While the IRC for 4-methoxystyrene is almost symmetrical around the transition state it forms a plateau for 4-(trifluoromethyl)styrene flanked by a steep decay marking the onset of the C-H bond cleavage. Computationally, the series was expanded to 4-nitrostyrene due to the large positive Hammett parameter of the NO2 substituent (σ -= +1.27). In this extreme case, complete deconvolution of the rearrangement and C-H activation of the bound substrate into two separate transition states was obtained (TS-3e, ΔG ‡ = 9.4 kcal/mol; TS-3e', ΔG ‡ = 2.1 kcal/mol; see Figure in the Supporting Information). This trend as well as the calculated activation barriers can be traced back to the resonance stabilizing effect of the para-substituent. A comparison of TS-3 down the series shows that the C(α)-C(Ar 1 ) bond distances are getting shorter indicating an increasing double bond character (TS-3b: 1.412 Å, TS-3e: 1.384 Å). At the same time, NBO analysis of TS-3 reveals the negative charge accumulation at the α-C decreases in the same order (TS-3b: -0.59, TS-3e: -0.48). This ability to stabilize the negative charge at the α-C appears to be the key factor for the reactivity. To some extent, these findings resemble recent studies on the transition from a stepwise to a concerted behavior of SNAr reactions. The same effect was observed for the MgBr2-promoted reaction of 2-vinylpyridine with 1 (Figure ). Binding of the Lewis acid to the pyridine nitrogen allows for an extreme resonance structure resulting in a stepwise process and an even lower barrier than for 4-nitrostyrene. The emergent intermediate (INT-3d', ΔG = -3.7 kcal/mol) is just slightly lower in energy than the adjacent barriers ((TS-3d, ΔG = 2.5 kcal/mol; TS-3d', ΔG = 0.4 kcal/mol).