Surface-Mediated Reactions. 4. Hydrohalogenation of Alkynes

Article abstract of DOI:10.1021/jo00090a031

The use of appropriately prepared silica gel and alumina has been found to mediate the addition of hydrogen halides to alkynes.The technique has been rendered even more convenient by the use of various organic and inorganic acid halides that react in the presence of silica gel or alumina to generate hydrogen halides in situ.Treatment in this fashion of 1-propynylbenzene (1), which underwent no reaction in CH2Cl2 solution saturated with HCl, readily afforded the syn addition product, alkenyl chloride (E)-4a.On extended treatment (E)-4a underwent subsequent iaomerization to the thermodynamically more stable Z isomer.Thus either isomer of 4a could be obtained in good yield depending on the reaction conditions.In a similar way bromides (E)- and (Z)-4b were obtained without competing formation of the radical products (E)- and (Z)-5, which occurred in solution.In contrast with solution-phase hydroiodination of alkyne 1, which slowly afforded iodide (E)-4c, surface-mediated addition readily afforded (E)-4c, followed by isomerization to the Z isomer.E ->/<- Z equilibration of the alkenyl halides 4 was shown to involve, at last in part, addition-elimination via the gem-dihalides 13.Analogous behavior was exhibited by the phenylalkynes 2 and 3 on surface-mediated hydrohalogenation.Surface-mediated addition of HBr and HI to the internal alkylalkyne 14 afforded principally the anti addition products (Z)-15b,c.Treatment of the terminal alkynes 17 and 22 with (COBr)2 over alumina gave the dibromides 20 and 24/25, respectively, whereas use of acetyl bromide as the HBr precursor afforded the alkenyl bromides 18b and 23.