Assembly and characterization of novel hydrogen-bond-induced nanoscale rods

Article abstract of DOI:10.1021/jo0486037

A class of bis-urea compounds with perylene bisimide was synthesized and characterized successfully. ¹H NMR and fluorescence spectra confirmed that strong hydrogen-bonding interactions between neighboring urea groups were formed. Interestingly, the photocurrent measurement showed that the self-assembled films of bis-urea compounds could produce steady and rapid anodic photocurrent responses. The TEM images indicated that well-defined nanoscale rods with uniform diameter distribution could be fabricated by self-assembly of hydrogen-bonding interactions and π-π stacking interactions of perylene rings.

Full text of DOI:10.1021/jo0486037

Assembly and Characterization of Novel Hydrogen-Bond-Induced
Nanoscale Rods
Yang Liu,†,‡ Yongjun Li,†,‡ Li Jiang,†,§ Haiyang Gan,†,‡ Huibiao Liu,† Yuliang Li,*,†
Junpeng Zhuang,† Fushen Lu,†,‡ and Daoben Zhu*,†
CAS Key Laboratory of Organic Solids, Center for Molecular Sciences, Institute of Chemistry,
Chinese Academy of Sciences, Beijing 100080, P.R. China, Graduate School of Chinese
Academy of Sciences, and Department of Applied Physics, Harbin Institute of Technology,
Heilongjiang 150001, P.R. China
ylli@iccas.ac.cn
Received August 10, 2004
A class of bis-urea compounds with perylene bisimide was synthesized and characterized
successfully. 1H NMR and fluorescence spectra confirmed that strong hydrogen-bonding interactions
between neighboring urea groups were formed. Interestingly, the photocurrent measurement showed
that the self-assembled films of bis-urea compounds could produce steady and rapid anodic
photocurrent responses. The TEM images indicated that well-defined nanoscale rods with uniform
diameter distribution could be fabricated by self-assembly of hydrogen-bonding interactions and
π-π stacking interactions of perylene rings.
Introduction
between donor and acceptor unites.4 The urea functional-
ity has been utilized to create highly organized hydrogen-
bonded molecular assemblies.5 A class of compounds that
is particularly well suited for the spatial organization of
functional entities is bis-urea compounds.6 Bis-urea
compounds with the thiophene moiety self-assembled into
ribbons and fibers through multiple hydrogen bonds with
neighboring molecules have been synthesized.7 A class
of bis-urea macrocycles that self-assembled into columnar
nanotubes via urea-urea hydrogen bonding was re-
There is growing interest in supramolecular chemistry
in attempts to construct organic molecules into well-
defined functional aggregates.1 Well-defined nanosized
aggregates should be very useful for the development of
novel functional materials and nanoelectronic devices. In
particular, hydrogen bonding is a very useful means of
constructing the supramolecular system and has been
used for the design of various molecular aggregates.2
Meanwhile, energy- and electron-transfer processes have
also been investigated in the assembled supramolecular
systems through the interaction of hydrogen bonds.3
Hydrogen bonding assemblies are most promising to
fabricate a controllable molecular array and shape for
efficient intermolecular energy and electron transfer
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Int. Ed. 2001, 40, 2382. (b) Schenning, A. P. H. J.; Herrikhuyzen, J.
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† Chinese Academy of Sciences.
‡ Graduate School of Chinese Academy of Sciences.
§ Harbin Institute of Technology.
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10.1021/jo0486037 CCC: $27.50 © 2004 American Chemical Society
Published on Web 11/11/2004
J. Org. Chem. 2004, 69, 9049-9054
9049

Liu et al.
FIGURE 1. Possible superstructure arrangements of 7 through hydrogen-bonding and π-π stacking interactions.
ported.8 Perylene bisimides have been extensively studied
as organic semiconductors, in electronic and optical
application such as field-effect transistors,9 fluorescent
solar collectors,10 electrophotographic devices,11 photo-
voltaic devices,12 dye lasers,13 and molecular switches.14
They have outstanding chemical, thermal, and photo-
chemical stability.15 Recently, we discussed the photo-
current generation of the self-assemblies of [60]fullerene
derivative with perylene bisimide and perylene derivative
with perylene bisimide through hydrogen-bonding inter-
actions.16 Here, we report a class of bis-urea compounds
with perylene bisimides that self-assembly into nanoscale
rodlike superstructures through 3-centered intermolecu-
lar hydrogen-bonding interactions of urea-urea groups
and π-π stacking interactions of perylene rings (Figure
1) and study their photocurrent responses. We hope that
the supramolecular systems can demonstrate the energy
and electron transfer and can provide effective conversion
from photons to electrons.
FIGURE 2. 1H NMR shifts of the urea N-H protons of 7a
and 7b as a function of concentration in CDCl3 solution at room
temperature.
Results and Discussion
The bis-urea compounds with perylene bisimide were
synthesized in four steps from 1,7-dibromoperylene-
3,4,9,10-tetracarboxylic acid dianhydride 1, as shown in
Scheme 1. Briefly, compound 1 was converted to the
corresponding perylene bisimide 4 using 2,6-diisopropyl-
aniline. Subsequently, compound 5 was prepared by the
nucleophilic substitution of the two bromine atoms using
N-(tert-butoxycarbonyl)tyramine 3. Compound 6 was
obtained by removing the protective units of amino
groups. Finally, the bis-urea compounds 7a and 7b were
synthesized by the reaction of corresponding isocyanates
and amino-groups of compound 6, respectively. In con-
trast to the reported bis-urea thiophenes,7a the solu-
bility of compounds 7a and 7b is rather good in organic
solvents such as chloroform or ethyl acetate at room
temperature.
We studied the formation of hydrogen bonds of bis-urea
compounds in CDCl3 by a 1H NMR titration experiment.
As shown in Figure 2, an increase of the concentration
(8) (a) Ranganathan, D.; Lakshmi, C.; Karle, I. L. J. Am. Chem. Soc.
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Meiners, C.; Enkelmann, V.; Greerts, Y.; Mu¨llen, K. J. Mater. Chem.
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9050 J. Org. Chem., Vol. 69, No. 26, 2004

Novel Hydrogen-Bond-Induced Nanoscale Rods
SCHEME 1. Synthesis of Bis-Urea Compounds with Perylene Bisimidesa
a (a) 2,6-Diisopropylaniline, propionic acid, nitrogen, reflux, 26 h, 44%. (b) Compound 3, anhydrous K2CO3, 18-crown-6, toluene, nitrogen,
reflux, 2 h, 75%. (c) CF3COOH, CH2Cl2 (anhydrous), at room temperature, 2 h, 95%. (d) 7a: octyl isocyanate, CH2Cl2 (anhydrous), at
room temperature, 6 h, 99%. 7b: hexadecyl isocyanate, CH2Cl2 (anhydrous), at room temperature, 12 h, 79%.
of bis-urea compounds in CDCl3 resulted in a clear
downfield shift of the spectral position of the urea N-H
protons since the electron densities of the protons in-
volved in hydrogen bonds were decreased and conse-
quently their NMR signals were shifted to lower magnetic
fields. When the concentration of 7a increased from 1.3
to 30 mM, the chemical shift for the urea N-H protons
changed from 4.27 parts per million (ppm) to 4.58 ppm.
While the concentration of 7b increased from 1.3 to 30
mM, the chemical shift for the urea N-H protons
changed from 4.29 to 4.49 ppm. These results indicated
that strong 3-centered intermolecular hydrogen bonds
between neighboring urea-urea groups were formed.
Moreover, hydrogen-bonding interactions between the
urea groups of 7a were stronger than those of 7b since
the chemical shift for the urea N-H protons of 7a was
bigger than that of 7b. This indicated that the length of
the alkyl chains connected with the urea groups could
affect the interactions of hydrogen bonds between urea
groups.17
amide-II band of the urea groups generally show clear
shifts upon the formation of hydrogen bonds. When 7a
was dissolved in chloroform at the concentration of 5 ×
10-5 M, this solution showed two absorptions at 3457 (N-
H) and 1522 (amide-II) cm-1, which were characteristic
for the presence of non-hydrogen-bonded urea groups18
(Table 1). The peak in the amide-I region of urea groups
was not precisely determined because the absorption
band of amide-I overlapped that of the CdO stretch
region of the diimide units. As shown in Table 1,
increasing the concentration of 7a from 5 × 10-5 M to
2.7 × 10-3 M caused a shift of the N-H stretch absorption
toward a shorter wavenumber and a shift of the amide-
II band toward a higher wavenumber. These concentra-
tion-dependent spectral changes clearly indicated the
formation of intermolecular hydrogen bonds between
urea groups. Similar changes were observed in the
(17) (a) de Loos, M.; Ligtenbarg, A. G. J.; van Esch, J.; Kooijman,
H.; Spek, A. L.; Hage, R.; Kellogg, R. M.; Feringa, B. L. Eur. J. Org.
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J. 2003, 9, 3008.
(18) (a) Dobrowolsky, J. C.; Jamro´z, M. H.; Mazurek, A. P. Vib.
Spectrosc. 1994, 8, 53. (b) Schoonbeek, F. S.; van Esch, J. H.; Hulst,
R.; Kellogg, R. M.; Feringa, B. L. Chem. Eur. J. 2000, 6, 2633.
The formation of hydrogen bonds of bis-urea com-
pounds in chloroform was further studied by means of
infrared spectroscopy since the N-H stretch and the
J. Org. Chem, Vol. 69, No. 26, 2004 9051

Liu et al.
FIGURE 3. (a) Fluorescence emission spectra of (a) 7a, (b) 7b, and (c) 4 in CHCl3 at different concentrations. (Arrows indicate
increasing concentrations from 10-6 to 10-4 mol L-1, respectively; λem ) 420 nm).
TABLE 1. Infrared Spectroscopy of 7a and 7b in
Chloroform
hydrogen-bonding resulting in the red-shifting of the
fluorescence of 7, compound 4 not containing hydrogen-
bonded groups was utilized as a comparative object.
Figure 3c showed that the fluorescence of compound 4
was red-shifted by 10 nm from 547 to 557 nm with the
increasing concentration from 10-6 to 10-4 M. The red-
shifting of the fluorescence of compound 4 was mainly
caused by molecular collision at increased concentrations.
These results indicated that the red-shifting of the
fluorescence of 7 upon the increasing concentration might
be produced as a result of corporate interactions of both
molecular collision and hydrogen bonding.20 A similar
phenomenon was observed in the fluorescence spectrum
of 7b in chloroform (Figure 3b).
We first study the photoinduced electron-transfer
property of the self-assembled 7 on the basis of the
hydrogen-bond-induced nanostructure system. The an-
odic photocurrent responses of 0.2 and 0.06 µA cm-2 at
100 mW cm-2 white light irradiation were produced
(Figure 4) as the irradiations of the 7a and 7b films
were switched on and off, respectively. The responses to
on/off cycling were prompt and reproducible; five cycles
are shown in Figure 4. The photocurrent stabilities in
two systems were good in the monitored time. The
introduction of a perylene unit in the urea compound led
to the occurrence of efficient charge-transport pathways,
concn
NH stretch
amide-II
compound
[10-4 M]
[ν cm-1
]
[ν cm-1
]
7a
0.5
1
3457
3455
3447
3446
3444
3459
3456
3449
3447
1522
1522
1526
1527
1541
1527
1527
1532
1535
5
10
27
1
5
10
27
7b
infrared spectra of 7b in chloroform, and spectral shifts
were smaller than those of 7a. This indicated that
hydrogen-bonding interactions between urea groups of
7b were weaker than those of 7a, consistent with the
1
results of H NMR titration experiments.
Little attention has been paid to the study of the
fluorescence of bis-urea compounds.19 We also studied the
fluorescence spectra of 7a and 7b in chloroform at
different concentrations. As shown in Figure 3a, while
the concentration of 7a increased from 10-6 to 10-4 M,
not only was the intensity of its fluorescence strength-
ened but also its fluorescence was red-shifted by 12 nm
from 572 to 584 nm. To investigate the interactions of
(19) Ricks, H. L.; Shimizu, L. S.; Smith, M. D.; Bunz, U. H. F.;
Shimizu, K. D. Tetrahedron Lett. 2004, 45, 3229.
(20) Huang, C. H.; Li, F. Y.; Huang, Y. Y. Ultrathin Films for Optics
and Electronics; Peking University Press: Beijing, 2001; p 163.
9052 J. Org. Chem., Vol. 69, No. 26, 2004

Novel Hydrogen-Bond-Induced Nanoscale Rods
assembled 7a in p-xylene can form large quantity of
rodlike nanostructures with diameters in the range of
30-80 nm, while their lengths were extended to several
micrometers. Because of the hydrogen-bonding interac-
tions of the bis-urea groups of compound 7a, self-
assembled aggregates tend to form hydrogen-bonded
chains. Moreover, the π-π stacking interactions of
perylene rings and the interactions of alkyl chains can
also facilitate aggregates to form one-dimensional rodlike
supramolecular structures. However, similar rodlike
nanostructures were not observed in the electron micro-
graph of 7b in p-xylene. This indicated that because
hydrogen-bonding interactions between the urea groups
of 7b were weaker than those of 7a, 7b could not self-
assembly into rodlike nanostructures as 7a did through
3-centered intermolecular hydrogen bonding interactions
of urea-urea groups. The results also suggested that the
length of the alkyl chains connected with the urea groups
could affect the interactions of hydrogen bonds between
urea groups.
FIGURE 4. Time dependence of the photocurrent responses
of the self-assembled 7a and 7b films upon irradiation by 100
mW cm-2 white light in 0.5 M KCl solution.
Conclusions
In conclusion, two bis-urea compounds with perylene
bisimide were synthesized and characterized successfully.
1H NMR and fluorescence spectra confirmed the existence
of strong hydrogen-bonding interactions between neigh-
boring urea groups. The TEM images indicated that well-
defined nanoscale rods could be fabricated by self-
assembly due to hydrogen-bonding interactions and π-π
stacking interactions of perylene rings. Interestingly, the
photocurrent measurement showed that the self-as-
sembled films of bis-urea compounds could produce
steady and rapid anodic photocurrent responses. There-
fore, these perylene bisimides modified with bis-urea
units, with the capability for self-assembly through hy-
drogen-bonding interactions, are promising candidates
for inclusion in photoelectric devices based on organic
semiconducting layers.
Experimental Section
1,7-Dibromoperylene-3,4,9,10-tetracarboxylic acid dianhy-
dride 1 and N-(tert-butoxycarbonyl)tyramine 3 were prepared
according to literature procedures.21,22
N,N′-Bis[(2,6-diisopropyl)phenyl]-1,7-dibromoperylene-
3,4,9,10-tetracarboxylic Acid Bisimide (4). 2,6-Diisopro-
pylaniline (2.832 g, 16 mmol) was added to a suspension of
1,7-dibromoperylene-3,4,9,10-tetracarboxylic acid dianhydride
1 (2.2 g, 4 mmol) in propionic acid (40 mL), and the mixture
was refluxed under a nitrogen atmosphere for 26 h. Unreacted
starting material was removed by filtering the hot mixture.
The product precipitated from the cooled filtrate was collected
by filtration, washed with methanol, and dried in a vacuum
at 100 °C to give 4 as a red solid (1.53 g, 44%): 1H NMR (300
MHz, CDCl3, 25 °C) δ 9.57 (d, 2H, J ) 7.8 Hz), 9.02 (s, 2H),
8.80 (d, 2H, J ) 7.8 Hz), 7.52 (t, 2H, J ) 7.8 Hz), 7.37 (d, 4H,
J ) 7.8 Hz), 2.74 (m, 4H), 1.19 (d, 24H); UV-vis (CHCl3) λmax
533 nm, 497 nm, 467 nm, 396 nm; fluorescence (CHCl3) λmax
555 nm; FT-IR (KBr), ν [cm-1] 1711 (CdO), 1673 (CdO); MS
(MALDI-TOF) 866.8 (M-). Elemental analysis calcd (%) for
FIGURE 5. (A) Transmission electron micrograph (TEM)
images of self-assembled 7a in p-xylene. (B-D) High magni-
fication of TEM image.
similar to the introduction of the thiophene unit.7a It can
be seen that the photocurrent response of 7a was stronger
than that of 7b at the same experimental condition. This
indicated that hydrogen-bonding interactions between
the urea groups of 7a were stronger than those of 7b,
1
consistent with the results of H NMR titration experi-
(21) (a) Rohr, U.; Schlichting, P.; Bo¨hm, A.; Gross, M.; Meerholz,
K.; Bra¨uchle, C.; Mu¨llen, K. Angew. Chem., Int. Ed. 1998, 37, 1434.
(b) Rohr, U.; Kohl, C.; Mu¨llen, K.; van de Craats, A.; Warman, J. J.
Mater. Chem. 2001, 11, 1789.
(22) Nutt, R. F.; Chen, K. M.; Joullie´. M. M. J. Org. Chem. 1984,
49, 1013.
ments.
Figure 5 showed the transmission electron micrograph
(TEM) images of self-assembled 7a in p-xylene. From the
electron micrograph in Figure 5 it is clear that self-
J. Org. Chem, Vol. 69, No. 26, 2004 9053

Liu et al.
C48H40N2O4Br2 (866.1): C, 66.37; H, 4.64; N, 3.22. Found: C,
66.08; H, 4.81; N, 3.01.
anhydrous dichloromethane (20 mL). The resulting mixture
was stirred for 6 h at room temperature. The solvent was
removed by reduced pressure and the crude product was
obtained. Purification was accomplished by column chroma-
tography on silica with CH2Cl2/CH3OH (20/1, v/v) to give 7a
(64 mg, 99%): 1H NMR (300 MHz, CDCl3, 25 °C) δ 9.65 (d,
2H, J ) 7.8 Hz), 8.72 (d, 2H, J ) 7.8 Hz), 8.39 (s, 2H), 7.47 (t,
2H, J ) 7.8 Hz), 7.35 (m, 8H), 7.13 (d, 4H, J ) 7.8 Hz), 4.33
(br, 4H), 3.48 (t, 4H, J ) 6.5 Hz), 3.11 (m, 4H), 2.86 (t, 4H, J
) 6.5 Hz), 2.73 (m, 4H), 1.46 (s, 6H), 1.26 (m, 24H), 1.14 (d,
24H); UV-vis (CHCl3) λmax 550 nm, 518 nm, 403 nm; fluores-
cence (CHCl3) λmax 584 nm; FT-IR (KBr), ν [cm-1] 3365
(N-H), 1707 (CdO), 1668 (CdO); MS (MALDI-TOF) 1292.2
(M-). Elemental analysis calcd (%) for C82H94N6O8 (1290.7):
C, 76.25; H, 7.34; N, 6.51. Found: C, 76.16; H, 7.84; N, 6.23.
N,N′-Bis[(2,6-diisopropyl)phenyl]-1,7-bis[4-(2-(3-hexa-
decyl-ureido)ethyl)phenoxy]perylene-3,4,9,10-tetracar-
boxylic Acid Bisimide (7b). Hexadecyl isocyanate (40 mg,
0.15 mmol) was added to a solution of compound 6 (49 mg,
0.05 mmol) in anhydrous dichloromethane (20 mL). The
resulting mixture was stirred for 6 h at room temperature.
The solvent was removed by reduced pressure, and the crude
product was obtained. Purification was accomplished by
column chromatography on silica with CH2Cl2/CH3OH (20/1,
v/v) to give 7b (59.8 mg, 79%): 1H NMR (300 MHz, CDCl3,
25 °C) δ 9.68 (d, 2H, J ) 7.8 Hz), 8.79 (d, 2H, J ) 7.8 Hz),
8.42 (s, 2H), 7.49 (t, 2H, J ) 7.8 Hz), 7.37 (m, 8H), 7.16 (d,
4H, J ) 7.8 Hz), 4.31 (br, 4H), 3.51 (t, 4H, J ) 4.5 Hz), 3.14
(m, 4H), 2.89 (t, 4H, J ) 4.5 Hz), 2.74 (m, 4H), 1.47 (s, 6H),
1.29 (m, 56H), 1.18 (m, 24H); UV-vis (CHCl3) λmax 550 nm,
518 nm, 403 nm; fluorescence (CHCl3) λmax 584 nm; FT-IR
(KBr), ν [cm-1] 3362 (N-H), 1707 (CdO), 1669 (CdO); MS
(MALDI-TOF) 1515.9 (M-). Elemental analysis calcd (%) for
C98H126N6O8 (1515.0): C, 77.64; H, 8.38; N, 5.54. Found: C,
76.88; H, 8.94; N, 5.36.
N,N′-Bis[(2,6-diisopropyl)phenyl]-1,7-bis{[4-(2-N-tert-
butoxycarbonyl)-aminoethyl]phenoxy}perylene-3,4,9,10-
tetracarboxylic Acid Bisimide (5). A mixture of N-(tert-
butoxycarbonyl)tyramine 3 (71 mg, 0.3 mmol), anhydrous
potassium carbonate (83 mg, 0.6 mmol), and 18-crown-6 (316
mg, 1.2 mmol) was stirred in toluene (30 mL) at room
temperature. Then compound 4 (80 mg, 0.09 mmol) was added.
The reaction mixture was refluxed under nitrogen with stirring
for 2 h. After cooling to room temperature, the solvent was
removed by reduced pressure, and the crude product was
obtained. Purification was accomplished by column chroma-
tography on silica with CH2Cl2/CH3OH (20/1, v/v) to give 5
(79.7 mg, 75%): 1H NMR (300 MHz, CDCl3, 25 °C) δ 9.64 (d,
2H, J ) 9.0 Hz), 8.76 (d, 2H, J ) 9.0 Hz), 8.43 (s, 2H), 7.47 (t,
2H, J ) 7.2 Hz), 7.33 (d, 4H, J ) 7.2 Hz), 7.12 (d, 4H, J ) 7.2
Hz), 6.77 (d, 4H, J ) 7.2 Hz), 4.60 (s, 2H), 3.35 (m, 4H), 2.82
(t, 4H, J ) 7.2 Hz), 2.70 (m, 4H), 1.43 (s, 18H), 1.15 (d, 24H);
UV-vis (CHCl3) λmax 548 nm, 515 nm, 403 nm; fluorescence
(CHCl3) λmax 580 nm; FT-IR (KBr), ν [cm-1] 3385 (N-H), 1707
(CdO), 1668 (CdO); MS (MALDI-TOF) 1182.4 (M-). Elemental
analysis calcd (%) for C74H76N4O10 (1180.6): C, 75.23; H, 6.48;
N, 4.74. Found: C, 74.40; H, 6.77; N, 4.35.
N,N′-Bis[(2,6-diisopropyl)phenyl]-1,7-bis[4-(2-amino-
ethyl)phenoxy]perylene-3,4,9,10-tetracarboxylic Acid
Bisimide (6). In a 50-mL flask, compound 5 (70.8 mg, 0.06
mmol) was dissolved in dichloromethane (20 mL). Then
CF3COOH (1 mL) was carefully added under stirring. The
reaction mixture was stirred for 2 h. The solvent was removed
under reduced pressure, and the residue was redissolved in
CH2Cl2. The resulting solution was treated with 100 mL of
5% aqueous sodium carbonate. The organic phase was col-
lected, washed with water, and dried over anhydrous sodium
sulfate. The product was obtained after the solvent was
removed in a vacuum: yield 55.7 mg (95%); 1H NMR (300
MHz, CDCl3, 25 °C) δ 9.66 (d, 2H, J ) 7.8 Hz), 8.77 (d, 2H, J
) 7.8 Hz), 8.44 (s, 2H), 7.48 (t, 2H, J ) 7.8 Hz), 7.34 (m,
8H), 7.13 (d, 4H, J ) 7.8 Hz), 3.01 (s, 4H), 2.75 (t, 4H, J ) 7.8
Hz), 2.70 (m, 4H), 1.16 (d, 24H); UV-vis (CHCl3) λmax 549 nm,
515 nm, 401 nm; fluorescence (CHCl3) λmax 582 nm; FT-IR
(KBr), ν [cm-1] 3423 (N-H), 1707 (CdO), 1668 (CdO); MS
(MALDI-TOF) 981.0 (M-). Elemental analysis calcd (%) for
C64H60N4O6 (980.5): C, 78.34; H, 6.16; N, 5.71. Found: C,
77.87; H, 6.29; N, 5.13.
Acknowledgment. This work was supported by the
Major State Basic Research development Program and
the National Natural Science Foundation of China
(20151002, 50372070, and 90101025).
Supporting Information Available: General experimen-
tal methods, mass spectra for compounds 4-7, and partial 1H
NMR spectra for compound 7a at different concentrations in
CDCl3. This material is available free of charge via the Internet
at http://pubs.acs.org.
N,N′-Bis[(2,6-diisopropyl)phenyl]-1,7-bis[4-(2-(3-octyl-
ureido)ethyl)phenoxy]perylene-3,4,9,10-tetracarboxyl-
ic Acid Bisimide (7a). Octyl isocyanate (23 mg, 0.15 mmol)
was added to a solution of compound 6 (49 mg, 0.05 mmol) in
JO0486037
9054 J. Org. Chem., Vol. 69, No. 26, 2004