Full Text
E. N. PRILEZHAEVA, N. P. PETUKHOVA,
Corresponding Member of the USSR Academy of Sciences M. F. SHOSTAKOVSKII
ON THE REACTION OF THIOLACETIC ACID WITH VINYL ETHERS
Although thiolacetic acid is a fairly strong acid \((K_{\alpha} = 4.7 \cdot 10^{-4})\) \((^{1})\), its addition to unsaturated systems usually proceeds exclusively against Markovnikov’s rule \((^{2})\). We have found that, owing to the nucleophilicity of the double bond of vinyl alkyl ethers, thiolacetic acid under certain conditions gives adducts of normal structure with them, thus displaying its comparatively high acidity. An increase in the aliphatic radical promotes electrophilic addition.
\[
\mathrm{CH_2{=}CH{-}OR + CH_3COSH \rightarrow CH_3CHOR}
\]
\[
\mathrm{\hspace{5.7em}|}
\]
\[
\mathrm{\hspace{5.1em}SCOCH_3}
\tag{I, II}
\]
\[ \mathrm{R = C_6H_{13}\ (I);\quad R = C_9H_{19}\ (II).} \]
The reaction is carried out in a stream of nitrogen in the presence of catalytic amounts of sulfurous anhydride or thionyl chloride, using hydroquinone as inhibitor, or without a catalyst in vacuo, removing oxygen. If the reaction between substances degassed in vacuo is carried out at a pressure of about 0.02 mm, the remaining amount of oxygen is not capable of catalyzing the free-radical process, and only \(\alpha\)-alkoxyethyl thiolacetates are formed. If the addition is conducted at a pressure of 2–5 mm, then the oxygen present is sufficient for a partial change in the nature of the interaction: a mixture of adducts of normal and abnormal structure is obtained.
Using thoroughly purified, freshly distilled reagents, even when mixing in air, one obtains a mixture of \(\alpha\)- and \(\beta\)-alkoxyethyl thiolacetates, although with a predominant content of the latter. If, however, reagents that have been stored for some time are used and the reaction is carried out in air, the only direction becomes addition against Markovnikov’s rule, just as was previously established for vinyl butyl ether \((^{3})\).
\[ \mathrm{CH_2{=}CHOR + CH_3COSH \rightarrow ROCH_2CH_2SCOCH_3} \tag{III, IV, V} \]
\[ \mathrm{R = C_6H_{13}\ (III);\quad R = C_9H_{19}\ (IV);\quad R = C_6H_5\ (V).} \]
Thus, depending on the conditions, the reaction can be directed toward obtaining individual \(\alpha\)- or \(\beta\)-alkoxyethyl thiolacetates or their mixtures.
For vinyl phenyl ether, whose double-bond nucleophilicity is decreased in comparison with vinyl alkyl ethers, the main reaction product, even in the presence of acidic catalysts, is \(\beta\)-phenoxyethyl thiolacetate.
The structure of the substances obtained was proved by alcoholysis to \(\alpha\)- or \(\beta\)-alkoxy- and aroxyethanethiols with known properties \((^{3,4})\), and in part by independent synthesis.
\[ \begin{gathered} \mathrm{ROCH_2CH_2SCOCH_3} \xrightarrow[\mathrm{H^+}]{\mathrm{ROH}} \mathrm{ROCH_2CH_2SH} \xrightarrow{\mathrm{HgCl_2}} \mathrm{ROCH_2CH_2SHgCl} \\ \hspace{2.8cm}(\mathrm{VI,VII,VIII}) \hspace{2.7cm}(\mathrm{IX,X}) \\[0.5em] \mathrm{R'OCH_2CH_2SH} \xleftarrow{\mathrm{H_2S}} \mathrm{ROCH{=}CH_2} \xrightarrow{\mathrm{HgCl_2}} \mathrm{ROCH_2CH_2SHgCl} \\ \mathrm{R=C_6H_{13}\ (VI,IX)} \\ \mathrm{R=C_9H_{19}\ (VII,X)} \\ \mathrm{R=C_6H_5\ (VIII)} \end{gathered} \]
\[ 3\,\mathrm{CH_3CH(SCOCH_3)(OR)} \xrightarrow[\mathrm{H^+}]{\mathrm{ROH}} 3\,[\,\mathrm{CH_3CH(SH)(OR)}\,] \longrightarrow \mathrm{ROH}+(\mathrm{CH_3CHS})_3 \]
On alcoholysis, β-alkoxythiols are formed in sufficiently high yields, which opens up the possibility of obtaining them by a simpler method than the addition of gaseous hydrogen sulfide to vinyl ethers\(^5\).
Experimental Part
“Normal” addition of thiolacetic acid to vinyl ethers
All reagents were used freshly distilled.
α-Hexyloxyethyl thiolacetate (I). a) Into a three-necked flask equipped with a stirrer, thermometer, gas-inlet tube, reflux condenser, and a jacketed dropping funnel were placed 11.4 g of thiolacetic acid in 70 ml of absolute ether, 0.5 ml of thionyl chloride, and a little hydroquinone. (The reaction was carried out in a stream of dry nitrogen.) To this mixture, cooled to \(-25^\circ\), 19.2 g of vinyl hexyl ether, cooled to \(-30^\circ\), was added slowly (over 4 hours). The reaction mixture was kept for 1 hour at \(-25^\circ\) and left overnight. The ethereal solution was washed once with water and dried over sodium sulfate. Distillation gave 20.8 g (67.9%) of (I), b.p. \(91\text{--}93^\circ\) (3 mm); \(n_D^{20}\) 1.4556; \(d_4^{20}\) 0.9419; found \(MR_D\) 58.80, calculated 58.00.
\[ \begin{array}{ll} \text{Found, \%:} & \mathrm{C}\ 59.05,\ 58.98;\quad \mathrm{H}\ 9.83,\ 9.90;\quad \mathrm{S}\ 15.29,\ 15.19\\ \mathrm{C_{10}H_{20}O_2S.}\ \text{Calculated, \%:} & \mathrm{C}\ 58.79;\quad \mathrm{H}\ 9.85;\quad \mathrm{S}\ 15.65 \end{array} \]
b) Into a three-necked flask were placed 3 g of thiolacetic acid in 20 ml of absolute ether and a little hydroquinone. The experiment was conducted in a stream of nitrogen. Sulfurous anhydride was passed for 2 minutes into the mixture cooled to \(-35^\circ\), with vigorous stirring; then 5 g of vinyl hexyl ether cooled to \(-35^\circ\) was slowly added. The reaction mixture was kept for 0.5 hour at \(-35^\circ\) and 1 hour at room temperature. Distillation afforded 5.9 g of (I) (73.7%), b.p. \(97\text{--}99^\circ\) (5.5 mm), \(n_D^{20}\) 1.4553.
c) 2.5 g of thiolacetic acid and 4.2 g of vinyl hexyl ether were successively condensed on a vacuum manifold into an ampoule at a residual pressure of 0.02 mm. After sealing off the ampoule under vacuum and subsequent heating for 2 hours at \(60^\circ\), distillation gave 6.5 g of (I) (95.5%), b.p. \(90\text{--}91^\circ\) (2 mm), \(n_D^{20}\) 1.4556.
α-Nonyloxyethyl thiolacetate (II) was obtained by method a) from 2.9 g of thiolacetic acid, 6.6 g of vinyl nonyl ether, 0.02 ml of thionyl chloride, and 40 ml of absolute ether. Yield 6.7 g (70.5%), b.p. \(130\text{--}131.5^\circ\) (7 mm), \(n_D^{20}\) 1.4600, \(d_4^{20}\) 0.9293; found \(MR_D\) 72.58, calculated 71.84.
\[ \begin{array}{ll} \text{Found, \%:} & \mathrm{C}\ 63.45,\ 63.48;\quad \mathrm{H}\ 10.49,\ 10.43;\quad \mathrm{S}\ 13.14,\ 13.20\\ \mathrm{C_{13}H_{26}O_2S.}\ \text{Calculated, \%:} & \mathrm{C}\ 63.36;\quad \mathrm{H}\ 10.63;\quad \mathrm{S}\ 13.01 \end{array} \]
“Anomalous” addition of thiolacetic acid to vinyl ethers.
β-Hexyloxyethyl thiolacetate (III). 1.9 g of thiolacetic acid was mixed in air with 3.2 g of vinyl hexyl ether. The reaction is exothermic; the temperature reaches 70°. After distillation, 3.9 g of (III) was obtained (76.9%); b.p. 108–110° (8 mm), $n_D^{20}$ 1.4622, $d_4^{20}$ 0.9552, found $MR_D$ 58.73, calculated 58.00.
$C_{10}H_{20}O_2S.$ Found, %: C 58.50, 58.71; H 9.77, 9.64; S 15.63, 15.35
Calculated, %: C 58.79; H 9.85; S 15.65
β-Nonyloxyethyl thiolacetate (IV) was obtained in the same way as (III), from 8 g of vinyl nonyl ether and 3.6 g of thiolacetic acid. Yield 10.7 g (83.1%); b.p. 148–148.5° (5 mm), $n_D^{20}$ 1.4639, $d_4^{20}$ 0.9370, found $MR_D$ 72.55, calculated 71.84.
$C_{13}H_{26}O_2S.$ Found, %: C 63.52, 63.26; H 10.41, 10.30; S 13.20, 12.90
Calculated, %: C 63.36; H 10.63; S 13.01
β-Phenoxyethyl thiolacetate (V) was obtained, as was (III), from 4.7 g of vinyl phenyl ether and 2.7 g of thiolacetic acid. Yield 6 g (81.1%), b.p. 130–131.5° (5 mm), m.p. 48–48.5° (from heptane).
$C_{10}H_{12}O_2S.$ Found, %: C 60.90, 61.04; H 6.30, 6.12; S 16.44, 16.12
Calculated, %: C 61.19; H 6.16; S 16.30
“Mixed” addition of thiolacetic acid to vinyl ethers.
a) 3 g of thiolacetic acid and 3.9 g of vinyl butyl ether were condensed on a vacuum line into an ampoule at a residual pressure of 4 mm. The ampoule was heated for 3 h at 60°. The following were obtained: fraction I—b.p. 93–95° (12 mm), 1.7 g, $n_D^{20}$ 1.4579. Fraction II—b.p. 95–100° (12 mm), 5 g, $n_D^{20}$ 1.4605. Total yield 97.1%.
Literature data: for α-butoxyethyl thiolacetate (4) b.p. 61.7–62.3° (3 mm), $n_D^{20}$ 1.4560; for β-butoxyethyl thiolacetate (3) b.p. 84–84.5° (5 mm), $n_D^{20}$ 1.4605.
b) 3 g of thiolacetic acid and 4.9 g of vinyl hexyl ether were condensed into an ampoule at a residual pressure of 4 mm. The ampoule was heated for 4 h at 60°. On distillation the following fractions were obtained: I—b.p. 90–91° (3 mm), $n_D^{20}$ 1.4565; II—b.p. 91–94° (3 mm), $n_D^{20}$ 1.4578, in an amount of 7 g (88.6%), representing a mixture of α- and β-hexyloxyethyl thiolacetates.
Alcoholysis of β-alkoxy-(aroxy)-ethyl thiolacetates.
β-Hexyloxyethanethiol (VI). Into a three-necked flask equipped with a stirrer, condenser, gas-inlet tube, and thermometer were placed 7.0 g of (III) and 70 g of 1.2 N methanolic HCl solution. The experiment was carried out under a stream of dry nitrogen. The mixture was boiled for 5 h, then diluted with water, the methanol was evaporated in vacuo, and the residue was extracted with ether and dried over sodium sulfate. 3.8 g (61.9%) of (VI) was obtained, b.p. 82–83° (9 mm), $n_D^{20}$ 1.4520, $d_4^{20}$ 0.8996, found $MR_D$ 48.58; calculated 48.45.
$C_8H_{18}OS.$ Found, %: C 58.92, 58.79; H 11.10, 11.37; S 19.17, 19.48
Calculated, %: C 59.23; H 11.18; S 19.73
By titration after treatment with iodine solution according to the known procedure (5), 99.2% thiol content was determined. The mercuric chloride (IX), after crystallization from boiling alcohol, had m.p. 124.5–125.5°.
$C_8H_{17}ClHgOS.$ Found, %: C 24.38, 24.22; H 4.43, 4.33; Hg 50.47, 50.15
Calculated, %: C 24.18; H 4.31; Hg 50.49
The mercuric chloride compound obtained from the thiol, which was synthesized by the known method \((^5)\) by addition of hydrogen sulfide to vinyl \(n\)-hexyl ether, had m.p. 124—124.5°; a mixed sample with (IX) gave no depression.
β-\(n\)-Hexyloxyoctanethiol (VII) was obtained in the same way as (VI) from 4.8 g of (IV) in 48 g of 1.1 \(N\) methanolic HCl. Yield 3 g (76.9%), b.p. 118—119° (8 mm), \(n_D^{20}\) 1.4560, \(d_4^{20}\) 0.8888. Found \(MR_D\) 62.54; calculated 62.33.
\[ \begin{aligned} &\text{Found, \%: } &&\mathrm{C}\ 64.92,\ 64.62;\quad \mathrm{H}\ 11.79,\ 12.01;\quad \mathrm{S}\ 15.74,\ 15.47\\ &\mathrm{C}_{11}\mathrm{H}_{24}\mathrm{OS}.\ \text{Calculated, \%: } &&\mathrm{C}\ 64.58;\quad \mathrm{H}\ 11.83;\quad \mathrm{S}\ 15.67 \end{aligned} \]
By titration with alkali after addition of an excess of 20% alcoholic sulema solution, 99.1% content of (VII) was determined. After crystallization from boiling alcohol, the mercuric chloride compound (X) had m.p. 123.5—124°.
\[ \begin{aligned} &\text{Found, \%: } &&\mathrm{C}\ 30.11,\ 30.12;\quad \mathrm{H}\ 5.31,\ 5.32;\quad \mathrm{Hg}\ 44.91,\ 45.07\\ &\mathrm{C}_{11}\mathrm{H}_{23}\mathrm{ClHgOS}.\ \text{Calculated, \%: } &&\mathrm{C}\ 30.06;\quad \mathrm{H}\ 5.27;\quad \mathrm{Hg}\ 45.65 \end{aligned} \]
β-Phenoxyoctanethiol (VIII) was obtained from 9 g of (V) in 90 g of 1.2 \(N\) methanolic HCl in an amount of 5.9 g (84.3%), with b.p. 108—109° (8 mm), \(n_D^{20}\) 1.5610. Literature data \((^7)\) for (VIII): b.p. 106—108.5° (7.5 mm), \(n_D^{20}\) 1.5610.
Alcoholysis of α-hexyloxyethyl thiolacetate (I). Into a three-necked flask were placed 7 g of (I) in 110 ml of a 5% solution of KOH in methanol. The alcoholysis was carried out in a stream of nitrogen, at room temperature for 24 hours. After work-up of the reaction mixture, distillation gave 1.9 g (50%) of a substance which, by constants and analysis, was analogous to \(n\)-hexyl alcohol, and a higher-boiling fraction. The aqueous extract and the distillation residue were combined and treated with gaseous HCl. This gave 1.3 g (52%) of a compound which, after crystallization from alcohol, had m.p. 123.5—124° and corresponded to β-trithioacetaldehyde \((^8)\).
Counter synthesis of β-hexyloxyethyl thiolacetate (IV). 5 g of (VI), obtained by addition of hydrogen sulfide to vinyl \(n\)-hexyl ether, were dissolved in 20 g of dry pyridine and cooled to −5°. 2.3 g of freshly distilled acetyl chloride were added. After work-up of the reaction mixture and distillation, 4 g (64.3%) of a substance were obtained, coinciding in constants with (III): b.p. 105—107° (5 mm), \(n_D^{20}\) 1.4622.
Institute of Organic Chemistry named after N. D. Zelinsky
Academy of Sciences of the USSR
Received
17 VI 1963
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