Selective Reduction of Aromatic Alkynes Catalyzed by Palladium with Formic Acid as the Hydride Source

Background: Transfer hydrogenation methods that employ non-H2 hydrogen sources have evolved as an attractive alternative for conventional hydrogenation approaches. Objective: In this study, we aimed at developing optimized conditions to induce the selective transfer hydrogenation reduction of aromatic alkynes catalyzed by PdCl2(PPh3)2 and using formic acid as the hydride source. Methods: The effect of various reaction parameters, such as the nature and amount of the catalyst, the H-donor/base couple, reaction time and temperature, and the nature of the solvent on the outcome of the alkyne reduction were investigated. Results: The reduction of the alkyne can be chemoselectively controlled by adjusting the reaction conditions. Among the tested catalysts, PdCl2(PPh3)2 was the most suitable, with 2% of the catalyst being the optimal amount. While the reduction was successful in different solvents of different polarities, THF was selected as the solvent of choice. The reduction of diphenylacetylene yielded the alkene both at 50°C and 80°C. When testing the optimized conditions on the reduction of 4- phenyl-3-butyne-2-one, quantitative partial reduction to the corresponding α,β-unsaturated ketone was obtained at 50°C, while the saturated ketone was produced as the major product at 80°C. Conclusion: The chemoselective reduction of aromatic alkynes was performed successfully with complete conversion using 2% PdCl2(PPh3)2 as a catalyst, formic acid/NEt3 as the H-donor/base couple, THF as the solvent, at 50°C and 80°C.