Abstract
Full Text
Chemistry
L. L. Shchukovskaya, M. G. Voronkov, and O. V. Pavlova
New Routes for the Synthesis of B-Monohalogen-Substituted N-Dialkylborazenes and N-Trialkylborazanes
(Presented by Academician A. V. Topchiev, 18 XI 1961)
Recently \((^{1})\) a method was described for obtaining the previously rather inaccessible N-alkyl-B-monohaloborazanes by the action of the corresponding hydrogen halide on N-alkylborazanes according to the scheme:
\[ \mathrm{R}_n\mathrm{H}_{3-n}\mathrm{N}\cdot \mathrm{BH}_3 + \mathrm{HX} \rightarrow \mathrm{R}_n\mathrm{H}_{3-n}\mathrm{N}\cdot \mathrm{BH}_2\mathrm{X} + \mathrm{H}_2, \]
\[ \mathrm{X}=\mathrm{Cl},\ \mathrm{Br},\ \mathrm{J}^* \]
Along with hydrogen halides, free halogens (chlorine, bromine, and iodine), as well as boron halides, were used. Reduction of boron halides with lithium or sodium borohydrides leads to complete replacement of all halogens at the boron atom by hydrogen \((^{2})\), and compounds containing both hydrogen and halogens at the boron atom cannot be obtained. We have developed a new method for the synthesis of B-monohalogen-substituted borazenes and borazanes, based on the reduction of readily available B-trihalogen-N-trialkylborazanes (complexes of boron trihalides with trialkylamines) and N-dialkyl-B-dihalogenoborazenes by compounds containing the Si—H bond. As such compounds we used triethylsilane, which readily reduces halogen derivatives of a series of elements \((^{3,4})\).
The reaction proceeds smoothly in the presence of catalytic amounts of colloidal nickel according to the scheme:
\[ \mathrm{R}_3\mathrm{N}\cdot \mathrm{BX}_3 + (\mathrm{C}_2\mathrm{H}_5)_3\mathrm{SiH} \xrightarrow{\mathrm{Ni}} \mathrm{R}_3\mathrm{N}\cdot \mathrm{BH}_2\mathrm{X} + (\mathrm{C}_2\mathrm{H}_5)_3\mathrm{SiX}. \]
Small amounts of N-trialkylborazane \(\mathrm{R}_3\mathrm{N}\cdot \mathrm{BH}_3\) and \(\mathrm{R}_3\mathrm{N}\cdot \mathrm{HCl}\) are formed as by-products. Colloidal nickel is obtained directly in the reaction vessel by reduction of nickel chloride with triethylsilane. The reduction reaction of boron tribromide proceeds very vigorously even on cooling to \(-80^\circ\). We carried out the reduction by adding \(\mathrm{BBr}_3\) dropwise to a mixture of triethylsilane and triethylamine. It turned out that the reaction leads to the formation, in equal yield (\(\sim 35\%\)), of both \((\mathrm{C}_2\mathrm{H}_5)_3\mathrm{N}\cdot \mathrm{BH}_3\) and \((\mathrm{C}_2\mathrm{H}_5)_3\mathrm{N}\cdot \mathrm{BH}_2\mathrm{Br}\). The latter compound was obtained for the first time.
Reduction of the complex \((\mathrm{C}_2\mathrm{H}_5)_3\mathrm{N}\cdot \mathrm{BCl}_3\) at room temperature proceeds over many days. When heated to \(130^\circ\), the reaction proceeds much more rapidly, and, upon distillation of excess triethylsilane and the triethylchlorosilane formed from the filtrate, a crystallizing residue of N-triethyl-B-chloroborazane is rapidly obtained (yield 42%).
In the reduction of N-dialkyl-B-dihalogenoborazenes, previously unknown N-dialkyl-B-halogenoborazenes are formed:
\[ \mathrm{R}_2\mathrm{N}=\mathrm{BX}_2 + (\mathrm{C}_2\mathrm{H}_5)_3\mathrm{SiH} \xrightarrow{\mathrm{Ni}} \mathrm{R}_2\mathrm{N}=\mathrm{BHX} + (\mathrm{C}_2\mathrm{H}_5)_3\mathrm{SiX}. \]
In this way, for example, we obtained N-dimethyl-B-bromoborazene in 30% yield.
* In the case \(X=\mathrm{F}\), the reaction proceeds differently:
\[ \mathrm{RNH}_2\cdot \mathrm{BH}_3 + 3\mathrm{HF} \rightarrow \mathrm{RNH}_2\cdot \mathrm{BF}_3 + 3\mathrm{H}_2, \]
\[ \mathrm{R}=\mathrm{C}(\mathrm{CH}_3)_3, \]
with formation of N-alkyl-B-trifluoroborazane.
Experimental Part
Starting reagents: boron tribromide was obtained by the reaction of amorphous boron with bromine at 700–750° (5). Yield 74%, calculated on the starting bromine. $\mathrm{BCl_3}$ was prepared by heating a mixture of $\mathrm{AlCl_3}$ and $\mathrm{KBF_4}$ (6). Triethylsilane was synthesized by the action of ethylmagnesium bromide on ethyldichlorosilane. It was purified by distillation over sodium. B.p. 107–108°; $n_D^{20}$ 1.4117.
N-Dimethyl-B-dibromoborazene was prepared by the procedure of (7) and had b.p. 45.5–47° (10 mm); 162–163° (772 mm); $n_D^{20}$ 1.4965; $d_4^{20}$ 1.755. Yield 53.5%. Literature data (8): b.p. 163°, m.p. 21.5°.
N-Triethyl-B-trichloroborazane was obtained according to Lappert (9) and, after recrystallization from 80% alcohol, had m.p. 89–91°. Literature data: m.p. 91–92°.
Reduction of N-dimethyl-B-dibromoborazene, $(\mathrm{CH_3})_2\mathrm{N}=\mathrm{BBr_2}$. In a 0.1-liter three-necked flask equipped with a thermometer, dropping funnel, and reflux condenser connected to a trap cooled to −70°, 45.0 g of triethylsilane was placed, and at room temperature 25.0 g of N-dimethyl-B-dibromoborazene was gradually added dropwise. No warming was observed. The reaction mixture was left for 4 days. Even after this, and after 4 hours’ boiling (119–120°), no visible interaction of the components was observed. When a catalytic amount (~0.15 g) of $\mathrm{NiCl_2}$ was introduced into the flask, crystals immediately separated on its bottom and walls. To complete the reaction, the reaction mixture was heated to boiling (121°). After cooling, the precipitated crystals were separated, washed with dry petroleum ether, and sublimed twice in vacuo (68–70° at 10–11 mm). This gave 6 g (30% yield) of white, lustrous, needle-shaped crystals with m.p. 122–125°, possessing a camphor-like odor.
The substance is insoluble in water, readily soluble in hot petroleum ether, from which, after recrystallization, it has m.p. 122–124° (lustrous plates).
Found, %: N 10.42; 10.48; C 18.14; 18.17; H 5.37; 5.43; B 7.96; 8.19; Br 60.07
$\mathrm{C_2H_7BBrN}$. Calculated, %: N 10.31; C 17.68; H 5.19; B 7.96; Br 58.86
On distillation of the filtrate, 35.0 g of the starting triethylsilane was obtained (b.p. 106–110; $n_D^{20}$ 1.4145) and 18.0 g of triethylbromosilane (b.p. 160–165°; $n_D^{20}$ 1.4550). Along with these, a small amount of dimethylamine hydrobromide and 2.0 g of a liquid higher-boiling substance, which was not further investigated, were isolated.
Catalytic reaction of boron tribromide with triethylsilane in the presence of triethylamine. In an apparatus analogous to that described above, connected to a trap cooled with liquid nitrogen, 40 g of freshly distilled dry triethylsilane and ~0.2 g of $\mathrm{NiCl_2}$ were placed. The mixture was boiled until the $\mathrm{NiCl_2}$ was reduced. After cooling, 12 g of dry triethylamine was added to it, after which the apparatus was swept with dry nitrogen purified of oxygen. Then, with the flask cooled with solid carbon dioxide, 25 g of $\mathrm{BBr_3}$, also cooled with solid carbon dioxide, was added dropwise over the course of 40 min.
On the following day the precipitate that had formed, $(\mathrm{C_2H_5})_3\mathrm{N}\cdot\mathrm{HBr}$ (1.7 g), was filtered off and purified by sublimation in vacuo, m.p. 250°. The filtrate was distilled in vacuo. This gave 20 g of unchanged $(\mathrm{C_2H_5})_3\mathrm{SiH}$ and 30 g of $(\mathrm{C_2H_5})_3\mathrm{SiBr}$. The residue in the distillation flask with $n_D^{20}$ 1.4855 was fractionated twice in vacuo (<0.1 mm). In this process two substances were isolated: N-tri-
ethylborazane \((\mathrm{C}_2\mathrm{H}_5)_3\mathrm{N}\cdot \mathrm{BH}_3\) (I) (bath temperature \(90\text{–}95^\circ\)) \(n_D^{20}\) 1.4420; \(d_4^{20}\) 0.7885; yield 4 g (35%, calculated on the starting \(\mathrm{BBr}_3\)).
Found, %: C 62.27; 62.30; H 15.66; 15.85; B 9.36
\(\mathrm{C}_6\mathrm{H}_{18}\mathrm{BN}\). Calculated, %: C 62.70; H 15.70; B 9.40
Literature data for \((\mathrm{C}_2\mathrm{H}_5)_3\mathrm{N}\cdot \mathrm{BH}_3\) \((^{10})\): b.p. \(42^\circ\) \((10^{-4}\ \mathrm{mm})\); m.p. \(-2^\circ\).
\(N\)-Triethyl-\(B\)-bromoborazane \((\mathrm{C}_2\mathrm{H}_5)_3\mathrm{N}\cdot \mathrm{BH}_2\mathrm{Br}\) (II) (bath temperature \(150\text{–}155^\circ\)); crystals, after two recrystallizations from dry petroleum ether, m.p. \(65\text{–}67^\circ\); yield 7.0 g (36%, calculated on the starting \(\mathrm{BBr}_3\)).
Found, %: C 37.14; 37.28; H 8.80; 8.87; B 5.37; 5.57; Br 41.35; 41.65; N 7.20
\(\mathrm{C}_6\mathrm{H}_{17}\mathrm{BNBr}\). Calculated, %: C 37.15; H 8.76; B 5.57; Br 41.28; N 7.22
Reduction of \(N\)-triethyl-\(B\)-trichloroborazane. To a suspension of colloidal nickel in 12.0 g of triethylsilane there was added at room temperature, all at once, 7.0 g of unrecrystallized \((\mathrm{C}_2\mathrm{H}_5)_3\mathrm{N}\cdot \mathrm{BCl}_3\). The reaction mixture was heated for 6 h at \(85^\circ\), after which over 5 h the temperature was raised to \(130^\circ\). Upon cooling, the precipitate \((\mathrm{C}_2\mathrm{H}_5)_3\mathrm{N}\cdot \mathrm{HCl}\) (m.p. \(253^\circ\)) was filtered off (0.2 g). The filtrate was fractionated in vacuo. This gave, along with the starting \((\mathrm{C}_2\mathrm{H}_5)_3\mathrm{SiH}\) and the formed \((\mathrm{C}_2\mathrm{H}_5)_3\mathrm{SiCl}\), 2.0 g of crystals of \(N\)-triethyl-\(B\)-chloroborazane, which after two recrystallizations from dry petroleum ether had m.p. \(42\text{–}43^\circ\). Yield 42%. Literature data \((^1)\): m.p. \(43^\circ\).
In addition to \((\mathrm{C}_2\mathrm{H}_5)_3\mathrm{N}\cdot \mathrm{BH}_2\mathrm{Cl}\), 0.5 g of \((\mathrm{C}_2\mathrm{H}_5)_3\mathrm{N}\cdot \mathrm{BH}_3\) with \(n_D^{20}\) 1.4438, distilling in vacuo at a bath temperature of \(110^\circ\), was isolated.
The analyses were carried out in the analytical laboratory of the Institute of High-Molecular Compounds of the Academy of Sciences of the USSR under the direction of A. I. Lebedeva, to whom the authors express their deep gratitude.
Institute of Silicate Chemistry
Academy of Sciences of the USSR
Received
18 XI 1961
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