V. M. VDOVIN, Corresponding Member of the USSR Academy of Sciences N. S. NAMETKIN,
P. L. GRINBERG

SILICACYCLOBUTANES

SYNTHESIS OF 1,1-SUBSTITUTED SILICACYCLOBUTANES

There is only a small amount of information in the literature concerning the synthesis and properties of silicacyclobutane compounds ($^1$).

In the present work, the reaction (A) of organometallic ring closure of readily available ($^{2,3}$) $\gamma$-chloropropyl derivatives of silicon was studied

[
\mathrm{Cl_2RSiCH_2CH_2CH_2Cl}
\xrightarrow[\text{solvent}]{\mathrm{M}}
\mathrm{ClRSi(CH_2)_3}
\tag{A}
]

[
\mathrm{R = CH_3;\ Cl;\ M = Na;\ Li;\ Mg}
]

Numerous experiments carried out with unactivated magnesium in ether led to low yields of silicacyclobutanes (usually less than 10%), and even these yields were difficult to reproduce. Changing the temperature and reaction time, using tetrahydrofuran and dibutyl ether as solvents, and sodium and lithium as the metals did not lead to positive results. In these cases either a polymer was formed—

[
\left[
\begin{array}{c}
\mathrm{R}\
|\
-\mathrm{SiCH_2CH_2CH_2}-\
|\
\mathrm{Cl}
\end{array}
\right]_x,
]

or the starting $\gamma$-chlorides remained unchanged. Satisfactory (and well reproducible) yields,

Table 1

* Amount of hydrolyzable chlorine, %.
* Data ($^5$) for 1,1-dimethylsilicacyclobutane obtained from $\mathrm{BrCH_2CH_2CH_2Si(CH_3)_2Cl}$: b.p. 81°, $n_D^{20}$ 1.4270, $d_4^{20}$ 0.7746.
* Data ($^4$) for 1-methyl-1-ethylsilicacyclobutane: b.p. 109 (745 mm), $n_D^{20}$ 1.4388; $d_4^{20}$ 0.7965.
*** Molecular weight found by the cryoscopic method in benzene: 124.0, 125.0; calculated 126 for $\mathrm{C_7H_{14}Si}$.

chlorine-substituted silacyclobutanes only when using magnesium activated with iodine vapor.

Earlier attempts(^{4,5}) to cyclize γ-halopropylchlorosilanes with magnesium led to a high yield of the silacyclobutane derivative only when γ-bromopropyldimethylchlorosilane was used, which, however, is obtained by a complicated multistage synthesis(^{5}).

We further established that the chlorine-substituted silacyclobutanes obtained by us can be successfully used for preparing silacyclobutanes with alkyl, aryl, alkenyl, and alkylaryl substituents at silicon. In the reaction

[
\mathrm{Cl(CH_3)Si(CH_2)_3}
\ \xrightarrow[\text{ether}]{\mathrm{R'MgX}}\
\mathrm{R'(CH_3)Si(CH_2)_3}
\tag{B}
]

[
\mathrm{R'}=\mathrm{CH_3;\ C_2H_5;\ } n\text{-}\mathrm{C_4H_9;\ -CH_2CH{=}CH_2;\ -CH_2{-}C_6H_5;\ -C_6H_5;\ X=Cl,\ Br,\ I}
]

chloro-, bromo-, and iodo-derived organomagnesium compounds were successfully used.

Table 2

[
\begin{aligned}
&^{}\ \text{The calculation was carried out using the table of group refractions }(^{10}).\
&^{*}\ \text{In the compound } \mathrm{(CH_3)ClSiCH_2CH(CH_3)CH_2}.
\end{aligned}
]

Noticeable cleavage of the Si—C bond in the silacyclobutane grouping under the reaction conditions was not observed(^*).

We also investigated the possibility of obtaining silacyclobutane hydrocarbons by the combined ring-closure—alkylation scheme

[
\mathrm{Cl_2(CH_3)SiCH_2CH_2CH_2Cl + 2Mg + R'X
\to R'(CH_3)Si(CH_2)_3}
\tag{B}
]

[
\mathrm{R'X=C_2H_5Br;\ } n\text{-}\mathrm{C_4H_9Br;\ CH_3I}
]

However, the corresponding silacyclobutanes were obtained in low yields and contained impurities that were difficult to separate.

The properties of 1,1-substituted silacyclobutanes and the yields of products obtained by reactions (A)—(B) are presented in Table 1.

In calculating (MR_D) for the compounds obtained, we noted the presence of stable exaltation. From the data presented in Table 2 it is evident that, both in our examples and in silacyclobutanes known from the literature, such exaltation is observed.

[
^{*}\ \text{As is known from literature data }(^{6,7}),\ \text{when magnesium iodoalkyls act on allylchlorosilanes }
(\mathrm{CH_2{=}CHCH_2})n\mathrm{SiCl},
\text{ cleavage of the Si—C}{\text{allyl}}\text{ bond occurs. In the case of methallylchlorosilanes }
[\mathrm{CH_2{=}C(CH_3)CH_2})_n\mathrm{SiCl}]
\text{ the Si—C bond is cleaved even under the action of magnesium chloroalkyls.}
]

However, if the value of (MR_D) for the (\mathrm{Si}-(\mathrm{CH}_2)_3-) group is taken not as 17.64 (according to Warrick ((^8))), but, with the correction calculated by us ((+0.50)), as 18.14, then quite satisfactory agreement is obtained between the calculated and found values of (MR_D).

Such a considerable exaltation in (MR_D) in silicacyclobutanes apparently has the same nature as in other “strained” rings, in particular cyclopropanes ((^9)). In this connection it is interesting to note that, in substituted silicacyclopentanes and silicacyclohexanes, a noticeable exaltation of (MR_D) is absent.

The (MR_D) values of several silicacyclopentane and silicacyclohexane compounds whose purity is reliable are given in Table 2. The starting (\gamma)-chlorides were synthesized by the method of addition of hydrosilanes to allyl chloride with an (\mathrm{H}_2\mathrm{PtCl}_6) catalyst ((^3)).

Synthesis of 1-chloro-1-methylsilicacyclobutane by method (A). To 144 g (6 mol) of magnesium in a two-necked vessel, isolated from atmospheric moisture, 10 g of crystalline iodine was sublimed. The magnesium thus activated, 1200 g (6.2 mol) of (\gamma)-chloropropylmethyldichlorosilane, and 800 ml of ether were heated on a water bath for (\sim 40) h. Stirring (until the formation of a monolithic mass) was continued for (\sim 30) h. During heating of the reaction mixture, an additional 2.5–3 l of ether was added. After filtration from the precipitate, triple washing of the precipitate with ether, and fractionation on a column, 470 g (3.9 mol) of 1-chloro-1-methylsilicacyclobutane, b.p. 103.5–104°, and 200 g of the starting (\gamma)-chloride were isolated.

The yield of 1-chloro-1-methylsilicacyclobutane can be increased to 75% by using an excess of activated magnesium and adding it gradually to the reaction mixture.

Synthesis of 1-methyl-1-benzylsilicacyclobutane by method (B) was carried out from 40 g (1.7 mol) of magnesium, 140 g (1.11 mol) of benzyl chloride, and 54 g (0.448 mol) of 1-chloro-1-methylsilicacyclobutane in 500 ml of ether. The reaction mixture was heated for 4 h and decomposed with water. After the usual workup and vacuum distillation, 69 g (0.39 mol) of 1-benzyl-1-methylsilicacyclobutane was obtained, b.p. 73.5–74°/5 mm Hg.

Synthesis of 1-ethyl-1-methylsilicacyclobutane by method (B) was carried out from 191.5 g (1 mol) of (\gamma)-chloropropylmethyldichlorosilane, 55 g (2.3 mol) of magnesium, and 115 g (1.05 mol) of ethyl bromide by boiling the reaction mixture in 700 ml of ether. After decomposition of the reaction mixture with water, 24.5 g (0.21 mol) of 1-ethyl-1-methylsilicacyclobutane was isolated, b.p. 60–64°/168 mm, and 69.5 g (yield 41.5%) of sym-di-((\gamma)-chloropropyl)-dimethyldiethyldisiloxane, b.p. 134–140°/5 mm.

Omission of the stage of decomposition of the reaction mixture with water does not lead to a substantial improvement in the yield of 1-ethyl-1-methylsilicacyclobutane.

Institute of Petrochemical Synthesis
Academy of Sciences of the USSR

Received
12 II 1963

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