Synthesis of an o-Carboranyl Derivative of 4-[5-(4-Methyl-1-piperazinyl)-2,5′-bi-1H-benzimidazol-2′-yl]phenol

Article abstract of DOI:10.1021/ic980505e

Synthesis of an o-carboranyl derivative of 4-[5-(4-methyl-l-piperazinyl)-2,5′-bi-1H-benzimidazol-2′-yl]phenol (1), a candidate for application in boron neutron capture therapy for cancer treatment, is described. Decaborane was introduced into 3-(p-cyanophenoxy)-1-propyne (11) to form l-(4-cyanophenoxymethyl)-1,2-dicarba-dayododecaborane(12) (12), which was transformed into the corresponding imidate 10 in order to be coupled with 4-cyano-ophenylenediamine (13) to give 1-[4-(5′-cyano-1H-benzimidazol-2′-yl)phenoxymethyl]-1,2-dicarba- c7o5ododecaborane(12) (14). The latter was reacted to the related imidate salt 15 and condensed with 5-(4-methyl-1-piperazinyl)-o-phenylenediamine (6) to the title compound 1-{4-[5-(4-methyl-l-piperazinyl)-2,5′-bi-1H-benzimidazol-2′-yl] phenoxymethyl}-1,2-dicarba-closo-dodecaborane(12) (2).

Full text of DOI:10.1021/ic980505e

6018
Inorg. Chem. 1998, 37, 6018-6022
Synthesis of an o-Carboranyl Derivative of
4-[5-(4-Methyl-1-piperazinyl)-2,5′-bi-1H-benzimidazol-2′-yl]phenol
Mario Argentini,† Deborah F. Dos Santos,‡ Regin Weinreich,*,† and Hans-Ju1rgen Hansen‡
Institut fu¨r Medizinische Radiobiologie der Universita¨t Zu¨rich und des Paul Scherrer Instituts,
CH-5232 Villigen-PSI, Switzerland, and Organisch-Chemisches Institut der Universita¨t Zu¨rich,
Winterthurerstrasse 190, CH-8057 Zurich, Switzerland
ReceiVed May 4, 1998
Synthesis of an o-carboranyl derivative of 4-[5-(4-methyl-1-piperazinyl)-2,5′-bi-1H-benzimidazol-2′-yl]phenol (1),
a candidate for application in boron neutron capture therapy for cancer treatment, is described. Decaborane was
introduced into 3-(p-cyanophenoxy)-1-propyne (11) to form 1-(4-cyanophenoxymethyl)-1,2-dicarba-closo-
dodecaborane(12) (12), which was transformed into the corresponding imidate 10 in order to be coupled with
4-cyano-o-phenylenediamine (13) to give 1-[4-(5′-cyano-1H-benzimidazol-2′-yl)phenoxymethyl]-1,2-dicarba-closo-
dodecaborane(12) (14). The latter was reacted to the related imidate salt 15 and condensed with 5-(4-methyl-
1-piperazinyl)-o-phenylenediamine (6) to the title compound 1-{4-[5-(4-methyl-1-piperazinyl)-2,5′-bi-1H-
benzimidazol-2′-yl]phenoxymethyl}-1,2-dicarba-closo-dodecaborane(12) (2).
Introduction
Boron neutron capture therapy (BNCT) is a binary process:
the stable boron isotope 10B has to be introduced and trapped
in the tumor cell, and after neutron irradiation, it emits an R
7
Figure 1. 4-[5-(4-Methyl-1-piperazinyl)-2,5′-bi-1H-benzimidazol-2′-
yl]phenol (1, “Hoechst 33258”).
particle and a Li nucleus. Both particles have a high linear
energy transfer (LET) and may consequently induce disturbances
in the tumor cell division. This process was proposed in 19361
strategy of many research groups is to load potential DNA
ligands with as many boron atoms as possible.
and has been used for therapeutic applications for many years.
The main advantages of this therapy are that no radioactive
Boron-containing DNA ligands such as the following com-
materials are involved and that, in principle, the supply of
pounds were synthesized: (i) pyrimidine derivatives that act as
neutrons can be controlled with respect to their energy, intensity,
and space distribution.2
surrogates for thymidine and, hence, can be introduced directly
into the DNA of the tumor cell5 and (ii) well-known dyes that
can adhere to the DNA. Among these were boronated netropsin,
distamycin, some phenanthridine and acridine derivatives, and
the fluorescence marker 4-[5-(4-methyl-1-piperazinyl)-2,5′-bi-
1H-benzimidazol-2′-yl]phenol (1, in the literature known as
Hoechst 33258).6
1 (Figure 1) was developed in the 1960s for potential
application as a chemotherapeutically active drug.7 So far,
however, it is merely used as a fluorescent stain for in vitro
DNA studies.8 It binds to the minor grooves of DNA,
preferentially to A-T base pairs. As is shown by X-ray
structure analyses,9 hydrogen bridges are formed between the
Many attempts have been made to design an efficient tumor-
targeting compound as a boron carrier.3 To date, only two
compounds, the 10B-enriched disodium undecahydromercapto-
10
closo-dodecaborate, Na2 B12H11SH (sodium borocaptate, BSH),
and the 10B-enriched L-4-(dihydroxyboryl)phenylalanine (BPA)
are routinely used in BNCT, the former for glioma and the latter
for skin melanoma treatment. An increase in the radiothera-
peutic effectiveness of 10B, however, could be expected if a
carrier compound were designed which enters the cell nucleus
or, most favorably, binds directly to the DNA.4 Thus, the
* To whom correspondence should be addressed. Fax: +41-56-310-
4435. E-mail: Regin.Weinreich@psi.ch.
† Paul Scherrer Institut.
(5) For a review, see: Goudgaon, N. M.; El-Kattan, G. F.; Schinazi, R.
F. Nucleosides Nucleotides 1994, 13, 849-880.
‡ Universita¨t Zu¨rich.
(1) Locher, G. L. Am. J. Roentgenol. Radium Ther. 1936, 36, 1-13.
(2) For reviews, see: Barth, R. F.; Soloway, A. H.; Fairchild, R. G.;
Brugger, R. M. Cancer 1992, 70, 2995-3007. Carlsson, J.; Sjo¨berg,
S.; Larsson, B. S. Acta Oncol. 1992, 31, 803-813. Barth, R. F.;
Soloway, A. H.; Brugger, R. M. Cancer InVest. 1996, 14, 534-550.
(3) For reviews, see: Hawthorne, M. F. Pure Appl. Chem. 1991, 63, 327-
334. Hawthorne, M. F. Angew. Chem., Int. Ed. Engl. 1993, 32, 950-
984. Gabel, D. Chim. Oggi 1996, October, 37-42. Larsson, B.,
Crawford, J., Weinreich, R., Eds. BNCT-Related Chemistry. AdVances
in Neutron Capture Therapy, Vol. II, Chemistry and Biology; Elsevier
Scientific: Amsterdam, 1997; Chapter 1, pp 1-145.
(6) Kelly, D. P.; Bateman, S. A.; Martin, R. F.; Reum, M. E.; Rose, M.;
Whittaker, A. R. D. Aust. J. Chem. 1994, 47, 247-262. Yamamoto,
Y.; Cai, J.; Nakamura, H.; Sadayori, N.; Asao, N.; Nemoto, H. J. Org.
Chem. 1995, 60, 3352-3357. Sjo¨berg, S.; Carlsson, J.; Ghaneol-
hosseini, H.; Gedda, L.; Hartman, T.; Malmquist, J.; Naeslund, C.;
Olsson, P.; Tjarks, W. J. Neuro-Oncol. 1997, 33, 41-52.
(7) (a) Farbwerke Hoechst. Fr. Patent 1.519.964, April 5, 1968. (b)
Farbwerke Hoechst. Fr. Patent 6.681 M, March 17, 1969. (c) Loewe,
H.; Urbanietz, J. Arzneim.-Forsch. 1974, 24, 1927-1933.
(8) Hilwig, I.; Gropp, A. Exp. Cell Res. 1973, 81, 474-477. Weisblum,
B.; Haenssler, E. Chromosoma 1974, 46, 255-260. Hilwig, I.; Gropp,
A. Exp. Cell Res. 1975, 91, 457-460. Latt, S. A. J. Reprod. Med.
1976, 17, 41-52. Zimmer, C.; Wa¨hnert, U. Prog. Biophys. Mol. Biol.
1986, 47, 31-112.
(4) Kobayashi, T.; Kanda, K. Radiat. Res. 1982, 91, 77-94. Gabel, D.;
Foster, S.; Fairchild, R. G. Radiat. Res. 1987, 111, 14-25. Hartman,
T.; Carlsson, J. Radiother. Oncol. 1994, 31, 61-75.
10.1021/ic980505e CCC: $15.00 © 1998 American Chemical Society
Published on Web 10/22/1998

Synthesis of an o-Carboranyl Derivative
Inorganic Chemistry, Vol. 37, No. 23, 1998 6019
Scheme 1a
a (a) acetyl chloride, reflux; (b) 1-methylpiperazine, K2CO3, reflux; (c) 6 M HCl, reflux; (d) Ru (5%)/charcoal, N2H4‚H2O/EtOH, reflux; (e) 6,
AcOH, reflux; (f) Pd (10%)/charcoal, 1 M AcOH, reflux; (g) AcOH, reflux.
NH groups of the bis(benzimidazole) system and the adenine
and thymine bases of the DNA. The piperazine group, however,
tends to bind to the cytosine and guanine residues of the DNA.
Consequently, it was the aim of this work to synthesize a
DNA ligand containing as many boron atoms as possible
because of the potential value for BNCT. On the basis of the
X-ray structure data,9 it seemed advantageous to choose the
compound 1-{4-[5-(4-methyl-1-piperazinyl)-2,5′-bi-1H-benz-
imidazol-2′-yl]phenoxymethyl}-1,2-dicarba-closo-dodecaborane-
(12) (2) as an o-carboranyl derivative of 1.10,11 In this molecule,
the bulky residue of the o-carborane is located far away from
the bis(benzimidazole) region. Thus, no appreciable steric
hindrances in the coupling to the DNA should occur, and the
formation of the above-mentioned hydrogen bridges should
generally be possible.
Results and Discussion
Our previous attempts to synthesize an o-carboranyl derivative
of 1 in a way similar to that described in the patents of 17 were
unsuccessful (Scheme 1).11
The synthesis carried out according to Scheme 2 gave the
expected results. In this new route, the first step consisted of
the condensation of the commercially available p-hydroxyben-
zonitrile with propargyl bromide in the presence of potassium
carbonate in acetone to form 3-(p-cyanophenoxy)-1-propyne
(11). From this by condensation with the previously prepared
decaborane/bis(acetonitrile) complex12 in boiling toluene, 1-(4-
cyanophenoxymethyl)-1,2-dicarba-closo-dodecaborane(12) (12)
was obtained.13 This reaction gave many side products that were
not investigated in detail. It might be of interest to mention
(9) Pjura, P. E.; Grzeskowiak, K.; Dickerson, R. E. J. Med. Biol. 1987,
197, 257-271. Teng, M.-K.; Usman, N.; Frederick, C. A.; Wang, A.
H.-J. Nucleic Acids Res. 1988, 16, 2671-2690. Carrondo, M. A. A.
F. C. T.; Coll, M.; Aymami, J.; Wang, A. H. J.; Van der Marel, G.
A.; Van Boom, J. H.; Rich, A. Biochemistry 1989, 28, 7849-7859.
(10) Argentini, M.; Dos Santos, D. F.; Weinreich, R.; Hansen, H.-J. In
Hadrontherapy in Oncology; Amaldi, U., Larsson, B., Eds.; Elsevier:
Amsterdam, 1994; pp 555-559.
(11) Argentini, M.; Dos Santos, D. F.; Weinreich, R.; Hansen, H.-J. In
AdVances in Neutron Therapy, Vol. II, Chemistry and Biology; Larsson,
B., Crawford, J., Weinreich, R., Eds.; Elsevier Scientific: Amsterdam,
1997; pp 82-84.
(12) Schaeffer, R. J. Am. Chem. Soc. 1957, 79, 1006-1007.

6020 Inorganic Chemistry, Vol. 37, No. 23, 1998
Argentini et al.
Scheme 2a
methyl]-1,2-dicarba-closo-dodecaborane(12) (14) crystallized
directly from the reaction mixture as very pure fine colorless
needles.
The transformation to the corresponding 1-{4-[5′-(imino-
(ethoxy)methyl)-1H-benzimidazol-2′-yl]phenoxymethyl}-1,2-di-
carba-closo-dodecaborane(12) (15) was achieved in nearly
quantitative yields as well when dry hydrogen chloride was
bubbled through a dry ethanol/benzene mixture. The final step,
consisting of the condensation between the 5-(4-methyl-1-
piperazinyl)-o-phenylenediamine (6), which was freshly pre-
pared by reduction of 5-(4-methyl-1-piperazinyl)-2-nitroaniline
(5) with hydrazine hydrate and ruthenium on charcoal in ethanol,
and the imidate 15, was carried out in boiling glacial acetic
acid, from which the o-carboranyl derivative 2 crystallized in
pure form.
Experimental Section
General. Starting materials and reagents were purchased from
Aldrich Chemical Co., Fluka Chemie AG, or Merck AG as reagent
grade. The decaborane was purchased from Alfa, a subsidiary of
Johnson Matthey Co. The decaborane/bis(acetonitrile) complex (6,9-
CH3CN)2B10H12 was prepared according to a method described previ-
ously.12 All melting points were determined on a Bu¨chi 535 apparatus,
and the values are uncorrected. IR spectra were recorded on a Perkin-
Elmer 298 spectrometer using KBr pellets, and the signals are given
1
in cm-1. The H NMR spectra were measured with a Varian Gemini
2000 at 300 MHz. The signals are given in ppm, δ, and are referred
to the residual 1H present in deuterated solvents, and the coupling
constants J are given in Hz. The 11B NMR spectra were measured
with a Bruker AMX-600 spectrometer, and the signals are given in
ppm, δ. The mass spectra (MS) were measured in the following way:
EI with a Varian MAT 112s at 70 eV and ESI with a Fison VG TRIO
2000 at 40 eV, with ions in m/z (rel %). Column chromatography was
performed with silica gel (Merck grade 60, 35-70-mesh ASTM) and
TLC by using silica gel (60 F254, Merck, coated plates 0.25-mm thick).
The carboranyl compounds were stained with a solution of palladium
chloride (300 mg of palladium chloride was dissolved in 12.5 mL of
concentrated hydrochloric acid, diluted with 500 mL of water, and
filtered).17 Stain visualization was improved by heating the coated
plates. The closo-o-carborane is evidenced by a brownish-black area
on the plate. In the presence of the nido-o-carborane, the staining
appears nearly immediately, without heating.17 HPLC analysis was
carried out using a Waters 600E multisolvent delivery system fitted
with a Nova-Pak C18 column (4 µm, 3.9 × 150 mm). The UV spectra
were obtained by an Ultrospec 2000 spectrometer (Pharmacia S.A.,
Uppsala, Sweden).
Synthesis of 5-Chloro-2-nitroacetanilide, 3. A mixture of 5-chloro-
2-nitroaniline (145 mmol, 25 g) and acetyl chloride (100 mL) was
refluxed for 2 h. The reaction mixture was cooled to room temperature,
and acetyl chloride was removed under reduced pressure. The residue
was then dissolved in acetone and evaporated to dryness at reduced
pressure. This step was repeated until no more acetyl chloride was
detectable. Product 3 crystallized as pale yellow needles from ethanol.
Yield: 28 g (90%). Mp: 117-118 °C [115 °C (ethanol)18].
a (a) propargyl bromide, K2CO3, acetone, reflux; (b) (6,9-
CH3CN)2B10H12, toluene, reflux; (c) EtOH-benzene/dry HCl; (d)
AcOH, reflux; (e) EtOH-benzene/dry HCl; (f) AcOH, reflux.
that when the decaborane/bis(acetonitrile) complex is prepared
in situ, no reaction occurs. To avoid a substantial decrease in
the yield of 10, or even the total lack of formation of any 14,
careful purification of 12 is important.
The transformation of highly purified 12 to 1-{4-[imino-
(ethoxy)methyl]phenoxymethyl}-1,2-dicarba-closo-dodecaborane-
(12) (10) with dry hydrogen chloride in a dry ethanol/benzene
mixture14 was achieved in a smooth reaction and led to a very
pure salt in nearly quantitative yield. The condensation in
glacial acetic acid of this pure product with 4-cyano-1,2-
phenylenediamine15 (13, obtained by reduction of commercially
available 4-cyano-2-nitroaniline with hydrazine hydrate and
ruthenium on charcoal in ethanol16) is also a straightforwarded
reaction, and 1-[4-(5′-cyano-1H-benzimidazol-2′-yl)phenoxy-
Synthesis of 5-(4-Methyl-1-piperazinyl)-2-nitroaniline, 5.
A
mixture of 3 (125 mmol, 26.75 g), 1-methylpiperazine (300 mmol, 30
g), and potassium carbonate (130 mmol, 18 g) was refluxed for 2 h.
After being cooled to room temperature, the reaction mixture was
poured into distilled water (500 mL). The brownish oil formed in this
step solidified on standing overnight at room temperature. The resulting
crude product, a mixture of 5-(4-methyl-1-piperazinyl)-2-nitroacetanilide
(4) and 5-(4-methyl-1-piperazinyl)-2-nitroaniline (5), was filtered,
washed with water, and refluxed for 2 h in 6 M HCl (500 mL). After
the solution was cooled to room temperature, its pH was adjusted to
9-10 using solid NaOH pellets. The resulting precipitate was collected
(13) Fein, M. M.; Bobinski, J.; Mayes, N.; Schwartz, N.; Cohen, M. S.
Inorg. Chem. 1963, 2, 1111-1115. Heying, T. L.; Ager, J. W.; Clark,
S. L.; Mangold, D. J.; Goldstein, H. L.; Hillman, M.; Polak, R. J.;
Szymanski, J. W. Inorg. Chem. 1963, 2, 1089-1092.
(14) Allen, R. E.; Schumann, E. L.; Day, W. C.; Van Campen, M. G. J.
Am. Chem. Soc. 1958, 80, 591-598.
(15) King, F. E.; Acheson, R. M. J. Chem. Soc. 1949, 1396-1400.
(16) Pietra, S. Ann. Chim. 1962, 52, 727-730. Argentini, M.; Wiese, C.;
Weinreich, R. J. Fluorine Chem. 1994, 68, 141-144.
(17) Soloway, A. H. Personal communication, 1996.
(18) Beilstein, F.; Kurbatow, A. Justus Liebigs Ann. Chem. 1876, 182, 94-
112.

Synthesis of an o-Carboranyl Derivative
Inorganic Chemistry, Vol. 37, No. 23, 1998 6021
on a Bu¨chner funnel, washed with water, and dried in vacuo. The
final product recrystallized from ethyl acetate in yellow prisms.
Yield: 25 g (85%). Mp: 155-156 °C [155 °C (not cryst)7a,c]. 1H
NMR (DMSO-d6): 2.20 (s, 3 H, N-CH3), 2.38 [t, J ) 5.2, 4 H,
(-H2C)2N-CH3]; 3.32-3.29 [t, J ) 4.99, 4 H, (-H2C)2N-Ar], 6.21
(d, J4,6 ) 2.6, 1 H, H-6), 6.36 (dd, J4,6 ) 2.6, J3,4 ) 9.7, 1 H, H-4),
7.26 (s, 2 H, Ar-NH2), 7.78 (d, J3,4 ) 9.7, 1 H, H-3). IR: 3440,
3280, 3150, 2930, 2840, 2800, 1620, 1570, 1500, 1470, 1440. EI
MS: 236.2 (7.7, M+), 71.2 (37), 70.2 (100), 65.1 (14.4). Anal. Calcd
for C11H16N4O2 (236.28): C, 55.93; H, 6.78; N, 23.73. Found: C,
56.08; H, 6.56; N, 23.46.
acid into 300 mL of sulfuric acid)19 was bubbled through for 2 h. The
reaction mixture was left for 5 h at room temperature. Then the solvent
was removed under reduced pressure, and the residue was triturated in
dry diethyl ether (500 mL) and was left to stand overnight at room
temperature. The colorless hydrochloride (C12H23B10NO2‚HCl) obtained
was filtered, washed with diethyl ether, and dried in vacuo. Yield:
12.2 g (94%). This compound was used in the next step without further
purification. 1H NMR (DMSO-d6): 1.46 (t, J ) 6.9, 3 H, -CH3),
4.58 (q, J ) 6.9, 2 H, -CH2-CH3), 4.80 (s, 2 H, -OCH2-), 5.45 (s,
1 H, -CB10H10C-H), 7.24 [AA′XX′, JA(JX) ) 9, 2 H, H-3 and H-5],
8.13 [AA′XX′, JX(JA) ) 9, 2 H, H-2 and H-6]. IR: 2980, 2580, 1600,
1440, 1385, 1255, 1190, 1030, 840.
Synthesis of 5-(4-Methyl-1-piperazinyl)-o-phenylenediamine, 6.
5 (8.5 mmol, 2 g) was reduced in the presence of hydrazine hydrate
(99%, 2.2 mL) and ruthenium (5%) on charcoal (25%, 0.5 g) in ethanol
(50 mL) at reflux for ≈1.5 h or until the foam of the reaction mixture
became colorless. After being cooled, the reaction mixture was filtered,
and the filtrate was evaporated to dryness. The residue was suspended
in water (100 mL) and extracted with chloroform (3 × 100 mL). The
organic phase was dried (sodium sulfate), and the combined organic
phases were evaporated under reduced pressure to give a yellow-
brownish oil which rapidly crystallized. This product 6 can be used
without further purification in the following reaction step. Yield: 1.56
g (89%).
Synthesis of 3-(p-Cyanophenoxy)-1-propyne, 11. p-Hydroxyben-
zonitrile (80 mmol, 9.6 g) was allowed to react with propargyl bromide
(80 mmol, 6.4 mL) in the presence of potassium carbonate (80 mmol,
11.2 g) in acetone for 24 h under reflux. After being cooled to room
temperature, the solution was evaporated to dryness. The residue was
recrystallized from ethanol/water (1:1, v/v) to give pale yellow needles.
Yield: 11.3 g (90%). Mp: 109-110 °C. 1H NMR (DMSO-d6): 2.57
(t, J ) 2.4, 1 H, -CtC-H), 4.75 (d, J ) 2.4, 2 H, -O-CH2-C),
7.04 [AA′XX′, JA(JX) ) 9, 2 H, H-3 and H-5], 7.61 [AA′XX′, JX(JA)
) 9, H-2 and H-6]. IR: 3394, 3100, 2918, 2124, 1574, 1372, 1330,
1296, 1120, 968, 840, 812, 746, 720, 665. EI MS: 157.1 (42, M+),
156.1 (94), 129.1 (53), 128.0 (100). Anal. Calcd for C10H7NO
(157.1): C, 76.43; H, 4.46; N, 8.92. Found: C, 76.35; H, 4.51; N,
8.78.
Synthesis of 1-(4-Cyanophenoxymethyl)-1,2-dicarba-closo-dode-
caborane(12), 12. 3-(p-Cyanophenoxy)-1-propyne (11; 20 mmol, 3.1
g) and the decaborane/bis(acetonitrile) complex (40 mmol, 8.2 g) were
allowed to react in toluene (500 mL) for 5 h under reflux. After being
cooled to room temperature, the reaction mixture was filtered off and
evaporated to dryness. The residue was dissolved in chloroform and
purified by column chromatography on silica gel 60 (35-70 mesh,
length ) 20 cm, diameter ) 8 cm) with chloroform. This process
was repeated, generally three times, or until only one spot was detected
by TLC (silica gel, chloroform). 12 crystallized from toluene in
colorless crystals. Yield: 2.2 g (40%). Mp: 194-196 °C. 1H NMR
(DMSO-d6): 4.72 (s, 2 H, -O-CH2-), 5.33 (s, 1 H, -CB10H10C-
H), 7.16 [AA′XX′, JA(JX) ) 8, 2 H, H-3 and H-5], 7.80 [AA′XX′,
JX(JA) ) 8, 2 H, H-2 and H-6]. IR: 2580, 2200, 1605, 1505, 1300,
1250, 1170, 1030, 825, 710. EI MS: 277.2 (40), 276.6 (91.3), 275.3
(100), 274.2 (72.7), 273.3 (36.4), 272.2 (14.4). Anal. Calcd for
C10H17B10NO (277.2): C, 43.32; H, 6.13; N, 5.05. Found: C, 43.45;
H, 6.00; N, 4.98.
Synthesis of 4-Cyano-o-phenylenediamine, 13. 4-Amino-3-ni-
trobenzonitrile (38.17 mmol, 5 g) was reduced in the presence of
hydrazine hydrate (99%, 6 mL) and ruthenium (5%) on charcoal (28%,
1.4 g) in ethanol (100 mL) at reflux for ≈1.5 h or until the foam of the
reaction mixture became colorless. After being cooled, the reaction
mixture was filtered, and the filtrate was evaporated to dryness, giving
a white powder that turned rapidly reddish-brown by air oxidation. This
compound was used without further additional purification. Yield: 3.9
g (77%). Mp: 139-140 °C [140.0-140.5 °C (not cryst)20].
Synthesis of 1-[4-(5′-Cyano-1H-benzimidazol-2′-yl)phenoxy-
methyl]-1,2-dicarba-closo-dodecaborane(12), 14. 10 (28 mmol, 10
g) and 4-cyano-o-phenylenediamine (13; 28 mmol, 3.7 g) were allowed
to react in glacial acetic acid (150 mL) under reflux for 5 h and then
were left standing overnight at room temperature. 14 crystallized from
glacial acetic acid as fine colorless needles. Yield: 7.9 g (68%). Mp:
278-279 °C. 1H NMR (DMSO-d6): 4.76 (s, 2 H, -O-CH2-), 5.4
(br, 1 H, -CB10H10C-H), 7.25 [AA′XX′, JA(JX) ) 8.7, 2 H, Ar-H
ortho to -O-CH2-], 7.68 (dd, J4′,6′ ) 1.4, J6′,7′ ) 8.4, 1 H, H-6′),
7.79 (d, J ) 8.4, 1 H, H-7′), 8.17 (m, 1 H, H-4′), 8.2 [AA′XX′, JX(JA)
) 8.7, 2 H, Ar-H meta to -O-CH2-]. 11B NMR (DMSO-d6):
-3.99, -5.83, -10.30, -12.09, and -13.80. IR: 3042, 2586, 2220,
Synthesis of 4-Amino-3-nitro-benziminoethyl Ester Hydrochlo-
ride, 7. In a flask (250 mL; equipped with a magnetic stirrer and a
gas inlet tube), 4-amino-3-nitrobenzonitrile (61.35 mmol, 10 g) was
dissolved in a mixture of dry ethanol (100 mL) and dry benzene (100
mL) under stirring. At room temperature, dry hydrogen chloride was
bubbled through the solution until saturation was attained. The reaction
mixture was left to stand overnight at room temperature, and diethyl
ether (300 mL) was added. The crystals were filtered off, washed with
diethyl ether, and dried at 60 °C. Yield: 10.84 g (72%). Mp: 288.2-
289.4 °C. 1H NMR (DMSO-d6): 1.46 (t, J ) 7.0, 3 H, -CH3), 4.58
(q, J ) 7.0, 2 H, -O-CH2-), 7.18 (d, J ) 9.2, 1 H, H-5), 8.11 (dd,
J5,6 ) 9.3, J2,6 ) 2.3, 1 H, H-6), 8.36-8.40 (s, 2 H, -NH2), 8.79 (d,
J2,6 ) 2.3, 1 H, H-2). IR: 3420, 3240, 3150, 1670, 1625, 1490.
Synthesis of 2-(4-Amino-3-nitrophenyl)-5-(4-methyl-1-pipera-
zinyl)-1H-benzimidazole, 8. A mixture of 6 (1.7 mmol, 0.35 g) and
7 (1.6 mmol, 0.4 g) was refluxed in glacial acetic acid (55 mL) for 2
h. Glacial acetic acid was removed under reduced pressure, and the
residue was dissolved in water and filtered off. The crude product
was precipitated with a concentrated aqueous NH3 solution. The
product was purified by redissolution in a mixture of MeOH (250 mL)
and CH3COOH (30 mL) and reprecipitation with a concentrated aqueous
NH3 solution. Yield: 0.44 g (73%). Mp: 183.3-185.4 °C. 1H NMR
(DMSO-d6): 2.4 (s, 3 H, N-CH3), 2.5 [br, 4 H and DMSO,
(-H2C)2N-CH3], 3.10 [br, 4 H, Ar-N(-CH2)2], 6.92 (d, J6,7 ) 8.6, 1
H, H-6), 6.94 (s, 1 H, H-4), 7.13 (d, J5,6 ) 8.9, 1 H, H-5 ortho to -NH2),
7.40 (d, J6,7 ) 8.6, 1 H, H-7), 7.80 (s, 2 H, -NH2), 8.15 (d, J5,6 ) 8.9,
1 H, H-6 para to -NO2), 8.76 (s, 1 H, ortho to -NO2). IR: 3485,
3450, 3358, 2970, 2925, 2860, 2840, 2800, 1635, 1588, 1560, 1505,
1480, 1350, 1240.
Synthesis of 2-(3,4-Diaminophenyl)-5-(4-methyl-1-piperazinyl)-
1H-benzimidazole, 9. A mixture of 8 (16.34 mmol, 5.75 g) and
palladium (10%) on charcoal (≈20%, 1.15 g) in 1 M aqueous acetic
acid (70 mL) was hydrogenated at 2.5 kg/cm2 pressure for 24.5 h. The
mixture was filtered on Hyflo (prewashed with ethanol and conditioned
with 1 M aqueous acetic acid). The solvent was removed, and the
residue was dissolved in water. Concentrated aqueous ammonia
solution was added under stirring, the precipitate was filtered off, and
the red crystals obtained were washed with diethyl ether. The
precipitation was repeated twice. Yield: 3.85 g (73%). Mp: 259.5-
264 °C [268 °C (not cryst)7c decomp7a,b]. IR: 3420, 3400, 3310, 2960,
2920, 2870, 2800, 1625, 1585, 1500, 1455, 1415, 1370, 1150.
Synthesis of 1-{4-[Imino(ethoxy)methyl]phenoxymethyl}-1,2-di-
carba-closo-dodecaborane(12) Hydrochloride, 10. 12 (36.1 mmol,
10 g) was dissolved in a mixture of dry ethanol (100 mL) and benzene
(150 mL). After the solution was cooled to 5-10 °C, dry hydrogen
chloride (prepared by dropping 200 mL of concentrated hydrochloric
(19) Organic Syntheses; Wiley & Sons: New York, 1941, Collect. Vol. I,
p 293.
(20) Kelly, D. P.; Bateman, S. A.; Hook, R. J.; Martin, R. F.; Reum, M.
E.; Rose, M.; Whittaker, A. R. D. Aust. J. Chem. 1994, 47, 1751-
1769.

6022 Inorganic Chemistry, Vol. 37, No. 23, 1998
Argentini et al.
1698, 1608, 1500, 1394, 1252, 1188, 1058, 820, 620. EI MS: 393.3
(49.5), 392.3 (45), 391.3 (100), 390.2 (59.6), 389.3 (29.5).
Yield: 4 g (62%). Mp: g290 °C. UV spectra (H2O/ethanol, 90:10,
v/v) for 1 (“Hoechst 33258”): λmax 261 nm (log ꢀ ) 4.24) and 342 nm
(log ꢀ ) 4.45), λmin 295 nm (log ꢀ ) 3.97). UV spectra for 2 (“Hoechst-
Carborane”): λmax 262 nm (log ꢀ ) 4.25) and 342 nm (log ꢀ ) 4.41),
Synthesis of 1-{4-[5′-(Imino(ethoxy)methyl)-1H-benzimidazol-2′-
yl]phenoxymethyl}-1,2-dicarba-closo-dodecaborane(12), 15. 14 (12.7
mmol, 5 g) in a mixture of dry ethanol/benzene was treated according
to the preparation of 10. The resulting compound, 15, proved to be a
white substance and was used without further purification for the next
step. Yield: 5.9 g (98%). 1H NMR (MeOH-d4): 1.86 (t, J ) 7.2, 3
H, -CH3), 4.55 (q, J ) 7, 2 H, -CH2CH3), 4.96 (s, 2 H, -O-CH2-),
5.10 (-CB10H10C-H under HDO), 7.55 [AA′XX′, JA(JX) ) 9, 2 H,
Ar-H ortho to -O-CH2], 8.24 (dd, J4′,7′ ) 0.6, J5′,7′ ) 9, 1 H, H-7′),
8.35 [AA′XX′, JX(JA) ) 9, 2 H, Ar-H meta to -O-CH2], 8.42 (dd,
J4′,6′ ) 1.8, J6′,7′ ) 9, 1 H, H-6′), 8.73 (dd, J4′,7′ ) 0.6, J4′,6′ ) 1.8, 1 H,
H-4′). No further spectral data were taken.
Synthesis of 1-{4-[5-(4-Methyl-1-piperazinyl)-2,5′-bi-1H-benz-
imidazol-2′-yl]phenoxymethyl}-1,2-dicarba-closo-dodecaborane-
(12), 2. The freshly prepared 6 (11.2 mmol, 2.3 g) and 15 (11.2 mmol,
5.3 g) were allowed to react in glacial acetic acid (250 mL) under reflux
for 6 h. After 15-20 min, a yellow substance began to precipitate.
After 6 h, the heat was switched off, and the reaction mixture was
allowed to stand overnight at room temperature. The crystallized yellow
precipitate was collected in a Bu¨chner funnel, washed with glacial acetic
acid, and dried in vacuo (if necessary, it can be recrystallized from
glacial acetic acid in the dark, because this substance is light-sensitive).
λmin 295 (log ꢀ ) 3.95). HPLC analysis [acetonitrile/methanol/20 mM
KH2PO4 buffer (pH 3.36), 50:15:35, v/v, flow rate 1 mL/min, retention
time 5.3 min] showed that 2 was obtained in g99% purity. 1H NMR
(MeOH-d4 and TFA-d): 3.23 (t, J ) 12, 2 H, H3CN-CH2-), 3.36 (t,
J ) 15, 2 H, H3CN-CH2-), 3.69 (d, J ) 12, 2 H, ArN-CH2-), 4.00
(d, J ) 15, 2 H, ArN-CH2-), 4.76 (s, 2 H, -OCH2-), 4.87
(-CB10H10C-H under HDO), 7.35 [AA′XX′, JA(JX) ) 9, 2 H, Ar-H
ortho to -OCH2], 7.38 (d, J4′,6′ ) 1.8, 1 H, H-4′), 7.47 (dd, J4′,6′ ) 1.8,
J6′,7′ ) 9, 1 H, H-6′), 7.78 (d, J6′,7′ ) 9, 1 H, H-7′), 8.11 (d, J4,7 ) 0.6,
J6,7 ) 9, 1 H, H-7), 8.22 [AA′XX′, JX(JA) ) 9, 2 H, Ar-H meta to
-OCH2], 8.28 (dd, J4,6 ) 1.8, J6,7 ) 9, 1 H, H-6), 8.60 (dd, J4,7 ) 0.6,
J4,6 ) 1.8, 1 H, H-4). 11B NMR (DMSO-d6): -3.99, -5.83, -10.28,
-12.20, and -13.77. ESI MS: 584.1 (30), 583 (36), 582.2 (75), 581.3
(50), 579.1 (32).
Acknowledgment. The authors are indebted to the Krebs-
forschung Schweiz (KFS 176-9-1995) for generous support.
IC980505E