Preparation of a novel group of hybrid compounds N-benzyl aminoboronbenzylphosphonic and N,N′-ethylenedi(aminoboronbenzylphosphonic) acids

Article abstract of DOI:10.1016/j.jorganchem.2010.09.002

Two groups of new boronic acids containing aminophosphonate functions were synthesized and characterized by NMR spectroscopy and ESI-MS. Both groups of compounds were obtained by simple reactions of prepared in situ tris(trimethylsilyl) phosphite with a corresponding imine. The synthesized compounds may serve as a potential new class of building blocks, BNCT agents and supramolecular host molecules.

Full text of DOI:10.1016/j.jorganchem.2010.09.002

Journal of Organometallic Chemistry 696 (2011) 457e460
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Communication
Preparation of a novel group of hybrid compounds N-benzyl
aminoboronbenzylphosphonic and N,N0-ethylenedi
(aminoboronbenzylphosphonic) acids
*
Piotr M1ynarz , Agata Rydzewska, Ziemowit Pok1adek
_
ꢀ
Department of Bioorganic Chemistry, Faculty of Chemistry, Wrocław University of Technology, Wybrzeze Wyspianskiego 27, 50-370 Wrocław, Poland
a r t i c l e i n f o
a b s t r a c t
Article history:
Two groups of new boronic acids containing aminophosphonate functions were synthesized and char-
acterized by NMR spectroscopy and ESI-MS. Both groups of compounds were obtained by simple reac-
tions of prepared in situ tris(trimethylsilyl) phosphite with a corresponding imine. The synthesized
compounds may serve as a potential new class of building blocks, BNCT agents and supramolecular host
molecules.
Received 4 May 2010
Received in revised form
19 August 2010
Accepted 1 September 2010
Available online 15 September 2010
Ó 2010 Elsevier B.V. All rights reserved.
Keywords:
Boronic acids
Aminophosphonate compounds
ESI-MS
NMR spectroscopy
Fragmentation pattern
1. Introduction
2. Results and discussion
Recently boronic compounds have attracted much attention due
to their supramolecular and biomedicinal functions. They are
recognized as potent receptors to diols, saccharides, hydroxy-
carboxylates, dicarboxylates [1,2], also acting as inhibitors of
proteases (with BORTEZOMIB being the most spectacular example),
glycosidases and agents in Boron Neutron Capture Therapy (BNCT)
[3e8]. Boronic acids have been customarily applied in organic
chemistry as building blocks in Suzuki-Miyaura and Petasis reac-
tions or as water tolerant Lewis acids, or Brønsted acids catalysts
[9e11]. Similar functions are carried out by phosphonic acids and
their derivatives, which could be considered as good supramolec-
ular host molecules for carbohydrates, catecholamines, amino
acids, peptides and proteins [12]. Furthermore this group of
compounds finds many applications as enzyme inhibitors towards
proteolytic enzymes and antiosteoporotic drugs [13,14].
For the synthesis of the new boronate compounds the most
convenient classical hydrophosphonylation of imines procedure for
aminophosphonates was applied [16]. However, during acidic
hydrolysis of the phosphonate ester groups the cleavage of boro-
nate entities was observed. Therefore, tris(trimethylsilyl) phosphite
generated by application of trimethylphosphite and TMSBr (bro-
motrimethylsilane) in the ratio 1:3 was used (Fig. 1) [17]. The in situ
formed tris(trimethylsilyl) phosphite was cooled down to 5 ꢀC and
blended with corresponding imine suspended in dry DCM. The
reaction mixture was stirred up to 48 h and evaporated. In order to
cleave trimethylsilyl esters methanolic solution was added and left
stirring overnight. After 24 h the formed yellow precipitate was
filtered off, washed with diethyl ether and the desired product was
purified by dissolving in 1 M NaOH and reprecipitated by a drop-
wise addition of 1 M HCl. The formed white precipitate was washed
with water and/or methanol and air-dried (1e3). The same proce-
dure was used for the preparation of the second group of
compounds (4e6) based on ethylenediamine skeleton. However,
due to low solubility of the formed Schiff bases the protected forms
of 3- and 4-formylphenylboronic acids esterified with diethanol-
amine were applied. The rest of the synthetic steps were the same
as for the compounds 1e3 (see Figs. 2 and 3). These molecules were
expected to form two pairs of diastereoisomers of SS/RR and/or
Recently we have published the unexpected pathway of
formation of a hybrid compound containing both boronic and
aminophosphonate units [15]. In this paper we describe the unes-
terified forms of its analogues starting from benzylamine and
ethylenediamine.
* Corresponding author. Tel./fax: þ48 71 3204597.
E-mail address: piotr.mlynarz@pwr.wroc.pl (P. M1ynarz).
0022-328X/$ e see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2010.09.002

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P. Młynarz et al. / Journal of Organometallic Chemistry 696 (2011) 457e460
The ESI-MS studies were performed in order to resolve the
a
CHO
+
fragmentation pattern of the new group of compounds. All mole-
cules bearing aminobenzyl group exhibited as a main molecular
signal at 320 (ꢁ) m/z. The exception was compound 1, in which
most probably immediate dehydration occurs responsible for
appearance of signal at 302 (ꢁ) m/z. The same peak was found for
the remaining compounds showing that dehydration was standard
upon MS conditions. Subsequently, the formation of dimers upon
abstraction of two water molecules 605 (ꢁ) m/z was observed. The
molecular decomposition was also visible by detachment of the
boronate group 276 (ꢁ) m/z and fragmentation of the phosphonate
unit 240 (ꢁ) m/z. An interesting fragmentation pattern was found
for di(aminoboronbenzylphosphonic) acids 4e6. The molecule 4
substituted in ortho position exhibited only traces of molecular
signal at 487 (ꢁ) m/z, which were present in the case of compounds
5e6. They showed characteristic stepwise dehydration reaction
under ESI-MS conditions. If considering the compound containing
a boronic group in the para position the observed peak is at 469 (ꢁ)
m/z (one molecule of water is released), while for meta 469 (ꢁ) m/z
and 451 (ꢁ) m/z (two signals: one and two water molecules
released). For ortho derivative three and four water molecules
dissociate at 433 (ꢁ) m/z and 415 (ꢁ) m/z. This phenomenon could
be related to structural alignment of amine group or phosphonate
moiety, which can prompt dehydration reaction. Additional indi-
cation of the essential role of this acidic group is the disappearance
of molecular signals for boronic acids being substituted at ortho
position for 1 and 4. This may also be explained regarding the direct
trigonal boron binding to amine nitrogen B(OH)eN or B(OH)eOeP.
N
(HO)2B
NH2
(HO)2B
b, c
H
N
PO3H2
1-3
(HO)2B
Fig. 1. Synthesis of aminoboronbenzylphosphonates 1e3. a) DCM (using DeaneStark
adapter), b) DCM, P(OSiCH3)3, c) 10% MeOH in H2O.
meso SR/RS configurations [18,19]. In fact, this phenomenon is most
noticeable at neutral pH in which two phosphorous signals were
observed for compounds 4e6. In this environment proton spectra
became more complicated most probably due to the presence of
stereoisomers and possible formation of intra- and/or intermolec-
ular hydrogen bonds. Therefore the NMR measurements for these
structures were performed at pH 11 to simplify the 1H NMR signals.
In such conditions double phosphorous peak remains only for
diastereoisomers of molecule 4. This 31P NMR differentiation might
be induced by intramolecular NeB bond formation or steric
hindrance of closely located boronic moieties and rigid amino-
phosphonate entities. The 11B NMR signal is not diagnostic for
stereochemical consideration because of large broadening of the
signals even over 900 Hz.
B(OH)2
(HO)2B
N
N
B(OH)2
a
2
+
CHO
H2N
NH2
b, c
H2O3P
PO3H2
(HO)2B
NH
HN
B(OH)2
4
Fig. 2. Synthesis of N,N-ethyl aminoboronbenzylphosphonate 4. a) MeOH, b) DCM, P(OSiCH3)3, c) 10% MeOH in H2O.
H
a
N
CHO
CHO
+
HO
OH
O
B
(HO)2B
NH
O
b, c
N
H2O3P
PO3H2
NH
N
H
N
d
O
O
B(OH)2
B
(HO)2B
B
HN
NH
O
5-6
O
Fig. 3. Synthesis of N,N-ethyl aminoboronbenzylphosphonates 5e6. a) THF, 2-propanol, b) ethylenediamine, MeOH, c) DCM, P(OSiCH3)3, d) 10% MeOH in H2O.

P. Młynarz et al. / Journal of Organometallic Chemistry 696 (2011) 457e460
459
The examination of the 11B NMR data performed with addition
(D2O þ NaOD, 192 MHz,
d
(ppm)): 3.07. Yield 44.3%, 0.95 g,
of NaOH showed the same differentiation in chemical shifts: for
compounds 2, 3, 5 and 6 the signal appears at ca. 3 ppm, while for 1
and 4 it is close to 10 ppm showing different behaviour of ortho
substituted compounds, most likely resulting from intramolecular
cyclization caused by BeN or BeOeP binding.
3.0 mmol, decomposing at 261 ꢀC.
4.2.3. N-Benzylamino-4-boronbenzylphosphonic acid 3
1H NMR (D2O þ NaOD, 600 MHz,
d
(ppm)): 3.43 (d, J ¼ 13.2 Hz,
1H), 3.56 (d, J ¼ 12.7 Hz, 1H), 3.58 (d, 1H, JHeP ¼ 17.7 Hz,), 7.10e7.31
(m, 7H), 7.39 (d, J ¼ 7.9 Hz, 2H). 13C NMR (D2O þ NaOD, 150 MHz,
3. Conclusions
d
(ppm)): 51.38 (d, JCeP ¼ 13.38 Hz), 63.11 (d, JCeP ¼ 135.5 Hz),
127.23, 128.00 (d, JCeP ¼ 5.1 Hz), 128.62, 128.77, 130.75 (bs), 137.12
In this work the simple and convenient synthetic routes for
the new compounds bearing aminoboronphosphonic units were
developed. The obtained molecules revealed interesting fragmen-
tation pattern under ESI-MS conditions, most likely depending
upon position of the boronic unit with respect to amino-
phosphonate group. These compounds are interesting as potential
supramolecular hosts, BNTC agents and building blocks for
Suzuki-Miyaura or Petasis reactions.
(d, JCeP ¼ 4.8 Hz), 139.36. 13P NMR (D2O þ NaOD, 243 MHz,
d
(ppm)): 16.80. 11B NMR (D2O þ NaOD, 192 MHz,
d (ppm)): 3.44.
Yield 61%, 1.3 g, 4.0 mmol, decomposing at 308 ꢀC.
4.3. General procedure e preparation of a Schiff base for compound 4
A mixture of 2-formylphenylboronic acid (1.0 g, 6.7 mmol),
ethylenediamine (0.18 g, 3 mmol) and 50 cm3 of methanol was
stirred for 3 days. The solvent was then removed under reduced
pressure resulting in a yellow residue containing a Schiff base,
which was not further purified.
4. Experimental
4.1. General procedure e preparation of a Schiff base for
compounds 1e3
4.4. Preparation of N,N0-ethylenedi(amino-2-
boronbenzylphosphonic) acid 4
A
mixture of adequate formylphenylboronic acid (1.0 g,
6.7 mmol), benzylamine (0.8 g, 7.3 mmol) and 50 cm3 of
dichloromethane (methylene chloride) was refluxed for 7 h. After
that time the volatiles were removed under reduced pressure
resulting in a yellow residue containing a Schiff base, which was not
further purified.
A mixture of trimethyl phosphite (1.5 cm3, 1.57 g, 12.6 mmol)
and bromotrimethylsilane (7.6 cm3, 8.78 g, 57.4 mmol) was dis-
solved in 50 cm3 of dried methylene chloride, cooled to ꢁ5 ꢀC and
stirred for 1 h. After this time 0.93 g of a crude Schiff base was
suspended in 10 cm3 of dried methylene chloride and added to the
solution of in situ generated tris(trimethylsilyl) phosphite. This
solution was stirred for 48 h at a room temperature. The reaction
mixture was evaporated and the obtained yellow residue was
treated with methanolic solution (10 cm3 of methanol and 60 cm3
water) and stirred overnight. Crude product was filtered off and
dissolved in 1 M of sodium hydroxide. 1 M of hydrochloric acid was
subsequently added dropwise until a white precipitate was formed.
It was filtered off and washed with diethyl ether.
4.2. General procedure e preparation of N-benzyl
aminoboronbenzylphosphonic acids 1e3
A mixture of trimethyl phosphite (0.87 cm3, 0.9 g, 7.3 mmol) and
bromotrimethylsilane (4.4 cm3, 5.1 g, 33.3 mmol) was dissolved
in 40 cm3 of dried methylene chloride, cooled to ꢁ5 ꢀC and stirred
for 45 min. After this time 1.6 g of a crude Schiff base was sus-
pended in 15 cm3 of dried methylene chloride and added to the
solution of in situ generated tris(trimethylsilyl) phosphite. This
solution was stirred for 48 h at a room temperature. After that time
the reaction mixture was evaporated and the yellow residue was
treated with a methanolic solution (5 cm3 of methanol and 15 cm3
water) and stirred overnight. Crude product was filtered off and
dissolved in 1 M solution of sodium hydroxide. Subsequently,1 M of
hydrochloric acid was added dropwise until a white precipitate was
formed which was filtered off and washed with diethyl ether.
1H NMR (D2O þ NaOD, 300 MHz,
d (ppm)): 2.86e2.97 (m, 2H),
3.17e3.29 (m, 2H), 3.88 and 3.95 (d, 2H all, JHeP ¼ 14.6 Hz and
JHeP ¼ 15.8 Hz), 7.17e7.31 (m, 6H), 7.32e7.41 (m, 2H). 13C NMR
(D2O
þ
NaOD, 75 MHz, d (ppm)): 46.14 (bs), 64.63 (d,
JCeP ¼ 124.3 Hz), 124.72 (bs), 126.51 (bs), 127.31, 129.12 (bs), 142.41
(d, JCeP ¼ 5.0 Hz). 13P NMR (D2O þ NaOD, 121 MHz,
d
(ppm)): 12.96
and 13.06. 11B NMR (D2O þ NaOD, 192 MHz,
d (ppm)): 10.3. Yield
29%, 0.425 g, 0.87 mmol, decomposing over 300 ꢀC.
4.5. Formation of formylphenyl boronic acids diethanolamine esters
4.2.1. N-Benzylamino-2-boronbenzylphosphonic acid 1
of compounds 5e6
1H NMR (D2O þ NaOD, 600 MHz,
d
(ppm)): 3.26 (d, J ¼ 13.8 Hz,
1H), 3.84 (d, JHeP ¼ 17.2 Hz, 1H), 4.10 (d, J ¼ 13.8 Hz, 1H each),
To an adequate formylphenylboronic acid (1.0 g, 6.7 mmol)
dissolved in 15 cm3 of tetrahydrofuran (THF) the solution of
diethanolamine (0.77 g, 7.3 mmol) in 5 cm3 of 2-propanol was
added. After that a yellow precipitate was formed. The whole
mixture was stirred for 3 h in a room temperature to complete the
reaction. Next, the precipitate was filtered off, washed with 5 cm3
of THF and dried. Yield compound: 5 e 99% (1.45 g, 6.6 mmol);
6 e 46% (6.8 g, 3.1 mmol).
7.10e7.19 (m, 3H), 7.25e7.38 (m, 6H). 13C NMR (D2O þ NaOD,
150 MHz,
d
(ppm)): 51.96 (d, JCeP
¼
7.7 Hz), 62.63 (d,
JCeP ¼ 125.8 Hz), 124.95 (bs), 126.59 (bs), 127.35 (bs), 128.14, 128.96,
129.53 (bs),135.70,142.22 (d, JCeP ¼ 5.18 Hz). 13P NMR (D2O þ NaOD,
243 MHz,
d
(ppm)): 13.9. 11B NMR (D2O þ NaOD, 192 MHz,
d (ppm)):
9.9. Yield 44.2%, 0.95 g, 3.0 mmol, decomposing at 310 ꢀC.
4.2.2. N-Benzylamino-3-boronbenzylphosphonic acid 2
1H NMR (D2O þ NaOD, 600 MHz,
d
(ppm)): 3.44 (d, J ¼ 12.9 Hz,
4.6. General procedure e preparation of a Schiff base for
compounds 5e6
1H), 3.55 (d, J ¼ 13.2 Hz, 1H), 3.60 (d, JHeP ¼ 17.7 Hz, 1H), 7.10e7.36
(m, 8H), 7.42 (bs, 1H). 13C NMR (D2O þ NaOD, 150 MHz,
d (ppm)):
51.45 (d, JCeP ¼ 13.41 Hz), 63.57 (d, JCeP ¼ 134.4 Hz), 126.40 (d,
JCeP ¼ 4.7 Hz), 126.81 (bs), 127.13 (bs), 128.59, 128.72, 129.12 (bs),
132.06 (d, JCeP ¼ 5.39 Hz), 138.62 (d, JCeP ¼ 4.37 Hz), 139.66. 13P
A mixture of the proper formylphenyl diethanolamine boronic
ester (5 e 1.45 g, 6.6 mmol; 6 e 0.395 g, 1.8 mmol) and ethylenedi-
amine (5 e 0.2 g, 3.3 mmol; 6 e 0.054 g, 0.9 mmol) in 50 cm3 of
methanol was stirred at a room temperature up to 48 h. The volatiles
NMR (D2O
þ
NaOD, 243 MHz, d
(ppm)): 17.19. 11B NMR

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P. Młynarz et al. / Journal of Organometallic Chemistry 696 (2011) 457e460
were then removed under reduced pressure resulting in a white
Acknowledgments
residue containing a Schiff base, which was not further purified.
The authors would like to express gratitude to Prof. Pawe1
Kafarski for stimulating discussions and valuable remarks as well as
The Polish Ministry of Science and Higher Education for grant no
N N 204 134 837.
4.7. Preparation of N,N0-ethylenedi(aminoboronbenzyl-
phosphonic) acids 5e6
Mixtures of trimethyl phosphite (5 e 0.76 cm3, 0.8 g, 6.45 mmol;
6 e 0.16 cm3, 0.171 g, 1.38 mmol) and bromotrimethylsilane (5 e
4.3 cm3, 4.9 g, 32.3 mmol; 6 e 0.9 cm3, 1.05 g, 6.88 mmol) were
dissolved in 50 cm3 of dried methylene chloride, cooled to ꢁ5 ꢀC
and stirred for 1 h. After this time a proper amount (1.36 g or 0.29)
of a crude Schiff base was suspended in 15 cm3 of dried methylene
chloride and added to the solution of in situ generated tris(tri-
methylsilyl) phosphite. This solution was stirred for 48 h at a room
temperature. After that time the reaction mixture was evaporated
and the yellow residue was treated with a 20 cm3 of methanol and
stirred, evaporated again, treated with 20 cm3 of water and left
stirring for the next day. After 24 h a pale precipitate was filtered off
and washed with water and methanol.
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4.7.1. N,N0-Ethylenedi(amino-3-boronbenzylphosphonic) acid 5
1H NMR (D2O þ NaOD, 300 MHz,
d (ppm)): 2.70e2.91 (m, 4H),
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3.76 and 3.79 (d, each 1H, J ¼ 16.12 Hz), 7.13e7.28 (m, 4H),
7.39e7.53 (m, 4H). 13C NMR (D2O þ NaOD, 75 MHz,
d
(ppm)): 45.04
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¼
9.76 Hz), 45.30 (d, JCeP
¼
10.0 Hz), 63.29 (d,
JCeP ¼ 131.2 Hz), 63.45 (d, JCeP ¼ 131 Hz), 126.67 (bs), 127.29, 130.26,
131.83 (d, JCeP ¼ 3.6 Hz), 135.83 (d, JCeP ¼ 4.1 Hz) 135.90 (d,
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JCeP ¼ 4.13 Hz). 13P NMR (D2O þ NaOD, 121 MHz,
d (ppm)): 13.37
d (ppm)): 4.1. Yield 33%,
(bs). 11B NMR (D2O þ NaOD, 192 MHz,
0.29 g, 0.6 mmol, decomposing over 300 ꢀC.
4.7.2. N,N0-Ethylenedi(amino-4-boronbenzylphosphonic) acid 6
1H NMR (D2O þ NaOD, 300 MHz,
d (ppm)): 2.46 (bs, 4H) 3.50 (d,
2H, JHeP ¼ 17.48 Hz), 7.14 (d, 4H, J ¼ 7.64 Hz), 7.35 (d, 4H,
J ¼ 7.64 Hz). 13C NMR (D2O þ NaOD, 75 MHz,
d (ppm)): 47.38
(d, JCeP ¼ 11.1 Hz), 63.86 (d, JCeP ¼ 134.4 Hz), 127.78 (d,
JCeP ¼ 4.63 Hz), 130.67, 137.52 (d, JCeP ¼ 3.23 Hz), 137.59 (d,
JCeP ¼ 3.39 Hz). 13P NMR (D2O þ NaOD, 121 MHz,
d (ppm)): 16.50.
11B NMR (D2O þ NaOD, 192 MHz,
d (ppm)): 3.3. Yield 34%, 1.12 g,
2.3 mmol, decomposing over 300 ꢀC.