Anion binding by fluorescent biimidazole diamides

Article abstract of DOI:10.1021/jo020098v

Six 2,2′-biimidazoles with various amide groups at the 4- and 4′-positions were prepared from 5-propyl-1H-imidazole-4-carboxylic acid ethyl ester. In the final step of the synthesis, biimidazole C2-C2′ bond formation was accomplished in 33-45% yield by palladium(0)-catalyzed homocoupling of the corresponding 2-iodoimidazoles. Four of the biimidazoles were studied by X-ray diffraction. In the solid state, all display coplanar imidazole rings, an anti relationship of amide groups, and intramolecular (NH_amideN_imid) and intermolecular (NH_imidO_amide) hydrogen bonding. In CH₂Cl₂, the emission intensity of the biimidazoles is quenched by the presence of dihydrogenphosphate and chloride anions, but no shifts in λ_emiss are observed. Binding constants for 1:1 biimidazoleanion complexation (K_assoc) are on the order of 10⁴ M^-1 for H₂PO₄^- and Cl^-. One of the receptors (bearing 3,5-difluorobenzylamides) is selective for chloride. The participation of the amide NH atoms in anion binding was established by ¹H NMR.

Full text of DOI:10.1021/jo020098v

An ion Bin d in g by F lu or escen t Biim id a zole Dia m id es
Corey P. Causey and William E. Allen*
Department of Chemistry, East Carolina University, Greenville, North Carolina 27858-4353
allenwi@mail.ecu.edu
Received February 11, 2002
Six 2,2′-biimidazoles with various amide groups at the 4- and 4′-positions were prepared from
5-propyl-1H-imidazole-4-carboxylic acid ethyl ester. In the final step of the synthesis, biimidazole
C2-C2′ bond formation was accomplished in 33-45% yield by palladium(0)-catalyzed homocoupling
of the corresponding 2-iodoimidazoles. Four of the biimidazoles were studied by X-ray diffraction.
In the solid state, all display coplanar imidazole rings, an anti relationship of amide groups, and
intramolecular (NHamide‚‚‚Nimid) and intermolecular (NHimid‚‚‚Oamide) hydrogen bonding. In CH2Cl2,
the emission intensity of the biimidazoles is quenched by the presence of dihydrogenphosphate
and chloride anions, but no shifts in λemiss are observed. Binding constants for 1:1 biimidazole-
anion complexation (Kassoc) are on the order of 104 M-1 for H2PO4 and Cl-. One of the receptors
-
(bearing 3,5-difluorobenzylamides) is selective for chloride. The participation of the amide NH atoms
1
in anion binding was established by H NMR.
In tr od u ction
are shown to simultaneously serve as multiple H-bond
donors7 and as anion-sensitive fluorophores.
Artificial receptors that are capable of selectively
binding anionic species show promise in the diagnosis
and treatment of diseases, and in environmental reme-
diation.1 Sensors for inorganic phosphate, for example,
could be used to monitor ATP synthesis/hydrolysis or
kinase-dependent cell signaling. “Carrier” molecules can
enhance through-membrane transport of chloride ion,
which is a goal of cystic fibrosis research. Extraction of
nitrate from rivers and lakes is expected to inhibit
eutrophication, and the associated oxygen depletion and
fish kills. Several anion receptors have been constructed
from five-membered heterocycles,2 (thio)amides,3 or both,4
because these groups form relatively strong NH‚‚‚anion
hydrogen bonds. Previous work has also established that
coupling luminescent moieties to H-bond donors can yield
sensors that operate at low anion concentrations.5 We
describe here a series of anion receptors that incorporate
all of these features into a single molecular unit.6
Specifically, electrically neutral biimidazole diamides 1
Resu lts a n d Discu ssion
Synthesis of the biimidazoles is shown in Scheme 1.
In the first step, the known ester 28 was hydrolyzed to a
carboxylic acid in quantitative yield with concentrated
aqueous HCl at reflux. Elemental analysis confirmed that
the product is isolated as a hydrochloride salt. Repeated
attempts to couple acid 3 to amines (using DCC, BOP,
or HBTU)9 failed to produce amides, so this compound
was instead converted to the corresponding acid chloride
4 by reaction with excess oxalyl chloride in CH3CN at
reflux.10 Once dried, solid 4 could be stored in a tightly
sealed container at room temperature for several months
(1) (a) Beer, P. D.; Gale, P. A. Angew. Chem., Int. Ed. 2001, 40, 486.
(b) Gale, P. A. Coord. Chem. Rev. 2000, 199, 181. (c) Snowden, T. S.;
Anslyn, E. V. Curr. Opin. Chem. Biol. 1999, 3, 740. (d) Antonisse, M.
M. G.; Reinhoudt, D. N. Chem. Commun. 1998, 443. (e) Beer, P. D.
Acc. Chem. Res. 1998, 31, 71. (f) Schmidtchen, F. P.; Berger, M. Chem.
Rev. 1997, 97, 1609.
(2) (a) Sessler, J . L.; Davis, J . M. Acc. Chem. Res. 2001, 34, 989. (b)
Cafeo, G.; Kohnke, F. H.; La Torre, G. L.; White, A. J . P.; Williams, D.
J . Chem. Commun. 2000, 1207. (c) Anzenbacher, P., J r.; J urs´ıkova´,
K.; Sessler, J . L. J . Am. Chem. Soc. 2000, 122, 9350. (d) Miyaji, H.;
Sato, W.; Sessler, J . L. Angew. Chem., Int. Ed. 2000, 39, 1777. (e)
Sessler, J . L.; Allen, W. E. CHEMTECH 1999, 29, 16. (f) Sato, K.; Arai,
S.; Yamagishi, T. Tetrahedron Lett. 1999, 40, 5219.
(5) (a) Liao, J .-H.; Chen, C.-T.; Fang, J .-M. Org. Lett. 2002, 4, 561.
(b) Gunnlaugsson, T.; Davis, A. P.; Glynn, M. Chem. Commun. 2001,
2556. (c) Sun, S.-S.; Lees, A. J . Chem. Commun. 2000, 1687. (d) Beer,
P. D.; Szemes, F.; Balzani, V.; Sala`, C. M.; Drew, M. G. B.; Dent, S.
W.; Maestri, M. J . Am. Chem. Soc. 1997, 119, 11864.
(6) (a) Sessler, J . L.; Maeda, H.; Mizuno, T.; Lynch, V. M.; Furuta,
H. Chem. Commun. 2002, 862. (b) Anzenbacher, P., J r.; Try, A. C.;
Miyaji, H.; J urs´ıkova´, K.; Lynch, V. M.; Marquez, M.; Sessler, J . L. J .
Am. Chem. Soc. 2000, 122, 10268. (c) Black, C. B.; Andrioletti, B.; Try,
A. C.; Ruiperez, C.; Sessler, J . L. J . Am. Chem. Soc. 1999, 121, 10438.
(7) Fortin, S.; Beauchamp, A. L. Inorg. Chem. 2000, 39, 4886.
(8) Paul, R.; Brockman, J . A.; Hallett, W. A.; Hanifin, J . W.; Tarrant,
M. E.; Torley, L. W.; Callahan, F. M.; Fabio, P. F.; J ohnson, B. D.;
Lenhard, R. H.; Schaub, R. E.; Wissner, A. J . Med. Chem. 1985, 28,
1704.
(3) (a) Ayling, A. J .; Pe´rez-Paya´n, M. N.; Davis, A. P. J . Am. Chem.
Soc. 2001, 123, 12716. (b) Choi, K.; Hamilton, A. D. J . Am. Chem. Soc.
2001, 123, 2456. (c) Miyaji, H.; Sessler, J . L. Angew. Chem., Int. Ed.
2001, 40, 154. (d) J agessar, R. C.; Burns, D. H. Chem. Commun. 1997,
1685.
(9) Li, P.; Xu, J .-C. Tetrahedron Lett. 2000, 41, 721.
(4) Gale, P. A.; Camiolo, S.; Tizzard, G. J .; Chapman, C. P.; Light,
M. E.; Coles, S. J .; Hursthouse, M. B. J . Org. Chem. 2001, 66, 7849.
(10) Collman, J . P.; Bro¨ring, M.; Fu, L.; Rapta, M.; Schwenninger,
R. J . Org. Chem. 1998, 63, 8084.
10.1021/jo020098v CCC: $22.00 © 2002 American Chemical Society
Published on Web 07/12/2002
J . Org. Chem. 2002, 67, 5963-5968
5963

Causey and Allen
SCHEME 1
F IGURE 1. Structure of 1a in the crystal. The molecule sits
on a crystallographic inversion center. Thermal ellipsoids are
scaled to the 50% probability level.
without decomposition. In addition, 4 is stable toward
TLC analysis on silica gel plates with EtOAc as the
eluent. Amides 5a -f were prepared by treating 4 with a
series of neat liquid amines. Removal of excess amine
followed by recrystallization or column chromatography
afforded the desired amides as solids that are freely
soluble in CH3OH and moderately soluble in CH2Cl2 or
EtOAc. Installation of a halogen at the imidazole 2-posi-
tion of 5a -f, required for subsequent biimidazole bond
formation,11 was accomplished by treating the amides
with N-iodosuccinimide (NIS) in THF at reflux. In the
final step of the synthesis, a palladium(0)-promoted
homocoupling reaction, originally developed for prepara-
tion of biimidazole diesters,11 was employed. Here, iodides
6a -f were treated with N,N-diisopropylethylamine and
a catalytic amount of Pd(PPh3)4 in toluene at 110 °C. The
six biimidazole products 1a -f display a characteristic
blue fluorescence under short-wave UV irradiation on
silica gel TLC plates.
Four of the final products (1a -d ) were analyzed by
single-crystal X-ray diffraction. In the solid state, the
imidazole rings and amide CONH atoms of these four
derivatives are essentially congruent, with no deviations
in bond lengths or angles greater than 2%. Significant
differences across the series appear only in the conforma-
tions of the 5,5′-dipropyl side chains and in the alkyl/
benzyl groups attached to the amide NH units. Therefore,
only a representative biimidazole (1a ) is described here
in detail. This molecule adopts a conformation such that
the amide groups are anti to one another, and the
heterocyclic rings are coplanar (Figure 1). These struc-
tural features have also been observed in bipyrrole-
containing compounds.12 Although imidazoles typically
F IGURE 2. Representative emission spectra of 1c (2.5 × 10-6
M) in aerated CH2Cl2 during titration with Bu4N+ H2PO4-. λexcit
) 300 nm.
display NH tautomerism, only one tautomer is readily
apparent in crystals of 1a . This form allows for intramo-
lecular hydrogen bonding between the amide N8-H8
atoms and the lone pair electrons of imidazole nitrogens
N3 (Hamide‚‚‚Nimid ≈ 2.3 Å). Furthermore, each molecule
of 1a H-bonds to four adjacent molecules by donating two
NHimid‚‚‚Oamide bonds and accepting two NHimid‚‚‚Oamide
bonds (see the Supporting Information).
With their ability to donate hydrogen bonds estab-
lished, the fluorescence behavior of 1b-f13 was examined
in the presence of anionic species. As an example, Figure
2 shows the evolution of the emission bands of 1c (2.5 ×
10-6 M in CH2Cl2)14 as tetrabutylammonium dihydro-
genphosphate (Bu4N+ H2PO4-) was introduced. The first
addition of anion (to give a solution with a 4-fold excess
(13) Quantum yields ΦF for 1c, 1d , and 1e were measured to be
0.29, 0.32, and 0.32 ((10%), respectively, with excitation at 300 nm
in N2-purged absolute ethanol at room temperature (relative to
7-amino-4-methylcoumarin). These determinations were carried out
according to: Fery-Forgues, S.; Lavabre, D. J . Chem. Educ. 1999, 76,
1260. For examples of 2,2′-bithiazoles that display intense fluorescence,
see: Wagner, M.; Engrand, P.; Regnouf-de-Vains, J .-B.; Marsura, A.
Tetrahedron Lett. 2001, 42, 5207.
(11) Allen, W. E.; Fowler, C. J .; Lynch, V. M.; Sessler, J . L. Chem.
Eur. J . 2001, 7, 721.
(12) (a) Che, C.-M.; Wan, C.-W.; Lin, W.-Z.; Yu, W.-Y.; Zhou, Z.-Y.;
Lai, W.-Y.; Lee, S.-T. Chem. Commun. 2001, 721. (b) Andrievsky, A.;
Ahuis, F.; Sessler, J . L.; Vo¨gtle, F.; Gudat, D.; Moini, M. J . Am. Chem.
Soc. 1998, 120, 9712.
5964 J . Org. Chem., Vol. 67, No. 17, 2002

Anion Binding by Fluorescent Biimidazole Diamides
TABLE 1. Bin d in g Con sta n ts, Ka ssoc (M-1), for 1b-f w ith
Dih yd r ogen p h osp h a te a n d Ch lor id e in CH2Cl2 a t 23 °C
changes in ionic strength, or protonation of imidazole
nitrogen atoms by the H2PO4 ion (a potential source of
-
H+). For the first titration, the solvent system was
changed to CH2Cl2-CH3OH (9:1 v/v). The presence of an
anion-solvating, hydrogen-bond donor solvent is expected
to render anion binding less favorable.19 Indeed, no
quenching of the fluorescence of biimidazole 1d was
observed when Cl- was added, even in 300-fold excess.
In the second control experiment, the emission intensity
of 1c (in CH2Cl2) remained constant during treatment
with trifluoroacetic acid.
-
biimidazole
H2PO4
Cl-
1b
1c
1d
1e
1f
6.8 × 104
4.6 × 104
4.7 × 104
4.1 × 104
2.0 × 104
1.4 × 105
3.7 × 104
2.7 × 104
2.5 × 104
4.0 × 103
of H2PO4- relative to 1c) caused the integrated emission
intensity F to decrease by about 15%. At the conclusion
-
of the titration, the presence of 75 equiv of H2PO4
Proton NMR experiments performed upon biimidazole
1c in a mixture of CD2Cl2 and acetone-d6 (2:1 v/v)
emphasize the role of the amide NH atoms in anion
coordination. (The modest solubility of 1c in neat dichlo-
romethane prevented this solvent from being used in the
NMR studies, as it was in the fluorescence experiments.)
quenched slightly more than 40% of the initial fluores-
cence. Chloride ion was found to be a less effective
quencher, with 75 equiv of this anion effecting a decrease
of just 15% in the total emission of 1c in CH2Cl2.15 No
shifts in the wavelengths of the emission peaks were
observed.
As the amount of added Bu4N+ H2PO4 was gradually
-
For each of 1b-f, plots of F/F0 versus [H2PO4-] or [Cl-]
are hyperbolic, demonstrating saturation behavior. Table
1 summarizes the 1:1 biimidazole-anion binding con-
stants (Kassoc) for dihydrogenphosphate and chloride that
were derived from fluorescence titrations.16 For all but
raised from 0 to 0.9 equiv, the amide protons of sensor
1c shifted downfield by about 0.55 ppm. Further addition
of dihydrogenphosphate induced small upfield shifts in
δ
NH. No CH protons of 1c were found to shift dramatically
during these titrations. For example, the 4-methylbenzyl
CH2 nuclei, which are constrained to lie in proximity to
a bound anion, moved upfield by 0.04 ppm. The data were
best fit20 to a binding model in which the predominant
complex has 2:1 receptor-anion stoichiometry when [1c]
-
1b, the Kassoc values for H2PO4 are higher than those
for Cl-, a result that has been observed with other
nonmacrocyclic receptors.5d,6 On the basis of its larger size
alone, binding of dihydrogenphosphate ion was expected
to be sensitive to the identity of the substituents attached
to the amide N atoms. Instead, chloride binding showed
> [H2PO4-]. The overall binding constant â for 1c2‚H2PO4
-
was calculated to be 1.8 × 105 M-2. A J ob plot also
indicated 2:1 binding in CD2Cl2-acetone-d6 (2:1 v/v).
However, the calculated concentration profiles20 for the
-
a higher degree of “tunability”.17 Affinities for H2PO4
are clustered in the range (2-7) × 104 M-1, while Cl- is
bound nearly 2 orders of magnitude more strongly by 1b
than 1f. The apparently anomolous behavior of 1b18
prompted further titrations with nitrate and bromide;
however, neither anion formed an unusually robust
-
1c + H2PO4 titration show the binding stoichiometry
to be largely 1:1 when [biimidazole] , [anion]. Although
the use of different solvents for the fluorescence and NMR
binding titrations prevents the results from being directly
compared, the observation of 1:1 binding in the presence
of excess anion is in accord with the fluorescence data.
-
complex with this receptor. Kassoc for 1b + NO3 and 1b
+ Br- were both determined to be <2 × 104 M-1. Diether
1e was designed to exploit additional OHphosphate‚‚‚Oether
interactions with H2PO4-, but the presence of such
H-bonds cannot be established from the Kassoc value.
Biimidazole 1f, which lacks amide NH atoms, interacts
with anions most weakly of all the receptors tested.
Two control titrations were performed to confirm that
the decrease in fluorescence arises from actual anion
binding, and not from nonspecific collisional quenching,
As expected, more polar solvents lead to smaller anion-
induced downfield shifts of the amide NH protons, and
to weaker binding. When a titration of 1c with Bu4N+
H2PO4- was performed in CD2Cl2-DMSO-d6 (2:1 v/v), the
amide NH resonance shifted by 0.31 ppm. Simple 1:1
binding stoichiometry was operative in this case, with
Kassoc ) 2.7 × 103 M-1. Introduction of a 15-fold excess of
Bu4N+ Cl- to a solution of 1e in DMSO-d6 caused a
change in δNH of just 0.08 ppm; Kassoc for the 1:1 interac-
tion was calculated to be ≈5 M-1. A single set of
biimidazole peaks appeared throughout these experi-
ments, indicative of fast exchange. Severe broadening of
the imidazole NH peaks precluded their use in Kassoc
determinations.
Among several possible anion binding modes for the
biimidazole unit, two are shown in Figure 3. Assuming
that the anti configuration observed in the solid state is
the predominant biimidazole conformation in solution,
anti-2 complexation requires minimal reorganization of
receptors 1. In principle this mode could accommodate
1:2 biimidazole-anion binding stoichiometry, although
complexes of this type were not observed even under
conditions of excess anion. Maximal H-bond donation to
(14) The emission spectra of 1b-f are sensitive to concentration.
When excited at 300 nm, aerated CH2Cl2 solutions of 1b-f of
concentrations greater than approximately 5 × 10-6 M emit light of
wavelengths 310-460 nm in a single, relatively featureless emission
band (λmax ≈ 365 nm). Below about
3
× 10-6 M, however, the
fluorescence spectra display three maxima near 322, 337, and 353 nm.
These results are consistent with biimidazole-biimidazole self-as-
sociation occurring. Aggregation was particularly problematic in the
case of dimethyl derivative 1a , for which the emission spectrum did
not become resolved into three peaks even at a concentration as low
as 9 × 10-7 M.
(15) For discussions of the factors which could account for the
emission quenching, see: (a) de Silva, A. P.; Gunaratne, H. Q. N.;
Gunnlaugsson, T.; Huxley, A. J . M.; McCoy, C. P.; Rademacher, J . T.;
Rice, T. E. Chem. Rev. 1997, 97, 1515. (b) Lakowicz, J . R. Principles of
Fluorescence Spectroscopy, 2nd ed.; Kluwer Academic/Plenum: New
York, 1999.
(16) Connors, K. A. Binding Constants; Wiley: New York, 1987.
(17) For analysis of how steric properties influence receptor solvation
and binding energetics, see: Haj-Zaroubi, M.; Mitzel, N. W.; Schmidtch-
en, F. P. Angew. Chem., Int. Ed. 2002, 41, 104.
(18) Anion-π interactions may be operative for 1b. See: Mascal,
M.; Armstrong, A.; Bartberger, M. D. J . Am. Chem. Soc. 2002, 124,
6274.
(19) Uppadine, L. H.; Drew, M. G. B.; Beer, P. D. Chem. Commun.
2001, 291.
(20) Hynes, M. J . J . Chem. Soc., Dalton Trans. 1993, 311.
J . Org. Chem, Vol. 67, No. 17, 2002 5965

Causey and Allen
with saturated aqueous NaCl and was dried over Na2SO4 or
MgSO4, as appropriate. Filtration, rotary evaporation of the
filtrate, and drying for 24 h under high vacuum afforded the
crude amide as a yellow-orange oil or solid. Further purifica-
tion was performed as described below.
5-P r op yl-1H -im id a zole-4-ca r b oxylic Acid Met h yl-
a m id e (5a ). By using a solution of 40% aqueous methylamine,
0.31 g (80%) of amide 5a was obtained as colorless prisms after
1
recrystallization from CH3CN-CH3OH. Mp 173-175 °C; H
NMR (CD3OD) δ 0.97 (t, 3H), 1.65 (m, 2H), 2.55 and 2.90 (both
s, 3H total), 2.95 (t, 2H), 7.50 (s, 1H), 7.85 (br t, 1H); 13C NMR
(CD3OD) δ 14.0, 23.8, 25.8, 27.8, 135.0, 137.7, 166.7; LRMS
(CI+) m/z (%) 168 (100) [M + H]+; HRMS (CI+) calcd for
C8H14N3O 168.1137, found 168.1141.
F IGURE 3. Possible structures of biimidazole-anion com-
plexes.
an anionic guest is only possible if both imidazole NH
tautomerism and rotation about the central C2-C2′ bond
occur, as in syn-4. Each heterocycle-amide “half” of the
biimidazole would provide two convergent NH groups,4
allowing 1 to define a crescent-shaped binding pocket.
5-P r op yl-1H-im id a zole-4-ca r boxylic Acid 3,5-Diflu o-
r oben zyla m id e (5b). By using 3,5-difluorobenzylamine, crude
amide 5b was obtained as a viscous yellow oil after flash
column chromatography on silica gel with EtOAc as the eluent.
Recrystallization from Et2O-hexanes afforded 0.32 g (50%) of
pure 5b as colorless needles. Mp 104-105 °C; TLC (EtOAc)
1
Rf ) 0.33; H NMR (CDCl3) δ 0.95 (t, 3H), 1.63 (m, 2H), 2.97
Con clu sion s
(t, 2H), 4.59 (d, 2H), 6.60 (t, 1H), 6.80 (d, 2H), 7.32 (s, 1H),
8.00 (br t, 1H); 13C NMR (CDCl3) δ 13.8, 22.8, 27.2, 42.2, 110.0,
129.5, 133.4, 137.2, 143.1, 161.8, 164.7; LRMS (CI+) m/z (%)
280 (100) [M + H]+; HRMS (CI+) calcd for C14H16F2N3O
280.1261, found 280.1263. Anal. Calcd for C14H15F2N3O: C
60.21, H 5.41, N 15.05. Found: C 60.21, H 5.43, N 14.98.
5-P r op yl-1H -im id a zole-4-ca r b oxylic Acid 4-Met h yl-
ben zyla m id e (5c). By using 4-methylbenzylamine, 0.53 g
(90%) of amide 5c was obtained as an off-white solid after flash
column chromatography on silica gel with EtOAc as the eluent.
Mp 131-133 °C; TLC (EtOAc): Rf ) 0.28; 1H NMR (CDCl3) δ
0.85 (t, 3H), 1.65 (m, 2H), 2.30 (s, 3H), 3.00 (t, 2H), 4.57 (d,
2H), 7.10 (dd, 4H), 7.25 (s, 1H), 7.62 (br t, 1H); 13C NMR
(CDCl3) δ 13.9, 21.7, 22.5, 27.1, 43.0, 133.6, 135.8, 137.7, 164.8;
LRMS (CI+) m/z (%) 258 (100) [M + H]+; HRMS (CI+) calcd
for C15H20N3O 258.1606, found 258.1600.
Biimidazole diamides serve as electrically neutral,
relatively unselective sensors for anions in organic solu-
tion. The intrinsic fluorescence of the biimidazole unit
obviates the need for luminescent metal ions or covalently
appended, dedicated chromophores. Incorporation of bi-
imidazole units into macrocycles and/or spherical recep-
tors, thereby constraining the biimidazoles to adopt only
the syn conformation, is expected to improve the anion
affinities and selectivities of this receptor class.
Exp er im en ta l Section
5-P r op yl-1H-im id a zole-4-ca r boxylic Acid Hyd r och lo-
r id e (3). 5-Propyl-1H-imidazole-4-carboxylic acid ethyl ester
2 (2.00 g; 11.0 mmol) and concentrated aqueous HCl (40 mL)
were combined and heated to reflux with stirring for 24 h.
After cooling to room temperature, the light tan solution was
diluted with water (100 mL) and extracted with EtOAc. The
aqueous layer was then evaporated under reduced pressure.
The residue was dissolved in 2-propanol (20 mL) and cooled
to -78 °C. Upon addition of Et2O, a white precipitate formed.
This material was collected by filtration and dried under
vacuum to afford 2.10 g (100%) of 3. 1H NMR (DMSO-d6) δ
0.87 (t, 3H), 1.69 (m, 2H), 2.92 (t, 2H), 9.17 (s, 1H); 13C NMR
(DMSO-d6) δ 13.9, 22.4, 26.4, 120.9, 135.7, 140.0, 160.7. Anal.
Calcd for C7H11ClN2O2‚0.66(C3H9O)‚0.33(H2O): C 45.67, H
7.24, N 11.84, Cl 14.98. Found: C 45.27, H 7.17, N 11.86, Cl
14.68.
5-P r op yl-1H-im id a zole-4-ca r bon yl Ch lor id e (4). 5-Prop-
yl-1H-imidazole-4-carboxylic acid hydrochloride 3 (1.0 g; 5.3
mmol) was added to nitrogen-purged CH3CN (10 mL). Oxalyl
chloride (2.5 mL; 29 mmol) was then added, and the mixture
was heated to reflux under N2 for 1 h. The brown solution was
then allowed to cool, and the product began to precipitate. Ice-
cold dry Et2O (30 mL) was added to the reaction flask to
complete the precipitation. The green-gold solid was collected
by suction filtration, washed with Et2O, and dried under
vacuum to afford 0.65 g (71%) of 4. 1H NMR (DMSO-d6) δ 0.90
(t, 3H), 1.70 (m, 2H), 2.90 (t, 2H), 8.70 (s, 1H); 13C NMR
(DMSO-d6) δ 13.6, 21.3, 29.4, 118.1, 137.9, 150.46, 155.8.
Gen er a l P r oced u r e for P r ep a r a tion of Am id es 5a -f.
Under N2, a round-bottomed flask containing a stir bar was
charged with 5-propyl-1H-imidazole-4-carbonyl chloride 4 (0.40
g, 2.3 mmol) and 3-4 mL of a neat liquid amine. The mixture
was stirred with gentle warming from a heat gun for 5 min,
during which time a cloud of white vapor appeared in the flask.
The mixture was partitioned between 100 mL of water and
100 mL of an organic solvent (CH2Cl2 or EtOAc-THF (1:1 v/v),
as required for solubility). The organic phase was then washed
5-P r op yl-1H-im id a zole-4-ca r boxylic Acid Hexyla m id e
(5d ). By using 1-hexylamine, 0.49 g (90%) of amide 5d was
obtained as light yellow plates after recrystallization from CH3-
1
CN. Mp 103-104 °C; H NMR (CDCl3) δ 0.87 (br t, 6H), 1.30
(m, 6H), 1.61 (m, 2H), 1.65 (m, 2H), 3.00 (t, 2H), 3.37 (q, 2H),
7.22 (br t, 1H), 7.48 (s, 1H); 13C NMR (CDCl3) δ 14.1, 14.2,
22.8, 22.9, 27.2, 30.0, 31.3, 31.8, 39.1, 132.8, 136.0, 164.2;
LRMS (CI+) m/z (%) 238 (100) [M + H]+; HRMS (CI+) calcd
for C13H24N3O 238.1919, found 238.1912. Anal. Calcd for
C
13H23N3O: C 65.79, H 9.77, N 17.70. Found: C 65.89, H 9.38,
N 17.72.
5-P r op yl-1H-im id a zole-4-ca r boxylic Acid (3-Meth oxy-
p r op yl)a m id e (5e). By using 3-methoxypropylamine, 0.44 g
(85%) of amide 5e was obtained as yellow crystals after flash
column chromatography with EtOAc-CH3OH (9:1 v/v) as the
eluent. Mp 61-63 °C; TLC (EtOAc-CH3OH, 9:1) Rf ) 0.30;
1H NMR (CDCl3) δ 0.82 (t, 3H), 1.55 (m, 2H), 1.75 (m, 2H),
2.90 (t, 2H), 3.21 (s, 3H), 3.40 (m, 4H), 7.35 (s, 1H), 7.44 (br t,
1H); 13C NMR (CDCl3) δ 13.9, 23.7, 27.7, 30.5, 37.3, 58.9, 71.5,
134.1, 138.3, 164.6; LRMS (CI+) m/z (%) 226 (100) [M + H]+;
HRMS (CI+) calcd for C11H20N3O2 226.1556, found 226.1553.
(5-P r o p y l-1H -im id a zo l-4-y l)p y r r o lid in -1-y lm e t h a -
n on e (5f). By using pyrrolidine, 0.21 g (44%) of amide 5f was
obtained as yellow needles after recrystallization from CH3-
CN. Mp 121-122 °C; 1H NMR (CDCl3) δ 0.85 (t, 3H), 1.61 (m,
2H), 1.90 (br m, 4H), 2.84 (t, 2H), 3.60 (br s, 2H), 3.90 (br s,
2H), 7.40 (s, 1H); 13C NMR (CDCl3) δ 13.5, 22.5, 23.8, 44.9,
46.3, 46.7, 48.8, 131.0, 136.5, 164.5; LRMS (CI+) m/z (%) 208
(100) [M + H]+; HRMS (CI+) calcd for C11H18N3O 208.1450,
found 208.1445. Anal. Calcd for C11H17N3O: C 63.74, H 8.27,
N 20.27. Found: C 63.57, H 8.30, N 20.26.
Gen er a l P r oced u r e for P r ep a r a tion of 2-Iod o Am id es
6a -f. An imidazole amide 5 (1-2 mmol) was dissolved in dry
THF (25 mL) in a round-bottomed flask containing a stir bar.
N-Iodosuccinimide (NIS) (2.0 equiv) was added in one portion,
5966 J . Org. Chem., Vol. 67, No. 17, 2002

Anion Binding by Fluorescent Biimidazole Diamides
and the flask was covered with foil to exclude light. The
mixture was heated to reflux for 24 h under N2, then was
allowed to cool to room temperature. Saturated aqueous
NaHSO3 (7 mL) was added dropwise to destroy the excess
iodine reagent. The solvents were removed under reduced
pressure, and the residue was partitioned between water and
an organic solvent (CHCl3 or EtOAc, as determined by solubil-
ity). The organic phase was dried over Na2SO4 and filtered,
and the filtrate was evaporated to yield the 2-iodo product as
a solid. In most cases, this crude material was used in the
subsequent Pd(0)-coupling step without additional purification.
turned dark red-brown. Subsequent isolation and purification
of products 1a -f varied, as described below.
5,5′-Dip r op yl-1H,1′H-[2,2′]biim id a zolyl-4,4′-d ica r boxy-
lic Acid Bis(m eth yla m id e) (1a ). Iodide 6a (0.35 g; 1.2 mmol)
was used as starting material. After cooling to room temper-
ature, the reaction mixture was diluted with CH3OH and
filtered. The filtrate was evaporated under vacuum, and the
residue was dissolved in CH2Cl2. Upon standing, a white solid
precipitated. This material was collected by filtration and was
washed with ice-cold CH2Cl2 to afford 0.080 g (43%) of 1a . Mp
1
>260 °C; TLC (EtOAc) Rf ) 0.32; H NMR (DMSO-d6) δ 0.83
(t, 6H), 1.61 (m, 4H), 2.73 (d, 6H), 2.89 (t, 4H), 7.50 (br t, 2H);
13C NMR (DMSO-d6) δ 13.1, 21.9, 24.7, 30.2, 130.3, 135.6,
163.0; UV/vis (CH3OH) λmax (ꢀ M-1cm-1) 286 (18500), 294
(18200), 309 (9860). Anal. Calcd for C16H24N6O2‚0.25(CH2-
Cl2): C 55.19, H 6.98, N 23.76. Found: C 54.93, H 6.71, N
23.74.
2-Iod o-5-p r op yl-1H-im id a zole-4-ca r boxylic Acid Me-
th yla m id e (6a ). Starting from 5a (0.30 g; 1.8 mmol), 0.45 g
(87%) of 6a was obtained as a yellow solid. H NMR (CD3OD)
δ 0.89 (t, 3H), 1.61 (m, 2H), 2.63 and 2.82 (both s, 3H total),
2.91 (t, 2H), 7.85 (s, 1H); 13C NMR (CD3OD) δ 13.9, 23.7, 25.9,
27.7, 134.5, 142.0, 165.3; LRMS (CI+) m/z (%) 294 (100) [M +
H]+; HRMS (CI+) calcd for C8H13IN3O 294.0103, found 294.0010.
1
5,5′-Dip r op yl-1H,1′H-[2,2′]biim id a zolyl-4,4′-d ica r boxy-
lic Acid Bis(3,5-d iflu or oben zyla m id e) (1b). Iodide 6b (0.44
g; 1.1 mmol) was used as starting material. After cooling to
room temperature, the reaction mixture was diluted with CH3-
OH and filtered. The filtrate was evaporated under vacuum,
and the residue was dissolved in CHCl3. Slow addition of CH3-
CN caused a brown solid to precipitate. This material was
collected by filtration and washed with CH3CN to afford 0.12
g (40%) of 1b as an off-white solid. Mp 173-174 °C; TLC (CH2-
Cl2-EtOAc, 1:1) Rf ) 0.51; 1H NMR (DMSO-d6) δ 0.85 (t, 6H),
1.60 (m, 4H), 2.90 (t, 4H), 4.42 (d, 4H), 6.99 (d, 4H), 7.05 (t,
2H), 8.20 (br t, 2H); 13C NMR (DMSO-d6) δ 14.1, 23.1, 30.2,
41.8, 110.9, 131.0, 141.7, 145.5, 161.3, 163.8, 164.7; UV/vis
(CH3OH) λmax (ꢀ M-1 cm-1) 287 (21800), 294 (20700), 309
(10700); LRMS (CI+) m/z (%) 557 (100) [M + H]+; HRMS (CI+)
calcd for C28H29F4N6O2 557.2288, found 557.2292.
2-Iod o-5-p r op yl-1H-im id a zole-4-ca r boxylic Acid 3,5-
Diflu or oben zyla m id e (6b). Starting from 5b (0.32 g; 1.1
mmol), 0.39 g (84%) of 6b was obtained as yellow solid. 1H
NMR (CDCl3) δ 0.91 (t, 3H), 1.62 (m, 2H), 2.99 (t, 2H), 4.58
(d, 2H), 6.68 (t, 1H), 6.84 (d, 2H), 7.64 (br t, 1H), 9.14 (br s,
1H); 13C NMR (CDCl3) δ 13.9, 22.8, 27.1, 42.3, 110.5, 133.8,
141.7, 143.0, 161.6, 163.0, 165.1; LRMS (CI+) m/z (%) 406 (100)
[M + H]+; HRMS (CI+) calcd for C14H15F2IN3O 406.0228, found
406.0219.
2-Iod o-5-p r op yl-1H-im id a zole-4-ca r boxylic Acid 4-Me-
th ylben zyla m id e (6c). Starting from 5c (0.30 g; 1.2 mmol),
0.38 g (85%) of 6c was obtained as a yellow solid. 1H NMR
(CDCl3) δ 0.93 (t, 3H), 1.64 (m, 2H), 2.33 (s, 3H), 3.01 (t, 2H),
4.56 (d, 2H), 7.17 (dd, 4H), 7.40 (br t, 1H); 13C NMR (CDCl3)
δ 14.0, 21.3, 22.8, 29.8, 42.9, 128.1, 129.5, 135.6, 137.1, 141.0,
162.5; LRMS (CI+) m/z (%) 290 (100), 384 (10) [M + H]+; HRMS
(CI+) calcd for C15H19IN3O 384.0573, found 384.0585.
5,5′-Dip r op yl-1H,1′H-[2,2′]biim id a zolyl-4,4′-d ica r boxy-
lic Acid Bis(4-m eth ylben zyla m id e) (1c). Iodide 6c (0.45 g;
1.2 mmol) was used as starting material. After cooling to room
temperature, the reaction mixture was diluted with CH3OH
and filtered. The filtrate was evaporated under vacuum, and
the residue was triturated with CH3CN. The solid that
remained undissolved was collected by filtration and washed
with CH3CN to afford 0.30 g (38%) of 1c as an off-white
microcrystalline solid. An analytical sample was recrystallized
from CH3OH. Mp 226-227 °C; TLC (CH2Cl2-EtOAc, 1:1) Rf
) 0.60; 1H NMR (CD2Cl2) δ 0.80 (t, 6H), 1.59 (m, 4H), 2.21 (s,
6H), 2.86 (t, 4H), 4.39 (d, 4H), 7.05 (dd, 8H), 7.22 (br t, 2H);
13C NMR (DMSO-d6) δ 14.2, 21.4, 23.3, 27.0, 49.6, 128.3, 129.9,
2-Iod o-5-p r op yl-1H-im id a zole-4-ca r boxylic Acid Hex-
yla m id e (6d ). Starting from 5d (0.30 g; 1.3 mmol), 0.42 g
1
(92%) of 6d was obtained as a yellow solid. H NMR (CDCl3)
δ 0.81 (br t, 6H), 1.23 (m, 6H), 1.57 (m, 4H), 2.96 (t, 2H), 3.35
(q, 2H), 7.16 (br t, 1H), 11.39 (br s, 1H); 13C NMR (CDCl3) δ
13.7, 14.0, 22.6, 26.7, 26.9, 29.6, 31.5, 39.0, 134.0, 140.8, 162.5;
LRMS (CI+) m/z (%) 364 (100) [M + H]+; HRMS (CI+) calcd
for C13H23IN3O 364.0886, found 364.0877.
2-Iod o-5-p r op yl-1H -im id a zole-4-ca r b oxylic Acid (3-
Meth oxyp r op yl)a m id e (6e). Starting from 5e (0.56 g; 2.5
mmol), the crude product obtained from the General Procedure
was further purified by flash column chromatography on silica
gel with EtOAc as the eluent. Fractions containing the fast
spot were combined and evaporated to afford 0.55 g (63%) of
131.0, 137.0, 138.7, 164.2; UV/vis (CH3OH) λmax (ꢀ M-1 cm-1
)
287 (25100), 295 (25100), 310 (13700); LRMS (CI+) m/z (%)
513 (100) [M + H]+; HRMS (CI+) calcd for C30H37N6O2
513.2978, found 513.2981. Anal. Calcd for C30H36N6O2‚0.33-
(H2O): C 68.68, H 7.17, N 16.02. Found: C 68.58, H 7.17, N
16.11.
1
6e as a white solid. TLC (EtOAc) Rf ) 0.48; H NMR (CD3-
OD) δ 0.80 (t, 3H), 1.52 (m, 2H), 1.76 (m, 2H), 2.85 (t, 2H),
3.23 (s, 3H), 3.29 (m, 2H), 3.37 (t, 2H), 7.70 (br t, 1H); 13C NMR
(CD3OD) δ 13.9, 23.7, 27.7, 30.5, 58.9, 71.5, 134.5, 142.1, 164.6;
LRMS (CI+) m/z (%) 352 (100) [M + H]+; HRMS (CI+) calcd
for C11H19IN3O2 352.0522, found 352.0528.
5,5′-Dip r op yl-1H,1′H-[2,2′]biim id a zolyl-4,4′-d ica r boxy-
lic Acid Bis(h exyla m id e) (1d ). Iodide 6d (0.45 g; 1.1 mmol)
was used as starting material. After cooling to room temper-
ature, the reaction mixture was diluted with CH3OH and
filtered. The filtrate was evaporated under vacuum, and the
residue was recrystallized from CH3OH to afford 0.12 g (45%)
of 1d as a white solid. Mp >260 °C; TLC (CH2Cl2-EtOAc, 1:1)
Rf ) 0.51; 1H NMR (CDCl3) δ 0.88 (br t, 6H), 0.97 (t, 6H), 1.31
(br m, 12H), 1.60 (m, 4H), 1.72 (m, 4H), 3.06 (t, 4H), 3.38 (q,
4H), 7.00 (br t, 2H); 13C NMR (DMSO-d6) δ 14.2, 14.6, 22.8,
23.1, 26.9, 30.2, 31.7, 38.7, 131.3, 137.2, 163.5; UV/vis (CH3-
OH) λmax (ꢀ M-1 cm-1) 287 (26000), 295 (25900), 309 (14100);
LRMS (CI+) m/z (%) 473 (100) [M + H]+; HRMS (CI+) calcd
for C26H45N6O2 473.3604, found 473.3601. Anal. Calcd for
(2-Iodo-5-pr opyl-1H-im id a zol-4-yl)p yr r olid in -1-ylm eth -
a n on e (6f). Starting from 5f (0.21 g; 1.0 mmol), 0.30 g (89%)
1
of 6f was obtained as a yellow solid. H NMR (CDCl3) δ 0.89
(t, 3H), 1.60 (m, 2H), 1.90 (br m, 4H), 2.80 (t, 2H), 3.61 (br s,
2H), 3.85 (br s, 2H), 9.30 (s, 1H); 13C NMR (CDCl3) δ 13.7,
21.0, 22.6, 23.9, 26.4, 46.6, 49.0, 136.5, 141.9, 163.0; LRMS
(CI+) m/z (%) 334 (100) [M + H]+; HRMS (CI+): calcd for
C
11H17IN3O 334.0416, found 334.0419.
Gen er a l P r oced u r e for P r ep a r a tion of Biim id a zole
Dia m id es 1a -f. An iodide 6 (1.0-1.6 mmol) and dry toluene
(30 mL) were placed in a thick-walled pressure tube containing
a stir bar. The solution was purged with N2 for 5 min, then
N,N-diisopropylethylamine (2.0 equiv) and tetrakis(triphen-
ylphosphine)palladium(0) (0.040 equiv) were added to the
reaction mixture. The tube was sealed, covered in foil, and
heated at 110 °C for 48 h, during which time the reaction
C
26H44N6O2: C 66.07, H 9.38, N 17.78. Found: C 66.26, H 9.54,
N 17.83.
5,5′-Dip r op yl-1H,1′H-[2,2′]biim id a zolyl-4,4′-d ica r boxy-
lic Acid Bis[(3-m eth oxyp r op yl)a m id e] (1e). Iodide 6e (0.55
g; 1.6 mmol) was used as starting material. After cooling to
room temperature, the reaction mixture was diluted with CH3-
J . Org. Chem, Vol. 67, No. 17, 2002 5967

Causey and Allen
OH and filtered. The filtrate was evaporated under vacuum,
and the residue was recrystallized from EtOAc to afford 0.13
g (33%) of 1e as colorless prisms. Mp >260 °C; TLC (EtOAc)
kbiimid is equal to F0/[biimid]0. Results reported in Table 1 are
averages of at least two replicate titrations.
1H NMR Bin d in g Stu d ies. A 0.0132 M solution of 1c in
CD2Cl2-acetone-d6 (2:1 v/v) was prepared. A portion (0.75 mL)
of this solution was transferred to an NMR tube; to the
remainder of the solution was added Bu4N+ H2PO4- such that
its concentration was 0.189 M. A known volume of the anion-
containing solution was added to the NMR tube, the tube was
inverted several times to mix the contents, and the chemical
shift of the amide NH protons of 1c was determined with use
of a 500-MHz instrument. This process was repeated until
these protons ceased to move downfield. Procedures similar
to the one above were also used to assess the binding of 1c to
1
Rf ) 0.31; H NMR (CDCl3) δ 0.91 (t, 6H), 1.65 (m, 4H), 1.82
(m, 4H), 3.00 (t, 4H), 3.34 (s, 6H), 3.48 (m, 8H), 7.29 (br t,
2H); 13C NMR (DMSO-d6) δ 14.3, 24.1, 26.7, 32.3, 61.9, 72.3,
137.5, 145.1, 167.3; UV/vis (CH3OH) λmax (ꢀ M-1 cm-1) 287
(25600), 295 (25400), 309 (13800); LRMS (CI+) m/z (%) 449
(100) [M + H]+; HRMS (CI+) calcd for C22H37N6O4 449.2876,
found 449.2875. Anal. Calcd for C22H36N6O4: C 58.91, H 8.09,
N 18.74. Found: C 59.00, H 8.09, N 18.78.
[5,5′-Dip r op yl-4′-(p yr r olid in e-1-ca r bon yl)-1H,1′H-[2,2′]-
biim id a zolyl-4-yl]p yr r olid in -1-ylm eth a n on e (1f). Iodide 6f
(0.34 g; 1.0 mmol) was used as starting material. After cooling
to room temperature, the reaction mixture was diluted with
CH3OH and filtered. The filtrate was evaporated under
vacuum, and the residue was recrystallized from CH3OH to
afford 0.080 g (38%) of 1f as yellow prisms. Mp >260 °C; TLC
(EtOAc) Rf ) 0.14; 1H NMR (DMSO-d6) δ 0.84 (t, 6H), 1.59
(m, 4H), 1.80 (br m, 8H), 2.80 (t, 4H), 3.55 (br m, 4H), 3.90 (br
m, 4H); 13C NMR (DMSO-d6) δ 14.3, 23.4, 24.2, 26.9, 31.4, 46.8,
-
H2PO4 in CD2Cl2-DMSO-d6 (2:1 v/v), and the binding of 1e
to Cl- in DMSO-d6. Binding constants were calculated with
the nonlinear curve fitting program WinEQNMR.20
The stoichiometry of 1c + H2PO4- complexation in the NMR
concentration regime was determined by the method of
continuous variations (i.e., a J ob analysis). Thus, equimolar
solutions of 1c and Bu4N+ H2PO4- were prepared (in CD2Cl2-
acetone-d6 (2:1 v/v)) and were mixed in varying ratios in a
series of NMR tubes such that the total number of moles of
49.0, 132.9, 138.0, 163.7; UV/vis (CH3OH) λmax (ꢀ M-1 cm-1
)
287 (21200), 294 (20900), 309 (sh); LRMS (CI+) m/z (%) 208
(100), 413 (70) [M + H]+; HRMS (CI+) calcd for C22H33N6O2
413.2665, found 413.2662. Anal. Calcd for C22H32N6O2‚0.33-
(H2O): C 63.13, H 7.87, N 20.08. Found: C 63.21, H 7.82, N
20.08.
-
1c + H2PO4 was the same in each tube. The chemical shift
of the amide NH protons of 1c was recorded for each sample.
A plot of {(δobs - δ0)/δmax}[1c] vs mole fraction of 1c was then
generated; it displayed a maximum at a mole fraction 1c of
≈0.65.
F lu or escen ce Titr a tion s. Biimidazole diamides 1b-f
were dissolved in spectrophotometric grade dichloromethane
such that the concentration of each solution was between 1 ×
10-6 and 3 × 10-6 M (sonication was required to effect complete
dissolution in some cases). An electronic absorption spectrum
was acquired for each sample to ensure that the optical density
was less than 0.1. A 3.0-mL sample of biimidazole diamide
solution was transferred to a quartz cuvette and placed into
the fluorometer. The sample was excited at 300 nm, and an
emission spectrum from 310 to 460 nm was recorded. Upon
completion of the scan, the area contained under the emission
band (F) was computed. Aliquots of an approximately 0.03 M
solution of anion (as tetrabutylammonium (Bu4N+) salt) in
CH2Cl2 were then injected into the sample solution through a
small hole in the cap. The sample solution was magnetically
stirred for 1 min after each addition, then was scanned again.
This process was repeated until the change in fluorescence
intensity became insignificant. The total volume of the sample
solution changed by less than 2% over the course of the
experiment.
Ack n ow led gm en t. The authors thank Daniel K.
Barnhill (ECU) for determining the binding constants
for 1b with NO3- and Br-. Rebecca S. Zimmerman and
Dr. Vincent M. Lynch (UT-Austin) performed mass
spectrometry and X-ray crystallography, respectively.
Dipyrrolylquinoxalines (used in fluorescence control
experiments) were a gift from Prof. Pavel Anzenbacher,
J r. (Bowling Green State University). W.E.A. acknowl-
edges the Research and Creative Activity Committee of
ECU for funding a portion of this work. C.P.C. was
supported, in part, by a Burroughs-Wellcome Fellow-
ship.
Su p p or tin g In for m a tion Ava ila ble: A crystal packing
diagram for 1a , and ORTEP views of single molecules of 1b-
d ; tables of crystal data, bond lengths and angles, atomic
coordinates, and thermal parameters for 1a -d ; and binding
Binding constants (Kassoc) were derived from plots of F/F0
1
plots from fluorescence and H NMR titrations. This material
vs [anion].16 During iterative fitting to the equation F/F0
(1 + (kcomplex/kbiimid)Kassoc[anion])/(1 + Kassoc[anion]), the values
of kcomplex and Kassoc were allowed to vary freely. The constant
)
is available free of charge via the Internet at http://pubs.acs.org.
J O020098V
5968 J . Org. Chem., Vol. 67, No. 17, 2002