Ruthenium-Catalyzed Hydrocarboxylation of Internal Alkynes

The application of the highly efficient ruthenium catalyst [Ru(CO)(2){P(p-CF3-C6H4)(3)}(2)(O2CPh)(2)] (1) in the selective synaddition of carboxylic acids to internal alkynes, yielding valuable trisubstituted enol esters with (E)-configuration, is described. All reactions feature excellent stereoselectivities and good regioselectivities. The regioselectivity is dictated by electronic and steric aspects of the alkyne substituents and the acidity of the carboxylic acid. The catalytic activity can be significantly increased by the addition of catalytic amounts of B(C6F5)(3). Relative to known catalysts for the synthesis of (E)-enol esters, this methodology offers improved selectivity and requires lower catalyst loadings. Moreover, a broad range of alkynes and carboxylic acids can be successfully converted to their corresponding (E)-enol esters in high yields.