Aminophosphinic acids in a pyridine series: Cleavage of pyridine-2- and pyridine-4-methyl(amino)phosphinic acids in acidic solutions

Article abstract of DOI:10.1080/10426500500271865

The synthesis of a series of new pyridine aminomethylphosphinic acids is described. These compounds were obtained in the reaction of the corresponding pyridine aldehydes with primary amines and with ethyl phenylphosphinate, or methylphosphinate, in the presence of bromotrimethylsilane. In aqueous, strong acid solutions, pyridine aminophosphinic acids were split, forming the phenyl-, or methylphosphonic, acid and the corresponding secondary pyridyl-alkylamines. The kinetics of some observed cleavages were measured, and a mechanism of the cleavage has been proposed. Copyright Taylor & Francis Group, LLC.

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Aminophosphinic Acids in a
Pyridine Series: Cleavage of
Pyridine-2- and Pyridine-4-
methyl(amino)phosphinic Acids
in Acidic Solutions
Bogdan Boduszek a , Tomasz Olszewski a , Waldemar
Goldeman a & Magdalena Konieczna a
a Department of Organic Chemistry , Wroclaw
University of Technology , Wroclaw, Poland
Published online: 15 Aug 2006.
To cite this article: Bogdan Boduszek , Tomasz Olszewski , Waldemar Goldeman &
Magdalena Konieczna (2006) Aminophosphinic Acids in a Pyridine Series: Cleavage
of Pyridine-2- and Pyridine-4-methyl(amino)phosphinic Acids in Acidic Solutions,
Phosphorus, Sulfur, and Silicon and the Related Elements, 181:4, 787-795, DOI:
10.1080/10426500500271865
To link to this article: http://dx.doi.org/10.1080/10426500500271865
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Phosphorus, Sulfur, and Silicon, 181:787–795, 2006
Copyright © Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500500271865
Aminophosphinic Acids in a Pyridine Series: Cleavage
of Pyridine-2- and Pyridine-4-methyl(amino)phosphinic
Acids in Acidic Solutions
Bogdan Boduszek
Tomasz Olszewski
Waldemar Goldeman
Magdalena Konieczna
Department of Organic Chemistry, Wroclaw University of Technology,
Wroclaw, Poland
The synthesis of a series of new pyridine aminomethylphosphinic acids is described.
These compounds were obtained in the reaction of the corresponding pyridine alde-
hydes with primary amines and with ethyl phenylphosphinate, or methylphosphi-
nate, in the presence of bromotrimethylsilane. In aqueous, strong acid solutions,
pyridine aminophosphinic acids were split, forming the phenyl-, or methylphos-
phonic, acid and the corresponding secondary pyridyl-alkylamines. The kinetics of
some observed cleavages were measured, and a mechanism of the cleavage has been
proposed.
Keywords Aminophosphinic acids; bromotrimethylsilane; cleavage reactions; phospho-
nic acids; pyridine aminophosphinic acids; pyridyl amines
INTRODUCTION
Pyridine aminophosphinic acids are compounds of great importance due
to potential biological activity.1 Some of parent compounds, i.e., the pep-
tidyl pyridine aminophosphonates, were tested as enzyme inhibitors for
serine proteases.2 Also some of pyridine aminophosphonic acids were
used as powerful binding agents for transition metals.3 There is an ex-
pectancy that the pyridine aminophosphinic acids will be products with
valuable biological and chemical properties, like the parent aminophos-
phonic acids.
Received May 31, 2005; accepted June 9, 2005.
The authors are thankful to the Faculty of Chemistry, Wroclaw University of Tech-
nology, Wroclaw, Poland.
Address correspondence to Bogdan Boduszek, Department of Organic Chemistry, Fac-
ulty of Chemistry, Wroclaw University of Technology, Wyb. Wyspianskiego 27, 50-370
Wroclaw, Poland. E-mail: bogdan.boduszek@pwr.wroc.pl
787

788
B. Boduszek et al.
A few years ago, we described a new, convenient procedure for the
synthesis of pyridine aminophosphonic acids by the use of a mixture
composed of trimethyl phosphite and bromotrimethylsilane for the ad-
dition to aldimines.4 We found that such a procedure can be applied
for the synthesis of aminophosphinic acids, with a slight modification.
When H-phenyl- and H-methylphosphinate ethyl esters are treated
with bromotrimethylsilane, the corresponding trimethylsilyl esters are
formed. The obtained mixture, comprising mainly of the phosphinite
trimethylsilyl esters, could be used directly for additional reactions to
the aldimines.
RESULTS AND DISCUSSION
The title pyridine aminophosphinic acids were synthesized by a se-
quence of reactions from the corresponding pyridine aldehydes, primary
amines, appropriate phosphinic esters, and bromotrimethylsilane, ac-
cording to Scheme 1.
SCHEME 1
The procedure was as follows: the pyridine aldehydes 1 were
mixed with butyl- or benzylamine to form the corresponding pyridine
aldimines 2, which were treated with a mixture composed of phenyl- or
methylphosphinic acid ethyl ester and bromotrimethylsilane. Interme-
diates, i.e., the silylated phosphinates 3, were easily formed in this case,
and they were then transformed to the final pyridyl aminophosphinic
acids 4 by hydrolysis with methanol. Hydrolysis of the silyl phospho-
nate esters to free phosphonic acids by use of methanol, was previously
utilized by Boduszek4 and Zon´.5

Aminophosphinic Acids in a Pyridine Series
789
The present method of synthesis of the pyridine aminophosphinic
acids is a one-pot convenient and simple procedure leading directly to
the products 4 with a high yield and purity.
All the synthesized products were characterized by means of 1H NMR
and 31P NMR spectroscopy. Analytical data of the obtained aminophos-
phinic acids 4 are given in Table I.
The synthesis of some obtained compounds (4b, 4f, and 4j) already
has been published in a preliminary communication.10
The prepared aminophosphinic acids 4a–d and 4i–l underwent inter-
esting transformations in aqueous solutions of mineral acids. Under the
influence of strong acids, the aminophosphinic acids were split to form
the secondary amines and aryl(alkyl)phosphonic acids. A similar way
of splitting was found for 2- and 4-pyridylmethyl(amino)phosphonic
acids,6 and other heterocyclic phosphonate compounds.8,9 Thus, for ex-
ample, a solution of 4b in 10% aq. sulfuric acid at r.t. decomposed grad-
ually to form the amine 5b and phenylphosphonic acid (Scheme 2).
After about 20 h, the cleavage of 4b was complete. Likewise, the 4-
pyridyl phosphinic acid 4j was cleaved in the same manner, however
much slower, than the corresponding 2-pyridyl derivatives. Cleavage
of the aminophosphinic acids can be considerably accelerated at el-
evated temperature; thus, boiling a 10% H2SO4 solution of 4a–d or
4i–l caused complete decomposition of the aminophosphinic acids dur-
ing one hour. As a result, formation of a mixture of secondary amines
5a–d and phenyl- (6), or methylphosphonic acid (7) was obtained
(Scheme 2).
SCHEME 2
Phosphinic 3-pyridyl derivatives 4e–h were not affected by strong
mineral acids at these conditions.
31P NMR spectroscopy allowed measurement of some kinetics of
the cleavages. The measurement of kinetic rates for selected com-
pounds was executed likewise as it was done for the corresponding
phosphonic acids.7 Obtained pseudo-first-order rate constants (kobs) for

790
B. Boduszek et al.
TABLE I Analytical Data of Pyridylmethyl(amino)phosphinic Acids 4
31P NMR
(D2O)
Compound Yield
M.p.
◦C
1H NMR (D2O)
no.
%
δ, ppm
δ, ppm
4a
34
179–180 8.34 (d, 1H, J = 4.6 Hz, py-6), 7.85 (t, 1H,
J = 7.8 Hz, py-4), 7.45–7.28 (m, 7H,
20.48 (s)
arom.), 4.70 (d, 1H, CH-P, J = 12.0 Hz),
2.92–2.90 (m, 2H, CH2N), 1.50 (m, 2H),
1.15 (m, 2H), 0.70 (t, 3H, CH3, J = 7.6 Hz)
185–187 8.41–8.37 (m, 2H, py-4,6), 7.85–7.79 (m, 2H.
py-3,5), 7.41–7.15 (m, 10H, 2xPh),
4b
4c
81
54
20.03 (s)
30.68 (s)
4.78–4.74 (d, 1H, J = 11.8 Hz, CH-P),
4.40–4.20 (m, 2H, CH2N)
159–163 8.68 (d, 1H, J = 5.41 Hz, py-6), 8.30 (t, 1H,
J = 7.95 Hz, py-4), 7.87 (d, 1H, J = 7.99
Hz, py-3), 7.77 (t, 1H, J = 6.63 Hz, py-5),
4.70 (d, 1H, J = 20.27 Hz, CH-P), 3.01 (m,
2H, CH2N), 1.59 (m, 2H, CH2), 1.25 (d, 3H,
J = 14.32 Hz, P-CH3), 1.20 (m, 2H, CH2),
0.76 (t, 3H, J = 7.32 Hz, CH3)
4d
4e
4f
79
51
84
49
217–218 8.51 (d, 1H, J = 4.84 Hz, py-6), 7.77 (t, 1H,
J = 7.80 Hz, py-4), 7.36–7.23 (m, 7H,
32.54 (s)
arom.), 4.30 (d, 1H, J = 13.99 Hz, CH-P),
4.16 (dd, 2H, J = 13.17 Hz, CH2Ph),
1.06 (d, 3H, J = 14.17 Hz, P-CH3)
242–243 8.37 (bs, 1H, py-6), 8.15 (s, 1H, py-2), 7.69 (d, 21.49 (s)
1H, py-4, J = 7.85 Hz), 7.44–7.32 (m, 6H,
arom.), 4.70 (d, 1H, CH-P, J = 12 Hz), 2.87
(m, 2H, CH2N), 1.49 (m, 2H), 1.13 (m, 2H),
0.70 (t, 3H, CH3, J = 7.6 Hz)
225–227 8.56 (d, 1H, J = 5.7 Hz, py-6.), 8.29 (s, 1H.
py-2.), 8.18 (d, 1H, py-4.), 7.42 (m, 1H,
py-5), 7.32–7.10 (m, 10H, 2xPh), 4.57 (d,
1H, J = 11.1 Hz, CH-P), 4.20 (bs, 2H,
CH2N)
21.18 (s)
4g
168–169 8.87 (s, 1H, py-2), 8.81 (d, 1H, J = 5.73 Hz,
py-6), 8.65 (d, 1H, J = 8.28 Hz, py-4),
8.09 (m, 1H, py-5), 4.82 (d, 1H, J = 18.6
Hz, CH-P), 2.97 (m, 2H, CH2N), 1.58 (m,
2H), 1.29 (d, 3H, J = 8.32 Hz, P-CH3),
1.20 (m, 2H), 0.77 (t, 3H, J = 7.34 Hz, CH3)
235–236 8.37 (d, 1H, py-6, J = 5.77 Hz), 8.32 (s, 1H,
py-2), 8.19 (d, 1H, py-4, J = 8.27 Hz),
7.67 (m, 1H, py-5), 6.92 (m., 5H, Ph),
4.42 (d, 1H, J = 10.45 Hz, CH-P), 3.94 (m.,
2H, CH2N), 1.01 (d, 3H, J = 14.44 Hz,
P-CH3), (in 1M D2SO4 in D2O)
30.05 (s)
4h
78
35.55 (s)

Aminophosphinic Acids in a Pyridine Series
791
TABLE I Analytical Data of Pyridylmethyl(amino)phosphinic Acids 4
(Continued)
31P NMR
(D2O)
Compound Yield
M.p.
◦C
1H NMR (D2O)
no.
%
δ, ppm
δ, ppm
4i
47
208–210 8.32 (d, 2H, py-2,6, J = 4.9 Hz), 7.44–7.31 (m, 20.99(s)
5H, arom.), 7.15 (d, 2H, py-3,5, J = 4.9 Hz),
4.75 (d, 1H, CH-P J = 12 Hz), 2.84 (m, 2H,
CH2N), 1.47 (m, 2 H), 1.11 (m, 2H), 0.69 (t,
3H, CH3, J = 7.1 Hz).
4j
80
48
171–173 8.44 (d, 2 H, J = 6.5 Hz, py-2,6.), 7.53 (d, 2H,
J = 5.7 Hz, py-3,5.), 7.45–7.12 (m, 10H,
2xPh), 4.69–4.65 (d, 1H, J = 11.2 Hz,
20.52(s)
CH-P), 4.24 (d, 2H, J = 3.1 Hz, CH2N).
205–208 8.79(d, 2H, J = 6.71 Hz, py-2,6), 8.06(d, 2H,
J = 5.56 Hz, py-3,5), 4.80(d, 1H, J = 10.96
Hz, CH-P), 3.02(m., 2H, CH2N), 1.59(m,
2H), 1.29(d, 3H, J = 14.28 Hz, P-CH3),
1.22(m, 2H), 0.78(t, 3H, J = 7.36 Hz, CH3)
197–198 8.28(d, 2H, py-2,4, J = 6.13 Hz), 7.53(d, 2H,
py-3,5, J = 6.44 Hz), 6.91(m, 5 H, Ph),
4k
29.37(s)
4l
62
34.37(s)
4.43(d, 1H, J = 11.65 Hz, CH-P), 3.93(m,
2H, CH2N), 0.98(d, 3H, J = 14.49 Hz,
P-CH3) (in 1M D2SO4 in D2O)
cleavages of the aminophosphinic acids 4 are given in Table II. Ac-
cording to the obtained results, constant rates (kobs) were strongly de-
pended on the concentration of sulfuric acid. For example, the rates for
aminophosphinic acids (4b,4c, and 4j) increased faster than could be
expected from direct proportionality to the concentration of sulfuric acid
(Table II).
A small kinetic isotope effect (kH/kD) was observed in the cleavage.
An existence of the kH/kD > 1 shows that protons are involved in a
rate-determining step. However, the kH/kD was somewhat less than
unity for the measured pyridine aminomethylphosphinic acids (4c, and
4d, Table II). All the kinetic data indicate that the observed cleavages
are a kind of an electrophilic substitution observed at α-carbon in the
aminophosphinic acids. In this case, an electrophile (H+) replaces a
phosphinic moiety in the aminophosphinic acid giving as a result the
secondary amine. The metaphosphate-like moiety reacts with the sol-
vent (water) to form the phenyl- or methylphosphonic acid (Scheme 2).
Kinetic measurements for the cleavage of 4b were additionally car-
ried out at 35◦C and 45◦C in order to calculate the activation parameters
(Ea, ꢀH=, and ꢀS=, Table II). Low activation energy (Ea) calculated

792
B. Boduszek et al.
TABLE II Kinetics for the Cleavage of Pyridylmethyl(amino)-
Phosphinic Acids
Kinetic and thermodynamic
parameters
Conc. of compd. Conc. of acid Temp.
Compound
mol L−1
mol L−1
◦C
kobs h−1
t1/2 kH/kD
a
4b
0.06
0.08
0.08
0.06
0.08
0.08
0.21
0.21
0.21
0.21
0.18
0.18
0.18
0.18
0.18
0.07
0.08
0.08
0.07
0.18
0.18
0.18
0.18
0.5 (H2SO4)
1.0 (H2SO4)
1.0 (D2SO4)
2.0 (H2SO4)
1.0 (H2SO4)
1.0 (H2SO4)
0.5 (H2SO4)
1.0 (H2SO4)
2.0 (H2SO4)
1.0 (D2SO4)
0.5 (H2SO4)
1.0 (H2SO4)
2.0 (H2SO4)
1.0 (D2SO4)
1.0 (H2SO4)
0.5 (H2SO4)
1.0 (H2SO4)
1.0 (D2SO4)
2.0 (H2SO4)
0.5 (H2SO4)
1.0 (H2SO4)
2.0 (H2SO4)
1.0 (D2SO4)
0.038
0.126
0.108
0.356
0.265
0.634
0.001b
0.006b
0.013b
0.007b
0.002b
0.012b
0.031b
0.015b
0.053b
0.007
0.020
0.018
0.049
0.001b
0.002b
0.004b
0.002b
t1/2 = 5.5 h
kH/kD = 1.17
25
Ea = 73.7 kJ/mol
35
45
ꢀH= = 71.2 kJ/mol
ꢀS= = −91.7 J/mol.st
4c
4d
25
t1/2 = 119 h
kH/kD = 0.86
25
45
25
t1/2 = 73.7h
kH/kD = 0.80
4j
4l
t1/2 = 34.3 h
kH/kD = 1.10
t1/2 = 315 h
kH/kD = 1.0
25
aKinetic data for 1M H2SO4 solutions.
bKinetics measured in 50% aqueous methanol.
for the cleavage of 4b is significant, and it confirms the observed facility
of the splitting of the aminophosphinic acid.
Taking the obtained data into account, a mechanism for the cleav-
age of pyridyl aminophosphinic acids in acidic conditions is pro-
posed and shown in Scheme 3, for example, of the cleavage of 2-
pyridylmethyl(amino)phenylphosphinic acid 4b. A similar mechanism
was postulated previously for pyridine aminophosphonic acids.7
Protonation of the nitrogen atoms in the aminophoshinic acid
molecule set up an inductive effect additionally backed up by a res-
onance effect in the pyridine ring (Scheme 3). It causes, in conse-
quence, a rupture of a C-P bond and the leaving of a positively
charged metaphosphate-like moiety B. The introduction of a proton
to the α-methine group in the moiety A leads to the amine 5b.

Aminophosphinic Acids in a Pyridine Series
793
SCHEME 3
The metaphosphate-like moiety B can react with water to give the
phenylphosphonic acid 6 (Scheme 3).
EXPERIMENTAL
1H NMR and 31P NMR spectra were measured on a Bruker Avance
300 MHz spectrometer using TMS as an internal standard. Melting
points were determined using an Electrothermal 9200 apparatus and
a Boetius hot-stage apparatus and were uncorrected. Elemental analy-
ses were done in the Laboratory of Instrumental Analysis in the Insti-
tute. Reagents used were obtained from the Sigma-Aldrich Company
(Poznao`, Poland). Solvents were of commercial quality and purchased
from a local supplier (POCH Gliwice, Poland).
Ethyl methylphosphinate was prepared from methyldichlorophos-
phine according to the procedure published in.11
General Procedure for the Preparation of the Pyridine
Aminophosphinic Acids 4a–l
The procedure for the synthesis of pyridine N-benzylaminophosphinic
acids10 was followed with some modifications. Thus, a mixture of the
corresponding pyridinecarboxaldehyde 1 (1.07 g, 10 mmol) and benzyl,
or butylamine (11 mmol) in methylene chloride (25 mL) was prepared.
The mixture was kept at r.t. for 24 h; anhydrous sodium sulfate was

794
B. Boduszek et al.
added (5.0 g), stirred, and filtered. The filtrate was mixed with ethyl
phenylphosphinate, or ethyl methylphosphinate (10 mmol), followed by
bromotrimethylsilane (4.6 g, 30 mmol), and left for the next 24 h at r.t.
After this, the mixture was evaporated to dryness; and the resulting oily
product was kept at 70◦C for 1 h and treated with methanol (10 mL).
The mixture was refrigerated for several hours until crystals of the
aminophosphinic acid 4 separated. If necessary, diethyl ether (20 mL)
was added to the mixture to cause precipitation of the products. The
products 4a–l were collected by filtration, washed with diethyl ether,
and dried. Yields and analytical data of the 4 are given in the Table I.
Elemental anal. for 4a, calc.: N, 9.21; P, 10.18. Found: N, 9.21; P, 10.27%.
For 4b, calc.: N, 8.28; P, 9.15. Found: N, 8.21; P, 9.11%. Lit.[10]. For 4c,
calc.: N, 11.56; P, 12.79. Found: N, 11.44; P, 12.81%. For 4d, calc.: N,
10.14; P, 11.21. Found: N, 10.09; P, 11.22%. For 4e, calc.: N, 9.21; P,
10.18. Found: N, 9.20; P, 10.21%. For 4f, calc.; N, 8.28; P, 9.15. Found:
N, 8.19; P, 9.10%. Lit.[10]. For 4g, calc.: N, 11.56; P, 12.79. Found: N,
11.48; P, 12.78%. For 4h, calc.: N, 10.14; P, 11.21. Found: N, 10.09; P,
11.14. For 4i, calc.: N, 9.21; P, 10.18. Found: N, 9.11; P, 10.13. For 4j,
calc.: N, 8.28; P, 9.15. Found: N, 8.17; P, 9.14%. Lit.[10]. For 4k, calc.:
N, 11.56; P, 12.79. Found: N, 11.44; P, 12.76%. For 4l, calc.: N, 10.14; P,
11.21. Found: N, 10.05; P, 11.16%.
Cleavage of 4a–d and 4i–l and Isolation of the Products
A sample of pyridine-2-yl aminophosphinic acid 4a–d (1.0 mmol) was
dissolved in aqueous 10% sulfuric acid (10 mL), and the solution was
kept for 1–3 days at r.t. In the case of pyridine-4-yl aminophosphinic
acids, the solution of the 4i–l in 10% aq. H2SO4 was left for 7–10 days at
r.t. After this, the solution was alkalized with sodium carbonate (1.6 g,
15 mmol) and extracted with methylene chloride (25 mL). The extract
was dried (anh. Na2SO4), filtered, and evaporated to give the amines
5a–d, as thick oils. Yield: 70–90%. The amines 5a–d are known com-
pounds and were characterized as oxalate salts and by comparison with
the literature data.6
The remaining aqueous layer was acidified with 10% aq.
H2SO4(5 mL) and evaporated to dryness. The residue was treated with
absolute ethanol (25 mL), warmed to70◦C, and filtered; the filtrate evap-
orated to give the phosphonic acids, 6 and 7, respectively.
Phenylphosphonic acid 6, (0.12–0.15 g, 76–95%). Methylphosphonic
acid 7 (0.075–0.09 g, 78–94%).
The phosphonic acids 6 and 7 are known, commercial compounds
and their analytical data are given elsewhere.12

Aminophosphinic Acids in a Pyridine Series
795
Procedure for Kinetic Runs
Stock aqueous, or stock 50% aq. methanol solutions of sulfuric acid,
with the concentration of H2SO4; c = 0.5, 1.0, and 2.0 mol.L−1, were
prepared. Similarly, a solution of deuterated sulfuric acid was prepared
with a conc. of D2SO4 c = 1.0 mol.L−1. Samples of aminophosphinic
acids 4 (10–20 mg) and sulfuric acid solutions (0.5 mL) were mixed in
NMR tubes and thermostated at 25◦C, 35◦C, or 45◦C for a specified
period of time (1, 2, 4, 8, and 16 h), and the 31P NMR spectra were
recorded. The pseudo-first-order rate constants (kobs) were determined
from 31P NMR spectra by plotting the dependence of log(a-x) on time
(where the “a − x” represents concentration of unreacted (4). In most
cases, a dependence of log(a-x) on time was obeyed very well.
REFERENCES
[1] V. P. Kukhar and H. R. Hudson, In Aminophosphonic and Aminophosphinic Acids:
Chemistry and Biological Activity (John Wiley & Sons, Chichester, 2000), Chapters
12–18, pp. 407–615.
[2] B. Boduszek, Phosphorus, Sulfur, and Silicon, 144–146, 433 (1999).
[3] R. Lipinski, L. Chruscinski, P. Mlynarz, B. Boduszek, and H. Kozlowski, Inorg.
Chim. Acta, 322, 157 (2001).
[4] B. Boduszek, Polish J. Chem., 75, 663 (2001).
[5] J. Zon´, Polish J. Chem., 55, 643 (1981).
[6] B. Boduszek, Tetrahedron, 52, 12483 (1996).
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(2000).
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[10] B. Boduszek, Synth. Commun., 33, 4087 (2003).
[11] J. Kehler, E. Bjarke, and P. Krogsgaard-Larsen, Tetrahedron, 55, 771 (1999).
[12] Methylphosphonic acid; Beil. 4, IV, 3498. Phenylphosphonic acid; Beil. 16, IV, 1068.