Alkoxysulfonium Salts Unlock Access to New Aryl Sulfonium Salts for Cross‐Coupling

ABSTRACT Aryl sulfonium salts are indispensable partners in cross‐coupling reactions, allowing access to aryl metal or aryl radical intermediates, and offering advantages over aryl halides or other pseudohalides. Routes to aryl sulfonium salts are limited; the standard approach involves activation of arenes using a sulfoxide‐derived electrophile. While the activation of non‐prefunctionalized arene partners is attractive, the method is limited to arenes that are sufficiently electron‐rich to engage with sulfur electrophiles and is often dominated by para ‐selectivity when monosubstituted benzenes are used. This limited access to aryl sulfonium salts leads to biased substrate scopes and restrictions on accessible chemical space. We describe a route to otherwise hard‐to‐access (hetero)aryl sulfonium salts that exploits bench‐stable alkoxysulfonium salts in combination with the well‐established magnesiation of readily available aryl bromides. The approach allows access to aryl sulfonium salts bearing electron‐deficient aryl and heteroaryl rings, or electron‐rich systems in which the sulfonium motif resides at an alternative position on the ring to that delivered by electrophilic arene sulfenylation. To illustrate their value, the new sulfonium salts have been used to expand the scope of a selection of coupling processes.