Aerobic Co-/ N-Hydroxysuccinimide-Catalyzed Oxidation of p-Tolylsiloxanes to p-Carboxyphenylsiloxanes: Synthesis of Functionalized Siloxanes as Promising Building Blocks for Siloxane-Based Materials

Article abstract of DOI:10.1021/jacs.8b12600

Synthesis of organosilicon products with a "polar" functional group within organic substituents is one of the most fundamentally and practically important challenges in today's chemistry of silicones. In our study, we suggest a solution to this problem, viz., a high-efficiency preparative method based on aerobic Co-/N-hydroxysuccinimide (NHSI) catalyzed oxidation of p-tolylsiloxanes to p-carboxyphenylsiloxanes. This approach is based on "green", commercially available, simple, and inexpensive reagents and employs mild reaction conditions: Co(OAc)₂/NHSI catalytic system, O₂ as the oxidant, process temperature from 40 to 60 °C, atmospheric pressure. This reaction is general and allows for synthesizing both mono- and di-, tri-, and poly(p-carboxyphenyl)siloxanes with p-carboxyphenyl groups at 1,1-, 1,3-, 1,5-, and 1,1,1-positions. All the products were obtained and isolated in gram amounts (up to 5 g) and in high yields (80-96%) and characterized by NMR, ESI-HRMS, GPC, IR, and X-ray data: p-carboxyphenylsiloxanes in crystalline state form HOF-like structures. Furthermore, it was shown that the suggested method is applicable for the oxidation of organic alkylarene derivatives (Ar-CH₃, Ar-CH₂-R) to the corresponding acids and ketones (Ar-C(O)OH and Ar-C(O)-R), as well as hydride silanes ([Si]-H) to silanols ([Si]-OH). The possibility of synthesizing monomeric (methyl) and polymeric (siloxane-containing PET analogue, Sila-PET) esters based on 1,3-bis(p-carboxyphenyl)disiloxane was studied. These processes occur with retention of the organosiloxane frame and allow to obtain the corresponding products in 90 and 99% yields.

Full text of DOI:10.1021/jacs.8b12600

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Article
Aerobic Co- / N-hydroxysuccinimide- catalyzed oxidation of p-
tolylsiloxanes to p-carboxyphenylsiloxanes: synthesis of functionalized
siloxanes as promising building blocks for siloxane-based materials
Irina K. Goncharova, Kseniia P. Silaeva, Ashot V. Arzumanyan, Anton A.
Anisimov, Sergey A. Milenin, Roman A. Novikov, Pavel N. Solyev, Yaroslav V.
Tkachev, Alexander D. Volodin, Alexander A. Korlyukov, and Aziz M. Muzafarov
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b12600 • Publication Date (Web): 08 Jan 2019
Downloaded from http://pubs.acs.org on January 9, 2019
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Aerobic Co- / N-hydroxysuccinimide- catalyzed oxidation of p-
tolylsiloxanes to p-carboxyphenylsiloxanes: synthesis of
functionalized siloxanes as promising building blocks for siloxane-
based materials.
Irina K. Goncharova,a Kseniia P. Silaeva,a,b Ashot V. Arzumanyan,*a Anton A. Anisimov,a Sergey
A. Milenin,c Roman A. Novikov,d,f Pavel N. Solyev,d Yaroslav V. Tkachev,d Alexander D. Volodin,a
Alexander A. Korlyukova,e and Aziz M. Muzafarova,c
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a
Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, 28 Vavilova St., Moscow
b
119991, Russian Federation. Mendeleev University of Chemical Technology of Russia, 9 Miusskaya Sq., Moscow
125047, Russian Federation c Enikolopov Institute of Synthetic Polymeric Materials, Russian Academy of Sciences, 70
d
Profsoyuznaya St., Moscow 117393, Russian Federation Engelhardt Institute of Molecular Biology, Russian
Academy of Sciences, 32 Vavilova St., Moscow 119991, Russian Federation Pirogov Russian National Research
Medical University, 1 Ostrovityanov St., Moscow 117997, Russian Federation Zelinsky Institute of Organic
e
f
Chemistry, Russian Academy of Sciences, 47 Leninsky Pr., Moscow 119991, Russian Federation
ABSTRACT: Synthesis of organosilicon products with a “polar” functional group within organic substituents is one of the
most fundamentally and practically important challenges in today’s chemistry of silicones. In our study we suggest a
solution to this problem, viz., a high-efficiency preparative method based on aerobic Co / N-hydroxysuccinimide (NHSI)
catalyzed oxidation of p-tolylsiloxanes to p-carboxyphenylsiloxanes. This approach is based on “green”, commercially
available, simple and inexpensive reagents and employs mild reaction conditions: Co(OAc)2 / NHSI catalytic system, О2 as
о
the oxidant, process temperature from 40 to 60 С, atmospheric pressure. This reaction is a general and allows for
synthesizing both mono- and di-, tri-, and poly(p-carboxyphenyl)siloxanes with p-carboxyphenyl groups at 1,1-, 1,3-, 1,5-
and 1,1,1-positions. All the products were obtained and isolated in gram amounts (up to 5 g) and in high yields (80 – 96%),
and characterized by NMR, ESI-HRMS, GPC, IR and X-Ray data: p-сarboxyphenylsiloxanes in crystalline state form HOF-
like structures. Furthermore, it was shown that the suggested method is applicable for the oxidation of organic alkylarene
derivatives (Ar–CH3, Ar–CH2–R) to the corresponding acids and ketones (Ar–C(O)OH and Ar–C(O)–R), as well as hydride
silanes ([Si]–H) to silanols ([Si]–OH). The possibility of synthesizing monomeric (methyl) and polymeric (siloxane-
containing PET analogue, Sila-PET) esters based on 1,3-bis(p-carboxyphenyl)disiloxane was studied. These processes
occur with retention of the organosiloxane frame and allow one to obtain the corresponding products in 90 and 99%
yields.
to attract special attention of chemists and material
scientists. This is due to one of the central problems of
the modern chemistry of organosiloxanes, namely, the
preparation of organosilicon products with a “polar”
(−С(O)OH, −OH, −NH2, etc.) functional group in an
organic substituent.
INTRODUCTION
Organosilicon compounds, and especially materials
based thereon, i.e., silicones (siloxanes), are among the
most popular products that find use in the most
important fields of today’s science, technology, medicine,
aviation, aerospace industry and construction. This is due
to the unique spectrum of the physicochemical properties
of siloxanes: low glass transition point and high thermal
stability (wide range of working temperatures), high
hydrophobicity, gas permeability, biocompatibility,
radiation stability, low dielectric constant, etc. 1
The fact is that incorporation of a “polar” function into
organosilicon compounds opens quite unique
opportunities for their subsequent modification and
2
preparation of new copolymers, MOFs,3 HOFs,4 etc. In
addition, modification by incorporation of polar groups
would also allow other problems to be solved, namely, the
low mechanical strength and “incompatibility” of silicones
with organic polymers (polyesters, polyamides, etc.).5
The possibility of synthesizing hybrid and composite
materials based on siloxanes with integration of the
characteristics mentioned above allowed unique materials
to be created. However, the potential of their application
has not been revealed completely to date and continues
Moreover, functionalized organosilicon monomers are
promising building blocks in organic synthesis6 where a
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Page 16 of 25
new C–Hal, C–O, C–N or C–C bonds are to be created rather expensive and toxic reagents (these methods have
1
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8
(Hiyama, Hiyama-Denmark couplings, etc., which occur
through cleavage of a Si–C-bond).7
not been implemented in a catalytic variant); or rather
complex and time-consuming experimental techniques
and harsh reaction conditions that have a degrading
effect on the organosilicon core (using oxidants, acids,
bases, etc.). All these factors generally impose restrictions
on the synthetic approaches.
To date, only
a
few successful examples of
incorporating “polar” functional groups into an organic
substituent in siloxanes have been reported. On the one
hand, these examples confirm the possibility of such a
synthesis, but, on the other hand, they show the
limitations of the real synthetic methods aimed at solving
this problem. 8
In one of our previous studies, we made an attempt to
improve
the
conditions
for
synthesizing
p-
9
carboxyphenylsiloxanes using the MnCl2/tBuOOH
oxidizing system. It showed that this transformation is
possible in the [М]-catalyzed version, though it was not
very successful.14 d
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Using classical methods for synthesizing organosilicon
monomers and subsequently, polymers based thereon do
not allow one to obtain functionalized organosilicon
substrates,9,10 with rare exceptions.11 As a rule, these
methods are either suitable just for a narrow range of
substrates, or being excessively time-consuming, require
multiple stages involving the “attaching/removal” of
protective groups, and present other disadvantages.
Thus, the problem of development of new highly
efficient
methods
for
synthesizing
p-
carboxyphenylsiloxanes based on “green”, simple and
available reagents/conditions15 remains unresolved to
date.
However, it might seem that “polar” functional groups
can be easily incorporated into finished organosilicon
products. Despite appearing to be simple, this problem
has not been solved so far, except in rare cases.12 The
classic methods for the incorporation of functional groups
into an organic substituent used in organic chemistry
prove to be inefficient for organosilicon substrates due to
relatively harsh reaction conditions where these
substrates decompose with cleavage of Si−O, Si−Cl, Si−N
and Si−C bonds to give hardly separable product
mixtures, with ultimately low yields of the target product.
In recent years there have been an increasing number
of publications on the oxidation of organic compounds
involving molecular oxygen as a “green”, simple and
available oxidant that allows one to obtain valuable
products widely used in science, technology and
medicine. Furthermore,
a
number of industrially
important processes are based on aerobic oxidation:
production of K/A oil, benzoic acid, terephthalic acid,
phenol and acetone, Waker process, etc.16 However, the
processes indicated above generally occur with low
selectivity and under rather drastic conditions (elevated
temperature, high pressure, etc.).
The most promising results were obtained in syntheses
of such functionalized organosilicon compounds as p-
carboxyphenylsiloxanes using following main approaches:
(A) modification of p-bromophenylsiloxanes (Prof Davies,
Prof. Lickiss et al);12c,13 (B) oxidation of p-tolylsiloxanes
(Prof. Unno et al; Scheme 1).12a,14
Still, the use of the [M]-/Organo-catalyst combination
allowed to solve these problems, i.e., considerably soften
the reaction conditions and achieve high regio- and
chemoselectivity, in the aerobic liquid-phase oxidation of
the С–Н and Si–H groups.17 In fact, this approach was
utilized to implement the production of adamantanediol
Scheme 1. The main approaches to the synthesis of
p-carboxyphenylsiloxanes
and –triol that are used as components of photoresist
17a,b
polymer materials.
Moreover, we have shown in our
Prof. Lickiss, Prof. Davies,
Prof. Unno et al
This work: Aerobic Oxidation
recent study that it is principally possible to perform the
aerobic M-/Organo-catalyzed oxidation of a Si–H group
to a Si–OH group with retention of the organosiloxane
core. 17g
HO
O
HO
O
X
Conditions
Co(OAc)2 / NHSI, O2
Thus, the purpose of this study was to develop a
method of aerobic [M]-/Organo-catalyzed oxidation of p-
tolylsiloxanes to p-carboxyphenylsiloxanes (Scheme 1).
(A) and (B)
MeCN, 40 - 60 o
C
[Si]
[Si]
[Si]
X = CH3
Conditions:
(A) X = Br
(B) X = CH3
1. CuCN, H2O, NaOH
2. BuLi, CO2
1. CrO3, AcOH/Ac2O, H2SO4
2. KMnO4 / Pyr
3. NBS / AgNO3 / HCO2H
RESULTS AND DISCUSSION
At the first stage of our study, we performed a detailed
optimization of the conditions of aerobic oxidation of p-
tolylsiloxane 1c to p-carboxyphenylsiloxane 2c, then the
optimum conditions were extended to other p-
tolylsiloxanes 1a-b,d-g. Additionally, we estimated the
applicability of this method to organic toluene derivatives
1h-o and hydride silanes 3a-g. Furthermore, using certain
p-carboxyphenylsiloxanes as examples, we showed that it
is possible to use them as HOF-like structures and
monomers for the synthesis of copolymers (Sila-PET).
These studies contributed considerably to the
development of methods for the synthesis of p-
carboxyphenylsiloxanes and confirmed the potential
possibility of their synthesis and subsequent use in the
design of new materials based on organosilicon building
blocks: HOFs, MOFs, etc.12c,13
However, synthetic techniques presented in these
works generally require the use of excess amounts of
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Aerobic
Co-/NHSI-catalyzed
oxidation
of
0.015 / 0.6 equiv., which allows one to reach 96%
conversion of the p-CH3C6H4[Si] group to p-
HO(O)CC6H4[Si] (Table 1; Supp. Inf.).
1
2
3
4
5
6
7
8
p-tolylsiloxanes to p-carboxyphenyldsiloxanes
We chose the commercially available and inexpensive
N-hydroxysuccinimide (NHSI) as organocatalyst, which
was previously reported to be relatively low-active in [M]-
free aerobic oxidation of alkylbenzenes,17a benzylamines,
etc.18 NHSI attracted our attention due to good solubility
in aprotic and low-polar solvents (which is an important
factor in operations with organosilicon substrates) and
facility of separation from reaction products (p-
carboxyphenylsiloxanes) in comparison with other
organocatalysts. Commercially available forms of
abundant metals were chosen as the [M]-catalysts
(precatalysts).
The choice of solvent is also a key factor that affects the
result of the process. In fact, the conversion of p-
CH3C6H4[Si] group to p-HO(O)CC6H4[Si] in various
solvents (AcOH, MeNO2, EtOAc, EtOH, HCCl3, 1,2-
dichloroethane (DCE), octafluorotoluene (Tol-F8),
1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) and their mixtures
in various ratios), except for MeCN, does not exceed 2%.
However, if the reaction mixture was diluted or
concentrated twofold with respect to the original
concentration (1c concentration ~ 0.32 М), the conversion
of 1с did not change considerably (4 – 6% decrease; Table
1; Supp. Inf.).
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Table 1. Effect of aerobic oxidation conditions of 1с
on the yield of 2сa
The process temperature is also among key factors:
when the temperature was lowered from 40 to 25 – 30 оС,
the conversion of p-CH3C6H4[Si] group to p-
HO(O)CC6H4[Si] did not exceed 2%. Increasing the
O
O
Me
Me
HO
OH
x [M] / y NHSI, O2
Sol, T o
о
temperature from 40 to 60 С lead to 100% conversion of
C
p-CH3C6H4[Si] to p-HO(O)CC6H4[Si] group / byproducts,
but the content of byproducts also increased from 1% (at
40 oC) to 6% (Table 1; Supp. Inf.).
Si
Si
Si
Si
1c
2c
O
O
p-CH3C6H4[Si] /
Evolution of the effect of the process time (from 0 to 48
h) showed that the process involves an induction period.
In fact, in the 0.015 Сo(OAc)2 / 0.6 NHSI system in MeCN
at 60 оС, no conversion of substrate 1c was observed
during initial 3 h, however, conversion of p-CH3C6H4[Si]
to p-HO(O)CC6H4[Si] group / byproducts after 5 and 7 h
already reached 56 and 93%, respectively. After that, the
conversion was increasing, but sufficiently slower: 15 h –
99%; 24 and 48 h – 100%. This difference in conversion
from 93 to 100%, which is small at first glance, is an
important factor in syntheses of di-, tri- and
polyfunctional substrates.
Entry
[M]
Solvent T, oC p-HO(O)CC6H4[Si] /
byproducts, %
1
Cu(OAc)2 MeCN
Co(acac)2 MeCN
Co(OAc)2 MeCN
Co(OAc)2 EtOAc
Co(OAc)2 MeCN
Co(OAc)2 MeCN
40
40
40
40
40
60
95/0/5b
100/0/0b
11/77/12b
99/0/1b
3/96/1c
0/94/6,c,d 90e
2
3
4
5
6
a
1c (0.318 mmol, 1 eq.), NHSI (0.064 – 0.254 mmol, 0.2 – 0.8 eq.),
[M] (1.59 – 19.1 μmol, 0.005 – 0.06 eq.), MeCN (1 mL), 25 – 60 °C, 15 h,
Apparently, the induction period is due to the
1
b
O2 (1 atm). The yield of 2c was determined by H NMR. Co(OAc)2
formation of
a reactive catalytic form from the
c
(12.72 μmol, 0.04 eq.), NHSI (0.127 mmol, 0.4 eq.). Co(OAc)2 (4.77
precatalysts (Сo(OAc)2 and NHSI) and to the demand of a
certain concentration of succinimido-N-oxyl radicals
(SINO; “N–O•” radical) to be present in the reaction
mixture, which are both initiators and carriers of the
radical chain process by a mechanism similar to that
reported by Prof. Ishii et al. (Figure 1, Scheme 2; Supp.
Inf.).17a
μmol, 0.015 eq.), NHSI (0.191 mmol, 0.6 eq.). d 24 h. e Isolated yield of
2c. For the scaled reactions the temperature of oxidation was
increased leading to the increase in NHSI / p-HO(O)CC6H4[Si]-
adduct content.
It was found that the type of the metal (M) and anion
(X) in the precatalyst (MXn = [M]) are the key factors in
the process if the [M] / NHSI / O2 oxidizing system is
used for conversion of p-tolylsiloxane 1c to p-
carboxyphenylsiloxane 2c. In fact, the conversion of p-
CH3C6H4[Si] group to p-HO(O)CC6H4[Si] for reagent 2c
1
(according to H NMR), with 0.04[M] / 0.4NHSI ratio in
MeCN at 40 С in 15 hours using CuCl2, MnCl2, NiCl2,
о
FeCl3, CrCl3, CoCl2, Cu(OAc)2, Mn(OAc)2, Mn(OAc)3,
Ni(acac)2, Fe(OAc)2, Fe(OAc)2, Cr(OAc)3, Co(acac)2,
Co(acac)3, CoSO4 and CAN ((NH4)2Ce(NO3)6), was no
higher than 1 – 2%, but it was 77% in the case of Co(OAc)2
(Table 1; Supp. Inf.).
At the next stage, we performed screening of the
Сo(OAc)2 / NHSI ratio within (0.005 – 0.06) / (0.2 – 0.8)
equiv. It was found that the best Сo(OAc)2 / NHSI ratio is
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with the formation of the final product
Page 18 of 25
–
1
2
HO(O)CC6H4[Si] (Scheme 2).
3
4
5
6
7
8
Scheme 2. The proposed reaction mechanism
O
(NHSI)
O2
A
Co3+
N OH
G
Co2+
Ln
Ln
Ln
OO
[C] OH
O
or
or
or
O2
Co3+
Co3+
Co3+
Ln
OO
Ln
9
F
Co3+
Co3+
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59
60
Ln
Ln
OOH
OH
or
Co3+
or
Co2+
O
(SINO)
Ln
Ln
[C]
H
N O
B
O
[C] OOH
E
Figure 1. Free radical content of the reaction studied by EPR.
Spectra of the reaction mixture samples (1h, 3h, 6h, 10h and 15h, red
lines), and spectrum of SINO radical generated by treatment of
NHSI with oxygen in MeCN (blue line). Double integral value of the
signal region (shaded rectangle) depending on the reaction time is
shown on the right inset. Spectrum of SINO generated by oxidation
of NHSI with phenyliodine diacetate (PhI(OAc)2; PIDA), and
corresponding simulated spectrum (For simulation parameters see
Supp. Inf.).
[C]
[C] OO
NHSI
O2
D
C
The optimized conditions for 1c oxidation were found
to be applicable to the other p-tolylsiloxanes 1a-b, d-g
((A), Scheme 3; Supp. Inf.).
Electron Paramagnetic Resonance (EPR) spectra of
reaction mixture samples display radical signal near
g=2.005 (Figure 1, red line). The double integral intensity
of the signal significantly increases in course of reaction
(Figure 1, right inset). The signal shape is not well
resolved and suffers from baseline distortion, so it is
readily consistent with presence of detectable amounts of
free SINO only at 15 hours after reaction start, where its
width exceeds 28G, and the outer peak belonging to z-
manifold appears on the right. Before 15h, it cannot be
unequivocally attributed to SINO both due to its shape
and offset in g factor (2.005 instead of expected 2.010, as
observed in the spectrum of SINO radical generated by
treatment of NHSI with O2, blue line on Figure 1). The
width of the rigid limit SINO spectrum has low limit set
by double z-component of hyperfine splitting on 14N
nitrogen nucleus, estimated 2Azz = 28G (found by
simulation, left inset on Figure 1). These findings support
the theory that free SINO radical formation is hampered
on initial stages of reaction.
Scheme 3. Synthesis of 2a–o and 4a-g from 1a–o and
3a-g, respectivelya
Based on these findings and literature data, radical
mechanism of the oxidation was proposed.17a Accordingly,
for the initiation of reaction, the SINO is formed by
interaction of NHSI with complexes of [Co] A, or with O2.
Further interaction of SINO with the C–H group B (of
CH3C6H4[Si]) leads to the regeneration of NHSI and the
formation of the C-centered radical C (•CH2C6H4[Si]),
which then recombines with oxygen to give the peroxy
radical
D (•OOCH2C6H4[Si]). The radical D when
interacting with NHSI turns into the corresponding
hydroperoxide E (HOOCH2C6H4[Si]), the subsequent
interaction of which with the [Co] complexes F leads to
the reduction of hydroperoxide E to the corresponding
hydroxide G. This catalytic cycle can be repeated until
complete oxidation of the corresponding C–H groups,
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Journal of the American Chemical Society
HO
O
Me
This method makes it possible to oxidize p-
tolylsiloxanes 1a-g to p-carboxyphenylsiloxanes 2a-g (the
conversion is 100% according to 1H NMR) with high regio-
1
2
3
4
0.0075 Co(OAc)2, 0.3 NHSI, O2
MeCN, 60 o
(A)
C
Si
/chemoselectivity
and
with
retention
of
the
Si
R1
R3
1a-g
2a-g
R1
R3
R2
R2
organosiloxane core: no cleavage of Si–O, Si–Me and Si–
Ar bonds was observed.
Structures 2a-o, conversions according to 1H NMR for 1a-o (isolated yields of 2a-o), time:
5
6
7
8
9
The amphiphilic mono(p-carboxydiphenyl)di- (2а) and
trisiloxanes (2b) obtained in high yields (94 and 92%) are
of interest for synthesizing new types of siloxane
surfactants and modifiers of organic polymers.19
HO
O
O
O
O
HO
OH
HO
Si
Si
Si
Si
Si
Si
Si
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11
12
13
14
15
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O
O
O
O
We also estimated the applicability of the method not
only for preparation of bifunctional 1,3-bis(p-
carboxyphenyl)disiloxane 2с, but also “geminal” 1,1-bis-
(2d – 85%) and 1,1,1-tris(p-carboxyphenyl)disiloxanes (2e –
80%) and “isolated” 1,7-bis(p-carboxyphenyl)tetrasiloxane
(2f – 89%). These products may be of interest in the
syntheses of siloxane-containing copolymers and
materials based thereon, as we discuss below.
2c
, 100% (90%), 24 h
2b
2a
, 100% (92%), 24 h
HO
, 100% (94%), 24 h
O
O
HO
O
Si
Si
Si
Si
O
O
HO
2d
2e
, 100%
(85%), 24-48 h
, 100%
(80%), 24-48 h
HO
HO
O
Furthermore, the method proved to be suitable for
synthesis of such a polyfunctional product as poly(p-
carboxyphenyl)siloxane (2g) which can be obtained in
96% yield without degradation of the organosiloxane
chain (determination of the average number of structural
units in 1g and 2g (n ≈ 11) and the stability of the
organosiloxane core under the reaction conditions was
O
O
O
HO
O
HO
OH
Si
Si
Si
Si
Si
2g
O
Si
O
Si
11
O
O
O
2f
, 100% (89%), 24 h
, 100% (96%), 24 h
O
1
carried out according to H NMR data). This is the first
HO
O
O
example of polymeric p-carboxyphenylsiloxanes. Polymers
of this type may be promising for preparation of silicone
elastomers, surfactants, electrolytes, self-healing, liquid-
crystal and other materials. Furthermore, it is a way to the
design of silicones with enhanced mechanical strength.
(B)
R
O
2n + 2'n
2h m
-
2o
2h
2i
2j
2l
2'l
, R = 4-Me + , R = 4-C(O)OH, 100%, 12 h
, R = H, 98%, 16 h
2m
2'm
, R = 3-C(O)OH, 100%, 12 h
, R = 4-NO2, 75%, 72 h
, R = 4-Br, 95%, 24 h
, R = 3-Me +
The suggested method can be scaled to obtain p-
carboxyphenylsiloxanes 2a-g in gram amounts (up to 5 g;
Supp. Inf.) in 80 – 96% yields.
2n + 2'n
, 99%, 12 h
2k
2o
, 95%, 24 h
, R = 4-I, 99%, 24 h
0.01 Co(OAc)2, 0.2 NHSI, O2
MeCN, 60-80 o
(C)a
[Si]
H
HO [Si]
Aerobic Co-/NHSI-catalyzed oxidation of alkylarenes to
benzoic acid and arylketone derivatives
C
3a-g
4a-g
4a-g
Structures
, GLC yields, temperature and time:
Since liquid-phase aerobic oxidation is a powerful tool
for producing industrially important organic compounds
as mentioned above,16,17 we also estimated the
applicability of the suggested method for the oxidation of
organic derivatives, viz., alkylarenes 1h-o (Ar–CH3, Ar–
CH2–R) with various structures, to the corresponding
derivatives 2h-o (Ar–C(O)OH and Ar–C(O)–R) with
conversions up to 95 – 100%. It is noted in early studies on
the [M]-/Org-catalyzed oxidation of arylketones that
donor substituents at the aromatic ring, unlike acceptor
substituents, accelerate the oxidation process. In fact,
conversion can decrease to 0% in the case of
Si
Si
Et
O
O
Et
Et Si OH
Si OH
O
Si
Si OH
O
Si OH
Et Si
Et
O
O
Et
Si
b
Si
4a
4b
60 oC, 3 h
4d
, 85%,
, 92%,
4c
, 80%,
, 90%, 65%,
60 oC, 7 h
60 oC, 5 h
80 oC, 24 h
Si
O
O
Si
Si
O
O
Si
O
Si OH
Si
O
O
Si
Si
Si
OH
OH
4e
4f
4g
60 oC, 7 h
, 87%,
, 96%,
, 98%,
40 oC, 24 h
60 oC, 7 h
(A) 1a-g (6.36 mmol of Tol-groups, 1 eq.), NHSI (1.91 mmol, 0.3 eq.
per Tol-group), Co(OAc)2 (47.7 μmol, 0.0075 eq. per Tol-group),
MeCN (2.5 mL), 60 °C, 24 – 48 h, under O2 bubbling (≈ 5 –
7 mL / min).
(B) 1h-o (0.636 mmol, 1 eq.), NHSI (0.191 mmol, 0.3 eq.), Co(OAc)2
(4.77 μmol, 0.0075 eq.), MeCN (1 mL), 60 °C, 16 – 71 h, O2 (1 atm).
nitrotoluene.16,17Error!
Bookmark
not
defined.
However, the conversion of p-nitrotoluene under the
suggested conditions is about 40% in 24 h and 75% in 3
days ((B), Scheme 3; Supp. Inf.).
a
(C) 3a-g (0.452 mmol, 1 eq.), NHSI (0.0905 mmol, 0.2 eq.),
Co(OAc)2 (2.26 μmol, 0.005 eq.), MeCN (1 mL), 25 °C, 24 h, O2 (1
Aerobic Co-/NHSI-catalyzed oxidation of hydride silanes
to silanols
b
atm). Isolated yield of 4c after distillation under reduced pressure.
The ratios of reagents were taken from our previous article.17g
Hydride silanes ([Si]–H) with various structures are also
selectively oxidized to the corresponding silanols and
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siloxanols ([Si]–OH) with 80 - 98% conversion. As a rule,
the Si–H group undergoes oxidation much more easily
than the C–H group. The main difficulty in the synthesis
of silanols lies in the subsequent side process, i.e.,
condensation to give siloxanes (disiloxanes). However,
silanols are rather stable under the suggested reaction
conditions: for example, the fraction of disiloxane after 24
hours does not exceed 14% in the case of silanol 4c ((C),
Scheme 3; Supp. Inf.).
Page 20 of 25
1
2
3
4
5
6
7
8
9
Determination of the structure and study of the
properties of p-carboxyphenyldsiloxanes
10
11
12
13
14
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Unlike the starting oil-like p-tolylsiloxanes 1а-d,f,g, p-
carboxyphenylsiloxanes 2а-g are solid compounds at 25
оС, which is an additional confirmation that the
intermolecular interactions are stronger. The structures of
Crystal packing of 2’’c (hydrogen-bonded polymer,
HOF-like structure)
1
1
2а-g were confirmed by H, 13C, 29Si and H, 29Si-HMBC
NMR spectroscopy, IR spectroscopy, high resolution mass
spectrometry (ESI-HRMS) for 2a-f and X-Ray for 2a,c,d
and 1e (Figure 2; Supp. Inf.).
Like in the majority of carboxylic acid derivatives, the
supramolecular structures of p-carboxyphenylsiloxanes
2a-d are hydrogen-bonded systems: they are dimers (in
the case of 2a and 2’c – pseudo-cis-conformation of 2c),
like in, e.g., benzoic acid;20 polymers, i.e., HOF-like
12a
structures4 (in the case of 2d and 2’’c
conformation of 2c) (Figure 2).
– pseudo-trans-
Molecular structure of 2d
Molecular structure of 2a (hydrogen-bonded dimer)
Molecular structure of 2’c (pseudo-cis-
conformation of 2c; hydrogen-bonded dimer)
Crystal packing of 2d (hydrogen-bonded polymer,
HOF-like structure)
Figure 2. X-Ray structures of 2a,c,d. HOF-like structures
Bond lengths and angles in of 2a, 2’c, 2’’c and 2d are
typical for carbosiloxanes (Table 1s, X-Ray Supp. Inf.). Due
to the presence of carboxylic groups in all the molecules
studied, hydrogen bonds play a significant role in the
stabilization
Compounds 2’c and 2’’c,
of
their
being
crystal
packing.
Molecular structure of 2’’c (pseudo-trans-conformation of 2c)
conformational
polymorphs present the most interesting case. The
conformational changes in the latter structures are
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O
O
O
O
related to changes in the mutual orientation of the Ar-
groups, so molecules 2’c and 2’’c are pseudo-cis- and
pseudo-trans-isomers. According to theoretical
HO
OH
MeO
OMe
1
CDI, MeOH
THF, 25 o
2
C
3
4
5
6
7
8
9
calculations, the former is by 0.88 kJ/mol more
favorable. As a consequence, H-bonded dimers are
formed in the crystal packing of 2’c, while in the case
of 2’’c, infinite zigzag chains are present. The latter
packing motif is also observed in the crystal of 2d. Due to
the presence of only one carboxylic group in 2a,
centrosymmetric dimers are formed by two independent
molecules in the crystal. The parameters of H-bonds are
presented in ESI. Additionally, the crystal packing
of 2a, 2’c, 2’’c, 2d was analyzed in terms of pairwise
interactions (CE-B3LYP/6-31G(d,p) method). The energy
of pairwise interactions related to O-H…O bonding in a
crystal are 76.4 - 121.3 kJ/mol, whereas bonding between
the chains or dimers related to other types of
intermolecular bonds does not exceed 35 kJ/mol (X-Ray
Supp. Inf.).
Si
Si
Si
Si
O
O
2c
5
, 90%
0.5% Zn(OAc)2
OH
OH
200 o
C
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3JH-C=2.1 JH-C=2.2 JH-Si=6.7 JH-C=1.3
3
2
4
3JH-C=4.4
2JH-C=6.5
3JH-C=4.1
CH3
H
H
H
H
Si
Si
O
C
O
O
C
H
H
C
H2
n
H
H
O
O
4
3
3
3JH-C=5.0 2JH-C=3.0 JH-Si=1.5 JH-Si=5.2 JH-C=3.5 2JH-C=6.8
n
6,
X-Ray structure of 5
[ JH-C/Si] = Hz
Synthesis of monomeric (5) and polymeric (6) esters
1H
(Sila-PET)
from
p-carboxyphenyldsiloxane
(2c).
OCH2
Me
Ar
Determination of the structures of 5 and 6
As mentioned above, functionalized siloxanes are
important building blocks, in particular in syntheses of
copolymers from organic and organosilicon monomers.21
For example, our attention was attracted by the potential
possibility of synthesizing a siloxane analogue of PET, a
polymer produced on a large scale and used for the
production of various materials, such as fibers, films for
food packaging, and plastics used in building, medicine,
electronics, etc.22
13C
p-C
1H,13C HSQMBC
C=O
29Si
We studied the feasibility of synthesizing Sila-PET 6
from p-carboxyphenylsiloxane 2c (Scheme 4).
SiMe2
1H,29Si HSQMBC
Scheme 4. Synthesis of esters 5 and 6 (Sila-PET)
from 2c (top); H-C and H-Si coupling constants for
1
1
Sila-PET 6, and key fragments of H,13C and H,29Si
First, the dimethyl ester of p-carboxyphenylsiloxane 5
was obtained in 90% yield. Further, the reaction of 5 and
ethylene glycol in the presence of catalytic amounts of
Zn(OAc)2 gave polymeric ester 6 – Sila-PET – in 99%
yield. This process occurs with retention of the
organosiloxane frame and allows one to obtain Sila-PET 6
with Мn = 9700, Мw = 14900 and PDI = 1.53 (PSS; Supp.
Inf.).
HSQMBC spectra from it (bottom).
Products 5 is a solid and 6 is glassy polymer. The
structures of 5 and 6 were confirmed by H, C, 29Si and
1H, 29Si-HMBC NMR spectroscopy, IR spectroscopy, high
resolution mass spectrometry (ESI-HRMS) for 5, GPC for
6 and X-Ray for 5 (Scheme 4, top; Supp. Inf.). The
structure of Sila-PET 6 was additionally confirmed by a
representative set of 2D NMR correlation techniques
1
13
(COSY, NOESY, HSQC, HMBC, H-29Si-HMBC, DOSY).
1
Moreover, the accurate determination of long-range H-C
and H-Si spin-spin coupling constants was performed
using H,13C and H,29Si HSQMBC and IPAP-HSQMBC 2D
NMR experiments (coupling constants are shown on
Scheme 4, bottom). Sila-PET 6 despite its high molecular
weight has a highly ordered structure and gives narrow
1
1
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Page 22 of 25
peaks in NMR spectra, which allows to accurately Molecular Biology, Russian Academy of Sciences).
1
2
3
4
5
6
7
8
measure the coupling constants. Diffusion NMR
experiment (DOSY) additionally shows a high molecular
weight of the Sila-PET 6 in solutions.
Recording of NMR spectra conducted by Dr. Roman A.
Novikov and Dr. Yaroslav A. Tkachev were supported by
the Program of fundamental research for state academies
for 2013–2020 (No. 01201363817) (Engelhardt Institute of
Molecular Biology, Russian Academy of Sciences).
CONCLUSION
A
new highly efficient method for synthesizing
ACKNOWLEDGMENT
organosilicon products with a “polar” functional group in
organic substituents, p-carboxyphenylsiloxanes, has been
suggested. The method is based on aerobic Co- / NHSI-
Authors acknowledge Dr. Natalia Chumakova (Moscow
State University), and partial support from M.V.
Lomonosov Moscow State University Program of
Development; Kseniia Karnaukh (INEOS RAS) for
particiation in an oxidation of hydride silanes to silanols.
9
catalyzed
oxidation
of
p-tolylsiloxanes
to
p-
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carboxyphenylsiloxanes. This is the first preparative
method for synthesizing such products implemented in a
catalytic version and under mild and “green” reaction
conditions.
All X-Ray studies were carried out using equipment of
Center for molecule composition studies of INEOS RAS.
This approach makes it possible to obtain monomeric,
oligomeric and polymeric p-carboxyphenylsiloxanes with
various structures in 80 – 96% yields. The method was
also found suitable for the oxidation of organic
derivatives, namely alkylarenes, and hydride silanes to the
corresponding acids/ketones and silanols with high
conversions (75 – 100%).
REFERENCES
1. (a) Mark, J. E. Some interesting things about polysiloxanes Acc.
Chem. Res. 2004, 37 (12), 946-953; (b) Kickelbick, G. (ed.). Hybrid
materials: synthesis, characterization, and applications. John
Wiley & Sons, VCH: Weinheim, 2007; (c) Cobo, I.; Li, M.;
Sumerlin, B. S.; Perrier, S. Smart hybrid materials by conjugation
of responsive polymers to biomacromolecules. Nat. Mater. 2015,
14 (2), 143-159; (d) Zhang, Z.; Nguyen, H.T.H.; Miller, S.A.; Cohen,
p-Сarboxyphenylsiloxanes can become promising
building blocks for synthesizing unique silicone materials,
such as HOFs, MOFs, various copolymers, surfactants,
etc. In fact, a siloxane analogue of PET (Sila-PET),
obtained from one of the products (1,3-bis(p-
carboxyphenyl)disiloxane), was taken as an example.
S.M.
PolyMOFs:
A
Class
of
Interconvertible
Polymer-Metal-Organic-Framework Hybrid Materials. Angew.
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Arkhipov, D.E.;. Korlyukov, A.A.; Chernyshev, V.V.; Fitch, A.N.;
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M. P.; Salieb-Beugelaar, G. B.; Hunziker, P. PDMS with designer
Thus, considering the simplicity and efficiency of the
catalytic reaction and taking into account the commercial
availability and low cost of the reagents used, this method
has considerable prospects for application in the
syntheses of new functionalized organic and
organosilicon monomers, oligomers and polymers.
ASSOTIATED CONTENT
functionalities
– properties, modifications strategies, and
applications. Prog. Polym. Sci. 2018, 83, 97 – 134.
Supporting Information Available: Optimization
and experimental procedures, spectral and X-Ray data are
included in the supporting information. This material is
available free of charge via the internet at
http://pubs.acs.org.
2. (a) Vitale, A.; Touzeau, S.; Sun, F.; Bongiovanni, R.
Compositional Gradients in Siloxane Copolymers by
Photocontrolled Surface Segregation. Macromolecules. 2018, 51
(11), 4023–4031; (b) Racles, C.; Cozan, V.; Bele A.; Dascalu, M.
Polar silicones: structure-dielectric properties relationship. Des.
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Špírková, M.; Poręba, R.; Steinhart, M.; Ostojić, S.; Tešević, V.;
Pergal. M.V. Study of the Properties of Urethane–Siloxane
AUTHOR INFORMATION
Corresponding Author
Copolymers
Based
on
Poly(propylene
oxide)-b-
* Ashot V. Arzumanyan: aav@ineos.ac.ru
poly(dimethylsiloxane)-b-poly(propylene oxide) Soft Segments.
Ind. Eng. Chem. Res. 2016, 55 (14), 3960–3973. (d) Pouget, E.;
Tonnar, J.; Lucas, P.; Lacroix-Desmazes, P.; ois Ganachaud, F.;
Boutevin, B. Well-architectured poly (dimethylsiloxane)-
containing copolymers obtained by radical chemistry. Chem.
Rev. 2010, 110, 1233–1277; (e) Cheesmana, B.T.; Gates, P.J.; Castle,
T.C.; Cosgrove, T.; Prescott, S.W. Linear and star architecture
methacrylate-functionalised PDMS. Mater. Today Commun.
2015, 3, 122-129; (f) Dai, Z.; Yang, K.; Dong, Q. Synthesis and
Funding Sources
This work was supported by the grant of the Russian
Science Foundation (RSF grant 17-73-10462).
Products 2c, 5 and 6 were synthesized as part of work
supported by the grant of the Ministry for Science and
Higher Education of the Russian Federation, Grant of the
Government
of
the
Russian
Federation
characterization
polyether-polydimethylsiloxane-polyether
of
hydroxy-terminated
No. 14.W03.31.0018. Structural (X-Ray) studies of 2a,c,d
(PE-PDMS-PE)
were supported by the RFBR according to the research
triblock oligomers and their use in the preparation of
thermoplastic polyurethanes J. Appl. Polym. Sci. 2015, 132 (38),
42521; (g) Wang, J.; Cai, X.; Jia, P.; Han, Q.; Run, M. Synthesis
and characterization of the copolymers containing blocks of
polydimethylsiloxane in low boiling point mixtures. Mater.
project
№
18-29-04060. Mass-spectrometry studies
conducted by Dr. Pavel N. Solyev were supported by the
Program of fundamental research for state academies for
2013–2020 (No. 01201363818) (Engelhardt Institute of
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Si
Si
Reagents / Conditions are green, inexpensive,
commercially available & simple:
O
O
O
HO
O
Me
Sila-PET
n
O
O
Building Blocks for
Siloxane-Based
Materials
5
6
7
8
Co(OAc)2 O2
NHSI
40 - 60 o
C
Si
Si
R1
R2 Si
O
O
R1
R3
gram-scale
O
H
O
R1
R3
R2
HOF-like
structures
R2
Si R2
1a-g; R1, R2, R3 = O[Si], p-CH3-C6H4-, Me
2a-g, 80 - 96%
R1
O
H
O
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