Chemistry

Corresponding Member of the Academy of Sciences of the USSR K. A. Andrianov and G. Ya. Rumba

On Rearrangements of Hexamethyl- and Octamethylcyclosilazanes

There are no data in the literature on rearrangements of organocyclosilazanes. We found that in the series of dimethylcyclosilazanes a rearrangement is observed with a change in the silicon–nitrogen framework of the molecules. Thus, for example, when hexamethylcyclotrisilazane is acted upon by trimethylchlorosilane and ammonia, ring expansion already occurs at 20–40°. Along with the normal reaction products, namely octamethyltrisilazane and a polymer (with a molecular weight of about 2000), which is formed in an amount of 12.3 and 18.9% of the high-boiling reaction products, 36.5% of octamethylcyclotetrasilazane is formed:

\[ \chemfig{% Si(-[:120]CH_3)(-[:60]CH_3) (-[:-135]HN(-[:-90]Si(-[:180]CH_3)(-[:-150]CH_3)(-[:-45]NH))) (-[:-45]NH(-[:-90]Si(-[:0]CH_3)(-[:-30]CH_3)(-[:-135]NH))) } \;+\; (\mathrm{CH_3})_3\mathrm{SiCl}+\mathrm{NH_3} \;\longrightarrow \]

\[ \longrightarrow\; (\mathrm{CH_3})_3\mathrm{SiNHSi}(\mathrm{CH_3})_2\mathrm{NHSi}(\mathrm{CH_3})_3 \;+\; \chemfig{% Si(-[:90]CH_3)(-[:180]CH_3) (-[:0]NH-Si(-[:90]CH_3)(-[:0]CH_3)(-[:-90]NH-Si(-[:-90]CH_3)(-[:180]CH_3)-NH-)) (-[:-90]HN-Si(-[:-90]CH_3)(-[:180]CH_3)-NH-) } \;+\;\text{polymers} \]

When hexamethylcyclotrisilazane is heated for 48 hr at 250° in the presence of ammonium chloride, about 3% octamethylcyclotetrasilazane is formed.

The rearrangement reaction in the presence of trimethylchlorosilane and ammonia, or of ammonium chloride, may be represented as follows: under the conditions of our experiments, hydrogen chloride is formed upon the action of ammonia on trimethylchlorosilane, or as a result of the thermal dissociation of ammonium chloride. Hydrogen chloride, acting on the silicon–nitrogen bond, opens the ring:

\[ \chemfig{% Si(-[:120]CH_3)(-[:60]CH_3) (-[:-135]HN(-[:-90]Si(-[:180]CH_3)(-[:-150]CH_3)(-[:-45]NH))) (-[:-45]NH(-[:-90]Si(-[:0]CH_3)(-[:-30]CH_3)(-[:-135]NH))) } \;+\;\mathrm{HCl} \;\rightleftharpoons\; \chemfig{% Si(-[:120]CH_3)(-[:60]CH_3)(-[:0]Cl) (-[:-135]HN(-[:-90]Si(-[:180]CH_3)(-[:-150]CH_3)(-[:-45]NH))) (-[:-45]NH_2-[:0]Si(-[:0]CH_3)(-[:-30]CH_3)(-[:-135]NH)) } \]

Ammonia reacts with chlorine bound to silicon

\[ \begin{gathered} \text{chlorinated hexamethylcyclotrisilazane derivative} \;+\;\mathrm{NH_3} \rightleftarrows \text{amino hexamethylcyclotrisilazane derivative} \;+\;\mathrm{HCl}. \end{gathered} \]

Thus hydrogen chloride is regenerated, which cleaves the silicon–nitrogen bond according to the scheme:

\[ \begin{gathered} \text{amino hexamethylcyclotrisilazane derivative} \;+\;\mathrm{HCl} \rightleftarrows \text{chlorosilyl- and aminosilyl-containing linear intermediates} \\ \text{or} \\ \text{chlorosilyl- and aminosilyl-containing cyclic intermediates}. \end{gathered} \]

The intermediate compounds formed as a result of recombination give both hexamethylcyclotrisilazane and octamethylcyclotetrasilazane. The same result was obtained when hexamethylcyclotrisilazane was treated with triethylmethylammonium iodide at \(300^\circ\).

An analogous rearrangement reaction with ring expansion was also observed by us when hexamethylcyclotrisilazane was treated with catalytic amounts of caustic potash at \(160^\circ\). A small amount of octamethylcyclotetrasilazane was isolated. It is of special interest to note that, on heating octamethylcyclotetrasilazane with catalytic amounts of ammonium chloride at \(280^\circ\), rearrangement with ring contraction occurs, and hexamethylcyclotrisilazane is formed in low yield:

\[ 3\,\text{octamethylcyclotetrasilazane} \;\xrightarrow[\;]{\mathrm{NH_4Cl}}\; 4\,\text{hexamethylcyclotrisilazane}. \]

Rearrangement with ring contraction also occurs when octamethylcyclotetrasilazane is treated with aniline at \(250\text{—}300^\circ\). Here, along with the transamination reaction, rearrangement also takes place, and about \(20\%\) of tri-\(N\)-phenylhexamethylcyclotrisilazane is formed:

\[ 3\,\text{octamethylcyclotetrasilazane} +12\,\mathrm{C_6H_5NH_2} \rightarrow 4\,\text{tri-}N\text{-phenylhexamethylcyclotrisilazane} +12\,\mathrm{NH_3}. \]

The experimental conditions are given in Table 1.

Experimental Part

1. Aminolysis of trimethylchlorosilane in a solution of hexamethylcyclotrisilazane. In \(65.4\) g (\(0.3\) g-mol) of hexamethylcyclotrisilazane, \(7\) ml (\(0.06\) g-mol) of trimethylchlorosilane was dissolved.

With vigorous stirring, dry ammonia was introduced. During the reaction a white precipitate separated. The reaction mixture was filtered. On washing with petroleum ether, part of the precipitate dissolved. After evaporation of the resulting solution, 5.8 g (8.3%) of a crystalline substance with m.p. 96° was obtained. The filtrate was distilled, collecting fractions: 165–180°, 1.50 g, 28%, \(n_D^{20}\) 1.4300, corresponding to octamethyltrisilazane; 180–188°, 17.73 g, 33.6%, \(n_D^{20}\) 1.4443. From the residue 14.9 g (28.3%) of a crystalline substance with m.p. 96° was isolated. After removal of volatiles up to 250°, 6.5 g (12.3%) of a light-yellow, vaseline-like product with an average molecular weight of 2000 remained.

1. Heating hexamethylcyclotrisilazane with ammonium chloride. 21.9 g (0.1 g-mol) of hexamethylcyclotrisilazane and 0.1 g of ammonium chloride were heated for 48 h at 220–250°. After heating, 0.65 g of crystals with m.p. 96° was isolated.

2. Heating hexamethylcyclotrisilazane with triethylmethylammonium iodide. 21.9 g (0.1 g-mol) of hexamethylcyclotrisilazane and 0.1 g of triethylmethylammonium iodide were heated for 17 h at 300°. After heating, 0.8 g of crystals with m.p. 96° was isolated.

Table 1

1. Heating hexamethylcyclotrisilazane with caustic potash. 10.0 g of hexamethylcyclotrisilazane and 0.1 g of caustic potash were heated for 216 h at 160°. The reaction mixture was dissolved in petroleum ether, filtered, and evaporated. 4.9 g of tricyclotridecamethylheptasilazane and 0.1 g of crystals with m.p. 96° were isolated.

2. Heating octamethylcyclotetrasilazane with aniline. 29.2 g (0.1 g-mol) of octamethylcyclotetrasilazane and 18.6 g (0.2 g-mol) of aniline were heated for 24 h at 300°. The reaction mixture was distilled; from the distillate, 5.8 g of crystals with m.p. 248–249° was isolated.

Found, %: C 64.47, 64.37; H 7.87, 7.92; Si 18.65, 19.02; N 10.78, 10.75.
\(C_{24}H_{33}Si_3N_3\). Calculated, %: C 64.38; H 7.43; Si 18.80; N 9.38.

For tri-N-phenylhexamethylcyclotrisilazane, \(C_{24}H_{33}Si_3N_3\), calculated, %: C 64.38; H 7.43; Si 18.80; N 9.38.

1. Heating octamethylcyclotetrasilazane with ammonium chloride. 5 g of octamethylcyclotetrasilazane and 0.1 g of ammonium chloride were heated for 72 h at 300°. After cooling, a drop of liquid with \(n_D^{20}\) 1.4436 was pressed out.

29.2 g (0.1 g-mol) of octamethylcyclotetrasilazane, 1.6 g (0.01 g-mol) of hexamethylcyclotrisilazane, and 0.5 g of ammonium chloride were heated for 48 h at 245°. From the reaction mixture 2.52 g of liquid with b.p. 186–188° and \(n_D^{20}\) 1.4436 was distilled off.

Institute of Organoelement Compounds
Academy of Sciences of the USSR

Received
17 V 1962

CITED LITERATURE

1. S. D. Brewer, Ch. P. Haber, J. Am. Chem. Soc., 70, 3888 (1948).
2. E. Larsson, B. Smith, Acta chem. scand., 3, 487 (1949).