Based on the orbital energies (e), the only possible interaction is between the LUMO of the radical cation II-1a-2a and the HOMO of the alkyne 1. Hence, the coupling between II and 1 is best described as a formal [4+2] cycloaddition 34 of the C-N and C-C π-bonds with no involvement of the radical center. To match energetically the low lying LUMO of the radical cation II, the HOMO of the alkyne must be low enough in energy, favoring acceptor substitution at the alkyne. A good indication that the mechanistic hypothesis of a formal [4+2] is a suitable description is the observation that the FMO-gaps (De) are of predictive power for substrate selectivity. The correlation of De-values and the experimentally observed preference of II-1a-2a for electron-poor alkynes like 1k and 1p over 1a is readily explained with the smaller FMO-gap, implying a decreasing barrier of this elementary step. The signs of the interacting orbitals (Figure ) suggest that a concerted path is possible, leading to a highly resonance stabilized radical cation III that is significantly downhill in regard to the starting structures (ΔRH = -110.1 kcal/mol for II-1a-2a + 1p). Also, this concerted path explains the regioselectivity for the observed 1,2,5substituted III, which can delocalize the unpaired electron over the aryl ring of the former alkyne reactant, whereas the opposite regioisomer lacks this possibility (see. S.I. for details).