Chemistry

M. A. MIROPOL’SKAYA, S. Ya. MEL’NIK, T. S. FRADKINA, G. I. SAMOKHVALOV
and Corresponding Member of the Academy of Sciences of the USSR A. D. PETROV

SELECTIVE REDUCTION OF 6-METHYLHEPTA-3,5-DIEN-2-ONE WITH TRIALKOXY- AND TRIALKYLSILANE HYDRIDES

The ability of silicon hydrides to add to olefins under the influence of platinum, palladium, potassium chloroplatinate, and chloroplatinic acid is known \((^{1,2})\). One of us, jointly with S. I. Sadykh-Zade \((^3)\), has described the addition of trialkyl- and trialkoxysilanes to the conjugated system of \(\alpha,\beta\)-unsaturated carbonyl compounds.

In the present work we have studied the addition of triethoxy- and triethylsilanes to the conjugated diene system of 6-methylhepta-3,5-dien-2-one (I) under the influence of catalytic amounts of chloroplatinic acid, and a strictly selective 1,4-addition was observed, which first leads to substituted vinyl ethers (II)

\[ \begin{array}{ccc} \begin{array}{c} \mathrm{CH_3}\quad \mathrm{CH_3}\\ \backslash\quad/\\ \mathrm{C}\\ \Vert\\ \mathrm{HC}\\ |\\ \mathrm{HC}\\ \backslash\\ \mathrm{CH}\quad \mathrm{CH_3}\\ \text{(I)} \end{array} & \xrightarrow{\ \mathrm{HSiR_3}\ } & \begin{array}{c} \mathrm{CH_3}\quad \mathrm{CH_3}\\ \backslash\quad/\\ \mathrm{C}\\ \Vert\\ \mathrm{HC}\quad \mathrm{OSiR_3}\\ |\quad |\\ \mathrm{CH_2}\quad \mathrm{C}\\ \backslash\quad/\\ \mathrm{CH}\quad \mathrm{CH_3}\\ \text{(II)} \end{array} \ \longrightarrow\ \begin{array}{c} \mathrm{CH_3}\quad \mathrm{CH_3}\\ \backslash\quad/\\ \mathrm{C}\\ \Vert\\ \mathrm{HC}\quad \mathrm{O}\\ |\quad \Vert\\ \mathrm{CH_2}\quad \mathrm{C}\\ \backslash\quad/\\ \mathrm{CH_2}\quad \mathrm{CH_3}\\ \text{(III)} \end{array} \end{array} \]

Hydrolysis of (II) in the presence of acidified alcohol gives 6-methylhept-5-en-2-one (III).

The observed selective direction of the reaction forming silico ethers II may be represented, in simplified form, as the result of the initial electrophilic attack \((^{5-7})\) of the ion \(\mathrm{Si(OC_2H_5)_3^+}\) or, respectively, \(\mathrm{Si(C_2H_5)_3^+}\) formed on the carbonyl oxygen, with formation of a carbonium ion (IV), stabilized by conjugation with two double bonds, toward which the hydride hydrogen is directed.

It is also possible that, sterically, the most favorable is the six-membered cyclic transition complex (V)

\[ \begin{array}{cc} \begin{array}{c} \mathrm{CH_3}\quad \mathrm{CH_3}\\ \backslash\quad/\\ \mathrm{C}\\ \Vert\\ \mathrm{HC}\quad \mathrm{OSiR_3}\\ |\quad |\\ {}^{+}\mathrm{HC}\quad \mathrm{C}\\ \backslash\quad/\\ \mathrm{CH}\quad \mathrm{CH_3}\\ \text{(IV)} \end{array} & \qquad \begin{array}{c} \mathrm{Si}^{+}\\ /\quad\backslash\\ -\mathrm{H}\quad \mathrm{O}\\ \vdots\quad \Vert\\ \text{conjugated diene ketone fragment}\\ \text{(V)} \end{array} \end{array} \]

The reaction begins under the influence of a very small amount of catalyst and proceeds with strong heating, which may indicate its ionic-chain character.

Experimental Part

Formation of silyl ethers. 2-(6-Methylheptadien-2,5)-oxytriethoxysilane (II, \(R = OC_2H_5\)). To 12.4 g (0.1 mole) of 6-methylheptadien-3,5-one-2 (I), heated to \(90^\circ\), 18 g (0.11 mole) of triethoxysilane and 1.5 ml of a 0.1 M solution of \(H_2PtCl_6\) in absolute isopropyl alcohol are added simultaneously dropwise. Heating to \(162^\circ\) occurs. The mixture is then heated at \(155\text{--}160^\circ\) for 15 min, centrifuged from the catalyst, and distilled. 21.5 g (74.6%) of the silyl ether is obtained. B.p. \(89\text{--}91^\circ/1\) mm; \(n_D^{20}\) 1.4310, \(d_4^{20}\) 0.9466; \(MR_D\) found 78.72, calculated 78.92.

\[ \begin{aligned} &\text{Found, \%: } && C\ 58.27;\ 57.94;\quad H\ 9.87;\ 9.97;\quad Si\ 9.77;\ 10.07\\ &C_{14}H_{28}O_4Si.\ \text{Calculated, \%: } && C\ 58.33;\quad H\ 9.72;\quad Si\ 9.72 \end{aligned} \]

2-(6-Methylheptadien-2,5)-oxytriethylsilane (II, \(R = C_2H_5\)) is obtained at a temperature not above \(100^\circ\). Yield 73.2%. B.p. \(109\text{--}112^\circ/5\) mm; \(n_D^{20}\) 1.4605, \(d_4^{20}\) 0.8646; \(MR_D\) found 76.08, calculated 76.54.

2-(6-Methylheptadien-2,5)-oxymethyldiethylsilane is obtained at \(60\text{--}70^\circ\). Yield 66.2% of theory. B.p. \(73\text{--}74^\circ/3\) mm; \(n_D^{20}\) 1.4600; \(d_4^{20}\) 0.8590; \(MR_D\) found 72.07, calculated 71.89.

Hydrolysis. 10 g (0.35 mole) of the silyl ether (II, \(R = OC_2H_5\)) is added to 100 ml of ethyl alcohol acidified with 2.5 ml of concentrated hydrochloric acid and heated at reflux for 30 min. The alcohol is removed in vacuo, the residue is extracted with ether, the extract is washed with water and dried, and the solvent is removed. The substance is distilled. 3.4 g (76.6%) of 6-methylhepten-5-one-2 (III) is obtained. B.p. \(75\text{--}77^\circ/25\) mm; \(n_D^{20}\) 1.4410.

By gas–liquid chromatography, there is one peak, which increases upon addition of authentic 6-methylhepten-5-one-2. Semicarbazone, m.p. \(134\text{--}135.5^\circ\).

\[ \begin{aligned} &\text{Found, \%: } && C\ 59.00;\ 58.85;\quad H\ 9.58;\ 9.35;\quad N\ 23.00\\ &C_{19}H_{17}N_3O.\ \text{Calculated, \%: } && C\ 58.98;\quad H\ 9.35;\quad N\ 22.92 \end{aligned} \]

A mixed sample with the semicarbazone of authentic 6-methylhepten-5-one-2 gives no depression of the melting point.

All-Union Scientific Research
Vitamin Institute

Institute of Organic Chemistry named after N. D. Zelinsky,
Academy of Sciences of the USSR

Received
26 III 1962

References Cited

1. A. D. Petrov, V. F. Mironov, E. A. Chernyshev, Uspekhi khimii, 26, 292 (1957).
2. C. Eaborn, Organosilicon Compounds, London, 1960.
3. A. D. Petrov, S. I. Sadykh-Zade, Bull. Soc. Chim. France, 1957, 1932.
4. A. D. Petrov, S. I. Sadykh-Zade, USSR Author’s Certificate 119186, April 15, 1959.