New synthesis of trimethylsilyl esters of phosphorus(III) acids

Article abstract of DOI:10.1007/s00706-019-02511-6

Abstract: A novel synthetically important reaction has been developed for the quick, convenient, and high-yield preparation of trimethylsilyl esters of phosphorus(III) acids, synthetically valuable Michaelis–Arbuzov reaction precursors. Commercially and synthetically available initial compounds used are derivatives of propan-2-ol phosphorylated in the second position capable of long-term storage and available silylating reagents: hexamethyldisilazane, bis(trimethylsilyl)acetamide, and diethyl(trimethylsilyl)amine. Also, a stepwise reaction scheme of the starting compounds with silylating reagents has been proposed. Graphic abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s00706-019-02511-6

Monatshefte für Chemie - Chemical Monthly
https://doi.org/10.1007/s00706-019-02511-6
ORIGINAL PAPER
New synthesis of trimethylsilyl esters of phosphorus(III) acids
Vasily Morgalyuk1 · Tatyana Strelkova1 · Valery Brel1
Received: 1 June 2019 / Accepted: 11 October 2019
© Springer-Verlag GmbH Austria, part of Springer Nature 2019
Abstract
A novel synthetically important reaction has been developed for the quick, convenient, and high-yield preparation of tri-
methylsilyl esters of phosphorus(III) acids, synthetically valuable Michaelis–Arbuzov reaction precursors. Commercially
and synthetically available initial compounds used are derivatives of propan-2-ol phosphorylated in the second position
capable of long-term storage and available silylating reagents: hexamethyldisilazane, bis(trimethylsilyl)acetamide, and
diethyl(trimethylsilyl)amine. Also, a stepwise reaction scheme of the starting compounds with silylating reagents has been
proposed.
Graphic abstract
Keywords Phosphorus(III) trimethylsilylether · Phosphorus compounds · Reaction mechanisms · Retro reactions
Introduction
compounds with trimethylchlorosilane [10], which requires
careful monitoring of the reaction conditions, or interaction
with hexamethyldisilazane in harsh conditions [11, 12] or
in the presence of catalysts [13]. Therefore, the search for
new methods for the synthesis of trimethylsilyl ethers of
III-valent phosphorus remains relevant.
Synthetic methods developed in the feld of organophospho-
rus chemistry are of great importance for modern organic,
medical, and pharmaceutical chemistry [1–4]. However,
the number of ways to create a phosphorus–carbon bond in
organophosphorus chemistry itself remains limited [5, 6].
One of the key reactions in the synthesis of organophos-
phorus compounds is the Michaelis–Arbuzov reaction. The
introduction of trimethylsilyl esters of phosphorus(III) acids
into it was shown to lead to better yields of target products
in comparison to alkyl analogs. Therefore, trimethylsilyl
esters of acids of III-valent phosphorus are popular precur-
sors of the Michaelis–Arbuzov reaction that are used for
the synthesis of organophosphorus compounds of various
structures [5–9]. However, the number of methods for pro-
ducing trimethylsilyl esters of III-valent phosphorus acids
remains limited by the interaction of hydrophosphoryl
Results and discussion
We showed previously [8, 9] that trimethylsilyl diphe-
nylphosphinite (1a), a promising precursor of the Michae-
lis–Arbuzov reaction, could be prepared quickly and in high
yield by the reaction of stable (1-hydroxy-1-methylethyl)-
diphenylphosphine oxide (2a) with silylating reagents: hex-
amethyldisilazane (HMDS), diethyl(trimethylsilyl)amine
(DETS), and bis(trimethylsilyl)acetamide (BSA) in 1,4-diox-
ane at 100–110 °C. Therefore, it is of practical importance to
study the possibility of using this reaction for preparing also
synthetically valuable trimethylsilyl phosphinites and tri-
methylsilyl phosphites by the example of ethyl trimethylsilyl
phenylphosphonite (1b) and diethyl trimethylsilyl phosphite
(1c) starting from phosphinate and phosphonate analogs of
* Vasily Morgalyuk
morgaliuk@mail.ru
1
Nesmeyanov Institute of Organoelement Compounds,
Russian Academy of Sciences, ul. Vavilova 28,
Moscow 119991, Russia
Vol.:(0123456789)
1 3

V. Morgalyuk et al.
phosphine oxide 2a: ethyl (1-hydroxy-1-methylethyl)phe-
nylphosphinate (2b) and diethyl (1-hydroxy-1-methylethyl)-
phosphonate (2c).
1b was obtained in 82% yield only when phosphinate 2b
reacted with DETS. The reaction of phosphinate 2b with
HMDS and BSA led only to ethyl phenylphosphonite 3b in
82% and 74% yields, respectively (Scheme 2).
Initial compounds 2a–2c were prepared in high yields
using modifed method [14] (Abramov reaction) by the reac-
tion of hydrophosphoryl compounds 3a–3c with acetone in
the presence of KF on Al2O3 support (KF/Al2O3) as a cata-
lyst [15] (Scheme 1).
Phosphonate 2c reacted with HMDS and DETS to give
expected diethyl trimethylsilyl phosphite 1c in 78% and 86%
yields, respectively. The reaction of 2c with BSA under
the same conditions aforded a product of silylation at the
hydroxy group, diethyl [1-(trimethylsiloxy)-1-methylethyl]-
phosphonate (4c) in 62% yield (Scheme 2).
Since it can be expected that the two reaction prod-
ucts—phosphonite 1b and phosphite 1c—are more volatile
than phosphinite 1a, all attempts to synthesize compounds
1a–1c, unlike the initial reaction [8, 9], were performed in
the absence of 1,4-dioxane to minimize the loss of the prod-
ucts upon isolation. For this purpose, and since the silylat-
ing reagents HMDS, DETS, and BSA were also solvents
(in this case the ratio of 2a–c to them was 1:2), they were
being introduced into the reaction at 100–110 °C for 30 min
(Scheme 2).
The observed reactions of 2a–2c with silylating reagents
provided a possibility to suggest a stage-to-stage scheme for
the synthesis of compounds 1a–1c:
(1) It is obvious for phosphinate 2b that the thermal cleav-
age of phosphorus-carbon bond in the presence of
HMDS and BSA (retro Abramov reaction) to form
hydrophosphonite (H-phosphinate) 3b proceeds faster
than the silylation of 2b. The resultant hydrophospho-
nite 3b undergoes no further silylation under the reac-
tion conditions. Considering this result as a general fea-
ture of all compounds 2a–2c, we can draw a conclusion
that hydrophosphoryl compounds 3a–3c (Scheme 2)
are not intermediate products in the reaction of com-
pounds 2a–2c with silylating reagents (Scheme 3, route
1). It is also known from the literature that the interac-
tion of hydrophosphoryl compounds with silylating rea-
gents takes place only under fairly harsh conditions or
when interacting with hexamethyldisilazane, however,
in rather harsh conditions (when heated to 110° C for
2 h in an inert atmosphere [11, 12] or in the presence
of catalysts [13]).
Under these conditions, phosphine oxide 2a reacts, as
expected, with HMDS, DETS, and BSA to produce phos-
phinite 1a. The yield of 1a increases from 79% [8] to 82%
(HMDS), from 80% [8] to 89% (DETS) and from 71% [8] to
87% (BSA) when no solvent is used (Scheme 2).
A more complex situation is observed for the reaction
of phosphinate 2b and phosphonate 2c with the silylating
reagents. Expected ethyl trimethylsilyl phenylphosphonite
Scheme 1
(2) The reaction of phosphonate 2c with BSA leads to
the silylation of the hydroxy group to give diethyl
[(1-trimethylsiloxy)-1-methyl]phosphonate 4c, prob-
Scheme 2
1 3

New synthesis of trimethylsilyl esters of phosphorus(III) acids
Scheme 3
ably due to reduced nucleophilicity of the phosphoryl
group in 2c comparing to compounds 2a and 2b.
Resulting silyl ester 4c remains unchanged under reac-
tion conditions. This fact enables us to conclude that
silyl esters 4a–4c, like hydrophosphoryl compounds
3a–3c, are not intermediate products in the synthesis
of fnal compounds 1a–1c from 2a–2c (Scheme 3). To
confrm this assumption, we synthesized phosphonate
4c and its analogs, diphenyl[1-(trimethylsilyloxy)-
1-methylethyl]phosphine oxide 4a and ethyl phenyl[1-
(trimethylsiloxy)-1-methylethyl]phosphinate 4b, by the
reaction of silyl esters 1a–1c with acetone in the pres-
ence of CuBr (Scheme 4).
Conclusion
Thus, we have described a novel reaction (Scheme 3, route
3) which at the same time is a new, fast and convenient
method for the synthesis of silyl esters of phosphorus(III)
acids 1a–1c, reactive precursors of the Michaelis–Arbuzov
reaction. Additional advantage of the proposed method is
the availability of initial compounds 2a–2c (Scheme 1). This
method allows isolation of compounds 2a–2c from the reac-
tion mixture as analytically pure products. All initial com-
pounds 2a–2c can be stored without special precautions for
a long time without degradation.
Silyl esters 4a and 4b, like 4c, were found to be
unchanged under reaction conditions (in the presence of
HMDS or BSA) (Scheme 3, route 2). This indicates that,
as we supposed earlier [8], the stage-to-stage scheme for
the synthesis of fnal silyl esters 1a–1c begins with silyla-
tion of the phosphoryl group of initial compounds 2a–2c
to produce zwitterions 5a–5c whose subsequent rearrange-
ment afords fnal compounds 1a–1c (Scheme 3, route 3).
Experimental
1H, 31P, and 13C NMR spectra (CDCl3, 25 °C) were obtained
on a Bruker Avance 400 spectrometer operating at 400.13,
161.98, and 100.05 MHz, respectively. CDCl3 was distilled
over P2O5 and stored over K2CO3 at 0 °C in the dark. All
syntheses were conducted under an inert atmosphere. Cat-
alyst KF/Al2O3 was prepared by the procedure described
in the literature [15]. Elemental analysis for C, H, and N
Scheme 4
1 3

V. Morgalyuk et al.
was performed on a Carlo Erba 1106 automated analyzer.
Elemental analysis for P and Si was carried out by spectro-
photometry on a Cary 100 Scan instrument.
The reaction of phosphine oxide 2a with DETS (87%
yield of phosphinite 1a) and BSA (89% yield of phosphinite
1a) was performed in a similar manner. All obtained samples
of trimethylsilyl diphenylphosphinite 1a were combined.
Colorless needles; m.p.: 24–26 °C (Ref. [8, 9] 24–26 °C);
b.p.: 110–112 °C at 1 Torr (Ref. [8, 9] 110–112 °C at
Diethyl (1‑hydroxy‑1‑methylethyl)phosphonate (2c) Cata-
lyst KF/Al2O3 (1 g) was added to a mixture of 5 g of diethyl
phosphite (4c, 4.65 cm3, 36.5 mmol) and 5.3 g of acetone
(6.76 cm3, 91.2 mmol) under stirring. An exothermal reac-
tion began after 3–5 min. The reaction mixture tempera-
ture was maintained at 20–25 °C using external cooling.
After heat evolution ceased, the mixture was stirred at
20 °C for three units. The catalyst was separated by fl-
tration, washed with acetone (2 × 5 cm3), the solvent was
removed from the fltrate at 40 °C and 14 Torr to give 6.6 g
(92%) of phosphonate 2c. B.p.: 133–135 °C at 14 Torr
(Ref. [16] 132–134 °C at 14 Torr); 1H NMR (400.13 MHz,
1
1 Torr); H NMR (400.13 MHz, CDCl3): δ = 7.56–7.51
(m, 4H, o–H, Ph), 7.38–7.30 (m, 6H, m,p–H, Ph), 0.27 (s,
9H, Si(CH3)3) ppm; 31P{1H} NMR (161.98 MHz, CDCl3):
δ=94.90 s ppm [8].
The following reactions were performed in similar
manner:
Reaction of ethyl (1‑hydroxy‑1‑methylethyl)phenylphosphi‑
nate (2b) with DETS The yield of phosphonite 1b is 82%;
b.p.: 85–87 °C at 1 Torr (Ref. [19] 53 °C at 0.05 Torr);
1H NMR (400.13 MHz, CDCl3): δ = 7.59–7.56 (m, 2H,
o–H, Ph), 7.43–7.38 (m, 3H, m,p–H, Ph), 4.83 (ddq,
3
CDCl3): δ = 4.61 (s, 1H, OH), 4.07 (ddd, JH,P = 7.08 Hz,
2JH,H =10.16 Hz, 3JH,H =7.08 Hz, 4H, 2O-CH2-CH3), 1.38
(d, 3JH,P =15.56 Hz, 6H, 2CH3), 1.33 (t, 3JH,H =7.08 Hz, 6H,
2O-CH2-CH3) ppm; 31P{1H} NMR (161.98 MHz, CDCl3):
δ=27.65 s ppm.
2
3
3JH,P = 7.32 Hz, JH,H = 10.16 Hz, JH,H = 7.00 Hz, 1H,
O-CH2-CH3), 3.97 (ddq, 3JH,P =7.32 Hz, 2JH,H =10.16 Hz,
3JH,H =7.00 Hz, 1H, O-CH2-CH3), 1.21 (t, 3JH,H =7.00 Hz,
3H, O-CH2-CH3), 0.28 (s, 9H, Si(CH3)3) ppm; 31P{1H}
NMR (161.98 MHz, CDCl3): δ=145.32 s ppm [19].
Ethyl (1‑hydroxy‑1‑methylethyl)phenylphosphinate
(2b) Ethyl (1-hydroxy-1-methylethyl)phenylphosphinate
(2b) was obtained in a similar manner. Yield 94%; m.p.:
Reaction of phosphinate 2b with HMDS and BSA The yield
of ethyl phenylphosphonite 3b is 81% (HMDS) and 74%
(BSA). All obtained samples of ethyl phenylphosphinate 3b
were combined. B.p.: 96–98 °C at 1 Torr (Ref. [20] 94–99 °C
at 1 Torr); 1H NMR (400.13 MHz, CDCl3): δ=7.71–7.65
(m, 2H, o–H, Ph), 7.51–7.47 (m, 1H, m-H, Ph), 7.43–7.38
1
94–96 °C (Ref. [17] 94–96 °C); H NMR (400.13 MHz,
CDCl3): δ = 7.81–7.88 (m, 2H, o–H, Ph), 7.55–7.52 (m,
1H, p–H, Ph), 7.47–7.42 (m, 2H, m-H, Ph), 4.16 (ddq,
2
3
3JH,P = 7.08 Hz, JH,H = 10.20 Hz, JH,H = 7.08 Hz, 1H,
O-CH2-CH3), 3.97 (ddq, 3JH,P =7.08 Hz, 2JH,H =10.16 Hz,
3JH,3H =7.08 Hz, 1H, O-CH2-CH3), 3.51 (s, 1H, OH), 1.39
1
(m, 2H, p–H, Ph), 7.49 (d, JP,H = 563 Hz, 1H, P–H),
3
(d, JH,P = 14.20 Hz, 3H, CH3), 1.35 (d, JH,P = 14.16 Hz,
4.12–3.97 (m, 2H, O-CH2-CH3,), 1.27 (t, 3H, O-CH2-CH3,
3JH,H = 7.06 Hz) ppm [21]; 31P{1H} NMR (161.98 MHz,
CDCl3): δ=24.55 s ppm (Ref. [22] 23.5 ppm).
3H, CH3), 1.33 (t, 3JH,H =7.08 Hz, 3H, O-CH2-CH3) ppm;
31P{1H} NMR (161.98 MHz, CDCl3): δ=43.45 s ppm.
(1‑Hydroxy‑1‑methylethyl)diphenylphosphine oxide
(2a) (1-Hydroxy-1-methylethyl)diphenylphosphine oxide
(2a) was conducted in a similar manner, 92% yield of 2a as
colorless needles. M.p.: 136–138 °C (Ref. [18] 137–139 °C);
1H NMR (400.13 MHz, CDCl3): δ=8.02–7.96 (m, 4H, o–H,
Ph), 7.51–7.47 (m, 2H, m-H, Ph), 7.44–7.40 (m, 4H, p–H,
Ph), 3.61 (bs, 1H, OH), 1.40 (d, 3JP,H =13.56 Hz, 6H, 2CH3)
ppm; 31P{1H} NMR (161.98 MHz, CDCl3): δ=34.59 s ppm.
Reaction of diethyl (1‑hydroxy‑1‑methylethyl)phospho‑
nate (2c) with HMDS and DETS The yield of diethyl tri-
methylsilyl phosphite 3c is 78% (HMDS) and 86% (DETS).
B.p.: 62–65 °C at 14 Torr (Ref. [23] 61–63 °C at 14 Torr);
1H NMR (400.13 MHz, CDCl3): δ = 3.88–3.75 (m, 4H,
3
2CH2), 1.26 (t, JH,H = 7.04 Hz, 6H, 2CH3-CH2), 0.24 (s,
9H, Si(CH3)3) ppm; 31P{1H} NMR (161.98 MHz, CDCl3):
δ=127.51 s ppm.
Reaction of (1‑hydroxy‑1‑methylethyl)diphenylphosphine
oxide (2a) with HMDS HMDS (6.13 g, 38.2 mmol) was
added to 5.0 g of phosphine oxide 2a (19.2 mmol) and the
mixture was heated at 110 °C. Phosphine oxide 2a com-
pletely dissolved after 10 min. The mixture was heated
for additional 20 min, cooled to 25 °C, excess HMDS was
removed at 14 Torr. The residue was distilled twice under
reduced pressure to give 4.31 g (82%) of trimethylsilyl
diphenylphosphinite (1a).
Reaction of diethyl (1‑hydroxy‑1‑methylethyl)phosphonate
(2c) with BSA The yield of diethyl [1-(trimethylsiloxy)-
1-methylethyl]phosphonate (4c) is 62%. B.p.: 95–98 °C
1
at 1 Torr (Ref. [24] 70–72 °C at 0.07 Torr); H NMR
3
(400.13 MHz, CDCl3): δ = 4.10 (dq, JH,P = 7.08 Hz,
3JH,H =7.08 Hz, 4H, 2O-CH2-CH3), 1.42 (d, 3JH,P =15.60 Hz,
3
6H, 2CH3), 1.27 (t, JH,H = 7.10 Hz, 6H, 2O-CH2-CH3),
0.12 (s, 9H, Si(CH3)3) ppm; 31P{1H} NMR (161.98 MHz,
CDCl3): δ=25.24 s ppm [20].
1 3

New synthesis of trimethylsilyl esters of phosphorus(III) acids
Synthesis of diethyl [1‑(trimethylsiloxy)‑1‑methylethyl]‑
phosphonate (4c) Copper(I) bromide (0.13 g, 0.9 mmol,
5 mol %) was added with stirring to a solution of 3.2 g of
diethyl trimethylsilyl phosphite (1c, 18.2 mmol) in 4.2 g of
acetone (5.4 cm3, 72.4 mmol). After 15–20 min, the tem-
perature of the reaction mixture increased to 30–35 °C and
20–30 min later became 20 °C. The mixture was stirred for
additional 3 h at 20 °C, the excess acetone was removed,
20 cm3 of CH2Cl2 was added, the mixture was washed
with 5% aqueous NH3 (2×5 cm3) and 5 cm3 of water. The
organic layer was separated and dried with Na2SO4. The dry-
ing agent was separated by fltration, washed with 2×5 cm3
of CH2Cl2, the combined organic layer was concentrated
at 14 Torr, and the residue was purifed by distillation to
give 2.98 g (68%) of phosphonate 4c. B.p., 1H NMR, and
31P{1H} NMR of compound 4c synthesized by this method
coincide with those of 4c, synthesized by reaction compound
2c with BSA.
16.56 (d, 3JC,P =5.83 Hz, O-CH2-CH3), 2.40 (s, Si(CH3)3)
ppm.
Acknowledgements This work was performed with the fnancial sup-
port from the Ministry of Science and Higher Education of the Russian
Federation using the Equipment of Center for Molecular Composition
studies of INEOS RAS.
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4H, o–H, Ph), 7.44–7.34 (m, 6H, m,p–H, Ph), 1.42 (d,
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31P{1H} NMR (161.98 MHz, CDCl3): δ=31.76 s ppm; 13
C
NMR (100.05 MHz, CDCl3): δ=132.58 (d, 2JC,P =7.3 Hz,
4
o-C, Ph), 131.49 (d, JC,P = 2.9 Hz, p–C, Ph), 130.45 (d,
3
1JC,P = 91.9 Hz, ipso-C, Ph), 127.99 (d, JC,P = 10.9 Hz,
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1
m-C, Ph), 75.42 (d, JC,P = 97.0 Hz, P-C1), 25.10 (d,
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Ethyl [1‑(trimethylsilyloxy)‑1‑methylethyl]phenylphosphi‑
1
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9H, Si(CH3)3) ppm; 31P{1H} NMR (161.98 MHz, CDCl3):
δ=42.31 s ppm; 13C NMR (100.05 MHz, CDCl3): δ=133,62
(d, 2JC,P =8.75 Hz, o-C, Ph), 132.05 (d, 4JC,P =2.92 Hz, p–C,
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Publisher’s Note Springer Nature remains neutral with regard to
jurisdictional claims in published maps and institutional afliations.
3
Ph), 128.01 (d, JC,P = 116.72 Hz, ipso-C, Ph), 127.86 (d,
1
3JC,P = 11.68 Hz, m-C, Ph), 73.78 (d, JC,P = 127.66 Hz,
3
P-C1), 60.99 (d, JC,P = 7.29 Hz, O-CH2-CH3), 25.31 (d,
3JC,P =7.29 Hz, C1-CH3), 24.64 (d, 3JC,P =3.64 Hz, C1-CH3),
1 3