One-pot synthesis and fluorescence properties of 2-arylquinolines

Article abstract of DOI:10.1246/bcsj.20090305

The one-pot synthesis of 2-arylquinoline with arylamines, arylaldehyde, and 1,1-diethoxyethane were studied using a catalytic amount ytterbium triflate. Various 2-arylquinolines showed fluorescence properties and the fluorescence was quenched by introduci

Full text of DOI:10.1246/bcsj.20090305

© 2010 The Chemical Society of Japan
Bull. Chem. Soc. Jpn. Vol. 83, No. 4, 385–390 (2010)
385
One-Pot Synthesis and Fluorescence Properties of 2-Arylquinolines
Shunsuke Sueki,1 Chiharu Okamoto,1 Isao Shimizu,*1 Keisuke Seto,2 and Yukio Furukawa*2
1Department of Applied Chemistry, School of Advanced Science and Engineering, Waseda University,
3-4-1 Okubo, Shinjuku-ku, Tokyo 169-8555
2Department of Chemistry and Biochemistry, School of Advanced Science and Engineering, Waseda University,
3-4-1 Okubo, Shinjuku-ku, Tokyo 169-8555
Received November 24, 2009; E-mail: shimizui@waseda.jp
The one-pot synthesis of 2-arylquinoline with arylamines, arylaldehyde, and 1,1-diethoxyethane were studied using
a catalytic amount ytterbium triflate. Various 2-arylquinolines showed fluorescence properties and the fluorescence was
quenched by introducing the nitro group into aryl moiety of 2-arylquinolines.
Creation of new materials and development of their synthetic
processes are fundamental problems in organic synthesis.
Various multi-component coupling reactions (MCRs) forming
carbon-carbon and carbon-heteroatom bonds in one-pot have
been developed as useful tools to achieve with ease a chemical
diversity of products, which are concerned with discovery of
functional molecules with efficient acquisition and fine tuning
of their chemical properties. Quinolines and their derivatives
are a very important class of compounds and are widely
used from medicines to functional materials. So far a variety
of synthetic methods such as Skraup,1 Doebner-von Miller,2
Friedländer,3 Combes,4 Povarov,5 and Pfitzinger6 reactions
have been reported.7 However, most synthetic methods proceed
under strong acid or basic conditions, therefore the methods
cannot be applied to molecules having acid-sensitive functional
groups. We are interested in transition or rare earth metal
catalyzed carbon-carbon bond forming reactions which pro-
ceed under mild conditions in an environmentally benign way.8
In recent years we have found that iridium- and ytterbium
complexes catalyze one-pot three-component coupling reac-
tions to give multi-substituted quinolines9 and 2-arylquinolines
without substituents at the 3-position with strong fluorescence
properties. In this paper we describe a facile synthetic method
for 2-arylquinolines and the relationship between structure and
fluorescent properties.
Table 1. One-Pot Synthesis of 2-Phenylquinolines with
Various Catalysts
NH2
CHO
EtO
Me
OEt
H
+
+
OMe
1a
2a
3
1.0 equiv
1.2 equiv
1.5 equiv
MeO
catalyst
N
DMSO, 90 °C,
under O2 (1 atm)
4a
Entry
Catalyst (mol %)
Yb(OTf)3 (5)
Yielda),b)
1
2
3
4
5
55
27c)
42d)
37
[IrCl2(H)(cod)]2 (5)
Ir(acac)3 (5)
49
6
7
8
[RhCl(cod)]2 (5)
Ru(acac)3 (5)
ZnCl2 (5)
29
37
54
9
SnCl2 (5)
30
10
11
TiCl4 (5)
HCl in 1,4-dioxane (5)
33
46
Results and Discussion
a) The yields were determined by GC. b) 1,2,4-Trimethlyben-
zene was used as an internal standard. c) Acetaldehyde was
used instead of 3. d) Ethyl vinyl ether was used instead of 3.
One-Pot Synthesis of 2-Arylquinolines in the Presence
of Various Catalysts.
At first we optimized the one-pot
three-component coupling reaction with various metal or
acid catalysts and results of the reactions of p-anisidine (1a),
benzaldehyde (2a), and 1,1-diethoxyethane (3) are shown in
Table 1. When the reaction was carried out using Yb(OTf)3 in
DMSO under oxygen at 90 °C, 6-methoxy-2-phenylquinoline
(4a) was obtained in 55% yield (Entry 1). 1,1-Diethoxyethane
(3) was effective as a precursor of enolizable acetaldehyde
equivalent. However, in the reaction using acetaldehyde and
ethyl vinyl ether the yields were decreased to 27% and 42%
respectively (Entries 2 and 3). The reaction with trivalent
iridium complexes [IrCl2(H)(cod)]2 and Ir(acac)3 gave 4a
in 37% and 49% yields respectively (Entries 4 and 5).
Reactions with late transition metal complexes [Rh(cod)Cl]2
and Ru(acac)3 did not give satisfactory results to afford 4a in
29% and 37% yields respectively (Entries 6 and 7). Other
Lewis acids such as SnCl2 and TiCl4 did not give satisfactory
results, i.e., 4a was obtained in 30% and 33% yields (Entries 9
and 10). ZnCl2 is an efficient catalyst to give 4a (54% yield,
Published on the web March 26, 2010; doi:10.1246/bcsj.20090305

386
Bull. Chem. Soc. Jpn. Vol. 83, No. 4 (2010)
2-Arylquinolines and Fluorescence Properties
-EtOH
EtO OEt
OEt
Me
H
MeO
NH2
O
Yb cat.
MeO
N
H
-H2O
N
OMe
Imine
[Yb]
OEt
OEt
MeO
Friedel-Crafts
cyclization
MeO
MeO
-EtOH
N
H
N
H
H
N
H
-Amino aldehyde equivalent
Tetrahydroquinoline
β
MeO
under O2
-2[H]
N
Scheme 1. Plausible reaction mechanism of formation of 6-methoxy-2-phenylquinoline.
Entry 8), however, it is considerably hygroscopic. Recently,
Baba et al. reported that a small amount of HCl catalyzed
quinoline synthesis.10 When a catalytic amount of HCl in 1,4-
dioxane was used, 4a was obtained in 46% yield (Entry 11).
Although Ir(acac)3 and ZnCl2 have enough Lewis acidity for
activation of the imine moiety to give 4a in similar yields,
Yb(OTf)3 is more convenient to handle because of its stability
and tolerance to air and moisture. Therefore synthesis of other
quinolines were carried out with Yb(OTf)3.
A plausible reaction mechanism of 6-methoxy-2-phenyl-
quinoline synthesis is shown in Scheme 1. At first, dehydration
of p-anisidine and benzaldehyde proceeds to give the imine.
The vinyl ether formed by elimination of ethanol from
the acetal attacks the Yb-activated imine, and subsequent
Friedel-Crafts cyclization of corresponding ¢-amino aldehyde
equivalent proceeds to give tetrahydroquinoline. Finally auto-
oxidation proceeds to give 6-methoxy-2-phenylquinoline.
The atmospheric oxygen plays a role as an oxidant for
dehydrogenation in this reaction mechanism. As shown in
the mechanism the reaction requires Yb(OTf)3 catalyst and
oxygen as a “green” oxidant and ethanol is generated as sole
by-product.
Ytterbium-Catalyzed One-Pot Synthesis of 2-Arylquino-
lines. The reactions of p-anisidine (1a), various arylaldehyde
2, and 1,1-diethoxyethane (3) in the presence of a catalytic
amount of Yb(OTf)3 at 90 °C under oxygen atmosphere were
carried out to afford the corresponding 6-methoxy-2-arylquino-
lines 4 in satisfactory yields. The results are summarized in
Table 2 and various substituted quinolines were obtained.
The reaction with 4-(methoxymethoxy)benzaldehyde (2b)
gave the corresponding MOM-protected 2-arylquinoline 4p in
14% yield, and a deprotected 2-arylquinoline 4q in 25% yield,
which was caused by acid hydrolysis of 4p with Yb(OTf)3 or
trace amount of trifluoromethanesulfonic acid from the catalyst
(Scheme 2).
The reduction of the nitro group in 4c using Pd/C-H2 gave the
quinoline amine 4r in 76% yield (Scheme 3).
The reaction with 2,4-dimethoxyaniline (1b) and N,N-
dimethyl-p-phenylenediamine (1c), instead of p-anisidine, gave
the corresponding 2-phenylquinoline derivatives 4s and 4t in
52% and 27% yields (Table 3).
Synthesis of 2-Aryl-N-methylquinolinium and 2-Aryl-
quinoline N-Oxide Derivatives.
For the comparison of
fluorescent properties 2-arylquinoline derivatives, 5a and 5b,
were prepared. As shown in Scheme 3, N-methylation of 6-
methoxy-2-phenylquinoline (4a) with methyl trifluoromethane-
sulfonate gave the corresponding 2-aryl-N-methylquinolinium
triflate salt 5a in 80% yield.11 On the other hand, 6-methoxy-2-
phenylquinoline N-oxide (5b) was obtained via the treatment of
4a with mCPBA in 56% yield12 (Scheme 4).
Fluorescent Properties of 2-Arylquinolines. Fluorescence
of synthesized 2-arylquinolines was measured in CHCl3 or
DMSO solutions (The details of the measurement conditions
can be found in Supporting Information). Most of the 2-aryl-
quinolines were excited at 320 nm. All fluorescence quantum
efficiencies were obtained with the following equation (F
denotes the area under the fluorescence band (F =-Iex(-),
where Iex(-) is the fluorescence intensity at each emission
wavelength), A denotes the absorbance at the excitation wave-
length, and n denotes the refractive index of the solvent).13
ꢀ
ꢁ ꢀ ꢁ ꢀ
ꢁ
2
FX
Fst
Ast
nX
nst
ꢀX ¼ ꢀst ꢀ
ꢀ
ꢀ
ð1Þ
2
AX
As shown in Table 4, 2-phenylquinoline (6) had a large
Stokes shift and does not show fluorescence, however by
introducing a methoxy group in the 6-position of the quinoline
ring, ¯ of 4a is increased because of the increase of electron
density of the quinoline rings (Entries 1 and 2). More electron-
rich quinolines 4s and 4t also showed good ¯s and 4t showed a
large Stokes shift (Entries 3 and 4). From these results, the
electron density of the quinoline ring may cause the high fluo-
rescence intensity. Surprisingly, 3-ethyl-6-methoxy-2-phenyl-
For comparison of fluorescence properties of 2-arylquino-
lines, several 2-arylquinoline derivatives were also synthesized.

S. Sueki et al.
Bull. Chem. Soc. Jpn. Vol. 83, No. 4 (2010)
387
Table 2. Yb-Catalyzed One-Pot Synthesis of 6-Methoxy-2-arylquinolines with Various Arylaldehydes
MeO
NH2
CHO
Yb(OTf)3 (5 mol%)
EtO
Me
OEt
H
+
+
DMSO, 90 °C,
under O2 (1 atm)
N
R
R
OMe
1a
2
3
4
1.0 equiv
1.2 equiv
1.5 equiv
Product (Yielda))
MeO
MeO
MeO
NO2
MeO
MeO
NO2
N
N
N
N
NO2
NO2
4a (50%)
4b (39%)
4c (49%)
4d (11%)
MeO
MeO
MeO
S
N
N
N
N
CN
CF3
4e (50%)
4f (46%)
4g (50%)
4h (48%)
MeO
MeO
MeO
MeO
Cl
Cl
N
N
N
N
CO2Me
Cl
Cl
4i (24%)
4j (54%)
4k (55%)
4l (52%)
MeO
MeO
MeO
N
N
N
OMe
OMe
NO2
NO2
4n (43%)
4o (40%)
4m (43%)
a) Isolated yields based on 1a.
NH2
CHO
MeO
Yb(OTf)3 (5 mol%)
EtO
Me
OEt
H
+
+
N
DMSO, 90 °C,
under O2 (1 atm)
OR
OMe
1a
OMOM
2b
4p (R=MOM, 14%)
4q (R=H, 25%)
3
Scheme 2. Synthesis of 4-(6-methoxyquinolin-2-yl)phenol.
MeO
MeO
Pd/C, H2 (1 atm)
N
N
AcOH, rt
NO2
NH2
4r (76%)
4c
Scheme 3. Hydrogenation reduction of 4c.
quinoline14 (7) and 2-(2-chlorophenyl)-6-methoxyquinoline
(4k) showed low fluorescence (Entries 5 and 6). The dihedral
angle of the quinoline scaffold and aryl moiety had a possi-
bility of correlation with fluorescence. Then the dihedral angles
were calculated with 6-31G**/B3LYP by Gaussian 98.15 From
the results of calculations, deconjugation between quinoline
scaffold and aryl moiety occurred with the increase of dihedral
angle, which may cause low fluorescence (Figure 1). In recent
years, Nagano et al. reported notable fluorescein-xanthene
fluorescence probes. In these probes, benzene moiety and

388
Bull. Chem. Soc. Jpn. Vol. 83, No. 4 (2010)
2-Arylquinolines and Fluorescence Properties
Table 3. Yb-Catalyzed One-Pot Synthesis of 2-Phenylquinolines with Various Arylamines
NH2
CHO
R1
Yb(OTf)3 (5 mol%)
EtO
Me
OEt
H
+
+
DMSO, 90 °C,
under O2 (1 atm)
N
R1
3
4
2a
1
1.0 equiv
1.2 equiv
1.5 equiv
Product (Yielda))
MeO
MeO
Me2N
N
N
N
OMe
4a (50%)
4s (52%)
4t (27%)
a) Isolated yields based on arylamine 1.
MeO
MeO
MeO
MeOTf
N
N
CH2Cl2, 60 °C
OTf
Me
5a (80%)
4a
MeO
mCPBA
N
O
N
CHCl3, 0 °C - rt
5b (56%)
4a
Scheme 4. Synthesis of 2-aryl-N-methylquinolinium and 2-arylquinoline N-oxide derivatives.
in this series. The synthesized 6-methoxy-2-arylquinolines
4r, 4o, and 4q having electron-donating groups of the aryl
moiety (Entries 1, 2, and 4) showed good ¯s (0.57, 0.61, and
0.54) relatively, and 2-arylquinolines 4f, 4j, 4l, and 4h having
electron-withdrawing group of the aryl moiety (Entries 5, 6, 7,
and 8) showed ¯s in 0.23, 0.44, 0.48, and 0.41, however other
quinolines 4g, 4i, and 4e showed good ¯s (0.77, 0.60, and
0.64) (Entries 9, 10, and 11). These differences were caused
by steric hindrance or electron repulsion of the aryl moiety.
Furthermore, quenching of fluorescence was observed by
introducing nitro group(s) in the 2-aryl moiety (Entries 12,
13, 14, and 15).
Furthermore, 2-aryl-N-methylquinolinium and 2-arylquino-
line N-oxide derivatives 5a and 5b showed lower fluorescence
than the corresponding quinoline 4a (Table 6). As shown
above, 2-arylquinolines including a nitro group in aryl moiety
had no fluorescence in spite of energy absorption. In recent
years, Albini et al. reported intramolecular electron transfer in
the photochemistry of some nitrophenyldihydropyridines.17 We
supposed that the photochemical reaction with the nitro group
might occur in 2-arylquinoline and the fluorescence would be
quenched by generation of nitro radical species. Nagano et al.
succeeded to observe radical species of fluorescein under UV
irradiation with ESR.18 To confirm the radical species, we also
tried to measure ESR spectra, but in this case no mean signals
were observed.
Figure 1. Structures of 3-alkyl-2-arylquinoline and 2-aryl-
quinolines and its dihedral angles; (a) 6-methoxy-2-
phenylquinoline (4a) (Entry 2 in Table 4), (b) 3-ethyl-6-
methoxy-2-phenylquinoline (7) (Entry 5 in Table 4), and
(c) 2-(2-chlorophenyl)-6-methoxyquinoline (4k) (Entry 6
in Table 4) (Green: C, White: H, Red: O, Blue: N, Purple:
Cl, dihedral angle is shown in the parenthesis).
fluorophore were orthogonal to each other and played an
important role to photoinduced electron transfer (PeT).16
Next, 6-methoxy-2-arylquinolines 4 were ordered in LUMO
energies levels (calculated with 6-31G**/B3LYP by Gaussian
9815). As shown in Table 5, 4r showed a large Stokes shift

Table 4. Fluorescence Properties of 2-Arylquinoline Derivatives
MeO
MeO
Me2N
MeO
MeO
Cl
N
N
N
OMe
N
N
N
7
4k
6
4a
4s
4t
Entry
1
2
3
4
5
6
-
-
abs,max/nm
258
264
370
39616
0.39
272
400
44462
0.74
276
433, 443
32364
0.76
242
360
32393
0.04
256
369
28475
0.12
em,max/nma)
443.5
36299
0.002
¾/M¹1 cm
¹1
b)
¯
a) Excited at 320 nm. b) Obtained by calculation, based on the quinine sulfate as the standard (¯q = 0.55).
Table 5. Fluorescence Properties of 6-Methoxy-2-arylquinolines
MeO
N
R1
CO2Me
CN
OMe
NO2
Me
NH2
OMe
OH
Cl
CF3
NO2
NO2
O2N
NO2
Cl
Cl
S
4r
4o
4a
4q
4f
4j
4l
4h
4g
4i
4e
4m
12
270
®
4n
4c
4d
Entry
1
2
3
264
370
4
280
396
5
284
388
21307
0.23
6
264
369
38239
0.44
7
8
264
370
36808
0.41
9
264
378
55529
0.77
10
276
379
33751
0.60
11
276
380
36240
0.64
13
264
®
14
242
®
30110
0.004
15
240
®
-
-
abs,max/nm
240, 296 276
425 382
21372 28647 39616 30676
0.57 0.61 0.39 0.54
270
372
38263
0.48
em,max/nma)
¾/M¹1 cm
38437
0.003
42409
0.000
14449
0.000
¹1
b)
¯
LUMO energy
/hartreesc)
0.01287 0.00404 0.00263 0.00051 ¹0.00840 ¹0.01343 ¹0.0248 ¹0.02712 ¹0.03598 ¹0.04372 ¹0.05242 ¹0.07604 ¹0.08568 ¹0.0895
¹0.11535
a) Excited at 320 nm. b) Obtained by calculation, based on the quinine sulfate as the standard (¯q = 0.55). c) LUMO energy levels were calculated with 6-31G**/B3LYP by Gaussian
98.

390
Bull. Chem. Soc. Jpn. Vol. 83, No. 4 (2010)
2-Arylquinolines and Fluorescence Properties
Table 6. Fluorescence Properties of 2-Aryl-N-methylquinolinium and 2-Arylquinoline N-Oxide Derivatives
MeO
MeO
MeO
N
N
N
O
OTf
Me
4a
5a
5b
Entry
1
2
3
-
-
abs,max/nm
264
370
39616
0.39
260
459
30288
0.15
242, 268
371.5
23740
0.01
em,max/nma)
¾/M¹1 cm
¹1
b)
¯
a) Excited at 320 nm. b) Obtained by calculation, based on the quinine sulfate as the standard (¯q = 0.55).
Li, D. Wang, Y.-J. Chen, J. Org. Chem. 2006, 71, 6592. m) Y.
Wang, X. Xin, Y. Liang, Y. Lin, R. Zhang, D. Dong, Eur. J. Org.
Conclusion
Chem. 2009, 4165. n) H. Vander Mierde, N. Ledoux, B. Allaert, P.
In conclusion, we have found Yb(III)-catalyzed one-pot
synthesis of 2-arylquinolines in satisfactory yields and also
synthesized 2-aryl-N-methylquinolinium and 2-arylquinoline
N-oxide derivatives. Most synthetic 2-arylquinolines had
fluorescence properties, although the 2-arylquinolines which
have a nitro group in 2-aryl moiety showed no fluorescence.
Further application for the fluorescent probe is undergoing.
Van Der Voort, R. Drozdzak, D. De Vos, F. Verpoort, New J. Chem.
2007, 31, 1572. o) H. Vander Mierde, P. Van Der Voort, D. De Vos,
F. Verpoort, Eur. J. Org. Chem. 2008, 1625. p) V. V. Kouznetsov,
C. M. M. Gómez, J. M. U. González, E. E. Stashenko, A. Bahsas,
J. Amaro-Luis, J. Heterocycl. Chem. 2007, 44, 551. q) N. P.
Selvam, C. Saravanan, D. Muralidharan, P. T. Perumal, J.
Heterocycl. Chem. 2006, 43, 1379. r) K. Niknam, M. A.
Zolfigol, A. Dehghani, Heterocycles 2008, 75, 2513.
8
S. Sueki, T. Igarashi, T. Nakajima, I. Shimizu, Chem. Lett.
This work is financially supported in part by the Global COE
program “Center for Practical Chemical Wisdom” by MEXT.
2006, 35, 682.
9
a) T. Nakajima, T. Inada, T. Igarashi, T. Sekioka, I.
Supporting Information
Shimizu, Bull. Chem. Soc. Jpn. 2006, 79, 1941. b) T. Inada, T.
Nakajima, I. Shimizu, Heterocycles 2005, 66, 611.
10 S. Tanaka, M. Yasuda, A. Baba, J. Org. Chem. 2006, 71,
800.
11 T. Nakajima, T. Inada, I. Shimizu, Heterocycles 2006, 69,
497.
Typical synthesis procedure for Yb-catalyzed 2-arylquinoline
synthesis, other synthesis procedures for 2-arylquinoline deriva-
tives, spectral data, fluorescent properties, and parameters of the
coordinates of optimized structures of 4a, 7, and 4k. This material
is available free of charge on the web at: http://www.csj.jp/
journals/bcsj/.
12 M. Yao, H. Inoue, N. Yoshioka, Chem. Phys. Lett. 2005,
402, 11.
13 H. Sunahara, Y. Urano, H. Kojima, T. Nagano, J. Am.
Chem. Soc. 2007, 129, 5597.
References
1
2
H. Skraup, Chem. Ber. 1880, 13, 2086.
O. Doebner, W. von Miller, Ber. Dtsch. Chem. Ges. 1881,
14 3-Ethyl-2-phenylquinoline was synthesized according to
Ref. 9a.
14, 2812.
15 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria,
M. A. Robb, J. R. Cheeseman, V. G. Zakrzewski, J. A.
Montgomery, Jr., R. E. Stratmann, J. C. Burant, S. Dapprich,
J. M. Millam, A. D. Daniels, K. N. Kudin, M. C. Strain, O. Farkas,
J. Tomasi, V. Barone, M. Cossi, R. Cammi, B. Mennucci, C.
Pomelli, C. Adamo, S. Clifford, J. Ochterski, G. A. Petersson, P. Y.
Ayala, Q. Cui, K. Morokuma, D. K. Malick, A. D. Rabuck, K.
Raghavachari, J. B. Foresman, J. Cioslowski, J. V. Ortiz, A. G.
Baboul, B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I.
Komaromi, R. Gomperts, R. L. Martin, D. J. Fox, T. Keith,
M. A. Al-Laham, C. Y. Peng, A. Nanayakkara, M. Challacombe,
P. M. W. Gill, B. Johnson, W. Chen, M. W. Wong, J. L. Andres, C.
Gonzalez, M. Head-Gordon, E. S. Replogle, J. A. Pople, Gaussian
98, Revision A.9, Gaussian Inc., Pittsburgh, PA, 1998.
16 Y. Urano, M. Kamiya, K. Kanda, T. Ueno, K. Hirose, T.
Nagano, J. Am. Chem. Soc. 2005, 127, 4888.
3
4
5
P. Friedländer, Ber. Dtsch. Chem. Ges. 1882, 15, 2572.
A. Combes, Bull. Soc. Chim. Fr. 1888, 49, 89.
L. S. Povarov, B. M. Mikhailov, Izv. Akad. Nauk SSSR, Ser.
Khim. 1963, 953.
6
7
W. Pfitzinger, J. Prakt. Chem. 1886, 33, 100.
For recent research: a) C. S. Cho, W. X. Ren, S. C. Shim,
Tetrahedron Lett. 2006, 47, 6781. b) L. Zhang, J. Wu, Adv. Synth.
Catal. 2007, 349, 1047. c) M. A. Zolfigol, P. Salehi, A. Ghaderi,
M. Shiri, Catal. Commun. 2007, 8, 1214. d) S. Matsumoto, K.
Ogura, Tetrahedron Lett. 2007, 48, 1117. e) C. S. Cho, W. X. Ren,
N. S. Yoon, J. Mol. Catal. A: Chem. 2009, 299, 117. f) A.-H. Li, E.
Ahmed, X. Chen, M. Cox, A. P. Crew, H.-Q. Dong, M. Jin, L. Ma,
B. Panicker, K. W. Siu, A. G. Steinig, K. M. Stolz, P. A. R.
Tavares, B. Volk, Q. Weng, D. Werner, M. J. Mulvihill, Org.
Biomol. Chem. 2007, 5, 61. g) F. Portela-Cubillo, J. S. Scott, J. C.
Walton, J. Org. Chem. 2008, 73, 5558. h) M. Dabiri, M.
Baghbanzadeh, M. S. Nikcheh, Monatsh. Chem. 2007, 138,
1249. i) C. S. Cho, W. X. Ren, J. Organomet. Chem. 2007, 692,
4182. j) H.-M. Wang, R.-S. Hou, H.-T. Cheng, L.-C. Chen,
Heterocycles 2009, 78, 487. k) V. V. Kouznetsov, Tetrahedron
2009, 65, 2721, and references therein. l) Y.-C. Wu, L. Liu, H.-J.
17 E. Fasani, M. Fagnoni, D. Dondi, A. Albini, J. Org. Chem.
2006, 71, 2037.
18 T. Ueno, Y. Urano, K. Setsukinai, H. Takakusa, H. Kojima,
K. Kikuchi, K. Ohkubo, S. Fukuzumi, T. Nagano, J. Am. Chem.
Soc. 2004, 126, 14079.