Hydrocarboxylation of alkynes with formic acid over multifunctional ligand modified Pd-catalyst with co-catalytic effect

Atmospheric hydroxycarbonylation of alkynes with HCOOH over Pd-catalyst modified by the multiple-functional ligand of L2 . • Atmospheric hydrocarbonylation of alkynes with formic acid free of CO and water. • The multifunctional ligand modified Pd-catalyst featured with co-catalytic effect. • Formic acid not only serving as CO-surrogate but also as proton/formate donor. • Synergetic activation of formic acid by the multifunctional ligand. Hydrocarboxylation of terminal alkynes with formic acid (FA) was accomplished over a multifunctional ligand ( L2 ) modified Pd-catalyst, advantageous with 100% atom-economy, free use of CO and H 2 O, mild reaction conditions, and high yields (56–89%) of α,β-unsaturated carboxylic acids with 100% regioselectivity to the branched ones. The multifunctional ligand of L2 as a zwitterion salt containing the phosphino-fragment (-PPh 2 ), Lewis acidic phosphonium cation and sulfonate group (-SO 3 − ), was constructed on the skeleton of 1.1′-binaphthyl-2.2′-diphenyl phosphine (BINAP) upon selective quaternization by 1,3-propanesultone. It was found that L2 conferred to the Pd-catalyst the co-catalytic effect, wherein the phosphino-coordinated Pd-complex was responsible for activation of all the substrates (including CO, FA and alkyne), and the incorporated phosphonium cation was responsible for synergetic activation of FA. The 1 H NMR spectroscopic analysis supported that FA was truly activated by the incorporated Lewis acidic phosphonium cation in L2 via “acid-base pair” interaction. The in situ FT-IR spectra demonstrated that, the presence of Ac 2 O and NaOAc additives in the catalytic amount could dramatically promote the in situ release of CO from FA, which was required to initiate the hydrocarboxylation.