C(spⁿ)−X (n=1–3) Bond Activation by Iron

The iron-catalyzed oxidative addition of C(spⁿ)−X bonds (n=1–3 and X=H, CH₃, Cl) in archetypal model substrates H₃C−CH₂−X, H₂C=CH−X and HC≡C−X to Fe(CO)₄ was investigated using relativistic density functional theory at ZORA-OPBE/TZ2P. The C(spⁿ)−X bonds become substantially stronger going from C(sp³)−X to C(sp²)−X to C(sp)−X, whereas the oxidative addition reaction barrier decreases along this series. Our activation strain and energy decomposition analyses expose that the decreased reaction barrier for the oxidative addition going from sp³ to sp² to sp stems from a relief of the destabilizing (steric) Pauli repulsion between the catalyst and substrate. This originates from the decreasing coordination number of the carbon atom that goes from four in C(sp³)−X to three in C(sp²)−X to two in C(sp)−X. In analogy with our previous results on palladium-catalyzed oxidative additions, this enhances the stabilizing catalyst–substrate interaction, which is able to overcome the more destabilizing strain associated with the stronger C(spⁿ)−X bonds. This work again demonstrates that iron-based catalysts can resemble the behavior of their well-known palladium analogs in the oxidative addition step of cross-coupling reactions.