CHEMISTRY

M. Ya. Khorlina and V. N. Kost

HOMOLYTIC ISOMERIZATION OF 2-BROMO-3,3-DICHLOROBUTENE-1

(Presented by Academician A. N. Nesmeyanov, 23 XI 1960)

As was shown earlier, 2-bromo-3,3,3-trichloropropene, after a certain induction period or upon irradiation with ultraviolet light, isomerizes to 3-bromo-1,1,2-trichloropropene-1. This transformation was interpreted as the result of a homolytic rearrangement (¹).

In the present work, the ability of 2-bromo-3,3-dichlorobutene-1 to isomerize was investigated; it was obtained according to the scheme

\[ \mathrm{CH_3CCl_2—CH_2CH_2Br \xrightarrow{KOH} CH_3CCl_2CH = CH_2,} \]

\[ \mathrm{CH_3CCl_2—CH = CH_2 \xrightarrow[(CH_3COOH)]{Br_2} CH_3CCl_2CHBr—CH_2Br,} \]

\[ \mathrm{CH_3CCl_2—CBrH—CH_2Br \xrightarrow{KOH} CH_3CCl_2CBr = CH_2.} \]

On standing or upon irradiation with ultraviolet light, a sample of the compound \(\mathrm{CH_3CCl_2CBr = CH_2}\) isomerizes to \(\mathrm{CH_3CCl = CCl—CH_2Br}\), which with thiourea forms a bromine-free derivative isolated in the form of a picrate. This isomerization may be described by scheme (A), entirely analogous to the rearrangement scheme for 2-bromo-3,3,3-trichloropropene (¹):

\[ \mathrm{CH_3CCl_2—CBr = CH_2 \xrightarrow{Br} CH_3CCl_2—\dot{C}Br—CH_2Br,} \]

\[ \mathrm{CH_3CCl_2\dot{C}Br—CH_2Br \xrightarrow{\text{rearrangement}} CH_3\dot{C}Cl—CClBr—CH_2Br, \ (A)} \]

\[ \mathrm{CH_3\dot{C}Cl—CClBr—CH_2Br \rightarrow CH_3CCl = CCl—CH_2Br + Br \ etc.} \]

Addition of hydroquinone prevents this isomerization. Under the action of \(\mathrm{SbCl_5}\), \(\mathrm{CH_3CCl_2CBr = CH_2}\) undergoes an anionotropic allylic rearrangement according to the scheme

\[ \mathrm{CH_3CCl_2CBr = CH_2 \xrightarrow{SbCl_5} CH_3CCl = CBr—CH_2Cl.} \]

2-Bromo-1,3-dichlorobutene-2 was isolated in the form of the picrate of an isothiuronium derivative.

The structure of \(\mathrm{CH_3CCl_2CBr = CH_2}\) was confirmed by obtaining the dimethylamine derivative \(\mathrm{CH_3CCl = CBr—CH_2N(CH_3)_2}\), identified in the form of its picrate. The same amine \(\mathrm{CH_3CCl = CBr—CH_2N(CH_3)_2}\) was obtained by two routes: by the interaction of an excess of dimethylamine with 1,2-dibromo-3,3-dichlorobutane according to the scheme

\[ \mathrm{CH_3CCl_2CHBr—CH_2Br \rightarrow CH_3CCl = CBr—CH_2N(CH_3)_2} \]

and by the interaction of 2,3-dibromo-1,3-dichlorobutane, obtained by bromination of \(\mathrm{CH_3CCl = CH—CH_2Cl}\), with dimethylamine.

Next, the interaction of hydrogen bromide with \(\mathrm{CH_3CCl_2CBr = CH_2}\) in the presence of benzoyl peroxide was studied. As the sole reaction product, in 90% yield, a mixture of isomers of structure was obtained

$\mathrm{CH_3CCl{=}CCl{-}CH_2Br}$. The indicated reaction probably proceeds according to a scheme analogous to (A). In the present case hydrogen bromide is the source of bromine atoms initiating the process of homolytic isomerization.

From a mixture of $\mathrm{CH_3CCl{=}CCl{-}CH_2Br}$, by reaction with dimethylamine, the amines $\mathrm{CH_3CCl{=}CCl{-}CH_2N(CH_3)_2}$ were obtained, isolated as picrates, which contain no bromine; this confirms the indicated structure of the dichlorobromobutene. In all the reactions cited above, in which unsaturated compounds of the structure $\mathrm{CH_3CCl{=}CX{-}CH_2Y}$ are formed, where $\mathrm{X = Cl}$ or $\mathrm{Br}$, $\mathrm{Y = Br}$ or $\mathrm{N(CH_3)_2}$, the appearance of cis-trans isomers was noted. However, in the present work the stereochemistry of these reactions was not studied in detail.

Thus, substances of the structure $\mathrm{XCCl_2{-}CBr{=}CH_2}$, where $\mathrm{X{-}Cl}$ (1), $\mathrm{CH_3}$, under the influence of radical agents undergo homolytic isomerization to $\mathrm{XCCl{=}CCl{-}CH_2Br}$, accompanied by 1,2-migration of chlorine in the radicals formed as intermediates:

\[ \mathrm{XCCl_2{-}\dot{C}Br{-}CH_2Br \rightarrow X\dot{C}Cl{-}CClBr{-}CH_2Br.} \]

\[ \begin{array}{cc} 1 & 2 \end{array} \]

Radicals of structure 2 are stabilized with formation of the unsaturated $\mathrm{XCCl{=}CCl{-}CH_2Br}$, which, apparently, is associated with the ease of elimination of a Br atom from a system of the type

\[ >\!\dot{\mathrm{C}}{-}\mathrm{C}\!< \quad \begin{array}{c} |\\[-2mm] \mathrm{Br} \end{array} \]

Experimental Part

1. Preparation of 3,3-dichlorobutene-1. 135 g of 1-bromo-3,3-dichlorobutane was dehydrobrominated with 45.9 g of KOH in ethyl cellosolve with stirring at 0°. After the usual work-up, distillation on a column gave 35.5 g of 3,3-dichlorobutene-1 (43% of theory); b.p. 45° at 105 mm; $n_D^{20}$ 1.4460; $d_4^{20}$ 1.1043; found $MR$ 30.16, calculated 29.94.

\[ \begin{array}{rll} & \text{Found \%:} & \mathrm{C}\ 38.25;\ 38.44;\ \mathrm{H}\ 4.96;\ 4.76\\ \mathrm{C_4H_6Cl_2}. & \text{Calculated \%:} & \mathrm{C}\ 38.43;\ \mathrm{H}\ 4.84 \end{array} \]

On heating with thiourea for several minutes in alcoholic solution, the thiourea derivative was isolated as the picrate

\[ \mathrm{CH_3CCl{=}CH{-}CH_2{-}S{-}C} \begin{array}{c} \diagup\!\!\mathrm{NH}\\[-1mm] \diagdown\!\!\mathrm{NH_2} \end{array} \cdot \mathrm{C_6H_3N_3O_7} \]

with m.p. 168.5–170° (from alcohol).

\[ \begin{array}{rll} & \text{Found \%:} & \mathrm{C}\ 33.33;\ 33.56;\ \mathrm{H}\ 3.19;\ 3.14;\ \mathrm{N}\ 17.74;\ 17.60\\ \mathrm{C_{11}H_{12}ClN_5O_7S}. & \text{Calculated \%:} & \mathrm{C}\ 33.55;\ \mathrm{H}\ 3.07;\ \mathrm{N}\ 17.78 \end{array} \]

A mixed sample with the picrate of the isothiourea derivative obtained from $\mathrm{CH_3CCl{=}CH{-}CH_2Cl}$ gave no depression of the melting point.

2. Bromination of 3,3-dichlorobutene-1. To 90 g of $\mathrm{CH_3CCl_2CH{=}CH_2}$ in 100 ml of glacial acetic acid, 34.2 ml of bromine was added dropwise. After the usual work-up, 1,2-dibromo-3,3-dichlorobutane was distilled on a column in the range 79–80° at 4 mm, weight 158 g (77% of theory); $n_D^{20}$ 1.5435; $d_4^{20}$ 1.9810; found $MR$ 45.35, calculated 45.94.

\[ \begin{array}{rll} & \text{Found \%:} & \mathrm{C}\ 16.77;\ 17.00;\ \mathrm{H}\ 2.08;\ 2.18\\ \mathrm{C_4H_6Br_2Cl_2}. & \text{Calculated \%:} & \mathrm{C}\ 16.52;\ \mathrm{H}\ 2.12 \end{array} \]

On treatment with an excess of dimethylamine, an unsaturated amine, CH₃CCl = CBr—CH₂N(CH₃)₂, was obtained, distilling in the range 82–87° at 20 mm, \(n_D^{20}\) 1.4996–1.5006. Picrates with m.p. 153–154° (from alcohol) were isolated. Found %: N 12.62; 12.68. Found m.p. 160–161°.

\[ \begin{gathered} \text{Found \%: N } 12.62;\ 12.82\\ \mathrm{C}_{12}\mathrm{H}_{14}\mathrm{BrClN}_4\mathrm{O}_7.\ \text{Calculated \%: N } 12.68\quad (\text{cf. experiment 4}) \end{gathered} \]

1. Preparation of 2-bromo-3,3-dichlorobutene-1 and its properties. 127 g of 1,2-dibromo-3,3-dichlorobutane was dehydrobrominated with 34.2 g of KOH in ethylene glycol, analogously to experiment 1. Distillation through a column gave 58 g (59% of theory) of 2-bromo-3,3-dichlorobutene-1 with b.p. 61–62° at 25 mm; \(n_D^{20}\) 1.5120; \(d_4^{20}\) 1.6363; found \(MR\) 37.88, calculated 37.70.

\[ \begin{gathered} \text{Found \%: C } 24.00;\ 23.82;\ \text{H } 2.53;\ 2.44\\ \mathrm{C}_4\mathrm{H}_5\mathrm{BrCl}_2.\ \text{Calculated \%: C } 23.56;\quad \text{H } 2.47 \end{gathered} \]

On standing without the addition of hydroquinone, the substance becomes tear-producing, \(n_D^{20}\) 1.5300; with hydroquinone it does not isomerize. 2-Bromo-3,3-dichlorobutene-1, previously distilled (\(n_D^{20}\) 1.5130), was subjected to UV irradiation. After 10 min \(n_D^{20}\) 1.5280; after another 20 min \(n_D^{20}\) 1.5320. The substance was distilled in the range 77–80° at 25 mm; \(n_D^{20}\) 1.5340; \(d_4^{20}\) 1.6674; found \(MR\) 38.01, calculated 37.70.

The picrate of the isothiourea derivative melted at 194.5–195° (from alcohol).

\[ \begin{gathered} \text{Found \%: C } 30.47;\ 30.73;\ \text{H } 2.55;\ 2.77;\ \text{N } 16.54;\ 16.54\\ \mathrm{C}_{11}\mathrm{H}_{11}\mathrm{Cl}_2\mathrm{N}_5\mathrm{O}_7\mathrm{S}.\ \text{Calculated \%: C } 30.85;\quad \text{H } 2.59;\quad \text{N } 16.35 \end{gathered} \]

On treatment of 2-bromo-3,3-dichlorobutene-1 with SbCl₅, an isomer was obtained which distilled in the range 75–80° at 24 mm; \(n_D^{20}\) 1.5380, from which, by reaction with thiourea, the isothiourea derivative was obtained as the picrate with m.p. 185° with decomposition (from alcohol).

\[ \begin{gathered} \text{Found \%: C } 28.44;\ 28.55;\ \text{H } 2.33;\ 2.52\\ \mathrm{C}_{11}\mathrm{H}_{11}\mathrm{ClBrN}_5\mathrm{O}_7\mathrm{S}.\ \text{Calculated \%: C } 27.95;\quad \text{H } 2.35 \end{gathered} \]

1. Reaction of 2-bromo-3,3-dichlorobutene-1 with dimethylamine. a) A solution of 7.9 g of 2-bromo-3,3-dichlorobutene-1, obtained as described in experiment 3, in 20 ml of alcohol was heated in a sealed ampoule with an excess of dimethylamine for several hours at 100°. After the usual workup, dimethyl-2-bromo-3-chlorobuten-2-ylamine was distilled at 81–83° at 20 mm, 5.6 g (68% of theory); \(n_D^{20}\) 1.4994; \(d_4^{20}\) 1.3507; found \(MR\) 46.23, calculated 46.01. Picrate m.p. 153–154° (from alcohol).

\[ \begin{gathered} \text{Found \%: C } 32.60;\ 32.84;\ \text{H } 3.13;\ 3.19;\ \text{N } 12.81;\ 13.00\\ \mathrm{C}_{12}\mathrm{H}_{14}\mathrm{BrClN}_4\mathrm{O}_7.\ \text{Calculated \%: C } 32.66;\quad \text{H } 3.19;\quad \text{N } 12.68 \end{gathered} \]

A mixed sample of this picrate with a sample of the corresponding compound from experiment 2 showed no depression of the melting point.

b) The picrate of an amine of structure CH₃CCl = CBr—CH₂N(CH₃)₂, stereoisomeric with that described above, was obtained by reaction of an excess of dimethylamine with 1,3-dichloro-2,3-dibromobutane, which in turn had been obtained by bromination of 1,3-dichlorobutene-2. Constants of CH₃CClBr—CHBr—CH₂Cl: b.p. 88° at 8 mm; \(n_D^{20}\) 1.5502; \(d_4^{20}\) 1.9910; found \(MR\) 45.59, calculated 45.94.

\[ \begin{gathered} \text{Found \%: C } 16.98;\ 16.90;\ \text{H } 2.03;\ 2.01;\ \text{Cl } 25.08;\ \text{Br } 55.76\\ \mathrm{C}_4\mathrm{H}_6\mathrm{Br}_2\mathrm{Cl}_2.\ \text{Calculated \%: C } 16.52;\quad \text{H } 2.12;\quad \text{Cl } 24.89;\ \text{Br } 56.11 \end{gathered} \]

The amine obtained was distilled at 58–60° at 4 mm; \(n_D^{20}\) 1.5030; m.p. of the picrate 166–167° (from alcohol).

\[ \begin{array}{ll} \text{Found, \%:} & \mathrm{C}\ 32.66;\ 32.68;\quad \mathrm{H}\ 3.11;\ 3.18;\quad \mathrm{N}\ 13.08;\ 13.07 \\ \mathrm{C}_{12}\mathrm{H}_{14}\mathrm{BrClN}_{4}\mathrm{O}_{7}. \text{ Calculated, \%:} & \mathrm{C}\ 32.63;\quad \mathrm{H}\ 3.19;\quad \mathrm{N}\ 12.68 \end{array} \]

Reaction of HBr with \(\mathrm{CH_3CCl_2CBr{=}CH_2}\) in the presence of benzoyl peroxide. A solution of 36.5 g of 2-bromo-3,3-dichlorobutene-1 and 0.5 g of benzoyl peroxide in 112 g of carbon tetrachloride was saturated with hydrogen bromide for 30 min. The reaction product was washed with soda solution and with water, and dried over \(\mathrm{CaCl_2}\). After removal of the solvent and distillation on a column, 31.4 g (90% of theory) of 1-bromo-2,3-dichlorobutene-2 (a mixture of cis–trans isomers) was obtained, with b.p. 64–70° at 15 mm; \(n_D^{20}\) 1.5310–1.5323; \(d_4^{20}\) 1.6519–1.6775; found \(MR\) 38.20–37.88, calculated 37.70.

\[ \begin{array}{ll} \text{Found, \%:} & \mathrm{C}\ 23.60;\ 23.76;\quad \mathrm{H}\ 2.32;\ 2.42 \\ \mathrm{C}_{4}\mathrm{H}_{5}\mathrm{BrCl}_{2}. \text{ Calculated, \%:} & \mathrm{C}\ 23.56;\quad \mathrm{H}\ 2.47 \end{array} \]

By reaction with thiourea there was obtained

\[ \mathrm{CH_3CCl{=}CCl{-}CH_2S{-}C} \begin{matrix} \ \ //\mathrm{NH}\\[-2pt] \ \ \backslash \mathrm{NH_2} \end{matrix} , \]

isolated in the form of the picrate with m.p. 197° (from alcohol).

\[ \begin{array}{ll} \text{Found, \%:} & \mathrm{N}\ 16.56;\ 16.50;\quad \mathrm{Cl}\ 16.48;\ 16.33 \\ \mathrm{C}_{11}\mathrm{H}_{11}\mathrm{Cl}_{2}\mathrm{N}_{5}\mathrm{O}_{7}\mathrm{S}. \text{ Calculated, \%:} & \mathrm{N}\ 16.35;\quad \mathrm{Cl}\ 16.56 \end{array} \]

On treating a mixture of 1-bromo-2,3-dichlorobutene-2 with dimethylamine in alcohol, a mixture of stereoisomeric amines of structure \(\mathrm{CH_3CCl{=}CCl{-}CH_2N(CH_3)_2}\) was obtained in good yield; from this a fraction was isolated with b.p. 65–68° at 20 mm; \(n_D^{20}\) 1.4720; m.p. of the picrate 144–145° (from alcohol).

Found, %: C 36.25; 36.25; H 3.30; 3.37; N 14.39; 14.29, and a fraction with b.p. 75–77° at 20 mm; \(n_D^{20}\) 1.4750; m.p. of the picrate 160–161° (from alcohol).

\[ \begin{array}{ll} \text{Found, \%:} & \mathrm{C}\ 35.90;\ 35.89;\quad \mathrm{H}\ 3.68;\ 3.54;\quad \mathrm{N}\ 14.17;\ 14.19 \\ \mathrm{C}_{12}\mathrm{H}_{14}\mathrm{Cl}_{2}\mathrm{N}_{4}\mathrm{O}_{7}. \text{ Calculated, \%:} & \mathrm{C}\ 36.28;\quad \mathrm{H}\ 3.55;\quad \mathrm{N}\ 14.11 \end{array} \]

Institute of Organoelement Compounds
Academy of Sciences of the USSR

Received
10 XI 1960

CITED LITERATURE

1. A. N. Nesmeyanov, R. Kh. Freidlina, V. N. Kost, 113, 828 (1957).