Hydrosilylation of aromatic azomethines

Article abstract of DOI:10.1023/B:RUGC.0000031855.77108.f6

The reactions of aromatic azomethines with methyldichlorosilane, phenyldichlorosilane, and dimethylchlorosilane, performed in the presence of Speier's, Wilkinson's, and Karstedt's catalysts and a series of Pt(II) complexes LL'PtCl₂, give hydrosilylation and reduction products whose ratio depends on the catalyst used. The highest yield of hydrosilylation products is attained with Pt(II) complexes as catalysts.

Full text of DOI:10.1023/B:RUGC.0000031855.77108.f6

Russian Journal of General Chemistry, Vol. 74, No. 4, 2004, pp. 548 550. Translated from Zhurnal Obshchei Khimii, Vol. 74, No. 4, 2004,
pp. 599 602.
Original Russian Text Copyright
2004 by Zuev, Kovaleva, Skvortsov.
Hydrosilylation of Aromatic Azomethines
V. V. Zuev, O. P. Kovaleva, and N. K. Skvortsov
Institute of Macromolecular Compounds, Russian Academy of Sciences, St. Petersburg, Russia
St. Petersburg State Institute of Technology, St. Petersburg, Russia
Received March 25, 2003
Abstract The reactions of aromatic azomethines with methyldichlorosilane, phenyldichlorosilane, and
dimethylchlorosilane, performed in the presence of Speier’s, Wilkinson’s, and Karstedt’s catalysts and a series
of Pt(II) complexes LL PtCl2, give hydrosilylation and reduction products whose ratio depends on the catalyst
used. The highest yield of hydrosilylation products is attained with Pt(II) complexes as catalysts.
Aromatic azomethines are important and the most
accessible mesomorphic compounds; they play a
decisive role in development of high technologies in
computer engineering and optoelectronics [1]. Modi-
fication of these compounds is a promising route to
mesomorphic materials with a new phase behavior
and a set of valuable properties. One of modification
procedures may be hydrosilylation with various sil-
anes. However, hydrosilylation of azomethines has
been studied insufficiently [2, 3]. It is known [4] that
hydrosilylation of benzalaniline with alkyl- and aryl-
hydrosilanes in the presence of various catalysts is
nonselective and is accompanied by reduction and
reductive silylation followed by cleavage, silylation,
and hydrogenolysis of N-benzylaniline, etc., with the
yield of the desired hydrosilylation product being
extremely low. Therefore, it is necessary to look for
new catalysts allowing preparative synthesis of azo-
methine hydrosilylation products.
Preparative isolation of hydrosilylation products
obtained with chlorosilanes is complicated by their
ready hydrolysis on contact with atmospheric mois-
ture, followed by condensation. Andrianov et al. [4]
analyzed the products of reactions of azomethines
with hydrosilanes by GLC. However, a shortcoming
of this method is the possibility of secondary transfor-
mations of the products in the vaporizer and column.
Therefore, we chose NMR spectroscopy. Since both
substrates contain methyl groups, all the transforma-
tion products can be reliably detected. The signal
assignment in the NMR spectra of the substrates and
hydrosilylation products in given in Table 1.
Experiments were performed at the substrate : re-
agent : catalyst molar ratio of 1 : 3 : 0.01. Performing
the reaction at low temperatures generally leads to its
higher selectivity. However, the total conversion of
azomethines I and II in their reaction with methyldi-
chlorosilane at 20 C in the presence of the Wilkinson
catalyst was less than 3% in 24 h. Therefore, in the
subsequent experiments the reactions were performed
at 80 C for 24 h, which ensured high total conversion
(Table 2).
Our goal was comparative study of hydrosilylation
of mesomorphic azomethine I (p-ethoxybenzylidine-
p-methylaniline) and azomethine II containing a
methyl substituent at the C=N bond (acetophenone
anil). As hydrosilanes we used readily available di-
methylchlorosilane, methyldichlorosilane, and phenyl-
dichlorosilane, which allowed variation of the reactiv-
ity of both the substrate and reagent. As catalysts we
used Speier’s catalyst (1% H2PtCl6 in isopropyl alco-
hol), Pt(0) siloxane complex (so-called Karstedt’s cat-
alyst), and a series of Pt(II) complexes of the general
formula LL PtCl2, tested previously in hydrosilylation
of olefins, ketones, and vinylsiloxanes [5 8]. Also we
tested Wilkinson’s rhodium catalyst (Ph3P)3RhCl,
since previous studies [2, 3] showed that this catalyst
was the most selective in hydrosilylation of azometh-
ines; it ensured predominant formation of hydrosilyla-
tion and reduction products.
The reaction of azomethines with hydrosilanes is a
complex process involving hydrosilylation (1), reduc-
tion (2), and reduction silylation followed by product
cleavage (3) [4]:
SiR3
1
ArCH(R ) NAr
III, IV
2
R3SiH + ArC(R )=NAr
ArCH(R )NHAr
I, II
V
3
ArCH2R + Ar N(SiR3)2
VI
VII
When the reaction is performed in a solvent
1070-3632/04/7404-0548 2004 MAIK Nauka/Interperiodica

HYDROSILYLATION OF AROMATIC AZOMETHINES
Table 1. 1H NMR spectra of I VII
549
Comp.
no.
Silane
, ppm, (J, Hz) (CDCl3)
I
II
III
2.37 s (3H, ArCH3), 1.45 t (3H, CH3CH2, J 6.7), 4.10 q (2H, OCH2, J 6.7), 8.39 s (1H, CH=N)
2.25 s (CH3)
2.28 s (3H, ArCH3), 1.35 t (3H, CH3CH2, J 6.7), 3.98 q (2H, OCH2, J 6.7), 4.50 s (1H, CH2N),
0.95 s (3H, CH3Si)
HSiCl2CH3
III
HSiCl(CH3)2
2.28 s (3H, ArCH3), 1.34 t (3H, CH3CH2, J 6.7), 3.97 q (2H, OCH2, J 6.7), 4.52 s (1H, CH2N),
0.82 s (6H, CH3Si)
III
IV
IV
IV
V
HSiCl2Ph
2.28 s (3H, ArCH3), 1.35 t (3H, CH3CH2, J 6.7), 3.98 q (2H, OCH2, J 6.7), 4.54 s (1H, CH2N)
1.79 d (3H, CCH3, J 7.0), 4.56 q (1H, CHN, J 7.0), 0.92 s (3H, CH3Si)
1.67 d (3H, CCH3, J 7.0), 4.54 q (1H, CHN, J 7.0), 0.80 s (6H, CH3Si)
1.57 d (3H, CCH3 , J 7.0), 4.60 q (1H, CHN, J 7.0)
2.29 s (3H, ArCH3), 1.37 t (3H, CH3CH2, J 6.7), 4.02 q (2H, OCH2, J 6.7), 4.29 s (1H, CH2N),
4.80 s (1H, NH)
HSiCl2CH3
HSiCl(CH3)2
HSiCl2Ph
VI
2.24 s (3H, ArCH3), 1.33 t (3H, CH3CH2, J 6.7), 3.96 q (2H, OCH2, J 6.7)
2.26 s (3H, ArCH3), 1.16 s (6H, CH3Si)
2.26 s (3H, ArCH3), 1.12 s (12H, CH3Si)
VII HSiCl2CH3
VII HSiCl(CH3)2
VII HSiCl2Ph
2.26 s (3H, ArCH3)
(CDCl3), all the three processes occur simultaneously,
irrespective of the catalyst used (Table 2). The reac-
tion mixture acquires a dark cherry-red color (a maxi-
mum was observed in the UV spectrum at 355 nm),
suggesting partial transformation of silylated amines
formed by pathway 3 into azo compounds. When the
reaction is performed without a solvent, pathway 3
(cleavage of hydrosilylation reduction products) is
eliminated (Table 2), which is favorable for obtaining
the desired products.
Table 2. Product yields (%) in hydrosilylation of azo-
methine I
Catalyst
I
III
V
Methyldichlorosilane
Wilkinson’s
Karstedt’s
Wilkinson’s
Karstedt’s
0.5a
1.5a
31.3
45.0
53.5
38.5
14.0
31.3
59.4
59.25
30.6
42.4
55.9
56.3
44.5
46.5
61.5
86.0
68.7
40.6
40.75
69.4
57.6
44.1
<0.1
<0.1
<0.1
<0.1
<0.1
<0.1
<0.1
<0.1
<0.1
Speier’s
Tables 2 and 3 show that azomethines I and II
strongly differ in the reactivity. Whereas the conver-
sion of unsubstituted azomethine I is 100% irrespec-
tive of the catalyst used, the total conversion of com-
pound II containing a methyl substituent at the C=N
bond is always considerably lower. The Pt(II)-based
catalysts are optimal in this case; some of them ensure
almost quantitative yield of the hydrosilylation prod-
ucts. At the same time, in this case virtually no reduc-
tion products are formed (as judged from the NMR
spectra, sensitivity 1%). This fact is important, as it
opens prospects for preparing optically active silyl-
ated amines using chiral catalysts [9].
Pt(P(i-Pr)3)2Cl2
Pt(Bz2S)2Cl2
Pt(DMSO)(C5H5N)Cl2
Pt(DMSO)(Ph3PS)Cl2
Pt(DESO)2Cl2
Pt(Ph3As)2Cl2
Dimethyldichlorosilane
Wilkinson’s
Pt(Ph3Sb)2Cl2
Speier’s
Pt(P(i-Pr)3)2Cl2
Pt(Bz2S)2Cl2
41.55
6.25
52.5
32.5
84.0
93.0
82.1
4.5
<0.1
<0.1
63.0
16.0
7.0
<0.1
17.9
Phenyldichlorosilane
Wilkinson’s
Karstedt’s
Speier’s
Pt(P(i-Pr)3)2Cl2
Pt(Bz2S)2Cl2
<0.1
<0.1
<0.1
<0.1
<0.1
50.8
60.9
18.0
41.0
38.5
49.2
39.1
82.0
59.0
61.5
The relative activity of the catalysts depends on
particular silane, and no clear regular trends can be
revealed. On the whole, for each particular silane there
is a specific optimal catalyst [2, 3]. Speier’s catalyst
shows high activity in hydrosilylation with such an in-
active hydrosilane as dichloromethylsilane (Tables 2,
3). However, when this catalyst is used with azometh-
ine I, the reduction prevails. In this case, Speier’s
a
Solvent CDCl ; with Wilkinson’s and Karstedt’s catalyst, the
3
yield of VII is 11.9 and 9.0%, respectively. No solvent in
other cases.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 74 No. 4 2004

550
ZUEV et al.
Table 3. Product yields (%) in hydrosilylation of azo-
methine II
Phenyldichlorosilane was prepared as described in
[10], and azomethines, as described in [11]. The cata-
lysts were prepared according to [5 8] and used as
0.1% solutions.
Catalyst
II
IV
Hydrosilylation was performed in sealed ampules
at 80 C; the azomethine : silane : catalyst ratio was
1 : 3 : 0.01.
Methyldichlorosilane
Wilkinson’s
Karstedt’s
Speier’s
Pt(P(i-Pr)3)2Cl2
Pt(Bz2S)2Cl2
48.7
31.6
53.8
16.7
11.1
51.3
68.4
46.2
83.3
88.9
>99
>99
96.2
ACKNOWLEDGMENTS
The study was financially supported by the Russian
Foundation for Basic Research.
Pt(DMSO)(C5H5N)Cl2
Pt(DMSO)(Ph3PS)Cl2
Pt(DESO)2Cl2
<1
<1
3.8
REFERENCES
Pt(Ph3As)2Cl2
Pt(Ph2PCH=CH2)2Cl2
7.4
50.6
92.6
49.4
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Wilkinson’s
Pt(Ph3Sb)2Cl2
Speier’s
Pt(P(i-Pr)3)2Cl2
Pt(Bz2S)2Cl2
93.0
92.6
53.8
10.7
7.0
7.4
46.2
89.3
26.9
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29.3
54.0
26.3
1.5
70.7
46.0
73.7
98.5
88.5
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a catalyst ensuring more than 50% yield of the hydro-
silylation product, which opens prospects for prepara-
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1
The H NMR spectra were recorded on a Bruker
AC-200 spectrometer. The UV spectra were taken on
a Specord UV-Vis spectrophotometer.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 74 No. 4 2004