Non-reversible (2+4) addition. In absence of the Lewis acid additive 2-vinylpyridine forms a (2+4) cycloaddition product with 1 (Figure ). Addition of 2-vinylpyridine to a solution of 1 in tolune-d8 at -35 °C resulted in the formation of 5d in ca. 85 % NMR yield alongside with 4d as minor side product. The 1 H NMR spectrum of 5d recorded at -40 °C shows a sharp triplet resonance at δH = 3.81 ppm diagnostic for the α-CH group of the (2+4) bound substrate. Warming the reaction solution to room temperature resulted in the slow decomposition of 5d (see Supporting Information) and did not convert into 2d. These results suggest that the (2+4) addition represents a competitive pathway preventing the C-H activation reaction. The analogue product 5e was obtained from the reaction of 1 with methyl acrylate at room temperature, with the (E)-C-H activation product 4e also being formed in a 15 % NMR yield. In the case of 5e, crystals suitable for X-ray diffraction could be grown confirming the structure of the (2+4) cycloaddition product with the β-CH2 attached to Fe and the oxygen of the former carbonyl group bound to Al (Figure ).