Synthetic, spectroscopic and antitumor activity studies on phosphacyanoboranes

Article abstract of DOI:10.1016/S0020-1693(02)01191-X

Triphenylphosphinecyanoborane (1), diphenyl(2-methoxyphenyl)phosphinecyanoborane (2), diphenyl(p-tolyl)phosphinecyanoborane (3), methyldiphenylphosphinecyanoborane (4) were synthesized either by Lewis-base exchange reaction or by the reaction of the sodiu

Full text of DOI:10.1016/S0020-1693(02)01191-X

Inorganica Chimica Acta 343 (2003) 383Á386
/
www.elsevier.com/locate/ica
Note
Synthetic, spectroscopic and antitumor activity studies on
phosphacyanoboranes
Kamesh Vyakaranam a, Geeta Rana a, Kurt Grelck a, Bernard F. Spielvogel a,1,
John A. Maguire b, Narayan S. Hosmane a,*
a Department of Chemistry and Biochemistry, Michael Faraday Laboratories, Northern Illinois University, DeKalb, IL 60115, USA
b Department of Chemistry, Southern Methodist University, Dallas, TX 75275, USA
Received 6 June 2002; accepted 12 July 2002
Abstract
Triphenylphosphinecyanoborane (1), diphenyl(2-methoxyphenyl)phosphinecyanoborane (2), diphenyl(p-tolyl)phosphinecyano-
borane (3), methyldiphenylphosphinecyanoborane (4) were synthesized either by Lewis-base exchange reaction or by the reaction of
the sodium cyanoborohydride with the corresponding phosphine hydrochloride in 54Á88% yields. All the products were
/
characterized by spectroscopic techniques and elemental analyses. These compounds were evaluated for their antitumor activity.
# 2002 Elsevier Science B.V. All rights reserved.
Keywords: Lewis-base exchange reaction; Antitumor activity; Substituted borane adducts; BNCT; Phosphacyanoboranes
1. Introduction
ity [3], has encouraged us to extend our studies on the
syntheses, properties and antitumor activities of the
phosphineboranes. A parallel interest in studying such
compounds is their potential application in boron
A number of interesting, and potentially useful,
compounds of phosphorus and boron have been
synthesized by the coupling of electron donor phospho-
rus groups with electron acceptor borane fragments. The
observation that a number of boron analogues of the
phosphonoacetates functioned as effective hypolipidae-
mic agents [1] has led us to extend our studies to other
neutron capture therapy (BNCT) [4Á9]. Herein, we
/
report the syntheses, spectroscopic characterization
and antitumor activity studies of triphenylphosphine-
cyanoborane (1), diphenyl(2-methoxyphenyl)phosphine-
cyanoborane (2), diphenyl(p-tolyl)phosphinecyanobor-
ane (3) and methyldiphenylphosphinecyano-borane (4).
PÃ
/
B linked compounds, such as the phosphineboranes.
CN,
The first phosphineboranes, PPh3BH2X (Xꢀ
/
COOH, COOEt and CONHEt), was reported by Martin
and coworkers using a somewhat cumbersome, and low
yield, procedure involving the synthesis, and subsequent
solvolysis, of PPh3BH2CN [2]. Moreover, the synthesis
of substituted-phenylphosphineboranes has not yet been
reported. Our recent report of a new and high-yield
procedure for the synthesis of triphenylphosphinecarbo-
methoxyborane, which exhibited good antitumor activ-
2. Experimental
2.1. Materials
Me3NBH2CN was prepared by literature methods
[10]. Triphenylphosphine, methyldiphenylphosphine, di-
phenyl(2-methoxyphenyl)phosphine, diphenyl(p-tolyl)-
phosphine were obtained commercially (Aldrich) and
used without further purification. Monoglyme (1,2
dimethoxyethane, DME) and tetrahydrofuran (THF)
were distilled from CaH2 and stored over molecular
sieves. All other solvents and reactants were of reagent
grade and were used as received.
* Corresponding author. Tel.: ꢁ
4802
E-mail address: nhosmane@niu.edu (N.S. Hosmane).
/
1-815-753 3556; fax: ꢁ1-815-753
/
1
Contribution from Metallo-Biotech International, Inc., DeKalb,
IL 60115, USA.
0020-1693/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 0 2 0 - 1 6 9 3 ( 0 2 ) 0 1 1 9 1 - X

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K. Vyakaranam et al. / Inorganica Chimica Acta 343 (2003) 383Á386
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2.2. Spectroscopic and analytical procedures
dissolved in anhydrous DME (35 ml) under N2 atmo-
sphere. The mixture was heated to reflux and the extent
of the reaction was monitored by 11B NMR spectro-
scopy. After 11 h, the reaction was complete and the
reaction mixture was filtered to remove a white pre-
cipitate (NaCl). The solvent was removed from the
filtrate under reduced pressure to give the product as a
creamy solid, which was subsequently recrystallized
from CH2Cl2 to obtain white crystals of
(C6H5)2P(C6H4OCH3)BH2CN (0.83 g, 73% yield,
Proton, boron-11, phosphrus-31 and carbon-13 NMR
spectra were obtained on Brucker Fourier-Transform
NMR spectrometer at 200, 64.2, 80.2 and 50.3 MHz,
respectively. Infrared spectra were recorded using a
Nicolet Magna 550 FT-IR spectrophotometer with
OMNIC software. Elemental analyses were obtained
in house using PerkinÁ
analyser. All NMR samples were prepared in d6-
DMSO with BF3×OEt2 as an internal standard.
/Elmer 2400 CHN elemental
/
slightly soluble in polar organic solvents; m.p. 110Á
/
1
112 8C). Spectroscopic and analytical data for 2: H
NMR (DMSO, relative to Me4Si) d 3.85 [s, OCH3],
2.3. Synthetic procedures
7.10Á
to BF3.OEt2) d ꢂ
J(BP)
117 Hz]; 13C NMR (DMSO, relative to Me4Si) d
58.30 [OCH3], 122.70Á
128.30 [phenyl C’s]; 31P NMR
(DMSO, relative to H3PO4) d 9.07 [q, J(BP) 115 Hz];
IR (KBr pellet, cmꢂ1) 2422, 2380 [n(BÃ
H)]; Anal. Calc.
/
7.63 [m, aromatic H]; 11B NMR (DMSO, relative
All experiments were carried out in 250 ml Pyrex-glass
two-necked round-bottom flasks, each containing a
magnetic stirring bar and nitrogen inlet. All known
compounds were identified by comparing their IR and
NMR spectra with those of authentic samples. The
hydrochloride salts of methyldiphenylphosphine, diphe-
nyl(2-methoxyphenyl)phosphine and diphenyl(p-to-
lyl)phosphine were prepared by dissolving the
particular phosphine in Et2O and then bubbling HCl
gas into the solution until saturation was obtained. The
hydrochloride salts were then recrystallized from 95%
EtOH solution.
/36.20 [d of t, BH2, J(BH)
ꢀ111 Hz,
/
ꢀ
/
/
ꢀ
/
/
for C20H19POBN: C, 72.43; H, 5.73; N, 4.23. Found: C,
72.66; H, 5.92; N, 4.31%.
2.6. Diphenyl(p-tolyl)phosphinecyanoborane (3)
Equimolar amounts of diphenyl(p-tolyl)phosphine
hydrochloride (1.00 g, 3.20 mmol) and sodium cyano-
borohydride (0.20 g, 3.20 mmol) were dissolved in
anhydrous DME (50 ml) under N2 atmosphere. The
mixture was heated to reflux and the reaction monitored
by 11B NMR spectroscopy. After completion (8.5 h), the
mixture was filtered to remove a white precipitate,
identified as NaCl. The solvent was then removed
from the filtrate under reduced pressure to give a white
2.4. Triphenylphosphinecyanoborane (1)
Equimolar amounts of triphenylphosphine (1.00 g,
3.81 mmol) and trimethylaminecyanoborane (0.50 g,
3.83 mmol) were dissolved in anhydrous DME (50 ml)
under N2 atmosphere. The mixture was heated to reflux
and the extent of the reaction was monitored by 11B
NMR spectroscopy. After 18 h, the solvent was removed
under reduced pressure and the crude white solid
solid that was recrystallized from CH2Cl2ÁC6H14 (1:1)
/
to produce off-white crystals of (C6H5)2P(C6H4-
CH3)BH2CN (0.54 g, 54% yield, sparingly soluble in
product was then recrystallized from CH2Cl2Á
(9:1) to produce off-white needles of PPh3BH2CN (0.75
g, 65% yield, soluble in polar and slightly soluble in
/
C5H12
polar solvents; m.p. 117Á
analytical data for 3: 1H NMR (DMSO, relative to
Me4Si) d 2.15 [unresolved, CH3], 7.20Á7.80 [m, aro-
matic H]; 11B NMR (DMSO, relative to BF3×
OEt2) d ꢂ
38.00 [d of t, BH2, J(BH) 102 Hz, J(BP)
112 Hz]; 13C
NMR (DMSO, relative to Me4Si) d 40.10 [CH3],
123.00Á
131.50 [phenyl C’s]; 31P NMR (DMSO, relative
to H3PO4) d 10.45 [q, J(BP) 110 Hz]; IR (KBr pellet,
cmꢂ1
2453, 2372 [n(BÃH)]; Anal. Calc. for
/
119 8C). Spectroscopic and
/
nonpolar organic solvents; m.p. 170Á
scopic and analytical data for 1: 1H NMR (DMSO,
relative to Me4Si) d 7.42Á
7.66 [m, aromatic H]; 11B
NMR (DMSO, relative to BF3×OEt2) d ꢂ31.90 [d of t,
BH2, J(BH) 107 Hz, J(BP)
109 Hz]; 13C NMR
(DMSO, relative to Me4Si) d 127.00Á129.00 [phenyl
C’s]; 31P NMR (DMSO, relative to H3PO4) d 10.81 [q,
J(BP)
/
172 8C). Spectro-
/
/
ꢀ
/
ꢀ
/
/
/
/
/
ꢀ
/
ꢀ
/
ꢀ
/
/
)
/
C20H19PBN: C, 76.11; H, 6.03; N, 4.44. Found: C,
ꢀ
/
109 Hz]; IR (KBr pellet, cmꢂ1) 2415, 2389 [n(BÃ
/
75.96; H, 6.10; N, 4.37%.
H)]; Anal. Calc. for C19H17PBN: C, 75.78; H, 5.64; N,
4.65. Found: C, 75.42; H, 5.72; N, 4.69%.
2.7. Methyldiphenylphosphinecyanoborane (4)
2.5. Diphenyl(2-methoxyphenyl)phosphinecyanoborane
(2)
Methyldiphenylphosphine hydrochloride (5.00 g,
21.14 mmol) and sodium cyanoborohydride (1.33 g,
21.16 mmol) were dissolved in anhydrous DME (50 ml)
under N2 atmosphere. As in the syntheses of 2 and 3, the
mixture was heated to reflux and the reaction followed
by 11B NMR spectroscopy. After completion (9 h) the
Equimolar amounts of diphenyl(2-methoxyphe-
nyl)phosphine hydrochloride (1.00 g, 3.04 mmol) and
sodium cyanoborohydride (0.19 g, 3.04 mmol) were

K. Vyakaranam et al. / Inorganica Chimica Acta 343 (2003) 383Á
/
386
385
mixture was filtered to remove NaCl. Solvent was
removed from the filtrate under reduced pressure and
the resulting crude colorless oil product was purified by
Table 1
GI50, TGI and LC50 values (mol dmꢂ3) of the phosphineboranes
Compound panel/cell line
affected
GI50ꢃ105 TGIꢃ105 LC50ꢃ105
flash chromatography on silica using Et2OÁC6H14 (1:1)
/
eluent to obtain (C6H5)2P(CH3)BH2CN (4.5 g, 88%
yield, soluble in polar solvents). Spectroscopic and
analytical data for 4: 1H NMR (DMSO, relative to
Non-small cell lung cancer
A549/ATCC
NCI-H226
4.9
7.7
3.3
3.0
ꢀ10.0
ꢀ10.0
ꢀ10.0
ꢀ10.0
ꢀ10.0
ꢀ10.0
ꢀ10.0
ꢀ10.0
Me4Si) d 2.02 [s, CH3], 7.44Á
NMR (DMSO, relative to BF3×
BH2, J(BH) 99.1 Hz, J(BP)
(DMSO, relative to Me4Si) d 20.10 [CH3], 126.00Á
130.80 [phenyl C’s]; 31P NMR (DMSO, relative to
H3PO4) d 3.90 [q, J(BP) 103.5 Hz]; IR (KBr pellet,
cmꢂ1
2478, 2395 [n(BÃH)]; Anal. Calc. for
/
7.93 [m, aromatic H]; 11B
OEt2) d ꢂ37.50 [d of t,
102.9 Hz]; 13C NMR
NCI-H23
NCI-H522
/
/
ꢀ
/
ꢀ
/
Small cell lung cancer
DMS 114
DMS 273
/
6.9
3.9
ꢀ10.0
ꢀ10.0
ꢀ10.0
ꢀ10.0
ꢀ
/
CNS cancer
SF-268
SF-295
2.3
1.9
3.5
1.3
2.8
1.4
7.18
ꢀ10.0
ꢀ10.0
)
/
4.53
3.83
5.79
6.36
2.82
C14H15PBN: C, 70.26; H, 6.27; N, 5.85. Found: C,
70.33; H, 6.15; N, 6.03%.
SNB-75
SNB-78
U251
9.74
ꢀ10.0
ꢀ10.04
5.88
XF 498
2.8. Antitumor activity studies
Leukemia
RPMI-8226
0.0047
0.14
ꢀ10.0
ꢀ10.0
ꢀ10.0
In vitro tests for anti-tumor activity against a number
Renal cancer
of human cancer cells were conducted on compounds 1Á
/
RXF-393
ꢀ10.0
ꢀ10.0
4 through the National Cancer Institute’s (NCI) Devel-
opmental Therapeutics Program [11]. Preliminary
screening results, in terms of the concentrations that
cause 50% growth inhibition (GI50), total growth
inhibition (TGI), and death of 50% of the cell popula-
tion (LC50) are listed in Table 1.
Leukemia
CCRF-CEM
HL-60 (TB)
4.5
2.5
ꢀ10.0
ꢀ10.0
9.91
Melanoma
LOX IMBI
MALME-3M
6.6
4.9
ꢀ10.0
ꢀ10.0
ꢀ10.0
ꢀ10.0
Ovarian cancer
IGROV1
OVCAR-3
6.6
4.9
ꢀ10.0
ꢀ10.0
ꢀ10.0
ꢀ10.0
Renal cancer
ACHN
CAKI-1
3. Results and discussion
8.1
2.2
ꢀ10.0
8.0
ꢀ10.0
ꢀ10.0
Prostate cancer
3.1. Syntheses
PC-3
3.1
ꢀ10.0
ꢀ10.0
Breast cancer
MCF7
MCF7/ADR-RES
The phosphineboranes were synthesized using two
different methods as outlined in Scheme 1. Triphenyl-
phosphinecyanoborane (1) was synthesized by the direct
Lewis-base exchange reaction between triphenylpho-
sphine and trimethylaminecyanoborane in a 1:1 molar
ratio using DME as a solvent. Although a yield of 65%
was an improvement over the method of Martin and
coworkers [2], the long reaction time of 18 h is some-
what of a disadvantage if the method is to be generally
applied. Therefore, an alternative, two-step method was
4.4
3.5
ꢀ10.0
ꢀ10.0
ꢀ10.0
ꢀ10.0
3.2. Characterization
All the compounds were characterized by their IR
spectra, 1H, 11B, 13C, and 31P NMR spectra and
elemental analyses. All of these data are consistent
with the formulations given in Scheme 1. The proton-
developed for the syntheses of 2Á4; this is also shown in
/
decoupled 31P NMR spectra of 1Á
the range of dꢀ3.9Á12.45 ppm, corresponding to a
phosphorus atom bonded to a boron atom. The proton-
/
4 showed a quartet in
Scheme 1. The new route consisted of first preparing the
particular phosphine hydrochloride salt and then react-
ing it with sodium cyanoborohydride, NaBH3CN, to
give the phosphineborane and sodium chloride. The
/
/
decoupled 11B NMR spectrum of each phosphineborane
showed a doublet in the range of dꢀ
/
ꢂ
/
29 and ꢂ38
/
overall yields of 54Á88% are comparable with those
/
ppm, while their proton-coupled spectra showed doublet
of triplet patterns, consistent with a boron being split by
hydrogen and phosphorous coupling. These spectra are
obtained from the phosphine/amine exchange reactions,
but the reaction times were about halved. Purification of
the solid products (1Á/3) was achieved by recrystalliza-
tion, while 4 was purified by chromatography.
quite similar to the 31P and 11B resonances of dꢀ
12.45
/

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K. Vyakaranam et al. / Inorganica Chimica Acta 343 (2003) 383Á386
/
4. Conclusions
This work reports the synthesis and spectroscopic
characterizations of four new phosphineboranes. The
two types of synthetic methods described, direct base
exchange or hydrochloride/sodium cyanoborohydride
reaction, are general ones that should be applicable to
other phosphineboranes. The compounds were screened
for anti-tumor activity against a number of human
cancer cell lines. All showed some anti-tumor activity
and triphenylphosphinecyanoborane (1) demonstrated a
high selectivity. However, with the exception of diphe-
nyl(2-methoxyphenyl)phosphinecyanoborane (2), all
compounds possessed GI50, TGI, and LC50 concentra-
tions that were too large to be of clinical interest.
Acknowledgements
This work was supported by grants from the National
Science Foundation (CHE-9988045), the Robert A.
Welch Foundation (N-1322 to JAM), the donors of
the Petroleum Research Fund, administered by the
American Chemical Society, and Northern Illinois
University through Presidential Research Professorship
(to NSH).
Scheme 1.
and
ꢂ
/
29.5
ppm,
respectively,
found
for
PPh3BH2COOCH3 that has been structurally character-
ized [3]. The IR spectra of 1Á4 showed all the expected
/
bands consistent with the structures, as did their 13C
NMR spectra.
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3.3. Antitumor activity studies
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tional Cancer Institute’s Developmental Therapeutics
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values of the concentrations of the phosphineboranes
that cause 50% growth inhibition (GI50), total growth
inhibition (cytostatic) (TGI), and the concentration that
is lethal to 50% of the cell population (LC50). The results
show that all the compounds exhibit some anti-tumor
activity and that triphenylphosphinecyanoborane (1)
shows a high degree of specificity within the central
nervous system (CNS) tumor cell lines. However, the
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10ꢂ5 M) are generally too
/
[9] P. Lemmen, L. Weissfloch, T. Auberger, T. Probst, in: Y.
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which exhibited a clinically acceptable GI50 value of
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/
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4.74ꢃ
/
10ꢂ8 M against the leukemia RPMI-8226 cell
line.