GB1581856A

GB1581856A - Process for the manufacture of alkoxymethylsilazanes

Info

Publication number: GB1581856A
Application number: GB15851/77A
Authority: GB
Original assignee: TH Goldschmidt AG
Current assignee: Evonik Operations GmbH
Priority date: 1977-04-15
Filing date: 1977-04-15
Publication date: 1980-12-31
Also published as: DE2807230A1; DE2807230B2; DE2807230C3; ES467849A1

Description

(54) PROCESS FOR THE MANUFACTURE OF ALKOXYMETHYLSILAZANES (71) We, TH. GOLDSCHMIDT AG., a German Body Corporate of Goldschmidtstrasse 100, 4300 Essen, Germany, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to a process for the manufacture of compounds of the formula

wherein X is an alkoxy group derived by removal of the hydroxyl hydrogen atom from a primary or secondary alcohol with up to 4 carbon atoms and preferably is methoxy, Y is identical to X or is a methyl group and a is 0 or 1.

In preparative organic chemistry, the silylation reaction has gained considerable importance as a reaction for introducing protective groups. Since this reaction takes place under mild conditions, it is frequently used in reactions of natural products, antibiotics or other compounds which are sensitive to reactions.

Generally, hexamethyldisilazane or derivatives prepared therefrom are used for carrying out silylation reactions. For the preparation of hexamethyldisilazane, however, trimethylchlorosilane is required which is obtained only in small amounts during the Rochow synthesis. The production of trimethylchlorosilane is not increased because the bulk of the halogenosilanes formed cannot be utilised commercially in this quantity. Thus. hexamethyldisilazane is only available in limited amounts so that there is a demand for other compounds which can likewise be used for silylation and which can be prepared from more readily accessible halogenosilanes. The alkoxymethylsilazanes of the formula II indicated hereinafter are examples of such halogenosilanes which can be used.

It is known to prepare the types of compound of formula I by reacting a dimethylalkoxychlorosilane or a methyldialkoxychlorosilane with ammonia in the presence of an inert solvent, for example n-hexane or methylene chloride. Dimethylalkoxvchlorosilanes are usually obtained from dimethyldichlorosilane by reaction with an alcohol, the ratio of quantities in the reaction selected so that only one Cl radical is exchanged for the alkoxy group. Methyldialkoxychlorosilanes are usually obtained from methyltrichlorosilane likewise by reaction with an alcohol: the ratio of quantities in the reaction are selected so that two Cl radicals are exchanged for the alkoxy group, and a slight excess of alcohol can be desirable in order to keep the content of methyldichloroalkoxysilanes as low as possible.

Hitherto. the use of inert solvents in the preparation of dialkoxytetramethyldisilazane or tetraalkoxydimethyldisilazane appeared to be necessary because the ammonium chloride formed during the reaction hinders or prevents thorough stirring of the reaction mixture unless sufficient amounts of solvent are used. The ammonium chloride precipitates in a voluminous form and additionally impairs the course of the reaction because it occludes the starting silanes.

The additional use of solvents. however adversely affects the working-up of the reaction product since a fractional distillation is necessary to separate the solvents from the dialkoxytetramethyldisilazane or tetraalkoxydimethyldisilazane which can also be distilled.

Furthermore, the solvent necessarily reduces the throughput in an apparatus of given size.

The present invention provides a process for the manufacture of a silazane of the formula

wherein X is an alkoxy group derived by removal of the hydroxyl hydrogen atom from a primary or secondary alcohol with up to 4 carbon atoms, Y is identical to X or is a methyl group and a is 0 or 1, which process comprises reacting a silane of the formula

and optionally up to 50 mol %, based on the silane of formula II of a silane of formula

wherein X and Y are as defined above, with ammonia, to form the product silazane and ammonium chloride. a preformed silazane of formula I present from the beginning of the reaction being used as solvent. and separating the silazane and ammonium chloride by washing out the ammonium chloride or by distillation or centrifugation.

By means of this process, silazanes of formula I can be prepared without using extraneous solvents and with a high space-time yield. Moreover, the ammonium chloride obtained in the reaction as a by-product can be formed in coarsely crystalline form so that the reaction batch can be stirred more readily and, at the same time, the danger of occlusion of unconverted reaction products can be avoided.

The dimethylalkoxychlorosilanes and methyldialkoxychlorosilanes which are crude products of the reaction between dimethyldichlorosilane and methyltrichlorosilane respectively, and the relevant alcohols are preferably employed in the process according to the invention. Such crude products usually contain relatively large amounts of dimethyldialkoxysilanes or methyltrialkoxysilanes.

A preferred process is one in which a silazane of formula I. which optionally can contain a dimethyldialkoxysilane or a methyltrialkoxysilane, is introduced initially into a reaction vessel, gaseous ammonia is passed in until the mixture is saturated, and, whilst further ammonia is passed in, a silane of the formula

wherein X and Y are as defined above, is added, the mol ratio of ammonia : silane being at least 1.5 l and the reaction temperature not exceeding 90"C. and. preferably, any unconverted silane is then distilled off and subsequently either the ammonium chloride formed is washed out with water or the resulting silazane is separated off from the ammonium chloiide by distillation.

Preferably, silazane as solvent is introduced initiallv in at most that quantity which corresponds to the amount of silazane to be formed. The minimum amount of silazane to be introduced initially depcnds on the characteristics of the apparatus. It is mainly determined by the smallest amount which can still be intensively agitated and stirred through in a vessel with the existing stirrer device.

In the above preferred process, in order to accelerate and to ensure the saturation of the silazane, introduced initially, with ammonia it is advisable to pass ammonia into the disilazane, whilst maintaining a slight excess pressure. It has been found that an excess pressure of ammonia of about 100 to 200 mm Hg is sufficient for this purpose. Ammonia and further silane are now introduced simultaneously into the silazane saturated with ammonia. Heat is liberated during the reaction. Care is taken that the temperature does not exceed 90"C, preferably 75"C, by cooling the reaction mixture, in order to prevent decomposition and sublimation of the ammonium chloride. This is especially important since subliming ammonium chloride blocks the pipelines and leads to the decomposition of ammonium chloride with the formation of unwanted by-products.

The mol ratio of ammonia : silane should preferably be at least 1.5 : 1 corresponding to the equation

An excess of ammonia is desirable; however, an excess of halogenosilane should generally be avoided.

In the preferred process, ammonia and silane are introduced into the reaction vessel until the latter has reached a maximum filling level which still permits the reaction mixture to be worked up. If appropriate, unconverted silane is then distilled out of the reaction mixture at temperatures which should not exceed 90"C, preferably 75"C. If the boiling point of the silane at normal pressure is at or above this temperature, the pressure during the distillation must be correspondingly reduced.

The further working-up of the reaction product in this preferred process can be carried out either by washing the ammonium chloride out of the reaction mixture, for example with water or by separating off the resulting silazane from the ammonium chloride wholly or partially by distillation or by centrifuging off the ammonium chloride.

The ammonium chloride can be washed out with water, dilute ammonia or salt solutions.

In order to prevent a hydrolytic attack on the disilazane formed during this step, it is advisable to keep the temperature as low as possible during washing out. Moreover, the product/water mixture should not be stirred with excessive intensity during washing out, since the risk of hydrolysis is thus increased. If the ammonium chloride is washed with water, as stated above, great care must be taken during the preceding step of distilling off the unconverted dimethylalkoxychlorosilane or methyldialkoxychlorosilane, that the halogenosilanes contained in the reaction mixture are quantitatively removed in order to prevent hydrolysis reactions and hence the formation of by-products in the reaction mixture.

The silazane which has been freed from ammonium chloride by washing out, can now be dried in a manner which is in itself known and customary, for example using sodium-sulphate, free from water of crystallisation, or with the aid of molecular sieves.

If the silazane is separated off from the ammonium chloride by distillation, care must again be taken that the distillation is carried out at a temperature which does not exceed 90"C, preferably 75"C. In order to maintain the desired distillation temperature, the pressure should be reduced correspondingly.

It is also possible. however, to separate off the ammonium chloride by means of a centrifuge. This can be effected either continuously during the reaction, the filtrate then being continuously recycled to the reactor, or after the reaction of the chlorosilanes with NH3 is complete.

Advantageouslv. the process according to the invention is operated with recycle. A part of the silazane formed is re-used in the next reaction batch as the solvent. The distillate which had been obtained in a preceding reaction during the removal of the unconverted silane can be added to this silazane. If a crude dimethylalkoxychlorosilane or methyl dialkoxychlorosilane which still contained proportions of dimethyldichlorosilane and dimethyldiakoxysilane or methylalkoxydichlorosilane and methyltrialkoxysilane was used. the proportions of dimethyldialkoxysilane or methyltrialkoxysilane are also present in this distillate. It is an advantage of the process according to the invention that it is possible to use crude dimethylalkoxychlorosilane or methyldialkoxychlorosilane which has been obtained by reacting dimethyldichlorosilane with approximately equimolar amounts of alcohol, or by reacting 1 mol of methyltrichlorosilane with approximately 2 mols of alcohol, without a fractional distillation of the products of the process. If dimethyldichlorosilane is present in the crude dimethylalkoxychlorosilane, this forms 1,5-dialkoxy-1,1,3,3,5,5- hexamethyltrisilazane in accordance with the equation:

If the crude mcthyldialkoxychlorosilane contains methylalkoxydichlorosilane, this forms 1,1 ,3,3,5,5-hexaalkoxy- 1 ,3,5-trimethyltrisilazane in accordance with the equation:

These trisilazanes can likewise be used as silylating agents and do not reprcsent impurities which adversely affect the utilisation of the product of the process, but rather are useful products of the process of the present invention (a = 1).

It was surprising that the synthesis of the silazanes mentioned can be carried out without inert extraneous solvents and that a reaction mixture which can be readily worked up is obtained. It was not foreseeable that an industrially useful conversion takes place in the salt scurry if merely thorough mixing of the latter is provided. Furthermore, it was surprising that the end product can be separated out of this salt slurry by a water wash without noticeable hydrolysis, or that the end product can be separated off by distillation from the salt which has a strong tendency to sublime and which, furthermore, is capable of occluding the end product. In an attempt to separate the end product out by filtration, however. only a very low yield is obtained. The remaining salt slurry firmly occludes the bulk of the end product. In this connection it is also surprising that the ammonium chloride can be separated off by centrifuging.

Especially surprisingly, it has been found that the ammonium chloride obtained as the by-product is coarsely crystalline whenever the silazane is initially introduced as the indigenous solvent and care is taken that the reaction of the silane with ammonia takes place in at least equimolar amounts, but preferably with a certain excess of ammonia. Those skilled in the art could not foresee that. in contrast to other solvents hitherto used, the silazanes mentioned have these properties which affect crystallisation in the case of ammonium chloride.

The process according to the invention is illustrated by the Examples which follow.

The dimethylalkoxychlorosane required in Examples 1 to 6 was obtained from dimethyldichlorosilane by reaction with the corresponding alcohol. l mol of alcohol being employed per mol of dimethyldichlorosilane. This synthesis may be briefly described as follows: dimethyldichlorosilane was initiallv introduced into the reaction vessel. The corresponding alcohol was then added dropwise. whilst stirring. in the course of 2 hours.

During this period. the sump temperature falls to about () C. The mixture was then slowlv heated up to a sump temperature of 6()0C and this temperature was maintained for one hour. After cooling. the diniethylalkoxychlorosilane prepared in this way was filled into containers.

The methyldicthoxvchlorosilnne also required as a starting material was prepared in the same manner as just described. but in this preparation t mol of methyltrichlorosilane was reacted with 2.2 mols (I (l() excess) of ethanol.

Example 1 300 g of 1,3-dimethoxy-1,1,3,3-tetramethyldisilazane (composition, determined by gas chromatography: 1.2% of dimethoxydimethylsilane, 93.0% of 1,3-dimethoxy-1,1,3,3tetramethyldisilazane and 5.0% of 1 ,5-dimethoxy-1 1,3,3,5 ,5-hexamethyltrisilazane) and 300 g of distillate (composition, determined by gas chromatography: 4.0% of dimethylmethoxychlorosilane, 88.2% of dimethoxydimethylsilane and 7.8% of 1,3-dimethoxy 1,1,3,3-tetramethyldisilazane) were initially introduced into a reactor (useful volume 4 1.) cooled with water. Thereafter, the container was evacuated and ammonia was then passed into the liquid phase via a dip tube, whilst stirring, until an excess pressure of ammonia of about 150 mm mercury gauge was established. Subsequently, 1,000 g of dimethylmethoxychlorosilane (composition, determined by gas chromatography: 6.2% of dimethyldichlorosilane, 82.7% of dimethylmethoxychlorosilane and 8.3% of dimethyldimethoxysilane) were added via a dropping funnel in the course of 4 hours, an amount of ammonia always equivalent to the dimethylmethoxychlorosilane being passed in at the same time. For metering in the dimethylmethoxychlorosilane, an excess pressure of nitrogen of about 250 mm mercury gauge was applied to the dropping funnel. An excess pressure of ammonia of 150 - 200 mm mercury gauge was maintained during this entire phase. The internal temperature rose up to 56"C. After all the dimethylmethoxychlorosilane had been added dropwise, the mixture was stirred for a further 30 minutes under the quoted excess pressure of ammonia. This excess pressure of ammonia was also maintained during the subsequent cooling phase down to room temperature. After the reactor had been vented, 1,290 g of water were added, whilst stirring, at an internal temperature of 18"C. The mixture was now stirred for 2 minutes. A phase separation occurred within 1 minute after the stirrer had been switched off. The lower phase consisted of an aqueous solution of ammonium chloride and was withdrawn. The remaining product phase contained small amounts of water. The product phase was dried by introducing 14 g of anhydrous sodium sulphate and was subsequently filtered. 1,345 g of product were obtained.

This product had the following composition. determined by gas chromatography: 25.4% of dimethoxydimethylsilane 1.3% of 1,3-dimethoxy-l ,1,3,3-tetramethyldisiloxane 60.8% of 1,3-dimethoxy-l,l ,3,3-tetramethyldisilazane 10.3% of 1,5-dimethoxy-1,1,3,3,5,5-hexamethyltrisilazane and 2.2% of further compounds The end product had an active proportion of silazane of 71.1%. The Si yield of active substances thus was 93.29 of theory, relative to dimethyldichlorosilane and dimethylmethoxychlorosilane employed.

Example 2 300 g of 1,3-dimethoxy-1,1,3,3-tetramethyldisilazane (composition, determined by gas chromatography: 1.2% of dimethoxydimethylsilane, 93.0% of 1,3-dimethoxy-1,1,3,3tetramethyldisilazane and 5.0% of 1,5-dimethoxy-1,1,3,3,5,5-hexamethyltrisilazane) were initially introduced into a reactor (useful volume 3 l.) cooled with water. The containerwas then evactuated and subsequently ammonia was passed into the liquid phase via a dip tube, whilst stirring, until an excess pressure of ammonia of about 150 mm mercury gauge was established. 1,000 g of dimethylmethoxychlorosilane (composition, determined by gas chromatography: 6.2% of dimethyldichlorosilane, 82.7% of dimethylmethoxychlorosilane and 8.3% of dimethvldimethoxysilane) were then added via a dropping funnel in the course of 5 hours, an amount of ammonia always equivalent to the dimethylmethoxychlorosilane being passed in at the same time. For metering in the dimethylmethoxychlorosilane, an excess pressure of nitrogen of about 250 mm mercury gauge was applied to the dropping funnel. An excess pressure of ammonia of 150 - 200 mm mercury gauge was maintained during this entire phase. The internal temperature rose up to 580C. After all the dimethvlmethoxvchlorosilane had been added dropwise, the mixture was stirred for a further 30 minutes under the quoted excess pressure of ammonia. No cooling was applied during these 3() minutes.

78 g were subsequently taken off from the reaction mixture by simple distillation. This distillation was started at a reduced pressure of 100 mm mercury gauge which was lowered to 80 mm mercury gauge towards the end. During the distillation. the maximum internal temperature was 75"C.

The distillate had the following composition. determined by gas chromatography: 5.1% of dimethylmethoxychlorosilane 80.2% of dimethyoxydimethylsilane and 14.7% of 1 ,3-dimethoxy-1 ,1,3 ,3-tetramethyldisilazane.

Aftcl the end of the distillation, the reaction mixture was cooled to 22"C, the reactor being charged at the same time with NH3, until an excess pressure of ammonia of about 150 mm mercury gauge was established. After the reactor had been vented, 1,283 g of water were added, whilst stirring. The mixture was now stirred for 2 minutes. A phase separation occurred within 1 minute after the stirrer had been switched off. The lower phase consisted of an aqueous solution of ammonium chloride and was withdrawn. The remaining product phase contained small amounts of water. This was dried by introducing 11 g of anhydrous sodium sulphate and was subsequently filtered. 976 g of product were obtained.

This product had the following composition, determined by gas chromatography: 2.2% of dimethoxydimethylsilane 1.2% of l,3-dimethoxy-1,1,3,3-tetramethyldisiloxane 79.4% of 1 ,3-dimethoxy-1 ,l ,3,3-tetramethyldisilazane 14.2% of 1,5-dimethoxy-1,1,3,3,5,5-hexamethyltrisilazane and 3.0% of further compounds.

The Si yield of silazanes suitable for silylations thus was 93.9% of theory, relative to dimethyldichlorosilane and dimethylmethoxychlorosilane employed. The distillate is again included since this is used as the indigenous solvent in the next reaction batch.

Example 3 300 g of 1,3-di-n-butoxy-l,1,3,3-tetramethyldisilazane (composition, determined bv gas chromatography: 4.6% of di-n-butoxydimethylsilane and 95.4% of 1,3-di-n-butoxy-1,1,3,3tetramethyldisilazane) and 3()() g of distillate (composition, determined by gas chromatography: 2.00/0 of dimethyl-n-butoxychlorosilane and 98.0% of di-n-butoxydimethylsilane) were initially introduced into a reactor (useful volume 3 1) cooled with water. The container was then evacuated and ammonia was passed into the liquid phase via a dip tube, whilst stirring, until an excess pressure of ammonia of about 150 mm mercury gauge was established. I ,000 g of dimethyl-n-butoxychlorosilane (composition, determined by gas chromatography: 3.1% of dimethyldichlorosilane, 83.8lie of dimethyl-n-butoxychlorosilane and 12.5% of dimethyl-di-n-butoxysilane) were then added via a dropping funnel in the course of 3 hours, an amount of ammonia always equivalent to the dimethyl-n-butoxychlorosilane being passed in at the same time. The further procedure followed was analogous to Example 2.

The ammonium chloride formed was washed out with 963 g of water. After drying with anhydrous sodium sulphate and subsequent filtration, 1,418 g of product were obtained.

This product had the following composition, determined by gas chromatography: 30.OC/e of di-n-butoxydimethylsilane ().6% of 1,3-di-n-butoxy-l.l ,3 ,3-tetramethyldisiloxane 63. 2% of 1 ,3-di-n-butoxy- 1,1,3 ,3-tetramethyldisilazane 5.80/e of 1.5-di-n-butoxy-1,1,3,3.3,5-hexamethyltrsilazane and 0.4% of further compounds.

The end product contained a proportion of active substances of 69Yc of silazanes. The Si yield of active substances thus was 96.3% of theory. relative to dimethyldichlorosilane and dimethyl-n-butoxychlorosilane employed.

Comparative Exa/71pkS 1,200 g of dimethylmethoxychlorosilane (composition, determined by gas chromatography: 4.4% of dimethyldichlorosilane, 84.26Ac of dimethylmethoxychlorosilane and 10.2% of dimethyldimcthoxvsilane) were initiallv introduced into a reactor (useful volume 3 1) cooled with water. The container was now evacuated and ammonia was then passed into the liquid phase via a dip tube. whilst stirring, until an excess pressure of ammonia of about 150 mm mercury gauge was established. The internal temperature rose up to 75"C. After 180 g of ammonia had been fed in in the course of 8 hours, that is to sav. at the stage of 75% of theory. a higher excess pressure of ammonia built up very rapidly. Further NH3 was only absorbed very slowly. The contents of the reactor no longer moved even though the stirrer was running. The resulting mixture of product and ammonium chloride was no longer properly mixed through. The vessel was evacuated and distillate was taken off. After 19() g had been taken off. the stirrcr stopped. Large amounts of ammonium chloride had deposited in the line systems and this had resulted in blockages. Smooth operation was thus no longer ensured. The 190 g of distillate had the following composition, determined by gas chromatography: 29.8% of dimethoxydimethylsilane 60.7% of dimethylmethoxychlorosilane and 9.5% of 1 ,3-dimethoxy-1 ,1,3 ,3-tetramethyldisilazane.

Thus, a reasonable yield of the desired end product could not be obtained for this reason.

When the distillation residue was worked up by washing with water, complete hydrolysis of the silazane occurred.

WHAT WE CLAIM IS: 1. A process for the manufacture of a silazane of the formula

wherein X is an alkoxy group derived by removal of the hydroxyl hydrogen atom from a primary or secondary alcohol with up to 4 carbon atoms; Y is identical to X or is a methyl group and a is 0 or 1, which process comprises reacting a silane of the formula

and optionally up to 50 mol %, based on the silane of formula II, of a silane of formula

wherein X and Y are as defined above, with ammonia, to form the product silazane and ammonium chloride, a preformed silazane of formula I present from the beginning of the reaction being used as solvent, and separating the silazane and ammonium chloride by washing out the ammonium chloride or by distillation or centrifugation.

2. A process according to claim 1 in which X is methoxy.

3. A process according to claim l or 2 in which an amount of silazane is introduced initially as solvent, which is at most equal to the amount of silazane to be formed and at least such that the reaction mixture can be stirred.

4. A process according to any one of the preceding claims, in which at least a part of the product silazane is recycled for use a solvent in the process.

5. A process according to any one of the preceding claims, in which unconverted silane distilled out of the reaction mixture is recycled for use as solvent in the process.

6. A process according to any one of the preceding claims, in which the silazane and ammonium chloride are separated by washing out the ammonium chloride or by distillation.

7. A process according to claim 6. in which a silazane of formula I, is introduced initially into a reaction vessel gaseous ammonium is passed in until the mixture is saturated and, whilst further ammonia is passed in. a silane of formula

wherein X and Y are as defined in claim I or 2 is added, the mol ratio of ammonia : silane being at least 1.5 : I and the reaction temperature not exceeding 9() C, and the silazane and the ammonium chloride formed are separated by washing out the ammonium chloride with water or by distillation at a temperature of not more than 90"C.

8. A process according to claim 7. in which the silazane of formula I introduced initially

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. no longer ensured. The 190 g of distillate had the following composition, determined by gas chromatography: 29.8% of dimethoxydimethylsilane 60.7% of dimethylmethoxychlorosilane and 9.5% of 1 ,3-dimethoxy-1 ,1,3 ,3-tetramethyldisilazane. Thus, a reasonable yield of the desired end product could not be obtained for this reason. When the distillation residue was worked up by washing with water, complete hydrolysis of the silazane occurred. WHAT WE CLAIM IS:

1. A process for the manufacture of a silazane of the formula

2. A process according to claim 1 in which X is methoxy.

contains a dimethyldialkoxysilane or methyltnalkoxysilane.

9. A process according to claim 7 or 8, in which the silazane formed is distilled off from the ammonium chloride at a temperature of not more than 75"C.

10. A process according to any one of claims 7 to 9, in which unconverted silane is distilled out of the reaction mixture at a temperature of not more than 90"C.

l l. A process according to claim 10. in which unconverted silane is distilled out of the reaction mixture at a temperature of not more than 75"C.

12. A process according to claim 6 substantially as described in any one of the Examples.

13. A silazane of formula l as defined in claim 1 or 2 whenever prepared by a process as claimcd in any one of claims 1 to 5.

14. A silazane of formula I as defined in claim 1 or 2 whenever prepared by a process as claimed in any one of claims 6 to 12.