Ruthenium(IV)-Catalyzed Markovnikov Addition of Carboxylic Acids to Terminal Alkynes in Aqueous Medium

The dimeric bis(allyl)ruthenium(IV) complex [{RuCl(μ-Cl)(η 3:η 3 -C 10 H 16 )} 2 ] (C 10 H 16 = 2,7-dimethylocta-2,6-diene-1,8-diyl) ( 5 ) and several mononuclear species trans -[RuCl 2 (η 3:η 3 -C 10 H 16 )(L)] (L = two-electron-donor ligand) ( 6 ) derived from 5 have been checked as catalysts for the addition of carboxylic acids onto terminal alkynes using water as a green reaction medium. The best results in terms of activity and regioselectivity were obtained with the mononuclear derivative trans -[RuCl 2 (η 3:η 3 -C 10 H 16 )(PPh 3 )] ( 6a ), which was able to promote the selective Markovnikov addition of both aromatic and aliphatic carboxylic acids to a large variety of terminal alkynes, enynes, and diynes as well as propargylic alcohols. In this way, a wide number of enol esters and β-oxo esters could be synthesized in moderate to good yields under mild conditions (60 °C) in an aqueous medium.