Synthesis of phosphonates from phenylphosphonic acid and its monoesters

Article abstract of DOI:10.1080/00397911.2019.1637894

Possibilities for the mono- and diesterification of phenylphosphonic acid were evaluated considering the microwave(MW)-assisted direct esterification, and the alkylating esterification. It was found that regarding the monoesterification, the reaction with 15-fold alcohol excess in the presence of [bmim][BF₄] additive utilizing MWs is superior than the approach by alkylation. At the same time, for the conversion of the monoester intermediate to the diester, the reaction with alkyl halides in the presence of triethylamine as the base, again under MW irradiation, was found to be the method of choice. Phosphonates with both identical and different alkoxy groups were made available.

Full text of DOI:10.1080/00397911.2019.1637894

Synthetic Communications
An International Journal for Rapid Communication of Synthetic Organic
Chemistry
ISSN: 0039-7911 (Print) 1532-2432 (Online) Journal homepage: https://www.tandfonline.com/loi/lsyc20
Synthesis of phosphonates from
phenylphosphonic acid and its monoesters
Réka Henyecz, Adrienn Kiss, Virág Mórocz, Nóra Zsuzsa Kiss & György
Keglevich
To cite this article: Réka Henyecz, Adrienn Kiss, Virág Mórocz, Nóra Zsuzsa Kiss & György
Keglevich (2019): Synthesis of phosphonates from phenylphosphonic acid and its monoesters,
Synthetic Communications, DOI: 10.1080/00397911.2019.1637894
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https://doi.org/10.1080/00397911.2019.1637894
Synthesis of phosphonates from phenylphosphonic acid
and its monoesters
R ꢀe ka Henyecz, Adrienn Kiss, Vir ꢀa g M oꢀ rocz, N oꢀ ra Zsuzsa Kiss, and Gy o€ rgy Keglevich
Department of Organic Chemistry and Technology, Budapest University of Technology and Economics,
Budapest, Hungary
ABSTRACT
ARTICLE HISTORY
Possibilities for the mono- and diesterification of phenylphosphonic
acid were evaluated considering the microwave(MW)-assisted direct
esterification, and the alkylating esterification. It was found that
regarding the monoesterification, the reaction with 15-fold alcohol
Received 24 May 2019
KEYWORDS
Alkylation; esterification;
microwave; phosphonate;
phosphonic acid;
4
excess in the presence of [bmim][BF ] additive utilizing MWs is
superior than the approach by alkylation. At the same time, for the
conversion of the monoester intermediate to the diester, the reac-
tion with alkyl halides in the presence of triethylamine as the base,
again under MW irradiation, was found to be the method of choice.
Phosphonates with both identical and different alkoxy groups were
made available.
phosphonic monoester
GRAPHICAL ABSTRACT
Introduction
Phosphinic and phosphonic derivatives, especially the corresponding phosphinates and
phosphonates may be important intermediates in organophosphorus chemistry. The
“
classical” method for their synthesis involves the reaction of phosphinic chlorides and
[
1–5]
phosphonic dichlorides with alcohols.
However, the use of P-chlorides cannot be
regarded “green”, and the introduction of the P¼O group via the chloro derivatives is
not atomic efficient. Keglevich and Kiss were who elaborated the microwave(MW)-
[
6,7]
assisted direct esterification of phosphinic acids.
To eliminate the shortcoming
ꢁ
meant by the relatively high temperature (ꢀ200 C) required, the procedure was refined
further. It was found that certain ionic liquid (IL) additives had a beneficial effect
CONTACT Gy €o rgy Keglevich
gkeglevich@mail.bme.hu
Department of Organic Chemistry and Technology,
Budapest University of Technology and Economics, Budapest, 1521, Hungary.
Supplemental data for this article can be accessed publisher’s website.
ß 2019 The Author(s). Published with license by Taylor & Francis Group, LLC
This is an Open Access article distributed under the terms of the Creative Commons Attribution-NonCommercial-NoDerivatives License
(http://creativecommons.org/licenses/by-nc-nd/4.0/), which permits non-commercial re-use, distribution, and reproduction in any medium,
provided the original work is properly cited, and is not altered, transformed, or built upon in any way.

2
R. HENYECZ ET AL.
[
9]
Scheme 1. Earlier method for the synthesis of dialkyl phenylphosphonates.
allowing a lower reaction temperature, shorter reaction time and complete conver-
[
8]
sions. As regards the preparation of dialkyl phenylphosphonates, a three-step proced-
ure starting from phenyl-H-phosphinic acid (I) was developed (Scheme 1). According to
this, phosphinic acid I was subjected to MW-assisted direct esterification, the alkyl phe-
nyl-H-phosphinates (II) so obtained were converted to the corresponding phenylphos-
phonic-acid-esters (III) by oxidation using meta-chloroperbenzoic acid, and finally
[
9]
monoesters III were esterified utilizing MWs.
However, we were not satisfied with this method, since the oxidation step was critical,
as the purification of monoester III could not be solved perfectly, and the overall yield
of the 3-step procedure was rather poor. As another possibility, we wished to start from
phenylphosphonic acid. The preliminary experiments carried out in the presence of
[
2]
[
bmim] ionic liquids as the catalyst were encouraging in respect of monoesterification.
For this, we wished to develop a generally applicable method for the conversion of
phenylphosphonic acid to dialkyl phenylphosphonates.
Results and discussion
Esterification of phenylphosphonic acid
MW-assisted direct esterification of phenylphosphonic acid with alcohols
First, we wished to overview and extend our MW-assisted method elaborated for the
monoesterification of phenylphosphonic acid. It was found that irradiating the mixture
ꢁ
of phenylphosphonic acid and 15 equivalents of butanol or octanol at 200/220 C for
4
h, the conversions were not complete, and the corresponding monoesters (2a and 2c,
respectively) were the main components (92/75%) beside the diesters (3a and 3c)
[
9]
formed as minor by-products (Table 1, entries 1 and 5). According to our recent
finding, the use of 10% of [bmim][BF ] allowed complete conversion at a lower reaction
4
ꢁ
temperature of 180 C after a shorter reaction time of 45 min (Table 1, entries 2 and 6).
In these cases, the monoesterifications were more selective, and products 2a and 2c
[
10]
were isolated in a yield of 82% and 90%, respectively.
Our procedure seemed to be
of a more general value, as it worked also for the monoesterification with ethanol.
ꢁ
Volatility of EtOH allowed a maximum reaction temperature of 165 C, and after an
irradiation of 8 h in the presence of the ionic liquid additive, ethyl ester 2b was obtained
in a yield of 70% (Table 1, entry 4). The comparative experiment carried out in the
absence of the additive led to a low conversion of 30% (Table 1, entry 3).
O-Alkylation of phenylphosphonic acid
Then, the alkylating esterification was evaluated as another alternative. Phenylphosphonic
acid was reacted with 1 equivalent of butyl bromide in the presence of triethylamine

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Table 1. Direct esterification of phenylphosphonic acid with different alcohols under MW conditions.
a
Composition
a
Conversion
(%)
Monoester
2 (%)
Diester
3 (%)
Yield of the
monoester (%)
ꢁ
R
IL
T ( C)
t (h)
Ref.
[9]
Entry
b
Bu
Bu
Et
–
200
180
165
165
220
180
4
0.75
8
8
4
83
100
30
82
96
100
92 (2a)
95 (2a)
98 (2a)
94 (2b)
75 (2c)
96 (2c)
8 (3a)
5 (3a)
2 (3a)
6 (3b)
25 (3c)
4 (3c)
75 (2a)
82 (2a)
1
2
3
4
5
6
c
[10]
10% [bmim][BF
4
4
4
]
]
]
–
Et
10% [bmim][BF
–
10% [bmim][BF
70 (2b)
–
90 (2c)
[9]
Oct
Oct
a
[10]
0.75
31
On the basis of relative P NMR intensities.
No change in the conversion on further irradiation.
In a comparative thermal experiment, the conversion was 22% [composition: 93% (2a) and 7% (3a)].
b
c
Table 2. Attempted monoalkylation of phenylphosphonic acid with butyl bromide under differ-
ent conditions.
a
Composition
ꢁ
a
Base
Et
PTC
T ( C)
t (h)
Conversion (%)
Monoester 2a (%)
Diester 3a (%)
Entry
3
N
–
80 (MW)
0 (D)
00 (MW)
4
4
58
47
62
7
76
85
61
71
24
15
39
29
1
2
3
4
8
b
1
2
c
2
K CO
3
TEBAC (5%)
80 (MW)
4
a
31
Based on relative P NMR intensities.
b
No change on further irradiation.
c
There was no reaction in the comparative thermal experiment.
ꢁ
ꢁ
under MW irradiation. However, both at 80 C for 4 h, and at 100 C for 2 h, the
conversion remained incomplete (58/62%), and the selectivity of the formation of the
monoester (2a) was only 76/61%, as 24/39% of the dibutylester (3a) was also formed
(Table 2, entries 1 and 3). The comparative thermal experiment led to a lower conversion
of 47% (Table 2, entry 2). In the solid–liquid phase transfer catalytic (PTC) version
applying K CO as the base, the conversion was even lower (7%) (Table 2, entry 4).
2
3
It was envisaged that the use of BuBr in excess may promote the formation of dibutyl
phenylphosphonate (3a). For this, the alkylation of phenylphosphonic acid was per-
ꢁ
formed at 100 C for 4 h using 2.2 equivalents of BuBr in the presence of TEA on MW
irradiation. Sure enough, the expected diester (3a) was the major component (68%)
together with 32% of the monoester (2a), but the conversion was not complete
ꢁ
(
Table 3, entry 1). Elevating the temperature to 120 and 150 C did not help (Table 3,

4
R. HENYECZ ET AL.
Table 3. Attempted dialkylation of phenylphosphonic acid with butyl bromide under differ-
ent conditions.
a
Composition
Monoester
2a (%)
Yield of the
diester (%)
ꢁ
a
Base
Et
PTC
BuBr (equiv.) T ( C) Conversion (%)
Diester 3a (%)
Entry
3
N
–
2.2
100
120
150
120
120
95
94
94
100
11
32
35
39
17
73
68
65
61
83
27
1
2
3
4
5
2
2
5
.2
.2
69 (3a)
K
2
CO
3
TEBAC (5%)
2.2
a
31
Based on relative P NMR intensities.
entries 2 and 3). However, a drastic increase of the molar ratio of the phosphonic acid
1) and BuBr to 1:5 led to a complete conversion, and to a better selectivity of 83% for
(
the dibutylester (3a) that was isolated in a yield of 69% (Table 3, entry 4). The phase
transfer catalytic version applying K CO as the base was not successful, as the conver-
2
3
sion was rather low (11%), moreover the monoester (2a) was the major product
Table 3, entry 5).
(
Esterification of phenylphosphonic monoesters
It was shown that the monoesters of phenylphosphonic acid (2) may be best synthesized
by MW-assisted direct esterification in the presence of [bmim][BF ] as the catalyst. It
4
was a question for us, if the monoesters (2) prepared can be esterified further to afford
the diesters (3).
MW-assisted direct esterification of phenylphosphonic acid monoesters
The first series of the diesterification attempts involved MW-assisted direct esterification
of the monoesters (2). Without an IL additive, the MW-promoted reaction of the
corresponding PhP(O)(OR)(OH) with 15-fold ROH took place in only low conversions.
ꢁ
Esterification of monobutylester 2a with BuOH at 180 or 220 C for 6 h led to a conver-
sion of 7% and 17%, respectively (Table 4, entries 1 and 2), while the similar reaction
ꢁ
of the monooctyl derivative (2c) with octanol at 200, 220 and 235 C proceeded with
conversions of 22%, 27% and 28%, respectively (Table 4, entries 5–7). Repetitions in the
ꢁ
ꢁ
presence of 10% of [bmim][BF ] were more successful at 220 C for 6 h, and at 235 C
4
for 3 h, but the conversions of 45% and 72% still meant incomplete transformations
(
Table 4, entries 3 and 8) that could not be increased. The use of [bmim][PF ] instead
6
of [bmim][BF ] was not helpful (Table 4, entries 4 and 9). It can be seen that the
4
monoesters (2) cannot be converted efficiently to the diesters (3) by MW-assisted direct
esterification.

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Table 4. Direct esterification of phenylphosphonic monoesters.
ꢁ
a,b
R
IL
T ( C)
t (h)
Conversion (%)
Entry
Bu (2a)
–
–
180
220
220
220
200
220
235
235
235
6
6
6
6
6
6
6
3
3
7 (3a)
17 (3a)
45 (3a)
40 (3a)
22 (3c)
27 (3c)
28 (3c)
72 (3c)
65 (3c)
1
2
3
4
5
6
7
8
9
c
1
1
4
0% [bmim][BF ]
0% [bmim][PF ]
6
Oct (2c)
–
–
–
c
1
0% [bmim][BF
0% [bmim][PF ]
4
]
1
6
a
31
Based on relative P NMR intensities.
b
Average of three parallel measurements.
No increase on further irradiation.
c
Table 5. Synthesis of dialkyl phenylphosphonates by the alkylation of phenylphosphonic monoesters.
ꢁ
R
RX
Base
Et
Solvent
PTC
T ( C)
t (h)
Yield (%)
Entry
b
a
Bu (2a)
BuBr
3
N
–
MeCN
–
MeCN
–
–
85
100
85
100
100
100
0.5
80 (3a)
75 (3a)
92 (3b)
76 (3b)
72 (3c)
58 (3c)
1
2
3
4
5
6
a
K
2
CO
3
3
3
TEBAC (5%)
–
TEBAC (5%)
–
1
Et (2b)
Oct (2c)
a
EtI
Et
3
N
CO
0.5
a
K
2
1
OctBr
Et
3
N
1
1
K
2
CO
MeCN
TEBAC (5%)
No change on further irradiation.
b
In the comparative thermal experiment the yield was 57%.
Alkylating esterification of phenylphosphonic monoesters
The other option was to try the monoester (2) ! diester (3) conversion by O-alkyl-
ation. Hence, the monoesters (2) were first reacted with the corresponding alkyl halides
ꢁ
in the presence of triethylamine and in the absence of any solvent at 85 C for 30 min
under MW irradiation. The alkylation of half-esters 2a, 2b and 2c with BuBr, EtI and
OctBr furnished diesters 3a, 3b and 3c in yields of 80%, 92%, and 72%, respectively
(
Table 5, entries 1, 3 and 5). The alternative protocol applying K CO3 and
2
tetrabutylammonium chloride (TEBAC) in acetonitrile as the solvent provided

6
R. HENYECZ ET AL.
Table 6. Synthesis of phenylphosphonates with different alkyl groups by the alkylation of phenyl-
phosphonic monoesters.
0
ꢁ
R
R X
EtI
Base
Et
Solvent
PTC
T ( C)
t (h)
Yield (%)
Entry
b
Bu (2a)
3
N
–
MeCN
–
MeCN
–
MeCN
–
MeCN
–
–
85
100
85
100
85
100
85
100
85
100
0.5
1
71 (3d)
62 (3d)
68 (3e)
56 (3e)
48 (3f)
38 (3f)
72 (3g)
54 (3g)
54 (3h)
37 (3h)
72 (3d)
67 (3d)
1
2
3
4
5
6
7
8
9
b
K
2
CO
N
3
3
3
3
3
3
TEBAC (5%)
n
PrBr
Et
3
–
0.5
1
K
2
CO
TEBAC (5%)
i
a
PrBr
Et
3
N
–
0.5
K
Et
2
CO
TEBAC (5%)
1
0.5
1
n
Et (2b)
PrBr
3
N
–
K
2
CO
TEBAC (5%)
–
TEBAC (5%)
–
i
PrBr
Et
3
N
1
a
K
Et
2
CO
MeCN
–
MeCN
1
10
11
12
BuBr
3
N
85
100
0.5
1
K
2
CO
TEBAC (5%)
a
No change on further irradiation.
Diethyl phenylphosphonate by-product was formed at a higher temperature.
b
the diesters (3a–c) in somewhat lower yields of 58–76% (Table 5, entries 2, 4 and 6).
Worth of mentioning that the comparative thermal experiment resulted in ester 3a in a
lower yield of 57% (Table 5, entry 1/footnote b). It is concluded that the MW-assisted
alkylating esterification is more advantageous for the conversion of the monoesters (2)
to the diesters (3), than the MW-promoted direct esterification.
Finally, we aimed at the synthesis of phenylphosphonates with different alkyl groups.
In the first series of experiments, the monobutyl ester of phenylphosphonic acid 2a was
i
reacted with EtI, PrBr and PrBr in the presence of triethylamine without any solvent at
ꢁ
8
5 C. After a 0.5 h’s irradiation, work-up and purification, the corresponding phospho-
nates with two different alkyl groups (3d, 3e and 3f) were obtained in yields of 71%,
8%, and 48%, respectively (Table 6, entries 1, 3 and 5). The similar alkylation of the
6
i
monoethyl ester of phenylphosphonic acid 2b with PrBr, PrBr and BuBr almost as
above, furnished diesters 3g, 3h and 3d, in yields of 72%, 54%, and 72%, respectively
i
(
Table 6, entries 7, 9 and 11). The reaction with the hindered PrBr required a longer
reaction time of 1 h. In the second round, monoesters 2a and 2b were alkylated in the
ꢁ
presence of K CO and TEBAC in acetonitrile at 100 C for 1 h under MW. After work-
2
3
up and chromatography, the solid–liquid accomplishments provided the mixed diesters
d–h in yields of 37–67% (Table 6, entries 2, 4, 6, 8, 10 and 12). The preparations in
3
homogeneous phase were more efficient in all cases than the heterogeneous-phase
syntheses. It is noteworthy that no matter if the butyl ethyl diester (3d) was prepared
from the monobutyl ester (2a) or from the monoethylester (2b), the outcomes were
comparable (See Table 6, entry 1 vs. entry 11, and entry 2 vs. entry 12).

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Scheme 2. The new protocol elaborated for the synthesis of dialkyl phenylphosphonates.
Taking into account all experiences described above, the 2-step conversion including
a MW-assisted direct esterification in the first step, and an alkylating esterification in
the second step seems to be the optimum solution for the stepwise esterification of
phenylphosphonic acid (Scheme 2).
Conclusions
The possibilities for the conversion of phenylphosphonic diacid to the monoester
PhP(O)(OR)(OH) and to the dialkyl phosphonates were explored. Principally, both the
monoesterification, and the esterification of the monoester may be performed either by
MW-assisted reaction with an alcohol, or by alkylation with an alkyl halide. The latter
step may be achieved in homogeneous or heterogeneous phase using triethylamine or
K CO as the base, respectively. It was found that while for the monoesterification, the
2
3
MW-assisted and [bmim][BF ]-catalyzed direct esterification is the method of choice,
4
the conversion of the monoester to the dialkyl phosphonate may be performed best by
alkylation in the homogeneous phase. Phosphonates with both identical and different
alkyl groups could be prepared by the novel 2-step protocol.
Experimental
General procedure for the direct esterification of phenylphosphonic acid (1)
A mixture of 0.10 g (0.63 mmol) of phenylphosphonic acid (1) and 9.45 mmol of an
alcohol (0.55 mL of ethanol, 0.87 mL of n-butanol, or 1.5 mL of n-octanol) was meas-
ured in a sealed tube, and irradiated in the MW reactor applying 50–150 W irradiation
in the presence of 0.012 mL (0.060 mmol) [bmim][BF ] at the temperatures and for the
4
times shown in Table 1. (The pressure developed was in the range of 1–18 bar.) The
excess of alcohol was removed under reduced pressure, and the residue purified by
column chromatography (silica gel/ethyl acetate) to afford products 2a–c in yields
shown in Table 1.
General procedure for the alkylating esterification of phenylphosphonic acid
monoesters (2)
To 0.75 mmol of phenylphosphonic acid monoalkyl ester (2a: 0.16 g, 2b: 0.14 g, 2c:
0.20 g) was added 0.75 mmol of an alkyl halide (0.060 mL of ethyl iodide, 0.068 mL of
propyl bromide, 0.070 mL of isopropyl bromide, 0.081 mL of butyl bromide, or 0.13 mL
of octyl bromide) and 0.12 mL (0.83 mmol) of triethylamine. The contents of the closed
vial were irradiated in the MW reactor at 10–20 W. The resulting mixture was
filtered, and removal of the volatile components provided the crude product that was

8
R. HENYECZ ET AL.
purified by column chromatography (Brockmann I type neutral aluminum oxide/ethyl
acetate–hexane) to afford diesters 3a–h in purities of >99% as colorless oils (Table 5,
entries 1, 3 and 5, as well as Table 6, entries 1, 3, 5, 7, 9 and 11).
Esters 3d–h are new compounds, while derivatives 2a–c and 3a–c have been
[
11–15]
described in the literature.
See the Supplementary Information for the
characterization data of products 2a–c and 3a–h.
Funding
This project was sponsored by the National Research, Development and Innovation Office
(K119202). R. Henyecz is grateful for the fellowship provided by the Gedeon Richter’s Talentum
Foundation. N.Z.Kiss was supported by the J ꢀa nos Bolyai Research Scholarship of the Hungarian
Academy of Sciences (BO/00130/19/7).
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[
1] Quin, L. D. A Guide to Organophosphorus Chemistry; Wiley: New York, 2000. ISBN: 978-
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0
[
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[
[
[
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Assisted Direct Esterification Steps. Curr. Org. Synthesis 2014, 11, 767–772. doi:10.2174/
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