One-step synthesis of triethynylvinylmethanes and tetraethynylmethanes by GaCl₃-promoted diethynylation of 1,4-enynes and 1,4-diynes

The treatment of 1,4-enynes with chlorotriethylsilylacetylene at 130 °C in the presence of GaCl₃ gave triethynylvinylmethanes by the diethynylation at the allylic methylene moiety. The addition of 2,6-di(tert-butyl)-4-methylpyridine and tert-butyldiphenylsilanol considerably improved the yields of the products by reducing the decompositions of the substrates and products. The reaction should involve the initial formation of allylgallium from the enynes and GaCl₃, where GaCl₃ activated hydrocarbon C-H to generate a nucleophilic organogallium intermediate. Carbometalation with chloroacetylene followed by β-elimination then led to the ethynylated product. Triethynylvinylmethanes were obtained by the repeated regioselective ethynylation at the 3-position of the enynes. The reaction of allylsilanes with the chloroacetylene also gave diethynylvinylmethanes, in which 1,4-enynes were formed in situ by the addition-elimination of allylgallium formed from allylsilane and GaCl₃. Tetraethynylmethanes were obtained by reacting 1,4-diynes with the chloroacetylene at 150 °C. The structure and amount of silanol can be used to control the reactivity of GaCl₃.