Iron(III)‐Catalyzed Hydration of Unactivated Internal Alkynes in Weak Acidic Medium, under Lewis Acid‐Assisted Brønsted Acid Catalysis

Alkylarylalkynes are converted with full regioselectivity into the corresponding arylketones by formal hydration of the triple bond under weak acidic conditions, at times and temperatures (≤95 °C) comparable to those used for terminal alkynes. The process catalyzed by Fe2(SO4)3nH2O in glacial acetic acid exhibits good functional group compatibility, including that with bulky triple bond substituents, and can be extended to the one‐pot transformation of aryltrimethylsilylacetylenes into acetyl derivatives via a desilylation‐hydration sequence. The overall reactivity pattern along with proton affinity data indicate that the triple bond is activated by proton transfer rather than by π‐interaction with the metal ion. This mechanistic feature, at variance with that of noble metal catalysts, accounts for the total regioselectivity and the insensitivity to steric hindrance exhibited by the Fe2(SO4)3nH2O/AcOH catalytic system.