Phosphine catalyzed addition of long-chain dialkyl phosphites to electron-deficient alkenes

Article abstract of DOI:10.1080/00397911.2020.1799015

Conjugate addition of the long chain dialkyl phosphites to electron-deficient alkenes under PBu₃ catalysis afforded the corresponding phosphonates in good yields in a short reaction time. Long chain alkyl groups in the phosphites were well tolerated in this transformation. The products of this reaction are of particular interest as components of lubrication oil composition.

Full text of DOI:10.1080/00397911.2020.1799015

Synthetic Communications
An International Journal for Rapid Communication of Synthetic Organic
Chemistry
ISSN: (Print) (Online) Journal homepage: https://www.tandfonline.com/loi/lsyc20
Phosphine catalyzed addition of long-chain dialkyl
phosphites to electron-deficient alkenes
Anton Il’in , Arthur Gubaev , Anna Antonova , Arthur Khannanov & Vladimir
Galkin
To cite this article: Anton Il’in , Arthur Gubaev , Anna Antonova , Arthur Khannanov & Vladimir
Galkin (2020): Phosphine catalyzed addition of long-chain dialkyl phosphites to electron-deficient
alkenes, Synthetic Communications, DOI: 10.1080/00397911.2020.1799015
To link to this article: https://doi.org/10.1080/00397911.2020.1799015
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https://doi.org/10.1080/00397911.2020.1799015
Phosphine catalyzed addition of long-chain dialkyl
phosphites to electron-deficient alkenes
Anton Il’in , Arthur Gubaev , Anna Antonova, Arthur Khannanov, and
Vladimir Galkin
A.M. Butlerov Institute of Chemistry, Kazan Federal University, Kazan, Russian Federation
ABSTRACT
ARTICLE HISTORY
Received 13 June 2020
Conjugate addition of the long chain dialkyl phosphites to electron-
deficient alkenes under PBu3 catalysis afforded the corresponding
phosphonates in good yields in a short reaction time. Long chain
alkyl groups in the phosphites were well tolerated in this transform-
ation. The products of this reaction are of particular interest as com-
ponents of lubrication oil composition.
KEYWORDS
Long-chain dialkyl
phosphites; phospha-
Michael addition;
phosphine-catalyzed reac-
tions; phosphonates;
Pudovik reaction
GRAPHICAL ABSTRACT
Introduction
The carbon–phosphorus bond formation is the fundamental reaction in organophos-
phorus chemistry. One of the classical approaches for its construction is the Pudovik
reaction, also known as the phospha-Michael addition, to be found in the conjugate
addition of nucleophiles to electron-deficient alkenes. These processes are an efficient
way to produce a variety of functionalized products using simple and easily available
starting materials.[1] Usually the addition of P(O)–H bonds to electron-deficient alkenes
proceeds in the presence of basic catalysts—the alkali metal alkoxides in alcohol solu-
tions. It is well known that the addition of the long chain dialkyl phosphites to activated
C¼C bonds in the presence of alkali metal alkoxides proceeds less satisfactory and com-
plicated by transesterification reactions.[2]
Due to the intriguing possibilities for metabolic regulation or perturbation the phos-
phonic acids and their derivatives have been used as analogues of naturally occurring
phosphates.[3] Phosphonic esters not only exert pivotal biological activities, but have
also found a wide range of important industrial and practical applications. Noteworthy,
CONTACT Anton Il’in
antonilin.1989@mail.ru
A.M. Butlerov Institute of Chemistry, Kazan Federal University,
Kremlevskaya Str. 18, Kazan 420008, Russian Federation.
Supplemental data for this article can be accessed on the publisher’s website.
ß 2020 Taylor & Francis Group, LLC

2
A. V. IL’IN ET AL.
the phosphonates containing long chain alkoxy groups at the phosphorus atom are of
particular interest in terms of extraction of rare earth metals and natural carboxylic
acids.[4] Currently, the properties of compounds containing P–C bond have attracted
the attention of several well-known oil refining and lubricants creating companies. The
phosphonic acids and their derivatives that have long chain linear alkoxy groups (9–18
carbon atoms) at phosphorus atom are a part of lubricating oil composition for a slid-
ing guide surface.[5] A lower coefficient of friction, better abrasion resistance, and
extreme pressure properties can be achieved using the compositions mentioned above.
Lubricating oil compositions comprising refrigerants and phosphonic acid esters can
be used with polyalkylene glycols, ester oils and fluorinated oils for refrigerators.[6,7]
Diundecyl phosphite and homologs can be included in the composition of the photo-
sensitive resin as an antioxidant,[8] and can also be used as a polymer modifier.[9]
Therefore, the development of methods for the preparation of phosphonates contain-
ing long chain alkoxy groups at the phosphorus atom is of interest. In this work, the
Pudovik reaction was chosen as the main method for obtaining these compounds.
Currently, many alternative methods for catalysis of the Pudovik reaction have been
developed, the use of which avoids undesirable transesterification reactions. Strongly
basic amines and their derivatives,[10] as well as inorganic bases[11] are often used as a
catalyst for these reactions. The Pudovik reaction can be carried out in the presence of
radical initiators,[12] transition metal complexes,[13] Brønsted/Lewis acids,[14] microwave
irradiation,[15] tertiary phosphines,[16] etc.
In this article, the dialkylphosphonates with long-chain alkoxy groups at the phos-
phorus atom were obtained by the phosphine-catalyzed Pudovik reaction, because long-
chain alkoxy groups in the dialkyl phosphite were well tolerated in this
transformation.[17]
Result and discussion
The alkoholysis reaction of dialkyl phosphites is widely used for the preparation of less
common dialkyl phosphites with alkyl chains of higher carbon atom number.[18]
Diundecyl and didecyl phosphites were obtained by a modified literature procedure
(Scheme 1).[19] This direct route (Scheme 1) provided the compounds in good yield
(80–82%). The full experimental procedure is presented in Supplementary Material.
To increase the product yield, ethanol was distilled off from the reaction mixture.
The oil bath temperature was gradually increased from 185 to 230 ꢀC.
Next, the resulting dialkylphosphonates were introduced into the phosphine-catalyzed
addition of alkenes activated by electron-withdrawing groups through double bonds.
Usually phosphine-catalyzed hydrophosphorylation reactions of electron-deficient
alkenes using bis(2-ethylhexyl) phosphite, diheptyl phosphite, and di-i-butyl phosphite
easily proceeded under mild conditions developed for lower dialkyl phosphites.
As previously shown,[17] polar solvents such as MeCN, DMF can significantly facili-
tate the phosphine catalyzed Pudovik reaction; the best results were obtained using
acetonitrile as a solvent. The reactions of the long-chain dialkyl phosphites with elec-
tron-deficient alkenes in acetonitrile were difficult due to low solubility of the phospho-
nates in this solvent. Therefore, an exploratory study using model reactions of

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SYNTHETIC COMMUNICATIONSV
3
Scheme 1. Preparation of diundecyl 1b and didecyl phosphites 1c.
Table 1. Overview of catalysts and solvents for the reaction of diundecyl phosphite with methyl
acrylate and methyl methacrylate.
Entry
Catalyst (%)
Alkene
Solvent
Time (h)
Conversiona (%)
1
2
3
4
5
6
7
8
PBu3 (10)
PBu3 (75)
PBu3 (10)
PBu3 (5)
2a
2b
2b
2a
2b
2a
2b
2a
2b
2a
2b
2a
2b
2a
2b
2a
2b
2a
2b
DMF
DMF
DMF
DMF
DMF
i-PrOH
i-PrOH
DMSO
DMSO
C6H6
1
3
3
1
3
1
3
1
3
1
3
1
3
1
3
1
3
1
3
89
64
traces
81
54
88
35
7
8
4
6
0
0
6
8
0
0
2
16
PBu3 (50)
PBu3 (10)
PBu3 (75)
PBu3 (10)
PBu3 (75)
PBu3 (10)
PBu3 (75)
PBu3 (10)
PBu3 (75)
PBu3 (10)
PBu3 (75)
PPh3 (10)
PPh3 (75)
P(NMe2)3 (10)
P(NMe2)3 (75)
9
10
11
12
13
14
15
16
17
18
19
C6H6
Hexane
Hexane
CH2Cl2
CH2Cl2
DMF
DMF
DMF
DMF
aConversion determined by 31P NMR.
diundecyl phosphite 1b with methyl acrylate 2a and methyl methacrylate 2b in different
solvents was conducted (Table 1; Scheme 2).
The products yield in reactions with methyl methacrylate 2b was highly sensitive to
the solvent. In the reaction of methyl acrylate 2a the best results were obtained using
isopropyl alcohol and dimethylformamide as solvents, but when isopropyl alcohol was
used in reactions with methyl methacrylate 2b, the conversion to the product was
greatly reduced. Probably, this is due to the interception of the zwitterion formed dur-
ing the reaction by a proton donor solvent, which reduces the rate of the reaction.
Then the reactions were carried out with the participation of various tertiary phos-
phines as catalysts. Less nucleophilic triphenylphosphine failed to catalyze this addition.
The use of hexamethyltriaminophosphine P (NMe2)3 as a catalyst led to a low conver-
sion of methyl methacrylate to the phosphorylation product. However, this phosphine
failed to catalyze the reaction with methyl acrylate (Table 1). The best results were
obtained when the reactions were performed using nucleophilic aliphatic phosphine-tri-
n-butylphosphine (PBu3) as a catalyst.
With the optimized conditions in hand, we found that didecyl and diundecyl phos-
phites readily reacted with activated alkenes affording anticipated products 11–20 in

4
A. V. IL’IN ET AL.
Scheme 2. Model reactions of diundecyl phosphite 1b with methyl acrylate 2a and methyl methacryl-
ate 2b.
Scheme 3. PBu3-catalyzed addition reaction of the long chain dialkyl phosphites 1b, 1c to
alkenes 2a–e.
Table 2. Substrate scope for the PBu3-catalyzed addition of the long chain dialkyl phosphites to acti-
vated alkenes.
Entry
R1
R2
R3
Y
% PBu3
Conversiona (%)
Time (h)
Yieldb (%)
3a
3b
3c
3d
3e
3f
3g
3h
3i
C11H23
C11H23
C11H23
C11H23
C11H23
C10H21
C10H21
C10H21
C10H21
C10H21
C11H23
C11H23
H
Me
H
H
H
H
H
H
H
H
H
H
H
CO2Me
CO2Me
CN
CONH2
CO2Me
CO2Me
CO2Me
CN
CONH2
CO2Me
CO2Me
CO2Me
10
75
10
35
50
10
75
10
35
89
64
76
94
65
79
72
74
88
1
3
1
1
2
1
3
1
1
82
59
70
90
60
74
67
68
84
69
0
CH2CO2Me
H
Me
H
H
3j
3k
3l
CH2CO2Me
H
Me
Ph
50
100
100
75
trace
trace
2
48
48
H
H
0
aConversion determined by 31P NMR;
bIsolated yield.
good yields. (Scheme 3). The results are indicated in Table 2. The reaction is compatible
with other commonly used electron-withdrawing groups, such as CN and CONH2 and
C(O)OR. The reactivity of a-substituted esters methyl acrylate and dimethyl itaconate
was noticeably lower than unsubstituted alkenes. This lower activity is in agreement
with the known reluctance of a-substituted activated alkenes to undergo other phos-
phine-catalyzed reactions.[20] The reaction rate was successfully improved by using
higher concentrations of PBu3. As seen from Table 2, typically 10–75 mol % of the cata-
lyst was required for these reactions depending on the reactivity activated alkene to

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SYNTHETIC COMMUNICATIONSV
5
Scheme 4. Proposed reaction mechanism.
furnish corresponding addition products in good yields in a short reaction times
(0.5–3 h). The reaction was monitored by 31P NMR spectroscopy.
Unfortunately, attempts to involve b-substituted activated alkenes trans-methyl croto-
nate and trans-methyl cinnamate into phosphine-catalyzed reaction were unsuccessful.
Even under harsh reaction conditions (DMF, under Ar, 48 h, 100 mol % of PBu3), the
target phosphonates formed only in trace amounts (Table 2. Entry 3k, 3l). This result
was expected, since the inertness of b-substituted alkenes in phosphine-catalyzed reac-
tions is well known. Most likely the inertness of b-substituted activated alkenes in phos-
phine-catalyzed reactions is generally explained by slow addition of tertiary phosphines
to these substrates due to steric hindrance of the terminal carbon atom.[19]
A possible mechanism of the long-chain dialkyl phosphites to electron-deficient
alkenes addition under PBu3 is presented in Scheme 4. As in the case of other phos-
phine-catalyzed processes, the reaction starts from the nucleophilic addition of tertiary
phosphine to the electron-deficient double bond to form phosphonium zwitterion A.[19]
It is most likely that the dependence of the product yield on the catalyst concentration
is caused by the increase in the zwitter-ionic intermediate concentration, which was
favored by an increase in the tertiary phosphine amount in the mixture. Hereafter, this
intermediate deprotonates the phosphites 1b or 1c and generates reactive species C. By
all appearances, the more acidic trivalent three-coordinated tautomeric form of the
phosphites 1b, 1c takes part in this stage.
The resulting anion C adds to electron-deficient alkene to give anion D. This anion
deprotonates the next molecule of compound 1b, 1c with the formation of the reaction

6
A. V. IL’IN ET AL.

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SYNTHETIC COMMUNICATIONSV
7
product and anion C, which re-participates in the reaction with the unsaturated deriva-
tive, thereby carrying out a catalytic cycle. Another pathway involves deprotonation of
phosphonium intermediate B that also yields the final product E and recovers the cata-
lyst from zwitterion A.
As mentioned earlier, multifunctional phosphonates containing long alkoxy groups at
the phosphorus atom are of particular interest as a component of lubrication oil com-
position. The use of lubricating oils normally involves heating, so the investigations of
their thermal characteristics and stability are very important to support their industrial
applications. Thermogravimetric analysis (TGA) (Fig. 1) and first derivative of weight
loss (DTG data is presented in Supplementary material) are widely used to evaluate the
thermal properties of materials and to show the mechanism by which a material loses
weight as a result of controlled heating.
This data shows the TGA curves of the phosphonates 3a–j. The degradation started
at 200 ꢀC and continued to above 300 ꢀC with a weight loss of more than 50 wt.%. The
corresponding weight loss can be referred to a complex process including loss of long
alkyl chain. Apparently, weight loss at 120–200 ꢀC of sample 3e associated with insuffi-
ciently high purification of this compound. The remaining compounds are stable in this
range. In the future, it will allow us to study their properties oil additives.
Conclusion
In summary, the PBu3-catalyzed addition of the long-chain dialkyl phosphites to elec-
tron-deficient alkenes has been developed. This methodology provides efficient access to
a range of practically useful phosphonates under neutral conditions with high atom
economy. The synthesis procedure is simple to implement and allows you to remove
the catalyst from the reaction mixture. Further research will focus on the use of sub-
stances 3a–j as additives for lubricating oils.
Experimental
General considerations
Materials and methods
All solvents and reagents used in the synthesis were supplied by Sigma Aldrich,
Acros Organics or Alfa Aesar and were of the highest quality available. All the electron-
deficient alkenes except for acrylamide were redistilled under reduced pressure in the
presence of small amounts of hydroquinone immediately before use. Acrylamide was
recrystallized from ethyl acetate.
NMR spectra were recorded at room temperature in CDCl3 solution on a Brucker
1
1
Avance III 400 spectrometer (400 MHz for H, 100.6 MHz for 13Cf Hg, 161.9 MHz for
31Pf Hg). 85% H3PO4 was used as external reference (0.0 ppm) for 31Pf Hg NMR spec-
tra. IR spectra were recorded on a Spectrum two PERKIN ELMER spectrometer using
an UATR (Single Reflection Diamond) for the Spectrum Two sampling surface. Mass
spectrometry was performed on a AB Sciex Triple TOF 5600 spectrometer with samples
dissolved in MeOH, with positive ion polarity.
1
1

8
A. V. IL’IN ET AL.
Thin-layer chromatography (TLC) was carried out on commercial silica gel plates
(Sorbfil, F254) and visualized using UV light (254 nm) and/or iodine. Column chroma-
tography was performed on silica gel (Acros Organics, 0.060–0.200 mm, 60 Å).
General reaction procedure for the synthesis of phosphonates 3a–j
To a stirring mixture diundecyl or didecyl phosphite (5 mmol (1.95 g diundecyl or
1.81 g didecyl phosphite), 3 ml of DMF) with appropriate amount of PBu3 (10–75 mol
%, see Table 2; 0.10 g—10 mol%, 0.35 g—35 mol%, 0.50 g—50 mol%, 0.76 g—75 mol%
PBu3) during 15 min was added dropwise 5 mmol of an activated alkene (0.43 g methyl
acrylate, 0.50 g methyl methacrylate, 0.27 g acrylonitrile, 0.36 g acrylamide, 0.79 g
dimethyl itaconate) in 7 ml of DMF; more or less noticeable heat evolution was
observed in each reaction. Then the stirring was continued for 1–3 h (see Table 2). The
reaction was monitored by 31P NMR spectroscopy. After completion, the DMF from
the filtrate was removed under reduced pressure, and compounds 3a–c, 3e–h and 3j
were purified by column chromatography (EtOAc/Hex). PBu3 can be recovered by vac-
uum distillation from the corresponding fraction (Rf ¼ 0.99).
In case of products 3d and 3i obtained from acrylamide, the residue was washed with
cold Et2O to give white solid, filtered off, and dried in vacuo. PBu3 extracted by Et2O
can be recovered by vacuum distillation.
Methyl 3-(bis(undecyloxy)phosphoryl)propanoate (3a)
1
Yield 82%, colorless oil, Rf ¼ 0.42 (EtOAc:C6H14 1:1).  NMR: d (ppm) 0.85 (t, 6,
3J
¼ 6.7 Hz, ‘ ‘3), 1.20–1.36 (m, 32, O‘ ‘2(‘2)8), 1.62 (quintet, J
3

2
2

¼ 6.8 Hz, 4 O‘ ‘2), 2.00–2.10 (m, 2, ‘ ‘2), 2.53–2.62 (m, 2,
2
2
13
1
‘ ‘2), 3.68 (s, 3, OCH3), 3.95–4.03 (m, 4, POCH2). ‘f Hg NMR: d (ppm)
2
1
14.2 (s, ‘ ‘3), 21.0 (d, JP‘ ¼ 144.8 Hz, ‘ ‘2), 22.8 (s, ‘2), 25.6 (s, ‘2),
2
2
2
27.5 (d, JP‘ ¼ 3.9 Hz, ‘ ‘2), 29.3 (s, ‘2), 29.4 (s, ‘2), 29.6 (s, ‘2), 29.7
2
3
(broadened s, overlapped two ‘2), 30.6 (d, JP‘ ¼ 6.2 Hz, O‘ ‘2), 32.0 (s,
2
3
2
‘2), 52.1 (s, Ž‘3), 66.0 (d, JP‘ ¼ 6.6 Hz, PO‘2), 172.8 (d, JPC ¼ 18.8 Hz,
‘¼Ž). 31Pf Hg NMR: d (ppm) 30.0. IR ꢀ, cmꢁ1: 1743 (C¼O, strong); 1244 (P¼Ž,
1
strong); 996 (P–O–C, strong). HRMS (ESI): calcd. for C26H54O5P [M þ H]: 477.3709,
found: 477.3709.
Supplemental data of this article includes experimental detail for synthesis the phos-
1
1
1
phites 1b, c and phosphonates 3b–3j, copies of H, 13Cf Hg, 31Pf Hg NMR spectra for
3a–j, and DTG data of compounds 3a–j. This material can be found via the
“Supplementary Content” section of this article’s webpage.
Disclosure statement
The authors declare no conflict of interest.
Funding
The reported study was funded by the Russian Federation President Grant [ŒR-1822.2020.3].
The support of subsidy allocated to Kazan Federal University for the State Assignment in the
Sphere of Scientific Activities [0671-2020-0063] is also gratefully acknowledged by V.I.G.

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SYNTHETIC COMMUNICATIONSV
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ORCID
Anton Il’in
Arthur Gubaev
http://orcid.org/0000-0002-2706-9898
http://orcid.org/0000-0001-5119-8449
Vladimir Galkin
http://orcid.org/0000-0003-1035-3427
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