Full Text
Chemistry
Z. I. Sergeeva, D. D. Tsitovich, and M. G. Voronkov
A New Reaction of Trialkylsilanes with Acid Chlorides of Aliphatic Monocarboxylic Acids in the Presence of Aluminum Chloride
(Presented by Academician A. V. Topchiev, June 3, 1960)
Trialkylsilanes in the presence of $\mathrm{AlCl_3}$ readily reduce haloalkyls to the corresponding hydrocarbons ($^{1-3}$), and acid chlorides of aromatic acids to aldehydes ($^4$). The reaction proceeds according to the general scheme:
\[ \mathrm{R_3SiH + R'X \xrightarrow{AlCl_3} R_3SiX + R'H,} \]
where $\mathrm{R'}$ is a hydrocarbon or acyl radical, and $\mathrm{X}$ is a halogen.
It seemed of interest to study this reaction as applied to the acid chlorides of aliphatic monocarboxylic acids. To this end, we investigated the reduction by triethylsilane of the acid chlorides of acetic, n-butyric, trimethylacetic, and $\beta$-trimethylsilylpropionic acids. It was found that in the absence of aluminum chloride practically no interaction of the reagents is observed. However, upon addition to the mixture of triethylsilane and the acid chloride of catalytic amounts of $\mathrm{AlCl_3}$ (2–3 mol. %), strong heating of the mixture occurs immediately. The spontaneous boiling of the reaction mixture was subsequently maintained by gentle heating and was carried out for about four hours.
Table 1
Products of the reaction of triethylsilane with acid chlorides of aliphatic monocarboxylic acids in the presence of 2–3 mol. % $\mathrm{AlCl_3}$
| Starting $\mathrm{RCOCl}$ | Reaction product $\mathrm{RCOOCH_2R^*}$: compound | Reaction product $\mathrm{RCOOCH_2R^*}$: b.p., °C (mm Hg) | $d_4^{20}$ | $n_D^{20}$ | $MR_D$ found | $MR_D$ calc. | $\mathrm{RCOOCH_2R^*}$ yield, % | $(\mathrm{C_2H_5})_3\mathrm{SiCl}^{**}$ yield, % |
|---|---|---|---|---|---|---|---|---|
| $\mathrm{CH_3COCl}$ | $\mathrm{CH_3COOC_2H_5}$ | a) 76–79 (759) | 0.8990 | 1.3727 | 22.34 | 22.28 | 54.2 | 66.3 |
| $\mathrm{\mathit{n}\text{-}C_3H_7COCl}$ | $\mathrm{C_2H_7COOCH_2C_3H_7}$ | b) 164–165 (762) | 0.8719 | 1.4073 | 40.66 | 40.78 | 60.0 | 91.6 |
| $\mathrm{(CH_3)_3CCOCl}$ | $\mathrm{(CH_3)_3CCOOCH_2C(CH_3)_3}$ | c) 162–165 (761.5) | 0.8431 | 1.4060 | 50.20 | 50.29 | 31.4 | 78.5 |
| $\mathrm{(CH_3)_3SiCH_2CH_2COCl}$ | $\mathrm{(CH_3)_3SiCH_2CH_2COOCH_2CH_2CH_2Si(CH_3)_3}$ | 143–144 (21) | 0.8684 | 1.4368 | 78.56 | 78.72 | 54.6 | 76.3 |
* Literature data for the esters: a) b.p. 77.15°; $n_D^{20}$ 1.37239; $d_4^{20}$ 0.90063 ($^5$); b) b.p. 166.25°; $n_D^{15}$ 1.4087; $d_4^{15}$ 0.8712 ($^6$); c) b.p. 162–164° ($^7$).
** In all cases the triethylchlorosilane obtained had b.p. 143–147°; $n_D^{20}$ 1.4309 (10); $d_4^{20}$ 0.8932. Literature data ($^8$): b.p. 146.5°; $n_D^{20}$ 1.4313; $d_4^{20}$ 0.8968.
Contrary to expectation and in contradiction to the data of Jenkins and Post \(^{(4)}\), we did not observe the formation of the corresponding aldehydes, despite the fact that the starting triethylsilane was converted in 66–92% yield into triethylchlorosilane. The products of reduction of the acid chlorides proved to be the corresponding esters, isolated in good yields. The results of the experiments and the properties of the substances formed are given in Table 1.
The results obtained make it possible to represent the new catalytic reaction, discovered by us, between trialkylsilanes and the acid chlorides of aliphatic monocarboxylic acids by the following equation:
\[ 2\mathrm{R}_3\mathrm{SiH} + 2\mathrm{R}'\mathrm{COCl} \xrightarrow{\mathrm{AlCl}_3} 2\mathrm{R}_3\mathrm{SiCl} + \mathrm{R}'\mathrm{COOCH}_2\mathrm{R}', \]
where \(\mathrm{R}=\mathrm{C}_2\mathrm{H}_5\), \(\mathrm{R}'=\mathrm{CH}_3,\ n\text{-}\mathrm{C}_3\mathrm{H}_7,\ (\mathrm{CH}_3)_3\mathrm{C},\ (\mathrm{CH}_3)_3\mathrm{SiCH}_2\mathrm{CH}_2\).
The mechanism of this reaction has not yet been definitively established. Apparently, an intermediate reduction of the acid chloride by triethylsilane to the corresponding aldehyde takes place here, the aldehyde then reacting with an excess of acid chloride according to the scheme:
\[ \mathrm{R}_3\mathrm{SiH} + \mathrm{R}'\mathrm{COCl} \rightarrow \mathrm{R}'\mathrm{CHO} + \mathrm{R}_3\mathrm{SiCl}, \]
\[ \mathrm{R}'\mathrm{CHO} + \mathrm{R}'\mathrm{COCl} \rightarrow \mathrm{R}'\mathrm{COOCHClR}'^{*}, \]
\[ \mathrm{R}'\mathrm{COOCHClR}' + \mathrm{R}_3\mathrm{SiH} \rightarrow \mathrm{R}'\mathrm{COOCH}_2\mathrm{R}' + \mathrm{R}_3\mathrm{SiCl}. \]
However, the possibility of direct ester condensation of the aldehydes formed cannot be excluded.
Experimental Part
Starting reagents. The acid chlorides of \(n\)-butyric (I), trimethylacetic (II), and \(\beta\)-trimethylsilylpropionic (III) acids were obtained by the action of thionyl chloride on the corresponding acids and had the following constants: I—b.p. 101.0–101.5°, \(n_D^{20}\) 1.4120; II—b.p. 103–106°; III—b.p. 70–71° (25 mm), \(n_D^{20}\) 1.4381.
Acetyl chloride was a commercial preparation with b.p. 51–52.5°. Triethylsilane, obtained by an organomagnesium synthesis, had b.p. 107–109°; \(n_D^{20}\) 1.4120. Aluminum chloride was a commercial “purified” preparation.
Reaction of triethylsilane with acetyl chloride. Into a round-bottomed flask fitted with a ground-joint reflux condenser and a dropping funnel were placed 23.6 g (0.3 mole) of acetyl chloride and 0.8 g of aluminum chloride. A slight warming was observed. Then, over the course of 30–40 min, 34.9 g (0.3 mole) of triethylsilane was added. The reaction mixture became strongly heated and boiled spontaneously. Boiling was maintained by gentle heating for four hours, after which the reaction mixture was distilled through an efficient fractionating column.
On distillation there were obtained 7.15 g (54.6%) of ethyl acetate with b.p. 76–79° (759 mm); \(n_D^{20}\) 1.3737; \(d_4^{20}\) 0.8990, and 30 g (66.3%) of triethylchlorosilane with b.p. 143–145°; \(n_D^{20}\) 1.4309.
Reaction of triethylsilane with butyryl chloride. To a mixture of 64 g (0.6 mole) of butyryl chloride and 1.64 g of aluminum chloride, over the course of one and a half hours, 70 g (0.6 mole) of triethylsilane was added. In this case also self-heating of the reaction mixture occurred; it was then boiled for four hours.
On distillation there were obtained: 24.2 g (60.0%) of butyl butyrate with b.p. 164–165° (762 mm); \(n_D^{20}\) 1.4073; \(d_4^{20}\) 0.8719, and 83.1 g (91.6%) of triethylchlorosilane with b.p. 143–147° (762 mm), \(n_D^{20}\) 1.4310.
* The condensation of aldehydes with acid chlorides according to the indicated scheme is described in the literature \(^{(9)}\).
Reaction of triethylsilane with trimethylacetic acid chloride. When 1.2 g of aluminum chloride was added to 36.15 g (0.3 mole) of trimethylacetic acid chloride, vigorous gas evolution was observed, accompanied by a sharp decrease in the temperature of the reaction mixture. The evolution of gas ceased immediately upon addition of 34.8 g (0.3 mole) of triethylsilane. Further conduct of the reaction was analogous to the preceding experiments.
Distillation gave 16.2 g (31.4%) of the β-trimethylethyl ester of trimethylacetic acid, b.p. 162–165.5° (761.5 mm); \(n_D^{20}\) 1.4160; \(d_4^{20}\) 0.8431, and 35.4 g (78.5%) of triethylchlorosilane, b.p. 143–147°; \(n_D^{20}\) 1.4309.
The ester was saponified according to Scheffer \((^{10})\) by boiling for six hours with a 1 \(N\) ethylene glycol solution of caustic potash. Titration was carried out with a double indicator (neutralization to thymolphthalein and titration of the potassium salt to bromophenol blue).
Ester number found: 324.1; 324.5; calculated 325.1. Molecular weight (cryoscopically in benzene), found: 173.1; 171.1, \(C_{10}H_{20}O_2\) calculated: 172.28.
Reaction of triethylsilane with β-trimethylsilylpropionic acid chloride. To a mixture of 50 g (0.3 mole) of β-trimethylsilylpropionic acid chloride and 1.2 g of aluminum chloride was added 46.5 g (0.4 mole) of triethylsilane. The mixture heated strongly. Boiling of the mixture was maintained for four hours.
Distillation gave 21.6 g (54.6%) of the γ-trimethylsilylpropyl ester of β-trimethylsilylpropionic acid, b.p. 143–144° (21 mm); \(n_D^{20}\) 1.4368; \(d_4^{20}\) 0.8634, and 46 g (76.3%) of triethylchlorosilane, b.p. 143–146°; \(n_D^{20}\) 1.4308; \(d_4^{20}\) 0.8932.
Saponification of the ester was carried out as in the preceding experiment. Ester number found: 212.8; 213; calculated 215. Molecular weight (cryoscopically in benzene), found: 255.8; 265.4; \(C_{12}H_{28}Si_2O_2\), calculated 260.48.
Found, %: Si 21.21; 21.53; C 55.81; 55.76; H 11.28; 11.69
Calculated, %: Si 21.68; C 55.23; H 10.83
The results of experiments with a change in the order of addition of the reagents (the acid chloride was added to a mixture of triethylsilane and aluminum chloride) are analogous.
Institute of Silicate Chemistry
Academy of Sciences of the USSR
Leningrad State University
named after A. A. Zhdanov
Received
13 VI 1960
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