The addition of terminal alkynes to carboxylic acids in the presence of magnetically separable ruthenium catalysts

Enol ester structures are important building block in synthethic organic chemistry. These intermediates can be used in synthesis of halo ketones, cycloaddition and cyclopropanation reactions. The formation of C-O bonds using highly reactive alkynes and carboxylic acids catalyzed by transition metal complexes generates enol-ester derivatives via markovnikov addition. Ruthenium based catalysts show excellent selectivity and activity in carboxylic acid addition to alkynes. Ruthenium precursors such as Ru3(CO)12, (arene)RuCl2PCy3 were used for markovnikov addition reactions. On the otherhand, the reaction mechanism can be manipulated to reverse the selectivity to anti-markovnikov addition using different initatiators such as Ru(Ph2P(CH2)nPPh2)(η3-CH2CMe=CH2)2. Although several ruthenium based initiators were used for carboxylic acid addition reactions to alkynes, the number of studies that were focused on reusable catalytic systems for this reaction is limited. In this study, we report the catalytic activity of ammonium tagged Hoveyda-Grubbs catalysts that were supported on magnetically separable core/shell silica supported on carboxylic acid addition to terminal alkynes. The support material was synthesized using Stöber method by coating magnetic α-Fe2O3 particles with a thin layer of SiO2, followed by coating with a thick second SiO2 layer. Ammonium tagged Hoveyda-Grubbs catalysts were supported on core/Shell support and showed excellent activity and selectivity on acetic acid addition to phenyl acetylene. In addition the catalyst was shown to be reusable up to eleven times without a significant loss in catalytic activity. Keywords: Ruthenium, Core/Shell, Silica, Carboxylic Acid Addition, Alkynes