Phenyl cation: A versatile intermediate

Phenyl cations are short-lived electrophiles and can be smoothly generated by photolysis of aromatic halides (chlorides, fluorides) and esters (sulfonates, sulfates and phosphates) in protic media. Indeed, the heterolytic cleavage of an Arylsingle bondLG bond (LG = leaving group; X, OR) allows for the generation of a triplet aryl cation (3Ar+) that chemoselectively reacts with π-bond nucleophiles to perform transition metal-free arylations. Aryl cations can be also exploited in the generation of other high energy intermediates, such as diradicals (including ortho-arynes and α,n-didehydrotoluenes), potentially useful for medicinal applications. Since the generation of phenyl cations is always accompanied by the release of a strong acid (e.g. sulfonic acids), the phenyl cation precursors are actually non ionic photoacid generators (PAGs) that find useful applications in nanolithography.