Isolation of Protonated Arenes (Wheland Intermediates) with BAr^F and Carborane Anions. A Novel Crystalline Superacid

Abstract

Protonated arenes are widely accepted intermediates in electrophilic aromatic substitution chemistry.1 Known as Wheland intermediates, but more correctly ascribed to Pfeiffer and Wizinger,2,3 they have been characterized by isolation and spectroscopy at low temperatures in superacid media.4,5 However, obtaining an X-ray crystal structure of a protonated arene salt is an unrealized experimental challenge,5 and thermal instability has prevented their development as useful reagents. In this paper, we show how modern anions lead to readily crystallized salts of remarkable thermal stability. The salt of protonated benzene is a crystalline superacid with distinct advantages over existing superacid media.