Tetrahedron 2007, 63,
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h) W. Li, M. Shi, J. Org. Chem. 2008, 73, 6698–6705; i) W. Li, M.
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[11] For the preparation of VDCP diesters 1, see: M. J. Campbell, P. D.
Pohlhaus, G. Min, K. Ohmatsu, J. S. Johnson, J. Am. Chem. Soc.
2008, 130, 9180–9181.
[12] CCDC-731462 (3ae) and 719445 (4aa) contain 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.
2009CB825300), and the National Natural Science Foundation of China
are thanked for their financial support (20872162, 20672127, 20821002
and 20732008) and also thank Mr. Sun Jie for performing X-ray diffrac-
tion.
Keywords: cycloaddition · homogeneous catalysis · indoles ·
nitrones · ytterbium
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Received: September 11, 2009
Published online: December 18, 2009
1152
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Chem. Eur. J. 2010, 16, 1149 – 1152