Efficient regioselective synthesis of indole N-carboximidamides and N-carboximidoates by a sequential aza-wittig/Ag(I)-catalyzed cyclization

Article abstract of DOI:10.1021/jo901362c

(Chemical Equation Presented) An efficient Ag(I)-catalyzed regioselective cyclization of (2-alkynylphenyl)guanidine or (2-alkynylphenyl)isourea to indole N-carboximidamides or N-carboximidoates has been developed. The approach has the advantages of high regioselectivity, mild reaction conditions, easily accessible starting materials, and good yields. 2009 American Chemical Society.

Full text of DOI:10.1021/jo901362c

pubs.acs.org/joc
matory drugs,6 and perfume ingredients.7 However, indole
Efficient Regioselective Synthesis of Indole
N-Carboximidamides and N-Carboximidoates by a
Sequential Aza-Wittig/Ag(I)-Catalyzed Cyclization
N-carboximidamides and N-carboximidoates are rarely
investigated probably due to the fact that they are not easily
accessible by routine synthetic methods.8
The synthesis and functionalization of indoles has been a
major area of focus for synthetic organic chemists, and
numerous well-established classical methods for the prepara-
tion of indoles have been developed.9 Recently, one of the
most convenient methods for catalytic intramolecular cycli-
zation of 2-alkynylaniline derivatives into polysubstituted
indole derivatives has been paid particular attention.10 The
reaction could be catalyzed by a strong base, such as
NaOEt,11 KOt-Bu,12 and KH,13 or by Lewis acidic iodine
complexes,14 or by various late transition metal complexes
involving Pd,15 Cu,16 In,17 Au,18 Fe,19 Ag,20 etc. However, to
the best of our knowledge, analogous intramolecular cycli-
zation of a (2-alkynylphenyl)guanidine (or isourea) has
never been reported.
Nian-Yu Huang,†,‡ Ming-Guo Liu,‡ and Ming-Wu Ding*,†
†Key Laboratory of Pesticide & Chemical Biology of
Ministry of Education, Central China Normal University,
Wuhan 430079, People’s Republic of China, and ‡Hubei Key
Laboratory of Natural Products Research and Development,
China Three Gorges University, Yichang 443002, People’s
Republic of China
ding5229@yahoo.com.cn
Received June 26, 2009
The aza-Wittig reactions of iminophosphoranes have
received increased attention in view of their utility in the
synthesis of nitrogen-containing heterocyclic compounds.21
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An efficient Ag(I)-catalyzed regioselective cyclization of
(2-alkynylphenyl)guanidine or (2-alkynylphenyl)isourea
to indole N-carboximidamides or N-carboximidoates has
been developed. The approach has the advantages of high
regioselectivity, mild reaction conditions, easily accessi-
ble starting materials, and good yields.
The chemistry of indoles has received particular attention
because the indole moiety is a structural component of a vast
number of bioactive natural and synthetic compounds.1
Some N-aryl or N-acyl indoles exhibit wide pharmacological
activity and could be used as selective hPPARγ receptor
agonists,2 high-affinity reagents for the 5-HT6 receptor,3 and
cyclooxygenase (COX) inhibitors.4 Indole-1-carboxamides
could also be used as agrochemical fungicides,5 anti-inflam-
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6874 J. Org. Chem. 2009, 74, 6874–6877
Published on Web 08/07/2009
DOI: 10.1021/jo901362c
r
2009 American Chemical Society

Huang et al.
JOCNote
SCHEME 1
SCHEME 3
SCHEME 2
TABLE 1. Yields of Compounds 6a-w
compd
R
Ar
Y
yield (%)a
6a
6b
6c
6d
6e
6f
6g
6h
6i
H
H
H
H
H
H
H
H
Ph
Ph
Ph
Ph
4-F-Ph
4-Cl-Ph
4-Cl-Ph
4-Cl-Ph
3-Me-Ph
3-Me-Ph
3-Me-Ph
Ph
NEt2
N(Pr-n)2
morpholin-4-yl
pyrrolidin-1-yl
NEt2
84
74
72
76
85
74
71
70
80
78
71
72
81
87
83
72
88
77
78
72
88
93
68
N(Pr-n)2
morpholin-4-yl
piperidin-1-yl
N(Pr-n)2
H
H
H
6j
6k
6l
morpholin-4-yl
pyrrolidin-1-yl
morpholin-4-yl
piperidin-1-yl
pyrrolidin-1-yl
pyrrolidin-1-yl
morpholin-4-yl
piperidin-1-yl
piperidin-1-yl
OMe
Me
Me
Me
Me
Me
Me
Me
H
Recently we have been interested in the synthesis of
thienopyrimidinones,22 triazolopyrimidinones,23 gua-
nines,24 quinazolinones,25 and imidazolinones26 via the
aza-Wittig reaction, with the aim of evaluating their
biological activities. In this study, we first report a mild
and efficient synthesis of indole N-carboximidamides
and N-carboximidoates by Ag(I)-catalyzed intramole-
cular cyclization of (2-alkynylphenyl)guanidines (or iso-
ureas), which were obtained by aza-Wittig reaction and
further reaction of the carbodiimides with amines or
alcohols.
6m
6n
6o
6p
6q
6r
6s
6t
Ph
Ph
4-F-Ph
4-Cl-Ph
4-Cl-Ph
3-Me-Ph
Ph
4-Cl-Ph
4-Cl-Ph
Ph
H
H
Me
OMe
OEt
OMe
OEt
6u
6v
6w
Me
4-Cl-Ph
aIsolated yield.
The iminophosphoranes 3 (Scheme 1) were synthesized
according to standard protocols.27 Sonogashira coupling of
2-iodoaniline derivatives 1 with the phenyl acetylene led to
2-phenylethynylaniline derivatives 2 in excellent yields,28
which were converted to the corresponding iminophosphor-
anes 3 via reaction with triphenylphosphine, hexachloro-
ethane, and triethylamine.
Reaction of iminophosphorane 3 with aromatic isocya-
nates at room temperature furnished the required carbodi-
imides 4 (Scheme 2), which were allowed to react with
nucleophiles (HY, such as secondary amines or alcohols)
to provide the (2-alkynylphenyl)guanidine (or isourea) inter-
mediates 5. Even in the presence of a strong base, such as
NaOEt, KOt-Bu, and NaH, under refluxing conditions, 5
did not cyclize. However, when a catalytic amount of AgNO3
was added, 5 was converted easily to indole N-carboximid-
amides or N-carboximidoates 6 in moderate to excellent
yields (68-93%) at room temperature. The results are listed
in Table 1.
(21) (a) Lertpibulpanya, D.; Marsden, S. P.; Rodriguez-Garcia, I.;
Kilner, C. A. Angew. Chem., Int. Ed. 2006, 45, 5000. (b) Palacios, F.; Alonso,
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Bonacorso, H. G.; Martins, M. A. P.; Wessjohann, L. A. J. Org. Chem. 2006,
71, 6996. (d) Palacios, F.; Herran, E.; Alonso, C.; Rubiales, G.; Lecea, B.;
Ayerbe, M.; Cossio, F. P. J. Org. Chem. 2006, 71, 6020. (e) Chan, J.; Faul, M.
Tetrahedron Lett. 2006, 47, 3361.
(22) (a) Ding, M. W.; Xu, S. Z.; Zhao, J. F. J. Org. Chem. 2004, 69, 8366.
(b) Liu, M. G.; Hu, Y. G.; Ding, M. W. Tetrahedron 2008, 64, 9052.
(23) Zhao, J. F.; Xie, C.; Xu, S. Z.; Ding, M. W.; Xiao, W. J. Org. Biomol.
Chem. 2006, 4, 130.
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Bioorg. Med. Chem. Lett. 2009, 19, 831.
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18, 3872.
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Chem. 2006, 4170. (b) Ding, M. W.; Fu, B. Q.; Cheng, L. Synthesis 2004,
1067. (c) Li, H. X.; Xie, C.; Ding, M. W.; Liu, Z. M.; Yang, G. F. Synlett
2007, 2280.
(27) (a) Schmittel, M.; Steffen, J.-P.; Engels, B.; Lennartz, C.; Hanrath,
M. Angew. Chem., Int. Ed. 1998, 37, 2371. (b) Schmittel, M.; Rodrı
Steffen, J.-P. Angew. Chem., Int. Ed. 2000, 39, 2152. (c) Schmittel, M.;
Rodrıguez, D.; Steffen, J.-P. Molecules 2000, 5, 1372. (d) Alajarın, M.;
Molina, P.; Vidal, A. J. Nat. Prod. 1997, 60, 747.
´
guez, D.;
(28) (a) Takahashi, S.; Kuroyama, Y.; Sonogashira, K.; Hagihara, N.
Synthesis 1980, 627. (b) Roshchin, A. I.; Bumagin, N. A. Chem. Heterocycl.
Compd. 1999, 35, 171. (c) Schmittel, M.; Mahajan, A.; Steffen, J.-P. Synthesis
2004, 415.
´
´
J. Org. Chem. Vol. 74, No. 17, 2009 6875

Huang et al.
JOCNote
SCHEME 4
TABLE 2. DifferentCatalytic Abilities among Pd(II), Cu(I), and Ag(I)a
entry
catalyst
solvent
time (h)
yield (%)b
1
2
3
4
5
6
7
8
AgNO3
CH3CN
CH3CN
DMF
CH3CN
DMF
CH3CN
DMF
CH3CN
0.5
24
24
24
24
24
3
84
0
Pd(PPh3)2Cl2
Pd(OAc)2
Pd(OAc)2
CuI
CuI
PdCl2
PdCl2
30
35
27
34
30
35
1
aReactions were carried out with 0.4 mmol of 5a and 5 mL
of solvent in the presence of 5 mol % of catalyst at 25 °C.
bIsolated yield.
yields, and their structure was confirmed by spectral and
X-ray crystallographic analysis. The reasons for the above
regioselectivity are not yet very clear.
In summary, a highly efficient cyclization of (2-alkynyl-
phenyl)guanidine (or isourea) intermediates to indole N-
carboximidamides or N-carboximidoates was developed by
using Ag(I) as a catalyst under mild reaction conditions. The
versatile one-pot approach has the advantages of high
regioselectivity, mild reaction conditions, easily accessible
starting materials, and good yields.
The intramolecular cyclization of 2-alkynylaniline deri-
vatives to a triple bond has been extensively investigated by
using various late transition metal reagents as effective
catalysts; however, the cyclization often requires thermal
conditions or needs a solvent with Lewis basicity or a
general base. Considering the good nucleophilicity of the
guanidine or isourea group in intermediate 5, the cycliza-
tion of 5 to 6 was achieved at mild room temperature. We
therefore compared the different catalytic abilities among
Pd(II), Cu(I), and Ag(I) at room temperature. Cyclization
of the (2-(phenylethynyl)phenyl)guanidine (5a) was exam-
ined first (Table 2). In the presence of 5 mol % of AgNO3,
the reaction of 5a proceeded smoothly in a short time (0.5 h
at 25 °C) in CH3CN to give 6a in 84% isolated yield (entry
1, Table 2). Through control experiments, we found Pd-
(PPh3)2Cl2 was inactive (entry 2, Table 2); however, the
reaction could be slowly catalyzed by Pd(OAc)2 or CuI in
DMF or CH3CN for 24 h (entry 3-6, Table 2). The
cyclization could also take place by a reaction with a
catalyst such as PdCl2 for 1-3 h in DMF or CH3CN in
low yields probably due to side reactions (entries 7 and 8,
Table 2).
The crystal structure of indole 6c has been determined by
X-ray diffraction and is shown in Figure 1 in the Supporting
Information. The X-ray crystal analysis confirmed the struc-
ture of the product and the (Z) geometry about the imine
bond. On the basis of the results obtained and from the
related literature,29 a possible mechanism for the Ag(I)-
catalyzed intramolecular cyclization can be proposed
(Scheme 3). It presumably involves (i) the formation of
complex A through the coordination of the alkynyl moiety
of 5 with Ag(I), (ii) regioselective nucleophilic attack of the
activated triple bond by NH of the guanidine or isourea to
give the vinylsilver species B, and (iii) proton transfer with
lose of Ag(I), which enters a new catalytic cycle, and forma-
tion of indole product 6.
Experimental Section
Representative Procedure for the Preparation of Indole Deri-
vatives (6). To a solution of iminophosphorane (3) (3.0 mmol)
in dry methylene chloride (15 mL) was added aromatic iso-
cyanate (3.0 mmol) under nitrogen at room temperature. After
the reaction mixture was left unstirred for 8-12 h at 0-5 °C,
the solvent was removed under reduced pressure and Et2O/
petroleum ether (1:2, 12 mL) was added to precipitate triphe-
nylphosphine oxide. Removal of the solvent gave carbodi-
imides (4), which were used directly without further purifica-
tion. To a solution of carbodiimides (4) prepared above in
anhydrous acetonitrile (10 mL) was added secondary amines
(HNR2
,
3.0 mmol) or anhydrous alcohol (ROH,
2
5 mL) with several drops of sodium alkoxide (RONa) in the
corresponding ROH. After the reaction mixture was stirred
for 0.5-6 h, the guanidine or isourea intermediate (5) was
generated. Without further purification, the reaction mixture
was treated with AgNO3 (0.15 mmol, 25 mg) and stirred for
0.5 h at room temperature until the color of the reaction
mixture turned dark. The mixture was filtered to remove the
solid catalyst and the filtrate was evaporated under reduced
pressure. The crude product was purified by flash chromato-
graphy (5:1, petroleum ether:diethyl ether) to yield indole
derivatives (6).
(Z)-N,N-Diethyl-N0,2-diphenyl-1H-indole-1-carboxamidine
(6a): light yellow oil (yield 84%); IR (KBr, cm-1) 3058, 2947,
2933, 1625, 1589, 1455, 1421, 1343, 761, 748, 693; 1H NMR
(CDCl3, 600 MHz) δ (ppm) 7.54 (d, J=8.4 Hz, 1H), 7.37 (d,
J=7.8 Hz, 1H), 7.34-7.29 (m, 5H), 7.22-7.19 (m, 1H), 7.13
(t, J=7.2 Hz, 1H), 6.75 (t, J=7.8 Hz, 2H), 6.66 (t, J=7.2 Hz,
1H), 6.57 (s, 1H), 6.07 (d, J=7.8 Hz, 2H), 3.79-3.74 (m, 2H),
3.15-3.07 (m, 2H), 1.37 (t, J=6.9 Hz, 3H), 1.04 (t, J=6.9 Hz,
3H); 13C NMR (CDCl3, 100 MHz) δ (ppm) 147.6, 144.2,
139.4, 137.3, 131.8, 128.1, 127.8, 127.6, 127.2, 122.8, 121.7,
121.6, 121.0, 120.5, 111.2, 103.6, 41.9, 41.4, 14.0, 11.7; MS
(EI, 70 eV) m/z (%) 367 (Mþ, 42), 295 (3), 193 (19), 175 (100),
165 (15), 147 (11), 119 (37), 92 (3), 77 (14). Anal. Calcd for
It is noteworthy that this reaction shows very high regio-
selectivity. Presumably the cyclization of (2-alkynylphe-
nyl)guanidine (or isourea) intermediate 5 could produce
two products (Scheme 4). The indole derivatives 6 should
be obtained from the complex A, while the complex C could
cyclize into quinazoline derivatives 7. However, only one
exclusive product 6 was observed and isolated in satisfactory
C
6.93; N, 11.69.
25H25N3: C, 81.71; H, 6.86; N, 11.43. Found: C, 81.89; H,
(29) (a) Trost, B. M.; McClory, A. Angew. Chem., Int. Ed. 2007, 46, 2074.
(b) Xie, L.; Ma, J.; Ding, Y.-X. Tetrahedron Lett. 2008, 49, 847. (c) Rudisill,
D. E.; Stille, J. K. J. Org. Chem. 1989, 54, 5856.
(E)-Methyl 2-phenyl-N-phenyl-1H-indole-1-carbimidate (6s):
white solid (yield 78%); mp 100-101 °C; IR (KBr, cm-1) 3079,
6876 J. Org. Chem. Vol. 74, No. 17, 2009

Huang et al.
JOCNote
3062, 3001, 2949, 1685, 1595, 1454, 1440, 1370, 1320, 1271, 1214,
1127, 760, 741, 692; 1H NMR (CDCl3, 600 MHz) δ (ppm) 7.55
(d, J=7.8 Hz, 1H), 7.47 (d, J=7.8 Hz, 1H), 7.25-7.07 (m, 7H),
6.81-6.79 (m, 3H), 6.47 (s, 1H), 5.99-5.97 (m, 2H), 4.11 (s, 3H);
13C NMR (CDCl3, 150 MHz) δ (ppm) 147.1, 144.3, 139.3, 137.1,
131.8, 128.3, 128.1, 127.5, 127.0, 123.3, 123.2, 121.6, 121.4,
120.6, 111.5, 105.0, 55.5; MS (EI, 70 eV) m/z (%) 326 (Mþ,
97), 234 (5), 206 (100), 193 (19), 165 (28), 119 (67), 91 (16), 77 (9).
Anal. Calcd for C22H18N2O: C, 80.96; H, 5.56; N, 8.58. Found:
C, 81.17; H, 5.62; N, 8.73.
Acknowledgment. We greatly acknowledge financial sup-
port of this work from the National Natural Science Foun-
dation of China (No. 20772041) and the Key Project of
Chinese Ministry of Education (No. 107082).
Supporting Information Available: Full spectral details,
1
X-ray diffraction of 6c, and copies of the H and 13C NMR
spectra of compounds 6a-w. This material is available free of
charge via the Internet at http://pubs.acs.org.
J. Org. Chem. Vol. 74, No. 17, 2009 6877