Silver(I)-promoted intramolecular addition of N-heterocyclic carbenes towards unsaturated esters in water

Article abstract of DOI:10.1016/j.tet.2014.02.031

Silver(I)-promoted intramolecular addition reaction between N-heterocyclic carbene and unsaturated esters in water is described. The reaction leads to 1H-imidazo[1,2-a]indole derivatives in moderate to excellent yields.

Full text of DOI:10.1016/j.tet.2014.02.031

Tetrahedron 70 (2014) 3073e3077
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Silver(I)-promoted intramolecular addition of N-heterocyclic
carbenes towards unsaturated esters in water
,
a,
Xiaobo Shang a, Chao Chen b, Huayu Qiu b , Wanzhi Chen
*
*
a Department of Chemistry, Zhejiang University, Hangzhou 310028, China
b College of Material, Chemistry and Chemical Engineering, Hangzhou Normal University, Hangzhou 310012, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Silver(I)-promoted intramolecular addition reaction between N-heterocyclic carbene and unsaturated
esters in water is described. The reaction leads to 1H-imidazo[1,2-a]indole derivatives in moderate to
excellent yields.
Received 29 November 2013
Received in revised form 25 January 2014
Accepted 13 February 2014
Available online 18 February 2014
Ó 2014 Elsevier Ltd. All rights reserved.
Keywords:
N-Heterocyclic carbene
Silver carbonate
Intramolecular addition
Water
1. Introduction
Ph
N
R
R
Ph R
N
Ph
N
R
R
R
Ph
R
R
Ph
THF
N
R
+
N-Heterocyclic carbenes (NHCs) have found impressive uses not
N
N
N
N
-78oC
Ph
N
Ph
only as ligands in the catalysis and organometallic chemistry1 due
R
Ph
Ph
to their strong
s-donating ability but also as powerful organo-
R = CO2Me
catalysts in molecular chemistry2 and precision polymer synthesis
in macromolecular chemistry.3 The reactivity of stable dia-
minocarbenes towards CeC multiple bonds have been studied by
Enders and co-workers.4,5 The reactions of triazolylidene with di-
methyl fumarate, N-alkyl or N-aryl maleimides and benzyl bromide
generate the methylene triazoline derivatives. The reaction of tri-
azolylidene with dimethyl acetylenedicarboxylate afforded the
spiro compounds (Scheme 1) via nucleophilic addition of the car-
bene to the triple bond and subsequent 1,3-dipolar cycloaddition,
which rearranges to the more stable bicyclic compound upon
heating.6 Triazolylidene can also react with an excess of phenyl
isocyanate to generate the spiro compound (Scheme 1).6 The [4þ1]
cyclization of NHCs with vinyl isocyanates, vinyl ketenes and
diphenyltetrazine has also been reported leading to hydroindolone
derivatives.7 The multicomponent reactions of NHCs towards acti-
vated acetylenes and aldehydes are also known.8 Besides, intra-
molecular addition of NHC to imines giving access to new
heterocycles with the formation of six- and seven-membered
rings,9 CeH insertion reaction of NHC with the methyl group to
Ph
O
Ph
N
Ph
Ph
N
N
Ph
O
C
N
Ph
Ph-NCO
THF
RT
O
N
N
Ph
O
+
N
N
N
N
N
N
N
Ph
Ph
Ph
Ph
Ph
Ph
Scheme 1. The spiro compounds formed via the NHC addition.
generate the aminal10 and NHC bearing pushepull substituents and
aromatic aldehydes or acrylates resulting in the formation of im-
idazole derivatives11 have also been reported. More recently, Bie-
lawski and co-workers described the synthesis of a variety of
cyclopropanes, epoxides and diamidocyclopropenes by combining
DAC (N,N0-diamidocarbene) with a range of structurally and elec-
tronically diversed olefins, aldehydes and alkynes.12 In these re-
actions, DAC is used as a readily accessible and isolable [2þ1]
cycloaddition reagent.
NHCs are often generated through deprotonation of azolium
salts with NaH, KH, or KOt-Bu in THF.13 However, these methods
suffer from the drawbacks, such as sensitivity to air and moisture
and limited tolerance to various functionalities. The generation of
free NHCs through thermal decomposition of masked NHCs is
a good choice.14 The masked NHCs, such as 5-alkxytriazolines,4
* Corresponding authors. Tel.: þ86 571 88273314; e-mail addresses: hyqiu@hznu.
edu.cn (H. Qiu), chenwzz@zju.edu.cn (W. Chen).
0040-4020/$ e see front matter Ó 2014 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2014.02.031

3074
X. Shang et al. / Tetrahedron 70 (2014) 3073e3077
imidazolium-2-carboxylate,15
imidazolium-2-thioisocyanates,16
Ag(I)eNHC complexes,17 2-alkoxy,18 2-trichloromethyl,19 2-
pentafluorophenyl imidazolidines20 have been reported. Silver
oxide can easily react with imidazolium salts to give silvereNHC
complexes in which the AgeC bonds are labile, thus the nucleo-
philic addition of NHCs would proceed in the presence of basic
silver compounds. As an extension of our NHC chemistry,21 here,
we report a silver(I)-promoted intramolecular addition of NHC to-
wards unsaturated esters in water.
2. Results and discussion
Scheme 2. The effect of anion on the reaction. aAll reactions were carried out with 2
(0.5 mmol), Ag2CO3 (0.5 equiv) in water (2.5 mL) at 65 ꢀC for 18 h under nitrogen.
bYield of isolated product after flash column chromatography.
Initially, we treated imidazolium salt 2a with 0.5 equiv of Ag2O
in THF at 65 ꢀC, and 3a was produced in 78% yield. As shown in
Table 1, the yield decreased to 29% when 0.1 equiv of Ag2O was used
in the reaction. Obviously, 0.5 equiv Ag(I) is needed in this reaction.
A few other silver(I) salts were then screened in subsequent ex-
periments. When AgNO3 and AgOTf were used, the reaction
did not proceed at all, whereas when Ag2CO3 was used as the
deprotonation agent, the yield of 3a was increased to 82%. AgOAc is
also efficient. The reaction initially gave an unidentified product,
but it quickly transformed to 3a after flash column chromatography
in 80% yield. The effect of solvents was also examined, and the
results were summarized in Table 1. The results indicated that
when the solvent THF was replaced by acetonitrile, DMF and tol-
uene, the yield of 3a was sharply decreased to less than 40%. Sur-
prisingly, the reaction could proceed smoothly in water, and the
yield is comparable to that in THF. Thus we finally chose water as
the solvent owing to its environmental-friendly feature for further
studies. For comparison, the commonly used bases like NaH and
KOt-Bu were also examined, and the desired product was obtained
in 51% and 36% yields, respectively.
Under the optimized condition, a series of substrates were
subjected to the reaction and the results are summarized in Table 2.
All the imidazolium and benzoimidazolium salts in the presence of
Ag2CO3 could be converted to their corresponding products in
moderate to excellent yields. The reaction of isopropyl esters
afforded the best yield in up to 91%, whereas tert-butyl esters gave
relatively lower yield (Table 2, entries 4e6). In addition, the re-
actions of benzoimidazonium salt showed lower activity compared
to similar imidazolium salt (entry 7). The reaction of 2-
imidazoliumylcinnamic ester with Ag2CO3 did not yield the de-
sired product suggesting that only more electron-deficient double
bond could receive nucleophilic addition of NHC.
A plausible mechanism was tentatively proposed shown in
Scheme 3. The Ag(I)eNHC complex A was firstly formed upon
treatment of the imidazolium salt with Ag2CO3. The dynamic dis-
sociation of AgeC bond would generate free NHC species B, and
silvereolefin interaction would increase the electrophilicity of the
double bond. The subsequent [2þ1] cycloaddition of the carbene
with the double bond might generate the cyclopropane derivative
C,6a which would undergo rapid ring opening to afford the zwit-
terionic intermediate D. Direct Michael addition of NHC towards
the double bond giving zwitterionic D is also possible.22 Finally,
[1,2]-hydrogen shift form intermediate E, which quickly transferred
to the final product F.
Table 1
Optimization of the reaction conditions
3. Conclusions
Entry
Base (equiv)
Solvent
T (ꢀC)
Yield (%)
In summary, we have developed a silver-assisted intramolecular
cyclization of 2-imidazoliumylcinnamic esters selectively to 1H-
imidazo[1,2-a]indole derivatives in good to excellent yields. The
reaction was performed in water and the starting materials are
easily available. So far the synthesis and uses of imidazo[1,2-a]in-
doles has remained undeveloped.23 The present reaction offers
a reliable synthetic approach of the fused heterocyclic compounds
(Scheme 3).
1
2
3
4
5
6
7
8
Ag2O (0.5)
Ag2O (0.1)
THF
THF
THF
THF
THF
THF
Toluene
CH3CN
DMF
H2O
65
65
65
65
65
65
65
65
65
65
100
65
65
78
29
80
82
AgOAc (1.0)
Ag2CO3 (0.5)
AgNO3 (1.0)
AgOTf (1.0)
Ag2CO3 (0.5)
Ag2CO3 (0.5)
Ag2CO3 (0.5)
Ag2CO3 (0.5)
Ag2CO3 (0.5)
KOt-Bu (1.5)
NaH (1.5)
No reaction
No reaction
37
39
31
78
49
36
51
9
10
11
12
13
H2O
THF
THF
4. Experimental section
4.1. General
All reactions were carried out with 2 (0.5 mmol) for 18 h.
The materials and solvents were purchased from commercial
suppliers and used without additional purification. NMR spectra
were recorded on a Bruke Avance operating at for 1H NMR at
400 MHz, and 13C NMR at 100 MHz. Chemical shifts were given
relative to CDCl3 (7.26 ppm for 1H NMR, 77.0 ppm for 13C NMR)
and DMSO-d6 (2.50 ppm for 1H NMR, 39.5 ppm for 13C NMR).
Mass spectrometry data of the products were collected on an
HRMS-TOF instrument or a low-resolution MS instrument using
As shown in Scheme 2, imidazolium bromide 2ab and imida-
zolium iodide 2aa reacted smoothly with Ag2CO3 giving corre-
sponding imidazo[1,2-a]indoles in 65% and 78% yields, respectively.
However, under the same conditions, the PFꢁ6 and BF4ꢁ salts are
inactive, and no imidazo[1,2-a]indole was isolated. Probably,
[Ag(NHC)2]þ species generated from the PF6ꢁ and BFꢁ4 salts are inert
whereas imidazolium halides [Ag(NHC)X] are more labile.

X. Shang et al. / Tetrahedron 70 (2014) 3073e3077
3075
Table 2
N
The scope of the reactiona
N
COOMe
COOMe
N
N
AgX
N
N
COOMe
AgX
COOMe
COOMe
MeOOC
A
C
B
Entry
1
Reactant
Product
Yieldb (%)
N
N
N
N
N
N
COOMe
COOMe
MeOOC
COOMe
MeOOC
COOMe
78
E
F
D
Scheme 3. A proposed mechanism.
2
3
66
4.2.1. 1-(2-(3-Methoxy-2-(methoxycarbonyl)-3-oxoprop-1-en-1-yl)
phenyl)-3-methyl-1H-imidazol-3-ium iodide NMR
(2a). 1H
(400 MHz, DMSO-d6): 9.53 (s, 1H), 7.98 (s, 1H), 7.93 (s, 1H), 7.73 (d,
J¼8.8 Hz, 3H), 7.65 (s, 1H), 7.51 (d, J¼6.8 Hz, 1H), 3.97 (s, 3H), 3.76 (s,
3H), 3.70 (s, 3H); 13C NMR (100 MHz, DMSO):
165.0, 163.0, 138.1,
d
d
137.7, 133.5, 131.7, 131.0, 129.8, 128.9, 128.8, 127.0, 124.2, 123.6, 53.0,
52.8, 36.2; ESI-MS: m/z¼300.9 (positive ion, MeOH), m/z¼127.2
(negative ion, MeOH); IR (neat): 3092, 3018, 2957, 1715, 1361, 1260,
74
1236, 1066 cmꢁ1
.
4.2.2. 1-(2-(3-Methoxy-2-(methoxycarbonyl)-3-oxoprop-1-en-1-yl)
phenyl)-3-methyl-1H-imidazol-3-ium bromide (2b). 1H NMR
4
5
91
81
(400 MHz, DMSO-d6):
7.70e7.75 (m, 3H), 7.65 (s, 1H), 7.51 (d, J¼6.8 Hz, 1H), 3.98 (s, 3H),
3.77 (s, 3H), 3.70 (s, 3H); 13C NMR (100 MHz, DMSO-d6):
165.1,
d 9.58 (s, 1H), 8.00 (s, 1H), 7.93 (s, 1H),
d
163.1, 138.1, 137.8, 133.6, 131.7, 131.0, 129.8, 129.0, 128.8, 127.0, 124.2,
123.7, 53.0, 52.8, 36.2; ESI-MS: m/z¼300.9 (positive ion, MeOH), m/
z¼81.3 (negative ion, MeOH); IR (neat): 3095, 2891, 1718, 1443,
1363, 1263, 1237, 1070 cmꢁ1
.
4.2.3. 1-(2-(3-Ethoxy-2-(ethoxycarbonyl)-3-oxoprop-1-en-1-yl)phe-
nyl)-3-methyl-1H-imidazol-3-ium iodide (2c). 1H NMR (400 MHz,
DMSO-d6):
d
9.54 (s, 1H), 7.95 (d, J¼10.8 Hz, 1H), 7.67e7.80 (m, 3H),
6
63
68
7.63 (s, 1H), 7.54 (d, J¼6.8 Hz, 1H), 4.13e4.27 (m, 4H), 3.98 (s, 3H),
1.22 (t, J¼6.8 Hz, 3H), 1.09 (t, J¼6.8 Hz, 3H); 13C NMR (100 MHz,
DMSO-d6):
d 164.5, 162.6, 138.1, 137.3, 133.5, 131.6, 130.9, 130.6,
129.1, 129.0, 126.9, 124.2, 123.7, 61.8, 61.6, 36.2, 13.9, 13.6; ESI-MS:
m/z¼329.0 (positive ion, MeOH), m/z¼127.1 (negative ion,
MeOH); IR (neat): 3080, 2980, 1721, 1256, 1219, 1072, 1016 cmꢁ1
.
7
4.2.4. 1-(2-(3-Isopropoxy-2-(isopropoxycarbonyl)-3-oxoprop-1-en-
a
1-yl)phenyl)-3-methyl-1H-imidazol-3-ium iodide (2d). 1H NMR
All reactions were carried out with 2 (0.5 mmol), Ag2CO3 (0.5 equiv) in water
(2.5 mL) at 65 ꢀC for 18 h under nitrogen.
(400 MHz, DMSO-d6):
d
9.50 (s, 1H), 7.94 (d, J¼11.2 Hz, 2H),
b
Yield of isolated product after flash column chromatography.
7.68e7.77 (m, 3H), 7.52e7.59 (m, 2H), 4.96e5.08 (m, 2H), 3.97 (s,
3H), 1.23 (d, J¼5.6 Hz, 6H), 1.16 (d, J¼6.0 Hz, 6H); 13C NMR
(100 MHz, DMSO-d6): d 164.2, 162.1, 138.1, 136.3, 133.6, 131.6, 131.3,
EI (Agilent 5975) or ESI (Bruker Esquire 3000plus) ionization. In-
frared spectra were recorded on a Bruker ATR-FTIR spectrometer.
Melting points were measured with a WRS-1A digital point
apparatus.
130.9, 129.1, 129.0, 127.0, 124.2, 123.7, 69.6, 69.5, 36.2, 21.4, 21.1; ESI-
MS: m/z¼357.0 (positive ion, MeOH), m/z¼127.2 (negative ion,
MeOH); IR (neat): 3064, 2982, 1734, 1718, 1274, 1262, 1210, 1101,
1061 cmꢁ1
.
4.2.5. 3-Ethyl-1-(2-(3-isopropoxy-2-(isopropoxycarbonyl)-3-
4.2. General procedure for synthesis of 2aeh
oxoprop-1-en-1-yl)phenyl)-1H-imidazol-3-ium iodide (2e). 1H NMR
(400 MHz, DMSO-d6):
d
9.60 (s, 1H), 8.10 (d, J¼1.2 Hz, 1H), 7.95 (d,
Alkyl halide (5.0 equiv) was added to a solution of substituted
imidazole or benzoimidazole (3.0 mmol) in toluene (10 mL). The
mixture was stirred at 80 ꢀC overnight. Then the solvent was re-
moved in vacuo and recrystallized by diethylether to afford the final
imidazolium or benzoimidazolium salts.
J¼1.6 Hz, 1H), 7.68e7.81 (m, 3H), 7.52e7.59 (m, 2H), 4.96e5.07 (m,
2H), 4.33 (q, J¼7.2 Hz, 2H), 1.51 (t, J¼7.2 Hz, 3H), 1.22 (d, J¼5.2 Hz,
6H), 1.15 (d, J¼5.2 Hz, 6H); 13C NMR (100 MHz, DMSO-d6):
d 164.1,
162.1, 137.3, 136.4, 133.6, 131.6, 131.1, 130.9, 129.0, 129.0, 127.0, 123.9,

3076
X. Shang et al. / Tetrahedron 70 (2014) 3073e3077
122.8, 69.5, 69.4, 44.8, 21.4, 21.1, 15.0; ESI-MS: m/z¼359.0 (positive
ion, MeOH), m/z¼127.1 (negative ion, MeOH); IR (neat): 2987, 2901,
2H), 7.02 (t, J¼8.0 Hz, 1H), 6.64 (s, 1H), 5.19 (s, 1H), 3.78 (s, 9H); 13
C
NMR (100 MHz, CDCl3):
d 170.5, 140.9, 132.0, 123.5, 121.9, 120.9,
1707, 1375, 1241, 1106, 1071 cmꢁ1
.
116.6, 116.0, 109.8, 104.7, 73.7, 52.7, 47.9, 34.5; IR (neat): 2987, 2901,
1727, 1250, 1066, 1056 cmꢁ1; MS (EI): m/z (%): 300 (Mþ, 20), 241
4.2.6. 1-(2-(3-(tert-Butoxy)-2-(tert-butoxycarbonyl)-3-oxoprop-1-
(100), 181 (50); HRMS (EI-TOF) calcd for
300.1110, found: 300.1110.
C
16H16N2O4 (Mþ):
en-1-yl)phenyl)-3-methyl-1H-imidazol-3-ium iodide (2f). 1H NMR
(400 MHz, DMSO-d6):
7.69e7.76 (m, 3H), 7.58e7.64 (m, 1H), 7.40 (s, 1H), 3.98 (s, 3H), 1.45
(s, 9H), 1.41 (s, 9H); 13C NMR (100 MHz, DMSO-d6):
163.9, 161.8,
d 9.54 (s, 1H), 7.99 (s, 1H), 7.88 (s, 1H),
4.5.2. Diethyl
2-(1-methyl-1H-imidazo[1,2-a]indol-9-yl)malonate
d
(3c). Green solid, mp¼135e138 ꢀC, yield: 74%. 1H NMR (400 MHz,
138.0, 134.7, 133.5, 133.3, 131.4, 130.8, 129.3, 129.2, 127.0, 124.2,
123.5, 82.8, 82.4, 36.3, 27.6, 27.4; ESI-MS: m/z¼385.0 (positive ion,
MeOH), m/z¼127.0 (negative ion, MeOH); IR (neat): 2987, 2901,
CDCl3):
d
7.56 (d, J¼8.4 Hz, 1H), 7.49 (d, J¼8.0 Hz, 1H), 7.15e7.26 (m,
2H), 6.98 (t, J¼7.6 Hz, 1H), 6.60 (d, J¼2.0 Hz, 1H), 5.11 (s, 1H),
4.13e4.29 (m, 4H), 3.75 (s, 3H), 1.26 (t, J¼7.2 Hz, 6H); 13C NMR
1704, 1394, 1249, 1067 cmꢁ1
.
(100 MHz, CDCl3): d 170.1, 140.9, 132.1, 123.6, 121.9, 120.8, 116.9,
115.9, 109.7, 104.7, 74.0, 61.6, 48.4, 34.6, 14.1; IR (neat): 2987, 2901,
1741, 1714, 1255, 1057, 1028 cmꢁ1; MS (EI): m/z (%): 328 (Mþ, 29),
255 (100), 227 (55), 181 (38); HRMS (EI-TOF) calcd for C18H20N2O4
(Mþ): 328.1423, found: 328.1425.
4.2.7. 1-(2-(3-Methoxy-2-(methoxycarbonyl)-3-oxoprop-1-en-1-yl)
phenyl)-3-methyl-1H-benzo[d]imidazol-3-ium iodide (2g). 1H NMR
(400 MHz, DMSO-d6):
d
10.06 (s, 1H), 8.17 (d, J¼8.0 Hz, 1H),
7.75e7.87 (m, 4H), 7.65e7.73 (m, 1H), 7.57e7.64 (m, 2H), 7.46 (d,
J¼8.4 Hz, 1H), 4.21 (s, 3H), 3.76 (s, 3H), 3.65 (s, 3H); 13C NMR
4.5.3. Diisopropyl 2-(1-methyl-1H-imidazo[1,2-a]indol-9-yl)malo-
(100 MHz, DMSO-d6):
d
165.3, 163.0, 144.3, 138.4, 132.1, 131.7, 131.6,
nate (3d). Green solid, mp¼120e125 ꢀC, yield: 91%. 1H NMR
131.5, 130.3, 129.6, 129.3, 128.4, 127.3, 126.9, 114.1, 113.0, 52.9, 52.9,
33.6; ESI-MS: m/z¼351.0 (positive ion, MeOH), m/z¼127.0 (nega-
tive ion, MeOH); IR (neat): 2987, 2901, 1719, 1406, 1394, 1241,
(400 MHz, CDCl3):
d
7.56 (d, J¼8.0 Hz, 1H), 7.47 (d, J¼8.4 Hz, 1H),
7.12e7.26 (m, 2H), 6.97 (t, J¼7.6 Hz, 1H), 6.60 (d, J¼2.0 Hz, 1H),
4.99e5.11 (m, 3H), 3.75 (s, 3H), 1.18e1.30 (m, 12H); 13C NMR
1067 cmꢁ1
.
(100 MHz, CDCl3): d 169.6, 141.0, 132.2, 123.6, 121.9, 120.6, 117.2,
115.8, 109.6, 104.7, 74.1, 69.1, 49.0, 34.7, 21.6; IR (neat): 2986, 2924,
1723, 1568, 1096 cmꢁ1; MS (EI): m/z (%): 356 (Mþ, 23), 269 (63), 227
(100),181 (39), 149 (22); HRMS (EI-TOF) calcd for C20H24N2O4 (Mþ):
356.1736, found: 356.1739.
4.3. General procedure for synthesis of 2ac
To the saturated solution of ammonium hexafluorophosphate
(5.0 equiv), imidazolium or imidazolium halide dissolved in water
was added slowly, the white precipitate was produced immedi-
ately. The product was filtrated and dried.
4.5.4. Diisopropyl 2-(1-ethyl-1H-imidazo[1,2-a]indol-9-yl)malonate
(3e). Brown oil, yield: 81%. 1H NMR (400 MHz, CDCl3):
d 7.62 (d,
2ac: 1H NMR (400 MHz, DMSO-d6):
d
9.50 (s, 1H), 7.94 (s, 2H),
7.69e7.76 (m, 3H), 7.64 (s, 1H), 7.50e7.53 (m, 1H), 3.97 (s, 3H), 3.77
(s, 3H), 3.70 (s, 3H); 13C NMR (100 MHz, DMSO-d6):
165.1, 163.1,
J¼8.0 Hz, 1H), 7.48 (d, J¼8.0 Hz, 1H), 7.13e7.26 (m, 2H), 6.97 (t,
J¼7.6 Hz, 1H), 6.67 (s, 1H), 5.00e5.13 (m, 2H), 5.00 (s, 1H), 4.10 (q,
J¼7.2 Hz, 2H), 1.44 (t, J¼7.2 Hz, 3H), 1.15e1.35 (m, 12H); 13C NMR
d
138.1, 137.7, 133.6, 131.7, 131.0, 129.9, 129.0, 128.8, 127.0, 124.2, 123.7,
53.0, 52.8, 36.1; ESI-MS: m/z¼300.9 (positive ion, CH3CN), m/
z¼144.8 (negative ion, CH3CN); IR (neat): 2987, 2901, 1406, 1394,
(100 MHz, CDCl3): d 169.0, 140.2, 131.9, 123.8, 120.6, 119.3, 118.2,
115.8, 109.5, 105.0, 73.9, 69.0, 49.2, 42.0, 21.7, 21.6, 15.1; IR (neat):
2979, 1720, 1574, 1098 cmꢁ1; MS (EI): m/z (%): 370 (Mþ, 37), 283
(81), 241 (100),195 (42); HRMS (EI-TOF) calcd for C21H26N2O4 (Mþ):
370.1893, found: 370.1888.
1250, 1229, 1066, 1056 cmꢁ1
.
4.4. General procedure for synthesis of 2ad
4.5.5. Di-tert-butyl 2-(1-methyl-1H-imidazo[1,2-a]indol-9-yl)malo-
To the saturated solution of sodium fluoroborate (5.0 equiv),
imidazolium or benzoimidazolium salt dissolved in water was
added slowly, the white precipitate was produced immediately. The
pure product was obtained after filtration and drying.
nate (3f). Green solid, mp¼107e109 ꢀC, yield: 63%. 1H NMR
(400 MHz, CDCl3):
d
7.60 (d, J¼8.0 Hz, 1H), 7.49 (d, J¼7.6 Hz, 1H),
7.23 (d, J¼0.8 Hz, 1H), 7.18 (t, J¼8.0 Hz, 1H), 6.98 (t, J¼7.6 Hz, 1H),
6.63 (s, 1H), 4.96 (s, 1H), 3.79 (s, 3H), 1.49 (s, 18H); 13C NMR
2ad: 1H NMR (400 MHz, DMSO-d6):
d
9.48 (s, 1H), 7.93 (s, 2H),
7.71e7.76 (m, 3H), 7.64 (s, 1H), 7.62 (d, J¼7.2 Hz, 1H), 3.96 (s, 3H),
3.77 (s, 3H), 3.70 (s, 3H); 13C NMR (100 MHz, DMSO-d6):
165.1,
(100 MHz, CDCl3): d 169.3, 132.3, 129.8, 128.1, 121.9, 120.5, 117.6,
115.7, 115.5, 109.5, 104.6, 81.5, 50.7, 34.8, 29.7, 28.0, 27.7; IR (neat):
2974, 2923, 1738, 1720, 1126, 1066 cmꢁ1; MS (EI): m/z (%): 384 (Mþ,
81), 283 (28), 227 (100), 183 (72); HRMS (EI-TOF) calcd for
d
163.1, 138.1, 137.7, 133.6, 131.8, 131.0, 130.0, 129.0, 128.9, 127.0, 124.2,
123.8, 53.0, 52.8, 36.2; ESI-MS: m/z¼300.9 (positive ion, CH3CN), m/
z¼87.2 (negative ion, CH3CN), IR (thin film): 2987, 2901, 1706, 1406,
C
22H28N2O4 (Mþ): 384.2049, found: 384.2048.
1193, 1051 cmꢁ1
.
4.5.6. Dimethyl
2-(10-methyl-10H-benzo[4,5]imidazo[1,2-a]indol-
11-yl)malonate (3g). Green solid, mp¼124e126 ꢀC, yield: 68%. 1H
4.5. General procedure for synthesis of 3aeg
NMR (400 MHz, CDCl3):
d
7.71 (d, J¼8.0 Hz, 1H), 7.65 (d, J¼8.0 Hz,
1H), 7.53 (d, J¼8.0 Hz, 1H), 7.03e7.22 (m, 5H), 5.13 (s, 1H), 3.74 (s,
Silver carbonate (0.5 equiv) was added to the solution of 2
(0.5 mmol) in water (2.5 mL) at 65 ꢀC under nitrogen in the Schlenk
tube. The mixture was stirred for 18 h. After the completion of the
reaction detected by TLC, it was extracted by ethyl acetate. The
combined organic extracts were dried over anhydrous Na2SO4 and
evaporated under reduced pressure. The product was purified by
column chromatography.
3H), 3.70 (s, 6H); 13C NMR (100 MHz, CDCl3):
d
170.0, 142.6, 138.3,
132.5, 127.1, 125.8, 122.3, 121.3, 120.1, 118.4, 117.2, 110.3, 109.9, 107.9,
76.4, 52.8, 47.8, 31.0; IR (neat): 2987, 2901, 1731, 1255, 1066,
1055 cmꢁ1; MS (EI): m/z (%): 350 (Mþ, 63), 291 (100), 233 (42);
HRMS (EI-TOF) calcd for C20H18N2O4 (Mþ): 350.1267, found:
350.1272.
Acknowledgements
4.5.1. Dimethyl 2-(1-methyl-1H-imidazo[1,2-a]indol-9-yl)malonate
(3a). Green solid, mp¼111e116 ꢀC, yield: 78%. 1H NMR (400 MHz,
We thank the financial support from the National Nature
Science Foundation of China (21352002).
CDCl3):
d
7.57 (d, J¼8.0 Hz, 1H), 7.52 (d, J¼8.0 Hz, 1H), 7.19e7.29 (m,

X. Shang et al. / Tetrahedron 70 (2014) 3073e3077
3077
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Supplementary data
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dx.doi.org/10.1016/j.tet.2014.02.031.
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