Synthesis of alkyl- and arylphosphonic acid monoesters by direct esterification of dibasic phosphonic acids in the presence of an arsonic acid catalyst

Article abstract of DOI:10.1080/10426500490464032

Partial hydrolysis of a diester, hydrolysis of the monochloro monoester, or alcoholysis of a phosphonic acid anhydride generally is used to prepare monoesters of alkyl- and arylphosphonic acids. Limited cases have been reported for the esterification of a dibasic phosphonic acid to yield the monoester, and none of these methods are as simple as the analogous method for preparing carboxylic acid esters, in which the carboxylic acid is esterified with an alcohol in the presence of an acid catalyst. Described is a method for preparing monoesters of alkyl- and arylphosphonic acids by direct esterification with an alcohol in the presence of a catalytic amount of phenylarsonic acid. The water formed during the reaction is removed azeotropically. For example, methylphosphonic acid was esterified in good yield to give its isopropyl, butyl, cyclohexyl, bornyl, and octadecyl monoesters. Similarly prepared are the ethyl, butyl, hexyl, and 2-(ethylthio)ethyl monoesters of phenylphosphonic acid, as well as 2-isopropoxyethyl hydrogen ethylphosphonate and 2-methoxyethyl hydrogen benzylphosphonate.

Full text of DOI:10.1080/10426500490464032

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SYNTHESIS OF ALKYL- AND ARYLPHOSPHONIC ACID
MONOESTERS BY DIRECT ESTERIFICATION OF DIBASIC
PHOSPHONIC ACIDS IN THE PRESENCE OF AN ARSONIC
ACID CATALYST
Michael D. Crenshaw a
a Battelle Memorial Institute , Columbus, Ohio, USA
Published online: 16 Aug 2010.
To cite this article: Michael D. Crenshaw (2004) SYNTHESIS OF ALKYL- AND ARYLPHOSPHONIC ACID MONOESTERS BY DIRECT
ESTERIFICATION OF DIBASIC PHOSPHONIC ACIDS IN THE PRESENCE OF AN ARSONIC ACID CATALYST, Phosphorus, Sulfur, and
Silicon and the Related Elements, 179:8, 1509-1516, DOI: 10.1080/10426500490464032
To link to this article: http://dx.doi.org/10.1080/10426500490464032
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Phosphorus, Sulfur, and Silicon, 179:1509–1516, 2004
ꢀC
Copyright Taylor & Francis Inc.
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500490464032
SYNTHESIS OF ALKYL- AND ARYLPHOSPHONIC
ACID MONOESTERS BY DIRECT ESTERIFICATION
OF DIBASIC PHOSPHONIC ACIDS IN THE
PRESENCE OF AN ARSONIC ACID CATALYST
Michael D. Crenshaw
Battelle Memorial Institute, Columbus, Ohio, USA
(Received November 6, 2003; accepted January 1, 2004)
Partial hydrolysis of a diester, hydrolysis of the monochloro monoester,
or alcoholysis of a phosphonic acid anhydride generally is used to pre-
pare monoesters of alkyl- and arylphosphonic acids. Limited cases have
been reported for the esterification of a dibasic phosphonic acid to yield
the monoester, and none of these methods are as simple as the analogous
method for preparing carboxylic acid esters, in which the carboxylic
acid is esterified with an alcohol in the presence of an acid catalyst.
Described is a method for preparing monoesters of alkyl- and arylphos-
phonic acids by direct esterification with an alcohol in the presence
of a catalytic amount of phenylarsonic acid. The water formed during
the reaction is removed azeotropically. For example, methylphospho-
nic acid was esterified in good yield to give its isopropyl, butyl, cy-
clohexyl, bornyl, and octadecyl monoesters. Similarly prepared are the
ethyl, butyl, hexyl, and 2-(ethylthio)ethyl monoesters of phenylphospho-
nic acid, as well as 2-isopropoxyethyl hydrogen ethylphosphonate and
2-methoxyethyl hydrogen benzylphosphonate.
Keywords: Alkylphosphonic acid monoester; arylphosphonic acid mo-
noester; catalysis; esterification; phenylarsonic acid
INTRODUCTION
Monoesters of alkyl- and arylphosphonic acids are generally prepared
by partial hydrolysis of a diester,1−4 hydrolysis of the monochloro
monoester,5,6 or alcoholysis of a phosphonic acid anhydride or
dioxophosphorane.7−10 In 1999, Pienaar et al. found the alcoholysis
of the anhydride preferable to alkaline hydrolysis for the preparation
NMR spectra were acquired by Sue Harsh. Exact mass measurements were obtained
by Edwin Quinones and Dr. Aaron Frank.
Address correspondence to Michael D. Crenshaw, Battelle Memorial Institute, 505
King Avenue, Columbus, OH 43201, USA. E-mail: crenshaw@battelle.org
1509

1510
M. D. Crenshaw
of monoalkyl esters of alkylphosphonic acids.11 Other, less-often
used methods include the thermolysis or photolysis of phosphonyl
peroxides,12,13 oxidation of a phosphonite with chlorine or nitrogen
dioxide,14,15 and alcoholysis of a tetrachlorophosphorane.16
More recently, additional methods have been developed. Cleavage
of alkylphosphonate diesters with NaI in acetone or 2-butanone yields
the monoesters,17 while methylphosphonic acid monoesters have been
prepared by methanolysis of the alkyl trimethylsilyl diester.18
Three methods have been reported in which the monoester was de-
rived directly from the dibasic phosphonic acid. These three are the
reaction of a phosphonic acid with one-half molar equivalent of carbodi-
imide followed by addition of alcohol,19 reaction of the phosphonic acid
with trichloroacetonitrile and ethanol,20 and O-alkylation of a phos-
phonic acid bis(tetramethylammonium) salt.21 However, none of these
methods for preparing the monoesters of phosphonic acids are as simple
as the analogous method for preparing carboxylic acid esters, in which
the acid is esterified with an alcohol in the presence of an acid catalyst.
RESULTS AND DISCUSSION
Reported here is a simple preparative method, analogous to the Fischer
esterification reaction, for the direct monoesterification of alkyl- and
arylphosphonic acids with alcohols. This reaction is performed in the
presence of an arsenic catalyst, phenylarsonic acid at 3 mol%, while
azeotropically removing the water formed. In the absence of the ar-
sonic acid catalyst, or when it is substituted by p-toluenesulfonic acid,
no esterification occurs. As listed in Table I, the monoesters of both
alkylphosphonic and arylphosphonic acids were prepared in good-to-
excellent yield in this manner. This method was demonstrated to be
successful for the formation of the monoesters from both primary and
secondary alcohols, although the yields were greater with primary alco-
hols. Excess alcohol was required to obtain good yields when the boiling
point of the alcohol was less than that of the solvent. The higher-boiling
solvents, toluene and tetrachloroethylene, were found to give yields
of the monoesters superior to those from the lower-boiling solvents,
trichloroethylene and nitromethane.
Samples of isopropyl hydrogen methylphosphonate (1), butyl hydro-
gen methylphosphonate (2), and cyclohexyl hydrogen methylphospho-
nate (4) prepared as described here were found to be identical to authen-
tic samples prepared by the method of Petrov et al.7 1H- and 31P-NMR
(nuclear magnetic resonance) spectra for 1,1,11,18 2,1,18 4,1,10,11,18 7,5 and
813,22 agree well with those reported.

Alkyl- and Arylphosphonic Acid Monoesters
1511
TABLE I Reaction Conditions for the Preparation of Phosphonic
Acid Monoesters
Duration Yielda
No.
R
R’OH
Isopropanol
Equiv. R’OH
Solvent
(h)
(%)
1
Me
6.0
3.0
1.6
1.5
1.5
1.1
1.2
1.1
1.2
6.1
1.5
1.5
1.5
1.5
1.2
1.3
1.2
C2Cl4
76
76
28
28
28
28
28
97
28
76
28
28
28
28
28
19
28
64
26
18
90
50b
86
39
59
87
89
98
96
63b
58b
98
76
74
Toluene
Toluene
Toluene
C2HCl3
Toluene
Toluene
Toluene
Toluene
C2Cl4
2
Me
Butanol
3
4
5
6
7
8
Me
Me
Me
Et
Octadecanol
Cyclohexanol
[(1 S)-endo]-(-)-Borneol
2-Isopropoxyethanol
Ethanol
Ph
Ph
Butanol
Toluene
C2Cl4
C2HCl3
CH3NO2
Toluene
Toluene
Toluene
9
10
11
Ph
Ph
Hexanol
2-(Ethylthio)ethanol
PhCH2 2-Methoxyethanol
aIsolated and purified yields unless stated otherwise.
bYields based on 31P-NMR analysis in D2O.
To determine the extent that the diester could be produced under
these conditions, phenylphosphonic acid was reacted with 3.2 equiva-
lents of butanol and 3% arsenic catalyst (As2O5) in toluene. The reac-
tion was monitored, and the relative proportions of phenylphosphonic
acid, butyl hydrogen phenylphosphonate, and dibutyl phenylphospho-
nate were determined. Figure 1 shows the results from analysis of this
reaction. Phenylphosphonic acid is quickly consumed, and butyl hy-
drogen phenylphosphonate is produced in greater than 90% in 24 h.
Dibutyl phenylphosphonate is formed more slowly, and greater than
5 days are required for it to comprise more than 50% of the reaction
mixture, and about 20 days are required for it to reach 90%.
At this time, the mechanism of this reaction is not known. Clearly,
the arsonic acid is not acting as an acid catalyst. It seems likely that one
of two mechanisms is involved. In the first, the arsonic acid is esterified
with the alcohol, and then through transesterification the phosphonate
ester is formed, leaving the arsonic acid to become esterified again. The

1512
M. D. Crenshaw
FIGURE 1 Reaction composition from esterification of phenylphosphonic acid
with butanol as function of time.
second mechanism involves the formation of the mixed phosphonic-
arsonic anhydride, which then undergoes alcoholysis to yield the phos-
phonate ester and the arsonic acid. Investigations to determine the role
of the arsonic acid in the mechanism of this esterification reaction are
ongoing, as are studies to further the application of this reaction to
other phosphorus acids and the preparation of their aryl esters.
EXPERIMENTAL
All chemicals were purchased from Sigma-Aldrich (Milwaukee, WI,
USA), Pfaltz & Baur (Waterbury, CT, USA), or Lancaster Synthesis
(Windham, NH, USA). NMR spectra were obtained using a Varian Gem-
1
ini NMR spectrometer operating at 300 MHz for H and 121.5 MHz
1
for 31P. H-NMR are expressed in ppm relative to the internal stan-
dard, tetramethylsilane, and 31P-NMR are expressed in ppm relative
to the external standard, 85% H3PO4. 31P-NMR spectra are decoupled
with respect to protons. Prior to analysis by GC/MS, the monoesters
were methylated with trimethylsilyldiazomethane in the presence of
methanol to give the corresponding mixed alkyl methyl diester as de-
scribed below. Mass spectra (+EI) were obtained using a Shimadzu
QP-5000 GC/MS system. HRMS data was obtained from a VG 70S or a
VG AutoSpec Ultima S170 magnetic sector instrument.

Alkyl- and Arylphosphonic Acid Monoesters
1513
Procedure for the Preparation of Monoesters
of Alkyl- or Arylphosphonic Acids
In 200 ml of solvent were mixed alkyl- or arylphosphonic acid
(0.063 mol), phenylarsonic acid (3 mol%), and the selected alcohol (0.069
to 0.38 mol). The mixture was heated to reflux under an atmosphere of
dry nitrogen and the water/solvent azeotrope collected in a Dean-Stark
or similar receiver. After cooling, the solvent and any remaining volatile
alcohol were removed using a rotary evaporator. With the exception of
octadecyl methylphosphonic acid, the residue was mixed with water and
the pH adjusted to 13–14 with 2.5N NaOH. The alkaline solution was
then extracted with CHCl3. Concentrated HCl was then added to the
aqueous portion until the pH was 1. This acidic solution was extracted
with CHCl3. The combined CHCl3 extracts were dried over Na2SO4, fil-
tered, and the solvent removed using a rotary evaporator. Residual sol-
vent was removed under high vacuum (1 mmHg). Oils were not further
purified, while solids were recrystallized. Octadecyl methylphosphonic
acid was purified by removing the reaction solvent and recrystallizing
the residue. Purity of the products was found to be greater than 95%
1
based on H- and 31P-NMR analyses. 2-(Ethylthio)ethyl phenylphos-
phonic acid was determined to be 92% pure initially but degraded to
about 70% after six months of storage at 4◦C.
Procedure for Preparing Methyl Esters
of the Phosphonic Acids23
About 4 mg of phosphonic acid was dissolved in one milliliter of acetone
and then 100 µL of this solution was added to 700 µL of benzene and 200
µL of methanol. Addition of 20 µL of a 2.0 M solution of trimethylsilyl-
diazomethane in THF (tetrahydrofuran) at ambient temperature com-
pleted the methylation.
Isopropyl Hydrogen Methylphosphonate (1)
1H-NMR (CDCl3) ppm: 1.33 (6H, d, J = 6.2 Hz), 1.48 (3H, d, J =
17.9 Hz), 4.66 (1H, dsept, J = 8.3, 6.2 Hz), 11.49 (1H, s); 31P-NMR
(CDCl3) ppm: 33.07; MS (EI) of the methyl ester of 1 m/z 152 ([M]+·,
absent), 137 (31.11), 111 (87.46), 93 (100.00), 79 (25.96).
Butyl Hydrogen Methylphosphonate (2)
1H-NMR (CDCl3) ppm: 0.94 (3H, t, J = 7.3 Hz), 1.41 (2H, m), 1.50
(3H, d, J = 17.9 Hz), 1.66 (2H, m), 4.01 (2H, dt, J = 7.3, 6.6 Hz), 11.94
(1H, s); 31P-NMR (CDCl3) ppm: 34.02; MS (EI) of the methyl ester of
2 m/z 166 ([M]+·, absent), 137 (6.26), 111 (100.00), 93 (72.95), 79 (17.27).

1514
M. D. Crenshaw
Octadecyl Hydrogen Methylphosphonate (3)
1
m.p., 71–73◦C (isooctane); H-NMR (C6D6) ppm: 0.92 (3H, t, J =
6.7 Hz), 1.18–1.38 (33H, m) overlapping 1.23 (d, J = 17.7 Hz), 1.54 (2H,
m), 4.00 (2H, dt, J = 6.6, 7.3 Hz), 10.10 (1H, s); 31P-NMR (C6D6) ppm:
34.64; MS (EI) of the methyl ester of 3 m/z 362 ([M]+·, absent), 111
(100.00), 93 (9.51), 57 (14.49), 55 (21.96), 43 (35.76).
Cyclohexyl Hydrogen Methylphosphonate (4)
m.p., 37–39◦C (isooctane) [lit.7 m.p., 45–48◦C]; 1H-NMR (CDCl3)
ppm: 1.31 (3H, m), 1.45–16.0 (6H, m) overlapping 1.49 (d, J = 17.9
Hz), 1.75 (2H, m), 1.94 (2H, m), 4.37 (1H, m), 11.38 (1H, s). 31P-NMR
(CDCl3) ppm: 33.19; MS (EI) of the methyl ester of 4 m/z 192 ([M]+·,
0.03), 111 (100.00), 93 (41.27), 79 (13.63), 67 (21.31), 55 (17.34), 54
(22.73), 41 (37.66); HRMS: m/z: calc. for methyl ester of 4 (C8H17O3P):
192.0915. Found: 192.0894.
Bornyl Hydrogen Methylphosphonate (5)
m.p., 132–133◦C (isooctane) [lit.24 m.p., 129–131◦C]; 1H-NMR
(CDCl3) ppm: 0.87 (6H, s, br), 0.88 (3H, s), 1.25 (3H, m), 1.49 (3H, d,
J = 17.9 Hz), 1.66 (1H, m), 1.74 (1H, m), 1.95 (1H, m), 2.29 (1H, m),
4.55 (1H, m), 11.81 (1H, s); 31P-NMR (CDCl3) ppm: 34.10; MS (EI) of
the methyl ester of 5 m/z 246 ([M]+·, absent), 137 (38.77), 136 (30.33),
121 (32.98), 111 (70.34), 95 (30.95), 93 (100.00), 79 (25.74).
2-(Isopropoxy)ethyl Hydrogen Ethylphosphonate (6)
1H-NMR (CDCl3) ppm: 1.17 (6H, d, J = 6.1 Hz), 1.18 (3H, dt, J =
20.4, 7.7 Hz), 1.79 (2H, dq, J = 18.7, 7.7 Hz), 3.63 (3H, m), 4.13 (2H,
m), 11.36 (1H, s); 31P-NMR (CDCl3) ppm: 37.56; MS (EI) of the methyl
ester of 6 m/z 210 ([M]+·, absent), 151 (35.16), 138 (31.29), 125 (85.84),
110 (79.86), 107 (50.59), 79 (61.28), 58 (45.75), 43 (100.00).
Ethyl Hydrogen Phenylphosphonate (7)
1H-NMR (CDCl3) ppm: 1.29 (3H, dt, J = 7.0, 0.5 Hz), 4.07 (2H, dq,
J = 7.8, 7.0 Hz), 7.43 (2H, m), 7.53 (1H, m), 7.81 (2H, m), 12.18 (1H,
s); 31P-NMR (CDCl3) ppm: 21.04; MS (EI) of the methyl ester of 7 m/z
200 ([M]+·, 20.50), 173 (19.44), 172 (39.77), 171 (31.79), 155 (41.53), 141
(46.98), 91 (77.51), 79 (27.31), 78 (79.58), 77 (100.00); HRMS: m/z: calc.
for methyl ester of 7 (C9H13O3P): 200.0602. Found: 200.0596.
Butyl Hydrogen Phenylphosphonate (8)
1H-NMR (CDCl3) ppm: 0.86 (3H, t, J = 7.3 Hz), 1.36 (2H, m), 1.61
(2H, m), 3.99 (2H, dt, J = 6.9, 6.7 Hz), 7.42 (2H, m), 7.51 (1H, m), 7.79
(2H, m), 11.66 (1H, s); 31P-NMR (CDCl3) ppm: 21.82; MS (EI) of the

Alkyl- and Arylphosphonic Acid Monoesters
1515
methyl ester of 8 m/z 228 ([M]+·, 0.79), 173 (100.00), 155 (52.80), 141
(28.33), 91 (19.11), 78 (23.88), 77 (55.66); HRMS: m/z: calc. for methyl
ester of 8 (C11H17O3P): 228.0915. Found: 228.0905.
Hexyl Hydrogen Phenylphosphonate (9)
1H-NMR (CDCl3) ppm: 0.86 (3H, t, J = 6.7 Hz), 1.28 (6H, m), 1.64
(2H, m), 3.90 (2H, dt, J = 7.0, 6.7 Hz), 7.43 (2H, m), 7.53 (1H, m), 7.80
(2H, m), 12.22 (1H, s); 31P-NMR (CDCl3) ppm: 21.08; MS (EI) of the
methyl ester of 9 m/z 256 ([M]+·, 0.80), 173 (100.00), 155 (42.50), 141
(25.61), 77 (42.48); HRMS: m/z: calc. for methyl ester of 9 (C13H21O3P):
256.1228. Found: 256.1222.
2-(Ethylthio)ethyl Hydrogen Phenylphosphonate (10)
1H-NMR (CDCl3) ppm: 1.18 (3H, t, J = 7.4 Hz), 2.49 (2H, q, J =
7.4 Hz), 2.74 (2H, t, J = 7.1 Hz), 4.08 (2H, dt, J = 7.9, 7.1 Hz), 7.43
(2H, m), 7.52 (1H, m), 7.80 (2H, m), 11.96 (1H, s); 31P-NMR (CDCl3)
ppm: 20.55; MS (EI) of the methyl ester of 10 m/z 260 ([M]+·, absent),
173 (58.65), 155 (17.14), 88 (83.25), 77 (36.98), 60 (100.00).
2-(Methoxy)ethyl Hydrogen Benzylphosphonate (11)
1H-NMR (CDCl3) ppm: 3.07 (2H, d, J = 22.3 Hz), 3.32 (3H, s), 3.45
(2H, m), 3.91 (2H, m), 7.26 (5H, m), 11.27 (1H, s); 31P-NMR (CDCl3)
ppm: 30.20; MS (EI) of the methyl ester of 11 m/z 244 ([M]+·, 14.50), 187
(17.69), 186 (38.09), 153 (22.49), 109 (22.96), 104 (32.19), 91 (100.00), 65
(41.47); HRMS: m/z: calc. for methyl ester of 11 (C11H17O4P): 244.0864.
Found: 244.0853.
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