Self-assembly of molecular nanoball: Design, synthesis, and characterization

Article abstract of DOI:10.1021/jo061311g

The design and self-assembly of two new flexible supramolecular nanoballs are described. These assemblies incorporate two flexible tritopic amide and ester building blocks and were prepared in excellent yields (96-97%) via coordination driven self-assembl

Full text of DOI:10.1021/jo061311g

Self-Assembly of Molecular Nanoball: Design, Synthesis, and
Characterization
Sushobhan Ghosh and Partha Sarathi Mukherjee*
Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore-560012, India
psm@ipc.iisc.ernet.in
ReceiVed June 26, 2006
The design and self-assembly of two new flexible supramolecular nanoballs are described. These assemblies
incorporate two flexible tritopic amide and ester building blocks and were prepared in excellent yields
(96-97%) via coordination driven self-assembly. The first resulted from the reaction of 4 equiv of a
new tritopic ester ligand N,N′,N′′-tris(4-pyridylmethyl) trimesic ester and 3 equiv of C4 symmetric Pd-
(NO3)2. The second analogous structure was obtained by the self-assembly of a flexible N,N′,N′′-tris(3-
pyridylmethyl) trimesic amide and Pd(NO3)2. The assemblies were characterized with multinuclear NMR
spectroscopy, electrospray ionization mass spectroscopy, elemental analysis, and TGA. Mass spectrometry
along with NMR data and TEM view confirms the existence of the two assemblies. MM2 force field
simulations of the cages showed a ball shape with the diameter of the inner cavity of about 2.1 and 1.8
nm for 2a and 2b, respectively, which were also corroborated by TEM analysis.
Introduction
to guide the properties of the resulting assemblies. Several
functional groups4 like porphyrin, calixarene, and acid-based
receptors have been incorporated into self-assemblies. Recently,
we have introduced amide functionality into 3D Pd cages.5
However, incorporation of ester functionality to nanostructures
of Pd(II) and Pt(II) is rare except for a very few ester-based
open frameworks reported recently.6
Here we report the design and synthesis of a new tripodal
ester linker N,N′,N′′-tris(4-pyridylmethyl) trimesic ester (1a) and
its self-assembly with a C4 symmetric naked Pd(II)-nitrate to a
face-directed nanoscopic molecular ball (2a). To the best of our
knowledge 2a represents the first example of a 3D Pd(II) cage
(close framework) containing ester functionality. An analogous
cage (2b) was also prepared by using a tripodal amide linker
N,N′,N′′-tris(3-pyridylmethyl) trimesic amide (1b).
Stepwise covalent synthesis of large molecules is often time-
consuming and laborious and thus generally ends in a low yield
of the target product. It is also difficult to achieve a large desired
product where the controlling force is a nondirectional weak
interaction like hydrogen bonding, van der Waals, and π-π
interaction. Instead, by utilizing the stronger metal-ligand
directional coordination bonding approach, one can easily
prepare the desired large molecules using appropriate molecular
units.1 The major requirement for this coordination-driven self-
assembly approach is the use of rigid precursors of appropriate
size and shapes. Square-planar Pt(II) and Pd(II) have long been
used as favorite metals in this area.1 In a majority of the cases,
rigid organic linkers have been used to control the shape and
symmetry of the final assemblies.2 On the other hand, flexible
linkers are less predictable in self-assembly and have a tendency
to form undesired polymer. However, flexible linkers may
generate pseudorigid assemblies3 that can distort their shapes
to obtain a more thermodynamically stable conformation for
host-guest interactions. Moreover, properties of a molecule
depend on the existing functional group(s). Thus, incorporation
of functional groups in nanostructures may be an efficient way
Results and Discussion
Edge-directed and face-directed self-assembly are the two
distinct processes in the synthesis of supramolecular species.
Using the former one, several 3D assemblies like molecular
dodecahedron,7 cube,8 adamantanoid,9a double-square,9b and
trigonal bipyramidal5 cages have been synthesized. On the other
10.1021/jo061311g CCC: $33.50 © 2006 American Chemical Society
Published on Web 10/06/2006
8412
J. Org. Chem. 2006, 71, 8412-8416

Self-Assembly of a Molecular Nanoball
SCHEME 1. Synthesis of Tritopic Donor Linkers 1a and 1b
hand, face-directed assemblies are less common. In this
paradigm, some or all the faces of the target assemble are
spanned by the linkers themselves, which hold together the
overall structure.10 Our selection of the two tripodal linkers (1a
and 1b) is basically due to two reasons: the first is to introduce
ester/amide functionality and flexibility into the 3D nanostruc-
ture and the second is to use a pseudoflat tritopic linker that
can fit well on the eight trigonal faces of a ball.
To align eight tritopic donor linkers on the trigonal faces of
a ball, we prepared ligands 1a,b by the reaction of trimesyl
chloride and an appropriate alcohol/amine using excess triethy-
lamine (see Scheme 1).
When intense orange ligand 1a (0.04 mmol) was treated with
Pd(NO3)2 (0.03 mmol) in DMSO-d6 (1.5 mL) for 2 h at 60 °C,
the formation of a single product (Scheme 2) (very light yellow,
(1) (a) Seidel, S. R.; Stang, P. J. Acc. Chem. Res. 2002, 35, 972. (b)
Leininger, S.; Olenyuk, B.; Stang, P. J. Chem. ReV. 2000, 100, 853. (c)
Stang, P. J.; Olenyuk, B. Acc. Chem. Res. 1997, 30, 502. (d) Swiegers, G.
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M.; Fujita, N.; Kusukawa, T.; Biradha, K. Chem. Commun. 2001, 509. (i)
Uller, E.; Demleitner, I.; Bernt, I.; Saalfrank, R. W. Synergistic Effect of
Serendipity and Rational Design in Supramolecular Chemistry. In Structure
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p 149. (j) Caulder, D. L.; Raymond, K. N. J. Chem. Soc., Dalton Trans.
1999, 1185. (k) Caulder, D. L.; Raymond, K. N. Acc. Chem. Res. 1999,
32, 975. (l) Baxter, P. N. W.; Lehn, J.-M.; Baum, G.; Fenske, D. Chem.
Eur. J. 1999, 5, 102. (m) Fujita, M. Chem. Soc. ReV. 1998, 27, 417. (n)
Lehn, J.-M. Supramolecular Chemistry: concepts and perspectiVes; VCH:
New York, 1995.
SCHEME 2. Self-Assembly of Nanoballs (2a and 2b)a
(2) (a) Mukherjee, P. S.; Das, N.; Kryschenko, Y. K.; Arif, A. M.; Stang,
P. J. J. Am. Chem. Soc. 2004, 126, 2464. (b) Tominaga, M.; Suzuki, K.;
Murase, T.; Fujita, M. J. Am. Chem. Soc. 2005, 127, 11950. (c) Chi, K. W.;
Addicott, C.; Arif, A. M.; Das, N.; Stang, P. J. J. Org. Chem. 2003, 68,
9798. (d) Tabellion, F. M.; Seidel, S. R.; Arif, A. M.; Stang, P. J. J. Am.
Chem. Soc. 2001, 123, 7740. (e) Tabellion, F. M.; Seidel, S. R.; Arif, A.
M.; Stang, P. J. J. Am. Chem. Soc. 2001, 123, 11982. (f) Hiraoka, S.; Fujita,
M. J. Am. Chem. Soc. 1999, 121, 10239-10240. (g) Fujita, M.; Ibukuro,
F.; Seki, H.; Kamo, O.; Imanari, M.; Ogura, K. J. Am. Chem. Soc. 1996,
118, 899. (h) Fujita, M.; Aoyagi, M.; Ibukuro, F.; Ogura, K.; Yamaguchi,
K. J. Am. Chem. Soc. 1998, 120, 611. (i) Fujita, M.; Nagao, S.; Ogura, K.
J. Am. Chem. Soc. 1995, 117, 1649. (j) Fujita, M.; Kwon, Y. J.; Sasaki, O.;
Yamaguchi, K.; Ogura, K. J. Am. Chem. Soc. 1995, 117, 7287. (k) Bowyer,
P. K.; Cook, V. C.; Naseri, N. G.; Gugger, P. A.; Rae, D. A.; Swiegers, G.
F.; Willis, A. C.; Zank, J.; Wild, S. B. Proc. Natl. Acad. Sci. 2002, 99,
4877. (l) Das, N.; Mukherjee, P. S.; Arif, A. M.; Stang, P. J. J. Am. Chem.
Soc. 2003, 125, 13950. (m) Bourgeois, J-P.; Fujita, M.; Kawano, M.;
Sakamoto, S.; Yamaguchi, M. J. Am. Chem. Soc. 2003, 125, 9260.
(3) (a) Hiraoka, S.; Kubota, Y.; Fujita, M. Chem. Commun. 2000, 1509.
(b) Chand, D. K.; Fujita, M.; Biradha, K.; Sakamoto, S.; Yamaguchi, K.
Dalton Trans. 2003, 2750 and references therein. (c) Liu, H. K.; Tong, K.
Chem. Commun. 2002, 1316.
a Grey represents one of the eight trigonal faces each of which is occupied
by a tritopic linker (blue).
1
2a) was indicated by H NMR spectroscopic analysis (Figure
1). Similarly, reaction of Pd(NO3)2 with 1b in a 3:4 molar ratio
in DMSO-d6 yielded 2b, which was also characterized by NMR
and IR spectroscopy.
(4) (a) Fan, J.; Whiteford, J. A.; Olenyuk, B.; Levin, M. D.; Stang, P. J.
J. Am. Chem. Soc. 1999, 121, 2741. (b) Stang, P. J.; Fan, J.; Olenyuk, B.
J. Chem. Soc., Chem. Commun. 1997, 1453. (c) Drain, C. M.; Lehn, J. M.
J. Chem. Soc., Chem. Commun. 1994, 2313. (d) Ikeda, A.; Udzu, H.; Zhong,
Z.; Shinkai, S.; Sakamoto, S.; Yamaguchi, K. J. Am. Chem. Soc. 2001,
123, 3872. (e) Ikeda, A.; Yoshimara, M.; Tani, F.; Naruta, Y.; Shinkai, S.
Chem. Lett. 1998, 587. (f) Ikeda, A.; Yoshimara, M.; Udzu, H.; Fukuhara,
C.; Shinkai, S. J. Am. Chem. Soc. 1999, 121, 4296. (g) Whiteford, J. A.;
Stang, P. J.; Huang, S. D. Inorg. Chem. 1998, 37, 5595. (h) Schnebeck, R.
D.; Randaccio, L.; Zangrando, E.; Lippert, P. Angew. Chem., Int. Ed. 1998,
37, 119. (i) Moon, D.; Kang, S.; Park, J.; Lee, K.; John, R. P.; Won, H.;
Seong, G. H.; Kim, Y. S.; Kim, G. H.; Rhee, H.; Lah, M. S. J. Am. Chem.
Soc. 2006, 128, 3530.
The four proton signals (Ha-d) observed in 2a (Figure 1)
indicate that all the ligands are located in an identical fashion
in the product and are in a symmetry environment identical with
the parent ligand. The overall downfield shift (∆δpyR ) 0.6 and
0.7 ppm for 2a and 2b respectively) of the pyridinyl proton
(7) (a) Olenyuk, B.; Levin, M. D.; Whiteford, J. A.; Shield, J. E.; Stang,
P. J. J. Am. Chem. Soc. 1999, 121, 10434. (b) Two drops of the suspended
solution (made by ultrasound) of the appropriate cage was put on a Cu
grid and it was heated to evaporate the solvent and finally put under a
microscope.
(8) (a) Eaton, P. E.; Cole, F. W. J. Am. Chem. Soc. 1964, 86, 3157. (b)
Fleischer, E. B. J. Am. Chem. Soc. 1964, 86, 3889.
(5) Mukherjee, P. S.; Das, N.; Stang, P. J. J. Org. Chem. 2004, 69, 3526
and references therein.
(6) (a) Chatterjee, B.; Noveron, J. C.; Resendiz, M. J. E.; Liu, J.;
Yamamoto, T.; Parker, D.; Cinke, M.; Nguyen, C. V.; Arif, A. M.; Stang,
P. J. J. Am. Chem. Soc. 2004, 126, 10645. (b) Tashiro, S.; Tominaga, M.;
Kusukawa, T.; Kawano, M.; Sakamoto, S.; Yamaguchi, M.; Fujita, M.
Angew. Chem., Int. Ed. 2003, 42, 3267.
(9) (a) Schweiger, M.; Seidel, S. R.; Schmitz, M.; Stang, P. J. Org. Lett.
2000, 2, 1255. (b) Fujita, M.; Yu, S. Y.; Kusukawa, T.; Funaki, H.; Ogura,
K.; Yamaguchi, K. Angew. Chem., Int. Ed. 1998, 37, 2082.
(10) Johannessen, S. C.; Brisbois, R. G.; Fischer, J. P.; Grieco, P. A.;
Counterman, A. E.; Clemmer, D. E. J. Am. Chem. Soc. 2001, 123, 3818.
J. Org. Chem, Vol. 71, No. 22, 2006 8413

Ghosh and Mukherjee
Several attempts to grow suitable single crystals for X-ray
diffraction failed and only in the case of 2a microcrystalline
was product obtained. MM2 force field simulations9 were
employed to visualize the size and shapes of the cages. The
Pd6(1a)8(NO3)12 composition of 2a yielded the shape of a
nanoscopic ball (Figure 3) with six C4 symmetric centers each
occupied by a Pd(II) ion and eight C3 symmetric trigonal faces
occupied by 1a. A similar shape was also obtained for 2b (see
Supporting Information) by energy-minimized MM2 calcula-
tions. The predicted diameter of 2a is 2.1 nm and the volume
of the sphere is approximately 4851 Å3 while for 2b the diam-
eter is 1.8 nm. The opposite Pd-Pd distance measured the
diameter.
Transmission electron microscopy (TEM) has become a
useful technique in providing valuable information on the size
and shape of nanoensembles.7 A clear TEM7b image of 2a was
observed (Figure 4), strongly supporting the formation of a
2-2.5 nm molecular particle, which was also implied by MM2
force field calculation. TEM image of 2b also suggested the
formation of a spherical molecule of around 2 nm (Supporting
Information). Elemental analyses reveled the presence of 12
solvated DMSO molecules in 2a whereas no solvent was found
in 2b. TGA also confirmed the presence of 12 DMSO molecules
by the appropriate amount of weight loss between 190 and 260
°C. This weight loss was due to the removal of DMSO and
was also confirmed by mass analysis of the evolved molecule
by a mass spectrometer directly attached to the TG instrument.
The reversible solvation (DMSO) was found for the desolvated
2a.12 The cage is stable up to 270 °C (see the Supporting
Information). Upon formation, molecular cage 2a assumes a
less intense yellow color. The absorption spectrum of 2a showed
(see the Supporting Information) a significant decrease in
absorbance relative to 1a after complex formation. The absor-
bance in the electronic spectrum exhibited a near-UV transition,
which is red-shifted relative to 1a. Host-guest experiments
revealed that 2 (both 2a and 2b) could efficiently interact with
FIGURE 1. 1H NMR of 1a (top) and 2a (bottom).
signals in both cases indicated the coordination of the ligand to
metal ions. Hetero-COSEY revealed the diastereotopic nature
of the Hc protons of 2b (Supporting Information). Moreover,
in the IR spectrum of the cages the absorption arising from the
ester/amide group was observed. The CdO stretching frequency
of 1a appeared at 1706 cm-1, whereas the corresponding
stretching frequency in 2a was found at 1731 cm-1. Similarly,
the CdO stretching frequency in 2b is 1658 cm-1, which is 30
cm-1 more than the parent amide 1b. This shift of the CdO
stretching to higher frequency is consistent with the withdrawal
of electron density from the pyridine nitrogen by the Pd metal
center via the N-Pd dative bond formation. In case of 2b,
broadening of the proton peaks was probably due to inter-/
intramolecular hydrogen bonding.
While NMR spectroscopy provides a first insight about the
metal-ligand coordination and formation of a single product,
it does not provide any information about the shape of this kind
of product. ESI mass spectrometry has proven to be a useful
tool in the corroboration of structural assignments for this kind
of self-assemblies.11 Electrospray mass spectroscopy clearly
confirmed a Pd6(1)8(NO3)12 composition with the molecular
mass of 5250.36 and 5226.36 Da for 2a and 2b, respectively.
The ESI-mass spectra of 2a and 2b (Figure 1) showed signals
corresponding to the consecutive loss of nitrate counterions, [M
+
a cationic guest, NEt4 (3), despite their positive charge.
Treatment of excess (5 equiv) NEt4NO3 with a DMSO solution
of 2 resulted in the quantitative accommodation of one 3 per
molecule of cage 2. The inclusion complexes (2‚3)13+ were
identified by 1H NMR spectroscopy, which showed a new
downfield shifted set of signals for 3 with a 1:1 host-guest
ratio (see the Supporting Information). This downfield shift of
proton signals is due to the interaction of the protons of 3 with
the ester/amide moieties of the cages 2. However, no clear
NOESY correlation between the protons of 3 and phenyl rings
of the cages 2 was found. The inclusion complex was isolated
in a pure form by acetone addition and characterized.
In conclusion, we report the first flexible, Pd(II)-based close
framework cage incorporating ester functionality using a new
tripodal ester 1a. An analogous nanoball using N′,N′,N′′-tris-
(3-pyridylmethyl) trimesic amide linker was also prepared.
NMR, ESI-mass spectrometry, TEM, and MM2 force field
calculations support the formation of nanoballs with eight
trigonal faces occupied by C3 symmetric donor linkers. Despite
their tendency to form polymers, ligands 1a and 1b self-
assemble into cages. The use of flexible ester and amide linkers
with Pd(II)/Pt(II) metals and the formation of nanocages has
the potential to expand the coordination driven self-assembly
- 3NO3]3+, [M - 4NO3]4+, [M - 5NO3]5+, and [M - 6NO3]6+
;
for 2a, [M - 3NO3]3+ [m/z 1687.42 (calcd 1688.12)], [M -
4NO3]4+ [m/z 1250.25 (calcd 1250.59)], [M - 5NO3]5+ [m/z
987.75 (calcd 988.07)], [M - 6NO3]6+ [m/z 812.75 (calcd
813.02)]; and for 2b, [M - 3NO3]3+ [m/z 1680.33 (calcd
1680.12)], [M - 4NO3]4+ [m/z 1244.33 (calcd 1244.59)], [M
- 5NO3]5+ [m/z 983.17 (calcd 983.27)], [M - 6NO3]6+ [m/z
809.00 (calcd 809.06)]. Our attempt to prepare a similar cage
using rigid tripodal linker tris(4-pyridyl)methanol in combination
with Pd(II) was unsuccessful; only an insoluble polymeric
product was obtained. So, both the flat (aromatic) part of the
tritopic linker as well as the flexible ester/amide functionalities
are important factors for the formation of this kind of ball. The
former is required to fit on the trigonal faces of a ball and the
flexibility helps to maintain the C4 symmetry of the square-
planar Pd(II) center.
(11) (a) Jude, H.; Disteldorf, H.; Fischer, S.; Wedge, T.; Hawkridge, A.
M.; Arif, A. M.; Hawthorne, M. F.; Muddiman, D. C.; Stang, P. J. J. Am.
Chem. Soc. 2005, 127, 12131. (b) Jude, H.; Sinclair, D. J.; Das, N.; Sherburn,
M. S.; Stang, P. J. J. Org. Chem. 2006, 71, 4155.
(12) The desolvated 2b was soaked in DMSO for 30 min and then washed
several times with acetone and dried completely. TGA and elemental
analyses of the dry product showed the reversible solvation by DMSO.
8414 J. Org. Chem., Vol. 71, No. 22, 2006

Self-Assembly of a Molecular Nanoball
FIGURE 2. ESI-mass spectra of the assemblies 2a (left) and 2b (right).
FIGURE 3. Molecular view of the nanoballs 2a (left) and 2b (right) (green ) Pd, red ) O, blue ) N, white ) C).
FIGURE 4. Transmission electron microscopic view of 2a (left) and 2b (right).
refluxed for 12 h under nitrogen. The solution was washed with
for the construction of novel materials for practical application
water and extracted twice with saturated KHCO3 solution. The
organic part was dried over anhydrous K2CO3 and filtered. The
solvent was removed and the product was crystallized from a
methanol/dichloromethane mixture. Yield: 66%. Anal. Calcd for
C27H21O6N3: C, 67.08; H, 4.3; N, 8.6. Found: C, 67.33; H, 4.42;
like gas adsorption, catalysis, and host-guest chemistry.
Experimental Section
Synthesis of 1a. To a stirred solution of trimesyl chloride (0.66
g, 2.50 mmol) in dry dichloromethane (50 mL) was added
triethylamine (1.12 mL, 8 mmol) under nitrogen atmosphere
followed by the addition of 4-pyridylcarbinol (7.5 mmol, 0.82 g).
This mixture was then stirred under nitrogen for 1 h and then
1
N, 8.35. H NMR (DMSO-d6): δ 8.99 (s, 3H), 8.66 (d, 6H), 7.37
(d, 6H), 5.46 (s, 6H). 13C NMR (DMSO-d6): δ 163.0, 149.8, 143.2,
135.0, 130.0, 121.1, 63.1 ppm. IR (KBr): ν(CdO), 1706 cm-1
ESI: 484.2 [M + H+].
.
J. Org. Chem, Vol. 71, No. 22, 2006 8415

Ghosh and Mukherjee
Synthesis of 1b. To a stirred solution of trimesyl chloride (0.90
g, 3.39 mmol) in dry THF (50 mL) was added triethylamine (1.6
mL, 11.2 mmol) and then 3-aminomethylpyridine (1.09 g, 10.2
mmol) was added under nitrogen atmosphere. This mixture was
stirred for 1 h at room temperature and then refluxed for 24 h. The
white precipitate was filtered and washed several times with water
to remove the byproduct Et3NHCl. Then the white solid was dried
under vacuum. Yield: 60%. Anal. Calcd for C27H24O3 N6: C, 67.50;
2b: Yield: (26.0 mg) 96%. Anal. Calcd for Pd6C216H192
N60O60: C, 49.59; H 3.67; N, 16.07. Found C, 49.43; H, 3.62; N,
1
16.23. H NMR (DMSO-d6): δ 9.9 (br, 3H), δ 9.6 (br, 3H), 9.3
(br, 3H), 8.4(br, 3H), 8.1 (br, 3H), 7.7 (br, 3H), 4.6 (br, 6H) ppm.
13C NMR (DMSO-d6): δ 166.0, 151.0, 150.2, 139.9, 139.0, 134.7,
129.4, 126.8, 40.0 ppm. IR (KBr): ν(CdO), 1658 cm-1; ν(NH)
3042 cm-1
.
1
H, 4.00; N, 17.5. Found: C, 67.34; H, 4.86; N, 17.4. H NMR
(DMSO-d6): δ 9.5 (s, 3H), 8.70 (d, 3H), 8.60 (s, 3H), 8.56 (d,
3H), 7.80 (m, 3H), 7.50 (d, 3H), 4.60 (s, 6H). 13C NMR (DMSO-
d6): δ 166.3, 147.3, 146.7, 135.5, 134.2, 128.1, 123.1, 121.0, 40.0
ppm. IR (KBr): ν(CdO), 1628 cm-1; ν(NH), 3061 cm-1. ESI:
481.0 [M + H+].
General Procedure for the Preparation of Cage 2a and 2b.
To a 2 mL DMSO solution containing 7.98 mg (0.03 mmol) of
Pd(NO3)2 was added a DMSO solution (1 mL) of either 1a or 1b
(0.04 mmol) drop-by-drop with continuous stirring. The mixture
was stirred for 2 h and adding acetone precipitated the product.
Acknowledgment. Financial support from the DST, Gov-
ernment of India, and the Indian Institute of Science, Bangalore
is gratefully acknowledged. We thank Mr. B. R. Mohan of
Thermo Electron Corporation, Bangalore Division, for assistance
with the electrospray mass spectra.
Supporting Information Available: General experimental
1
method, H NMR (for 1b and 2b), and 13C NMR for 1a, 1b, 2a,
2a: Yield: (26.4 mg) 97%. Anal. Calcd for Pd6C240
240N36O96S12: C, 46.54; H, 3.8; N, 8.14; S, 6.2. Found: C, 46.32;
-
and 2b (five figures); TGA and UV-vis spectra for 2a. This
material is available free of charge via the Internet at http://
pubs.acs.org.
H
1
H, 3.92; N, 8.23; S, 6.38. H NMR (DMSO-d6): δ 9.26 (d, 6H),
8.70 (s, 3H), 7.60 (d, 6H), 5.55(s, 6H). 13C NMR (DMSO-d6): δ
163.1, 151.9, 150.0, 134.0, 130.0, 122.0, 63.1 ppm. IR (KBr): ν-
(CdO), 1731 cm-1
.
JO061311G
8416 J. Org. Chem., Vol. 71, No. 22, 2006