Ammonium Chloride/B(C6F5)3 System Catalyzed Selective Addition of Acids to Alkynes

Development of straightforward and selective approaches to functionalize vinyl groups is an important and continuing goal. A novel convenient route to vinylhalides or enol esters by a Markovnikov regioselective addition of hydrogen chloride or carboxylic acid to the C C bond of alkynes in the presence of an ammonium hydrochloride/B(C6F5)(3) catalytic system is reported. Thus, when treated with catalytic amounts of ammonium hydroborate ([TMPH](+)[ Cl-B(C6F5)(3)](-)), equimolar mixtures of hydrogen chloride and alkynes are converted into a variety of chloroalkenes as monoadducts. The yields of the monoadducts are usually higher than 90% for terminal aromatic alkynes, while for the terminal aliphatic alkynes they are considerably lower, with the worst observed for sterically hindered tert-butylacetylene (only 67%). NMR monitoring of the reaction mixtures reveals that under ambient conditions the main by-products are the corresponding diadducts (gem-dihalides). At higher temperatures (50 degrees C) for equimolar alkyne/HCl mixtures or at ambient temperature for alkyne-enriched mixtures, the diadduct formation can be nearly completely suppressed. Noteworthy, that both ammonium and borane (-ate) components of the catalytic system are essential for the conversion success. In the case of trifuoroacetic acid addition to alkynes, presence of the ammonium component is not required, with the reaction yields usually exceeding 95% for terminal aromatic alkynes and being modest to good for the aliphatic ones. The reported catalytic system presents the first example of the "metal-free" catalysts for the selective addition of acids to alkynes.