Cooperative Friedel–Crafts Alkylation of Electron‐Deficient Arenes via Catalyst Activation with Hexafluoroisopropanol**

Abstract A Friedel‐Crafts alkylation of electron‐deficient arenes with aldehydes through ′′catalyst activation′′ is presented. Through hydrogen bonding interactions, the solvent 1,1,1,3,3,3, ‐hexafluoroisopropanol (HFIP) interacted with the added Brønsted acid catalyst p TSA•H 2 O, increasing its acidity. This activated catalyst enabled the Friedel–Crafts alkylation of electron‐neutral as well as electron‐deficient arenes. Strongly electron withdrawing arenes including arenes with multiple halogen atoms, NO 2 , CHO, CO 2 R, and CN, groups acted as efficient nucleophiles in this reaction. DFT studies reveal multiple roles of solvent HFIP viz; increasing the Brønsted acidity of the catalyst p TSA•H 2 O, and stabilization of the transition states through a concerted pathway enabling the challenging reaction.