Cluster impact activation of high-energy chemical reactions is suggested by our simulations to provide an effective process. For four-center rearrangements it is shown that the reaction can proceed by both a concerted and a two-step mechanism. The relative importance of the two mechanisms is shown to be a stereodynamic effect, governed by the nature of the cluster atoms. The energetics of the two paths are rather similar, and one can therefore control the mechanism by the choice of initial conditions.