Advances for synthesis of bis-P,O-ligands and their stereo-electronic effect on Pd-catalyzed hydrocarboxylation of alkynes with formic acid without void decomposition

Hydrocarboxylation of alkynes using formic acid (FA) as carboxyl-source is highly challenging due to the competing decomposition of FA to H₂/CO₂ and to CO/H₂O to sacrifice the utilization efficiency of FA as well as to induce the side-reactions. Herein, a series of flexible and steric-bulky bis-P,O-ligands (L1-L4) were synthesized and characterized on purpose to promote this reaction by avoiding unwanted FA-decomposition. It was found that L2, characteristic with the most electron-rich nature, the bulkiest steric-hindrance, and the convenient P,P-chelation effect, enabled Pd(TFA)₂ highly active and selective for hydrocarboxylation of alkynes with FA under additive-free condition, affording the target α,β-unsaturated branched-carboxylic acids in the yield of 37–85 %. The in situ FT-IR analysis verified that the competing decomposition of FA like dehydrogenation to H₂/CO₂ and dehydration to CO/H₂O were completely inhibited, and the labile FA-based hybrid anhydride was the real product generated upon the catalysis of L2-modified Pd(TFA)₂ system.