many pericyclic reactions have similar stepwise radical processes connected with them. chemists disagree whether some reactions are pericyclic reactions. for example, it is not definitively known whether the [2+2] cycloaddition mechanism is concerted (or may depend on the reactive system). many pericyclic reactions have similar reactions that are metal-catalyzed. but these metal-catalyzed reactions are also not really pericyclic. the metal catalysts stabilize the reaction intermediates. so the reaction is not concerted, but rather metal-stabilized.