Double-stranded supramolecular assembly through salt bridge formation between rigid and flexible amidine and carboxylic acid strands

Article abstract of DOI:10.1021/jo902158v

(Chemical Equation Presented) A series of monomeric strands consisting of m-terphenyl backbones with chiral rigid C-linked (3) and flexible N-linked (5) formamidines and achiral carboxylic acid (4) and flexible carboxymethyl (6) residues were synthesized, and their duplex formations through amidinium-carboxylate salt bridges were investigated by NMR, circular dichroism (CD), and UV-visible spectroscopies. The salt bridge-derived duplex formation was largely dependent on the structures of the formamidine and carboxylic acid strands, and the C-linked formamidine strand 3 formed a more stable duplex with the complementary carboxylic acid strands (4 and 6) than did the flexibleN-linked formamidine strand 5. The single crystal X-ray analysis revealed that the duplex 5·4 has a skewed right-handed double helical structure. A complementary duplex dimer was also synthesized from the dimers of 5 and 4 joined by diacetylene linkers. Variable-temperature CD measurements indicated that the duplex possesses a dynamic double helical structure resulting from the flexible N-linked formamidine units.

Full text of DOI:10.1021/jo902158v

pubs.acs.org/joc
Double-Stranded Supramolecular Assembly through Salt Bridge
Formation between Rigid and Flexible Amidine and
Carboxylic Acid Strands
Hiroki Iida, Munenori Shimoyama, Yoshio Furusho, and Eiji Yashima*
Department of Molecular Design and Engineering, Graduate School of Engineering, Nagoya University,
Chikusa-ku, Nagoya 464-8603, Japan
yashima@apchem.nagoya-u.ac.jp
Received October 20, 2009
A series of monomeric strands consisting of m-terphenyl backbones with chiral rigid C-linked (3) and
flexible N-linked (5) formamidines and achiral carboxylic acid (4) and flexible carboxymethyl (6)
residues were synthesized, and their duplex formations through amidinium-carboxylate salt bridges
were investigated by NMR, circular dichroism (CD), and UV-visible spectroscopies. The salt
bridge-derived duplex formation was largely dependent on the structures of the formamidine and
carboxylic acid strands, and the C-linked formamidine strand 3 formed a more stable duplex with the
complementary carboxylic acid strands (4 and 6) than did the flexible N-linked formamidine strand 5.
The single crystal X-ray analysis revealed that the duplex 5 4 has a skewed right-handed double
3
helical structure. A complementary duplex dimer was also synthesized from the dimers of 5 and 4
joined by diacetylene linkers. Variable-temperature CD measurements indicated that the duplex
possesses a dynamic double helical structure resulting from the flexible N-linked formamidine units.
Introduction
information from generation to generation. Consequently,
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of DNA, that is, the complementarity of the strands.
The double helix of DNA has attracted significant interest
because of its key biological structure and functions, for
example, the storage and faithful transmission of genetic
*To whom correspondence should be addressed.
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DOI: 10.1021/jo902158v
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Published on Web 12/21/2009
J. Org. Chem. 2010, 75, 417–423 417
2009 American Chemical Society

Iida et al.
JOCArticle
Inspired by the complementary double helical structure of
DNA, we have recently designed and synthesized a comple-
mentary double helical oligomer 1 2 with a controlled helix-
3
sense that consists of two crescent-shaped m-terphenyl
strands containing optically active formamidine and achiral
carboxyl groups (Figure 1).8a The duplexation relies on the
amidinium-carboxylate salt bridges which are unique in
binding structures composed of double hydrogen bonds and
the electrostatic interaction between the basic amidines and
carboxylic acids.10 The complementary face-to-face associa-
tion between each m-terphenyl strand through the amidi-
nium-carboxylate salt bridges leads to the well-defined,
intertwined duplex. Due to the high association constant
FIGURE 1. Artificial double helix 1 2 assisted by face-to-face salt
bridge formation.
3
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E. Chem. Commun. 2009, 1650–1652.
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Furusho, Y.; Sakurai, S.-I.; Kumaki, J.; Okoshi, K.; Yashima, E. J. Am.
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FIGURE 2. (a) Structures of chiral formamidines (3 and 5) and
achiral carboxylic acids (4 and 6) and (b) schematic illustration of
amidinium-carboxylate salt bridge formation.
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Chem., Int. Ed. Engl. 1992, 31, 654–656. (b) Kirby, J. P.; Roberts, J. A.;
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and the rigid intertwined structure associated with the
chiral formamidines, the resulting duplex has a thermody-
namically stable one-handed double helical structure.8a,g On
the basis of this model oligomer study, the face-to-face salt
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D. G. J. Am. Chem. Soc. 2006, 128, 10474–10483. (i) Sanchez, L.; Sierra, M.;
Martin, N.; Myles, A. J.; Dale, T. J.; Rebek, J., Jr.; Seitz, W.; Guldi, D. M.
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Furusho, Y.; Yashima, E. Angew. Chem., Int. Ed. 2007, 46, 2435–2439. (k)
Otsuki, J.; Kanazawa, Y.; Kaito, A.; Islam, D. M. S.; Araki, Y.; Ito, O.
Chem.;Eur. J. 2008, 14, 3776–3784.
bridge-assisted duplex (3 4) formation by the C-linked
3
m-terphenyl formamidine 3 and the m-terphenyl formic acid
4 (Figure 2) has been used as a motif for creating various
complementary double helical oligomers with different
lengths and sequences,8b,d,e,g and polymers8c,f with a controlled
418 J. Org. Chem. Vol. 75, No. 2, 2010

Iida et al.
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SCHEME 1. Synthesis of the Strands 5 and 11
FIGURE 3. 1H NMR (500 MHz, CDCl3, rt) spectra of (a) 3
(4.9 mM), (b) 3 4b (1 mM), (c) 3 6 (1 mM), (d) 5a (117 mM), (e) 5a
(117 mM) with acetic acid (5 equiv), (f) 5a 4b (1 mM), and (g)
3
3
3
5a 6 (1 mM).
3
handedness. We anticipated that the modification of the
geometrical structure around the salt bridges would provide
a novel complementary double helical motif. In this study,
we designed and synthesized a series of novel skewed
duplexes, such as 3 6, 5 4, and 5 6, using chiral N-linked
m-terphenyl formamidine 5 and/or achiral m-terphenyl
acetic acid 6 in combination with the conventional strands
3 and 4 (Figure 2), and investigated their association
behaviors and chiroptical properties. In addition, we also
prepared a double helical dimer based on the skewed duplex
SCHEME 2. Synthesis of the Strand 6
3
3
3
ester 15, which was then hydrolyzed in an aqueous NaOH
solution to give 6 in 95% yield. All the monomers and dimers
were purified by column chromatography and characterized
motif 5 4 with the flexible N-linked formamidine units,
3
and its preferred-handed helical structure in solution as
compared to that of the dimer of 3 4 (1 2 in Figure 1) was
3
3
1
and identified with H and 13C NMR spectroscopies, ele-
investigated by means of NMR, circular dichroism (CD),
and UV-visible spectroscopies as well as the single crystal
mental analyses, and mass measurements (see the Experi-
mental Section or the Supporting Information (SI)).
Duplex Formation of Monomeric Strands. We first inves-
tigated the duplex formation of the formamidines 3 or 5a
X-ray analysis of the model duplex 5 4.
3
Results and Discussion
1
with the carboxylic acids 4b or 6 in CDCl3 by H NMR
Synthesis of Monomeric and Dimeric Strands. The new
chiral monomeric formamidine 5, in which the m-terphenyl
group is linked to one of the nitrogen atoms of the forma-
midine, and the corresponding dimer 11 joined by a diace-
tylene linker were prepared according to Scheme 1. The
optically active formamide 7, derived from (R)-1-phenyl-
ethylamine,11 was allowed to react with dibromoaniline 8
and thionyl chloride, producing the formamidine 9 in 48%
yield.12 The Suzuki-Miyaura coupling of 9 with the boro-
nate 10 afforded 5a. Subsequently, 5a was then treated with
Bu4NþF- to yield the monodesilylated formamidine 5c
(30%) along with the didesilylated formamidine 5b (7%
yield). 5c was then dimerized by the oxidative coupling with
use of Pd and Cu catalysts to give the dimeric strand 11 in
65% yield. The m-terphenyl acetic acid 6 was also prepared
according to Scheme 2. The Suzuki-Miyaura coupling of
the dichloride 12 with the boronic acid 13 catalyzed by
Pd(OAc)2 and the phosphine ligand 1413 yielded the methyl
spectroscopy. The formamidine 3 showed a complicated
spectrum due to the presence of several conformers originat-
ing from the E-Z isomerism of the CdN double bonds and
the restricted rotation about the C-C bond between the
formamidine and m-terphenyl groups.8a On the other hand,
1
the H NMR spectrum of 3 in the presence of 6 became
simple, indicating the formation of the skewed duplex 3 6 in
3
which the formamidine unit is most likely fixed in the E
configuration through the salt bridge formation as in the case
of the face-to-face duplex 3 4b (Figure 3, and in the Support-
3
ing Information Figures S1 and S2).8a The NH proton
resonances of the 3 4b appeared as a doublet peak at a low
3
magnetic field of approximately δ 13.3 ppm (Supporting
Information, Figure S1) due to the salt bridge formation,
while the corresponding NH protons were not observed for
3 6 probably due to the fast monomeric strand-duplex
3
equilibrium. The association constant of 3 and 6 estimated
by 1H NMR in CDCl3 at 25 °C14 was 1.7 ꢀ 105 M-1 (Support-
ing Information, Figure S6), which was lower than that of
(11) Iwata, M.; Kuzuhara, H. Chem. Lett. 1989, 18, 2029–2030.
(12) Gall, M.; McCall, J. M.; TenBrink, R. E.; VonVoigtlander, P. F.;
Mohrland, J. S. J. Med. Chem. 1988, 31, 1816–1820.
(13) Walker, S. D.; Barder, T. E.; Martinelli, J. R.; Buchwald, S. L.
Angew. Chem., Int. Ed. 2004, 43, 1871–1876.
(14) See the Supporting Information and: Connors, K. A. Binding Con-
stants: The Measurement of Complex Stability; John Wiley & Sons: New
York, 1987.
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FIGURE 4. CD spectra of 3, 3 4b, 3 6 (a) and 5a, 5a 4b, and 5a 6 (b) at ca. 25 °C in CDCl3 (1 mM).
3
3
3
3
3 and 4b (ca. 106 M-1),15 because of the relatively weaker acidity
of the alkyl carboxylic acid 6 compared to that of the aryl
carboxylic acid 4b.16
clear CD signals below 330 nm, while 3 exhibited a weak
Cotton effect in this region (Figure 4a). The CD intensity of
the second Cotton effect (Δεsecond) of 3 6 is about -94 M-1
3
The formamidine 5a also showed a complicated 1H NMR
spectrum caused by a similar E-Z isomerism and tautome-
rism of the CdN double bonds (Figure 3d, and in the
Supporting Information Figure S3). However, the spectrum
showed clear signals after the addition of excess acetic acid,
which can be ascribed to the formation of the amidinium-
acetate salt bridges (Figure 3e). The 1H NMR spectra of 5a
with 4b also exhibited similar simple peaks, indicating the
cm-1, which is approximately 3.5 times greater than that
of 3 (Δεsecond=-27 M-1 cm-1). Although the CD intensity
of 3 6 is relatively smaller than that of 3 4b (Δεfirst=-131
3
3
M-1 cm-1), the remarkable enhancement of the Cotton
effect upon the complexation of 3 with the achiral 6 and 4b
suggested the formation of a preferred-handed double helix
consisting of intertwined complementary m-terphenyl units.
In contrast, 5a showed a very weak Cotton effect, because 5a
possesses a one chiral phenylethyl group and the stereogenic
center is located relatively far from the m-terphenyl strand
(Figure 4b). However, once complexed with the achiral 4b,
the CD spectral pattern significantly changed and its inten-
formation of the complementary skewed duplex 5a 4b
3
(Figure 3f, and in the Supporting Information Figure S3),
although the broadening of the resonances and the lack of
NH protons due to the salt bridge formation suggest a rather
weak association of this system. In contrast, when 6 was used
for the complexation with 5a in CDCl3, the observed spec-
trum of the mixture remained complicated, suggesting in-
complete salt bridge formation (Figure 3g, and in the
Supporting Information Figure S4). The association con-
stants of 5a with 4b or 6 estimated by 1H NMR in CDCl314 at
25 °C were 1.2 ꢀ 104 and 7.5 ꢀ 102 M-1, respectively (Suppor-
ting Information, Figure S6). The weaker basicity of 5a
relative to 3 is likely caused by the lack of the N-alkyl
substituent on the formamidine group,17 resulting in the
lower association constants for 5a 4b and 5a 6 in compari-
sity drastically increased to form a duplex 5a 4b, in which
3
75% of 5a and 4b are estimated to be complexed. These
results suggest that the duplex likely adopts an excess one-
handed double helical structure, although the CD intensity
of the duplex was much weaker than that of the 3 4b. On the
3
other hand, no apparent Cotton effect enhancement was
observed for 5a 6, as expected from its low association
3
constant in CDCl3, indicating that 5a and 6 may be too
flexible to form a preferred-handed intertwined double
helical structure.
The X-ray single-crystal analysis was then carried out to
gain insight into the chiral structure of the skewed duplex
5 4. The single crystals suitable for the X-ray structural
3
3
son to those for 3 4b and 3 6, respectively.
The chiroptical properties of the chiral duplexes 3 4b, 3 6,
3
3
3
3
3
5a 4b, and 5a 6 were then investigated by CD and absorp-
3
analysis were obtained from a toluene/hexane solution of
the skewed duplex 5b 4b without the terminal trimethylsilyl
groups. The crystal structure revealed that the duplex 5b 4b
3
tion measured in CDCl3 at ca. 25 °C (1 mM) (Figure 4, and in
the Supporting Information Figure S5). The CD spectrum of
3
3
3 6 in CDCl3, in which 93% of 3 and 6 are hybridized under
3
adopts a right-handed double helical structure (Figures 5,
and in the Supporting Information Figure S7 and Table S1).
The formamidine and carboxy groups formed a salt bridge
with two hydrogen bonds with an average N-O distance of
the conditions based on the association constant, showed
(15) Dilution 1H NMR experiments were performed to estimate the
association constants of 3 with 4b and 11 with 16 in CDCl3. These constants
˚
2.69 A. Due to the N-linked formamidine residue, the two
were observed that upon the dilution of the complexes 3 4b (0.05 mM) and
3
monomeric strands were held together in a skewed manner;
the angle formed between two intersecting straight lines,
passing through C10 and C13 of 5b and C41 and C44 of 4b, is
123° (Figure 5c). The benzene rings of the formamidine-
bound m-terphenyl group were chirally twisted clockwise by
49° and 53°, whereas those of the carboxy-bound m-terphe-
nyl group were twisted counterclockwise by 56° and 61°. The
preferential clockwise twists were observed for all the ben-
zene rings of the m-terphenyl groups in the right-handed
11 16 (0.01 mM), no signals due to the dissociated amidine 3 and 11 could be
3
detected (Supporting Information, Figures S1 and S8). On the basis of the
results together with the detection limit, the association constants were
roughly estimated. See ref 8a.
(16) The acidities of phenylacetic acid (pKa = 13.5 and 11.6) in DMF
and DMSO are relatively weaker than those of benzoic acid (pKa = 12.3
and 10.9), respectively. See: Izutsu, K. Acid-Base Dissociation Constants in
Dipolar Aprotic Solvents; IUPAC Chemical Data Series, No. 35; Blackwell
Scientific: Oxford, UK, 1990.
(17) For examples of the substituent effects of amidines on basicity, see:
(a) Raczynska, E.; Oszczapowicz, J. Tetrahedron 1985, 41, 5175–5179. (b)
Oszczapowicz, J.; Ciszkowski, K. J. Chem. Soc., Perkin Trans. 2 1987, 663–
668.
double helical 1 2.8a Therefore, the relatively weak Cotton
3
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FIGURE 7. CD spectra of 11 (ca. 25 °C) and 11 16 (-55, -35, -15,
3
5, and 25 °C) and UV/vis spectra of 11, 16, and 11 16 (ca. 25 °C) in
3
CDCl3. The concentrations of 11, 16, and 11 16 are 0.1, 0.1, and
0.02 mM, respectively.
3
FIGURE 5. X-ray structure of 5b 4b: (a) top view and (b) side view.
Carbon, black or pale blue; oxygen, red; nitrogen, blue. The inter-
3
molecular hydrogen bonds between N and O with average N
˚
O
3 3 3
1
16, the H NMR spectrum became simple, indicating the
duplex formation of the dimer 11 16, although most of the
signals remained broadened. The salt bridge formation was
supported by the NH proton resonances that appeared as a
broad peak at a lower magnetic field of δ 13.4 ppm. We noted
that the corresponding NH proton resonances could not be
distances of 2.69 A are shown by light blue dashed lines. (c) Schema-
tic illustration of the geometry around the skewed salt bridge.
3
observed for the monomer duplex 5a 4b. Upon dilution of a
3
solution of the duplex (0.01 mM), no signals due to the
dissociated free 11 and 16 were observed. Thus, the dimeric
skewed complex 11 16 formed a rather stable duplex stabi-
3
lized by the double salt bridge formation. The association
constant of 11 and 16 in CDCl3 was roughly estimated to be
greater than 106 M-1 15
The duplex 11 16 showed weak, but apparent CD signals
.
3
below 370 nm, whereas 11 exhibited very weak Cotton effects
in this region (Figure 7). This remarkable enhancement of the
Cotton effects for the duplex 11 16, especially in the absorp-
3
tion region of the diacetylene linkages (ca. 300-370 nm),
indicated that the duplex 11 16 most likely adopted a pre-
ferred-handed double helical structure, as in the case of the
3
analogous duplex 1 2.8a Incontrast tothe thermodynamically
stable, right-handed double helix (R)-1 2, the 11 16 showed
3
3
3
significant changes in its CD intensity depending on tempera-
ture; the CD intensity in the diacetylene linkage region
gradually increased with the decreasing temperature from 25
to -35 °C, and was drastically enhanced at -55 °C not only in
the diacetylene linkage chromophore region, but also in the
m-terphenyl absorption region (250-300 nm). These results
FIGURE 6. 1H NMR (500 MHz, CDCl3, rt) spectra of (a) 11 (0.1
mM), (b) 16 (3.1 mM), and (c) 11 16 (0.1 mM).
3
effects observed in the 5a 4b may be ascribed to the opposite
3
twist-sense of the m-terphenyl groups.
Duplex Formation of Dimeric Strands of 5 and 4. On the
basis of systematic studies on the complexation of complemen-
tary monomeric strands with different formamidine and car-
boxyl groups together with the crystal structural analysis of
clearly indicated that the duplex 11 16 has a dynamic nature
3
and the diastereomeric right- and left-handed double helices
are under equilibrium, which shifts to an excess of helices at
low temperature. In addition, the dramatic increase in the
Cotton effect intensity in the m-terphenyl absorption region
suggests that the twist-sense of the m-terphenyl groups may
also be significantly biasedat-55 °C, although the twist-sense
5b 4b, it was found that the newly designed flexible N-linked
3
formamidine strand 5 could form a relatively stable intertwined
duplex with the m-terphenyl formic acid strand 4, therefore we
then prepared dimers of 5 and 4 joined by diacetylene linkers 11
and 16, respectively (Scheme 1), and its duplex formation was
investigated. 1-Octynyl chains were introduced as a solubility
enhancer in the dicarboxylic acid 16.8g
of the m-terphenyl groups of the monomeric duplex (5b 4b)
3
was opposite to each other in the solid state (Figure 5).
However, the observed CD intensity of 11 16 (Δεfirst = 14.4
3
M-1 cm-1 in CDCl3 at -55 °C) is still much lower than that of
Due to the presence of a variety of conformational iso-
1
mers, 11 showed a broad and quite complicated H NMR
spectrum in CDCl3 (Figure 6a, and in the Supporting
Information Figure S8). However, upon complexation with
the face-to-face complexed 1 2 (Δεfirst = -246 M-1 cm-1 in
3
CDCl3 at 20 °C).8a Therefore, the helix-sense excess of 11 16
3
may be lower than that of 1 2.
3
J. Org. Chem. Vol. 75, No. 2, 2010 421

Iida et al.
JOCArticle
Summary
CHN, 1H), 1.37 (d, CH3CHN, J = 6.3 Hz, 3H), 0.29 (s, TMS,
18H). 13C NMR (176 MHz, CDCl3, 5a (47 mM), CH3CO2H (5
equiv)): δ 156.9, 140.3, 138.7, 138.0, 133.7, 132.2, 130.4, 129.5,
129.0, 128.01, 127.96, 126.2, 122.6, 104.7, 95.3, 56.5, 21.9, 0.07.
MS (FAB): m/z 569 [M þ H]þ. Anal. Calcd for C37H40N2Si2: C,
78.12; H, 7.09; N, 4.92. Found: C, 77.98; H, 6.86; N, 4.79.
Synthesis of 5c and 5b. A solution of Bu4NþF- in THF (0.100
mM, 1.68 mL, 0.168 mmol) was added portionwise (140 μL at a
time) to a solution of 5a (163 mg, 0.287 mmol) and acetic acid
(36.7 mg, 0.611 mmol) in anhydrous THF (6.60 mL). After
stirring for 10 min, aqueous HCl (1 M, 1.0 mL) and water (50
mL) were sequentially added. The mixture was then extracted
with Et2O (2 ꢀ 50 mL). The organic extracts were washed with
water (50 mL) and brine (50 mL) and then dried over anhydrous
MgSO4. After filtration, the solvent was removed by evapora-
tion. The residue was subjected to SEC fractionation to obtain
5c (42.9 mg, 30%) as a yellowish solid and 5b (8.8 mg, 7.2%) as a
white solid.
In summary, we have synthesized a series of monomeric
and dimeric strands consisting of m-terphenyl backbones
with rigid or flexible chiral formamidines and achiral
carboxyl acid units. The duplex formation through the
amidinium-carboxylate salt bridges is highly dependent
on the structures of the formamidines and carboxylic acids,
and the C-linked formamidine strand appeared to form a
more stable duplex with the complementary carboxylic acid
strands than the N-linked formamidine strand. The dimeric
duplex 11 16 was found to possess a dynamic double helical
3
structure probably caused by the flexible skewed N-linked
formamidine units. This finding may be useful to develop
unique complementary double helical oligomers and poly-
mers with dynamic characteristics.9c,d,h,j,m,n
5c: Mp: 57.8-58.0 °C. IR (film, cm-1): 3418 (νN;H), 3294
(νCtCH), 2155 (νCtC), 1644 (νCdN). 1H NMR (700 MHz,
CDCl3, 5c (57 mM), CH3CO2H (5 equiv)): δ 7.54 (d, 2H, J =
8.1 Hz), 7.51 (d, J = 8.4 Hz, 2H), 7.40 (t, J = 7.6 Hz, 1H),
7.35-7.23 (m, 9H), 6.83 (d, J = 7.1 Hz, 2H), 6.63 (s, 1H), 4.23
(q, J = 6.8 Hz, 1H), 3.16 (s, 1H), 1.37 (d, J = 6.9 Hz, 3H), 0.29
(s, 9H). 13C NMR (176 MHz, CDCl3, 5c (57 mM), CH3CO2H (5
equiv)): δ 156.9, 140.0, 138.63, 138.57, 138.4, 137.9, 133.4, 132.4,
132.2, 130.5, 130.4, 129.6, 129.5, 129.1, 128.1, 128.0, 126.3,
122.6, 121.6, 104.7, 95.3, 83.3, 78.1, 56.5, 21.8, 0.08. HRMS
(ESIþ): m/z calcd for C34H33N2Si (M þ Hþ) 497.2413, found
497.2425.
Experimental Section
Materials. Copper(I) iodide (CuI) and palladium acetate
(Pd(OAc)2) were obtained from Kishida (Osaka, Japan).
Tetrakis(triphenylphosphine)palladium(0) (Pd(PPh3)4) and bis-
(triphenylphosphine)palladium(II) dichloride (Pd(PPh3)2Cl2)
were purchased from Tokyo Kasei (TCI, Tokyo, Japan). (R)-
N-(1-Phenylethyl)formamide (7),11 4-(trimethylsilylethynyl)-
phenylboronic acid (13),18 carboxylic acids 4a8a and 4b,8c and
dicarboxylic acid 168g were synthesized according to the pre-
viously reported methods.
Synthesis of (R)-N-(2,6-Dibromophenyl)-N0-(1-phenylethyl)-
formamidine (9). SOCl2 (121 mg, 1.02 mmol) was added drop-
wise to a solution of 2,6-dibromoaniline (8) (251 mg, 1.00 mmol)
and 7 (172 mg, 1.16 mmol) in anhydrous toluene (1.00 mL). The
resultant suspension was stirred at room temperature for 1 h and
at 65 °C for 16 h. After nitrogen was bubbled through the
solution for 1 h, the solvent was evaporated under reduced
pressure. The residue was then purified by column chromato-
graphy (NH-SiO2, hexane/CHCl3 = 9/1 to 4/1) to afford 9 (186
mg, 48%) as a white solid. Mp: 84.7-86.8 °C. IR (KBr, cm-1):
5b: Mp: 175.7-175.9 °C. IR (KBr, cm-1): 3435 (νN;H), 3292
(νCtCH), 2107 (νCtC), 1631 (νCdN). 1H NMR (500 MHz,
CDCl3, 5b (12.1 mM), CH3CO2H (10 equiv)): δ 7.54 (d, 4H,
J = 7.9 Hz), 7.42-7.31 (m, 7H), 7.31-7.22 (m, 3H), 6.84 (m,
2H), 6.66 (s, 1H), 4.25-4.18 (m, 1H), 3.16 (s, 2H), 1.36 (d, J =
6.9 Hz, 3H). 13C NMR (176 MHz, CDCl3, 5b (15.8 mM),
CH3CO2H (17 equiv)): δ 157.0, 140.2, 138.5, 138.4, 134.1,
132.4, 130.5, 129.7, 129.1, 128.1, 127.8, 126.3, 121.5, 83.3,
78.1, 56.4, 22.0. HRMS (ESIþ): m/z calcd for C31H25N2 (M þ
Hþ) 425.2018, found 425.2000.
1
3247 (νN-H), 1644 (νCdN). H NMR (300 MHz, CDCl3, 9 (69
mM), CH3CO2H (5 equiv)): δ 7.49 (d, J=8.0 Hz, 2H), 7.44-
7.18 (m, 6H), 6.83 (t, J = 7.8 Hz, 1H), 4.61 (q, J = 6.9 Hz, 1H),
1.65 (d, J = 6.8 Hz, 3H). 13C NMR (126 MHz, CDCl3, 9 (69
mM), CH3CO2H (5 equiv)): δ 158.5, 142.6, 142.5, 132.3, 128.9,
127.9, 127.1, 126.3, 120.3, 55.8, 23.3. Anal. Calcd for C15H14-
Br2N2: C, 47.15; H, 3.69; N, 7.33. Found: C, 46.96; H, 3.57;
N, 7.18.
Synthesis of 11. A mixture of 5c (24.0 mg, 48.3 μmol),
Pd(PPh3)2Cl2 (3.3 mg, 4.7 μmol), CuI (1.0 mg, 5.4 μmol), and
Et3N (0.060 mL) in anhydrous THF (1.18 mL) was stirred at
room temperature for 6 h. After evaporation to dryness, the
residue was then purified by SEC chromatography (Biobeads
SX-3, CHCl3) to afford 11 (15.5 mg, 65%) as a yellow powder.
Mp: 181 °C dec. [R]20D þ24.5 (c 0.29, CH2Cl2). IR (KBr, cm-1):
1
Synthesis of 5a. To a mixture of 10 (61.0 mg, 0.203 mmol), 9
(30.0 mg, 78.4 μmol), and Pd(PPh3)4 (9.06 mg, 7.84 μmol) were
added EtOH (0.32 mL) and anhydrous toluene (0.79 mL)
followed by an addition of aqueous Na2CO3 (2 M, 0.26 mL,
0.520 mmol). The mixture was refluxed for 17 h under stirring.
After cooling, water (10 mL) was added to the reaction mixture
and the mixture was extracted with Et2O (2 ꢀ 10 mL). The
organic extracts were washed with water (10 mL) and brine (10
mL) and then dried over anhydrous MgSO4. After filtration, the
solvent was removed by evaporation. The residue was then
purified by column chromatography (NH-SiO2, hexane/
CHCl3 = 10/0 to 4/1) to afford 5a (13.9 mg, 31%) as a yellowish
3422 (νN;H), 2156 (νCtC), 1647 (νCdN). H NMR (700 MHz,
CDCl3, 11 (14 mM), CH3CO2H (10 equiv)): δ 7.60 (d, J = 7.6
Hz, 4H), 7.52 (d, J = 7.6 Hz, 4H), 7.45-7.23 (m, 20H), 6.86 (d,
J = 5.7 Hz, 4H), 6.65 (s, 2H), 4.22 (m, 2H), 1.38 (d, J = 6.7 Hz,
6H), 0.29 (s, 18H). 13C NMR (176 MHz, CDCl3, 11 (14 mM),
CH3CO2H (10 equiv)): δ 157.0, 140.3, 139.0, 138.6, 138.3, 138.1,
134.1, 132.8, 132.2, 130.6, 130.4, 129.9, 129.6, 129.1, 128.1,
127.8, 126.2, 122.6, 121.1, 104.7, 95.3, 81.6, 74.9, 56.5, 22.1,
-0.1. Anal. Calcd for C68H62N4Si2: C, 82.38; H, 6.30; N, 5.65.
Found: C, 82.16; H, 6.39; N, 5.53.
Synthesis of 15. A mixture of 12 (542 mg, 2.47 mmol), 13 (2.00
g, 9.18 mmol), Pd(OAc)2 (13.0 mg, 57.5 mmol), K3PO4 (2.92 mg,
13.8 mmol), and 14 (52.1 mg, 0.123 mmol) in anhydrous toluene
(9.50 mL) was stirred at 100 °C for 10 h. Water (50 mL) was
added to the reaction mixture and the mixture was extracted
with Et2O (2 ꢀ 50 mL). The organic extracts were washed with
water (50 mL) and brine (50 mL) and then dried over anhydrous
MgSO4. After filtration, the solvent was removed by evapora-
tion. The residue was then purified by column chromatography
(SiO2, hexane/Et2O = 10/0 to 99/1) to afford 15 (399 mg, 34%)
as a white solid. Mp: 149.5-149.8 °C. IR (KBr, cm-1): 2158
solid. Mp: 111.5-111.7 °C. [R]20 þ14.9 (c 0.39, CH2Cl2). IR
D
(KBr, cm-1): 3422 (νN;H), 2156 (νCtC), 1639 (νCdN). 1H NMR
(500 MHz, CDCl3, 5a (117 mM), CH3CO2H (5 equiv)): δ 7.51
00
00
(d, H3, 5, 3 , 5 , J = 7.7 Hz, 4H), 7.38 (t, H5 , J = 6.7 Hz, 1H),
0
0
0
00
00
7.35-7.23 (m, H4 , 6 , 4, 6, 4 , 6 , m- and p-H of Ph, 9H), 6.83 (d,
o-H of Ph, J = 7.7 Hz, 2H), 6.63 (s, NdCH, 1H), 4.25-4.18 (m,
(18) Thoresen, L. H.; Jiao, G.-S.; Haaland, W. C.; Metzker, M. L.;
Burgess, K. Chem.;Eur. J. 2003, 9, 4603–4610.
422 J. Org. Chem. Vol. 75, No. 2, 2010

Iida et al.
JOCArticle
1
(νCtC), 1736 (νCdO). H NMR (500 MHz, CDCl3): δ 7.49 (d,
J = 8.5 Hz, 4H), 7.36 (t, J = 7.2 Hz, 1H), 7.25 (d, J = 8.4 Hz,
4H), 7.23 (d, J = 7.6 Hz, 2H), 3.45 (s, 3H), 3.42 (s, 2H), 0.26 (s,
18H). 13C NMR (126 MHz, CDCl3): δ 172.4, 142.9, 141.7, 131.8,
129.7, 129.3, 129.0, 127.0, 122.1, 104.8, 94.7, 51.8, 36.6, 0.0.
HRMS (ESIþ): m/z calcd for C31H34NaO2Si2 (M þ Naþ)
517.1995, found 517.2002.
Synthesis of 6. A mixture of 15 (30.3 mg, 62.9 mmol) and
aqueous NaOH (2.5 M, 0.78 mL) in EtOH (2.00 mL) was stirred
at room temperature for 5 days. After an addition of water (10
mL), the mixture was washed with Et2O (10 mL) and the
aqueous layer was acidified with aqueous HCl (1 M). The
precipitated solid was extracted with Et2O (2 ꢀ 10 mL) and
the organic extracts were dried over anhydrous MgSO4. After
filtration, the solvent was removed by evaporation. The residue
was then purified by SEC chromatography (Biobeads SX-3,
CHCl3) toafford6(20.1 mg, 95%) as a white powder. Mp: 202.3 °C
dec. IR (KBr, cm-1): 3195 (νO;H), 2103 (νCtC), 1715 (νCdO). 1H
NMR (500 MHz, CDCl3, 6 (40 mM)): δ 7.54 (d, J = 8.4 Hz, 4H),
7.39 (t, J = 7.4 Hz, 1H), 7.28 (d, J = 8.5 Hz, 4H), 7.25 (d,
J = 6.1 Hz, 2H), 3.50 (s, 2H), 3.13 (s, 1H). 13C NMR (126 MHz,
CDCl3, 6 (40 mM)): δ 176.9, 142.8, 141.8, 132.1, 129.4 129.1, 128.9,
127.2, 121.3, 83.3, 77.8, 36.3. Anal. Calcd for C24H16O2: C, 85.69; H,
4.79. Found: C, 85.39; H, 4.86.
Acknowledgment. This work was supported in part by a
Grant-in-Aid for Scientific Research (S) from the Japan
Society for the Promotion of Science (JSPS) and the Global
COE Program “Elucidation and Design of Materials and
Molecular Functions” of the Ministry of Education, Cul-
ture, Sports, Science, and Technology, Japan.
Supporting Information Available: Experimental proce-
dures and characterization data for the monomeric duplexes
3 4b, 3 6, 5a 4b, and 5a 6, and the dimeric duplex 11 16. This
3
3
3
3
3
material is available free of charge via the Internet at http://
pubs.acs.org.
J. Org. Chem. Vol. 75, No. 2, 2010 423