CHEMISTRY

R. Ya. LEVINA, Yu. S. SHABAROV, V. K. DAUKSHAS, and E. G. TRESHCHOVA

2,4-DIMETHYLPENTADIENE-1,3 IN THE SYNTHESIS OF ALKANES WITH TWO QUATERNARY CARBON ATOMS SEPARATED BY A CH₂ GROUP (DITERTIARYALKYLMETHANES)

(Presented by Academician A. N. Nesmeyanov, December 21, 1956)

In our previous communications ((^{1,2})), a method was described for the synthesis of ethylenic hydrocarbons with a quaternary carbon atom, consisting in the reaction between alkylmagnesium bromides and unsaturated tertiary bromides of the allylic type; the latter are readily obtained by hydrobromination of diene hydrocarbons of branched structure with a conjugated system of double bonds.

Thus, for example, the hydrobromide of 2,4-dimethylpentadiene-1,3 ((\mathrm{I};\ \mathrm{Hal}=\mathrm{Br})) served as the starting material in the synthesis of 2,4,4-trimethylalkenes-2 ((^{1})^*):

[
\mathrm{CH_3{-}C(CH_3){=}CH{-}C(CH_3){=}CH_2}
\ \xrightarrow{+\mathrm{HHal}}\
\mathrm{CH_3{-}C(CH_3){=}CH{-}C(CH_3)_2{-}Hal}
\ \xrightarrow{+\mathrm{RMgBr}}\
\mathrm{CH_3{-}C(CH_3){=}CH{-}C(CH_3)_2{-}R}
]

[
(\mathrm{I}) \qquad\qquad\qquad\qquad\qquad\qquad\qquad (\mathrm{II})
]

In the present work, using the same 2,4-dimethylpentadiene-1,3 as the starting substance, we have developed a new convenient route for the synthesis of very difficultly accessible paraffinic hydrocarbons with two quaternary carbon atoms separated by a (\mathrm{CH_2}) group—ditertiaryalkylmethanes (in the literature only the first member of this series of hydrocarbons, 2,2,4,4-tetramethylpentane ((^{3})), has been described). The above-mentioned reaction constituted the first stage of this synthesis; moreover, the use, instead of the bromide, of the hydrochloride of 2,4-dimethylpentadiene-1,3 ((\mathrm{I};\ \mathrm{Hal}=\mathrm{Cl})) made it possible to raise the yield of 2,4,4-trimethylalkenes-2 ((\mathrm{II})) from 30 to 45–50%, calculated on the initial diene.

Alkenes ((\mathrm{II})) ((\mathrm{R}=\mathrm{CH_3},\ \mathrm{C_2H_5},\ \mathrm{C_3H_7},\ \mathrm{C_4H_9})), already containing one quaternary carbon atom, were then converted by the action of hydrochloric acid into the corresponding tertiary chlorides, 2-chloro-2,4,4-trimethylalkanes ((\mathrm{III};) yield 90%), which were subsequently introduced into reaction with organomagnesium compounds in the presence of mercuric chloride (a catalyst used in the Grignard–Wurtz reaction between (\mathrm{RMgBr}) and tertiary chlorides ((^{4,5}))); the products of this last stage of the synthesis were alkanes with two quaternary

* By the same route from 3,5-dimethylheptadiene-2,4, 3,5-dimethyl-5-ethylalkenes-3 were obtained ((^{2})).

carbon atoms separated by a CH₂ group (IV; ditertiaryalkylmethanes:)

[
\begin{gathered}
\begin{array}{c}
\mathrm{CH_3}\[-2mm]
|\[-2mm]
\mathrm{R{-}C{-}CH{=}C{-}CH_3}\[-2mm]
| \qquad\quad |\[-2mm]
\mathrm{CH_3}\qquad \mathrm{CH_3}
\end{array}
\ \xrightarrow{+\mathrm{HCl}}\
\begin{array}{c}
\mathrm{CH_3}\qquad\ \mathrm{CH_3}\[-2mm]
| \qquad\quad |\[-2mm]
\mathrm{R{-}C{-}CH_2{-}C{-}Cl}\[-2mm]
| \qquad\quad |\[-2mm]
\mathrm{CH_3}\qquad \mathrm{CH_3}
\end{array}
\ \xrightarrow[\left(\mathrm{HgCl_2}\right)]{+\mathrm{RMgBr}}
\[2mm]
\text{(II)} \qquad\qquad\qquad\quad \text{(III)}
\[3mm]
\longrightarrow
\begin{array}{c}
\mathrm{CH_3}\qquad\ \mathrm{CH_3}\[-2mm]
| \qquad\quad |\[-2mm]
\mathrm{R{-}C{-}CH_2{-}C{-}R}\[-2mm]
| \qquad\quad |\[-2mm]
\mathrm{CH_3}\qquad \mathrm{CH_3}
\end{array}
\[-1mm]
\text{(IV)}
\[2mm]
\mathrm{R = CH_3,\ C_2H_5,\ C_3H_7,\ C_4H_9}
\end{gathered}
]

On reaction with organomagnesium compounds, the saturated tertiary chlorides (III) also eliminated hydrogen chloride, giving the starting alkenes (II) in 50–55% yield; these were again used for the synthesis of alkanes (IV). The alkenes (II) and alkanes (IV) were readily separated by distillation. The yield of alkanes was 15–25%, calculated on the tertiary chloride introduced into the reaction, and 30–50%, calculated on that consumed.

Experimental Part

Hydrochlorination of 2,4-dimethylpentadiene-1,3. The unsaturated monohydrochloride of 2,4-dimethylpentadiene-1,3 (I) was obtained by passing dry hydrogen chloride (to a gain in weight of 37 g) into the diene hydrocarbon cooled with snow and salt (96 g—1 mole; b.p. 92–93°/745 mm; (n_D^{20}) 1.4445; (d_4^{20}) 0.7375; literature data (⁶): b.p. 93–94°/755 mm; (n_D^{20}) 1.4448; (d_4^{20}) 0.7376). On distillation and on storage, the hydrochloride, like the hydrobromide (¹), eliminated hydrogen halide and was converted to a considerable extent into the starting diene and its dimer. Therefore, for obtaining the alkenes (II), the dried, but undistilled, hydrochloride was used immediately after its preparation.

Synthesis of 2,4,4-trimethylalkenes-2 (II). To an ethereal solution of alkylmagnesium bromide (1.5 moles of alkyl bromide, 36 g of magnesium, and 300 ml of absolute ether), with cooling by ice water, was gradually added an ethereal solution of the monohydrochloride of 2,4-dimethylpentadiene-1,3 (prepared from 1 mole of diene).

The reaction mixture was then heated for 5 hr; decomposition was carried out with dilute hydrochloric acid (2 N).

The yields of alkenes, calculated on the starting dimethylpentadiene, were 45–50%; their constants are given in Table 1.

Table 1

2,4,4-Trimethylalkenes-2

[
\mathrm{
\begin{array}{c}
\ \ \ CH_3\[-1mm]
\ \ \ |\[-1mm]
R{-}C{-}CH{=}C{-}CH_3\[-1mm]
\ \ \ | \qquad |\[-1mm]
\ \ \ CH_3\quad CH_3
\end{array}
}
]

In all experiments, a dimer of 2,4-dimethylpentadiene-1,3 with b.p. 90°/10 mm was isolated from the higher-boiling fractions.

Hydrochlorination of 2,4,4-trimethylalkenes-2

The synthesized alkenes were saturated with hydrogen chloride while being cooled with snow and salt, and then shaken for 15–20 hours at room temperature with concentrated hydrochloric acid saturated, under cooling, with hydrogen chloride. The resulting tertiary chlorides (III; 2-chloro-2,4,4-trimethylalkanes) were washed with water, dried over calcium chloride, and distilled in vacuo (yield 90%); their constants and analytical data are given in Table 2.

Table 2

2-Chloro-2,4,4-trimethylalkanes

* Literature data for 2-chloro-2,4,4-trimethylpentane (³): b.p. 53°/29 mm; $n_D^{20}$ 1.431. The other chlorides have not been described in the literature.

Synthesis of ditertiaryalkylmethanes (IV) from tertiary chlorides (2-chloro-2,4,4-trimethylalkanes)

To an ethereal solution of the organomagnesium compound (1.1 mol of alkyl bromide, 24.3 g of magnesium, 220 ml of abs. ether) was added 4 g of cuprous chloride, and, after it had completely dissolved, the tertiary chloride (0.5 mol) was added very slowly (over 6 h) with constant stirring and cooling of the reaction mixture to 13–15°. Stirring was then continued for a further 5 h at room temperature and for 2 h at 30–35°; decomposition of the reaction mixture was carried out with dilute hydrochloric acid.

Table 3

Alkanes of the general structure
$R—C(CH_3)_2—CH_2—C(CH_3)_2—R$ (ditertiaryalkylmethanes)

* Literature data for 2,2,4,4-tetramethylpentane (³) (obtained by the action of dimethylzinc on 2-chloro-2,4,4-trimethylpentane): m.p. −66.9; −67.1°; b.p. 122.3/760 mm; $n_D^{20}$ 1.4069; $d_4^{20}$ 0.7185. The other hydrocarbons have not been described in the literature.

(2 h). After distilling the ether from the washed and dried ethereal extract, the residue was boiled for 2 h with sodium, distilled from it, and subjected to fractional distillation on a column. A fraction of the starting 2,4,4-trimethylalkene-2 was collected (this fraction, whose yield in all cases was 50–55%, was subsequently used again for the synthesis of the corresponding alkane), together with a considerably higher-boiling alkane fraction—di-tert-alkylmethane. The yields of the alkanes ranged from 15% ($R = \mathrm{C_4H_9}$) to 25% ($R = \mathrm{CH_3}$), calculated on the tertiary chloride introduced into the reaction, or from 30 to 50%, calculated on the amount used; the constants and analytical data are given in Table 3.

The synthesized alkanes were studied by the method of combination light scattering.

Table 4 gives the spectra obtained, with intensities measured on a conventional visual scale in which the intensity of the frequency in the region 1440–1450 cm$^{-1}$ is taken as equal to 10 units.

Table 4

Combination-scattering spectra of di-tert-alkylmethanes

As is evident from the data presented, the spectra of all the synthesized di-tert-alkylmethanes contain a set of intense frequencies in the regions 700–750, 930, and 1200–1250 cm$^{-1}$, characteristic of complex branching in the chain ($^{7,8}$), i.e., of hydrocarbons with a quaternary carbon atom.

Study of the spectra also showed that the alkanes do not contain alkene impurities—frequencies in the region of 1600 cm$^{-1}$ were absent.

Synthesis of 3,3,5,5-tetramethylheptane from the ditertiary dichloride V (2,4-dichloro-2,4-dimethylpentane). The interaction of ethylmagnesium bromide (115 g of ethyl bromide, 24 g of magnesium, and 200 ml of abs. ether) with the ditertiary dichloride (V) (0.25 mole) in the presence of sublimate was carried out under the conditions described above for the reaction between $RMgBr$ and saturated tertiary monochlorides (III).

On distillation of the reaction products, 3,3,5,5-tetramethylheptane was isolated (in 5% yield, calculated on the dichloride) (b.p. 71–73°/12 mm; $n_D^{20}$ 1.4307, $d_4^{20}$ 0.7693), as well as 2,4-dimethylpentadiene-1,3 (b.p. 92–94°/750 mm; $n_4^{20}$ 1.4440) and its dimer (b.p. 92°/12 mm).

Received
15 XII 1956

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