Oxidative Alkoxycarbonylation of Alkynes by Means of Aryl α-Diimine Palladium(II) Complexes as Catalysts

The straightforward in situ synthesized Bis-(2,6-diisopropyl)-acenaphthenequinonediimine palladium triflate catalyst was generally employed for both the mono-alkoxycarbonylation of terminal alkynes, and the bis-alkoxycarbonylation of 1,2-disubstituted alkynes by using mild reaction conditions (PCO= 4 bar, Temp. 20°C). Utilizing low catalyst loading (down to 0.5 mol%), a variety of propiolic esters were synthesized with good to excellent isolated yields. Most importantly the system was very efficient not only with methanol but also with a range of different alcohols, starting from the less hindered benzyl alcohol to the most ones, such as isopropanol and tert-butanol. In addition aromatic and aliphatic 1,2-disubstituted alkynes were converted into maleic acid derivatives, together with acid-catalyzed isomerization reaction, showing modest to good selectivity and excellent combined yields. In particular 3-hexyne showed a satisfactory degree of selectivity for the maleic diesters of methanol, and benzyl alcohol, obtaining the corresponding products with good isolated yields.