Selective iodinated dipyrrolyldiketone BF2 complexes as potential building units for oligomeric systems

Article abstract of DOI:10.1039/b806161k

Selective iodination at the α-pyrrole positions of dipyrrolyldiketone BF₂ complexes is a key procedure to afford mono- and bisiodinated derivatives as the starting materials of the coupling reactions for various utility molecules and covalently linked oligomer systems. Iodination can also be applied to the phenylene-bridging receptor dimer to obtain the derivatives selectively iodinated at the terminal α-pyrrole position(s).

Full text of DOI:10.1039/b806161k

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PAPER
www.rsc.org/obc | Organic & Biomolecular Chemistry
Selective iodinated dipyrrolyldiketone BF2 complexes as potential
building units for oligomeric systems†
Hiromitsu Maeda*a,b and Yohei Haketaa
Received 11th April 2008, Accepted 5th June 2008
First published as an Advance Article on the web 2nd July 2008
DOI: 10.1039/b806161k
Selective iodination at the a-pyrrole positions of dipyrrolyldiketone BF2 complexes is a key procedure
to afford mono- and bisiodinated derivatives as the starting materials of the coupling reactions for
various utility molecules and covalently linked oligomer systems. Iodination can also be applied to the
phenylene-bridging receptor dimer to obtain the derivatives selectively iodinated at the terminal
a-pyrrole position(s).
the formation of ‘monomers’ at present, facile modifications of
Introduction
receptor frameworks and the extension of these frameworks to
include promising higher oligomers would require other efficient
synthetic strategies. b-Ethyl-substituted 2a (Fig. 1), possessing free
and fairly reactive apositions, is considered as a potential building
block for oligomeric and polymeric system;5 however, it is not so
easy to afford a–a oxidatively coupled compounds chemically in
a single step from 2a. In this report, the selective iodination of
2a and its subsequent coupling reaction to aryl-substituted anion
receptors are reported.
As ‘binding sites’ responsive to anions,1,2 boron complexes (e.g.
1a–c, Fig. 1)3–6 based on 1,3-dipyrrolyl-1,3-propanediones7–9 act as
efficient acyclic anion receptors by the ‘inversion’ of pyrrole rings
from the most stable conformations. The chemical modification
of the periphery of the receptors would control the anion binding
properties and behaviours related to the transformation of p-
conjugated systems. For example, a-aryl-substitution as observed
in 1b is an efficient strategy for obtaining utility anion-responsive
systems by using aryl moieties as the platforms for introducing
various substituents to the receptors. In fact, receptors with long
aliphatic chains at a-aryl units, such as 1c, constitute anion-
responsive supramolecular organogels based on p–p and van
der Waals interactions.6 However, the receptors 1b,c have been
prepared from aryl-substituted pyrroles as starting materials to
obtain the corresponding dipyrrolyldiketones as the intermediates
for BF2 complexes. As the use of a-arylpyrroles is limited to
Results and discussion
The treatment of 2a with 2.7 and 1.1 equiv. of N-iodosuccinimide
(NIS) in CH2Cl2 afforded diiodo-substituted 2a–I2 and monoiodo-
substituted 2a–I1 in 84% and 66% yields, respectively (Fig. 2).10
Bromination with N-bromosuccinimide (NBS) gave complicated
mixtures containing species with a bromo-substituent at the fairly
reactive ‘bridging (meso) carbon’. Iodination with other reagents
such as I2 could not be achieved. In the case of the iodination of
unsubstituted 1a using NIS, complicated mixtures were obtained
Fig. 1 (a) BF2 complexes of dipyrrolyldiketones (b-free 1a–c and
b-ethyl-substituted 2a) and (b) anion binding mode of 1b.
aCollege of Pharmaceutical Sciences, Institute of Science and Engineering,
Ritsumeikan University, Kusatsu, 525-8577, Japan. E-mail: maedahir@
ph.ritsumei.ac.jp; Fax: +81 77 561 2659; Tel: +81-77-561 5969
bPRESTO, Japan Science and Technology Agency (JST), Kawaguchi, 332-
0012, Japan
Fig. 2 Synthetic routes of 2b and 2c via iodinated derivatives 2a–I2
and 2a–I1, respectively: i) phenylboronic acid (2.2 equiv.), Pd(PPh3)4
(0.08 equiv.) and Na2CO3 (7.2 equiv.) in DME–water and ii) phenylboronic
acid (1.2 equiv.), Pd(PPh3)4 (0.08 equiv.) and Na2CO3 (5 equiv.) in
toluene–DMF–water.
† Electronic supplementary information (ESI) available: Complete 1H
NMR assignments of BF2 complexes, optimized structures and anion bind-
ing behaviors for 2b,c and X-ray structural analysis of 2a–I2 and 2b, CCDC
reference numbers 680227 and 680228. For ESI and crystallographic data
in CIF or other electronic format see DOI: 10.1039/b806161k
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due to the lack of selectivity for the reaction sites. However, the
halogenation of dipyrrolyldiketones as precursors with 1 equiv.
of NIS or NBS mainly afforded meso-halogenated species. Halo-
genated pyrroles are known to immediately decompose in solution
and are difficult to handle. Although iodinated 2a–I2 and 2a–
I1 are light-sensitive and gradually transformed into complicated
mixtures such as dimeric species in solution, we can readily isolate
and purify them by shielding them from light without significant
problems, in order to use them for subsequent reactions. In the next
step, Suzuki cross-coupling reactions of 2a–I2 and phenylboronic
acid (2.2 equiv.) afforded bisphenyl-substituted 2b in 35% yield,
while a similar procedure using 2a–I1 and phenylboronic acid
(1.2 equiv.) afforded monophenyl 2c in 34% yield.11–13 Chemical
identification of 2a–I1, 2a–I2 and 2b,c were performed by 1H NMR
and MALDI-TOF and ESI-TOF-MS. As compared to the UV–vis
absorption spectra of 1a (432 nm), 1b (500 nm) and 2a (452 nm),
those of 2b and 2c in CH2Cl2 were observed at 499 and 476 nm,
respectively. This suggested that the factors for the redshifts by
phenyl and ethyl moieties were partially cancelled by the distortion
of phenyl units due to the sterical effects of b-ethyl substituents.
Emissions in the same solvent were observed at, for example,
535 nm (2b) and 509 nm (2c), excited at each absorption maximum,
with high quantum yields of 0.94 (2b) and 0.70 (2c), respectively.
The use of NIS for the iodination of the ‘b-alkyl-substituted BF2
complex’ is crucial for activating a-pyrrole positions in order
to afford the desired aryl-substituted receptors. In addition, the
formation of monoaryl-substituted 2c from 2a–I1 appears to be a
more facile and efficient procedure than the synthetic route via the
mixture of dipyrrolyldiketones obtained from equivalent amounts
of a-unsubstituted and a-aryl-substituted pyrroles.
Fig. 3 Single-crystal X-ray structures (top and side views) of (a) 2a–I2
(two of the four independent conformations) and (b) 2b as dimeric forms
by hydrogen bonding. Atom colour code: brown, blue, pink, red, yellow,
green and purple represent carbon, nitrogen, hydrogen, oxygen, boron,
fluorine and iodine, respectively.
1H NMR spectral changes of, for example, 2b (1 mM), upon the
addition of Cl− as a tetrabutylammonium (TBA) salt (3 equiv.)
in CD2Cl2; the signals of NH, bridging CH and o-CH of the
aryl moieties at 9.49, 6.52 and 7.52 ppm, respectively, vanish
and the corresponding signals of the complexes are generated
at 11.95, 8.54 and 7.78 ppm, respectively. This result suggests
that 2b forms a pentacoordinated anion complex in solution,
as observed in 1b.6 The binding constants (Ka) of 2b and 2c,
summarized in Table 1, have been determined by the UV–vis
absorption spectral changes in CH2Cl2; the kmax values of 2b and
2c at 499 and 476 nm are moderately shifted to 502 and 477 nm,
respectively, upon the addition of Cl− with smaller absorbances.
Here, 2b,c exhibit high Ka values for anions, even though they are
linear non-‘preorganized’ receptors. In comparison with a-free 2a,
however, the decreased Ka values of 2b,c, e.g. 2700 mol−1 dm3 for
2b, 4200 mol−1 dm3 for 2c and 6800 mol−1 dm3 for 2a, suggest
that the introduction of aryl moieties to 2a is not effective for the
binding of anions, particularly large oxoanions for 2b. This trend
of Ka values obtained by aryl-substitutions is more remarkable
than that in the b-free receptors, 1a and 1b, which have comparable
Ka except for oxoanions. These results suggest that the terminal
o-CH is effective for spherical halide anions but does not work
efficiently in 2b,c due to the stercial hindrance by b-ethyl moieties.
Density functional theory (DFT) studies (B3LYP/6-31G**
level)14 have estimated the relative stabilities of preorganized
conformations, adequate for anion binding, possessing two ‘in-
verted’ pyrrole rings. The preorganized structures of 2b and
The solid-state structures of 2a–I2 and 2b have been deter-
mined by single-crystal X-ray analysis (Fig. 3).‡ Similar to other
derivatives,4,5 each molecule has a conformation of two pyrrole
NHs facing the carbonyl oxygen. The terminal phenyl rings are
tilted to the core plane consisting of 16 atoms at 24.29◦ and 31.89◦
for 2◦b, which are slightly larger but comparable to those of 1b
(20.0 and 28.6◦).6 This observation suggests that the aryl rings of
2b may be more canted to the core plane in the solution state than
in the solid state, and they partially disrupt the p-conjugation to
exhibit a more blue-shifted kmax than expected from the crystal
structure. Similar to 1a,b, b-ethyl-substituted derivatives form
dimers by N–H · · · F–B hydrogen bonding: the distances between
˚
˚
N(–H) · · · F are 2.785–2.914 A for 2a–I2 and 2.905 A for 2b. In
addition, self-assembled p–p stacking dimers, possibly formed by
the sterical effects of the b-substituents, are constructed in the solid
state in both cases, and these dimers are contrary to the infinite
stacking structures observed in b-free 1a and 1b.
Table 1 Binding constants (Ka, mol−1 dm3) of 2b and 2c as well as 1a,b
and 2a as references for anions as TBA salts in CH2Cl2, and the ratios of
binding constants of 1a,b and 2b,c to that of 2a (in parentheses)
The anion binding of aryl-substituted dipyrrolyldiketone BF2
complexes, as illustrated in Fig. 1b, has been suggested by the
1aa
1bb
2ac
6800
2b
2700
(0.40)
300
(0.25)
27 000
(0.13)
2200
2c
4200
(0.61)
600
(0.50)
98 000
(0.47)
36 000
(0.40)
‡ Crystal data for 2a–I2 (from CH2Cl2–hexane): C19H23BF2N2O2I2, Mw
=
Cl−
15 000
(2.2)
2100
(1.8)
930 000
(4.4)
30 000
(4.4)
2800
(2.3)
210 000
(1.0)
614.00, monoclinic, P21/n (no. 14), a = 16.022(5), b = 16.416(5), c =
◦
3
˚
˚
32.819(10) A, b = 101.164(5) , V = 8468(5) A , T = 123(2) K, Z = 16, Dc =
1.926 g cm−3, l(Mo-Ka) = 3.005 mm−1, 52 127 reflections measured, 19807
unique (Rint = 0.0363). R1 = 0.0658, wR2 = 0.1524, GOF = 1.253 (I >
2r(I)). CCDC 680227. Crystal data for 2b (from CH2ClCH2Cl–hexane):
Br−
1200
210 000
91 000
−
CH3CO2
¯
C31H33BF2N2O2, Mw = 514.40, triclinic, P1 (no. 2), a = 10.962(7), b =
−
H2PO4
270 000
(3.0)
72 000
(0.79)
◦
˚
11.536(6), c = 12.261(6) A, a = 77.123(17), b = 75.75(2), c = 62.248(19) ,
(0.024)
V = 1319.3(12) A , T = 123(2) K, Z = 2, Dc = 1.295 g cm−3, l(Mo-Ka) =
3
˚
0.090 mm−1, 13 095 reflections measured, 5969 unique (Rint = 0.0408). R1 =
0.0537, wR2 = 0.1485, GOF = 0.977 (I > 2r(I)). CCDC 680228.
a Ref. 4. b Ref. 6. c Ref. 5.
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2c are estimated to be less stable than structures with two
pyrrole NH facing carbonyl oxygens at 4.25 and 4.74 kcal mol−1
respectively, which is almost similar to b-ethyl-substituted 2a
(4.98 kcal mol−1)5 and smaller than b-free 1a (9.08 kcal mol−1)4
and 1b (8.71 kcal mol−1).6 These theoretical studies suggest that
the relative stabilities of the preorganized states in 2b,c due to
b-ethyl moieties may enhance the binding affinities of anions as
observed in 2a; however, the effects of preorganization on 2b,c
appear to be cancelled by the sterical effects between b-ethyl and
terminal phenyl units.
Examples of desired systems obtained using the procedure
mentioned in this report are efficient anion sensors and stimuli-
responsive p-conjugated oligomers.15 In fact, the covalently linked
dimer 3a (Fig. 4) has been synthesized in 46% yield by the
coupling reaction of 2a–I1 (2 equiv.) with 1,3-benzenediboronic
acid bis(pinacol)ester. By this procedure, a–a directly coupled
dimer 3b is also obtained in trace amount (ca. 0.5%). The kmax
value of 3a of 489 nm, due to the incomplete p-conjugation
at the meta-phenylene linkage, is in sharp contrast to the fairly
broad absorption of 3b observed around 520 nm (kmax). Further
iodination of phenylene-bridging dimer 3a afforded bisiodinated
dimer 3a–I2 and monoiodinated 3a–I1 in 84 and 41% using 2.3 and
1.0 equiv. of NIS, respectively. In these cases, iodinations at the
bridging phenylene moiety have not been observed. The iodinated
3a–I2 and 3a–I1 are also potential subunits for formation of anion-
responsive oligomers.
On the other hand, couplings with p-conjugated pentacycles, not
hexacycles as observed in 2b and 2c, have been found to give
somewhat higher yields due to smaller inner angles of pentacycles.
Based on these observations, the protocol in this report can be
applied to the formation of higher oligomers, which would exhibit
the fascinating properties such as anion-driven folding behaviours.
Experimental
Starting materials were purchased from Wako Chemical Co.,
Nacalai Chemical Co., and Aldrich Chemical Co. and used with-
out further purification unless otherwise stated. UV–visible spec-
tra were recorded on a Hitachi U-3500 spectrometer. Fluorescence
spectra and emission quantum yields were recorded on a Hitachi
F-4500 fluorescence spectrometer and a Hamamatsu Quantum
Yields Measurements System for Organic LED Materials C9920-
02, respectively. NMR spectra used in the characterization of prod-
ucts were recorded on a JEOL ECA-600 600 MHz spectrometer.
All NMR spectra were referenced to solvent. Matrix-assisted laser
desorption ionization time-of-flight mass spectrometry (MALDI-
TOF-MS) was recorded on a Shimadzu Axima-CFRplus using
negative mode. Electrospray ionization mass spectrometric studies
(ESI-MS) were recorded on a BRUKER microTOF using negative
mode ESI-TOF method. TLC analyses were carried out on
aluminium sheets coated with silica gel 60 (Merck 5554). Column
chromatography was performed on Wakogel C-200, C-300, and
Merck silica gel 60 and 60 H.
BF2 complex of 1,3-bis(3,4-diethyl-5-iodopyrrol-2-yl)-
1,3-propanedione, 2a–I2
To a CH2Cl2 (50 mL) solution of BF2 complex of 1,3-bis(3,4-
diethylpyrrole-2-yl)-1,3-propanedione (2a)5 (100.0 mg, 0.28mmol)
at room temperature was added N-iodosuccinimide (169.0 mg,
0.75 mmol). The mixture was stirred at room temperature for 3 h.
After confirming the consumption of the starting material by TLC
analysis, the mixture was washed with water and extracted with
CH2Cl2, dried over anhydrous MgSO4, and evaporated to dryness.
The residue was then chromatographed over silica gel flash column
(eluent: CH2Cl2) and recrystallized from CH2Cl2–hexane to afford
2a–I2 (142.2 mg, 84%). Rf = 0.49 (CH2Cl2). 1H NMR (600 MHz,
CDCl3, 20 ◦C): d (ppm) 9.45 (s, 2H, NH), 6.35 (s, 1H, CH), 2.78 (q,
J = 7.8 Hz, 4H, CH2CH3), 2.42 (q, J = 7.8 Hz, 4H, CH2CH3), 1.25
(t, J = 7.8 Hz, 6H, CH2CH3), 1.10 (t, J = 7.8 Hz, 6H, CH2CH3).
MALDI-TOF-MS: m/z (% intensity): 611.99 (15), 612.99 (88),
613.99 (100). HRMS (ESI-TOF): m/z (% intensity): 612.9864.
Calcd for C19H22BF2I2N2O2 ([M − H]−): 612.9841. This compound
was further characterized by X-ray diffraction analysis.
Fig. 4 Phenylene-bridging dimers 3a and 3a–I1,2 and direct-linked dimer
3b from monoiodinated 2a–I1.
Conclusions
We have exhibited the selectively iodinated BF2 complexes of
dipyrrolyldiketones for obtaining various aryl-substituted acyclic
anion receptors and their oligomers. It is remarkable to perform
the extension of p-conjugation using cross coupling reactions at
pyrrole a-positions, even though we have shown only low yields (ca.
30–50%) at the present moment. These moderate yields possibly
originate from the reaction conditions for the dipyrrolyldiketone
derivatives being difficult to optimise, the properties of these
derivatives, as well as the sterical hindrance at a-positions by
b-ethyl substituents. In fact, the coupling reactions in EtOH,
which were used by Setsune et al. for arylpyrrole derivatives,13
have afforded too small amounts of the desired compounds in
our case, due to the unstable boron unit under the conditions.
BF2 complex of 1-(3,4-diethyl-5-iodopyrrol-2-yl)-3-(3,4-
diethylpyrrol-2-yl)-1,3-propanedione, 2a–I1
To a CH2Cl2 (70 mL) solution of BF2 complex of 1,3-bis(3,4-
diethylpyrrol-2-yl)-1,3-propanedione (2a)5 (300.1 mg, 0.83 mmol)
at room temperature was added N-iodosuccinimide (203.3 mg,
0.90 mmol). After stirring at room temperature for 3 h, the
mixture was washed with water and extracted with CH2Cl2, dried
over anhydrous MgSO4, and evaporated to dryness. The residue
was then chromatographed over silica gel flash column (eluent:
CH2Cl2) and recrystallized from CH2Cl2–hexane to afford 2a–I1
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(235.2 mg, 58%). Rf = 0.40 (CH2Cl2). 1H NMR (600 MHz, CDCl3,
20 ◦C): d (ppm) 9.42 (s, 1H, NH), 9.34 (s, 1H, NH), 6.92 (d, J =
3.0 Hz, 1H, pyrrole-H), 6.42 (s, 1H, CH), 2.78 (qq, J = 7.8 Hz, 4H,
CH2CH3), 2.48 (q, J = 7.8 Hz, 2H, CH2CH3), 2.43 (q, J = 7.8 Hz,
2H, CH2CH3), 1.25 (tt, J = 7.8 Hz, 6H, CH2CH3), 1.21 (t, J =
7.8 Hz, 3H, CH2CH3), 1.10 (t, J = 7.8 Hz, 3H, CH2CH3). MALDI-
TOF-MS: m/z (% intensity): 486.09 (24), 487.09 (100), 488.09
(54). HRMS (ESI-TOF): m/z (% intensity): 487.0870. Calcd for
C19H23BF2IN2O2 ([M − H]−): 487.0874.
(kex[nm])): 509.0 (476.0). MALDI-TOF-MS: m/z (% intensity):
437.21 (35), 438.22 (100), 439.23 (26) HRMS (ESI-TOF): m/z
(% intensity): 437.2211. Calcd for C25H28BF2N2O2 ([M − H]−):
437.2222.
Phenylene-bridged dimer of 2a, 3a
A Schlenk tube containing 2a–I1 (450.0 mg, 0.92 mmol),
1,3-phenyldiboronic acid 1,3-bis(pinacol)ester16 (152.0 mg,
0.46 mmol), tetrakis(triphenylphosphine)palladium(0) (106.0 mg,
0.09 mmol), and Cs2CO3 (899.3 mg, 2.76 mmol) was flushed with
nitrogen and charged with a mixture of degassed DMF (9 mL),
toluene (9 mL), and water (0.2 mL). The mixture was heated at
90 ◦C for 12 h, cooled, then partitioned between water and CH2Cl2.
The combined extracts were dried over anhydrous MgSO4, and
evaporated. The residue was then chromatographed over silica gel
flash column (eluent: 0.5% MeOH–CH2Cl2) to give the dimer 3a
(167.5 mg, 46%) as a red solid. Rf = 0.30 (1% MeOH–CH2Cl2).
BF2 complex of 1,3-bis-(3,4-diethyl-5-phenylpyrrol-
2-yl)-1,3-propanedione, 2b
A Schlenk tube containing 2a–I2 (70.0 mg, 0.114 mmol),
phenylboronic acid (30.6 mg, 0.251 mmol), tetrakis-
(triphenylphosphine)palladium(0) (24.2 mg, 0.02 mmol),
and Na2CO3 (87.8 mg, 0.82 mmol) was flushed with nitrogen
and charged with a mixture of degassed 1,2-dimethoxyethane
(5 mL) and water (0.5 mL). The mixture was heated at 90 ◦C
for 18 h, cooled, then partitioned between water and CH2Cl2.
The combined extracts were dried over anhydrous MgSO4 and
evaporated. The residue was then chromatographed over silica gel
flash column (eluent: 10% EtOAc–hexane) to give 2b (17.3 mg,
35%) as a pink solid. Rf = 0.38 (10% EtOAc–hexane). 1H
NMR (600 MHz, CDCl3, 20 ◦C): d (ppm) 9.37 (s, 1H, NH),
7.53–7.51 (m, 4H, Ph-o-CH), 7.50–7.48 (m, 4H, Ph-m-CH),
7.43–7.39 (m, 2H, Ph-p-CH), 6.56 (s, 1H, CH), 2.86 (q, J =
7.8 Hz, 4H, CH2CH3), 2.62 (q, J = 7.8 Hz, 4H, CH2CH3),
1.35 (t, J = 7.8 Hz, 6H, CH2CH3), 1.20 (t, J = 7.8 Hz, 6H,
CH2CH3). UV–vis (CH2Cl2, kmax[nm] (e, 105 M−1 cm−1)): 499.0
(1.21). Fluorescence (CH2Cl2, kem[nm] (kex[nm])): 535.0 (499.0).
MALDI-TOF-MS: m/z (% intensity): 512.26 (8), 513.25 (44),
514.26 (100). HRMS (ESI-TOF): m/z (% intensity): 513.2536.
Calcd for C31H32BF2N2O2 ([M − H]−): 513.2536. This compound
was further characterized by X-ray diffraction analysis.
◦
1H NMR (600 MHz, CDCl3, 20 C): d (ppm) 9.39 (s, 2H, NH),
9.30 (s, 2H, NH), 7.64 (s, 1H, phenylene-CH), 7.59–7.58 (m, 1H,
phenylene-CH), 7.53 (m, 2H, phenylene-CH), 6.94 (s, 2H, pyrrole-
H), 6.53 (s, 2H, CH), 2.84–2.82 (m, 4H, CH2CH3), 2.82–2.80 (m,
4H, CH2CH3), 2.64 (m, 4H, CH2CH3), 2.48 (m, 4H, CH2CH3),
1.30 (m, 6H, CH2CH3), 1.27 (m, 6H, CH2CH3), 1.20 (m, 12H,
CH2CH3). UV–vis (CH2Cl2, kmax[nm] (e, 105 M−1 cm−1)): 489.0
(2.10). Fluorescence (CH2Cl2, kem[nm] (kex[nm])): 511.0 (489.0).
MALDI-TOF-MS: m/z (% intensity): 796.38 (47), 797.40 (100),
798.39 (50). HRMS (ESI-TOF): m/z (% intensity): 797.4045.
Calcd for C44H51B2F4N4O4 ([M − H]−): 797.4052.
Direct-linked dimer of 2a, 3b
Direct-linked dimer 3b was obtained in 0.5% yield as a dark purple
solid as one of the byproducts of the coupling reaction for 3a. Rf =
◦
1
0.33 (1% MeOH–CH2Cl2). H NMR (600 MHz, CDCl3, 20 C):
d (ppm) 9.32 (s, 1H, inner NH), 9.27 (s, 1H, outer NH), 6.98
(d, J = 3.0 Hz, 2H, pyrrole-H), 6.54 (s, 1H, CH), 2.87–2.84 (m,
4H, CH2CH3), 2.83–2.79 (m, 4H, CH2CH3), 2.61 (q, J = 7.8 Hz,
4H, CH2CH3), 2.49 (q, J = 7.8 Hz, 4H, CH2CH3), 1.34 (t, J =
7.8 Hz, 6H, CH2CH3), 1.28 (t, J = 7.8 Hz, 6H, CH2CH3), 1.22
(t, J = 7.8 Hz, 6H, CH2CH3), 1.16 (t, J = 7.8 Hz, 6H, CH2CH3).
UV–vis (CH2Cl2, kmax[nm]): 520.0. Fluorescence (CH2Cl2, kem[nm]
(kex[nm])): 601.0 (520.0). MALDI-TOF-MS: m/z (% intensity):
720.37 (43), 721.37 (100), 722.38 (39). HRMS (ESI-TOF): m/z
(% intensity): 721.3736. Calcd for C38H47B2F4N4O4 ([M − H]−):
721.3738.
BF2 complex of 1-(3,4-diethyl-5-phenylpyrrol-2-yl)-3-(5-
phenylpyrrol-2-yl)-1,3-propanedione, 2c
A Schlenk tube containing 2a–I1 (250.0 mg, 0.512 mmol),
phenylboronic acid (74.91 mg, 0.614 mmol), tetrakis-
(triphenylphosphine)palladium(0) (57.8 mg, 0.05 mmol), and
Na2CO3 (325.4 mg, 3.07 mmol) was flushed with nitrogen and
charged with a mixture of degassed toluene and DMF (1 : 1,
8 mL) and water (0.2 mL). The mixture was heated at 80 ◦C
for 12 h, cooled, then partitioned between water and CH2Cl2.
The combined extracts were dried over anhydrous MgSO4 and
evaporated. The residue was then chromatographed over silica gel
flash column (eluent: 0.5% MeOH–CH2Cl2) to give 2c (75.6 mg,
34%) as a yellow solid. Rf = 0.46 (1% MeOH–CH2Cl2). 1H NMR
(600 MHz, CDCl3, 20 ◦C): d (ppm) 9.32 (s, 1H, NH), 9.29 (s, 1H,
NH), 7.55–7.51 (m, 2H, Ph-o-CH), 7.50–7.47 (m, 2H, Ph-m-CH),
7.43–7.40 (m, 1H, Ph-p-CH), 6.94 (d, J = 3.0 Hz, 1H, pyrrole-H),
6.52 (s, 1H, CH), 2.85 (q, J = 7.8 Hz, 2H, CH2CH3), 2.80 (q, J =
7.8 Hz, 2H, CH2CH3), 2.61 (q, J = 7.8 Hz, 2H, CH2CH3), 2.49 (q,
J = 7.8 Hz, 2H, CH2CH3), 1.33 (t, J = 7.8 Hz, 3H, CH2CH3), 1.28
(t, J = 7.8 Hz, 3H, CH2CH3), 1.22 (t, J = 7.8 Hz, 3H, CH2CH3),
1.19 (t, J = 7.8 Hz, 3H, CH2CH3). UV–vis (CH2Cl2, kmax[nm]
(e, 105 M−1 cm−1)): 476.0 (1.12). Fluorescence (CH2Cl2, kem[nm]
Bisiodinated derivative of 3a, 3a–I2
To a solution of 3a (232.4 mg, 0.291 mmol) in CH2Cl2 (70 mL)
at room temperature was added N-iodosuccinimide (150.5 mg,
0.669 mmol). The mixture was stirred at room temperature for
7 h. After confirming the consumption of the starting material by
TLC analysis, the mixture was washed with water, extracted with
CH2Cl2, dried over anhydrous MgSO4, and evaporated to dryness.
The residue was then chromatographed over silica gel flash column
(eluent: 1.2% MeOH–CH2Cl2) to afford 3a–I2 (248.6 mg, 84%).
Rf = 0.25 (CH2Cl2). 1H NMR (600 MHz, CDCl3, 20 ◦C): d (ppm)
9.42 (s, 2H, NH), 9.41 (s, 2H, NH), 7.64 (s, 1H, phenylene-
CH), 7.63–7.60 (m, 1H, phenylene-CH), 7.56–7.54 (m, 2H,
3094 | Org. Biomol. Chem., 2008, 6, 3091–3095
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phenylene-CH), 6.48 (s, 2H, CH), 2.84 (q, J = 7.8 Hz, 8H,
CH2CH3), 2.65 (q, J = 7.8 Hz, 4H, CH2CH3), 2.43 (q, J = 7.8 Hz,
4H, CH2CH3), 1.34 (t, J = 7.8 Hz, 6H, CH2CH3), 1.29 (t, J =
7.8 Hz, 6H, CH2CH3), 1.20 (t, J = 7.8 Hz, 6H, CH2CH3), 1.11 (t,
J = 7.8 Hz, 6H, CH2CH3). MALDI-TOF-MS: m/z (% intensity):
1048.26 (52), 1049.20 (100), 1050.17 (48). HRMS (ESI-TOF): m/z
(% intensity): 1049.1985. Calcd for C44H49B2F4I2N4O4 ([M − H]−):
1049.1985.
Ministry of Education, Culture, Sports, Science and Technology
(MEXT), and the “Academic Frontier” Project for Private Univer-
sities, namely the matching fund subsidy from the MEXT, 2003–
2008. The author thanks Prof. Atsuhiro Osuka, Mr Yasuhide
Inokuma, and Mr Shohei Saito, Kyoto University, for the X-
ray analyses, Prof. Hiroshi Shinokubo, Kyoto University, for ESI-
TOF-MS measurements, and Prof. Hitoshi Tamiaki, Ritsumeikan
University, for helpful discussions.
Monoiodinated derivative of 3a, 3a–I1
References
To a solution of 3a (296.4 mg, 0.321 mmol) in CH2Cl2 (80 mL)
at room temperature was added N-iodosuccinimide (72.14 mg,
0.321 mmol). The mixture was stirred at room temperature for
14 h. After confirming the consumption of the starting material by
TLC analysis, the mixture was washed with water, extracted with
CH2Cl2, dried over anhydrous MgSO4, and evaporated to dryness.
The residue was then chromatographed over silica gel flash column
(eluent: 1% MeOH–CH2Cl2) to afford 3a–I1 (122.0 mg, 41%). Rf =
0.38 (3% MeOH–CH2Cl2). 1H NMR (600 MHz, CDCl3, 20 ◦C): d
(ppm) 9.39 (s, 3H, NH), 9.29 (s, 1H, NH), 7.64 (s, 1H, phenylene-
CH), 7.60 (m, 1H, phenylene-CH), 7.51–7.42 (m, 2H, phenylene-
CH), 6.93 (s, 1H, pyrrole-H), 6.51 (s, 1H, CH), 6.45 (s, 1H, CH),
2.82–2.78 (m, 8H, CH2CH3), 2.64–2.63 (m, 4H CH2CH3), 2.46
(m, 2H, CH2CH3), 2.42–2.41 (m, 2H, CH2CH3), 1.32 (m, 6H,
CH2CH3), 1.26 (m, 6H, CH2CH3), 1.20–1.17 (m, 9H, CH2CH3),
1.09 (t, J = 7.8 Hz, 3H, CH2CH3). MALDI-TOF-MS: m/z (%
intensity): 922.29 (44), 923.31 (100), 924.30 (62). HRMS (ESI-
TOF): m/z (% intensity): 923.3019. Calcd for C44H50B2F4IN4O4
([M − H]−): 923.3019.
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˚
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Acknowledgements
This work was supported by Grant-in-Aid for Young Scientists (B)
(No. 17750137) and Scientific Research in a Priority Area “Super-
Hierarchical Structures” (No. 18039038, 19022036) from the
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The Royal Society of Chemistry 2008
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