SURFACE-MEDIATED REACTIONS .4. HYDROHALOGENATION OF ALKYNES

The use of appropriately prepared silica gel and alumina has been foun d to mediate the addition of hydrogen halides to alkynes. The techniqu e has been rendered even more convenient by the use of various organic and inorganic acid halides that react in the presence of silica gel o r alumina to generate hydrogen halides in situ. Treatment in this fash ion of 1-propynylbenzene (1), which underwent no reaction in CH2Cl2 so lution saturated with HCl, readily afforded the syn addition product, alkenyl chloride (E)-4a. On extended treatment (E)-4a underwent subseq uent isomerization 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 ob tained without competing formation of the radical products (E)- and (Z )-5, which occurred in solution. In contrast with solution-phase hydri odination of alkyne 1, which slowly afforded iodide (E)-4c, surface-me diated addition readily afforded (E)-4c, followed by isomerization to the Z isomer. E reversible arrow Z equilibration of the alkenyl halide s 4 was shown to involve, at least in part, addition-elimination via t he gem-dihalides 13. Analogous behavior was exhibited by the phenylalk ynes 2 and 3 on surface-mediated hydrohalogenation. Surface-mediated a ddition of Hbr and HI to the internal alkylalkyne 14 afforded principa lly the anti addition products (Z)-15b,c. Treatment of the terminal al kynes 17 and 22 with (COBr)(2) over alumina gave the dibromides 20 and 24/25,respectively, whereas use of acetyl bromide as the Hbr precurso r afforded the alkenyl bromides 18b and 23.