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Reports of the Academy of Sciences of the USSR
- Volume 120, No. 3
Chemistry
Corresponding Member of the Academy of Sciences of the USSR D. N. KURSANOV, M. E. VOL’PIN
and I. S. AKHREM
REACTION OF TROPYLIUM SALTS WITH VINYL ETHERS
AND β-CHLOROMERCURIACETALDEHYDE
For the stable aromatic cycloheptatrienyl cation (tropylium), its ability to enter into reactions with nucleophilic reagents is characteristic. Thus, according to Doering and Knox \((^{1})\), tropylium salts readily react with \(\mathrm{NH_3}\), \(\mathrm{H_2S}\), \(\mathrm{OH^-}\), amines, and lithium- and magnesium-organic compounds.
It could be expected that the electrophilicity of the tropylium cation would be sufficient for reaction also with weaker nucleophilic reagents—vinyl ethers.
Indeed, it proved that tropylium salts in aqueous or alcoholic solutions, even in the cold, react very vigorously, with self-heating, with simple vinyl ethers.
The high reactivity of simple vinyl ethers in this reaction is a consequence of the influence of the electron-donating ether group
\[ \mathrm{CH_2{=}CH{-}\ddot{O}R} \]
Compounds with isolated or conjugated double bonds, but without activating substituents (for example, methylcyclohexene, cycloheptatriene, etc.), or with electron-acceptor substituents (cinnamic acid, acrylic acid, acrylonitrile, allyl chloride, etc.), do not react with tropylium salts under the same conditions.
In all cases, irrespective of the nature of the anion of the initial tropylium salt (bromide or perchlorate) and of the character of the alkyl group in the vinyl alkyl ethers (vinyl ethyl, vinyl isopropyl, vinyl butyl), one and the same product is formed—cycloheptatrienylacetaldehyde (I):
\[ \begin{gathered} \ce{[tropylium]+ X^- + CH2=CH-OR ->[R'OH][-HX] [II]} \\ \ce{[II] ->[H2O] I} \end{gathered} \]
\[ \text{(where } X = \mathrm{Br}\ \text{or}\ \mathrm{ClO_4};\quad R=\mathrm{C_2H_5},\ \text{iso-}\mathrm{C_3H_7},\ \text{or}\ \mathrm{C_4H_9};\quad R'=\mathrm{H}\ \text{or}\ \mathrm{C_2H_5}\text{).} \]
This reaction is analogous to the addition of mercury salts to vinyl ethers—
ram, discovered by A. N. Nesmeyanov and I. F. Lutsenko \((^2)\):
\[ \mathrm{HgX_2 + CH_2{=}CHOR} \ \xrightarrow[\ -HX\ ]{\ R'OH\ }\ \left[ \mathrm{XHgCH_2CH\left(OR\right)\left(OR'\right)} \right] \]
\[ \xrightarrow{\mathrm{H_2O}}\ \mathrm{XHgCH_2CH{=}O} \]
By analogy with the latter reaction, one may think that the mechanism of the interaction of tropylium salts with vinyl ethers includes attack by the \(\mathrm{C_7H_7^+}\) ion on the double bond of the vinyl ether with simultaneous (or consecutive) action of a solvent molecule (water or alcohol). The intermediate reaction product is probably an acetal or a hemiacetal (II).
On going from simple vinyl ethers to vinyl acetate, the rate of the reaction with tropylium salts decreases noticeably, which is apparently connected with partial withdrawal of electrons by the \(\mathrm{C{=}O}\) group.
\[ \begin{aligned} &\text{tropylium } \mathrm{X^-} + \mathrm{CH_2{=}CH{-}O{-}C(=O)CH_3} \xrightarrow{\mathrm{H_2O}} \mathrm{cycloheptatrienylacetaldehyde} + \mathrm{CH_3COOH} \end{aligned} \]
Cycloheptatrienylacetaldehyde was also obtained by a counter synthesis—the reaction of tropylium salts (bromide and perchlorate) with \(\beta\)-chloromercuriacetaldehyde.
\[ \mathrm{C_7H_7^+X^- + ClHgCH_2CH{=}O} \longrightarrow \mathrm{C_7H_7CH_2CH{=}O + ClHgX} \]
As a result of exchange of the mercury atom for a tropylium residue, an aldehyde was formed identical with that obtained from vinyl ethers. This reaction is the first studied example of the interaction of organomercury compounds with tropylium salts.
It is essential to note that the reaction of tropylium salts with \(\beta\)-chloromercuriacetaldehyde proceeds as a type of C-alkylation. As was shown by A. N. Nesmeyanov and I. F. Lutsenko \((^3)\), triphenylmethyl chloride reacts in exactly the same way, giving triphenylmethylacetaldehyde; at the same time acid halides O-acylate \(\beta\)-chloromercuriacetaldehyde.
Cycloheptatrienylacetaldehyde gives characteristic aldehyde reactions, forming the 2,4-dinitrophenylhydrazone and a dimedone derivative. The latter is especially convenient for identification of this aldehyde. The presence of three double bonds in cycloheptatrienylacetaldehyde was proved by its catalytic hydrogenation on platinum black; in this process 3 moles of \(\mathrm{H_2}\) are added and cycloheptylacetaldehyde is formed.
It is interesting to note that cycloheptatrienylacetaldehyde is also formed in the reaction of tropylium salts with acetaldehyde \((^4)\).
Experimental Part
Reactions of tropylium salts with vinyl ethers. A solution of 1.7 g (0.01 mole) of tropylium bromide in 10 ml of water was shaken for 15–20 min with 0.72 g (0.01 mole) of ethyl vinyl ether and extracted with ether. The ethereal extracts were washed with water and dried over \(\mathrm{MgSO_4}\). The ether was distilled off under reduced pressure, and the residue was distilled in vacuum. A fraction with b.p. 63–67°/2 mm was collected; \(n_D^{25}\) 1.5310. Yield 0.77 g (58%). After redistillation, b.p. of cycloheptatrienylacetaldehyde 62°/2 mm, \(n_D^{20}\) 1.5314, \(d_4^{20}\) 1.0204, \(\lambda_{\max}\) 260 m\(\mu\) \((\lg \varepsilon 3.63)\); \(\lambda_{\min}\) 237 m\(\mu\) \((\lg \varepsilon 3.43)\), \(\mathrm{MR}_{\text{calc.}} =\)
= 40.17 (without taking into account exaltation due to conjugation of the double bonds); \(MR_{\text{found}} = 40.70\)
\[ \begin{array}{rll} \mathrm{C}_9\mathrm{H}_{10}\mathrm{O}. & \text{Found, \%:} & \mathrm{C}\ 80.79;\ 80.63;\quad \mathrm{H}\ 7.46;\ 7.48 \\ & \text{Calculated, \%:} & \mathrm{C}\ 80.56;\quad \mathrm{H}\ 7.51 \end{array} \]
Cycloheptatrienylacetaldehyde gives characteristic aldehyde reactions: it reduces Fehling’s solution and ammoniacal silver nitrate solution; gives a red coloration with sodium nitroprusside and with benzidine. The 2,4-dinitrophenylhydrazone of cycloheptatrienylacetaldehyde is a yellow powder; decomposes above \(190^\circ\) (from nitromethane). The dimedone derivative: colorless needles, m.p. \(146.5^\circ\) (from alcohol). A mixed sample with dimedone melts at \(127—128^\circ\).
\[ \begin{array}{rll} \mathrm{C}_{25}\mathrm{H}_{32}\mathrm{O}_4 & \text{Found, \%:} & \mathrm{C}\ 75.90;\ 75.41;\quad \mathrm{H}\ 8.14;\ 8.13 \\ & \text{Calculated, \%:} & \mathrm{C}\ 75.72;\quad \mathrm{H}\ 8.13 \end{array} \]
Under the same conditions, from 7.8 g (0.041 mole) of tropylium perchlorate in 5 ml of water and 3 g (0.0416 mole) of ethyl vinyl ether, 3.24 g of cycloheptatrienylacetaldehyde was obtained (yield 59%). Dimedone derivative: m.p. \(146^\circ\).
Similarly, from 1.9 g (0.01 mole) of tropylium perchlorate and 0.86 g (0.01 mole) of isopropyl vinyl ether, 0.59 g of cycloheptatrienylacetaldehyde was obtained, yield 44%. Dimedone derivative: m.p. \(145—146^\circ\).
From 1.7 g (0.01 mole) of tropylium bromide and 0.86 g (0.01 mole) of isopropyl vinyl ether, 0.75 g of cycloheptatrienylacetaldehyde was obtained (yield 56%). Dimedone derivative: m.p. \(145^\circ\).
From 3.42 g (0.02 mole) of tropylium bromide and 2 g (0.02 mole) of butyl vinyl ether, 1.58 g of cycloheptatrienylacetaldehyde was obtained (yield 59%). Dimedone derivative: m.p. \(145—146^\circ\).
From 3.42 g (0.02 mole) of tropylium bromide and 1.72 g (0.02 mole) of vinyl acetate in water, after 24 hours 0.48 g of cycloheptatrienylacetaldehyde was isolated (yield 17%). Dimedone derivative: m.p. \(146^\circ\).
In all experiments with vinyl ethers, one and the same aldehyde is formed; mixed samples of the dimedone derivatives of the aldehyde from different experiments do not give a depression of the melting point.
Reaction of tropylium salts with \(\beta\)-chloromercuriacetaldehyde. 1.9 g (0.01 mole) of tropylium perchlorate and 2.79 g (0.01 mole) of \(\beta\)-chloromercuriacetaldehyde were shaken in a mixture of 10 ml of benzene and 5 ml of water for 2 days. The precipitate was separated by filtration and washed repeatedly with benzene. The benzene extracts were dried over \(\mathrm{MgSO}_4\). The benzene was distilled off under reduced pressure at a temperature not above \(30^\circ\). The residue was distilled in vacuo. 0.54 g of cycloheptatrienylacetaldehyde was obtained (yield 40%), b.p. \(64—66^\circ/2.5\) mm, \(n_D^{20}\ 1.5314\). The dimedone derivative (m.p. \(146.5^\circ\)) gives no depression of the melting point in admixture with the dimedone derivative of the aldehyde obtained from ethyl vinyl ether. Similarly, from 1.71 g (0.01 mole) of tropylium bromide and 2.79 g (0.01 mole) of \(\beta\)-chloromercuriacetaldehyde in a water–benzene mixture (1 : 1), 0.56 g of cycloheptatrienylacetaldehyde was obtained (yield 42%). The yield of cycloheptatrienylacetaldehyde is 50% when this same reaction is carried out with continuous stirring of the reaction mixture (water + \(\mathrm{CCl}_4\)) under a stream of \(\mathrm{N}_2\) and with periodic extraction of the aldehyde from the reaction mixture.
Hydrogenation of cycloheptatrienylacetaldehyde. 0.4097 g of cycloheptatrienylacetaldehyde was hydrogenated in alcohol over platinum black at \(21.5^\circ\) and 733.2 mm Hg. Hydrogenation was complete within 30 min. 225 ml of \(\mathrm{H}_2\) was absorbed, which amounts to 94.5% calculated for 3 double bonds. The catalyst was filtered off, and the alcoholic solution was evaporated in vacuo. The cycloheptylacetaldehyde formed was identified in the form of the 2,4-dinitrophenylhydrazone and the dimedone derivative.
2,4-Dinitrophenylhydrazone, m.p. 98° (from alcohol).
\[ \mathrm{C}_{15}\mathrm{H}_{20}\mathrm{N}_{4}\mathrm{O}_{4}. \quad \begin{array}{ll} \text{Found, \%:} & \mathrm{C}\ 56.30;\ 56.80;\quad \mathrm{H}\ 6.25;\ 6.51 \\ \text{Calculated, \%:} & \mathrm{C}\ 56.24;\quad \mathrm{H}\ 6.29 \end{array} \]
Dimedone derivative: m.p. 170.5° (from alcohol).
\[ \mathrm{C}_{25}\mathrm{H}_{38}\mathrm{O}_{4}. \quad \begin{array}{ll} \text{Found, \%:} & \mathrm{C}\ 74.58;\ 74.45;\quad \mathrm{H}\ 9.43;\ 9.50 \\ \text{Calculated, \%:} & \mathrm{C}\ 74.58;\quad \mathrm{H}\ 9.51 \end{array} \]
Institute of Organoelement Compounds
of the Academy of Sciences of the USSR
Received
14 I 1958
REFERENCES
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- A. N. Nesmeyanov, I. F. Lutsenko, N. I. Vereshchagina, Izv. AN SSSR, OKhN, 1947, 63; A. N. Nesmeyanov, I. F. Lutsenko, R. M. Khomutov, Izv. AN SSSR, OKhN, 1957, 942.
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