Use of MnCl2/tBuOOH oxidizing system for conversion of p-tolyldisiloxanes to p-carboxyphenyldisiloxanes

Article abstract of DOI:10.1016/j.jorganchem.2018.03.006

The possibility of Mn-catalyzed oxidation of p-tolyldisiloxanes 1a-b to the corresponding p-carboxyphenyldisiloxanes 2a-b was studied using a commercially available and inexpensive system based on MnCl₂/^tBuOOH. Products 2a and 2b were isolated in 51% and 32% yields, respectively.

Full text of DOI:10.1016/j.jorganchem.2018.03.006

Accepted Manuscript
t
Use of MnCl / BuOOH oxidizing system for conversion of p-tolyldisiloxanes to p-
2
carboxyphenyldisiloxanes
Irina K. Goncharova, Ashot V. Arzumanyan, Sergey A. Milenin, Aziz M. Muzafarov
PII:
S0022-328X(18)30164-5
10.1016/j.jorganchem.2018.03.006
JOM 20356
DOI:
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 23 January 2018
Revised Date: 26 February 2018
Accepted Date: 5 March 2018
Please cite this article as: I.K. Goncharova, A.V. Arzumanyan, S.A. Milenin, A.M. Muzafarov, Use of
t
MnCl / BuOOH oxidizing system for conversion of p-tolyldisiloxanes to p-carboxyphenyldisiloxanes,
2
Journal of Organometallic Chemistry (2018), doi: 10.1016/j.jorganchem.2018.03.006.
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ACCEPTED MANUSCRIPT
Use of MnCl2 / tBuOOH oxidizing system for conversion of p-tolyldisiloxanes
to p-carboxyphenyldisiloxanes
Irina K. Goncharovaa, Ashot V. Arzumanyan*a, Sergey A. Mileninb, Aziz M. Muzafarova,b
AUTHOR ADDRESS
a A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, 28
Vavilova st, Moscow 119991, Russian Federation
b
N.S. Enikolopov Institute of Synthetic Polymer Materials, Russian Academy of Sciences,
Profsoyuznaya 70, 117393 Moscow, RussianFederation
E-mail: aav@ineos.ac.ru
GraphicalAbstract
Abstract
The possibility of Mn-catalyzed oxidation of p-tolyldisiloxanes 1a-b to the corresponding p-
carboxyphenyldisiloxanes 2a-b was studied using a commercially available and inexpensive
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system based on MnCl2 / BuOOH. Products 2a and 2b were isolated in 51% and 32% yields,
respectively.
Keywords: functionalized siloxanes, peroxides, Mn-catalyzed oxidation, p-tolyldisiloxanes,
p-carboxyphenyldisiloxanes
1. Introduction

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Since 20th century, siloxanes (polyorganosiloxanes) were found to be very important
materials in various fields of science, technology, medicine and construction due to their unique
physico-chemical properties, e.g., low glass transition temperature, high thermal stability, low
temperature dependence of viscosity, gas permeability, biocompatibility,etc.1Despite all these
advantages, there are two main drawbacks limiting their application fields: low mechanical
strength and incompatibility with organic polymers. These issues arise fromthe weak
intermolecular interaction of siloxane chains. These problems can be solved by introducing of
"polar" functional group (e.g. −С(O)OH, −C(O)NH2, −OH, −NH2, etc.)into an organic
polyorganosiloxane substituent leading to an increase in intermolecular interaction in a
polysiloxane itself and in a polysiloxane combined with organic polymers and, consequently,
increasing the strength of siloxane-based materials.
The standard approaches to the synthesis of organosilicon compounds are not functional-
group tolerant and cannot provide monomers with "polar" organic substituents, so polysiloxanes
with "polar" edging are not readily available.2
It looks so promising to introduce "polar" groups into organic substituents of organosilicon
compounds synthesized by standard procedures,2but this post-modification challenge is still of
current interest despite its apparent simplicity: it was solved in only a few cases.3
The fact is that classic modification ways in organic chemistry become ineffective when
applied to organosilicon compounds because of drastic reaction conditions, e.g. usage of acids,
bases, oxidants, reductants, variable valency metals, and protonic solvents becomes fatal to Si-E-
groups (where E is O, N, C or Cl). So these approaches are usually destructive and lead to
complex mixtures of side products. Consequently, the methods for introduction of "polar"
functions known from organic chemistry are not applicable to the synthesis of functionalized
organosilicon substrates.3
One of the most attractive methods for synthesizingorganosilicon compounds containing
"polar" groups involvesthe oxidation of p-tolylsilyl derivatives to the corresponding p-
carboxyphenylsilylderivatives (scheme 1).4
Firstly, it is due to the relative ease of synthesizing p-tolylsilanes in
laboratory.4,5Furthermore, some of them are commercially available (e.g.tetra(p-tolyl)-, trichloro-
p-tolyl-, p-tolyltriethyl, triethoxy-p-tolylsilanes, etc.). Secondly, it was shown that it is
principallypossible to use the conditions of toluene oxidation to benzoic acid in the oxidation of
p-tolylsilanes to p-carboxyphenylderivatives (scheme 1, path. A).4 Thus, for example, using
methods (1) and (2) bis(p-carboxyphenyl)tetramethyldisiloxane 2b was prepared in the presence

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of 6 eq. of CrO3 in solution of AcOH / Ac2O / H2SO4 and 8 eq. of KMnO4 in solution of Pyr /
H2O at reflux, respectively.4a-b As for method (3),4c despite relatively high yields of octakis[(p-
carboxypheny)dimethylsilyl] silicate (77 - 78%), obtained from the corresponding octakis(p-
tolyl) derivative is used up to 68 eq. of N-bromsuccinimide (NBS; 8.5 eq. per one tolyl-group)
and 96 eq. of AgNO3 (12 eq. per one tolyl-group). The number of similar works is reduced to
several examples.4
Undoubtedly, these pioneering works made a serious contribution to the development of
methods for the preparation ofthe above-mentioned (symmetric)p-carboxyphenylsiloxanes and
confirmed the feasibility of this approach, but on the other hand showed the limitations of real
synthetic methods, in particular on other (asymmetric)p-carboxyphenylsiloxanes: the stringent
conditions of the process (in acid or base solution, high temperatures) lead to the formation of a
hardly separable product mixtures (decay products in Si-O- and Si-C-bonds); use of excess
quantities of expensive and toxic reagents and oxidants. Probably, for these reasons, these
methods have not received further serious development and practical application.
Scheme 1. Approaches to p-tolylsilanes oxidation
Thus, the development of new synthetic methods based on milder reaction conditions, in
particular catalytic ones, is required for the production of p-carboxyphenylsiloxanes, which are
most important for solving the above problems.
One of the most promising oxidizing systems for the functionalization of C-H or Si-H
bondsinvolvesa combination of a variable valence metal (mainly 3d-metals) as a catalyst, and a
peroxide as an oxidant, because of the low cost and commercial availability of the starting
reagents, as well as the relatively milder process conditions.6 In addition, in some studies(e.g.
concerning the production of silanols from the corresponding hydrosilanes using the [Cu] /
tBuOOH system) it was shown that the reaction occurs without breaking the Si-O- and Si-C
(sp3) bonds.7
2. Materials and methods

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All the starting materials were purchased from Acros and Sigma Aldrich company. Solvents
1
were dried and purified according to standard procedures. H, 13C, 29Si NMR spectra were
recorded using a Bruker Avance 400 NMR spectrometer in CDCl3, (CD3)2CO, сhemical shifts
are referenced to residual chloroform (7.26 ppm, 1H) and acetone (2.05 ppm, 1H).
3. Results and discussion
To evaluate the applicability of the [M]/R1OOR2system tothe oxidation of organosilicon
molecules,(asymmetric)p-tolyldisiloxane1a was selected as a model substrate (Table 1, note a;
SupportingInformation).
Table 1. Optimization of 1a-b oxidation
№
1 / 2 / side products,
%
[M]
[x]
1.
2.
0.0065 MnCl2
0.0065 CrCl3
2,75
2,75
9
54 / 30 / 16a
54/20/26a
25 / 55 / 20b
25 / 52 / 23b
20 / 58 / 22,b 51d
21/56/23,c 32d
3. 0.0065 MnCl2 + 0.0065 CrCl3
4. 0.0065 MnCl2 + 0.0065 AlCl3
9
5.
0.0065 MnCl2
0.013 MnCl2
9
18
a1a (0.151 g, 0.635mmol, 1 equiv.), tBuOOH (0.157 g, 1.746mmol, 2.75 equiv.,70% solution in
water),[M](0.0041mmol, 0.0065equiv.) and MeCN (1 mL)were stirredat 100 °C for 24 h. The 1 / 2 / side products
ratio was determined by 1H NMR ((CD3)2CO).
b1a (0.151 g, 0.635 mmol, 1 equiv.), tBuOOH (0.171 g, 1.905 mmol, 3 equiv., 70% solution in water), [M] (0.0041
mmol, 0.0065 equiv.) and MeCN (1 mL) were stirred at 100 °C for 5 h. Further, after adding tBuOOH (0.171 g,
1.905 mmol, 3 equiv., 70% solution in water), the reaction mixture was stirred at 100 °C for 5h (2 times at 5-hour
intervals). Next it was stirred at 100 °C for 14 h.
c1b (0.100 g, 0.317 mmol, 1 equiv.), tBuOOH (0.171 g, 1.905 mmol, 6 equiv., 70% solution in water), [M](0.0041
mmol, 0.013 equiv.) and MeCN (1 mL) were stirred in Schott culture tubes (H × diam. 160 mm × 16 mm) at 100 °C
for 5 h. Further, after adding tBuOOH (0.171 g, 1.905 mmol, 6 equiv., 70% solution in water), the reaction mixture
was stirred at 100 °C for 5h (2 times at 5-hour intervals).Next it was stirred at 100 °C for 14 h.

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dProducts2a-b were isolated by preparative chromatography using SiO2 as a sorbent, and a hexane / EtOAc system
as the eluent.
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At the first stage of optimization, the type of metal catalyst was varied, and BuOOH was
used as the oxidizing agent. It turned out that when MnCl2or CrCl3were used as the catalyst, a
product (p-carboxyphenyldisiloxane) 2a was formed in up to 30% and 20% yields respectively in
an acetonitrile solution, at 100 °C, in 24 h. However, in the presence of metal catalysts such as
CuCl2, NiCl2, CoCl2, FeCl3 and ceric ammonium nitrate (CAN), the yield of product 2a was
significantly lower and varied between 5 and 12% under the same conditions. Since the greatest
yield of product 2a was achieved using MnCl2as the catalyst, further optimization of the process
conditions was carried out with this metal catalyst.
Evaluation of the effect of the oxidant (peroxide) type on the yield of product 2a showed
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that BuOOH is the most active (20%), in contrast to other peroxides: H2O2aq, (NH4)2S2O8 and
tBuOOtBu which gives2ain yields up to 1%. The low efficiency of the latter peroxides can
probably be explained by the fact that H2O2aqis decomposed relatively quickly under the reaction
t
t
conditions (in comparison with BuOOH), while(NH4)2S2O8and BuOOtBu are rather stable and
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relatively less active (in comparison with H2O2aq and BuOOH) under appropriate reaction
conditions.
Further optimization of the solvent, ratio of the initial reagents, temperature and time of the
process showed no positive changes in product yields. Studying the effect of the sequence of
reagent addition on the yield of the reaction product was the next key stage in optimizing the
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reaction. It was shown that portion-wise addition of BuOOH increases the yield of product 2a
from 30% to 58%.Conversion remained incomplete in this stage, so we tried some combinations
of different metals as metal catalysts, but we failed to exceed the MnCl2 activity. The yield of
1
product 2a was 58% under optimal conditions (Table 1, line 5, determined by H NMR;
conversion of substrate 1a was 80% including 58% of product 2a and 22% of a difficultly
identifiable mixture of side products: decomposition cleavage by Si-Ar and Si-O bonds,
incomplete oxidation of 1a; no products of oxidation of Me-Si group were observed).Product 2a
was isolated in 51% yield by preparative chromatography using SiO2 as the sorbent and a hexane
/ EtOAc system as the eluent.
Further, we evaluated the applicability of oxidizing conditions to substrate 1b (bis(p-
tolyl)disiloxane). Substrate 1b was oxidized with a conversion of 79% to form a complex
mixture of up to 56% oxidation products with two (2b, bis(p-carboxyphenyl)disiloxane) or one
tolyl group and up to 23% side products. Compound 2b was also isolated in 32% yield by

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preparative chromatography using SiO2 as the sorbent and a hexane / EtOAc system as the
eluent.
4. Conclusions
Thus, the work demonstrates the principal possibility of using a catalytic approach to the
conversion of p-tolyldisiloxanes 1a-b to the corresponding p-carboxyphenyldisiloxanes 2a-b
using ainexpensive and commercially available oxidation system: MnCl2 – catalyst and tBuOOH
– oxidant. From a comparison of the conversions of the original model compounds and the yields
of the target products, it follows that, as the synthesis conditions are further optimized, the
achieved indices can be significantly improved. The demonstrated activity of the salts of some
transition metals, in particular in MnCl2, indicates the promise of further research in this
direction.
5. Experimental section
t
Method of 2a synthesis: 1a (0.151 g, 0.635 mmol, 1 eq.), BuOOH (0.171 g, 1.905 mmol, 3 eq.,
70% solution in water), MnCl2 (0.0005 g, 0.0041 mmol, 0.0065 eq.) and MeCN (1 mL) were
stirred in Schott culture tubes (H × diam. 160 mm × 16 mm) at 100 °C for 5 h. Further, after
t
adding of a BuOOH (0.171 g, 1.905 mmol, 3 eq., 70% solution in water), the reaction mixture
was stirred at 100 °C for 5h (2 times at 5-hour intervals). Next it was stirred at 100 °C for 14 h.
Then MeCN was evaporated (150-200 mbar, Тbath = 40 °C) and reaction mixture was dried (1
o
mbar, 50 C). Product 2a was isolated as white powder in a 51% yield (0.087 g) by preparative
chromatography using SiO2, as a sorbent, and a hexane / EtOAc system, as the eluent.
t
Method of 2b synthesis: 1b (0.100 g, 0.317 mmol, 1 eq.), BuOOH (0.171 g, 1.905 mmol, 6 eq.,
70% solution in water), MnCl2 (0.0005 g, 0.0041 mmol, 0.013 eq.) and MeCN (1 mL) were
stirred in Schott culture tubes (H × diam. 160 mm × 16 mm) at 100 °C for 5 h. Further, after
t
adding of a BuOOH (0.171 g, 1.905 mmol, 6 eq., 70% solution in water), the reaction mixture
was stirred at 100 °C for 5h (2 times at 5-hour intervals). Next it was stirred at 100 °C for 14 h.
Then MeCN was evaporated (150-200 mbar, Тbath = 40 °C) and reaction mixture was dried (1
o
mbar, 50 C). Product 2a was isolated as white powder in a 32% yield (0.037 g) by preparative
chromatography using SiO2, as a sorbent, and a hexane / EtOAc system, as the eluent.
Acknowledgements
This work was supported by a grant of the Russian Science Foundation (RSF grant no. 14-23-
00231).
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- Commercially available and inexpensive system based on MnCl2 / tBuOOH was investigated;
- Mn-catalyzed oxidation of p-tolyl-to p-carboxyphenyldisiloxanes was studied;
- The catalytic approach to the synthesis of p-carboxyphenyldisiloxanes was suggested.