Preparation of silanes

A process for the preparation of a silane of the formula EQU R.sub.a SiH.sub.b (OR').sub.4-(a+b) wherein PA1 R is optionally substituted alkyl, alkenyl or aryl with up to 7 C atoms, PA1 R' is alkyl with 1 to 4 C atoms, PA1 a and b each independently is 1, 2 or 3, and PA1 a+b is at most 4, comprising reacting an Si-H polysiloxane with a compound of the formula EQU R.sub.a Si(OR').sub.4-a in the presence of a conventional redox-stable siloxane polymerization catalyst which favors the exchange reaction. Advantageously the catalyst is AlCl.sub.3 and R and R' are methyl, ethyl, vinyl or phenyl.

The present invention relates to a process for the preparation of silanes 
of the general formula 
EQU R.sub.a SiH.sub.b (OR').sub.4-(a+b) (I) 
wherein 
R represents an optionally substituted alkyl, alkenyl or aryl radical with 
up to 7 C atoms, 
R' represents an alkyl radical with 1-4 C atoms and 
a and b represent 1, 2 or 3 and the sum of a+b is at most 4, 
by reaction of organopolysiloxanes, containing Si-H groups, with compounds 
of the general formula 
EQU R.sub.a Si(OR').sub.4-a 
wherein 
R, R' and a have the abovementioned meanings, in the presence of 
redox-stable siloxane polymerization catalysts which are in themselves 
known and which favor the exchange reaction. 
Silanes of the general formula I are in general obtained by reacting 
halogenosilanes of the formula 
EQU R.sub.a SiH.sub.b X.sub.4-(a+b) (X=Br, Cl) 
with alcohols (compare, for example, V. Bazant, Organosilicon Compounds 
(1965), page 125). 
However, frequently only unsatisfactory yields are achieved in this 
reaction. The reason for this is mainly that the hydrogen chloride formed 
destroys both the silicon-alkoxy group, giving the alcohol and the 
chloro-silane and also (particularly in the presence of alcohol) destroys 
the hydrosilane bond, with elimination of hydrogen and formation of a 
##STR1## 
bond. Furthermore, the hydrogen chloride forms, with the alkanols 
employed, chloroalkanes and, transiently, water which in its turn 
hydrolytically attacks the chlorosilanes and alkoxysilanes. 
These side reaction can largely be suppressed if the hydrogen chloride is 
rapidly removed from the reaction mixture or is trapped by means of bases. 
However, this requires complicated apparatus and complicated measures. 
Thus, for example, the use of an acid acceptor is described in DT-AD 
(German Published Specification) 1,162,365 and in DT-AS (German Published 
Specification) 2,304,503; the use of reduced pressure is described in U.S. 
Pat. No. 3,008,975; the use of pentane is described in U.S. Pat. No. 
3,806,549; the reaction in the vapor phase is described in DT-OS (German 
Published Specification) 2,144,748; the use of a jacketed tube is 
described in DT-OS (German Published Specification) 2,033,373 and the use 
of an inert gas and low temperatures is described in DT-AS (German 
Published Specification) 1,298,972. 
Furthermore, it is known to circumvent the side reactions described above 
by converting trimethylchlorosilane into trimethylethoxysilane, as 
described in British Pat. No. 653,237, and reacting the latter with 
methylhydrogenodichlorosilane in accordance with the following equation: 
EQU 2 Me.sub.3 SiOEt+MeHSiCl.sub.2 .fwdarw.2 Me.sub.3 SiCl+MeHSi(OEt).sub.2. 
The trimethylchlorosilane formed as a by-product is at the same time 
continuously distilled from the equilibrium. Furthermore, Japanese Patent 
Application 084,094 describes a controlled elimination of hydrogen in 
accordance with the equation: 
EQU RSiH.sub.3 +2 R'OH.fwdarw.RHSi(OR').sub.2 +2 H.sub.2. 
A further process, which however is also expensive, is to react triethyl 
orthoformate with methylhydrogenodichlorosilane (compare, for example, W. 
Noll, Chemie und Technologie der Silicone (Chemistry and Technology of the 
Silicones), Weinheim 1968, page 78). 
However, all the processes mentioned have some disadvantages which make 
them unsuitable for industrial use. 
Surprisingly, it has now been found that, according to the invention, 
silanes of the formula 
EQU R.sub.a SiH.sub.b (OR').sub.4-(a+b) 
can be obtained in a simple manner, and with good yields, if the 
alkylhydrogenopolysiloxanes, which in each case are readily available, are 
reacted, in the presence of a catalyst, with the appropriate 
alkoxysilanes, which are also readily available, so that the difficulties 
described above do not arise. In this reaction, one or more alkoxy groups 
of the monomeric silane undergo exchange with a hydrogen atom, bonded 
directly to the silicon atom, of the alkylhydrogenopolysiloxane. At the 
same time, polysiloxanes in which the Si-H hydrogen is partially or 
completely replaced by alkoxy groups, are formed. 
Suitable alkylhydrogenopolysiloxanes for the process according to the 
invention are those of the general formula 
##STR2## 
wherein R has the abovementioned meaning, and 
x can be 0 to 500 and 
y can be 2 to 500, preferably 3 to 70. 
The commercially available methylsiloxanes are preferred, but other 
siloxanes, such as, for example, also phenylsiloxanes, ethylsiloxanes, 
vinylsiloxanes and mixtures of these, are suitable as well. Halogen and/or 
C.sub.1-4 -alkyl or -alkoxy substituents may also be present, e.g. 
chloromethylsilanes. 
Furthermore, polymers can be employed as the starting substance, such as, 
for example, 
##STR3## 
wherein R, x and y independently of one another have the above meanings 
and 
z is 0-10, 
or cyclic compounds such as 
##STR4## 
The reaction takes place in the presence of siloxane polymerization 
catalysts which are in themselves known and are redox-stable under the 
particular reaction conditions. 
Examples of suitable catalysts are alkali metals, such as sodium, their 
alcoholates and hydroxides, such as sodium methylate and potassium 
hydroxide, tert.-amines, strong acids, bleaching earths and metal halides, 
such as, for example, AlCl.sub.3, or other compounds, such as, for 
example, Al acetylacetonate. Further suitable catalysts are described in 
W. Noll, loc. cit., page 81 and 188-197, the disclosure of which is 
incorporated herein by reference. 
The preparation according to the invention can be carried out in a simple 
manner in a conventional reaction vessel; the temperature in the reaction 
vessel must be selected so that the desired reaction product distils off 
rapidly. Reduced pressure or, where appropriate, excess pressure can also 
be used for this purpose. As a rule, however, the preparation is carried 
out under normal pressure. The reaction temperature is about 30.degree. C. 
to 300.degree. C., in particular about 100.degree. C. to 250.degree. C. It 
proves advantageous to carry out the reaction bhy initially mixing a part 
of the alkoxysilane with the catalyst, metering in the remaining reaction 
mixture and distilling off the reaction product at the rate at which it is 
formed; however, other ways of conducting the reaction are also possible. 
The amount of the catalyst can be varied in accordance with the desired 
rate of reaction; the range of about 0.1 to 10% by weight, relative to the 
starting compounds, has however proved advantageous. About 0.5-5% by 
weight is particularly preferred. 
Furthermore, it can be advantageous, for the purpose of reducing the 
viscosity, to work in an inert solvent of which the boiling point is 
higher than that of the desired product, such as, for example, to carry 
out the reaction in toluene or xylene.

The examples which follow are intended further to illustrate the process 
according to the invention. Percentage data are percentage by weight data, 
unless stated otherwise. The abbreviations Me and Et represent CH.sub.3 
and C.sub.2 H.sub.5 --respectively; M represents a (CH.sub.3).sub.3 
SiO--group and D represents a --(CH.sub.3)SiO--group. 
EXAMPLE 1 
10 g of AlCl.sub.3 (1.0%) and 100 g of CH.sub.3 Si(OC.sub.2 H.sub.5).sub.3 
(0.56 mol) were initially introduced into a 1 1 three-necked flask 
equipped with a thermometer, magnetic stirrer, dropping funnel, 
mirror-coated column (80 cm long, 8 cm diameter, 6 mm Raschig ring 
packing), distillation head and bubble counter at the apparatus outlet. 
The apparatus was flushed thorougly with dry N.sub.2 ; the material in the 
flask was then heated to 140.degree. C. and a mixture of 606 g of CH.sub.3 
Si(OC.sub.2 H.sub.5).sub.3 (3.4 mols) and 307 g of 
methylhydrogenopolysiloxane of the formula (CH.sub.3).sub.3 Si-O[HCH.sub.3 
SiO].sub.30 Si(CH.sub.3).sub.3 (about 4.7 mols of SiH) was then added 
dropwise over the course of 5 hours. At the same time, a distillate was 
taken off, at the rate at which it was formed, at a boiling point of 
between 95 and 103.degree. C.; according to analysis by gas 
chromatography, this distillate contained 94.4% of HCH.sub.3 Si(OC.sub.2 
H.sub.5).sub.2. 497 g of distillate were obtained (3.49 mols of HCH.sub.3 
Si(OC.sub.2 H.sub.5).sub.2 or 88% relative to CH.sub.3 Si(OC.sub.2 
H.sub.3).sub.3). The bottom temperature rose, during this time, to 
210.degree. C. The apparatus was then flushed with N.sub.2 until it had 
cooled. 507 g of a mobile liquid remain; according to analysis by NMR 
spectroscopy, this liquid had the approximate composition 
##STR5## 
EXAMPLE 2 
756 g of MeSi(OEt).sub.3 (4.24 mols), 20 g of AlCL.sub.3 (1.88%) and 307 g 
of a methylhydrogenopolysiloxane of the formula MD.sub.11.sup.H M (4.1 
mols of SiH) were reacted in the same manner as in Example 1. 
In this reaction, a distillate was obtained which according to analysis by 
gas chromatography consisted of 89.3% of MeHSi(OEt).sub.2. 590 g of 
distillate were obtained (3.92 mols of MeHSi(OEt).sub.2 or 95.7% relative 
to SiH employed). 
475 g of a mobile liquid remained; according to analysis by NMR 
spectroscopy, the liquid had the approximate composition 
EQU MD.sub.18.sup.OET M. 
EXAMPLE 3 
626 g of MeSi(OEt).sub.3 (2.95 mols), 10 g of aluminum acetylacetonate 
(1.3%) and 232 g of a methylhydrogenopolysiloxane of the formula 
MD.sub.11.sup.H M (3.1 mols of SiH) were reacted in the same manner as in 
Example 1. In this reaction, a distillate was obtained which according to 
analysis by gas chromatography consisted of 66.4% of MeHSi(OEt).sub.2. 346 
g of distillate were obtained (1.71 mols of MeHSi(OEt).sub.2 or 55.2% 
relative to SiH employed). 409 g of a mobile liquid remained; according to 
analysis by NMR spectroscopy, the liquid had the approximate composition 
MD.sub.23.sup.OEt M. 
EXAMPLE 4 
353 g of MeSi(OEt).sub.3 (1.98 mols), 2.5 g of KOH (0.5%) and 154 g of a 
methylhydrogenopolysiloxane of the formula MD.sub.11.sup.H M (2.06 mols of 
SiH) were reacted in the same manner as in Example 1. The yield of 
MeHSi(OEt).sub.2, according to analysis by gas chromatography, was 32.1%, 
relative to SiH. 
EXAMPLE 5 
180 g of MeSi(OEt).sub.3 (1.01 mols), 15.2 g of tributylamine (6.1%) and 70 
g of a methylhydrogenopolysiloxane of the formula MD.sub.54.sup.H M (1.1 
mols of SiH) were reacted in the same manner as in Example 1. The yield of 
MeHSi(OEt).sub.2, according to analysis by gas chromatography, was 13.2% 
relative to SiH. 
EXAMPLE 6 
180 g of MeSi(OEt).sub.3 (1.01 mols), 2 g of C.sub.4 F.sub.9 SO.sub.3 H 
(0.8%) and 70 g of a methylhydrogenopolysiloxane of the formula 
MD.sub.54.sup.H M (1.1 mols of SiH) were reacted in the same manner as in 
Example 5. The yield of MeHSi(OEt).sub.2, according to analysis by gas 
chromatography, was 68.4%, relative to SiH. 
EXAMPLE 7 
180 g of MeSi(OEt).sub.3 (1.01 mols), 25 g (10%) of bleaching earth and 70 
g of a methylhydrogenopolysiloxane of the formula MD.sub.54.sup.H M (1.1 
mols of SiH) were reacted in the same manner as in Example 5. The yield of 
MeHSi(OEt).sub.2, according to analysis by gas chromatography, was 33.9%, 
relative to SiH. 
EXAMPLE 8 
520 g of Me.sub.2 Si(OEt).sub.2 (3.51 mols), 37.5 (5.0%) of AlCl.sub.3 and 
230 g of a methylhydrogenopolysiloxane of the formula MD.sub.30.sup.H M 
(3.51 mols of SiH) were reacted in the same manner as in Example 1. 
In this reaction, a distillate was taken off at a boiling point of 
50.degree. to 60.degree. C.; according to analysis by gas chromatography, 
it contained 73.8% of Me.sub.2 HSiOEt. 376 g of distillate were obtained. 
(2.66 mols of Me.sub.2 HSiOEt or 75.9%, relative to Me.sub.2 Si(OEt).sub.2 
employed). 
It will be appreciated that the instant specification and examples are set 
forth by way of illustration and not limitation, and that various 
modifications and changes may be made without departing from the spirit 
and scope of the present invention.