A radical/polar crossover annulation between allyl‐substituted arenes and electron‐deficient alkenes is described. Cobalt‐catalyzed hydrogen atom transfer (HAT) facilitates tandem radical C−C bond formation that generates functionalized tetralin products in the face of potentially problematic hydrofluorination, hydroalkoxylation, hydrogenation, alkene isomerization, and radical polymerization reactions. The reactions proceed under mild conditions that tolerate many functional groups, leading to a broad substrate scope. This powerful ring‐forming reaction very quickly assembles complex tetralins that are the formal products of the largely infeasible Diels–Alder cycloadditions of styrenes.

A radical/polar crossover annulation between allyl‐substituted arenes and electron‐deficient alkenes is described. Cobalt‐catalyzed hydrogen atom transfer (HAT) facilitates tandem radical C−C bond formation that generates functionalized tetralin products in the face of potentially problematic hydrofluorination, hydroalkoxylation, hydrogenation, alkene isomerization, and radical polymerization reactions. The reactions proceed under mild conditions that tolerate many functional groups, leading to a broad substrate scope. This powerful ring‐forming reaction very quickly assembles complex tetralins that are the formal products of the largely infeasible Diels–Alder cycloadditions of styrenes.