Alkoxycarbonylation of 3,3,3‐Trifluoropropyne: an Intriguing Reaction to Prepare Trifluoromethyl‐Substituted Unsaturated Acid Derivatives

The addition of CO and methanol to 3,3,3‐trifluoropropyne is catalysed by Pd(OAc)₂ in the presence of (6‐methylpyrid‐2‐yl)diphenylphosphine and CH₃SO₃H. The main products of the reaction are the methyl esters of 2‐(trifluoromethyl)propenoic acid 1 and of 3‐(trifluoromethyl)propenoic acid 2 (4,4,4‐trifluorobut‐2‐enoic acid). The regioselectivity of the reaction can be controlled to a great extent by a suitable choice of the composition of the catalytic system and the reaction conditions. Thus, 1 can be obtained in 93% yield by using P(CO)=20 atm and high ligand/Pd and acid/Pd ratios. On the other hand, selectivity up to 85% in 2 can be achieved using P(CO)=80 atm and a low ligand/Pd ratio together with a high acid/Pd ratio. The reaction mechanism is also discussed.