Unusual reaction of chloroacetyl chloride with 1,2-dichloroethene. Synthesis and properties of 2-chlorovinyl dichloromethyl ketone

The reaction of chloroacetyl chloride with 1,2-dichloroethene in the presence of AlCl₃ unexpectedly led to the formation of (E)-1,1,4-trichlorobut-3-en-2-one whose structure was proved by ¹H and ¹³C NMR, IR, and mass spectra and independent synthesis. A probable reaction scheme was proposed, which involves transformation of initially formed 1,2,4-trichloro-3-oxobutan-2-yl cation by the action of AlCl₃. The high reactivity of the vinylic halogen atom in (E)-1,1,4-trichlorobut-3-en-2-one was demonstrated by its reactions with nitrogen-centered nucleophiles (triethylamine, aniline, 3,5-dimethyl-1H-pyrazole) and sodium sulfide. These reactions involved only the C-Cl bond in the vinyl fragment and afforded (4,4-dichloro-3-oxobut-2-en-1-yl)triethylammonium chloride, 1,1-dichloro-4-phenylaminobut-3-en-2-one, 1-(4,4-dichloro-3-oxobut-2-en-1-yl)-3,5-dimethyl-1H-pyrazole, and 4,4′-thiobis(1,1-dichlorobut-3-en-2-one), respectively. The reaction of 1,1,4-trichlorobut-3-en-2-one with benzylhydrazine gave a mixture of 1,3- and 1,5-disubstituted pyrazoles.